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Science  30 Apr 2010:
Vol. 328, Issue 5978, pp. 549
DOI: 10.1126/science.328.5978.549-b

Reductively coupling aldehydes with alkynes is a versatile means of preparing allylic alcohols. However, it has proven challenging to reliably select which of the two triply bonded carbon centers in the alkyne attacks the carbonyl. Modestly differing substituents at the two sites tend to induce a mixture of products, whereas substituents that favor one product have largely precluded access to the other by this route. Malik et al. show that careful ligand selection for a nickel catalyst can override such substrate preferences (or lack of preference) and thereby afford tunable control over the reaction's regiochemistry. They specifically screened a ligand set comprising three N-heterocyclic carbene scaffolds functionalized with a variety of bulky groups, and a fourth, less hindered cyclopropenyl derivative. The resulting catalysts permit selection of either alkynyl reaction site across a series of substitution patterns, with ratios in both directions ranging from 81:19 to >98:2. Yields for the room-temperature procedure ranged from 71 to 99%.

J. Am. Chem. Soc. 132, 10.1021/ja102262v (2010).

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