Aluminum Hydride Catalyzed Hydroboration of Alkynes

Abstract

An aluminum-catalyzed hydroboration of alkynes using either the commercially available aluminum hydride DIBAL-H or bench-stable Et Al⋅DABCO as the catalyst and H-Bpin as both the boron reagent and stoichiometric hydride source has been developed. Mechanistic studies revealed a unique mode of reactivity in which the reaction is proposed to proceed through hydroalumination and σ-bond metathesis between the resultant alkenyl aluminum species and HBpin, which acts to drive turnover of the catalytic cycle.

Comparing Neutral (Monometallic) and Anionic (Bimetallic) Aluminum Complexes in Hydroboration Catalysis: Influences of Lithium Cooperation and Ligand Set

“…Catalysis, using 10 mol % loadings in [D 8 ]toluene at 110 8C, in line with the reported reaction conditions using 6 (85 % conversion after 2 hours), [6] reveal that 1 and 3 catalyse the transformation to the anti-Markovnikov vinylboronate ester in yields of 71 % and 83 % respectively. Conversely, 4 as expected, does not function as a catalyst.…”

“…6 (10 mol %) is reported to convert diphenylacetylene to the boronic ester in 40 % yield after 2 hours at 110 8Ci n[ D 8 ]toluene, [6] whereas 1 is completely inactive, and 3 only reaches conversions of approximately 10 % after 2 hours, which is surprising given our preceding observations. One potential rationale for this marked reduction in ate reactivity with diphenylacetylene may be attributed to a steric effect (Scheme 4).…”

“…[5] More recently, Cowley, Thomas and Bismuto revealed that DIBAL(H), and Et 3 Al·DABCO can catalyse hydroboration of alkynes. [6] Our groups interests lie in exploiting the synergistic reactivity imparted by two distinct metal centres [7,8] installed within a bimetallic complex. In this regard we introduced ate complexes (Figure 1), detailing that heteroleptic lithium diamido-dihydridoaluminates and lithium monoamido-monohydrido-dialkylaluminates implicate that the alkali metal influences the ensuing "aluminum reactivity" in the hydroboration of aldehydes, ketones and terminal alkynes.…”

“…It is germane to note that Et 3 Al·DABCO can catalyse hydroboration of alkynes due to a redistribution reaction with HBpin generating the active Et 2 AlH species. [6] The structure of 7 (Figure 2, left) reveals a dimer wherein both iBu À groups of 4 have been replaced, by Ph 2 (H)CO À Scheme 1. Postulated insertion mechanism in Al-catalysed hydroboration.…”

“…With 6, 1-phenyl-propyne is only hydroborated in trace amounts, despite the intrinsically smaller CH 3 group with respect to diphenylacetylene. [6] On the other hand, 3 catalyses the transformation to a mixture of regio-isomers (60 % conversion overall) in favour of borylation at the least sterically hindered alkyne carbon atom, demonstrating once more the advantage of ate complexes in these catalytic transformations.…”