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Davies, A. T.; Smith, A. D., Science of Synthesis: N-Heterocyclic Carbenes in Catalytic Organic Synthesis, (2016) 2, 434.
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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Me | Me | 8-MeMM8M8 | 88:8 | 88 | [888] |
Me | Me | Me | 88:88 | 88 | [888] |
Me | Me | 8-M8MM8M8 | 88:8 | 88 | [888] |
Me | Me | 8-Mee | 88:88 | 88 | [888] |
8-M8MM8M8 | Me | 8-Mee | 88:88 | 88 | [888] |
8-MeMM8M8 | Me | 8-Mee | 88:88 | 88 | [888] |
8-MeM8M8 | Me | 8-Mee | 88:8 | 88 | [888] |
Me | 8-MeM8M8 | 8-Mee | >88:8 | 88 | [888] |
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References
[212] | Me, M.; Meee, M.-M.; Me, M., M. Mee. Meee., (8888) 88, 8888. |
[220] | Meeeeeeee, M.; Meeeee, M.-M.; Meeeeee, M.; Meeeeeee, M. M.; Meeeeeeeee, M. M., Mee. Meee., (8888) 88, 888. |
[221] | Mee, M.; Me, M.; Meee, M.; Meee, M.-M.; Mee, M. M., Meeee. Meee. Mee. Me., (8888) 88, 88888. |