Dialkylation of 1,3-Dienes by Dual Photoredox and Chromium Catalysis

Abstract

The direct conversion of feedstock chemicals into value-added products is of broad interest in chemical research. Herein, we present a regioselective and diastereoselective three-component dialkylation of feedstock 1,3-dienes with Hantzsch esters and aldehydes for the synthesis of homoallylic alcohols. The reaction is enabled by dual photoredox and chromium catalysis and can also be performed enantioselectively by employing chromium-bisoxazoline complexes.

Radical Carbonyl Propargylation by Dual Catalysis

“…Finally, UV‐visible (Scheme 5 E) and Stern–Volmer experiments (see supporting information for details) were performed and the results clearly show that only the photoredox catalyst (either PC‐1 or 4‐CzIPN ) absorbs visible light in the reaction system and the photoluminescence can be quenched exclusively by Hantzsch ester 3 a . Based upon these preliminary mechanistic results and our previous study, [21b] we propose that alkyl radical VII is generated from a suitable radical precursor following reductive quenching of the photoexcited PC‐1 or 4‐CzIPN (Scheme 5 F). Then the L n Cr(III) species is reduced in situ to L n Cr(II) ( E 1/2 =−0.65 V vs. SCE in H 2 O, E 1/2 =−0.51 V vs. SCE in DMF) [21b] by the reduced photocatalyst PC .− (for instance, organic dye 4CzIPN , E 1/2 (PC/PC .− )=−1.21 V vs. SCE in MeCN; [29] PC‐1 , E 1/2 (Ir II /Ir III )=−1.37 V vs. SCE in MeCN [30] ).…”

“…Based upon these preliminary mechanistic results and our previous study, [21b] we propose that alkyl radical VII is generated from a suitable radical precursor following reductive quenching of the photoexcited PC‐1 or 4‐CzIPN (Scheme 5 F). Then the L n Cr(III) species is reduced in situ to L n Cr(II) ( E 1/2 =−0.65 V vs. SCE in H 2 O, E 1/2 =−0.51 V vs. SCE in DMF) [21b] by the reduced photocatalyst PC .− (for instance, organic dye 4CzIPN , E 1/2 (PC/PC .− )=−1.21 V vs. SCE in MeCN; [29] PC‐1 , E 1/2 (Ir II /Ir III )=−1.37 V vs. SCE in MeCN [30] ). The alkyl radical VII could either reversibly add to a low‐valent L n Cr(II) species to form the resting state Cr(III)‐alkyl complex VIII or add to 1,3‐enynes to generate a propargylic radical in equilibrium with its allenyl equivalent.…”

“…Reaction development . In analogy with previous reports from our group, [21b] we investigated the reaction of 1,3‐enyne ( 1 a ), 3‐(4‐fluorophenyl)propanal ( 2 a ) and Hantzsch ester ( 3 a , E 1/2 =1.10 V vs. SCE in MeCN) [24] (Scheme 2). After exploring different reaction conditions, the best results were accomplished with CrCl 3 (10 mol %), PC‐1 (1 mol %, E 1/2 (*Ir III /Ir II )=1.21 V vs. SCE in MeCN) under irradiation with 450 nm light‐emitting diodes (30 W blue LEDs), affording three‐component coupling product 4 in 86 % isolated yield with 74:26 dr (entry 1).…”

“…In recent years, dual catalytic manifolds have emerged as attractive tools to promote energetically unfavorable reaction steps and achieve synthetically valuable transformations under increasingly mild conditions [20] . Since 2018, our group, [21a–c] Kanai, Mitsunuma and co‐workers [21d,e] have independently achieved allylation of aldehydes using dual photoredox and chromium catalysis [21f] . Compared to dual nickel/photoredox [20e–j] or copper/photoredox [20k–n] catalysis, dual chromium/photoredox catalytic mode is much less explored in synthetic chemistry, but provides a range of opportunities to achieve challenging radical‐to‐polar crossover‐based transformations.…”

Direct Access to Monoprotected Homoallylic 1,2-Diols via Dual Chromium/Photoredox Catalysis

“…Primary and secondary aliphatic aldehydes were equally reactive (entries 2a-d). Similar to previous reports, [31,33] tertiary aldehydes did not show reactivity presumably due to steric hindrance. A diversity of aromatic aldehydes could be efficiently converted to the respective products.…”