Abstract
The transition state structures for the gold-catalyzed transannular [4 + 3] cycloaddition reaction are located for two distinct mechanisms at the B3LYP/6-31G(d) level of theory. A direct [4 + 3] cycloaddition pathway is found to be favored over the step-wise cyclopropanation-Cope rearrangement pathway. In the former, partial bonding are apparent between the furan ring and the gold-stabilized allylic carbocation. The gold atom bearing the N-heterocyclic carbene ligand is bonded to the allylic system located near the center carbon. The acetate group, which is attached to C1 of the allylic carbocation, is anti to the gold atom. The C–Au distance in this transition state is 2.06 Å. The forming bonds between the furan ring and the allylic carbocation are 1.66 and 2.0 Å, respectively, indicating an asynchronous process. The calculated second pathway starts with a gold carbenoid intermediate, which must overcome two higher energy transition states in order to reach the same reaction product.
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