Name: preparation and use of carbonyl-decorated carbenes in the activation of white phosphorus
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Description: Watch this Scientific Journal Video about Preparation and Use of Carbonyl-decorated Carbenes in the Activation of White Phosphorus at JoVE.com

We are grateful to the Research Corporation for Science Advancement (20092), the National Science Foundation (CHE-1362140), and Texas State University for their generous support.

1. Synthesis of Diamidocarbene (Compound 1)
<ol>
	<li>Connect an oven-dried 100 ml Schlenk flask to a high performance vacuum manifold, evacuate it and flush with nitrogen. Add a stir bar to the flask and cap with a rubber septum. Weigh out N,N&#8217;-dimesitylformamidine12 (1.5 g, 5.35 mmol) and add it to the flask while flushing with nitrogen.
	<ol>
		<li>Add (via dry, deoxygenated syringes) 30 ml of dry, degassed dichloromethane (DCM) followed by triethylamine (1.1 ml, 8.0 mmol, 1.5 equiv.). Cool the resulting solution to 0 &#176;C in an ice bath.</li>
		<li>Add dropwise (via a dry, deoxygenated syringe) dimethylmalonyl dichloride (0.75 ml, 5.60 mmol, 1.05 equiv.) to the cooled solution. Upon addition, allow the solution to stir at 0 &#176;C for 1 hr under an atmosphere of nitrogen. Once the solution has stirred for 1 hr, remove all the volatile materials under vacuum.</li>
		<li>Add a solvent mixture of dry, degassed hexanes:DCM (2:1 by volume, 24 ml total volume) to the residue in the reaction flask. Let the mixture triturate for 10 min.</li>
		<li>Meanwhile, attach a filter tube equipped with a medium porosity glass frit to the top of an oven-dried 100 ml Schlenk flask (see Figure 1). To the filter tube, add enough oven-dried Celite to create a filter plug approximately 2 inches&#160;tall. Seal the filter tube with a rubber septum, and pull vacuum, on the system by attaching the Schlenk flask to the vacuum manifold. 
		<img alt="Figure 1" fo:content-width="2.5in" src="/files/ftp_upload/52149/52149fig1highres.jpg" width="250" /> 
		Figure 1. Filtration apparatus assembly for removal of [HNEt3][Cl] from carbene precursors 1-HCl and 2-HCl. This apparatus is also used to remove NaCl formed during the synthesis of carbenes 1 and 2.</li>
		<li>Filtration apparatus assembly for removal of [HNEt3][Cl] from carbene precursors 1-HCl and 2-HCl. This apparatus is also used to remove NaCl formed during the synthesis of carbenes 1 and 2.</li>
		<li>Once the filtration apparatus has been assembled and is under vacuum, transfer the white suspension in the reaction flask via cannula into the filter tube. Be sure to periodically pull vacuum on the collection Schlenk flask to ensure that all of the solution filters through the Celite.</li>
		<li>Wash the Celite by adding a solvent mixture of dry, degassed hexanes:DCM (2:1, volume, 18 ml total volume) using a syringe through the rubber septum. Again, periodically pull vacuum on the collection Schlenk flask to ensure that all of the solution filters through the Celite.</li>
		<li>Disconnect the collection Schlenk flask from the filter tube under a flush of nitrogen, then seal the collection Schlenk flask with a glass stopper. Remove all of the solvent from the collection Schlenk flask under vacuum to afford the precursor for diamidocarbene 1 (1-HCl) as an air/moisture sensitive white powder in approximately 92% yield (2.04 g). The product can be verified by 1H and 13C NMR spectroscopy (CDCl3)13,14. Transfer compound 1-HCl into a glove box for storage prior to the next step.</li>
	</ol>
	</li>
	<li>To prepare the diamidocarbene 1, first transfer an oven-dried 100 ml Schlenk flask equipped with a stir bar and a glass stopper into a nitrogen-filled glove box.
	<ol>
		<li>Weigh out carbene precursor 1-HCl (0.600 g, 1.45 mmol) and sodium hexamethyldisilazide (NaHMDS, 0.267 g, 1.46 mmol) and place both solids into the Schlenk flask.</li>
	</ol>
	</li>
	<li>Add dry, degassed benzene (25 ml) to the two solids in the Schlenk flask and then stopper the flask. At this point, the Schlenk flask can be removed from the glove box.</li>
	<li>Stir the solution of the carbene at RT for 30 min. Throughout the reaction, the solution will become a turbid yellow-orange color. During this time, set up a filtering apparatus similar to the one described above (using a 1 inch plug of Celite) for the synthesis of 1-HCl.</li>
	<li>Filter the carbene solution (to remove precipitated NaCl) as described for the precursor 1-HCl. Once the solution has been filtered, remove all of the volatiles using vacuum to afford the crude carbene 1 as a yellow-orange powder. Further purify carbene 1 by washing the solid with cold hexanes (~10 ml) to afford analytically pure compound as an air/moisture sensitive white powder in approximately 85% yield (0.462 g). Verify the product by 1H and 13C NMR spectroscopy (C6D6)13.</li>
</ol>
2. Synthesis of Monoamidocarbene (Compound 2)
<ol>
	<li>Connect an oven-dried 250 ml Schlenk flask to a high performance vacuum manifold, evacuate it and flush with nitrogen. Add a stir bar to the flask and cap with a rubber septum. Weigh out N,N&#8217;-dimesitylformamidine (3.00 g, 10.70 mmol) and add it to the flask while flushing with nitrogen.
	<ol>
		<li>Add (via dry, deoxygenated syringes) 125 ml of dry, degassed DCM followed by triethylamine (2.25 ml, 16.05 mmol, 1.5 equiv.). Cool the resulting solution to 0 &#176;C in an ice bath.</li>
		<li>Add dropwise (via a dry, deoxygenated syringe) 3-chloropivaloyl chloride (1.54 ml, 11.77 mmol, 1.1 equiv.) to the cooled solution. Upon addition, allow the solution to stir at 0 &#176;C for 30 min&#160;under an atmosphere of nitrogen. Then gradually warm the solution to RT, and then remove all the volatile materials under vacuum. After the solvent is removed, a white solid residue will remain.</li>
		<li>Add toluene (200 ml) to the white solid and allow the suspension to triturate for 1 hr. Then filter the mixture over a 1 inch plug of Celite using a medium porosity glass fritted B&#252;chner funnel.</li>
		<li>Transfer the toluene solution to a 500 ml round bottom flask equipped with a stir bar. Connect a reflux condenser to the flask and heat the solution to reflux (110 &#176;C) for 16 hr. Over the course of the reaction, a white precipitate will form.</li>
		<li>After 16 hr&#160;allow the suspension to cool to RT. During this time, more solid will precipitate from the solution. Collect the solid via vacuum filtration and wash the solid with cold toluene (3 x 20 ml).</li>
		<li>Dry the obtained white solid using vacuum to afford the precursor for monoamidocarbene 2 (2-HCl) as an air stable white powder in approximately 91% yield (3.32 g). The product can be verified by 1H and 13C NMR spectroscopy (CDCl3)15. Transfer compound 2-HCl into a glove box for storage prior to the next step.</li>
	</ol>
	</li>
	<li>To prepare the monoamidocarbene 2, first transfer an oven-dried 100 ml Schlenk flask equipped with a stir bar and a glass stopper into a nitrogen-filled glove box.
	<ol>
		<li>Weigh out carbene precursor 2-HCl (0.500 g, 1.25 mmol) and NaHMDS (0.241 g, 1.32 mmol) and place both solids into the Schlenk flask.</li>
		<li>Add dry, degassed benzene (45 ml) to the two solids in the Schlenk flask and then stopper the flask. At this point, the Schlenk flask can be removed from the glove box.</li>
		<li>Stir the solution of the carbene at RT for 30 min. During the reaction, the solution will become a turbid yellow color. Meanwhile, set up a filtering apparatus similar to the one described above for the synthesis of 1-HCl using a 1 inch plug of Celite.</li>
		<li>Filter the carbene solution (to remove precipitated NaCl) as described for carbene 1. Once the solution has been filtered, remove all of the volatiles using vacuum to afford the crude carbene 2 as a tan powder. Further purify carbene 2 by washing the solid repeatedly with pentane to afford analytically pure compound as an air/moisture sensitive white powder in approximately 62% yield (0.309 g). Verify the product by 1H and 13C NMR spectroscopy (C6D6)15.</li>
	</ol>
	</li>
</ol>
3. Synthesis of a Tris(phosphaalkenyl)phosphane (Compound 3)
Caution Statement: White phosphorus is extremely pyrophoric as well as toxic and should be handled cautiously in a glovebox whenever possible.
<ol>
	<li>To make a tris(phosphaalkenyl)phosphane (compound 3), weigh out diamidocarbene 1 (0.100 g, 0.266 mmol, 3 equiv.) and white phosphorus (P4, 0.011 g, 0.089 mmol, 1 equiv.) inside of a nitrogen-filled glove box with the lights turned off. Turn off as many lights in the lab during these first few steps as P4&#160;is light sensitive.</li>
	<li>Add the two solids to a 20 ml glass vial that is wrapped in aluminum foil. Add dry, degassed diethyl ether (Et2O, 10 ml) to the solids and then cap the vial. Stir the slurry in the dark for 2 hr. During the course of the reaction, a bright red-orange precipitate will form.</li>
	<li>Isolate the red solid via filtration using a 10 ml medium porosity glass fritted B&#252;chner funnel. Wash the red solid with Et2O (4 x 5 ml) and then dry it under vacuum to afford compound 3 as an analytically pure air stable compound in approximately 82% yield, 0.092 grams (based on P4). Verify the product by 1H and 31P NMR spectroscopy (C6D6)11.</li>
</ol>
4. Synthesis of a Carbene-stabilized P8&#160;Allotrope (Compound 4)
<ol>
	<li>Method A
	<ol>
		<li>To make a carbene-stabilized P8 allotrope (compound 4), weigh out monoamidocarbene 2 (0.100 g, 0.276 mmol, 3 equiv.) and P4 (11.4 mg, 0.092 mmol, 1 equiv.) inside of a nitrogen-filled glove box with the lights turned off. Turn off as many lights in the lab during these first few steps as P4&#160;is light sensitive.</li>
		<li>Add the two solids to a 20 ml glass vial that is wrapped in aluminum foil. Add dry, degassed diethyl ether (Et2O, 10 ml) to the solids and then cap the vial. Upon addition of the ether, the fleeting dark green color rapidly changes to bright orange. Stir the slurry in the dark for 2 hr. During the course of the reaction, a bright orange precipitate will form.</li>
		<li>Isolate the orange solid via filtration using a 10 ml medium porosity glass fritted B&#252;chner funnel. Wash the orange solid with Et2O (4 x 2 ml) and then dry it under vacuum to afford compound 4 as and analytically pure air stable compound in approximately 51% yield, 39.5 mg (based on P4). Verify the product by 1H and 31P NMR spectroscopy (THF-D8)11.</li>
	</ol>
	</li>
	<li>Method B
	<ol>
		<li>Weigh out monoamidocarbene 2 (0.100 g, 0.276 mmol, 2 equiv.) and P4 (17.1 mg, 0.138 mmol, 1 equiv.) inside of a nitrogen-filled glove box with the lights turned off. Turn off as many lights in the lab during these first few steps as P4&#160;is light sensitive.</li>
		<li>Add the two solids to a 20 ml glass vial that is wrapped in aluminum foil. Add dry, degassed hexanes (10 ml) to the solids and then cap the vial. Upon addition of the ether, the fleeting dark green color rapidly changes to bright orange. Stir the slurry in the dark for 2 hr. During the course of the reaction, a bright orange precipitate will form.</li>
		<li>Isolate the orange solid via filtration using a 10 ml medium porosity glass fritted B&#252;chner funnel, and then dry it under vacuum to afford compound 4 as and analytically pure air stable compound in approximately 75% yield, 87.7 mg (based on P4). Verify the product by 1H and 31P NMR spectroscopy (THF-D8).11</li>
	</ol>
	</li>
</ol>
5. Trapping a Transient E-1,2-bis(phosphaalkenyl)diphosphene via [4+2] Cycloaddition: Synthesis of Compound 5
<ol>
	<li>To prepare compound 5, weigh out monoamidocarbene 2 (0.300 g, 0.828 mmol, 2 equiv.) and P4 (51.3 mg, 0.414 mmol, 1 equiv.) separately inside of a nitrogen-filled glove box with the lights turned off. Turn off as many lights in the lab during these first few steps as P4&#160;is light sensitive.</li>
	<li>Add the P4 to a 20 ml glass vial, and then add dry, degassed hexanes (18 ml) to the vial. Next, add 2,3-dimethyl-1,3-butadiene (2 ml) to the P4 suspension in hexanes.</li>
	<li>While the hexane/P4 suspension is rapidly stirring, add the carbene 2 as a solid in one portion. The suspension will instantly become bright yellow. Over a period of about 10 min, all of the solids will dissolve followed by the precipitation of a bright yellow solid. At this point, let the suspension stir for 4 hr.</li>
	<li>After 4 hr, isolate the yellow solid via filtration using a 10 ml medium porosity glass fritted B&#252;chner funnel. This yellow solid is compound 5 (&#62; 90% purity by both 1H and 31P NMR). Concentrate the yellow supernatant solution to dryness and combine the yellow residue with the filtered yellow solid.</li>
	<li>To purify compound 5, recrystallize the combined yellow solids from a 1:3 (by volume) DCM:hexanes solution (12 ml total volume) in a -30 &#176;C freezer in the glovebox O/N. This procedure will afford 5 as analytically pure air stable yellow crystals in approximately 71% yield, 0.301 grams (based on P4). Verify the product by 1H and 31P NMR spectroscopy (C6D6)11.</li>
</ol>

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The authors have nothing to disclose.

A straightforward procedure for generating carbonyl-decorated carbenes and their application in the activation of white phosphorus is presented here. The critical steps in the protocol for synthesizing the carbenes are: (a) make sure all solvents are properly dried before use, (b) make sure the addition of acid chlorides to the formamidine is done very slowly, (c) if the Celite is not oven-dried for a minimum of 12 hr&#160;at 180 &#176;C, hydrolysis of the 1-HCl as well as carbenes 1&#160;and 2&#160;occur. In some cases, the white phosphorous will convert to red phosphorous. For the P4 activation reactions, it is imperative that the reactions be conducted in the dark or in foil-wrapped reaction vessels to ensure that the white phosphorus does not convert into red phosphorus.
There are no major limitations of the techniques described herein, and indeed these methods may be applied to the future synthesis of other carbenes. One significant advantage to our method of preparing the described carbenes is the utilization of NaHMDS as the base for the deprotonation of compounds 1-HCl and 2-HCl. NaHMDS is well-suited to the generation of carbenes as it is soluble in aromatic hydrocarbons in which the vast majority of carbenes are stable.

Stable carbenes have emerged as ubiquitous reagents in homogeneous catalysis1, organocatalysis2,&#160;materials science3,4, and more recently main group chemistry5-9.&#160;In the context of the latter, stable carbenes have recently been used in the activation and functionalization of white phosphorus (P4)5-9.&#160;The ability to directly convert P4&#160;into organophosphorus compounds has become a topical research objective in an effort to develop &#8220;greener&#8221; methods that circumvent the use of chlorinated or oxychlorinated phosphorus precursors. Despite their widespread use, the preparation and handling of carbenes and reactive compounds such as P4 can be a daunting task. For this reason, we have written this manuscript to provide a clear and concise protocol that will allow synthetic chemists of all skill levels to synthesize and manipulate two very unique stable carbenes. Additionally, the activation of P4&#160;using the described carbenes is detailed.
Herein we detail a protocol for the synthesis of two electron-deficient carbonyl decorated carbenes. We have chosen these carbenes because they differ only in their electrophilic properties, and not their steric parameters, making them ideal for studying the effects of carbene electronics on reactivity. The importance of carbene electronics with regard reactivity is exemplified by two similar compounds of the general formula carbene-P2-carbene that have been reported by Bertrand and Robinson5,8.&#160;Bertrand&#8217;s P2&#160;derivative is supported by two cyclic alkyl amino carbene (CAAC) ligands, and is structurally, photophysically, and electrochemically different than Robinson&#8217;s compound which is a P2 fragment supported by two N-heterocyclic carbenes (NHCs)5,8. Indeed, Bertrand&#8217;s P2&#160;complex is characterized as a yellow solid that features carbene-to-phosphorus double bonds in the solid state, whereas the derivative reported by Robinson is a dark red solid that contains NHC&#8594;P dative bonds. This structural difference also manifests itself electrochemically such that Robinson&#8217;s compound contains more electron-rich phosphorus centers that can undergo reversible 1- or 2-electron oxidations in contrast to Bertrand&#8217;s compound which can only undergo a single reversible oxidation10.
Based on the studies described above, we became interested in studying the activation of P4 using the highly electrophilic diamido- and monoamidoamino carbenes to determine if novel carbene-stabilized allotropes of phosphorus could be prepared. We focused on diamidocarbene (DAC) 1, and monoamidoamino carbene (MAAC) 2 which differ only in their respective electrophilicities to interrogate what role carbene electronics play in P4 activation. Interestingly when the more electrophilic DAC is used, a tris(phosphaalkenyl)phosphane (3) could be isolated as the exclusive product, whereas when a MAAC is used, a carbene-stabilized P8 allotrope (4) can be obtained11. We also interrogated the mechanism for the formation (4), and found that it is formed via a [2+2] cylcoaddition dimerization reaction of a transient diphosphene. The existence of this diphosphene was confirmed by trapping it with 2,3-dimethyl-1,3-butadiene to furnish the [4+2] cycloaddition adduct 5. The protocol for synthesizing these carbonyl-decorated carbenes and their corresponding P4 activated compounds is described herein.

<table><tbody><tr><td>2,4,6-Trimethylaniline</td><td>Alfa Aesar</td><td>AAA13049-0E</td><td>98%</td></tr><tr><td>Triethylorthoformate</td><td>Alfa Aesar</td><td>AAA13587</td><td>98%</td></tr><tr><td>Dimethylmalonyl dichloride</td><td>TCI</td><td>D2723</td><td>&gt;98%</td></tr><tr><td>3-Chloro-pivaloyl chloride</td><td>Aldrich</td><td>225703-25G</td><td>98%</td></tr><tr><td>Triethylamine</td><td>Alfa Aesar</td><td>AAA12646</td><td>Stored over dried, activated 3 &amp;Aring; molecular sieves</td></tr><tr><td>Celite&amp;trade; 545</td><td>EMD</td><td>CX0574-3D</td><td>Oven-dried at 180 &amp;deg;C for a minimum of 12 hr</td></tr><tr><td>Sodium hexamethyldisilazide</td><td>Across</td><td>200014-462</td><td>95+%</td></tr><tr><td>2,3-Dimethyl-1,3-butadiene</td><td>Alfa Aesar</td><td>AAAL04207-09</td><td>98%</td></tr><tr><td>Dichloromethane</td><td>EMD</td><td>DX0835-5</td><td>Purified through solvent purification system, or standard methods</td></tr><tr><td>Tetrahydrofuran</td><td>Mallinckrodt</td><td>8498-09</td><td>Purified through solvent purification system, or standard methods</td></tr><tr><td>Hexanes</td><td>EMD</td><td>HX0299-3</td><td>Purified through solvent purification system, or standard methods</td></tr><tr><td>Benzene</td><td>EMD</td><td>BX0220-5</td><td>Purified through solvent purification system, or standard methods</td></tr><tr><td>Toluene</td><td>BDH</td><td>1151-19L</td><td>Purified through solvent purification system, or standard methods</td></tr><tr><td>White phosphorus</td><td>Generously donated from the Texas A&amp;amp;M chemistry store room.</td><td></td><td>Purified through sublimation and transferred directly into a glovebox while under vacuum in the sublimator</td></tr></tbody></table>

preparation and use of carbonyl-decorated carbenes in the activation of white phosphorus

Here we present a protocol for the synthesis of two distinct carbonyl-decorated carbenes. Both carbenes can be prepared using nearly identical procedures in multi-gram scale quantities. The goal of this manuscript is to clearly detail how to handle and prepare these unique carbenes such that a synthetic chemist of any skill level can work with them. The two carbenes described are a diamidocarbene (DAC, carbene 1) and a monoamidoaminocarbene (MAAC 2). These carbenes are highly electron-deficient and as such display reactivity profiles that are atypical of more traditional N-heterocyclic carbenes. Additionally, these two carbenes only differ in their electrophilic character and not their steric parameters, making them ideal for studying how carbene electronics influence reactivity. To demonstrate this phenomenon, we are also describing the activation of white phosphorus (P4) using these carbenes. Depending on the carbene used, two very different phosphorus-containing compounds can be isolated. When the DAC 1 is used, a tris(phosphaalkenyl)phosphane can be isolated as the exclusive product. Remarkably however, when MAAC 2 is added to P4 under identical reaction conditions, an unexpected carbene-supported P8&#160;allotrope of phosphorus is isolated exclusively. Mechanistic studies demonstrate that this carbene-supported P8allotrope forms via a [2+2] cycloaddition dimerization of a transient diphosphene which has been trapped by treatment with 2,3-dimethyl-1,3-butadiene.

The ability to isolate a tris(phosphaalkenyl)phosphane such as 3&#160;or the P8-allotrope (4) from white phosphorus relies on the use of an electrophilic carbene to activate the P4&#160;tetrahedron11,16.&#160;Therefore, it is critical to prepare carbenes with enhanced &#960;-acidity, and by extension electrophilicity. Figure 2&#160;illustrates the synthesis of carbene precursor 1-HCl and its subsequent deprotonation to afford the diamidocarbene 113.&#160;The synthesis of diamidocarbene 1&#160;can be accomplished in a single day (approximately 6 hr&#160;from start to finish), and the carbene can be isolated as a white powder in 72% overall yield.
<img alt="Figure 2" fo:content-width="6in" src="/files/ftp_upload/52149/52149fig2highres.jpg" width="600" /> 
Figure 2.&#160;Synthesis of diamidocarbene 1 by coupling&#160;N,N&#8242;-dimesitylformamidine to dimethylmalonyl dichloride. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/52149/52149fig2highres.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
By removing one of the carbonyl moieties from carbene 1, the &#960;-acidity of the diamidocarbene can be attenuated. To accomplish this task, monoamidocarbene 2&#160;can be prepared in a similar manner to carbene 1 using 3-chloropivaloyl chloride and N,N&#8242;-dimesitylformamidine15.&#160;Figure 3&#160;describes the synthesis of 2&#160;which can be carried out in approximately two days. The free monoamidocarbene can be isolated as a white powder in 56% overall yield.
<img alt="Figure 3" fo:content-width="6in" src="/files/ftp_upload/52149/52149fig3highres.jpg" width="600" /> 
Figure 3.&#160;Synthesis of monoamidocarbene 2 by coupling&#160;N,N&#8242;-dimesitylformamidine to 3-chloropivaloyl chloride. To date there have been several reports that detail the activation of white phosphorus using stable carbenes. In these studies, it has been well-demonstrated that the electronic properties of the carbenes directly govern the identity of the activated phosphorus product5.&#160;To demonstrate this phenomenon, carbenes 1&#160;and 2, which differ only in their respective electrophilicities, can be used to activate P4&#160;to afford very different products. When the more electrophilic diamidocarbene 1 is used, the tris(phosphaalkenyl)phosphane (3) can be prepared as a red solid in 82% yield (Figure 4). However, when the less electrophilic monoamidocarbene 2&#160;is used, the P8-allotrope (4) can be isolated as an orange solid in yields ranging from 51-75% yield depending on the conditions used (Figure 4). <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/52149/52149fig3highres.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 4" fo:content-width="6in" src="/files/ftp_upload/52149/52149fig4highres.jpg" width="600" /> 
Figure 4.&#160;Synthesis of tris(phosphaalkenyl)phosphane 3 and carbene-stabilized P8&#160;allotrope 4&#160;starting from carbenes 1&#160;and 2, respectively (Mes = 2,4,6(CH3)3C6H2).
A mechanism has been proposed for the formation of compounds 3 and 4 (Figure 5) which describes how the differing electrophilicities of carbenes 1 and 2&#160;influences the reaction with P4. For both carbenes, zwitterionic intermediate A, which features two coordinated carbene ligands has been suggested as forming initially upon activation of the P4&#160;tetrahedron. When the more electrophilic diamidocarbene 1&#160;is used, intermediate A is sufficiently nucleophilic to add to the empty p-orbital of a third molecule of 1, eventually resulting in the formation of tris(phosphaalkenyl)phosphane 3&#160;through intermediate B. However, when the less electrophilic carbene 2&#160;is used, A is not sufficiently nucleophilic to add a third molecule of 2, and subsequently rearranges to afford the linear diphosphene intermediate C. Intermediate C then rapidly undergoes a [2+2] cycloaddition-dimerization to afford the P8-allotrope 4. It is proposed that intermediate C is the source of the dark green color observed in the synthesis of the 4. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/52149/52149fig4highres.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 5" fo:content-width="6in" src="/files/ftp_upload/52149/52149fig5highres.jpg" width="600" /> 
Figure 5.&#160;Proposed mechanism for the formation of compounds 3 and 4. The formation of the putative diphosphene intermediate C was verified by trapping with 2,3-dimethyl-1,3-butadiene to afford compound 5 (Figure 6). In a typical experiment, when the activation of P4&#160;using carbene 2&#160;is carried out in a large excess of 2,3-dimethyl-1,3-butadiene, compound 5&#160;can be isolated as a bright yellow solid in 71% yield. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/52149/52149fig5highres.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 6" fo:content-width="6in" src="/files/ftp_upload/52149/52149fig6highres.jpg" width="600" /> 
Figure 6.&#160;Synthesis of compound 5 by trapping intermediate C with 2,3-dimethyl-1,3-butadiene (Mes = 2,4,6(CH3)3C6H2). To demonstrate the effectiveness of these synthetic methods, we have provided 1H NMR spectra for carbenes 1&#160;and 2&#160;as well as 31P NMR spectra for compounds 3, 4, and 5&#160;(see Figures 7-11, respectively). <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/52149/52149fig6highres.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 7" fo:content-width="6in" src="/files/ftp_upload/52149/52149fig7highres.jpg" width="600" /> 
Figure 7.&#160;1H NMR (C6D6) of DAC 1&#160;prepared using the described protocol.
<a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/52149/52149fig7highres.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 8" fo:content-width="6in" src="/files/ftp_upload/52149/52149fig8highres.jpg" width="600" /> 
Figure 8.&#160;1H NMR (C6D6) of MAAC 2&#160;prepared using the described protocol. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/52149/52149fig8highres.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 9" fo:content-width="6in" src="/files/ftp_upload/52149/52149fig9highres.jpg" width="600" /> 
Figure 9.&#160;31P NMR (C6D6) of 3&#160;prepared using the described protocol.
<a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/52149/52149fig9highres.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 10" fo:content-width="6in" src="/files/ftp_upload/52149/52149fig10highres.jpg" width="600" /> 
Figure 10.&#160;31P NMR (THF-D8) of 4&#160;prepared using the described protocol. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/52149/52149fig10highres.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 11" fo:content-width="6in" src="/files/ftp_upload/52149/52149fig11highres.jpg" width="600" /> 
Figure 11.&#160;31P NMR (C6D6) of 5&#160;prepared using the described protocol. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/52149/52149fig11highres.jpg" target="_blank">Please click here to view a larger version of this figure.</a>

Watch this Scientific Journal Video about Preparation and Use of Carbonyl-decorated Carbenes in the Activation of White Phosphorus at JoVE.com

Preparation and Use of Carbonyl-decorated Carbenes in the Activation of White Phosphorus

Here, we present a protocol for the synthesis of two carbonyl-decorated carbenes. The protocol makes these interesting compounds readily available to chemists of all skill levels. In addition to the synthesis of these two carbenes, their use in the activation of white phosphorus is also described.

Here we present a protocol for the synthesis of two distinct carbonyl-decorated carbenes. Both carbenes can be prepared using nearly identical procedures ...

preparation-use-carbonyl-decorated-carbenes-activation-white

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13.6K Views.  Texas State University. Here, we present a protocol for the synthesis of two carbonyl-decorated carbenes. The protocol makes these interesting compounds readily available to chemists of all skill levels. In addition to the synthesis of these two carbenes, their use in the activation of white phosphorus is also described.

Video: Preparation and Use of Carbonyl-decorated Carbenes in the Activation of White Phosphorus

Reverse Microemulsion-mediated Synthesis of Monometallic and Bimetallic Early Transition Metal Carbide and Nitride Nanoparticles

A reverse microemulsion is used to encapsulate monometallic or bimetallic early transition metal oxide nanoparticles in microporous silica shells. The silica-encapsulated metal oxide nanoparticles are then carburized in a methane/hydrogen atmosphere at temperatures over 800 &#176;C to form silica-encapsulated early transition metal carbide nanoparticles. During the carburization process, the silica shells prevent the sintering of adjacent carbide nanoparticles while also preventing the deposition of excess surface carbon. Alternatively, the silica-encapsulated metal oxide nanoparticles can be nitridized in an ammonia atmosphere at temperatures over 800 &#176;C to form silica-encapsulated early transition metal nitride nanoparticles. By adjusting the reverse microemulsion parameters, the thickness of the silica shells, and the carburization/nitridation conditions, the transition metal carbide or nitride nanoparticles can be tuned to various sizes, compositions, and crystal phases. After carburization or nitridation, the silica shells are then removed using either a room-temperature aqueous ammonium bifluoride solution or a 0.1 to 0.5 M NaOH solution at 40-60 &#176;C. While the silica shells are dissolving, a high surface area support, such as carbon black, can be added to these solutions to obtain supported early transition metal carbide or nitride nanoparticles. If no high surface area support is added, then the nanoparticles can be stored as a nanodispersion or centrifuged to obtain a nanopowder.

A &#8220;removable ceramic coating method&#8221; is presented in visual format for the synthesis of non-sintered and metal-terminated monometallic and bimetallic early transition metal carbide and nitride nanoparticles with tunable sizes and crystal structures.

A reverse microemulsion is used to encapsulate monometallic or bimetallic early transition metal oxide nanoparticles in microporous silica shells. The ...

Preparation of Carbon Nanosheets at Room Temperature

Amphiphilic molecules equipped with a reactive, carbon-rich "oligoyne" segment consisting of conjugated carbon-carbon triple bonds self-assemble into defined aggregates in aqueous media and at the air-water interface. In the aggregated state, the oligoynes can then be carbonized under mild conditions while preserving the morphology and the embedded chemical functionalization. This novel approach provides direct access to functionalized carbon nanomaterials. In this article, we present a synthetic approach that allows us to prepare hexayne carboxylate amphiphiles as carbon-rich siblings of typical fatty acid esters through a series of repeated bromination and Negishi-type cross-coupling reactions. The obtained compounds are designed to self-assemble into monolayers at the air-water interface, and we show how this can be achieved in a Langmuir trough. Thus, compression of the molecules at the air-water interface triggers the film formation and leads to a densely packed layer of the molecules. The complete carbonization of the films at the air-water interface is then accomplished by cross-linking of the hexayne layer at room temperature, using UV irradiation as a mild external stimulus. The changes in the layer during this process can be monitored with the help of infrared reflection-absorption spectroscopy and Brewster angle microscopy. Moreover, a transfer of the carbonized films onto solid substrates by the Langmuir-Blodgett technique has enabled us to prove that they were carbon nanosheets with lateral dimensions on the order of centimeters.

We present the synthesis of an amphiphilic hexayne and its use in the preparation of carbon nanosheets at the air-water interface from a self-assembled monolayer of these reactive, carbon-rich molecular precursors.

Amphiphilic molecules equipped with a reactive, carbon-rich "oligoyne" segment consisting of conjugated carbon-carbon triple bonds self-assemble into ...

Palladium N-Heterocyclic Carbene Complexes: Synthesis from Benzimidazolium Salts and Catalytic Activity in Carbon-carbon Bond-forming Reactions

Detailed and generalized protocols are presented for the synthesis and subsequent purification of four palladium N-heterocyclic carbene complexes from benzimidazolium salts. Detailed and generalized protocols are also presented for testing the catalytic activity of such complexes in arylation and Suzuki-Miyaura cross-coupling reactions. Representative results are shown for the catalytic activity of the four complexes in arylation and Suzuki-Miyaura type reactions. For each of the reactions investigated, at least one of the four complexes successfully catalyzed the reaction, qualifying them as promising candidates for catalysis of many carbon-carbon bond-forming reactions. The protocols presented are general enough to be adapted for the synthesis, purification and catalytic activity testing of new palladium N-heterocyclic carbene complexes.

Palladium N-Heterocyclic Carbene Complexes: Synthesis from Benzimidazolium Salts and Catalytic Activity in Carbon-carbon Bond-forming Reactions

Detailed and generalized protocols are presented for the synthesis and subsequent purification of four palladium N-heterocyclic carbene complexes from benzimidazolium salts. The complexes were tested for catalytic activity in arylation and Suzuki-Miyaura reactions. For each reaction investigated, at least one of the four complexes successfully catalyzed the reaction.

Detailed and generalized protocols are presented for the synthesis and subsequent purification of four palladium N-heterocyclic carbene complexes from ...

Preparation and Reactivity of a Triphosphenium Bromide Salt: A Convenient and Stable Source of Phosphorus(I)

We present herein the optimized synthesis of a triphosphenium bromide salt. Apart from being a versatile metathesis reagent, this unusually stable low-valent-phosphorus-containing compound acts as a useful P+ transfer agent. Unlike traditional methods employed to access low-coordinate phosphorus species which usually require pyrophoric phosphorus-containing precursors (white phosphorus, Tris(trimethylsilyl)phosphine, etc.), or harsh reducing agents (alkali metals, potassium graphite, etc.), the current approach does not involve pyrophoric or explosive reagents and can be done on large scales (&#62;20 g) in excellent yields by undergraduates with basic air-free synthetic training. The bromide counter ion is readily exchanged with other anions such as tetraphenyl borate (described herein) using typical salt metathesis reagents to obtain materials with desired properties and reactivities. The versatility of this P+ transfer approach is exemplified by the reactions of these triphosphenium precursors with an N-heterocyclic carbene and an anionic bisphosphine, each of which readily displace the neutral bisphosphine to give an NHC-stabilized phosphorus(I) cation and a phosphorus(I) containing zwitterion, respectively.

Preparation and Reactivity of a Triphosphenium Bromide Salt: A Convenient and Stable Source of PhosphorusI

The synthesis of a triphosphenium bromide salt is described and its use as a P+ transfer agent is outlined by reactions with an N-heterocyclic carbene and an anionic bisphosphine, yielding an NHC-stabilized P(I) cation and a P(I) containing zwitterion, respectively.

We present herein the optimized synthesis of a triphosphenium bromide salt. Apart from being a versatile metathesis reagent, this unusually stable ...

Achieving Moderate Pressures in Sealed Vessels Using Dry Ice As a Solid CO2 Source

Herein is presented a general strategy to perform reactions under mild to moderate CO2 pressures with dry ice. This technique obviates the need for specialized equipment to achieve modest pressures, and can even be used to achieve higher pressures in more specialized equipment and sturdier reaction vessels. At the end of the reaction, the vials can be easily depressurized by opening at room temperature. In the present example CO2 serves as both a putative directing group as well as a way to passivate amine substrates, thereby preventing oxidation during the organometallic reaction. In addition to being easily added, the directing group is also removed under vacuum, obviating the need for extensive purification to remove the directing group. This strategy allows the facile &#947;-C(sp3)-H arylation of aliphatic amines and has the potential to be applied to a variety of other amine-based reactions.

Achieving Moderate Pressures in Sealed Vessels Using Dry Ice As a Solid CO2 Source

Here we present a protocol for performing reactions in simple reaction vessels under low-to-moderate pressures of CO2. The reactions can be performed in a variety of vessels simply by administering the carbon dioxide in the form of dry ice, without the need for costly or elaborate equipment or set-ups.

Herein is presented a general strategy to perform reactions under mild to moderate CO2 pressures with dry ice. This technique obviates the need for ...

Isolating Free Carbenes, their Mixed Dimers and Organic Radicals

Protocols for the isolation of the commonly employed cyclic (alkyl)(amino) carbene (CAAC) and N-heterocyclic carbene (NHC) are reported. Furthermore, the synthesis of their mixed CAAC&#8211;NHC &#8220;Wanzlick&#8221; dimer and the synthesis of the related stable organic &#8220;olefin&#8221; radical are presented. The main goal of this manuscript is to give a detailed and general protocol for the synthetic chemist of any skill level on how to prepare free heterocyclic carbenes by deprotonation using filter cannulas. Due to the air-sensitivity of the synthesized compounds, all experiments are performed under inert atmosphere using either Schlenk technique or a dinitrogen filled glovebox. Controlling Wanzlick&#8217;s equilibrium (i.e., the dimerization of free carbenes), is a crucial requirement for the application of free carbenes in coordination chemistry or organic synthesis. Thus, we elaborate on the specific electronic and steric requirements favoring the formation of dimers, heterodimers, or monomers. We will show how proton catalysis allows for the formation of dimers, and how the electronic structure of carbenes and their dimers affects the reactivity with either moisture or air. The structural identity of the reported compounds is discussed based on their NMR spectra.

We present protocols for the isolation of stable heterocyclic carbenes. The synthesis of a cyclic (alkyl)(amino) carbene (CAAC) and an N-heterocyclic carbene (NHC) is demonstrated using filter cannulas and Schlenk technique. We furthermore present the synthesis of the related oxygen-sensitive, electron-rich mixed &#8220;Wanzlick dimer&#8221; and the reduced stable organic radical.

Protocols for the isolation of the commonly employed cyclic (alkyl)(amino) carbene (CAAC) and N-heterocyclic carbene (NHC) are reported. Furthermore, the ...

[(DPEPhos)(bcp)Cu]PF6: A General and Broadly Applicable Copper-Based Photoredox Catalyst

Our group recently reported the use of [(DPEPhos)(bcp)Cu]PF6 as a general copper-based photoredox catalyst which proved efficient to promote the activation of a broad variety of organic halides, including unactivated ones. These can then participate in various radical transformations such as reduction and cyclization reactions, as well as in the direct arylation of several (hetero)arenes. These transformations provide a straightforward access to a range of small molecules of interest in synthetic chemistry, as well as to biologically active natural products. Altogether, [(DPEPhos)(bcp)Cu]PF6 acts as a convenient photoredox catalyst which appears to be an attractive, cheap and complementary alternative to the state-of-the-art iridium- and ruthenium-based photoredox catalysts. Here, we report a detailed protocol for the synthesis of [(DPEPhos)(bcp)Cu]PF6, as well as NMR and spectroscopic characterizations, and we illustrate its use in synthetic chemistry for the direct arylation of (hetero)arenes and radical cyclization of organic halides. In particular, the direct arylation of N-methylpyrrole with 4-iodobenzonitrile to afford&#160;4-(1-methyl-1H-pyrrol-2-yl)benzonitrile and the radical cyclization of N-benzoyl-N-[(2-iodoquinolin-3-yl)methyl]cyanamide to afford natural product luotonin A are detailed. The scope and limitations of this copper-based photoredox catalyst are also briefly discussed.

[DPEPhosbcpCu]PF6: A General and Broadly Applicable Copper-Based Photoredox Catalyst

Detailed and general protocols are presented for the synthesis of [(DPEPhos)(bcp)Cu]PF6, a general copper-based photoredox catalyst, and for its use in synthetic chemistry for the direct arylation of C-H bonds in (hetero)arenes and radical cyclization of organic halides.

Our group recently reported the use of [(DPEPhos)(bcp)Cu]PF6 as a general copper-based photoredox catalyst which proved efficient to promote the ...

Introduction to Catalysis

Source: Laboratory of Dr. Ryan Richards &#8212; Colorado School of Mines
Catalysis is among the most important fields of modern technology and presently accounts for approximately 35% of the gross domestic product (GDP) and sustenance of approximately 33% of the global population through fertilizers produced via the Haber process.1 Catalysts are systems that facilitate chemical reactions by lowering the activation energy and influencing the selectivity. Catalysis will be a central technology in addressing the energy and environmental challenges of modern times.

Catalysis: Homogeneous and Heterogeneous Catalysts

Wprowadzenie do katalizy

Source: Laboratory of Dr. Ryan Richards &#8212; Colorado School of Mines
Catalysis is among the most important fields of modern technology and presently ...

Education

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Organic Chemistry

&alpha;-Bromination of Carboxylic Acids: Hell&ndash;Volhard&ndash;Zelinski Reaction

The method to achieve &#945;-brominated carboxylic acids using a mixture of phosphorus tribromide and bromine is known as the Hell&#8211;Volhard&#8211;Zelinski reaction. The reaction is catalyzed by phosphorus tribromide, which can be used directly or produced in situ from red phosphorus and bromine. The mechanism comprises PBr3 catalyzed conversion of acid to acid bromide and hydrogen bromide. The acid bromide enolizes to its enol form in the presence of HBr. The nucleophilic enol attacks the bromine molecule to give an &#945;-bromo acid bromide. The resulting molecule, upon subsequent hydrolysis, yields the desired &#945;-bromo acid.

&amp;#945;-Bromination of Carboxylic Acids: Hell&amp;#8211;Volhard&amp;#8211;Zelinski Reaction

α-bromowanie kwasów karboksylowych: reakcja Hella-Volharda-Zelinskiego

The method to achieve &#945;-brominated carboxylic acids using a mixture of phosphorus tribromide and bromine is known as the ...

JoVE Core

Unit 1

&alpha;-Carbon Chemistry: Enols, Enolates, and Enamines

KEY TERMS AND CONCEPTS

Aldehydes and Ketones to Alkenes: Wittig Reaction Overview

The Wittig reaction is the conversion of carbonyl compounds&#8212;aldehydes and ketones&#8212;to alkenes using phosphorus ylides, or the Wittig reagent. The reaction was pioneered by Prof. Georg Wittig, for which he was awarded the Nobel Prize in Chemistry.
<img alt="Figure1" src="/files/ftp_upload/12335/12335_PT_Figure_1.svg" style="text-align: center; height: 75px;" />
Phosphorus ylide is a neutral molecule containing a negatively charged carbon directly bonded to a positively charged phosphorus atom. The molecule is stabilized by resonance.
<img alt="Figure2" src="/files/ftp_upload/12335/12335_PT_Figure_2.svg" style="text-align: center; height: 75px;" />
The Wittig reagents are synthesized from unhindered alkyl halides in two steps. At first, the alkyl halide undergoes an SN2 attack by a triphenylphosphine molecule generating a phosphonium salt. Next, in the presence of a strong base such as butyllithium, sodium hydride, or sodium amide, the salt undergoes deprotonation of the weakly acidic &#945; hydrogen, producing the carbanionic ylide nucleophile.
<img alt="Figure3" src="/files/ftp_upload/12335/12335_PT_Figure_3.svg" style="text-align: center; height: 75px;" />
Wittig reactions are regioselective, as the new C&#65309;C bond is formed explicitly at the carbonyl position. The stereoselectivity depends on the nature of the phosphorus ylide. Ylides with electron-withdrawing groups, such as carbonyl or aromatic rings that are stabilized by additional resonance structure, predominantly generate E alkenes. Alternatively, Wittig reagents with simple alkyl groups primarily form Z alkenes.
<img alt="Figure4" src="/files/ftp_upload/12335/12335_PT_Figure_4.svg" style="text-align: center; height: 200px;" />
The yield of Wittig reactions is influenced by steric crowding around the carbonyl group. Ketones that are sterically more hindered give poor yields compared to aldehydes. A variation of the Wittig reaction is the Horner&ndash;Wadsworth&ndash;Emmons reaction that involves a phosphonate ester reagent producing the E alkene as the major product.
<img alt="Figure5" src="/files/ftp_upload/12335/12335_PT_Figure_5.svg" style="text-align: center; height: 90px;" />

Aldehydy i ketony do alkenów: przegląd reakcji Wittiga

The Wittig reaction is the conversion of carbonyl compounds&#8212;aldehydes and ketones&#8212;to alkenes using phosphorus ylides, or the Wittig reagent. ...

Preparation and Use of Carbonyl-decorated Carbenes in the Activation of White Phosphorus

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Rozdziały w tym wideo

Reverse Microemulsion-mediated Synthesis of Monometallic and Bimetallic Early Transition Metal Carbide and Nitride Nanoparticles

Preparation of Carbon Nanosheets at Room Temperature

Palladium N-Heterocyclic Carbene Complexes: Synthesis from Benzimidazolium Salts and Catalytic Activity in Carbon-carbon Bond-forming Reactions

Preparation and Reactivity of a Triphosphenium Bromide Salt: A Convenient and Stable Source of Phosphorus(I)

Achieving Moderate Pressures in Sealed Vessels Using Dry Ice As a Solid CO₂ Source

Isolating Free Carbenes, their Mixed Dimers and Organic Radicals

[(DPEPhos)(bcp)Cu]PF₆: A General and Broadly Applicable Copper-Based Photoredox Catalyst

Wprowadzenie do katalizy

α-bromowanie kwasów karboksylowych: reakcja Hella-Volharda-Zelinskiego

Aldehydy i ketony do alkenów: przegląd reakcji Wittiga