A Dual Photoredox‐ and Cp₂TiCl₂‐Catalyzed Approach for the Direct Access to α‐Vinyl‐β‐hydroxy Esters

Abstract

For the first time, a dual photoredox and titanocene catalyzed methodology for the regioselective access to α‐vinyl‐ß‐hydroxy esters towards aldehyde allylation with 4‐bromobut‐2‐enoate is reported. The protocol is based on the Barbier‐type properties of the inexpensive and available Cp2TiCl2 in catalytic amount (5 mol%). The developed mild reaction conditions gave access to a library of differently functionalized α‐vinyl‐ß‐hydroxy esters in moderate diastereoselectivity, employing the commercially available ethyl 4‐bromobut‐2‐enoate and both aromatic and aliphatic aldehydes. The reaction was realized under visible light irradiation, in the presence of an organophotocatalyst (3DPAFIPN, 2 mol%) combined with Hantzsch’s ester as the sacrificial reductant. In contrast to other Barbier type reactions employing ethyl 4‐bromobut‐2‐enoate, the photoredox system ensures a better regioselectivity. Moreover, the use of preformed organometallic nucleophilic species (e.g., dienolborinates), and the need of metal reductants or stoichiometric amount of transition metals in low oxidation state for Barbier‐type reactions, is avoided. To support the experimental evidence, a detailed photophysical study shed light on the mechanism of the reaction.

Photoinduced Platinum-Catalyzed Reductive Allylation of α-Diketones with Allylic Carbonates

“…However, recent progress in the development of photoinduced transition metal catalysis has enabled the use of stable, readily available amines as reductants (Figure b). Titanium, cobalt, nickel, and chromium catalysts have so far been applied to photoinduced reductive allylation in combination with photoredox catalysts. Even so, these catalytic systems have been used mainly for the allylation of aldehydes, and only a few examples of the allylation of ketones have been reported to date .…”