An
unconventional cooperative catalysis for hydrogen-atom-transfer-mediated
acceptorless dehydrogenative cross-coupling is described. The combined
use of zwitterionic 1,2,3-triazolium amidate and an Ir-based photosensitizer
as catalysts enables C–H/C–H cross-couplings between
heteroatom-containing C–H donors and enamides or 1,1-diarylethenes
under visible-light irradiation without the need for any oxidants,
hydrogen evolution catalysts, or electrodes. A key to establishing
this catalysis is the susceptibility of the conjugate acid of the
triazolium amidate, amide triazolium, toward single-electron reduction
to complete the catalytic cycle.
Thioamidate ions are introduced as competent cocatalysts for multisite proton-coupled electron transfer (MS-PCET) catalysis. The effectiveness of these anions is ascribed to the high pKa values of their conjugate acids. The combined use of bench-stable tetrabutylammonium thioamidate with an appropriate Ir-based photosensitizer allows the C–H alkylation of a range of C–H donors, including simple cyclic alkanes, under visible-light irradiation.
A newly designed zinc Lewis acid/base hybrid catalyst was developed. By adjusting the Lewis acidity of the zinc center, aldol-type additions of 2-picolylamine Schiff base to aldehydes proceeded smoothly to afford syn-aldol adduct equivalents, trans-N,O-acetal adducts, in high yields with high selectivities. NMR experiments, including microchanneled cell for synthesis monitoring (MICCS) NMR analysis, revealed that anti-aldol adducts were formed at the initial stage of the reactions under kinetic control, but the final products were the trans-(syn)-N,O-acetal adducts that were produced through a retro-aldol process under thermodynamic control. In the whole reaction process, the zinc catalyst played three important roles: i) promotion of the aldol process (C-C bond formation), ii) cyclization process to the N,O-acetal product (C-O bond formation), and iii) retro-aldol process from the anti-aldol adduct to the syn-aldol adduct (C-C bond cleavage and C-C bond formation).
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