Identification of Bond‐Weakening Spirosilane Catalyst for Photoredox α‐C−H Alkylation of Alcohols

Abstract

The development of catalyst-controlled site-selective C(sp 3 )À H functionalization is a current major challenge in organic synthesis. This paper describes DFT-guided identification of pentavalent silicate species as a novel bond-weakening catalyst for the α-CÀ H bonds of alcohols together with a photoredox catalyst and a hydrogen atom transfer catalyst. Specifically, Martin's spirosilane accelerated α-CÀ H alkylation of alcohols.

Visible-Light-Driven Dehydrogenative Coupling of Primary Alcohols with Phenols Forming Aryl Carboxylates

“…When ethanol was irradiated under the reaction conditions in the presence of benzalmalononitrile 10 and ( n -Bu) 4 NBr as a bromide source instead of the phenol and the NiBr 2 catalyst, the adduct 12 was produced in 28% yield. The formation of 12 was explained by assuming that an α-hydroxy radical was generated from ethanol via hydrogen abstraction by a bromine radical and that it underwent conjugated addition to 10 , which was followed by intramolecular cyclization . The results shown in Scheme corroborate the mechanism shown in Scheme .…”

α-C–H Photoalkylation of a Glucose Derivative in Continuous Flow

“…Next to hydrogen bond-formation, several other methods have been reported, with the common theme that the electron density of the alpha-carbon is increased. Boron, tin and silicon are less electronegative than hydrogen, so diarylborinic acids (8) , borinates (9) , spirosilanes (10) and organostannanes (11) have all been used successfully, and are instrumental to induce regioselectivity in the alkylation, with the latter being applied to rhamnopyranosides. Alkylation via 1,6-HAT in a fructopyranoside has been reported as well.…”

The Dark Side of Photocatalysis: One Thousand Ways to Close the Cycle

“…These are based on photocatalysts operating via SET able to in situ generate a thermal H‐abstractor, which then takes care of promoting the desired chemistry . Inspired by a seminal work in the area by the MacMillan group, the use of a triple catalytic system, comprising of an Ir‐based photocatalyst ([Ir(dF(CF 3 )ppy) 2 (4,4'‐d(CF 3 )bpy)]PF 6 ), a quinuclidine derivative ( 6.4 ) and Martin's spirosilane ( 6.5 ), to promote the selective C‐alkylation of alcohols in the presence of electron‐poor olefins has been reported (Scheme ) . Here, PC* was used to oxidize quinuclidine 6.4 to 6.4 ·+ , which abstracted a hydrogen atom from hypervalent silyl derivative 6.6 – , in turn formed upon complexation of substrate 6.1 with 6.5 .…”

“…Once radical anion 6.7 ·– was formed, it underwent radical trapping by 6.2 to give intermediate 6.8 ·– . The latter species was finally in charge of closing the photocatalytic cycle to afford product 6.3 in excellent yield (94 %, dr 1:1; formed upon spontaneous cyclization of the initially formed acyclic adduct), while releasing spirosilane 6.5 prone to start a new cycle …”