Capel E, Rodríguez-Rodríguez M, Uria U, Pedron M, Tejero T, Vicario JL, Merino P. Absence of Intermediates in the BINOL-Derived Mg(II)/Phosphate-Catalyzed Desymmetrizative Ring Expansion of 1-Vinylcyclobutanols.
J Org Chem 2021;
87:693-707. [PMID:
34928602 PMCID:
PMC8749959 DOI:
10.1021/acs.joc.1c02699]
[Citation(s) in RCA: 3] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Abstract
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The catalyzed desymmetrizative
ring expansion of alkenylcyclobutanols
promoted by halofunctionalization of the alkene moiety with N-bromosuccinimide has been experimentally and computationally
studied. The reaction yields highly enantioenriched cyclopentanones
bearing two all-carbon quaternary stereocenters, one of them being
generated in the rearrangement of the cyclobutane ring and the other
by enantioselective desymmetrization. The reaction is competitive
with the formation of a spiroepoxide, but it turns completely selective
toward the cyclopentanone when a chiral bisphosphonium magnesium salt
is employed as a catalyst. Mechanistic studies support the formation
of an ion pair leading to a complex with only a unit of phosphoric
acid, which is the resting state of the catalytic cycle. Calculations
reproduce in an excellent way the observed reactivity and predict
the effect exerted by the substituents of the aromatic ring linked
to the double bond. The computational studies also revealed the reaction
as a highly asynchronous concerted process taking place as one kinetic
step but in two stages: (i) halogenation of the double bond and (ii)
rearrangement of the cyclobutane. No intermediates are present in
the reaction as energy minima. The experimental scope of the reaction
further confirms the predictions for the observed reactivity and selectivity.
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