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Closed Access item Conjugate Additions of Cyclic Oxygen-Bound Nickel Enolates to α,β-Unsaturated Ketones

Authors:Maya, Celia
Cámpora, Juan
Carmona Guzmán, Ernesto
Matas, Inmaculada
Palma Ramírez, Pilar
Gutiérrez-Puebla, Enrique
Monge, M. Angeles
Keywords:Cycloaddition reactions, Enolates, Lactones, Metallacycles, Michael addition, Nickel
Issue Date:26-Jan-2007
Citation:Chemistry - a European Journal 13(13): 3675- 3687 (2007)
Abstract:The reaction of nickel enolates displaying a metallacyclic structure with the α,β-unsaturated ketones methyl vinyl ketone (MVK) or methyl propenyl ketone (MPK) takes place in two stages, affording initially bicyclic adducts, which subsequently isomerize to the corresponding open-chain products. The former are generated with high stereoselectivity and can be considered as the products of the [2+4] cycloaddition of the enolate to the enone. The ring opening process involves a prototropic rearrangement that can be catalyzed by water. In the case of the reaction of the parent nickel enolate complex 1 (which displays an unsubstituted Ni-O=CR)CH2 arrangement) with MVK, a double-addition process has been observed, consisting of two successive cycloaddition/isomerization reactions. The carbonylation of the different cyclic and noncyclic products affords the corresponding lactones that retain the stereochemistry of the organometallic precursors. This methodology allowed trapping the primary product of the reaction of 1 with MPK as the corresponding organic lactone, demonstrating that the cycloaddition process takes place with exo selectivity. DFT modeling of the latter reaction provides further support for a quasi-concerted cycloaddition mechanism, displaying a nonsymmetric transition state in which the C-C and the C-O bond are formed in an asynchronous manner.
Publisher version (URL):http://dx.doi.org/10.1002/chem.200601357
E-ISSNmetadata.dc.identifier.doi = DOI:1521-3765
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