VARIABLE TRANSITION-STATE STRUCTURE IN 3,3-SIGMATROPIC SHIFTS FROM ALPHA-SECONDARY DEUTERIUM-ISOTOPE EFFECTS

The relative values of secondary a deuterium kinetic isotopic effects at the terminal and central carbons of 1,5-dienes undergoing the 3,3-shifts vary as a function of substitution. With radical stabilizing groups on C3, the transition state more resembles two weakly coupled allyl radicals. With radical stabilizing substituents at C2 and C5, the transition state more resembles cyclohexane-1,4-diyl. A thermochemical argument renders unlikely cyclohexane-1,4-diyl as an intermediate in the 3,3 shift of 1,5-hexadiene. Comparison of the KlEs at C4 and C6 in the rearrangement of 3-oxa-1,5-hexadiene (allyl vinyl ether to 4-pentenal) to appropriate equilibrium isotope effects reveals that the transition state “comes early” with more bond breaking than making. The transition state structures for many 3,3 shifts are discussed in terms of the relative stabilities of the two nonconcerted alternatives arising from either complete bond breaking or complete bond making. © 1979, American Chemical Society. All rights reserved.