31 P{ 1 H} NMR Studies of the Preparation of Dichlorotris(2,4,6-tribromophenoxy)phosphorane,Trichlorobis(2,4,6-tribromophenoxy)phosphorane and Tetrachloro(2,4,6-tribromophenoxy)phosphorane,and their Nonoxidative Chlorination Reactions with Dimethyl Phosphonate

Abstract

Dichlorotris(2,4,6-tribromophenoxy)phosphorane can serve as a nonoxidative chlorinating agent for the conversion of secondary phosphites into chlorophosphites. However, the route reported for the synthesis of this material only gives a 22% yield, and the product contains significant impurities. To better understand the reasons for this poor yield, detailed ³¹P{¹H} NMR studies of the reactions of phosphorus pentachloride with one, two and three equivalents of 2,4,6-tribromophenol have been carried out. These studies demonstrate that mixtures of the phosphoranes in which one, two and three of the chlorides are substituted by 2,4,6-tribromophenoxy groups are obtained even when a 3 : 1 ratio of the phenol to PCl₅ is used. Studies of the reactions of these mixtures with dimethyl phosphonate indicate that all three phosphoranes are equally capable of transforming dimethyl phosphonate into dimethyl chlorophosphonite in quantitative yield. The more highly substituted phosphoranes are very sensitive to water and difficult to handle suggesting that the monosubstituted phosphorane is the reagent of choice. To demonstrate the synthetic utility of this nonoxidative chlorination reaction, W(CO)₅{P(OMe)₂Cl} has been prepared in a one pot reaction involving the nonoxidative chlorination of dimethyl phosphonate with the monosubstituted phosphorane followed by the reaction of the dimethyl chlorophosphonite with W(CO)₅(MeCN).