Exploiting Transient Radical Cations as Brønsted Acids for Allylic C–H Heteroarylation of Enol Silyl Ethers

Abstract

Intermediary radical cations, generated through single-electron oxidation of enol silyl ethers by excited Irbased photocatalysts, can be exploited as Brønsted acids for the activation of heteroarylcyanides. This strategy enables the direct allylic CÀ H heteroarylation of enol silyl ethers under visible-light irradiation.

Formal β‐C−H Arylation of Aldehydes and Ketones by Cooperative Nickel and Photoredox Catalysis

“…Based on these experiments and literature precedence, [11] the following mechanism is proposed for this transformation (Scheme 4e). The excited 4-CzIPN first oxidizes the silyl enol ether 1 via SET to generate a radical cation A.…”

“…[10] Moreover, Ooi and Chang independently reported β-CÀ H arylation of aldehydes and ketones via singleelectron transfer (SET) of preformed silyl enol ethers and subsequent (hetero)arylation. [11] Although powerful, the limited scope with respect to βheteroarylation and insufficient functional group tolerance have hampered wider applications of these elegant methods. Given the importance of such privileged scaffolds, developing more general and environmentally benign synthetic methods for ketone and aldehyde β-arylation are still highly desirable.…”

Expanding the chemical space of enol silyl ethers: catalytic dicarbofunctionalization enabled by iron catalysis

“…2; inset). The list of compatible partners include electron-rich 2-ethoxy-4-pyridyl, N-Boc-protected indole and indoline, N-methyl indazole, benzofuran, dihydroxybenzofuran, [1,4]dioxine, and N-phenylcarbazole that deliver the desired cross-coupling products (39)(40)(41)(42)(43)(44)(45)(46)(47)(48)(49) in moderate to excellent yields (24-89% yield). Finally, to show the practical utility of the reaction, we executed the reaction on a 5.4 mmol scale, and isolated 4 in 83% yield (parenthesis).…”

“…1B). [37][38][39][40][41] Based on these reports, we hypothesized that this bisphosphine-iron mechanistic manifold is uniquely positioned to orchestrate a series of events that could allow for unprecedented [42][43][44][45][46][47] recombination of a-silyloxy radicals with organoiron species to selectively form two new C-C bonds with enol silyl ethers (Fig. 1C).…”

One-pot synthesis of 3-functionalized (Z)-silyl enol ethers from 1-arylallylic alcohols by C,O-difunctionalization of dipotassio α,β-dianion intermediates

“…2 Therefore, general and efficient synthetic methods for these compounds are sought after. Recently, MacMillan 3 and Ooi 2,4 independently developed elegant syntheses involving the direct C3–H functionalization 5 of silyl enol ethers under photoredox catalysis. However, these reactions proceeded through a radical mechanism, and only electron-deficient cyanoarenes and unsaturated bonds could be applied as electrophiles.…”