Single-electron transmetalation in organoboron cross-coupling by photoredox/nickel dual catalysis

By J. Tellis, D. Primer, G. Molander. Science, 2014

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A bright outlook for carbon coupling In contemporary organic chemistry, it is straightforward to forge bonds between unsaturated carbons (i.e., carbons already engaged in double bonds) using cross-coupling catalysis. The protocol runs into some trouble, however, if one or both starting carbon centers are saturated (purely single-bonded). Tellis et al. and Zuo et al. independently found that combining a second, light-activated catalyst with a nickel cross-coupling catalyst could achieve selective coupling of saturated and unsaturated reagents (see the Perspective by Lloyd-Jones and Ball). Their methods rely on single-electron transfer from the light-activated catalyst to the saturated carbon, thereby enhancing its reactivity more effectively than the twoelectron mechanisms prevailing in traditional protocols. Science, this issue p. 433, p. 437; see also p. 381 Combining two catalysts, one light-activated, facilitates bond formation between saturated and unsaturated carbons. [Also see Perspective by Lloyd-Jones and Ball] The routine application of Csp3-hybridized nucleophiles in cross-coupling reactions remains an unsolved challenge in organic chemistry. The sluggish transmetalation rates observed for the preferred organoboron reagents in such transformations are a consequence of the two-electron mechanism underlying the standard catalytic approach. We describe a mechanistically distinct single-electron transfer-based strategy for the activation of organoboron reagents toward transmetalation that exhibits complementary reactivity patterns. Application of an iridium photoredox catalyst in tandem with a nickel catalyst effects the cross-coupling of potassium alkoxyalkyl- and benzyltrifluoroborates with an array of aryl bromides under exceptionally mild conditions (visible light, ambient temperature, no strong base). The transformation has been extended to the asymmetric and stereoconvergent cross-coupling of a secondary benzyltrifluoroborate.

Subfields: Organic Chemistry

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