Visible Light Induced Brønsted Acid Assisted Pd‐Catalyzed Alkyl Heck Reaction of Diazo Compounds and N‐Tosylhydrazones

Abstract

A mild visible light-induced palladium-catalyzed alkyl Heck reaction of diazo compounds and N-tosylhydrazones is reported. A broad range of vinyl arenes and heteroarenes with high functional group tolerance, as well as a range of different diazo compounds, can efficiently undergo this transformation. This method features Brønsted acidassisted generation of hybrid palladium C(sp 3 )-centered radical intermediate, which allowed for new selective C À H functionalization protocol.Scheme 1. The reactivity of diazo compounds with alkenes.

Recent Advances in Visible Light Induced Palladium Catalysis

“…Mechanistically, this method showcased a switch from the conventional protic hydropalladation (A) to a hydridic hydropalladation (B) enabled by the enhanced excited-state hydricity [96] of Pd II H species. In line with Gevorgyan's previous work, [93] photoinduced homolysis of B generates electrophilic alkyl radical C, which upon trapping with relatively electron-rich alkene 38.2 leads to alkyl Heck type product 38.3. Mechanistic studies supported the enhanced hydricity of the photoexcited PdH species, which led to the switch of the chemoselectivity of the hydropalladation step.…”

“…In 2022, the Gevorgyan group disclosed a novel Brønsted acid‐assisted hydroalkenylation of diazo compounds 36.1 and N ‐tosylhydrazones 36.2 (Scheme 36). [93] In line with previously discussed alkyl Heck‐type transformations, [28,34,35] it involves formation of the alkyl Pd I hybrid radical intermediate C , which may form via several plausible pathways, including 1,2‐hydride shift of Pd II ‐carbene intermediate A formed upon denitrogenative reaction of 36.1 with Pd II H species, followed by photoinduced homolysis of C−Pd bond. Alternatively, intermediate C could be generated via an SET to diazonium salt D or a proton coupled electron transfer (PCET) to 36.1 .…”

Light-Driven Palladium-Radical Hybrid Species: Mechanistic Aspects and Recent Examples

“…The formation of radicals from diazo compounds usually leads to the generation of cyclopropanes due to the carbene-like reactivity of such species when reacting with alkenes, 56 which makes the use of diazo compounds for the generation of free radical intermediates scarce in the literature. 57 Regarding the generation of hybrid Pd I -radical species, a groundbreaking contribution by Gevorgyan and co-workers 58 exemplified the possible generation of such intermediates through the formation of Pd carbenes followed by a hydride shift and light-induced homolysis of the Pd–C bond. An alternative path was also proposed via proton-coupled electron transfer (PCET) between the diazo compound, or even direct SET via a diazonium ion.…”

Escape from Hydrofunctionalization: Palladium Hydride‐Enabled Difunctionalization of Conjugated Dienes and Enynes

“…We envisioned acrylic acid as a perfect triple-duty alkene substrate (see below) for this cross-coupling reaction (Scheme 1D). It was expected that more hydridic photoexcited PdH species, formed by reaction of Pd(0) and acrylic acid, would undergo hydropalladation of the double bond [10,11] of electrondeficient alkene to form Pd(II) intermediate. The latter, upon light irradiation would homolyze into hybrid radical species [12] capable of addition to conjugated dienes to form π-allyl Pd systems via radical-polar crossover (RPC) event, as was shown by us [13] and others.…”