Mechanism and selectivity of rhodium-catalyzed 1:2 coupling of aldehydes and allenes

Article abstract of DOI:10.1021/ja4014166

The rhodium-catalyzed highly regioselective 1:2 coupling of aldehydes and allenes was investigated by means of density functional theory calculations. Full free energy profiles were calculated, and several possible reaction pathways were evaluated. It is shown that the energetically most plausible catalytic cycle is initiated by oxidative coupling of the two allenes, which was found to be the rate-determining step of the overall reaction. Importantly, Rh-allyl complexes that are able to adopt both η³ and η¹ configurations were identified as key intermediates present throughout the catalytic cycle with profound implications for the selectivity of the reaction. The calculations reproduced and rationalized the experimentally observed selectivities and provided an explanation for the remarkable alteration in the product distribution when the catalyst precursor is changed from [RhCl(nbd)]₂ (nbd = norbornadiene) to complexes containing noncoordinating counterions ([Rh(cod)₂X]; X = OTf, BF₄, PF₆; cod = 1,5-cyclooctadiene). It turns out that the overall selectivity of the reaction is controlled by a combination of the inherent selectivities of several of the elementary steps and that both the mechanism and the nature of the selectivity-determining steps change when the catalyst is changed.

Full text of DOI:10.1021/ja4014166

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Article
Mechanism and Selectivity of Rhodium-
Catalyzed 1:2 Coupling of Aldehydes and Allenes
Genping Huang, Marcin Kalek, and Fahmi Himo
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/ja4014166 • Publication Date (Web): 25 Apr 2013
Downloaded from http://pubs.acs.org on May 5, 2013
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Journal of the American Chemical Society
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Mechanism and Selectivity of Rhodium-Catalyzed 1:2 Coupling of
Aldehydes and Allenes
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Genping Huang, Marcin Kalek, and Fahmi Himo*
Department of Organic Chemistry
Arrhenius Laboratory
Stockholm University
SEꢀ106 91 Stockholm, Sweden
Corresponding author: himo@organ.su.se
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Abstract
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The rhodiumꢀcatalyzed highly regioselective 1:2 coupling of aldehydes and allenes is investigated
by means of density functional theory calculations. Full free energy profiles are calculated and
several possible reaction pathways are evaluated. It is shown that the energetically most plausible
catalytic cycle is initiated by the oxidative coupling of the two allenes, which is found to be the
rateꢀdetermining step of the overall reaction. Importantly, Rhꢀallyl complexes that are able to adopt
both η3 and η1 configurations are identified as key intermediates present throughout the catalytic
cycle with profound implications on the selectivity of the reaction. The calculations reproduce and
rationalize the experimentallyꢀobserved selectivities and provide explanation for the remarkable
alteration in the product distribution upon changing the catalyst precursor from [RhCl(nbd)2]2 to
complexes containing nonꢀcoordinating counterions ([Rh(cod)2X], X = OTf, BF4, PF6). It turns out
that the overall selectivity of the reaction is controlled by a combination of the inherent selectivities
of several of the elementary steps, and that both the mechanism and the nature of the
selectivityꢀdetermining steps change when changing catalysts.
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Journal of the American Chemical Society
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1. Introduction
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Rhodium(I) complexes are wellꢀestablished catalysts for the threeꢀcomponent cycloadditions of
unsaturated compounds, yielding carbocycles with various ring sizes (for instance 6ꢀmembered,
Scheme 1a).1 Rhodium(I) also catalyzes hydroacylation reactions in which an acyl unit and the
hydrogen atom of an aldehyde are added across a CꢀC multiple bond (Scheme 1b).2 A reductive
version of the latter process, furnishing the corresponding alcohols, has also been developed
(Scheme 1c).3 In 2006 Krische et al. reported a novel reaction that can be considered as a crossover
between those presented in Scheme 1a and 1c. In its course, two molecules of acetylene are coupled
with one molecule of aldehyde, under hydrogen atmosphere, forming a linear allylic alcohol
product (Scheme 2a).4,5 Very recently, a combination of the in principle more fundamental
couplings from Schemes 1a and 1b has also been successfully accomplished.6,7 In particular, a
significant example comes from the laboratory of Murakami and consists of a 1:2 coupling of
aldehydes and allenes (Scheme 2b).6
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Scheme 1.
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