Cp₂Ti^IIICl Catalysis in a New Light

Abstract

Visible‐light‐mediated photogeneration of Cp2TiIIICl has opened new avenues for titanium catalysis. The synergistic utilization of dual photoredox and Cp2TiIIICl catalysis uses Hantzsch ester (HE) as an electron donor instead of traditional super‐stoichiometric metallic reductants. This new paradigm has found numerous applications in synthetic organic chemistry. The highly efficient photoinduced electron transfer process between light‐sensitive Cp2TiCl2 and an excited photocatalyst is the key to the success of such a photocatalytic system and ensures overall catalyst turnover. Comparisons between the metal reduction approach and photo reduction approach have been made.

Photocatalytic Generation of π‐Allyltitanium Complexes via Radical Intermediates

“…Metallaphotoredox catalysis has become a powerful tool in organic synthesis [14–19] . In this respect, we envisioned that under photoreductive conditions the Ti III , as well as the alkyl radical would be generated via the photoinduced SET process (Figure 1 c).…”

“…[13] Metallaphotoredox catalysis has become a powerful tool in organic synthesis. [14][15][16][17][18][19] In this respect, we envisioned that under photoreductive conditions the Ti III , as well as the alkyl radical would be generated via the photoinduced SET process (Figure 1 c). [20,21] The highly reactive allyl radical 2, resulting from the addition of an alkyl radical to a butadiene, is anticipated to be captured by the mild reducing agent Ti III , thus producing the nucleophilic p-allyltitanium complexes 1 that could be successively coupled with carbonyls.…”

Difunctionalization of 1,3-Butadiene via Sequential Radical Thiol-ene Reaction and Allylation by Dual Photoredox and Titanium Catalysis

“…This may be due to the strong nucleophilicity of thiols, which can sometimes hinder the reaction by coordinating with transition metals. − To overcome this limitation, herein, we report a radical-type three-component allylation of aldehydes with 1,3-butadiene and thiols by dual titanium and photoredox catalysis (Figure c). In this photocatalysis system, addition of the thiol radical to 1,3-butadiene generates allyl group radicals, which are then selectively trapped by Cp 2 TiCl to form active allyl-Ti IV species for aldehyde allylation. − This approach offers several advantages, including the use of the readily available raw materials of butadiene, simple post-treatment, mild reaction conditions, a wide range of substrates, high functional group tolerance, and excellent yields of the valuable allylic 1,3-thioalcohols with exceptional regio- and diastereoselectivity.…”

“…Initially, the excited state *Ir III [ E 1/2 (*Ir III /Ir IV ) = −0.96 V vs SCE] reduces Cp 2 TiCl 2 [ E 1/2 (TiI V /Ti III ) = −0.76 V vs SCE] to Cp 2 Ti III Cl while oxidizing the photosensitizer to Ir IV . The highly oxidizing Ir IV is then reduced by HE [ E 1/2 (HE •+ /HE) = 0.97 V vs SCE], returning to its ground state and completing the photocatalytic cycle. , Meanwhile, HE is converted into a radical cation (HE• + ) and participates in the photocatalytic process, ultimately leading to the formation of Hantzsch pyridine . Additionally, under light irradiation, the disulfide bonds undergo homolytic cleavage, resulting in the formation of thiyl radicals.…”