Aluminium‐Catalyzed C(sp)−H Borylation of Alkynes

Abstract

Historically used in stoichiometric hydroalumination chemistry, recent advances have transformed aluminium hydrides into versatile catalysts for the hydroboration of unsaturated multiple bonds. This catalytic ability is founded on the defining reactivity of aluminium hydrides with alkynes and alkenes: 1,2‐hydroalumination of the unsaturated π‐system. This manuscript reports the aluminium hydride catalyzed dehydroborylation of terminal alkynes. A tethered intramolecular amine ligand controls reactivity at the aluminium hydride centre, switching off hydroalumination and instead enabling selective reactions at the alkyne C−H σ‐bond. Chemoselective C−H borylation was observed across a series of aryl‐ and alkyl‐substituted alkynes (21 examples). On the basis of kinetic and density functional theory studies, a mechanism in which C−H borylation proceeds by σ‐bond metathesis between pinacolborane (HBpin) and alkynyl aluminium intermediates is proposed.

Aluminium complexes: next-generation catalysts for selective hydroboration

“…In this context, it is worth mentioning a recent report by Cowley, Thomas and co-workers on dehydrogenative C(sp)–H borylation catalysed by Al-complexes. 38 In situ generation of an amine tethered aluminium catalyst (FLP) can switch the alkyne hydroalumination reaction to C(sp)–H borylation. Thus, a suitable design of the Al-catalyst is capable of changing the typical reactivity of alkynes with HBpin to produce the expected hydroboration product.…”

“…For the sake of developing a sustainable world in general, recently Earth-abundant and less toxic metals have revived their importance in hydroboration and borylation chemistry, among which the suitable candidates are aluminium complexes as catalysts. 36–38 The reactivity of aluminium complexes can be controlled by suitably tuning their ligands, and selective reduction of various functionalities via hydroboration can be achieved (Scheme 2A). Moreover, the amount of catalyst can be decreased to as low as 0.5–1 mol% in several Al-catalysed hydroboration reactions.…”

Solvent-free hydroboration of alkynes catalyzed by an NHC–cobalt complex

“…[9][10][11] Recently, hydroboration of alkynes has become the most important method of obtaining these compounds. 12,13 B-H addition to alkene and alkynes can be catalyzed by the main group elements, such as sodium, 14 lithium, 15 magnesium, 16 aluminium 17,18 and boron. 19,20 Precious metal-based systems are commonly used in transformations of this type, however, a number of earth abundant metal catalysts have been developed and proved to be active in this reaction as well.…”

“…33 Among the most recently published methods leading to b-(E)-products of terminal alkyne hydroboration we can mention protocols based on transition metals such as: iron, 34 cobalt, 32,35,36 cooper, 37 zirconium 38 and silver. 39 What is more, several processes catalyzed by main-group elements [16][17][18][19][20] can lead to anti-Markovnikov products. However, they still suffer from some disadvantages such as high catalyst loading, elevated temperature and limited substrate scope.…”

Organosuperbase Catalyzed 1,1-Diboration of Alkynes

“…The aforementioned transition metal catalyzed diboration reactions of terminal alkynes typically proceed via a dehydrogenative monoboration at the C(sp)-H moiety, [24][25][26] followed by a direct or formal hydroborative addition reaction to the C-C triple bond to give 1,1-diborylalkenes. In the pursuit of an organocatalytic system that is capable of promoting the diboration reaction of terminal alkynes in a similar fashion, we envision that this new catalyst should have strong Brønsted basicity to break the weakly acidic C(sp)-H bond.…”