[BO2]? as a Synthon for the Generation of Boron-Centered Carbamate and Carboxylate Isosteres

Article abstract of DOI:10.1002/anie.202005674

Oxoborane carbamate and carboxylate analogues result from the in situ trapping of [BO₂]^? produced by elimination of 2,3-dimethyl-2-butene from a pinacolatoboryl anion.

Full text of DOI:10.1002/anie.202005674

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Accepted Article
Title: [BO2]- as a Synthon for the Generation of Boron-centered
Carbamate and Carboxylate Isosteres
Authors: Michael Stephen Hill, Anne-Frédérique Pecharman, Claire
McMullin, and Mary Mahon
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[BO2] as a Synthon for the Generation of Boron-centered
Carbamate and Carboxylate Isosteres
Anne-Frédérique Pécharman, Michael S. Hill,* Claire L. McMullin* and Mary F. Mahon
Abstract: Oxoborane carbamate and carboxylate analogues
result from the in situ trapping of [BO2] produced by elimination
of 2,3-dimethyl-2-butene from a pinacolatoboryl anion.
and generic syntheses of such species are currently
unavailable.[8] More attractive routes to anions such as A and B,
therefore, would emulate those applied in the synthesis of their
wholly organic analogues. The reaction of an organyl or amide
anion with CO2, for example, provides a classical means to
access carboxylate and carbamate anions, respectively. In
contrast, similar routes to boron analogues are precluded by the
unavailability of any suitable ‘boron dioxide’ synthon. While the
radical species, BO2, and the isoelectronic equivalent of CO2,
[BO2], have been identified spectroscopically as short lived
intermediates in borane flames or under matrix isolation
conditions,[10] and both have attracted significant theoretical
attention as highly oxidising ‘hypohalogens’,[11] these species
neither exist as discrete entities nor have they been implicated in
any productive synthesis. This lacuna is reminiscent of
oxoborane (BO) chemistry prior to Braunschweig’s report of
trans-[(Cy3P)2BrPt(BO)], which achieved the in situ generation of
a terminal B≡O ligand through the reversible elimination of
Organoboron oxides comprising three-coordinate boron have
been known since the mid-1930s,[1] and typically exist as
cyclotrimeric anhydrides, [RBO]3, of the corresponding
organoboronic acids.[2] Similarly, naturally occurring inorganic
metaborates such as NaBO2 comprise the trimeric [B3O6]3− unit
rather than discrete [BO2]− anions.[3] These observations are a
thermodynamic consequence of the strong B−O bond (809 kJ
mol−1) and the latent Lewis acidity of the boron center.[4] A number
of noteworthy recent advances in the chemistry of lower
nuclearity oxoborane derivatives, however, have been achieved
either through the incorporation of kinetically stabilizing
substituents,[5] for example in Aldridge’s isolated oxoborane
anion (1, Figure 1a),[6] or by saturation of the Lewis basic oxo and
Lewis acidic boron units.[7, 8] This latter approach is exemplified
by Rivard and co-workers’ isolation of [(IPr)(HO)B=OB(C6F5)3] (2,
IPr = N,N'-bis(2,6-di-isopropylphenylimidazol-2-ylidene, Figure
1a) in which the stability of the HOB=O unit is maintained through
the donor-acceptor combination of an N-heterocyclic carbene
(NHC) and the potent Lewis acid, B(C6F5)3.[8] Isoelectronic
carbon-for-boron replacement identifies compound 2 as a neutral
B(C6F5)3-stabilizedboron analogue of a carboxylic acid. Although
further chemistry of 2 is yet to be described, recognition of this
relationship prompts speculation that the conjugate bases of such
species (A, Figure 1b) may be exploited in a similar manner to
carboxylate anions, which are among the most commonly applied
tight angle bidentate or bridging ligands in coordination,
supramolecular, biomedical and bioinorganic chemistry.[9]
Me3SiBr from the B–Br oxidative addition product of Br2BOSiMe3
12]
and [Pt(PCy3)2].[4,
In this contribution, we demonstrate the
accessibility of [BO2] as a synthon through alkene elimination
from isolable magnesium pinacolatoboryl species and its in situ
trapping to provide boron-centered analogues of carbamate and
carboxylate anions.
Scheme 1: Synthesis of compounds 4 and 5.[13, 16]
Compound
2 is also a progenitor to other classes of
unprecedented boron-centered anions with isoelectronic organic
equivalents, for example, the carbamate analogue B (Figure 1b).
The current work emerged from our studies of magnesium-
centered boryl nucleophiles and,[13-15] specifically, their use in the
construction of B–B' bonds (Scheme 1).[16, 17] We have previously
reported that treatment of [(BDI)MgBu] (BDI = HC{C(Me)N-2,6-i-
Pr2C6H3}2] with bis(pinacolato)diboron (B2pin2) provides the
diboranate derivative, compound 3 (Scheme 1). The [B(sp2)–
B(sp3)] bond of compound 3 cleaves heterolytically when treated
with bases such as 4-dimethylaminopyridine (DMAP), providing
compound 4 that comprises a terminal boryl anion without the
direct use of a strong reductant (Scheme 1).[13,14,18] The boron
center of compound 4 displays nucleophilic character and reacts
with carbon- and boron-centered electrophiles to enable the
construction of C-B and B–B' bonds.[15-17] Although the copper(I)
derivative [(IPr)CuBpin] has recently been utilized in a similar
manner,[19] examples of unsymmetrical [B(sp2)–B(sp2)]
diboranes were previously limited to compounds obtained by the
Figure 1: (a) Compounds 1 and 2; (b) boron-centered isosteres of
carboxylate (A) and carbamate (B) anions.
Although viable quantities of 2 were achieved by Si-OH/B-Cl
metathesis between Ph3SiOH and the chloroboroxane,
[(IPr)ClB=OB(C6F5)3], the reaction required forcing conditions
Dr. Anne-Frédérique Pécharman, Prof. Michael S. Hill, Dr. Claire L.
McMullin and Dr. Mary F. Mahon
Department of Chemistry
desymmetrization of pre-existing diborane(4) B–B bonds.[20, 21]
A
University of Bath
Bath, BA2 7AY, UK.
E-mail: msh27@bath.ac.uk
notable case in point is pinB–B(Mes)2 (6), synthesized by
reaction of B2pin2 with mesityl magnesium bromide, which has
been shown to effect the activation of a variety of small molecule
substrates.[21-23]
Supporting information for this article is given via a link at the end of the
document and in CCDC 1997822-1997825.
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In an attempt to develop an alternative synthesis of compound 6,
therefore, we carried out the reaction of compound 4 with Mes2BF
(Mes = 2,4,6-trimethylphenyl). The formation of compound 6 was
identified in the resultant 1H NMRspectrum after five hours at 60
C. This analysis, however, also revealed that the adduct
complex, Mes2BFDMAP (7), identified by its independent
synthesis], accounted for ca. 50% of the initially added
fluoroborane. While minor quantities (<10%) of the anticipated
dimeric magnesium fluoride by-product, [(BDI)MgF]2 (8) were
observed,[24] the majority of the magnesium -diketiminate 1H
NMR resonances could be ascribed to a single new BDI-
containing product (9). Compound 9 was isolated in 48% yield by
fractional crystallization and identified by single crystal X-ray
diffraction as a dinuclear magnesium 2-fluoride in which charge
balance is maintained by a bridging [4Me2NC5H4NBO2] anion
(Scheme 2). Insight into the fate of the [pinB] anion of 4 and the
origin of the [4Me2NC5H4NBO2] ligand was provided by a further
experiment performed in C6D6. Although this reaction proceeded
identically, vacuum transfer of the volatile products delivered a
1H NMR spectra recorded at one hour intervals revealed the
emergence of a further BDI-magnesium species (10) [(1H) 4.97
ppm], which, although comprising ca. 20% of the total BDI signals
after three hours, diminished significantly in intensity during the
latter stages of the reaction. Two signals at almost identical
frequencies in the 19F{1H} NMR spectra ( 113.77 and 113.83
ppm) displayed an analogous increase and decrease in relative
intensity during the same time period and are also, therefore,
attributed to compound 10. Significantly, these latter resonances
appeared in a strict 1:4 ratio of intensities throughout the reaction
(Figure S5) and are, thus, assigned to natural abundance boron-
fluorine bonded 10B and 11B isotopomers of [(BDI)Mg{pinB–
BF(Mes2)}] (10).[27] Although
a
dimesitylfluorodiboranate
analogue of compound 5 (Scheme 1), which proved stable to
boron-to-magnesium hydride elimination,[17] compound 10
evidently degrades via boron-to-magnesium fluoride transfer and
elimination of Yamashita’s diborane (6).[21]
single component, most clearly manifested as
a singlet
resonance centered at 1.62 ppm in its 1H NMR spectrum, that
was readily identified as 2,3-dimethyl-2-butene.
Scheme 2: Synthesis of compound 9.
Figure 2: (a) ORTEP of the Mg1/Mg2-containing molecule of
compound 9 (30% probability ellipsoids). Hydrogen atoms, iso-propyl
methyl groups and occluded solvent are removed forclarity. Selected
bond lengths (Å)and angles ():B1-O1 1.320(4), B1-O2 1.324(4), B1-
N5 1.589(4), Mg1-O1 1.918(2), Mg2-O2 1.904(2), F1-Mg1 1.8953(18),
F1-Mg2 1.8865(19), O1-B1-O2 134.1(3), O1-B1-N5 113.7(3), O2-B1-
N5 112.2(3): (b) Calculated Natural Bond Orbital (NBO) surface for
HOMO-19 of compound 9.[28]
The structure of 9 comprises two effectively identical dinuclear
complexes (Figure 2, only the Mg1/Mg2-containing molecule is
discussed) in which the magnesium centers are connected by a
single 2-bridging fluoride and an unprecedented boron-centered
[4Me2NC5H4NBO2] monoanion. The MgO bond lengths in 9
[1.918(2); 1.904(2) Å] are somewhat shorter than those observed in
the only similarly dinuclear magnesium carbamates [ca. 1.95 – 2.0
Å], albeit the group 2 centers of these previously reported
compounds are five- rather than four-coordinate.[25] Like the
{CBO2} unit of compound 2, N5, B1, O1 and O2 in 9 are coplanar
and this plane subtends an angle of only 5.31 with the mean
plane defined by the DMAP ligand. Despite this near coplanarity,
the B1–N5 distance [1.589(4) Å] is elongated in comparison to
typical covalent B–N bonds (e.g. the borylamidinate, pinB-N(i-
Pr)HC=Ni-Pr, 1.42731(6) Å).[26] In contrast, the identical B1–O1
[1.320(4) Å] and B1–O2 [1.324(4) Å] distances are comparable to
the shorter of the B–O bonds [1.311(3) Å] of 2.[8] Natural bond
orbital (NBO) analysis of compound 9 also afforded Wiberg bond
indices for the B–O bonds (1.1365, 1.1390) that are closely
comparable to that reported for the shorter B–O linkage in 2
(1.123).[8] The values are indicative of multiple bond character,
such that the planarity of the dioxoborane unit is a consequence
of pronounced B(2p)-O(2p) - overlap across the {O–B–O} unit
(Figure 2b). These observations support the legitimacy of the
simple valence bond depiction (structure B in Figure 1b) of this
anion as a boron-centered carbamate analogue.
These observations lead us to suggest that the formation of
compound 9 is a consequence of two competitive pathways, the
thermodynamic viability of which have been confirmed by density
functional theory (DFT) calculations. Scheme 3 summarizes the
results of this analysis (see also Tables S2 and S3 in the SI).
Scheme 3: DFT computed pathway to compound 9. Free energies
Although the complexity of the reaction precluded more
quantitative analysis, re-examination of the aliphatic region of the
1
(kcal mol– , relative to 4) of computed structures are shown in
parenthesis.
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The route identified as pathway (i) requires the elimination of 2,3-
dimethyl-2-butene and the in situ generation of a [BO2]
equivalent. While [BO2] is not viable as a persistent species, its
immediate trapping by a molecule of DMAP provides a cogent
rationale for the generation of the [4Me2NC5H4NBO2] anion.
Although the intermediacy of a monomeric species, 11, is
questionable, 9 may be considered to result from its combination
with the putative three-coordinate magnesium fluoride,
compound 12. We suggest, however, that the concurrent
accumulation of minor quantities of the dimeric fluoride,
[(BDI)MgF]2 (8), provides circumstantial evidence for the
generation of 12 as a common intermediate.
The credibility of pathway (i) relies on the instability of the {Mg–
Bpin} unit toward alkene extrusion under the applied reaction
conditions. Yamashita and co-workers have reported that
treatment of pinB–B(Mes)2 (6) with 2,6-dimethylphenyl isonitrile
results in pinB ring contraction to provide a spirocyclic compound
Scheme 5: Synthesis of compounds 13 and 15. Free energies
(relative to 3 at 0.1 kcal mol1) for DFT computed structures are
shown in parenthesis.
A reaction of compound 13 and Mes2BF provided broadly
analogous observations to those resulting from the reaction with
compound 4, albeit the transformation was significantly more
facile and complete after only one hour at room temperature.
Approximately 50% of the Mes2BF was converted to the adduct
species, Mes2BF(i-Pr-NHC) (14), which was identified through its
independent synthesis and clearly characterized in the resultant
1H NMR spectrum by the emergence of two deshielded (1H)
multiplet resonances at  5.45 and 5.01 ppm and a series of
twelve differentiated (3H) methyl resonances. The formation of
2,3-dimethyl-2-butene was also clearly identifiable as a 12H
signal at  1.62 ppm, alongside the simultaneous production of
an approximately equimolar quantity of pinB-BMes2 [6,  2.35 (s,
12H), 2.15 (s, 6H), 1.07 (s, 12H) ppm]. Most significantly, these
transformations were accompanied by the emergence of a series
of broadened BDI-ligand resonances attributed to the formation
of a single new compound, identified by a subsequent X-ray
diffraction analysis as the dinuclear -diketiminato magnesium
complex(15). The overall stoichiometry of the reaction, therefore,
may be rationalized as depicted in Scheme 5.
comprising
a 4-membered cyclic {BOC2} 1,2-oxaboretane
structure.[23] In this earlier case, however, C-O bond cleavage
was deduced to proceed via a carbocationic mechanism. The
alkene elimination process identified in the formation of
compound 9, therefore, appears to be a unique observation that
could carry important implications for Bpin-related chemistry in
general.
Scheme 4: (a) Corey’s carbene decarboxylation strategy for the
synthesis of 2,3-dimethyl-2-butene;[30] (b) analogous elimination of
2,3-dimethyl-2-butene from the [pinB] anion.
Calculations by Schleyer and co-workers as long ago as 1995
highlighted that the stability of singlet model boryl derivatives,
X2B–Li, is predicated on not only electronegative X substitution
(e.g. F, O, N) but also the direct interaction of boron with the more
electropositive lithium.[29] These theoretical deductions were
foreshadowed by Corey’s even earlier demonstration that
desulfurization of
a cyclic pinacol-derived thionocarbonate
derivative (Scheme 4a) results in elimination of 2,3-dimethyl-2-
butene due to the relative instability of the resultant carbene
toward olefin and carbon dioxide formation.[30] DFT calculations
indicated that analogous transformation of the isoelectronic
[pinB] anion to 2,3-dimethyl-2-butene and [BO2] is significantly
exergonic (Gf = –79.5 kcal mol-1, Scheme 4b).
Figure 3: ORTEPs of (a) compound 13 and (b) compound 15 (30%
probability ellipsoids) Hydrogen atoms and iso-propyl methyl groups
are removed for clarity. Selected bond lengths (Å) and angles (): (13)
Mg1-C30 2.2929(15), Mg1-B1 2.3192(19), N1-Mg1-C30 102.52(6),
N1-Mg1-B1 124.92(7), N2-Mg1-N1 90.59(6), N2-Mg1-C30 113.71(6),
N2-Mg1-B1 108.07(6), C30-Mg1-B1 114.86(7); (15) Mg1-F1
1.8926(8), Mg1-O1 1.9131(9), Mg2-F1 1.8959(8), Mg2-O2, 1.9170(9),
O1-B1 1.3336(17), O2-B1 1.3324(17), C59-B1 1.6596(17), Mg1-F1-
Mg2 125.84(4), B1-O1-Mg1 125.17(8), B1-O2-Mg2 125.07(8), O1-
B1-C59 114.93(11), O2-B1-O1 130.54(11), O2-B1-C59 114.53(11).
The thermodynamic viability of this process prompted us to
attempt similar boryl decomposition to provide a boron-centered
carboxylate analogue akin to the NHC-based anion A (Figure 1).
The magnesium boryl [(BDI)Mg(Bpin)(i-Pr-NHC)] (13, i-Pr-NHC =
1,3-di-isopropyl-4,5-dimethylimidazol-2-ylidene)[31]
was,
therefore, prepared by an equimolar reaction of compound 3 and
the N-heterocyclic carbene (Scheme 5). Characterization by
single crystal X-ray diffraction analysis (Figure 3a) revealed that
the resultant MgB bond of compound 13 [2.3192(19) Å] is
closely comparable to that observed in compound 4 [2.324(2) Å]
indicating that incorporation of i-Pr-NHC results in only very
limited perturbation to the electronic character of the {Bpin} ligand.
Like 9, compound 15 (Figure 3b) comprises a dinuclear [(BDI)Mg-
2-F-Mg(BDI)] unit. In the case of 15, however, the coordination
environment of each Mg center is completed by a dioxoboron
monoanion in which the final bond to the trigonal boron is
provided by an equivalent of the i-Pr-NHC donor. This unit as a
whole, therefore, may be classified as a conjugate base analogue
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10.1002/anie.202005674
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The Mg-O bonds of 15 [1.9131(9), 1.9170(9) Å] are effectively
identical to the shorter of the comparable measurements in the
similarly four-coordinate carboxylate derivatives, [(BDI)Mg(-
O2CR)]2 [R = Me, 1.918(2), 1.941(2); R = Ph, 1.918(2), 1.958(2)
Å].[32] As in the case of compound 9, the BO bond distances are
similar [O1–B1 1.3336(17), O2-B1 1.3324(17) Å], while the NHC-
to-boron interaction [C59-B1 1.6596(17) Å] is only marginally
elongated in comparison to the CB distance [CB 1.636(3) Å]
reported for the formally charge neutral {CBO2} unit of compound
2.[8]
In conclusion, we report the synthesis of unique dioxoborane
analogues of the ubiquitous carbamate and carboxylate anions.
Both moieties result from the apparent in situ trapping of the
highly reactive [BO2] anion by a neutral N- or C-centered base,
subsequent to the kinetically facile and thermodynamically viable
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elimination
of 2,3-dimethyl-2-butene from well-defined
magnesium-coordinated boryl anions. We are continuing to study
this reactivity and to elaborate the more general coordination
chemistry of these unprecedented anions.
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Acknowledgements
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We thank the EPSRC (UK) for the support (EP/R020752/1). This
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Conflict of interest
The authors declare no conflict of interest.
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Keywords: magnesium • boryl • dioxoborane • main group
chemistry • density functional theory
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10.1002/anie.202005674
Angewandte Chemie International Edition
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Anne-Frédérique Pécharman, Michael
S. Hill,* Claire L. McMullin* and Mary F.
Mahon
Page No. – Page No.
[BO2] as a Synthon for the Generation
of Boron-centered Carbamate and
Carboxylate Isosteres
Playing the OBO: Oxoborane carbamate and carboxylate analogues result from the in situ trapping of [BO2] produced by elimination of 2,3-
dimethyl-2-butene from a magnesium pinacolatoboryl anion.
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MDeicphaaretml Se.nHt oillf Chemistry
DUneipvaertsmiteynotfoBfaCthemistry
This article is protected by copyright. All rights reserved.