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

Article abstract of DOI:10.1002/anie.201806168

Bimetallic lithium aluminates and neutral aluminum counterparts are compared as catalysts in hydroboration reactions with aldehydes, ketones, imines and alkynes. Possessing Li–Al cooperativity, ate catalysts are found to be generally superior. Catalytic a

Full text of DOI:10.1002/anie.201806168

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Accepted Article
Title: Comparing neutral (monometallic) and anionic (bimetallic)
aluminium complexes in hydroboration catalysis: influences of
lithium cooperation and ligand set
Authors: Robert E. Mulvey, Victoria A Pollard, Maria Angeles Fuentes,
Alan R Kennedy, and Ross McLellan
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201806168
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Comparing neutral (monometallic) and anionic (bimetallic)
aluminium complexes in hydroboration catalysis: influences of
lithium cooperation and ligand set
Victoria A. Pollard,[a] M. Ángeles Fuentes,[a] Alan. R. Kennedy,[a] Ross McLellan*[a] and Robert E.
Mulvey*[a]
Dedicated to Professor Dietmar Stalke on the happy occasion of his 60th birthday
Abstract: Bimetallic lithium aluminates and neutral aluminium
counterparts are compared as catalysts in hydroboration reactions
with aldehydes, ketones, imines and alkynes. Possessing Li-Al
cooperativity, ate catalysts are found to be generally superior.
Catalytic activity is also influenced by the ligand set, alkyl and/or
amido. Devoid of an Al-H bond, iBu2Al(TMP) operates as a masked
hydride reducing benzophenone through a  transfer process.
This catalyst library therefore provides an entry point into the future
design of Al catalysts targeting substrate specific transformations.
Cowley and Thomas revealed that DIBAL(H), and Et3Al∙DABCO
can catalyse hydroboration of alkynes.[6] Our group’s interests lie
in exploiting the synergistic reactivity imparted by two distinct
metal centres[7] installed within a bimetallic complex. In this
regard we introduced ate complexes (Fig. 1), detailing that
heteroleptic lithium diamido-dihydridoaluminates and lithium
monoamido-monohydrido-dialkylaluminates implicate that the
alkali metal influences the ensuing “aluminium reactivity” in the
hydroboration of aldehydes, ketones and terminal alkynes.[8]
The synthetic value of main group metal complexes aside from
the highly reactive and versatile organolithium and
organomagnesium reagents have, from a historical perspective,
been overshadowed by the illustrious reputation of transition
metal (notably precious metals) and lanthanide metal
counterparts especially in catalysis.[1] To a large extent main
group research has been driven by fundamental curiosity and
the understanding of the nature of chemical bonding and
structure. A step-change occurred when it was realised that
such main group metal species can act in homogeneous
catalytic roles, previously the exclusive province of transition
metal and lanthanide complexes. Emulating the high reactivity,
selectivity and versatility of the often toxic and scarce precious
Figure 1. Al complexes 1 – 6 assessed in this study: ates 1 – 3; neutral 4 – 6.
metal complexes is
a tantalising challenge that needs
addressing. In this regard, the pioneering work of Harder, Hill,
Jones, Okuda, Power, Roesky, Wright among others, are
expanding the vistas of main group complexes in homogeneous
catalysis.[1-2] Since aluminium is the most abundant metal in the
earth’s crust, and also benefits from low toxicity, harnessing its
reactivity is given high prominence in this main group uprising
with longer term sustainability being a key issue. Thus, recently
aluminium complexes have made significant strides forward in
important stoichiometric and catalytic transformations.[3] For
example, they are utilised in C-C cross coupling chemistries,
and in deprotonative metallation.[4] Catalytic hydroelementation
reactions have also witnessed impressive progress in the past
few years. Roesky demonstrated that a -diketiminato stabilised
aluminium hydride complex is an excellent catalyst for
hydroboration of alkynes and carbonyl groups.[5] More recently,
Further, the catalytic chemistry of LiAlH4 has recently been
explored by Cowley and Thomas in the challenging
hydroboration of alkenes, however the role of the alkali metal
was not elaborated.[9] Thus, the current state of the field dictates
that a systematic analysis of the secondary metal cooperative
effects and various ligand factors that contribute to efficient
hydroboration, is required in order to establish empirical rules for
a posteriori design of future catalysts.
Hydroboration of unsaturated substrates under aluminium
catalysis is gaining a foothold in the literature, and a variety of
neutral aluminium complexes are displaying excellent potential
in this role.[2a, 3, 10] Previously, we reported that bimetallic lithium
[iBu2AlTMP(H)Li]2, 1 and [(HMDS)2AlH(-H)Li∙3THF)], 2, are
both efficient bimetallic (pre)catalysts in the hydroboration of
aldehydes
and
ketones.[8]
However,
any
synthetic
advantages/disadvantages of using ate complexes are yet to be
fully uncovered, despite their potential. Thus, here, for the first
time ate complexes are compared with their neutral aluminium
counterparts to fully quantify their value in synthesis, and to
glean understanding of their modus operandi. Moreover, the
complexes chosen vary in their ligand constitution, i.e., alkyl
versus amido constituents, providing further comparison.
[a]
V. A. Pollard, Dr. M. Á. Fuentes, Dr. A. R. Kennedy, Dr. R. McLellan,
Prof. Dr. R. E. Mulvey
WestCHEM, Department of Pure and Applied Chemistry
University of Strathclyde
Glasgow G1 1XL (UK)
E-mail: r.e.mulvey@strath.ac.uk, ross.mclellan@strath.ac.uk
Supporting information for this article is given via a link at the end of the
document.
Mechanistically
a frequently postulated two-step reaction
pathway is: 1/ insertion of an unsaturated substrate into an Al-H
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bond; 2/ -bond metathesis with a borane, regenerating an
active species and liberating product (Scheme 1).
which can then follow the pathway represented in Scheme 1.
The second pathway is much lower in energy and describes a
concerted process containing a 6-membered transition state,
facilitating direct hydride transfer to the substrate.
7 (2.5 mol%) is shown to be catalytically active in a reaction with
benzophenone and HBpin, where quantitative hydroboration
occurs after 3 hours. Since 4 seems a reactivity outlier, showing
comparable reactivity to 1, they were both screened catalytically
with one aldehyde and two further ketones. In each case the
bimetallic complex 1 showed far superior activity.
Scheme 1. Postulated insertion mechanism in Al-catalysed hydroboration.
Catalytic activities were screened with aldehydes, ketones,
imines and alkynes, providing reaction scope to determine key
divergences in catalyst reactivity. We previously reported 1 and
2 in catalytic hydroboration and these are compared with the
neutral analogues 4 and 5, which differ by formal removal of LiH.
We prepared new complex 3, an all alkyl variant of 1 (via NMR
characterisation, including DOSY) by a simple co-complexation
procedure (see SI). 3 can be considered an ate version of
DIBAL(H), 6. Our results from comparative studies (reaction
conditions are identical between different catalysts) are
summarized in Table 1. Complexes 1-6 (5 mol%) were all tested
Scheme 2. (Top) Reaction between 4 and benzophenone, revealing formation
of the active catalytic species 7 via β-hydride elimination. (Bottom) Reaction
between 4 and benzophenone proceeding via a possible Meerwein-Ponndorf-
Verley type reaction.
in
hydroboration
reactions
with
benzophenone
and
pinacolborane (HBpin) at room temperature in J. Young’s tubes
in C6D6. Each bimetallic complex exhibits superior activity to its
monometallic counterpart, affording quantitative conversion after
30 mins, apart from 4, which is 94% complete after 30 mins.
This is surprising since 4 does not possess an Al-H bond.
Rationalising that an Al-H bond must form in situ during the
catalysis we performed a stoichiometric reaction between 4 and
benzophenone in hexane and C6D6, where clear, facile
quantitative reaction occurs rapidly at room temperature
(isobutene, the coproduct of -hydride elimination, is seen in the
1H NMR spectra). X-ray diffraction studies of colourless crystals
grown from the hexane solution revealed formation of
[(TMP){Ph2(H)CO}Al{-OC(H)Ph2}]2, 7 in a 45% isolated yield
(Scheme 2). It is germane to note that Et3Al∙DABCO can
catalyse hydroboration of alkynes due to a redistribution reaction
with HBpin generating the active Et2AlH species.[6] The structure
of 7 (Fig. 2 LHS) reveals a dimer wherein both iBu groups of 4
have been replaced, by Ph2(H)CO ligands, formed by apparent
-hydride elimination from the parent complex. -Hydride
elimination is known in alkyl-aluminium chemistry with
carbonyls,[11] but to our knowledge this is the first example in
hydroboration catalysis used to generate a transient aluminium
hydride. Thus 4 may be considered a masked hydride complex
in hydroboration of ketones. Elaborating this step further, it is
pertinent to consider the Meerwein-Ponndorf-Verley (MPV)
reduction,[11a,12],[13] employing aluminium alkoxides as the hydride
source to reduce ketones. Two competing mechanisms have
been studied in silico.[13] The first involves -hydride transfer
from the alkoxide ligand giving a high energy Al-H intermediate,
Furthermore, a control reaction employing LiH as a catalyst (5
mol%) for hydroboration of benzophenone gave a yield of only
10% after 4 h. This illustrates that, in this regard, the neutral
aluminium or lithium reagents in isolation deliver markedly
reduced reactivities compared with the bimetallic formulations.
Importantly, for the first time direct competition experiments
reveal the synthetic superiority of lithium aluminate complexes in
the context of hydroboration.
Figure 2. Molecular structures of 7 (LHS) and 8 (RHS). All hydrogen atoms
are omitted for clarity except those on the reduced benzophenone anions.
Thermal ellipsoids are drawn at 30% probability.
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Hypothesising that any “ate effect” would be magnified with
more challenging substrates we turned our attention to imines,
which hitherto have not been catalytically hydroborated with Al
complexes. That said, examples exist of main group complexes
catalysing this transformation, and of Al complexes catalysing
hydrosilylation or hydrogenation of imines, [2c, 10e, 14] suggesting
that imine hydroboration is a viable synthetic target.
amenable to deprotonation and therefore provides the possibility
of reaction proceeding via an alternative deprotonation pathway.
Furthermore, amido groups in 1 and 4 can be directly compared
with alkyl groups in 3 and 6. 1 and 3 achieve 73% and 80%
conversion after 2 h or 30 minutes respectively. 4 and 6 perform
poorly, showing no catalytic activity at room temperature,
prompting further consideration. Two stoichiometric reactions
between benzophenone imine and 1, and 4 were conducted,
wherein both exhibit amido basicity. In the latter case
[iBu2Al(N=CPh2)]2, 8 (Fig. 2 RHS), was isolated as single
Catalytic hydroboration reactions of N-benzylidenemethylamine,
using 1-6 showed lower reactivity at room temperature than with
aldehydes and ketones, however the same reactivity pattern
emerges, in that the bimetallic complexes are superior to
monometallic counterparts. After two hours, conversions are
with 1 (42%), 2 (35%), 3 (53%), 4 (3%), 5 (22%) and 6 (5%).
Nevertheless, these results with 1-3 constitute the first use of Al
complexes in imine hydroboration. Stoichiometric reactions
between 1, 3, 4 and 6 with the imine provide further insight. 4
forms only a coordination adduct with the imine in contrast to the
-hydride elimination product with benzophenone, whereas, 1, 3
and 6 add across the C=N double bond, with 6 displaying higher
insertion reactivity (see SI). Notably Stephan reported a dimeric
structure of an analogous reaction between 6 and a related
imine.[14a] However, faster substrate insertion does not translate
into fast catalytic transformation. Thus we infer that the -bond
metathesis step with HBpin is greatly facilitated by the additional
polarity imposed by the bimetallic ate constitution. Reinforcing
this hypothesis, Harder’s imine hydrogenation using catalytic
LiAlH4 illuminates the important role of the alkali metal, via DFT
studies, wherein Al-H-Li interactions are retained throughout the
proposed catalytic cycle.[14d]
crystals in
a
24% yield (1H NMR yield of 86% against
hexamethylcyclotrisiloxane as internal standard). In contrast to
the benzophenone case where catalysis proceeds after a -
hydride elimination step, the reactivity here ceases after an initial
deprotonation by the TMP basicity. Interestingly, both 3 and 6
display trace amounts of H2 evolution as evidenced by a low
1
intensity singlet resonance in the respective H NMR spectra at
 4.47 ppm.
The catalytic results with benzophenone imine merit further
comment. Both 1 and 4 exhibit deprotonation, suggesting that in
a catalytic regime, reaction (using 1) may proceed in the
pathway outlined in Scheme 3, that is, deprotonation followed by
hydroboration then protonolysis to liberate product and generate
a catalytically active species.
Table 1. Hydroboration catalysis results for carbonyls, imines and acetylenes
using 1 – 6 as catalysts.
1[a]97% 2 h; 4 40% 6 h
2[b]80% 3 h; 5 55% 5 h
3 99% 0.5 h
1[a]99% 0.5h; 4 94% 0.5h
2[b]99% 0.5h; 5 69% 5h
3 79% 0.5h; 6 17% 4h
1[a]93% 2.5 h; 4 53% 2 h
2[b]91% 2 h; 5 71% 1 h
3 98% 0.25 h
1[a,b]99% 0.25 h; 4 79% 1 h
2[b] 81% 2 h;5 88% 0.25 h
3 99% 0.25 h
1 42% 2 h; 4 3% 2 h
2 35% 2 h; 5 22% 2 h
3 53% 2 h; 6 5% 2 h
1 73% 0.5 h; 4 34% 5 h
2 78% 0.75 h; 5 56% 4 h
3 80% 0.5h; 6 33% 4h
Scheme 3. Suggested mechanism for hydroboration of benzophenone imine
catalysed by 1. RHS – Alternative intermediate for catalytic profile using 3.
1 71% 2 h; 4 0% 17 h
3 83% 2 h; 6[c]85% 2 h
R = Ph: 1 0%, 2 h
3 10%, 2 h; 6[c]40% 2 h
R = Me, 3 60% 2 h (2.2:1 ratio); 6[c] trace
That 1 is active and 4 is not, may be assigned to the nature of
deprotonation products, which clearly demonstrates the key role
of bimetallic (Li-Al) cooperativity. I is the proposed deprotonation
intermediate using 1 and I’ using 4, which corresponds to the
crystallographically authenticated 8. In I the alkali metal would
instil a different molecular charge distribution to that in I’. This
scenario clearly facilitates the hydroboration step, which is not
the case with I’. A final comment on imine hydroboration is that
in both cases 1 (73% 0.5 h) offers marginally less reactivity than
3 (80 % 0.5 h). This difference may describe a subtle alkyl vs
amido effect, whereby the replacement of one TMP anion for an
iBu anion imparts greater nucleophilicity onto the hydride,
Aldehyde/ketones 5 mol% [Al] cat. loading, C6D6, room temperature. Imines 10
mol% [Al] cat. loading, C6D6, room temperature. Alkynes 10 mol% [Al] cat.
loading, in d8-toluene at 110 oC. [a] data for 1 from reference 8b. [b] data for 2
(1 mol% cat.) from reference 8a. [c] data for 6 from reference 6; All yields
against 1H NMR internal standard hexamethylcyclotrisiloxane.
We next screened benzophenone imine in the catalysis with 1, 3,
4 and 6 (10 mol%), since this substrate has an acidic N-H atom
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priming it for addition across the unsaturated substrate.
Alternatively, the increased steric demand of TMP may slow
reactivity. Moreover, it is apparent that even when the
hydroboration catalysis. Elaborating further, we attempted one
further substrate in comparative catalytic experiments with 6 and
3. With 6, 1-phenyl-propyne is only hydroborated in trace
amounts, despite the intrinsically smaller CH3 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.
This study into hydroboration of aldehydes, ketones and imines
reveals that anionic ate complexes are important additions to the
main group catalyst toolbox, providing higher conversions in
shorter timescales. We attribute this superiority to the greater
polarisation of key reaction intermediates induced by the
heterobimetallic complexes. Moreover, a novel new catalytic
activation pathway was elucidated for ketone hydroboration
involving -hydride elimination. With internal alkynes the
scenario is different and mononuclear species are the catalysts
of choice when steric constraints override the ate effect. Overall
this study illuminated that while ate complexes are beneficial in
most cases, the mononuclear species are more effective in
deprotonation pathway is available (catalyst
benzophenone imine), the pathway that follows, insertion
(catalyst with benzophenone imine) is favoured, albeit
marginally.
1
with
3
Finally, we turned to acetylene hydroboration comparing
reactivity once more between 1, 3, 4 and 6. Stoichiometric
reactions of TMP-containing 1 and 4 with terminal alkyne
phenylacetylene (PhCCH) in C6D6, reveal deprotonation of
PhCCH at room temperature, in agreement with the fact that
hydroboration of PhCCH with 1 implicated deprotonation as a
key step.[8b] Alternatively 3 is unreactive with PhCCH, and 6 only
very slowly hydroaluminates PhCCH, at room temperature.
Catalysis, using 10 mol% loadings in d8-toluene at 110 °C, 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. Thus 3 is comparable to 6 however, for the first
time we note that a clear ate effect is not in operation.
Furthermore, 3 is a better catalyst than 1 underlying that
increased hydride nucleophilicity is more important,
mechanistically, than deprotonation, though reduced sterics may
also be a factor.
others.
Thus,
in
the
field
a
of
aluminium-catalysed
hydroelementation, there is
high degree of substrate
dependence, governing the appropriate choice of catalyst.
A
similar picture is seen with the internal alkyne
Acknowledgements
diphenylacetylene. (10 mol%) is reported to convert
6
diphenylacetylene to the boronic ester in 40% yield after 2 hours
at 110 °C in d8-toluene,[6] whereas 1 is completely inactive, and
3 only reaches conversions of ca. 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).
The referees are thanked for excellent suggestions. Support is
duly acknowledged from the EPSRC (DTP award to VAP) and
the Alexander von Humboldt foundation (Humboldt Research
Award to REM: host Universität Regensburg, Germany). The
data set underlying this research can be located at
http://dx.doi.org/10.15129/9f7efa41-0688-40ea-baf6-
c5e4431a4575
Keywords: aluminium • aluminates • bimetallic synergy •
homogeneous catalysis • hydroboration • lithium
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Comparing neutral (monometallic)
and anionic (bimetallic) aluminium
complexes in hydroboration
catalysis: influences of lithium
cooperation and ligand set
Team mATES: Aluminium works better as a hydroboration catalyst when teaming
up with lithium in ate complexes
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