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

Summary

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.

Abstract

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 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.

Introduction

Stable carbenes have emerged as ubiquitous reagents in homogeneous catalysis¹, organocatalysis², materials science^3,4, and more recently main group chemistry^5-9. In the context of the latter, stable carbenes have recently been used in the activation and functionalization of white phosphorus (P₄)^5-9. The ability to directly convert P₄ into organophosphorus compounds has become a topical research objective in an effort to develop “greener” methods that circumvent the use of chlorinated or oxychlorinated phosphorus precursors. Despite their widespread use, the pre....

Protocol

1. Synthesis of Diamidocarbene (Compound 1)

1. 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’-dimesitylformamidine¹² (1.5 g, 5.35 mmol) and add it to the flask while flushing with nitrogen.
   1. 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 °C in an ice bath.
   2. Add dropwise (via a dry, deoxygenated syringe) dimethylmalonyl dichloride (0.75 ml, 5.60 mmol, 1.05 equiv.....

Results

The ability to isolate a tris(phosphaalkenyl)phosphane such as 3 or the P₈-allotrope (4) from white phosphorus relies on the use of an electrophilic carbene to activate the P₄ tetrahedron^11,16. Therefore, it is critical to prepare carbenes with enhanced π-acidity, and by extension electrophilicity. Figure 2 illustrates the synthesis of carbene precursor 1-HCl and its subsequent deprotonation to afford the diamidocarbene <.......

Discussion

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 at 180 °C, hydrolysis of the 1-HCl as well as carbenes 1 

Materials

List of materials used in this article

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