The mechanism of cyclohexyne insertion into a C(O)-Ca bond of cyclic ketones, explored experimentally by the Carreira group, has been investigated using density functional theory.  B3LYP and M06-2X calculations were performed in both gas phase and THF (CPCM, UAKS radii).  The reaction proceeds through a stepwise [2+2] cycloaddition of cyclohexyne to the enolate, followed by ring opening of the cyclobutene alkoxide to give the product.  Surprisingly, our computational results for the model alkoxide and potassium alkoxide systems show that the allowed/forbidden energy gap in the ring opening is less than 1 kcal/mol, which is in sharp contrast to that of simple cyclobutenes.  In more complex systems, the barrier of the forbidden disrotatory ring opening is lower in energy than that of the allowed conrotatory ring opening by 4-8 kcal/mol.  This suggests that the thermodynamically more stable disrotatory product can be formed directly through a �forbidden� pathway.

Theoretical study of cyclohexyne addition to carbonyl-C<sub>α</sub> bonds: Allowed vs. forbidden electrocyclic ring openings of strained cyclobutenes

243rd National Meeting (2012)

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Theoretical study of cyclohexyne addition to carbonyl-Cα bonds: Allowed vs. forbidden electrocyclic ring openings of strained cyclobutenes

Theoretical study of cyclohexyne addition to carbonyl-C_α bonds: Allowed vs. forbidden electrocyclic ring openings of strained cyclobutenes