Structurally Defined Ring-Opening and Insertion of Pinacolborane into Aluminium-Nitrogen Bonds of Sterically Demanding Dialkylaluminium Amides

Article abstract of DOI:10.1002/ejic.202000919

Dialkylaluminium amides iBu₂Al(TMP) and iBu₂Al(HMDS) can perform catalytic hydroboration of ketones with pinacolborane to form the expected boronic esters. However, repeating the same reactions stoichiometrically without a ketone leads unexpectedly to ring-opening of pinacolborane and insertion of its open chain into the Al?N(amido) bond. To date there has been limited knowledge on decomposition pathways of HBpin despite its prominent role in hydroboration chemistry. X-ray crystallography shows these mixed Al?B products [iBu₂Al{OC(Me)₂C(Me)₂O}B(H)(NR₂)]₂ (NR₂=TMP or HMDS) form dimers with an (AlO)₂ core and terminal B?N bonds. Since the bond retention (B?H) and bond breaking (B?O) in these transformations seemed surprising, DFT calculations run using M11/6-31G(d,p) gave an energy profile consistent with a σ-bond metathesis mechanism where London dispersion interactions between iBu and (amide) Me groups play an important stabilising role in the final outcome.

Full text of DOI:10.1002/ejic.202000919

Communications
doi.org/10.1002/ejic.202000919
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Structurally Defined Ring-Opening and Insertion of
Pinacolborane into Aluminium-Nitrogen Bonds of Sterically
Demanding Dialkylaluminium Amides
Victoria A. Pollard,[a] Alan R. Kennedy,[a] Ross McLellan,[a] Duncan Ross,[a] Tell Tuttle,[a] and
Robert E. Mulvey*[a]
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Dedicated to Professor Alan Welch on the occasion of his retirement from Heriot-Watt University
work benzaldehyde derivative (HMDS)2Al(μ-OCH2Ph)2Li(THF)2,[5]
Dialkylaluminium amides iBu2Al(TMP) and iBu2Al(HMDS) can
and benzophenone derivative [(TMP){Ph2(H)CO}Al{μ-OC(H)
perform catalytic hydroboration of ketones with pinacolborane
to form the expected boronic esters. However, repeating the
Ph2}]2[6] (TMP=2,2,6,6-tetramethylpiperidide; HMDS=1,1,1,3,3,3-
hexamethyldisilazide). That notwithstanding,
a study by
same reactions stoichiometrically without
a ketone leads
Aldridge[7] on reducing CO2 using a range of NacNac-chelated
Al hydride complexes has indicated that the required Al–O/B–H
σ-bond metathesis is not always viable, thus thwarting the
turnover step in the catalytic cycle. Instead, following hydro-
alumination of the C=O group an Al boryloxy complex forms,
which in turn extrudes formaldehyde, with a significant differ-
ence in reaction rates between HBcat and 9-BBN implying that
the Al boryloxy complex is generated by a CÀ O/BÀ H σ-bond
metathesis.
Adding more complexity to the role/s of HBPin in catalysis,
Thomas provided convincing NMR and kinetic evidence estab-
lishing that hydroboration reactions of alkynes and alkenes
were catalysed by in situ generation of BH3 and borohydride
species from HBPin decomposition mediated by nucleophiles
such as nBuLi and nBu2Mg, previously thought to be the active
catalysts.[8] The mechanisms of these nucleophile-promoted
HBPin decompositions and co-products formed still remain to
be determined. Similarly and pre-dating this study, Harder
reported that calcium hydride, [DIPPNacNacCa(H)(THF)]2, acted as
a “Trojan horse” in facilitating decomposition of HBcat to
B2(cat)3, BH3 and other boron products, during hydroboration of
diphenylethene with HBCat, concluding that the true catalyst
was also BH3.[9] Clearly, more information is needed on
decomposition pathways involving HBPin especially in proc-
esses relevant to catalysis since the decomposition pathway of
the catalyst used will determine its maximum lifetime, and thus
the required catalyst loading. Towards this goal, here we report
the surprising outcomes of a combined experimental and
theoretical study probing the reactivity of HBpin towards the
neutral dialkylaluminium amides iBu2Al(TMP) 1 and iBu2Al
(HMDS) 2.
unexpectedly to ring-opening of pinacolborane and insertion of
its open chain into the AlÀ N(amido) bond. To date there has
been limited knowledge on decomposition pathways of HBpin
despite its prominent role in hydroboration chemistry. X-ray
crystallography shows these mixed AlÀ B products [iBu2Al{OC
(Me)2C(Me)2O}B(H)(NR2)]2 (NR2 =TMP or HMDS) form dimers
with an (AlO)2 core and terminal BÀ N bonds. Since the bond
retention (BÀ H) and bond breaking (BÀ O) in these trans-
formations seemed surprising, DFT calculations run using M11/
6-31G(d,p) gave an energy profile consistent with a σ-bond
metathesis mechanism where London dispersion interactions
between iBu and (amide) Me groups play an important
stabilising role in the final outcome.
Since Knochel introduced pinacolborane, HBpin, to synthetic
laboratories in 1992,[1] its use in synthesis and catalysis has
escalated as predicted in his seminal paper.[2] A successful
example of organic transformations catalysed by aluminium
compounds involving HBPin is the hydroboration of carbonyl
substrates.[3] Studied in depth, the reaction mechanism is
typically proposed to proceed via a two-step pathway: first an
aluminium hydride catalyst hydroaluminates the substrate (e.g.,
a ketone), while second a σ-bond metathesis occurs with HBpin
yielding the boronic ester product and regenerating the Al
hydride catalyst. Several intermediates potentially involved in
this second step have been isolated,[4] including from our own
[a] Dr. V. A. Pollard, Dr. A. R. Kennedy, Dr. R. McLellan, D. Ross, Prof. T. Tuttle,
Prof. 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
https://www.strath.ac.uk/staff/mulveyrobertprof/
Previously, hydride-free iBu2Al(TMP) 1, was reported to
catalyse hydroboration of ketones with HBpin at room temper-
ature via
a
β-hydride transfer mechanism.[6] Testing the
reproducibility of this initiation pathway to other alkylalumi-
nium amides, 2 was prepared by salt metathesis between
iBu2AlCl and Li(HMDS), isolating the desired compound as an oil
in high yield. A control reaction performed in a J. Young’s NMR
tube between iBu2Al(HMDS) and benzophenone in C6D6
Supporting information for this article is available on the WWW under
https://doi.org/10.1002/ejic.202000919
© 2020 The Authors. European Journal of Inorganic Chemistry published by
Wiley-VCH GmbH. This is an open access article under the terms of the
Creative Commons Attribution License, which permits use, distribution and
reproduction in any medium, provided the original work is properly cited.
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doi.org/10.1002/ejic.202000919
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solution monitored by H NMR spectroscopy revealed signals
consistent with isobutene [1H NMR: δ 1.58 (t, J=1.20 Hz, 6H);
4.72 (sept. J=1.20 Hz), 2H], the co-product of β-hydride trans-
fer. This implies that 2 acts as a masked hydride in the same
way as TMP analogue 1.
the amide has transferred from Al to B. From a general
perspective considering the relative strengths of such bonds,[10]
these bond transformations pose some interesting questions as
to the driving force behind the formation of 3.
To probe this surprising reactivity DFT calculations were
performed at the M11/6-31G(d,p)[11] level of theory using
Gaussian 16. For brevity full details are given in the SI.
Compound 3 is proposed to form through a σ-bond metathesis
mechanism involving initial complexation of HBPin and 1 to
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To try and uncover potential decomposition pathways in
hydroboration reactions utilising 1 as the catalyst, we per-
formed a stoichiometric reaction between iBu2Al(TMP) and
HBpin in C6D6 in a J. Young’s NMR tube. We expected a mixture
of products such as iBuBpin and (TMP)Bpin. Instead, crystals of
3 were obtained (56% crystalline yield, Scheme 1).
Reaction scale up and X-ray diffraction studies of 3 revealed
its novel molecular structure of formula [iBu2Al{OC(Me)2C
(Me)2O}B(H)(TMP)]2 (Figure 1).
Deposition Numbers 2019982 (3) and 2019983 (4) contain
the supplementary crystallographic data for this paper. These
data are provided free of charge by the joint Cambridge
Crystallographic Data Centre and Fachinformationszentrum
Karlsruhe Access Structures service www.ccdc.cam.ac.uk/struc-
tures.
Surprisingly, 3 retains the typically more reactive BÀ H bond
(416 kJ/mol dissociation energy), yet cleaves the doubly stron-
ger BÀ O bond (890 kJ/mol dissociation energy).[10] This is
significant as for hydroboration catalysis to proceed cleavage of
the B–H bond is necessary. The formula of 3 indicates that the
5-atom BOCCO ring of HBPin has opened during the reaction
and inserted into the AlÀ N bond of the amide. Concomitantly
~
form a Reaction complex (RC) ( H=À 14.2 kcal/mol, Figure 2).
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RC represents a local minimum on the potential energy
surface. From this point the non-covalently bound RC forms the
4-bond cyclic intermediate complex (Int1) in a barrierless
~
reaction. Int1 is surprisingly stable ( H=À 13.0 kcal/mol, rela-
tive to RC, Figure 2) primarily due to the flexible coordination of
B and Al, which can distort from a trigonal to a tetrahedral
geometry to support the extra bond formation (see SI for 3D
structures of all optimised species). The stability of Int1 is also
seen in the barrier to forming the product of the reaction. The
transition state (TS1) involves breaking concertedly the AlÀ N
and BÀ O bonds involved in the reactive core. While the
activation energy for this step is achievable under the room
~
temperature and STP reaction conditions ( H*=17.0 kcal/mol,
Figure 2) the relative stability of the product (Prod) drives this
reaction. Additional steric bulk provided by the substitution on
the TMP rings and iBu groups on Al would prima facie
destabilise the system. However, in forming Prod the additional
London dispersion interactions[12] between alkyl groups (Fig-
ure S21, H···H dispersion interactions of lengths 2.13, 2.26,
2.29 Å) helps stabilise the resulting intermediate from the bond
formation. This hypothesis was corroborated by modelling the
reaction with modified substrates using only (Me)2Al(piperidide)
as the reactant (see SI).
Scheme 1. Synthesis of the dimeric Bpin ring-opening product 3.
Figure 1. Molecular structure of 3. Hydrogen atoms, except BÀ H hydride
have been omitted for clarity. Thermal ellipsoids are drawn at 40%
probability. Symmetry operations used to generate equivalent atoms À x;
°
À y; 1À z. Selected bond lengths (Å) and angles ( ): Al1À O1, 1.877(1); AlÀ O1’,
1.862(1); Al1À C5, 1.979(2); Al1À C1, 1.987(2); B1À O2, 1.364(3); B1À N1, 1.422(3);
B1À H1, 1.07(2); C5À Al1À O1, 118.51(8); C1À Al1À O1, 110.59(8); C5À Al1À O1’,
115.71(8); C1À Al1À O1’, 110.40(8); C1À Al1À C5, 115.35(9); O1À Al1À O1’,
81.73(6); H1À B1À N1, 124(1); H1À B1À O2, 116(1); O2À B1À N1, 120.3(2). This
structure and that of 4 have been deposited in the CSD under codes
2019982 and 2019983.
Figure 2. Relative enthalpy surface (in kcal/mol) for the reaction between
HBPin and 1. Values in parentheses indicate the change in Gibb’s Free
energy ( G in kcal/mol) associated with each step.
~
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In this discussion we have focussed primarily on the relative
enthalpies of the reaction mechanism. Naturally, the reaction
rates and equilibria will depend on the corresponding Gibbs
Table 1. Hydroboration of benzophenone with HBpin catalysed by
selected aluminium catalysts.
1
2
3
4
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9
Catalyst (5 mol% [Al])[a]
Yield [%]
Time [h]
~
free enthalpies ( G), which have been computed by applying
iBu2Al(TMP), 1
iBu2Al(HMDS), 2
[iBu2Al{OC(Me)2C(Me)2O}B(H)(TMP)]2, 3
[iBu2Al{OC(Me)2C(Me)2O}B(H)(HMDS)]2, 4
94
97
35
24
0.5
3
17
21
the harmonic oscillator/rigid rotor approximation and are
~
G
provided in Figure 2 (values in parentheses). However, the
values should be viewed with some caution as the harmonic
oscillator/rigid rotor approximation is known to be problematic
in the case of weakly bound complexes, such as RC, due to the
large number of low-energy vibrational modes, which in turn
have a large contribution to the entropy. Nonetheless, it is
interesting to note the qualitative effect of the entropy on the
enthalpy profile. The inclusion of entropy disfavours the
formation of RC, with a decrease in the number of degrees of
freedom in the system, although the complex formation is only
[a] Conditions: 5 mol% [Al] catalyst loading, C6D6 solvent, room tempera-
ture. All yields estimated against 1H NMR internal standard hexameth-
ylcyclotrisiloxane.
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yield of Ph2CHOBpin, after 17 hours, in contrast to the near
quantitative conversion obtained within 0.5 hours using 1. As
expected from its similarity to 3, HMDS-borane 4 also performs
poorly as a pre-catalyst, with only 24% conversion after
21 hours at room temperature using 5 mol%. This implies that
both 3 and 4 could exist as off-cycle products from hydro-
boration with 1 and 2 as pre-catalysts, respectively. Therefore, 3
and 4 can be considered deactivation products from these
monometallic aluminium pre-catalysts. Though both possess a
B–H bond, they are occupied within bulky neutral environ-
ments, that greatly diminishes their reducing capability com-
pared to those of charged ate analogues such as the
trialkoxyborohydride formed via addition of nucleophilic NaOt-
Bu to HBpin.[14] However, note that when 1 is used as a pre-
catalyst in hydroborations no signals corresponding to 3 are
seen in 11B NMR spectra. This indicates that in the presence of
benzophenone the rate of hydroboration is greater than the
rate of B–O cleavage and formation of 3.
It is notable that such well-defined examples of elucidated
decomposition products of HBpin of relevance to catalysis are
very rare. B2pin3 is a known, crystallographically-characterised
decomposition product of HBpin.[15] The structure of B2pin3 has
a central O(Me2)CÀ C(Me2)O unit from ring-opening of a HBpin
molecule, but otherwise bears little resemblance to 3 or 4. Hill
reported observing trace amounts of B2pin3 in hydroboration of
imines, catalysed by DIPPNacNacMg(nBu), under forcing
conditions.[16] Other documented examples in main group
systems are known,[17–21] but for most examples of cleavage of
one or both B–O bonds in HBpin or HBcat one has to turn to
transition metal, lanthanide, or actinide complexes.[22–30] Ligand
redistribution reactions between alanes AlX3 and boranes BY3 to
AlX3-nYn and BY3-nXn are also well documented.[31,32] This
redistribution reactivity has been harnessed to generate the
active catalyst Et2AlH from a Et3Al pre-catalyst and HBpin in the
hydroboration of acetylenes.[4c] Similarly, iBu2AlH and HBpin also
undergo ligand scrambling generating iBuBpin and iBu3B,
amongst other products.
~
mildly endergonic on the free enthalpy surface ( G=0.8 kcal/
mol, Figure 2). The subsequent formation of Int1, TS, and Prod,
are relatively unaffected by the inclusion of entropy. Finally, the
formation of 3, while less favourable when entropy is consid-
~
ered, remains a strongly exergonic process ( G=À 24.4 kcal/
mol, Figure 2).
Crystalline 3 exists as a centrosymmetric dimer featuring a
planar kite-shaped AlÀ OÀ AlÀ O core. The Al centre exists in a
distorted tetrahedral geometry comprising two O and two C
°
atoms with bond angles spanning 110.96(7)–117.22(7) , whilst
the B centre is distorted trigonal planar with bond angles in the
°
range 117(2)–122.7(2) . TMP and Al components sit at opposite
1
ends of the ring-opened B-pinacol unit. The H NMR spectrum
of 3 displays the hydride resonance as a broad singlet at
4.75 ppm in C6D6 solution, which sharpens upon applying 11B
decoupling. In comparison, the HBpin hydride resonance is a
broad quartet at 4.17 ppm with a J coupling constant of
171.56 Hz, in C6D6. The 11B NMR spectrum of 3 displays a broad
singlet resonance at 30.0 ppm, while for HBpin this signal
occurs as a doublet at 28.1 ppm (J=174 Hz). The lack of
observed splitting in the 1H and 11B NMR spectra of 3 could be a
result of signal broadening due to a quadrupolar nucleus (11B)
in an asymmetrical environment. A 1H DOSY NMR experiment[13]
on 3 in C6D6 solution gave an estimated molecular weight of
830 g/mol, consistent with retention of the dimer in solution
(818.73 g/mol; À 1% error). The 13C{1H} NMR spectrum displayed
all the expected signals, though no signal was observed in the
27Al spectrum.
A similar outcome is seen reacting together iBu2Al(HMDS) 2
and HBpin in hexane. Crystalline [iBu2Al{OC(Me)2C(Me)2O}B(H)
(HMDS)]2, 4, is formed in a low 24% isolated yield. Its molecular
structure both in the crystal and in C6D6 are analogous to those
of 3 (see SI and Figure S13).
Next, 2, 3, and 4 were applied as pre-catalysts for hydro-
boration of benzophenone with HBpin to check their catalytic
viability (Table 1). The reduction of benzophenone with 5 mol%
of 2 as a pre-catalyst is slower than with 1, requiring 3 hours to
reach 97% conversion, compared to only 0.5 hours to reach
94% conversion with 1 as reported previously.[6] This suggests
that β-hydride transfer in forming the active Al hydride pre-
catalyst is significantly faster with 1 than with 2. Employing
5 mol% of TMP-borane 3 as a pre-catalyst managed only a 35%
In summary, light has been shed on the decomposition of
HBpin when used stoichiometrically with alkylaluminium
amides. When used catalytically these Al amides can hydro-
borate ketones with HBpin at room temperature via a putative
R2AlH intermediate. Such well-defined examples of the break-
down of HBpin with main group metal compounds are rare, but
they are important given the recent escalation of activity in
main group homogeneous catalysis. The products of these
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[5] V. A. Pollard, S. A. Orr, R. McLellan, A. R. Kennedy, E. Hevia, R. E. Mulvey,
reactions were unexpected since they formed via opening of
the HBpin ring and its concomitant insertion into the AlÀ N
(amide) bond. Adding intrigue, DFT calculations suggested that
attractive dispersion forces involving alkyl groups on the Al
compound are a key feature within the reaction coordinate. It is
rare for London dispersion forces to be mentioned in the
context of TMP-aluminate chemistry[33] or TMP’s general role in
synergistic bimetallic reactions,[34] but as recently highlighted in
a seminal review by Liptrot and Power,[35] such weak forces can
significantly impact inorganic and organometallic structures
involving bulky ligands. Future interrogation or re-interrogation
of TMP and related bimetallic compounds may find that London
dispersion forces are more significant than initially evidenced.
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Acknowledgements
We thank Prof. Eva Hevia and Dr David Nelson for insightful
comments, and to the EPSRC (DTP award EP/M508159 to V. A. P.)
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funded ARCHIE-WeSt High Performance Computer (www.archie-
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Conflict of Interest
The authors declare no conflict of interest.
Keywords: Aluminium
· Density functional calculations ·
Homogeneous catalysis · Hydroboration · Main group chemistry
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Manuscript received: October 1, 2020
Revised manuscript received: October 26, 2020
Accepted manuscript online: October 30, 2020
Eur. J. Inorg. Chem. 2021, 50–53
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