DEGENERATE SIDE-CHAIN APPROACH TO THE CARBON ANALOG OF THE CLAISEN REARRANGEMENT - FORMATION OF 9-MEMBERED RINGS BY A STERICALLY ACCELERATED ENE REACTION

The first step of the carbon Claisen rearrangement can be examined without reference to the second step through introduction of a suitably labeled ortho allyl substituent, which would be identifiably inserted into the butenyl side chain on reversal of the first step. We have prepared substrates containing the potentially degenerate side chain, labeled alternatively with deuterium and with methyl. Interchange of the side chains does not occur up to about 350 °C, so that the activation energy of the [3, 3] sigmatropic step of the carbon Claisen rearrangement must be at least 32-35 kcal mol-1. These results suggest that the first step, rather than the second step as previously thought, is rate determining. The effect of oxygen vs. carbon and of aliphatic vs. aromatic can then be readily understood in terms of perturbations on the frontier molecular orbitals of the starting materials. The substrates constructed with degenerate side chains produce, as the alternative reaction at 350 °C, a high-yield ene cyclization to form nine-membered rings. This unprecedented result for simple 1, 8-nonadienes is attributed to a steric acceleration arising from loss of rotational degrees of freedom on placement of the ene components in ortho-related side chains on a benzene ring. © 1979, American Chemical Society. All rights reserved.