Benzylic Thio and Seleno Newman–Kwart Rearrangements

The
thermally induced OBn → SBn and
OBn → SeBn 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.