General solvent-free highly selective N-tert-butyloxycarbonylation strategy using protic ionic liquid as an efficient catalyst

Abstract

A simple, rapid and solvent-free protocol is described for the chemo-selective transformation of amines to tert-butyloxycarbonyl protected derivatives (NHBoc) using Boc₂O and imidazolium trifluoroacetate protic ionic liquid (5–20 mol%). Unwanted side products such as isocyanate, urea or N,N-di-Boc were not detected. The scope of the protection strategy was successfully explored for substrate alcohols, phenols and thiol at elevated temperatures. Optically pure amino acids, amino acid esters and amino alcohols were efficiently converted to the corresponding N-Boc protected derivatives in excellent yields without racemization at the chiral center. The distinct advantages of this method are: operational simplicity, cleaner reaction, high selectivity, excellent yield, rapid reaction convergence, easy preparation and recyclability of the catalyst.