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.
Organic Chemistry Institute, University of Zürich, Switzerland.
David
D
Garayalde
Enrique
E
Gómez-Bengoa
Xiaogen
X
Huang
Andreas
A
Goeke
Cristina
C
Nevado
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