Angewandte
Communications
Chemie
Homogeneous Catalysis
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 Et3Al·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 s-bond metathesis
between the resultant alkenyl aluminum species and HBpin,
which acts to drive turnover of the catalytic cycle.
T
ransition-metal catalysis has revolutionized synthetic
chemistry. However, the increasing need for sustainable
chemical processes has led to a drive to replace transition
metals with earth-abundant, non-toxic, and environmentally
benign alternatives. Main-group elements offer many of these
properties, and are increasingly useful catalysts for synthetic
transformations.[1,2] In particular, the reductive functionaliza-
=
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tion of unsaturated polar bonds (e.g., C O, C NR) with
silicon and boron reagents is well-established, with both s-
and p-block based catalysts having been reported.[3–8]
Although aluminum compounds have found use as Lewis
acid catalysts and stoichiometric hydride reagents,[9] there is
a dearth of applications in wider catalysis.
Scheme 1. a) Established hydroalumination chemistry. b) Proposed cat-
alysis concept; functionalization and regeneration of alane. c) This
work: aluminum hydride catalyzed hydroboration.
À
Hydroalumination offers a controlled method to function-
alize alkenes and alkynes.[9–12] The stoichiometric reaction of
alanes with alkynes gives alkenyl aluminum reagents, which
can be functionalized through treatment with electrophiles.[9]
These reactions offer a powerful method for the stereoselec-
tive preparation of alkenes, but they are not ideal. The
requirement for stoichiometric amounts of inherently air- and
moisture-sensitive alanes makes hydroalumination chemistry
hazardous and wasteful. There is a clear need for novel
methods that avoid the direct handling of alanes and allow
substoichiometric amounts of alane to be used.
We envisaged a new class of catalytic hydrofunctionaliza-
tion reactions in which hydroalumination is the first step of
a catalytic cycle, and where turnover is achieved through
simultaneous alkene functionalization and aluminum hyride
regeneration (Scheme 1). In this way, stoichiometric quanti-
ties of aluminum hydride reagents would be avoided. Instead,
treatment of an alkenyl aluminum species with a suitable
hydride source would trigger s-bond metathesis to give the
functionalized alkene, whilst concurrently regenerating the
Al H bond, thereby enabling catalytic turnover (stoichio-
metric transmetallation from boron to aluminum is well
established).[13–16] We thus identified borane reagents as
possible reaction partners and a potential hydride source.
Ideally a readily available, air- and moisture-stable hydride
source would be used. Transmetallation of alkenyl aluminum
species with pinacol borane (HBpin), which fulfils these
criteria, would result in the formation of synthetically useful
alkenyl boronic esters. Here, an aluminum hydride would be
generated using a substoichiometric amount of alkyl alumi-
num reagent, with pinacol borane acting as the stoichiometric
hydride source and hydrofunctionalization reagent.
We began by investigating the catalytic activity of
commercially available dialkyl aluminum hydride reagents
in the presence of HBpin and 1-octyne to establish whether
our concept would lead to a viable process.[17] Using
diisobutylaluminum hydride [(iBu2AlH)2 or DIBAL-H,
10 mol %] as the catalyst gave the terminal boronic ester
with complete control of regiochemistry for the linear hydro-
boration product and in good yield (Table 1, entry 1), thus
demonstrating that an aluminum hydride is capable of
catalyzing alkyne hydroboration and showing that our
catalytic concept is valid.
We next investigated trialkyl aluminum reagents as
catalyst precursors. Use of these inexpensive and readily
available reagents would require an initial transmetallation
event to trigger catalysis through the transfer of H from
HBpin to AlR3 to generate the active HAlR2 catalyst.
Triethyl- and trimethylaluminum both successfully catalyzed
[*] A. Bismuto, Dr. S. P. Thomas, Dr. M. J. Cowley
University of Edinburgh, Joseph Black Building
David Brewster Road, Edinburg, EH9 3FJ (UK)
E-mail: Stephen.Thomas@ed.ac.uk
Supporting information and the ORCID identification number(s) for
Angew. Chem. Int. Ed. 2016, 55, 1 – 5
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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