Alkali-metal-assisted transfer of the carbamate group from phosphocarbamates to alkyl halides: a new easy way to alkali-metal carbamates and to carbamate esters

Abstract

Phosphocarbamates P(O₂CNR₂)_x(NR₂)_{3 –x}(R = alkyl; x= 1 or 2), easily obtainable by insertion of CO₂ in the P–N bond of the corresponding aminophosphines P(NR₂)₃, have been used as a source of carbamate groups in the reaction with alkyl halides, R′X, to afford carbamate esters. The reaction is mediated by alkali-metal halides, MY, and requires the presence of a suitable macrocyclic polyether (L). The overall reaction occurs in two steps: the carbamic group is first transferred to the alkali-metal cation to give a carbamate salt [ML][O₂CNR₂] which then reacts with the alkyl halide affording R₂NC(O)OR′. The yield and selectivity to the carbamate ester are remarkably influenced by the nature of MY. The influence of the nature of the alkali-metal salt in the overall process and the role of the macrocyclic ligand in modifying the reactivity of the R₂NCO₂^– anion in alkali-metal carbamates are discussed.