Benzylic Thio and Seleno Newman-Kwart Rearrangements

The thermally induced O_Bn → S_Bn and O_Bn → Se_Bn migration reactions facilitate the rearrangement of O-benzyl thio- and selenocarbamates [BnOC(=X)NMe₂] (X = S or Se) into their corresponding S-benzyl thio- and Se-benzyl selenocarbamates [BnXC(=O)NMe₂] (X = S or Se). A series of substituted O-benzyl thio- and selenocarbamates were synthesized and rearranged in good yields of 33-88%. The reaction rates are higher for substrates with electron-donating groups in the 2 or 4 position of the aromatic ring, but the rearrangement also proceeds with electron-withdrawing substituents. The rearrangement follows first-order reaction kinetics and proceeds via a tight ion pair intermediate consisting of the benzylic carbocation and the thio- or selenocarbamate moiety. Computational studies support these findings.