Regioselective and Stereospecific Azidation of 1,2- and 1,3-Diols by Azidotrimethylsilane via a Mitsunobu Reaction

A one-pot regio- and stereospecific azidation reaction of 1,2- and 1,3-diols with azidotrimethylsilane (Me 3 SiN 3 ) via a Mitsunobu reaction has been achieved. With 1,2- and 1,3-diols, the reaction of triphenylphosphine, diisopropyl azodicarboxylate, and Me 3 SiN 3 in dichloromethane gave regioselective azidation at C-2 and C-3, respectively, in good yield (74−90% combined yield of 1a + 1b or of 2a + 2b ). However, application of the same reaction conditions to a 1,4-diol led to the exclusive formation of the cyclic ether. The regioselectivity of this one-pot reaction is influenced by the solvent, the degree of steric bulk at C-2 of the 1,2-diol or at C-3 of the 1,3-diol, and the presence of electron-donating and electron-withdrawing groups near the secondary carbinol carbon. This selectivity is discussed in terms of the mechanistic model proposed by Mathieu-Pelta and Evans (Mathieu-Pelta, I.; Evans, S. A., Jr. J. Org. Chem . 1992, 57, 3409−3413), which involves reaction of the dioxaphospholane intermediate with Me 3 SiN 3 to form oxyphosphonium ions 4 and 5 .