Radical Migration–Addition
of <i>N</i>-<i>tert-</i>Butanesulfinyl Imines
with Organozinc Reagents
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Abstract
A novel migration–addition
sequence was discovered for the
reaction of enantioenriched <i>N</i>-<i>tert</i>-butanesulfinyl iminoacetate <b>1a</b> with functionalized
benzylzinc bromide reagents, producing <i>tert</i>-leucine
derivatives in excellent diastereoselectivity (dr 98:2). The absolute
configurations of two new chiral centers were unambiguously assigned
by chemical transformations and X-ray crystallography. In addition,
the regio- and diastereoselectivities of this novel reaction were
both explained through the key <i>N</i>-sulfinamine intermediate <b>M6</b> generated by the <i>tert</i>-butyl radical attack
on the imine. Computational analysis of this reaction process, which
was performed at the B3LYP/6-311++G(3df,2p)//B3LYP/6-31G*-LANL2DZ
level, also supported our proposed two-stage mechanism