On the Mechanism of Gold(I)-Catalyzed Ring Expansion of Cyclopropanols: Theoretical Calculations Uncover a Bottle-Neck 1,4-H Shift and Suggest Adequate Reaction Conditions

Autor: Diego Ardura, Tomás L. Sordo
Rok vydání: 2008
Předmět:
Zdroj: European Journal of Organic Chemistry. 2008:3004-3013
ISSN: 1099-0690
1434-193X
DOI: 10.1002/ejoc.200800043
Popis: The mechanism of the one-carbon ring-expansion reactions of 1-(1-propynyl)cyclopropanol (1; R = Me), (1R*,2R*)-1-ethynyl-2-isopropylcyclopropanol (3), and (1R*,2S*)-1-(phenylethynyl)-2-isopropylcyclopropanol (4) catalyzed by [AuP(Ph)3]+ to yield the corresponding 2-alkylidenecyclobutanones was theoretically investigated by B3LYP-PCM calculations by using the LANL2DZ relativistic effective core potential for Au and the 6-31G(d) basis set for the remaining atoms. The most favorable route for these rearrangements in dichloromethane solution is a two-step mechanism involving as the first step the coordination of the gold(I) complex to the alkyne moiety with subsequent evolution through a 1,2-alkyl shift. Activation of the reactive C–C bonds takes place mainly through hyperconjugative interactions of these bonds with the oxygen atom and the alkyne moiety; this latter interaction is reinforced by the presence of the cationic gold(I) complex. The second step is the rate-determining one and consists of a 1,4-H shift, which requires the assistance by a second molecule of cyclopropanol to become readily accessible. This second molecule plays a very efficient bifunctional catalytic role, which cannot be played by a dichloromethane molecule. The use of methanol, water, and HFIP as assisting molecules was also investigated. Calculations suggest that the process would run most favorably in water. In agreement with experimental data, we found that the most favorable ring expansion of 3 takes place through migration of the substituted carbon atom. The only product to be expected from the rearrangement of 4, however, is that corresponding to the migration of the nonsubstituted carbon atom.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)
Databáze: OpenAIRE
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