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A Theoretical Study of Cyclohexyne Addition to Carbonyl-C(alpha) Bonds: Allowed and Forbidden Electrocyclic and Nonpericyclic Ring-Openings of Strained Cyclobutenes

  • Author(s): Sader, Charles Avery
  • Advisor(s): Houk, Kendall N
  • et al.
Abstract

The mechanism of cyclohexyne insertion into a C(O)−Cα bond ofcyclic ketones, explored experimentally by the Carreira group, has beeninvestigated using density functional theory. B3LYP and M06−2X calculationswere performed in both gas phase and THF (CPCM, UAKS radii). The reactionproceeds through a stepwise [2 + 2] cycloaddition of cyclohexyne to the enolate,followed by three disparate ring-opening possibilities of the cyclobutene alkoxide togive the product: (1) thermally allowed conrotatory electrocyclic ring-opening, (2)thermally forbidden disrotatory electrocyclic ring-opening, or (3) nonpericyclicC−C bond cleavage. Our computational results for the model alkoxide andpotassium alkoxide systems show that the thermally allowed electrocyclic ring-opening pathway is favored by less than 1 kcal/mol. In more complex systemscontaining a potassium alkoxide (e−f), the barrier of the allowed conrotatory ring-opening is disfavored by 4−8 kcal/mol. This suggests that the thermodynamicallymore stable disrotatory product can be formed directly through a "forbidden" pathway. Analysis of geometrical parameters and atomic charges throughout the ring-opening pathways provides evidence for a nonpericyclic C−C bond cleavage, rather than a thermally forbidden disrotatory ring-opening. A true forbidden disrotatory ring-opening transition structure was computed for the cyclobutene alcohol; however, it was 19 kcal/mol higher in energy than the allowed conrotatory transition structure. An alternate mechanism in which the disrotatory product forms via isomerization of the conrotatory product was also explored for the alkoxide and potassium alkoxide systems.

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