Mechanistic insights in gold-stabilized nonclassical carbocations: gold-catalyzed rearrangement of 3-cyclopropyl propargylic acetates.
The reaction of 3-cyclopropyl propargylic carboxylates with Au(I) and Au(III) catalysts affords selectively 5-(E)-alkylidenecyclopentenyl acetates via [3,3]-sigmatropic rearrangement of the carboxylic moiety followed by cyclopropyl ring opening and cyclization. DFT calculations have been performed, supporting a two-step no-intermediate mechanism along the cyclization coordinate. The stereoselective formation of the exocyclic alkenes is kinetically controlled in the first of these events. Although stereospecific in nature through a gold-stabilized nonclassical carbocation, the chirality transfer in these cyclopentannulations is not complete. Computational and experimental evidence is provided for a Au-promoted cyclopropyl ring opening/epimerization/ring closure in both cis- and trans-cyclopropyl settings, which competes with the cyclization event, thus eroding the stereochemical information transfer. When tertiary acetates were used, products of both 1,2- and 1,3-acyloxy migration processes could be isolated, supporting the competitive coexistence of these two pathways along the reaction profile, as suggested also by DFT calculations.
Garayalde
David
D
Organic Chemistry Institute, University of Zürich, Switzerland.
Gómez-Bengoa
Enrique
E
Huang
Xiaogen
X
Goeke
Andreas
A
Nevado
Cristina
C
D002384:Catalysis; D002412:Cations / Q000737:chemistry; D003198:Computer Simulation; D003500:Cyclization; D003521:Cyclopropanes / Q000138:chemical synthesis* / Q000737:chemistry; D006046:Gold / Q000737:chemistry*; D008956:Models, Chemical; D015394:Molecular Structure; D010648:Phenylacetates / Q000138:chemical synthesis* / Q000737:chemistry; D013237:Stereoisomerism
C550574:3-((2-methylpropenyl)cyclopropylpropargyl)-1-phenylprop-2-ynyl acetate; D002412:Cations; D003521:Cyclopropanes; D010648:Phenylacetates; D006046:Gold