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2.8.9.5 N-Heterocyclic Carbene Catalyzed Sulfonyl Transfer

DOI: 10.1055/sos-SD-224-00223

Davies, A. T.; Smith, A. D.Science of Synthesis: N-Heterocyclic Carbenes in Catalytic Organic Synthesis, (20162434.

The addition of NHCs to imines has been documented in a number of reports;[‌220‌] for example, as an undesired side reaction which allows the free NHC to enter the catalytic cycle through equilibrium.[‌212‌] However, the synthetic potential of the intermediates thus generated is vastly underexplored. In 2013, the group of Chi documented an unusual process where N-sulfonylimines are used as sulfonyl-transfer agents, facilitating the β-sulfonation of chalcones to give β-sulfonyl ketones 58 (Scheme 44).[‌221‌] This is believed to proceed through addition of the NHC (e.g., ent-41) to the sulfonylimine, forming an aza analogue 59 of the Breslow intermediate. Then, a nitrogen lone pair from the NHC can promote the elimination of the sulfinic anion. Deprotonation of the final aza acyl azolium intermediate regenerates the catalyst and produces benzonitrile as a byproduct (Scheme 44). The authors believe that as no enantioselectivity is observed when using a chiral NHC, the sulfinic anion must be detached from the NHC during the C—S bond formation. As a result, they utilized a hydrogen-bond donor catalyst 57 to control facial selectivity.

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