The P-N bond cleavage during the phase transfer alkylation of N-alkyldiphenylphosphinic amides

Abstract

Phase transfer NH alkylation of N alkyldiphenyl phosphinic amides 1 is successful in the presence of an aqueous 50% 1 or solid2 alkali and a phase transfer cata lyst. It is worth noting that the reaction can proceed in the absence of a phase transfer catalyst, as demonstrated ear lier3 for alkylation of P(O)H acids. We synthesized the known4 dialkylamide 2 in high yield from N methylamide 1a in benzene without phase transfer catalyst, using aqueous 50% alkali or the anhydrous heterogeneous system NaOH—K2CO3/CH2Cl2 (Scheme 1). However, attempted alkylation of amide 1a with methyloxirane in aqueous alkali (50% NaOH—CH2Cl2) failed: we re covered the starting reagent only. With systems of stronger basicity, reactions of amide 1a with ethylene chlorohydrin (NaOH—K2CO3/C6H6) and methyloxirane (50% NaOH/DMSO) did not yield the expected N (2 hydroxyalkyl) derivatives; instead, we obtained diphenylphosphinic acid* (3) as a major reaction product (78—90%) arising from cleavage of the P—N bond. Attempted alkylation of N (2 hydroxy ethyl)diphenylphosphinic amide (1b) with methyl iodide (NaOH—K2CO3/DMSO, 35 °C, 5 h) also gave acid 3 in 76% yield.