The alkaline hydrolysis of sterically crowded phosphonium salts

Abstract

Di-t-butylphosphonium salts are extremely resistant to alkaline hydrolysis. Benzyl (p henyl)di-t-butylphosphonium bromide with aqueous alkali slowly gives isobutene and benzyl(pheny1) -t-butyfphosphine. With sterically crowded phosphonium salts, e.g. benzyl-cc-naphthylphenyl-t-butylphosphonium bromide, the normal rule, that hydrocarbon will be formed from that group most stable as the anion, does not apply. Factors which may be responsible for this are discussed.THE alkaline hydrolysis of phosphonium salts to give hydrocarbons and phosphine oxides is usually represented as in equation (1),1 the slow step being the irreversible loss of that group most stable as the anion from the conjugate base (2) of the initial adduct (1).Recent work2 has shown that at least the last two steps are concerted, the anion Rbeing protonated as formed. This mechanism accounts for the observed kinetics (rate oc [P+][OH-]a) and for the formation of hydrocarbon from that radical most stable as the anion.The observed inversion of configuration at phosphorus in the case of acyclic phosphonium salts is usually interpreted in terms of a trigonal bipyramidal intermediate (1) having the entering and leaving groups collinear and diapical. Steric effects have been held to be unimportant 334 on the basis of theoretical argument and of kinetic data on the hydrolyses of benzyltritolylphosphonium salts. We have shown in the case of phosphinate esters that while one t-butyl group has little effect on the rate of alkaline hydrolysis, two t-butyl groups attached to a phosphorus atom reduce this rate considerably, and we therefore sought similar effects in the hydrolysis of phosphonium salts. We report a study of the alkaline hydrolysis of di-t-butyl-and other sterically crowded phosphonium salts. Methyl(pheny1)di-t-butylphosphonium iodide proved to be extraordinarily resistant to alkaline hydrolysis. After 1 week in aqueous ethanolic 2~-sodium hydroxide at loo", over 60% was recovered unchanged. The half -life of a trimethylphenylphosphonium salt would be measured in seconds under these conditiom6 Benzyl-(phenylldi-t-butylphosphonium bromide was only slightly less resistant, 27% being unchanged after 2 days in refluxing aqueous ethanolic B~-sodium hydroxide. No toluene could be detected and the major product was benzyl(gheny1)-t-butylphosphine, isolated as the methiodide. The other product of a normal Hoffmann R. F. Hudson, Structure and Mechanism in Organophosphorus Chemistry,' Academic Press, New York, 1965.

A Fluorescence-Based Assay for High-Throughput Screening of Coupling Reactions. Application to Heck Chemistry

“…Pd(dba) 2 and the palladacycle 33 were prepared according to literature procedures. The following ligands were prepared according to literature methods: P(2,4-xyl) 3 , P(2-BTF) 3 , P(2-MOMPh) 3 , DPPBz, DTPF, D p -MeOPPF, D p -CF 3 PPF, D t -BPF, DPPR, DPPNp, , DPPDPE, DTPDPE, DPPX, DTPX, P(2-Me-4-FPh) 3 , ( t -Bu) 2 PPh, and ( t -Bu) 2 P( o -tol) . Synthesis of new ligands is described below.…”

First ever observation of the intermediate of phosphonium salt and ylide hydrolysis: P-hydroxytetraorganophosphorane

“…The mechanism that has been proposed for alkaline hydrolysis of phosphonium salts is shown in Scheme 1. [11][12][13][14]16,17,24 Nucleophilic attack of hydroxide at phosphorus gives a hydroxyphosphorane (with apical oxygen 8,25,26 ). This is deprotonated by hydroxide to give an oxyanionic phosphorane, which expels a carbanion (probably protonated in the process of its expulsion) to give phosphine oxide and alkane or arene.…”

“…Consistent with this interpretation are the results of hydrolysis reactions of phosphonium salts in which the phosphorus atom is in a cycle of five or fewer members (see the ESI for additional discussion on this topic). † Certain chiral small ring phosphonium salts (and also compound 2; see Chart 1) 24,36 have been shown to undergo hydrolysis with retention of configuration at phosphorus, 37 while other cyclic phosphonium salts undergo ring opening in the course of hydrolysis, 20,22,38 including dibenzophospholium salts such as 4 (Chart 1). 18,39 In TBP phosphoranes, the leaving group is obliged to depart from an apical site.…”