Alkali-metal-assisted Transfer of Carbamate Group from Phosphocarbamates to Alkyl Halides: a New Easy Way to Alkali-metal Carbamates and to Carbamate Esters

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.