Steric and kinetic effects of the t-butyl substituent at phosphorus

• Main Author: De'ath, Norman John
• Published: University of Leicester 1970
• Subjects: 540
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.673985

A study of both the steric and kinetic effects of the t-butyl substituent at phosphorus in phosphonium salts and thiophosphoiyl compounds has been made. In phosphonium salts one t-butyl substituent was found to have a relatively small effect on the rate of alkaline hydrolysis, whereas two t-butyl substituents completely inhibited attack at phosphorus and decomposition to olefin and phosphine became preferred over the normal decomposition to hydrocarbon and phosphine oxide. The reasons for the differing effects of one and two t-butyl substituents are discussed. With one t-butyl group nucleo- philic attack could occur with little difficulty opposite the t-butyl substituent to give an intermediate in which the t-butyl group occupied an apical position, whereas with two t-butyl groups attack must necessarily occur adjacent to one t-butyl group, a process which is greatly sterically hindered. The steric consequences of this explanation are profound. Alkaline hydrolysis of optically active benzyl- t-butylmethylphenylphosphonium iodide was found to proceed with predominant retention of configuration, instead of the usually observed inversion, and migration of a t-butyl group, presumably from an apical position in the intermediate, was observed in the alkaline hydrolysis of t-butyl- iodomethyldiphenylphosphonium iodide. Steric crowding at phosphorus was found to have a significant effect on the product distribution in the alkaline hydrolysis of a series of phosphonium salts. The kinetics of alkaline hydrolysis of the cyclic analogues of the t-butyl salts have been investigated, the rapid rates of alkaline hydrolysis being due to relief of ring strain on formation of the intermediate. The alkaline hydrolysis of optically active O-methyl t-butylphenylphosphinothioate was found to proceed with complete retention of configuration at phosphorus. This stereochemical result was found by studies to be due to predominant C-O bond cleavage rather than to the effect of the t-butyl substituent. Other attempts to elucidate the stereochemistry of substitution at a thiophosphoryl centre bearing a t-butyl substituent were unsuccessful because of the resistance of these compounds to nucleophilic substitution.