Selective Boryl-Anion Migration in a Vinyl sp2?sp3 Diborane Induced by Soft Borane Lewis Acids

Article abstract of DOI:10.1002/anie.201808216

An intramolecular 1,2-boryl-anion migration from boron to carbon has been achieved by selective activation of the π system in [(vinyl)B₂Pin₂)]^? using “soft” BR₃ electrophiles (BR₃=BPh₃ or 9-aryl-BBN). The soft character is key to ensure 1,2-migration proceeds instead of oxygen coordination/B?O activation. The BR₃-induced 1,2-boryl-anion migration represents a triple borylation of a vinyl Grignard reagent using only B₂Pin₂ and BR₃ and forms differentially protected 1,1,2-triborylated alkanes. Notably, by increasing the steric bulk at the β position of the vinyl Grignard reagent used to activate B₂Pin₂, 1,2-boryl-anion migration can be suppressed in favor of intermolecular {BPin}^? transfer to BPh₃, thus enabling simple access to unsymmetrical sp²?sp³ diboranes.

Full text of DOI:10.1002/anie.201808216

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Accepted Article
Title: Selective boryl-anion migration in a vinyl sp2-sp3 diborane
induced by soft borane Lewis acids
Authors: Michael Ingleson, Valerio Fasano, Jessica Cid, and Richard
Procter
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201808216
Angew. Chem. 10.1002/ange.201808216
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10.1002/anie.201808216
Angewandte Chemie International Edition
COMMUNICATION
Selective boryl-anion migration in a vinyl sp2-sp3 diborane
induced by soft borane Lewis acids
Valerio Fasano,a Jessica Cid,a Richard J. Procter,a Emily Ross,a and Michael J. Inglesona*
Abstract: A novel intramolecular 1,2-boryl anion migration from
boron to carbon has been achieved by selective activation of the π-
system in [(vinyl)B2Pin2)]− using “soft” BR3 electrophiles (BR3 = BPh3
or 9-Aryl-BBN). The soft character is key to ensure 1,2-migration
proceeds instead of oxygen coordination / B-O activation. The BR3
induced-1,2-boryl anion migration represents a triple borylation of a
vinyl Grignard reagent using only B2Pin2 and BR3 and forms
differentially protected 1,1,2-triborylated alkanes. Notably, by
increasing the steric bulk on the beta position of the vinyl Grignard
reagent used to activate B2Pin2, 1,2-boryl-anion migration can be
suppressed in favor of intermolecular {BPin}− transfer to BPh3, which
represents a simple way to access unsymmetrical sp2-sp3 diboranes.
activator. This approach is conceptually related to the Zweifel
reaction,18 but the use of borane Lewis acids and {BPin}- as the
migrating group will lead to differentially functionalised 1,1,2-
triborylated alkanes in one step. Related 1,1-diborylated alkanes
have emerged as highly versatile reagents used in selective C-C
bond formation by the Suzuki-Miyaura coupling reaction or via
deprotonation / deborylation of the diborylated carbon.19-22
The selective (for intramolecular 1,2-boryl-migration)
activation of [(vinyl)B2Pin2]- (complex B, Scheme 1 bottom),
requires judicious choice of the borane, BR3, as a range of
outcomes are feasible including: (i) vinyl anion transfer from B to
BR3; (ii) binding of BR3 to an oxygen in B and subsequent C-O
or B-O cleavage; (iii) {BPin}- anion transfer from B to BR3; (iv)
BR3 activation of the vinyl π-system and intramolecular {BPin}-
transfer. While (i) and (ii) are undesirable, pathway (iii) would be
an attractive route to unsymmetrical diboranes using commercial
Grignard reagents as activators. Equally notable and our primary
focus - intramolecular 1,2-boryl-migration (pathway iv) - would
be a new and simple route to 1,1,2-triborylated alkanes.
The coordination of a Lewis base (LB) to diborane(4)
compounds, such as B2Pin2 (1), generates an sp2-sp3 diborane
in which the boron–boron bond is polarised.1 This imparts
nucleophilic character to the sp2 boron, thereby enabling the
mild generation of a functional equivalent of {BPin}−.1,2 This has
become a powerful transition metal free methodology to borylate
organic substrates and generate desirable organoboronate
esters. Alkoxides or N-heterocyclic carbenes (NHCs) are the
typical LBs employed in the activation of 1,1-3 with the use of
carbanions (R-) having much less precedence,4-9 despite R-
being able to generate a more nucleophilic {BPin} moiety due to
their greater basicity relative to alkoxides and NHCs. Among the
limited examples in this area, recent work has showed that
complex A synthesised from 1 and nBu-MgL (L = β-diketiminato)
transfers a boryl anion to boranes forming new unsymmetrical
sp2-sp3 diboranes (Scheme 1, 1a).10 Indeed, transfer of a boryl
nucleophile to an external electrophile is the dominant reactivity
pathway reported for B2Pin2 activated by simple carbanions.10 It
is important to extend the chemistry of [(R)B2Pin2)]- to enable
new routes to highly functionalized organoboronates to be
discovered, as these will be desirable particularly if accessed
using readily accessible starting materials (e.g. RMgX / B2pin2).
Prior to this work, 1,2-boryl-anion migration from boron to
carbon in [(R)B2Pin2]- species had been limited to using
functionalized “R-” equivalents. For example, coordination of a
carbanion containing a Br or OCb group (or a diazoalkane), to 1
led to loss of [OCb]-, [Br]- (or N2) and formation of 1,1-
diborylalkanes (Scheme 1, 1b).11-17 We hypothesised that an
alternative route to induce intramolecular 1,2-boryl-anion
migration would be the activation of an unsaturated R- group
(e.g. -CH=CH2) in [(R)B2Pin2]- by a borane Lewis acid. This is
attractive as it avoids prefunctionalization of the carbanion
Scheme 1. Top, previous work on intermolecular / intramolecular {BPin}-
transfer in carbanion activated B2Pin2. Bottom, selective boryl-anion migration
in vinyl sp2-sp3 diboranes induced by soft borane Lewis acids.
Herein, we report that intramolecular 1,2-boryl-migration in
sp2-sp3 diboranes does not require preinstalled leaving groups in
the carbanion. Instead the formation of [(vinyl)B2Pin2]- followed
by selective activation of the π system by certain boranes forms
differentially functionalised (at boron) 1,1,2-triborylated alkanes.
The use of β-methyl vinyl Grignard reagent changes the reaction
outcome to intermolecular {BPin}- transfer to BR3, generating an
unsymmetrical diborane from simple starting materials.
[a]
V. Fasano, Dr. J. Cid, R. J. Procter, E. Ross, Prof. Dr. M. J. Ingleson
School of Chemistry, University of Manchester
Oxford Road, Manchester, M13 9PL (UK)
E-mail: michael.ingleson@manchester.ac.uk
Supporting information for this article is given via a link at the end of
the document. CCDC 1856184.
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10.1002/anie.201808216
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COMMUNICATION
We started our investigation by probing the accessibility of
the simplest vinyl adduct of 1, [(CH2=CH)B2Pin2]− ([2]-). This
could be generated as the major product by the addition of 1
equiv. of commercial vinyl magnesium bromide to 1 in THF at -
78oC (Scheme 2, left). The successful formation of [2]- was
indicated by 11B NMR spectroscopy where two new resonances
could be observed: one at 37.3 ppm (three coordinate boron)
and the other at 4.8 ppm (four coordinate boron), analogous with
that reported for [(Ph)B2Pin2]- (39.2 and 4.0 ppm, respectively).6
Since B(C6F5)3 can activate alkenes and alkynes even in the
presence of certain oxo-functionalities, the ability of B(C6F5)3 to
trigger the 1,2-boryl-migration was explored.23 Adding 1 equiv. of
B(C6F5)3 to [2]- (at -78oC) led after 2 hours to a single new 11B
resonance at -3.2 ppm, consistent with an [RO-B(C6F5)3]-
Figure 1. Left, solid-state structure of [3][(THF)2MgBr] with ellipsoids at 50%
probability (some hydrogens omitted for clarity). Right, schematic with select
bonds labelled, distances (in Å) a = 1.663(9), b = 1.571(7), c = 1.545(8), d =
1.554(8), e = 1.358(8), f = 1.417(7), g = 2.118(3) and h = 2.066(4).
species (in contrast [alkyl-B(C6F5)3]- anions have
a
11B
oxygen coordination to magnesium. This results in a modest
elongation of the B-O bonds involving oxygen coordinated to Mg
(compare e and f Fig. 1). Other distances and angles in
[3][(THF)2MgBr] are within the expected values, with C-BPin
bond distances shorter than the C-BPh3 distance (c, d Vs. a Fig.
1). In d8-THF solution, [3][(THF)2MgBr] shows two singlets in
the 1H NMR spectrum at 298 K for the methyl groups of the
pinacol groups, indicating the inequivalence of these protons on
the NMR timescale due to chelation to Mg. Cation metathesis
can be achieved using [Me4N][Cl] to form the air-stable product
[3][Me4N] in which the pinacol methyl groups now exhibit a
single resonance in the 1H NMR spectrum at 298 K (in THF). It is
noteworthy that the one-pot triborylation of a vinyl Grignard
reagent has not been reported to the best of our knowledge.
resonance ca. -15 ppm). The 19F NMR spectrum confirmed [RO-
B(C6F5)3]- formation, with ESI-MS analysis supporting the
formation of an [RO-B(C6F5)3]- species derived from ring opening
of one BPin moiety in [2]-. With two additional 11B resonances
observed at 48.0 and 29.2 ppm, we tentatively assign the
product as derived from B(C6F5)3 activation of pinacol bound to
the four coordinate boron (Scheme 2, top). This is consistent
with reports on BPin moieties in anionic borates undergoing B-O
cleavage on addition of electrophiles.24
Regarding the mechanism of formation, the arrangement of
boranes in [3]- excludes the possibility of vinyl transfer from [2]-
to BPh3, followed by diboration of the vinyl group in
[(CH2=CH)BPh3]- with B2Pin2 (or base activated B2pin2) since
this would yield a 1,2 arrangement of the BPin groups and not
1,1.1,2 To gain further insight into the reaction mechanism and
the disparity between BPh3 and B(C6F5)3, DFT calculations were
performed at the M06-2X/6-311G(d,p), with a solvent polarisable
continuum model (PCM, THF) level. Based on the structure of
[3][(THF)2MgBr], the cation [(THF)2MgBr]+ was included initially.
The formation of the neutral adduct 2′ from 1 and the vinyl
Grignard reagent is energetically favoured (∆G298K = -9.8 Kcal
mol-1), despite the adverse entropic contribution (Scheme 3).
Adduct 2′ showed a slightly elongated B-B bond relative to that
of 1 (1.73 and 1.70 Å, respectively), as reported for other sp2-sp3
diboranes.1,2 Addition of BPh3 to 2′ to yield the product
[3][(THF)2MgBr] is energetically downhill (∆G298K = -42.0 Kcal
mol-1). To gain insight into the disparate borane reactivity (B-O
activation vs π activation), the change in energy upon BR3
coordination to the oxygen of 2′ was probed. For BPh3 this
process is energetically uphill (∆G298K = 5.2 Kcal mol-1), in
agreement with the reduced electrophilicity and oxophilicity of
this borane relative to B(C6F5)3. Replacing BPh3 with B(C6F5)3
(Scheme 3, bottom), O-coordination become significantly
exergonic (∆G298K = -8.8 Kcal mol-1) consistent with the
observation of B-O cleavage on mixing [2]- and B(C6F5)3. Thus,
the correct tuning of the oxophilicity / electrophilicity of the
borane employed is a key aspect to selectively trigger 1,2-boryl-
migration. This is further emphasised by replacing B(C6F5)3 with
Scheme 2. Reaction of 1 with a vinyl Grignard reagent and B(C6F5)3 or BPh3.
The oxo-based reactivity of B(C6F5)3 with [2]- was
attributed to the high electrophilicity and oxophilicity of this
borane, therefore softer boron electrophiles were explored. In
particular BPh3, since this borane reacts with complex A to
generate [PinB-BPh3]- with no competitive reactivity at the oxo-
sites reported (Scheme 1, 1a).10 Adding 1 equiv. of BPh3 in THF
to the in-situ generated [2][(THF)nMgBr] (at -78oC), resulted in
the formation of the desired product [3]- formed from
intramolecular {BPin}- transfer (Scheme 2, bottom). [3]- has
diagnostic resonances in the 11B NMR spectrum (34.7 ppm for
1
the C-BPin moieties, and -9.5 ppm for [C-BPh3]-) and in the H
NMR spectrum (broad signal at 0.55 ppm for the CH(BPin)2),
with the formulation further confirmed by accurate mass
spectrometry. Performing the reaction at -78oC for 2 h and then
room temperature for 18 h resulted in complete consumption of
[2]-, yielding [3]- in 71% (in-situ conversion) as the major product.
Repeating the reaction on larger scale allowed for the isolation
of [3][(THF)2MgBr] as a white solid by solvent removal and
washing with Et2O (70% isolated yield). Single crystals of
[3][(THF)2MgBr] were obtained by layering pentane onto a THF
solution (Figure 1). In the solid state structure the cation is
chelated by the two pinacolato moieties of [3]- via
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10.1002/anie.201808216
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COMMUNICATION
BPh3
H
H
+ BPh3
H
H
B
H
H
O
O
B
O
O
O
O
B
B
O
+15.2
TS
O
[2]-
+0.4
0.0
G 298 K
(Kcal mol-1
H
H
B
H
)
BPh3
H
H
B
H
B
O
O
O
O
B
O
O
O
O
-
Scheme 3. Free energy profile for formation of 2′ and O-coordination of the
latter to the borane (the zero energy reference is set as 2′ + BR3 in each case).
[2B]
-45.3
+ BPh3
Scheme 4. Free energy reaction profile for BPh3 induced 1,2-boryl-migration.
the harder Lewis acid BF3, with O-coordination now becoming
much more exergonic (∆G298K = -26.4 Kcal mol-1 relative to 2’
and BF3). Attempts to crystallise [2][(THF)nMgBr] were
unsuccessful in our hands, thus due to the unknown exact
nature of the magnesium species coordinated to [2]- and to
facilitate more detailed computational studies, additional DFT
calculations were performed in absence of the counterion. It
should be noted that the calculated HOMO and HOMO-1 of [2]-
are analogous to that of 2′ indicating that while Mg coordination
will effect energies to some extent it does not drastically effect
the electronic distribution. The HOMO of [2]- has polarised σ B-B
character (consistent with the observed {BPin} nucleophilic
character), as well as some σ B-C(vinyl) and lone pair oxygen
character (Figure 2, left). The π C=C orbital instead contributes
to the HOMO-1, with the vinyl system almost completely aligned
with the B-B bond (B-B-C=C = 12.10o).
With an understanding of the reaction mechanism in hand,
other soft boron based Lewis acids were tested. The addition of
1 equiv. of 9-Ph-BBN to [2]- (at -78oC), gave the desired product
[4]-, with diagnostic peaks observed in the 11B NMR spectrum
(34.0 ppm for the -BPin moieties, and -15.3 ppm for [R(Ph)BBN]-
) and in the 1H NMR spectrum (upfield broad signal at 0.24 ppm
for the CH(BPin)2), with mass spectrometry confirming the
formulation for the anion [4]- (Scheme 5, top). [4][(THF)2MgBr]
was isolated in 52% yield (2 molecules of THF coordinated to
[MgBr]+ by 1H NMR spectroscopy). It is interesting to note that in
this case the tetra-coordinated boron centre in [4]- has restricted
rotation causing desymmetrization of the bicyclo moiety. Notably,
[4][(THF)2MgBr] could be selectively deborylated by the addition
of 1.1 equiv. of HNTf2, which yielded 9-Ph-BBN and
(PinB)2CHMe as the major products, indicating cleavage of the
C-(Ph)BBN bond dominates. In contrast, (PinB)2CHMe was
formed in low amounts from the addition of HNTf2 to the [2]-, with
formation of ethene and 1 dominating (Scheme 5, bottom).
HOMO
HOMO-1
Figure 2. The calculated HOMO and HOMO-1 of [2]- (isovalue = 0.04). [2]-
and 2′ showed similar geometry (particularly regarding the B-B-C=C dihedral
angle) and HOMOs, thus the former geometry is provided and not that of [2B]-.
The potential energy surface is flat where complex [2]- is
located, with different local minima obtained by rotation of the
vinyl group around the B-C(vinyl) bond. To trigger the
intramolecular 1,2-boryl-migration, a correct arrangement of the
vinyl moiety relative to the B-B bond is required for the trans-
addition of BPh3 and BPin to the C=C bond (Scheme 4). From
this arrangement ([2B]-) the reaction proceeds via transition state
TS with a low free energy barrier of 15.2 kcal mol-1 at 298 K. In
TS, the vinyl system is almost perpendicular to the B-B bond
(torsional angle B-B-C=C = 85.96o), with both the B-B and the
C=C bonds slightly elongated compared to [2B]- (1.75 vs 1.73 Å,
and 1.36 vs 1.33 Å, respectively). Bond Order analysis of TS
revealed that the reaction proceeds through an asynchronous
concerted mechanism, with the C-BPh3 bond formed to a greater
extent than the C-BPin bond (0.29 and 0.08, respectively).
Scheme 5. Top, reaction with 1, a vinyl Grignard reagent and 9-Ph-BBN.
Bottom, synthesis of 1,1-diboryl-ethane via protodeboronation of [4]- with this
product formed in low yield from direct protonation of [2]-.
This highlights the importance of using a soft Lewis acid to
selectively trigger the 1,2-boryl-migration over other potential
pathways. To further support that the reactivity difference
between B(C6F5)3 and BPh3 (or 9-Ph-BBN) is not due to steric
factors (as B(C6F5)3 is significantly bulkier than BPh3), 9-mesityl-
BBN and 9-o-tolyl-BBN were evaluated. While the former gave
no reaction with [2]- (presumably due to the large steric bulk
around boron), the addition of o-tolyl-BBN to [2]- in THF led to
the intramolecular 1,2-boryl anion migration product [5]- albeit
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10.1002/anie.201808216
Angewandte Chemie International Edition
COMMUNICATION
slower than when using 9-Ph-BBN. Importantly, no B-O
cleavage products were observed, with the mass balance at this
point being unreacted [2]- and o-tolyl-BBN. Thus with bulkier,
less Lewis acidic 9-aryl-BBN boranes the 1,2-boryl migration
Keywords: 1,2-migration • diboranes • Grignard reagents •
borylation • boranes
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With the aim to disfavour borane Lewis acids interacting with
the vinylic π system and thus switch selectivity from intra- to
inter-molecular {BPin}- transfer, we explored the effect of
increasing steric hindrance at the β-vinylic carbon. In particular,
using the adduct [7]-, which was generated in-situ by the
addition of 1 equiv. of (E/Z)-1-propenylmagnesium bromide to 1
in THF at -78oC. The subsequent addition of BPh3 to [7]-
resulted in suppression of 1,2-boryl-migration with [8]- detected
only in trace amounts (Scheme 6). In this case [PinB-BPh3]-
(40% yield) and (E/Z)-1-propenyl-BPin were observed as the
major new species after 18 h at room temperature, thus
confirming switching of selectivity from intra- to inter-molecular
{BPin}- transfer. This represents a simple route to access an
unsymmetrical sp2-sp3 diborane using only commercial reagents.
[4]
[5]
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[7]
[8]
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Scheme 6. Reaction of 1 with 1-propenyl-Grignard reagent and then BPh3.
The cation is assigned as [(THF)nMgBr]+ throughout.
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M. S. Hill, M. F. Mahon, Angew. Chem. Int. Ed. 2018,
10.1002/anie.201803607.
In summary, a novel intramolecular 1,2-boryl anion migration
has been induced by the addition of soft boranes to vinyl sp2-sp3
diboranes. Competitive strong oxygen coordination has to be
prevented, thus the softness of the borane is key in providing
selective boryl transfer. With BPh3 and 9-Ph-BBN, intramolecular
1,2-boryl migration enables the one-pot synthesis of differentially
protected 1,1,2-triborylated alkanes from simple starting
materials. Furthermore, the ability to switch {BPin}- transfer from
intra- to inter-molecular by increasing the steric hindrance in the
vinyl group allows access to unsymmetrical sp2-sp3 diboranes
using commercial Grignard reagents and B2Pin2.
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Acknowledgements
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We acknowledge the University of Manchester, the EPSRC
(EP/K039547/1)), and the Horizon 2020 Research and
Innovation Program (Grant no. 769599) for support. J.C
acknowledges a Marie Cure Fellowship (703227 – DIBOR).
Additional research data supporting this publication are available
as supplementary information accompanying this publication.
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Conflict of interest
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The authors declare no conflict of interest.
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10.1002/anie.201808216
Angewandte Chemie International Edition
COMMUNICATION
Entry for the Table of Contents
Valerio Fasano, Jessica Cid, Richard J.
Procter, Emily Ross and Michael J.
Ingleson
Page No. – Page No.
Selective boryl-anion migration in a
vinyl sp2-sp3 diborane induced by
soft borane Lewis acids
Selective Pi fishing: an intramolecular 1,2-boryl anion migration has been
achieved by selective activation of the π-system in vinyl sp2-sp3 diborane using
“soft” borane Lewis acids.
This article is protected by copyright. All rights reserved.