Dual Photoredox and Nickel Catalysed Reductive Coupling of Alkynes and Aldehydes

Abstract

A regioselective vinylation of aromatic and aliphatic aldehydes promoted by the merging of photoredox and nickel catalysis is here reported. A comprehensive investigation on the reaction conditions allowed the disclosure of a valid and reproducible protocol based on a nickel-mediated reductive coupling approach under visible light irradiation. The employment of 3CzClIPN (2,4,6-tris(carbazol-9-yl)-5-chloroisophthalonitrile) as the photocatalyst and Hantzsch's ester as the sacrificial organic reductant replace the use of boron-, silicon-or zinc-based reducing agents, making this method a worthy alternative to the already known protocols. The developed mild reaction conditions allow the access to a wide range of substituents decorating both the aldehyde and the alkyne. Moreover, careful photophysical investigations shed light on the mechanism of the reaction.

Reactivities of Photoredox Generated Nickel‐Nucleophilic Reactive Organometallic Species

“…We have selected 3CzClIPN (3CzClIPN = 2,4,6-tri(9H-carbazol-9yl)-5-chloroisophthalonitrile) as photocatalyst in combination with the Hantzsch ester (HE) as organic reductant. [75] This organic dye was evaluated in function of the values of the available reduction potential both for the ground and for the excited state, that are lower than other cianoarene-based photocatalysts commonly employed, i. e. 4CzIPN, or 3DPAFIPN [values are referred to SCE: E(4CzIPN * + /*4CzIPN) = À 1.18 V; E(3DPAFIPN * + /*3DPAFIPN) = À 1.38 V; E(3CzClIPN * + /*3CzClIPN) -= À 0.93 V. The lower reduction potential, both in excited that in the ground state is relevant when aromatic aldehydes are employed in conditions set to form Ni(0). To prevent any unwanted occurrence of parasitic pinacol coupling of the aromatic aldehyde in the presence of potent reducing photocatalysts [76] and a Brønsted acid, such as the oxidized Hantzsch ester, the photocatalyst was carefully chosen.…”

1,3-Difunctionalization of Propargyl Silanes with Concomitant 1,2-Silyl Shift: Synthesis of Allyl Functionalized Vinyl Silanes

“…The latter was employed further in the Suzuki–Miyaura reaction to yield stilbenes 14a , b in 62% and 56% yields, respectively, while simultaneously preserving the original double bond geometry throughout the sequence 4a → 10a → 13 → 14a , b . It is worth noting that synthesis of geometrically defined tri- and tetra-substituted olefins including those with vinylsilane moiety or with allylic functionality is still a challenging task. In this context, the newly obtained building blocks 2 – 4 have been demonstrated to be versatile triads for further derivatization, maintaining the double bond geometry, which originates from ( E )-selective 1,2-silyl group migration.…”