Copper‐Photocatalyzed Hydroboration of Alkynes and Alkenes

Abstract

The photocatalytic hydroboration of alkenes and alkynes is reported. The use of newly‐designed copper photocatalysts with B2Pin2 permits the formation a boryl radical, which is used for hydroboration of a large panel of alkenes and alkynes. The hydroborated products were isolated in high yields, with excellent diastereoselectivities and a high functional group tolerance under mild conditions. The hydroboration reactions were developed under continuous flow conditions to demonstrate their synthetic utility. The reaction mechanism was studied and suggested an oxidation reaction between an in situ formed borate and the Cu‐photocatalyst in its excited state for the boryl radical formation.

Copper‐Photocatalyzed Hydrosilylation of Alkynes and Alkenes under Continuous Flow

“…The incorporation of deuterium was only observed when D 2 O was added, demonstrating the hydrogen arise from water [25a] . However, taking into account the respective bond dissociation energies (BDE) of H 2 O and CH 3 CN, [25b] the direct H‐abstraction from H 2 O is unlikely and suggest the abstraction from a transient boron species generated from H 2 O, as previously observed [19] . The quantum yield ( Φ ) of 0.87 for the hydrosilylation of phenylacetylene was determined (Eq.…”

“…As part of our research program dedicated to the development of transformations using copper and copper photocatalysts, [18] we intend to develop a straightforward strategy to develop a copper photocatalyzed hydrosilylation of alkynes and alkenes. Recently, we reported the possible photoinduced oxidation of a boronate species, formed in situ from B 2 Pin 2 and a base, to carry out the hydroboration of alkynes and alkenes [19] . Thus, we questioned if a related photoinduced oxidation of the Suginome reagent (PhMe 2 Si−BPin) could be leveraged to perform the hydrosilylation of alkenes and alkynes [20] .…”

Overview of Regioselective and Stereoselective Catalytic Hydroboration of Alkynes

“…Cu catalysts have contributed significantly to hydroboration of alkynes in a highly stereo‐ and regioselective manner [63–77] . It is worthy of mention that the β‐ or α‐selective borylation of aromatic substituted unsymmetrical internal alkynes (such as 32 or 34 ) was achieved by varying the borylating reagents.…”

Cobalt-Catalyzed Hydroboration of Terminal and Internal Alkynes

“…In this area, although precious transition metals have played an important role, their high cost, low abundance, and environmental issues have lately promoted the use of more earth-abundant and sustainable base-metal catalysts . Particularly, Cu­(I) catalyzed borylation of alkynes with commercial boranes followed by protonolysis has gained significant momentum in recent years (Scheme a) . However, this methodology has some limitations: moisture- and air-sensitive bases such as NaO t -Bu or KO t -Bu are used and the inherent lack of atom-economy as stoichiometric amounts of boron-containing byproducts are generated.…”