Chemistry of boryllithium: Synthesis, structure, and reactivity

Article abstract of DOI:10.1021/ja8057919

A series of lithium salts of boryl anion, boryllithiums, were synthesized and characterized by NMR spectroscopy and crystallographic analysis. In addition to the parent boryllithium compound 35a, structural modification of boryllithium, using saturated C-C and benzannulated C=C backbones in the five-membered ring and mesityl groups on the nitrogen atoms, also allowed generation of the corresponding boryllithium. The solid state structures of boryllithium showed that the boron-lithium bond is polarized where the boron atom is anionic in all (35a?DME)₂, 35a?(THF)₂, 35b?(THF)₂, and 35c?(THF)₂ when compared to the structures of hydroborane 38a-c and optimized free boryl anion opt-46a-c. Dissolution of the isolated single crystals of (35a?DME)₂ and 35a?(THF)₂ in THF-d₈ showed that the boron-lithium bond remained in solution and free DME or THF molecules were observed. Temperature-dependent ¹¹B NMR chemical shift changes of 35a were observed in THF-d₈ or methylcyclohexane-d₁₄, suggesting a change of chemical shift anisotropy around the boron center. The HOMO of opt-35a?(THF)₂ had a lone pair character on the boron atom, as observed for phenyllithium, whereas the HOMO of hydroborane 38a corresponds to the π-orbital of the boron-containing five-membered heterocycle. The polarity of the B-Li bond, estimated by AIM analysis, was similar to that of alkyllithium. Boryllithiums 35a and 35b behave as a base or a boron nucleophile in reaction with organic electrophiles via deprotonation, S_N2-type substitution, halogen-metal exchange or electron-transfer, 1,2-addition to a carbonyl group, and S_NAr reaction. In the case of the reaction with CO₂, intramolecular cyclization followed by CO elimination from borylcarboxylate anion and subsequent protonation gave hydroxyboranes 64a and 64b. The characters of the carbonyl groups in the borylcarbonyl compounds 60a, 60b, 61, 62, and 63a, which were obtained from the reaction of boryllithiums 35a and 35b, were investigated by X-ray crystallography, IR, and ¹³C NMR spectroscopy to show that the boryl substituent weakened the C=O bond when compared to carbon substituted analogues.

Full text of DOI:10.1021/ja8057919

Published on Web 11/04/2008
Chemistry of Boryllithium: Synthesis, Structure, and
Reactivity
Yasutomo Segawa, Yuta Suzuki, Makoto Yamashita,* and Kyoko Nozaki*
Department of Chemistry and Biotechnology, Graduate School of Engineering, The UniVersity of
Tokyo 7-3-1, Hongo, Bunkyo-ku, 113-8656 Tokyo, Japan
Received July 24, 2008; E-mail: makotoy@chembio.t.u-tokyo.ac.jp; nozaki@chembio.t.u-tokyo.ac.jp
Abstract: A series of lithium salts of boryl anion, boryllithiums, were synthesized and characterized by
NMR spectroscopy and crystallographic analysis. In addition to the parent boryllithium compound 35a,
structural modification of boryllithium, using saturated CsC and benzannulated CdC backbones in the
five-membered ring and mesityl groups on the nitrogen atoms, also allowed generation of the corresponding
boryllithium. The solid state structures of boryllithium showed that the boron-lithium bond is polarized
where the boron atom is anionic in all (35a·DME)2, 35a·(THF)2, 35b·(THF)2, and 35c·(THF)2 when
compared to the structures of hydroborane 38a-c and optimized free boryl anion opt-46a-c. Dissolution
of the isolated single crystals of (35a·DME)2 and 35a·(THF)2 in THF-d8 showed that the boron-lithium
bond remained in solution and free DME or THF molecules were observed. Temperature-dependent 11B
NMR chemical shift changes of 35a were observed in THF-d8 or methylcyclohexane-d14, suggesting a
change of chemical shift anisotropy around the boron center. The HOMO of opt-35a·(THF)2 had a lone
pair character on the boron atom, as observed for phenyllithium, whereas the HOMO of hydroborane 38a
corresponds to the π-orbital of the boron-containing five-membered heterocycle. The polarity of the B-Li
bond, estimated by AIM analysis, was similar to that of alkyllithium. Boryllithiums 35a and 35b behave as
a base or a boron nucleophile in reaction with organic electrophiles via deprotonation, SN2-type substitution,
halogen-metal exchange or electron-transfer, 1,2-addition to a carbonyl group, and SNAr reaction. In the
case of the reaction with CO2, intramolecular cyclization followed by CO elimination from borylcarboxylate
anion and subsequent protonation gave hydroxyboranes 64a and 64b. The characters of the carbonyl
groups in the borylcarbonyl compounds 60a, 60b, 61, 62, and 63a, which were obtained from the reaction
of boryllithiums 35a and 35b, were investigated by X-ray crystallography, IR, and 13C NMR spectroscopy
to show that the boryl substituent weakened the CdO bond when compared to carbon substituted analogues.
Introduction
the boron atom can be considered as an isoelectronic analog of
singlet carbene.
Lithium salts of the second row p-block elements, LiF, LiOH,
LiNH2, and CH3Li,1 are widely used as anionic nucleophiles in
organic and inorganic syntheses. However, the corresponding
lithium salts of boron, boryllithium, have not been synthesized
or characterized yet.2 Boryllithium may be considered as a boryl
anion in which the boron center has an anionic charge because
of the wide difference between the electronegativities of boron
(2.04) and lithium (0.96).3 In conventional boron chemistry,
boron-containing reagents are usually considered as electro-
philes, because of their vacant p-orbital at the boron center and
the low-electronegativity of the boron atom. Therefore, using
boryllithium as a boron nucleophile in synthetic chemistry may
lead to the “umpolung“ concept. Unlike other lithium salts of
p-block elements, the boron atom in boryllithium has only six
valence electrons without satisfaction of the octet rule, where
Since 1952, there have been several reports on alkali metal
salts of boryl anion as a proposed intermediate (Scheme 1).
Auten and Kraus reported reduction of (n-Bu)2BCl (1) using a
sodium-potassium alloy to generate the corresponding boryl
anion, (n-Bu)2B-M+ (2, M ) Na or K),4 which could be trapped
with methyl iodide to form (n-Bu)2BMe (3). However, modern
spectroscopic characterization of the methylated product was
not available then. After 24 years, Smith reported that 2 had
rearranged to the corresponding boron-stabilized carbanion 4
through deprotonation of the alkyl group by an anionic boron
center, and then 4 aggregated to form a cyclic trimer 5 with a
characterization of the hydroborate moiety by IR spectroscopy.5
Diphenylboryl anion 7 was proposed in the photolysis of
tetraphenylborate 6, based on the formation of biphenyl.6
However, Schuster proved that irradiation of 6 produce a
biradical species, which rearranged to the cyclic borate 8
possessing a three-membered ring, followed by protonation to
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2008 American Chemical Society
J. AM. CHEM. SOC. 2008, 130, 16069–16079 16069

A R T I C L E S
Segawa et al.
Scheme 1. Proposed Boryl Anions as Intermediates
Scheme 3. Reaction of Base-Stabilized Boryl Anion as Boron
Nucleophiles
cyclic carbene, was isolated recently.11a The other case is a
“base-stabilized boryl anion”; that is, triethylamine- or tricy-
clohexylphosphine-coordinated haloborane could be reduced
using an alkali metal to form the corresponding sp3 boryl anion
species with the coordination of the Lewis base to the boron
atom. These species reacted with electrophiles such as CF3I,
TMSCl, and benzaldehyde in a nucleophilic manner to generate
trifluoromethylborane (19), trimethylsilylborane (22), and R-bo-
rylbenzylalcohol (23), respectively.13-15
Scheme 2. Reaction of Borylcopper as Boron Nucleophiles (Dip )
2,6-(i-Pr)2C6H3; Cy ) cyclohexyl)
Boryllithium is also considered as a low-valent boron(I)
species. Accessible low-valent group-14 element compounds
(carbene (24),16 silylene (25),17 germylene (26),18 and stannylene
(27)19) and their isoelectronic species, anionic group-13 element
compounds (gallyl anion (28)20,21), with a five-membered ring
structure are shown in Figure 1. As shown in Scheme 4, gallyl
anion 28 could be prepared by a reduction of the corresponding
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form biphenyl.7 Recently, Weber et al. investigated the reaction
of bromodihydrodiazaborole 9 with alkali metals in toluene-
d8.8 In that report, they mentioned that the intermediate of the
reduction of sodium-potassium alloy would be boryl radical
10 or boryl anion 11.
Some previously reported boron compounds which reacted
as boron nucleophiles are shown in Schemes 2 and 3. The
catalyst systems Cu(I)/diborane(4) reagent/oxygen ligand could
mediate nucleophilic borylation of R,ꢀ-unsaturated carbonyl
compounds (conjugate addition to form ꢀ-borylester 12),9 allyl
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tion to form 14), and benzaldehyde (1,2-diboration to form
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philic” boron reagents have not been characterized well, a
borylcopper complex, IPrCuBpin (16), possessing an N-hetero-
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Chemistry of Boryllithium
A R T I C L E S
can be considered as a dimer of boryl anion, sharing four
electrons between two boron atoms. The reason why boryl anion
does not form from the diborane(4) dianion is that the second-
row elements can form a stronger π-bond than the heavier main
group elements, such as gallium.23 A higher reactivity of boryl
anion than that of the anionic heavier group-13 element
compounds was also indicated by a computational study,24,25
which showed the higher HOMO energy of the boryl anion 29
than that of the anionic aluminum(I) species 30.25
Some problems can be expected in the synthesis of boryl-
lithium (Scheme 5). The general methods for generating
organolithium compounds are deprotonation from a C-H bond
and reductive dehalogenation from a C-halogen bond, but both
involve difficulties in generating boryllithium. In the reaction
of boryl-H with a base, it is difficult to deprotonate because
the hydrogen atom usually has a hydride character, since the
electronegativity of hydrogen atom is higher than that of boron
atom (H: 2.20, B: 2.04).3 Furthermore, the generation of a Lewis
acid-base adduct occurs in favor of deprotonation. Meanwhile,
one-electron reduction of a halogen-boron bond generates a
boryl radical, which is rapidly dimerized to form diborane(4)
species before the second electron transfer occurs to form
boryllithium.8,26
Figure 1. Low-valent group-14 and -13 element compounds possessing a
five-membered N-containing heterocycle (Mes ) 2,4,6-Me3C6H2).
Scheme 4. Synthesis of Gallyl Anion and Diborane(4) Dianion
Recently, we reported the synthesis of boryllithium by
reduction of an N-heterocyclic bromoborane precursor possess-
ing bulky 2,6-diisopropylphenyl groups on the nitrogen atoms.27,28
Herein, we report a further systematic study on the chemistry
of boryllithium, which includes the synthesis of boryllithium
from various precursors, modification of the boryllithium
skeleton, solid state structures, the bonding properties of the
boron-lithium bond with theoretical calculation, a broader study
on their reactivity with organic electrophiles, and the carbonyl
properties of the resulting borylcarbonyl compounds.
Scheme 5. Strategy for Generating a Boryl Anion
Results and Discussion
Generation of Boryllithium. Bromoborane 34a-Br was chosen
as the precursor of the corresponding boryllithium 35a. In our
previous study, 34a-Br was synthesized by reduction of N,N′-
diaryldiimine (36)29 with Mg followed by an addition of BBr3
(Scheme 6). We have also explored a more scalable procedure
digallane(4) 31; however, the reduction of the diborane(4)
species 32 led to the formation of isolable diborane(4) dianion
2Li·(Mes)2BB(Mes)Ph (33), having a BdB double bond
character with π-coordination to lithium cations. Dianionic 33
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4395–4408. (b) Jones, C.; Rose, R. P.; Stasch, A. Dalton Trans. 2007,
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Organometallics 2007, 26, 3424–3430. (d) Arnold, P. L.; Liddle, S. T.;
McMaster, J.; Jones, C.; Mills, D. P. J. Am. Chem. Soc. 2007, 129,
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Scheme 6. Synthesis of Bromoborane 34a-Br [Dip )
2,6-(i-Pr)2C6H3]
for synthesis of 34a-Br based on a report by DuMont and co-
workers for the preparation of a phosphorus-containing hetero-
cycle.30 A dilithium diamide, derived from the same starting
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A R T I C L E S
Segawa et al.
Scheme 7. Synthesis of Boryllithium 35a
lithiums, we examined modifications of the structure (Scheme
9).28b We synthesized various bromoboranes having an unsatur-
ated CdC bond, a saturated CsC bond, or a benzannulated
CdC bond in the five-membered ring and Dip (2,6-diisopro-
pylphenyl), Mes (2,4,6-trimethylphenyl), or t-Bu groups on each
nitrogen atom as precursors for boryllithium. Reduction of these
bromoboranes under the same conditions as the formation of
the original boryllithium 35a afforded the following results.
Bromoboranes 34b, 34c, and 34d could be reduced to the
corresponding boryllithium 35b-d, showing characteristic low-
field signals (35b, δB 51.9; 35c, δB 52.1; 35d, δB 45.0) in 11B
NMR spectra of reaction mixtures at room temperature. In the
reduction of 34e, a low-field shifted signal of 35e was observed
as a minor product (δB 52.4, 18%, Vide infra) at room
temperature, accompanied by a major high-field signal of 43e
Scheme 8. Syntheses and Reduction of 34a-Cl and 34a-I To Form
35a (DBB ) 4,4′-Di-tert-butylbiphenyl)
1
(δB 0.1, d, JBH ) 64 Hz, 82%). Hydroborate 43e was
characterized in solution by NMR spectroscopy and converted
to the corresponding neutral borane 44e (δB 39.6) during an
isolation procedure through loss of hydride from the borate
center. A low-temperature treatment of the reaction solution in
the reduction of 34e confirmed a quantitative formation of 35e
by low-temperature NMR spectroscopy. Two possible mecha-
nisms for the formation of 43e are illustrated in Scheme 10,
where boryllithium 35e adsorbed the benzylic proton to form
the corresponding benzyllithium derivative 45e. Then, the
benzylic carbon of 45e attacked the boron center having a
hydrogen atom to form the corresponding hydroborate 43e. One
of these two mechanisms, a radical pathway, may have a minor
or no contribution to the formation of hydroborate 43e because
of the quantitative formation of 35e from 34e at low temperature
and clean conversion of 35e to 43e. The difference in the
reactivities between CdC unsaturated 35d and CsC saturated
35e can be attributed to the difference in the flexibility of the
five-membered heterocycle, because the saturated C-C bond
in 35e can rotate to make the benzylic protons of the mesityl
group and the anionic boron center closer. On the other hand,
no low-field shifted 11B NMR signal above δB 40 was observed
in the reduction of tert-butyl-substituted bromoboranes with an
unsaturated CdC bond (34f) or a saturated CsC bond (34g),
as Weber has observed the formation of diborane(4) or hy-
droborane in the reduction of 34e with reducing agents such as
Na/Hg,32 Na/K,8 or K-mirror.8
Crystallographic Study: Solid State Structures of Boryllithium.
Crystallographic analysis of boryllithium revealed an ionic
character of the boron-lithium bond in the solid state. Four
structures of boryllithiums, (35a · DME)2, 35a · (THF)2,
35b(THF)2, and 35c(THF)2, were observed in single crystals
obtained at -45 °C from a hexane solution of the crude product
in the reduction using DME or THF solvent (Figures 2-5). The
selected structural parameters are summarized in Table 1 with
reference compounds, calculated free boryl anions (opt-46a-c),
and crystallographically analyzed hydroboranes (38a-c), which
were independently synthesized from 34a-Br, 34b,c with
LiAlH4, N-heterocyclic carbenes (47,33 49,34 5135), and their
N,N′-diaryldiimine 36 by reduction with elemental Li, was
protonated by triethylammonium chloride to form aminoimine
37. The reaction of the resulting aminoimine 37 with BBr3
followed by addition of iPr2EtN to trap generated HBr yielded
34a-Br in a good yield. The preparation of boryllithium 35a is
illustrated in Scheme 7. The B-Br bond of 34a-Br was reduced
using an excess amount of lithium powder in the presence of a
catalytic amount of naphthalene in THF or 1,2-dimethoxyethane
(DME) at -45 °C for 6 h to form the corresponding boryllithium
35a, showing a broad signal at δB 45.4 in the 11B NMR spectrum
with nearly quantitative conversion as judged by NMR spec-
troscopy (Vide infra). Biphenyl or 4,4′-di-tert-butylbiphenyl can
also be used as an electron mediator instead of naphthalene.
Leaving the resulting THF solution of 35a at room temperature
for a day or adding water to the solution resulted in the formation
of hydroborane 38a in a quantitative yield, which was inde-
pendently synthesized from a reaction of 34a-Br with lithium
aluminum hydride in 61% yield. The sensitivity of the THF
solution of 35a toward moisture indicates that the product seems
to have an anionic boron moiety. The formation of deuteriobo-
rane 38a-d1 in 97% yield, using D2O in place of water, shows
that the adsorbed proton did not come from solvent THF or
ligand backbone but from water.31 However, the THF solution
of boryllithium 35a could be stored at -45 °C for several
months. Boryllithium 35a could also be prepared with quantita-
tive conversion from the other haloboranes, 34a-Cl or 34a-I,
synthesized as shown in Scheme 8. Chloroborane 34a-Cl is more
attractive for large scale synthesis than 34a-Br because of its
stability against silica gel under an argon atmosphere. Although
the reduction of 34a-Cl with lithium naphthalenide requires
several days to complete, using lithium 4,4′-di-tert-butylbiphe-
nylide as a reducing agent reduces the reaction time to within
10 h.
Modification of Bulky Substituents and Backbone. To find
other applicable substituents and backbones to generate boryl-
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(29) Abrams, M. B.; Scott, B. L.; Baker, R. T. Organometallics 2000, 19,
4944–4956.
(33) Arduengo, A. J., III.; Krafczyk, R.; Schmutzler, R.; Craig, H. A.;
Goerlich, J. R.; Marshall, W. J.; Unverzagt, M. Tetrahedron 1999,
55, 14523–14534.
(30) Burck, S.; Gudat, D.; Nieger, M.; Du Mont, W.-W. J. Am. Chem.
Soc. 2006, 128, 3946–3955.
(31) In a previous communication, we reported 82% yield. We have
reexamined more carefully to prevent the decomposition of 35a with
water and moisture to get 97% yield. Detail for the experimental
procedure is described in the Supporting Information.
(34) Arduengo, A. J.; Goerlich, J. R.; Marshall, W. J. J. Am. Chem. Soc.
1995, 117, 11027–11028.
(35) Korotkikh, N. I.; Raenko, G. F.; Pekhtereva, T. M.; Shvaika, O. P.;
Cowley, A. H.; Jones, J. N. Russ. J. Org. Chem. 2006, 42, 1822–
1833.
9
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Chemistry of Boryllithium
A R T I C L E S
Scheme 9. Reduction of Various Bromoboranes (Dip ) 2,6-(i-Pr)2C6H3; Mes ) 2,4,6-Me3C6H2)
Scheme 10. Two Possible Mechanisms for the Formation of Hydroborate 43e
precursors, imidazolium salts (48,33 50,34 5235). In (35a·DME)2,
boryllithium 35a was crystallized with DME molecules, which
chelated to the central lithium atom, and one of two oxygen
atoms bridged two lithium atoms to form a dimeric structure
with a four-coordinate lithium atom. Another structure of 35a,
35a·(THF)2, contained two THF molecules coordinated to the
central lithium atom to form a three-coordinate lithium atom.
Other structures of boryllithiums, 35b·(THF)2 and 35c·(THF)2,
which have a saturated CsC bond or a benzannulated CdC
bond in the five-membered ring, have two THF molecules
coordinated to the central lithium atom, as does 35a·(THF)2.
The existence of a 2c-2e boron-lithium bond and sp2 hybrid-
ization around the planar boron center in all crystal structures
revealed that the products were boryllithiums, where the lithium
atom directly bonded to the boryl group. The boron-lithium
bond lengths [2.291(6) Å in (35a·DME)2; 2.276(5) Å in
35a · (THF)2; 2.271(4) Å in 35b · (THF)2; 2.218(9) Å in
35c·(THF)2] were 8.5, 7.9, 7.6, and 5.1% longer than the sum
of the covalent radii (2.11 Å),3 respectively. A similar elongation
of the CsLi bond was reported in the crystal structure of 2,3,4,5-
C6HF4Li·(THF)3.36 The CsLi bond [2.136(5) Å] in 2,3,4,5-
C6HF4Li·(THF)3 was also longer than the sum of the covalent
radii (2.00 Å). In the five-membered ring, the BsN bond lengths
[1.465(4) and 1.467(4) Å in (35a·DME)2; 1.474(3) and 1.480(4)
Å in 35a·(THF)2; 1.4547(18) Å in 35b·(THF)2; 1.474(4) Å
in 35c · (THF)2] and the NsBsN angles [99.2(2)° in
(35a · DME)2; 98.7(2)° in 35a · (THF)2; 101.89(16)° in
35b·(THF)2; 100.0(3)° in 35c·(THF)2] are closer to those in
the calculated free boryl anions opt-46a-c than to those in the
hydroboranes 38a-c, indicating that these boryllithiums have a
boryl anion character. The relationships between boryllithiums
and hydroboranes are quite similar to those between N-
heterocyclic carbenes (47,33 49,34 5135) and imidazolium salts
(48,33 50,34 5235), which are protonated compounds of N-
heterocyclic carbenes. In fact, in the structures of N-heterocyclic
carbenes, the CsN bonds are longer and the NsCsN angle is
smaller than those in the imidazolium salts.
(36) Kottke, T.; Sung, K. S.; Lagow, R. J. Angew. Chem., Int. Ed. Engl.
1995, 34, 1517–1519.
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A R T I C L E S
Segawa et al.
Figure 4. Crystal structure of 35b·(THF)2 (50% thermal ellipsoids;
hydrogen atoms are omitted for clarity; half of the entire structure constitutes
an asymmetric unit where the numbers with asterisks are in the second
asymmetric unit).
Figure 2. Crystal structure of (35a·DME)2 (50% thermal ellipsoids;
hydrogen atoms and the minor part of the disordered moieties were omitted
for clarity; half of the entire structure constitutes an asymmetric unit where
the numbers with asterisks are in the second asymmetric unit).
Figure 5. Crystal structure of 35c·(THF)2 (50% thermal ellipsoids;
hydrogen atoms are omitted for clarity; half of the entire structure constitutes
an asymmetric unit where the numbers with asterisks are in the second
asymmetric unit).
Figure 3. Crystal structure of 35a·(THF)2 (50% thermal ellipsoids;
hydrogen atoms are omitted for clarity).
chemical shift and half-width of the signal were the same as
those observed for the reaction mixture (δB 45.4, ν1/2 ) 535
Hz). This signal shifted from that in 38a (22.9 ppm, half-width
of ν1/2 ) 379 Hz). It has been already reported that the 13C
NMR signal of N-heterocyclic carbene 53 (213.7 ppm in THF-
d8) shifts to a lower field than that of the protonated carbene
(Figure 6), namely, imidazolium salt 54 (135.0 ppm in DMSO-
d6).37 This low-field shift was also observed for carbene 47
(220.6 ppm in C6D6)33 in comparison with imidazolium salt 48
(139.9 ppm in DMSO-d6, in this work), possessing the same
aromatic substituent on the nitrogen atoms to boryllithium 35a
and hydroborane 38a. Accordingly, boryllithium can be con-
sidered to have a similar electronic character to N-heterocyclic
carbene; that is, the boron-lithium bond is ionic and the boron
atom has a lone pair. In the other boryllithiums 35b-e, a
NMR Spectroscopic Study: Structure of Boryllithium and
the Bonding Properties of the B-Li Bond in Solution. To clarify
the solution structure of boryllithium, several spectroscopic
studies were performed. Dissolution of single crystals of
1
(35a·DME)2 and 35a·(THF)2 into THF-d8 gave identical H,
13C, and 11B NMR spectra consisting of signals assignable to
the N,N′-diaryldiazaborole moiety in 35a. In the 1H NMR
spectrum, two distinct methyl doublets were observed, reflecting
inhibited rotation around the Ar-N bond likely due to steric
repulsion. In contrast, the two 2,6-diisopropylphenyl rings are
equivalent in the spectrum, indicating that there is one 2-fold
rotation. The solvent molecules, which originally coordinated
to the central lithium atom in the solid state, were found to
dissociate from the lithium atom in solution [free DME (1 equiv)
or THF (2 equiv) molecules were observed in 1H NMR
spectroscopy], indicating that 35a forms a structure ligated by
some of the THF-d8 molecules. In the 11B NMR spectra, the
(37) Arduengo, A. J.; Dixon, D. A.; Kumashiro, K. K.; Lee, C.; Power,
W. P.; Zilm, K. W. J. Am. Chem. Soc. 1994, 116, 6361–6367.
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Chemistry of Boryllithium
A R T I C L E S
Table 1. Structural Comparison between Boryllithiums and Related Compounds
field (δB 38.5) at -100 °C (Figure 7). This chemical shift change
was reversible between 20 and -100 °C. A similar temperature
dependence on the 11B NMR chemical shift was observed in
methylcyclohexane-d14. In the methylcyclohexane-d14 solution,
THF molecules coordinated to the central Li atom were observed
1
at 1.47 and 3.19 ppm in the H NMR spectrum as broadened
Figure 6. Boryllithium, carbene, and their protonated compounds.
signals, which could be distinguished from those of free THF
molecules (1.74 and 3.61 ppm). Although it is difficult to clarify
the origin of the reversible chemical shift change, there are three
possible reasons: the dissociation/association equilibrium of THF
molecules to the lithium atom, changing aggregation of boryl-
lithium as observed for the alkyllithium species, and changing
strength of the B-Li bond, leading to a change of bond length.
The direction of the σ22 component of the chemical shielding
tensor on N-heterocyclic carbene 53 has been calculated to be
along with the lone pair of the central carbon (Figure 8).37 By
analogy with carbene, boryllithium 35a may have a similar σ22
component along the B-Li bond, which can be affected by a
situation around the central boron atom.
similarly broadened and low-field shifted 11B NMR signal was
observed (δB 51.9 for 35b, δB 52.1 for 35c, δB 45.0 for 35d,
δB 52.4 for 35e). Isolation of single crystals of boryllithium
enabled us to remove cogenerated lithium bromide for the
measurement of the 7Li NMR spectra. In each 7Li NMR
spectrum of 35a·(THF)2, 35b·(THF)2, and 35c·(THF)2, a
broad signal (δLi 0.46, ν1/2 ) 36 Hz for 35a·(THF)2, δLi 0.68,
ν1/2 ) 35 Hz for 35b·(THF)2, δLi 0.44, ν1/2 ) 50 Hz for
35c·(THF)2) was observed in contrast to that of the reference
compound, LiCl in D2O, which shows a very sharp signal (1.2
Hz). The large half-width may originate from the interaction of
the lithium with the quadrupolar boron nucleus,38 even in the
THF-d8 solvent. However, 6Li enrichment of boryllithium 35a
did not lead to an appearance of spin-spin coupling between
11B and 66Li nuclei39 in the 11B NMR spectrum even in
methylcyclohexane-d14 solvent and even at -100 °C, probably
due to the broadening of its 11B NMR signal.
DFT Study: Possible Solution Structures and the Lone
Pair Character in the HOMO Orbital. To understand the
characteristic B-Li bond, DFT calculation was performed.
Using the crystal structure of 35a·(THF)2 as an initial structure,
an optimized structure, opt-35a·(THF)2, was obtained with no
imaginary frequency in the vibrational analysis at the B3LYP/
6-31+G* level.40 The structural parameters, NPA charges of
B, Li, and N atoms (B3LYP/6-31+G*), and calculated 11B
NMR chemical shifts41 of opt-35a·(THF)2 (GIAO/6-311++G**)
During low-temperature NMR measurement, the 11B NMR
signal of 35a (δB 45.4 at 20 °C) in THF shifted to a higher
(38) Rath, N. P.; Fehlner, T. P. J. Am. Chem. Soc. 1988, 110, 5345–5349.
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A R T I C L E S
Segawa et al.
Table 2. Structural Parameters (Å and deg), NPA Charges of B,
Li, and N Atoms, and Calculated 11B NMR Chemical Shift (ppm)
for Optimized Boryllithium and Related Compounds
opt-46a
opt-35a
opt-35a•(THF)2 opt-35a•(THF)3
opt-38a
B-Li
B-N
2.159
1.467
2.268
1.481
2.363
1.491
1.487
98.77
0.084
0.768
-0.742
-0.739
56.9
1.495
1.436
N-B-N
B
Li
N
97.74
0.104
101.02
0.032
0.769
99.22
0.072
0.755
105.28
0.656
-0.770
-0.712
-0.728
-0.663
δB
51.3
36.1
41.4
19.6
where the positive charge of the central boron atom increased.
In all cases, nitrogen atoms directly connected to the boron atom
accepted the negative charge of “boryl anion” to stabilize these
boryllithiums and the free boryl anion, as Schleyer et al.
indicated the stabilization effect of nitrogen atoms through their
π-donor and σ-acceptor characters.43 The calculated 11B NMR
chemical shifts for opt-35a·(THF)2 (δB 41.4) and opt-35a (δB
36.1) were close to the two experimental values at 20 °C (δB
45.4) and -100 °C (δB 38.5). The chemical shifts for the free
anion opt-46a (δB 51.3) and opt-35a·(THF)3 (δB 56.9) did not
reproduce the experimental values. Since it is difficult to
consider that the free boryllithium opt-35a can exist in the THF
solution, boryllithium 35a may exist as 35a·(THF)2 in THF
solution. The HOMOs of the free phenyl anion (Ph-), free boryl
anion opt-46a, and opt-35a·(THF)2 are shown in Figure 8. Two
free anions, Ph- and opt-46a, have similar shapes of HOMO,
reflecting the lone-pair character of the central carbon and boron
atom, respectively. Complexation of opt-46a with Li and
solvation of two THF molecules to form opt-35a·(THF)2 did
not affect the lone pair character of the HOMO. This result also
suggested a polar character of the B-Li bond. On the other
hand, the HOMO of hydroborane opt-38a (Figure 9) corre-
sponds to the π-orbital of the electron-rich boron-containing
heterocycle. In other words, the localization of electrons in an
ionic B-Li bond to the boron center made this orbital become
the HOMO over the π-orbital of diazaborole, indicating a high
reactivity of boryllithium as a nucleophile at the boron center.
This similarity prompted us to compare the ionic nature of B-Li
and C-Li44 bonds by AIM analysis.45,46 Small F(r) values
Figure 7. 11B NMR spectra of single crystals of 35a·(THF)2 dissolved in
THF solution from 20 to -100 °C.
Figure 8. Shielding tensor primary component orientations in carbene 53
and boryllithium 35a.
are summarized in Table 2 with some reference compounds such
as free boryl anion opt-46a, nonsolvated boryllithium opt-35a,
opt-35a · (THF)3, and hydroborane opt-38a.42 The structural
parameters of calculated opt-35a·(THF)2 are close to the
experimentally obtained values for 35a·(THF)2 by crystal-
lographic study. Complexation of the free boryl anion opt-46a
with lithium cation, giving opt-35a, made the B-N bond shorter
and the N-B-N angle larger. Decrease of the positive charge
on B and the negative charge on N were simultaneously
observed by this B-Li complexation. Solvation by THF
molecules to the lithium cation of opt-35a, giving opt-
35a·(THF)2 or opt-35a·(THF)3, lengthened the B-Li bond,
3
5
(0.02889 e/a0 ) and positive 32F(r) values (0.08409 e/a0 ) at
the bond critical point of the B-Li bond in opt-35a·(THF)2
as alkyllithiums were calculated to have a polar C-Li bond.47
This result clearly indicates a similar bonding character of the
B-Li bond in boryllithium 35a to that of the C-Li bond in
alkyllithium.
Reaction of Boryllithium with Electrophiles. In addition to
our preliminary results for the reactivity of boryllithium, we
performed a systematic study on the reactivity of boryllithium
35a with general organic electrophiles (Scheme 11). Boryl-
(39) Del Bene, J. E.; Elguero, J. Magn. Reson. Chem. 2007, 45, 484–487.
(40) A four-membered bridging structure consists of -(Li-B)2-, which
corresponds to the -(Li-C)2- structure observed for alkyllithium species,
which is less probable because of the bulky substituents on the boron
center.
(41) The optimized B2H6 molecule at B3LYP/6-31+G* was used as a
reference (δB 16.6) for the 11B NMR chemical shift (GIAO/B3LYP/
6-311++G**). The chemical shift for B2H6 in the gas phase was
reported in the following reference: Onak, T. P.; Landesman, H.;
Williams, R. E.; Shapiro, I. J. Phys. Chem. 1959, 63, 1533–1535.
(42) A structure with one THF molecule, opt-35a·(THF)1, could not be
optimized to the minimum.
(43) Wagner, M.; van Eikema Hommes, N. J. R.; No¨th, H.; Schleyer, P. v.
R. Inorg. Chem. 1995, 34, 607–614.
(44) Lambert, C.; Schleyer, P. v. R. Angew. Chem., Int. Ed. Engl. 1994,
33, 1129–1140.
(45) Bader, R. F. W. Atoms In MoleculessA Quantum Theory; Oxford
University Press: New York, 1990.
(46) Bader, R. F. W. Chem. ReV. 1991, 91, 893–928.
(47) This result is consistent with the fact that the previously reported AIM
analyses on nonsolvated alkyllithiums have ionic C-Li bonds. See:
(a) Bader, R. F. W.; Macdougall, P. J. J. Am. Chem. Soc. 1985, 107,
6788–6795. (b) Ritchie, J. P.; Bachrach, S. M. J. Am. Chem. Soc.
1987, 109, 5909–5916.
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Chemistry of Boryllithium
A R T I C L E S
Figure 9. HOMOs of Free Phenyl Anion (Ph-), opt-46a, opt-35a·(THF)2, and opt-38a.
Scheme 11. Reactions of Boryllithium 35a with Various Electrophilesa
a General conditions: THF, room temperature, 10 min, unless otherwise noted. All yields were determined by 1H NMR compared with an internal standard.
Scheme 12. Plausible Mechanism for the Formation of R-Borylbenzyl Benzoate 59
lithium reacted with methyl trifluoromethanesulfonate, 1-chlo-
robutane, or benzyl chloride to form corresponding alkylborane
derivatives 55, 56, and 57, but in the case of benzyl chloride,
the chlorination product 34a-Cl was also generated as a major
compound, probably due to the halophilic attack or single
electron transfer from 35a to benzyl chloride including a radical
chain reaction. Reaction with more reactive n-butyl bromide or
benzyl bromide afforded the bromoborane 34a-Br as the sole
product. The reaction with carbonyl compounds gave the
corresponding products in the same way that carbanions react.
The reaction with 1 equiv of benzaldehyde followed by
protonation formed R-borylbenzylalcohol 58 in 81% yield,48,49
while the reaction with 3 equiv of benzaldehyde afforded
R-borylbenzyl benzoate 59 in 51% yield. The formation of 59
can be explained by the following two-step reaction: insertion
of the second benzaldehyde molecule into the lithium-oxygen
bond of R-borylbenzylalkoxide intermediate, followed by an
intermolecular hydride transfer to the third benzaldehyde
molecule to form lithium benzylalkoxide in the Oppenauer
oxidation manner (Scheme 12).50 The reaction with benzoyl
chloride, phenyl benzoate, and benzoic anhydride gave a
substituted product benzoylborane 60a51 in good yields. The
X-ray crystallographic study of 58 and 60a (Figures 10 and 11)
revealed that an oxygen atom does not interact (intra- or
intermolecularly) with the central boron atom to form three- or
six-membered rings and that there is no intermolecular hydrogen
bonding,52 probably due to the sterics of the two 2,6-diisopro-
pylphenyl groups on nitrogen atoms. The reaction with anhy-
drous carbonates afforded the corresponding borylcarboxylate
ester products, 61 and 62, for both t-Bu and Ph groups. Reaction
with carbon dioxide followed by protonation gave a borylcar-
boxylic acid, 63a, in a high yield with a small amount of
(49) Catalytic diboration of aldehyde to form R-borylalcohol was also
reported. See, refs 11b and 11d.
(50) Lithium alkoxides generated from N,N-dimethylformamide and car-
byllithiums were reported to transfer a hydride to benzaldehyde or
benzophenone in Oppenauer oxidation fashion. See: (a) Screttas, C. G.;
Steele, B. R. J. Org. Chem. 1988, 53, 5151–5153. The related Mg-
Oppenauer oxidation reactions were also reported. See: (b) Meerwein,
v. H.; Schmidt, R. Liebigs Ann. Chem. 1925, 444, 221–238. (c) Byrne,
B.; Karras, M. Tetrahedron Lett. 1987, 28, 769–772. (d) Kloetzing,
R. J.; Krasovskiy, A.; Knochel, P. Chem.sEur. J. 2007, 13, 215–
227.
(48) Several R-borylmethanol derivatives were synthesized by SN2 reaction
of halomethylborane with benzyloxide followed by deprotection. See:
Singh, R. P.; Matteson, D. S. J. Org. Chem. 2000, 65, 6650–6653.
(51) Yamashita, M.; Suzuki, Y.; Segawa, Y.; Nozaki, K. J. Am. Chem.
Soc. 2007, 129, 9570–9571.
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J. AM. CHEM. SOC. VOL. 130, NO. 47, 2008 16077

A R T I C L E S
Segawa et al.
Scheme 13. Reactions of C-C Saturated Boryllithium 35b with
Carbonyl Compounds (NMR Yield)
Scheme 14. Plausible Mechanism (a, Unsaturated; b, Saturated)
for the Formation of Hydroboranes 62a and 62b
Figure 10. Crystal structure of 58 (50% thermal ellipsoids; all hydrogen
atoms except OH and benzylic H and minor parts of the disordered moieties
were omitted for clarity).
atom to form the three-membered ring transition state 68, and
the following loss of carbon monoxide gave the boronate anion
69, which can be protonated to afford the hydroxyboranes 64a
and 64b. A similar reaction pathway was reported in the
computational study11c of Cu-catalyzed deoxygenation of carbon
dioxide in the presence of a diborane(4) reagent.11a The lower
reactivity of borylcarboxylate 67a in the rearrangement may
be explained by a loss of aromaticity in the transition state 68a
that would destabilize the transition state more than the
corresponding 68b.
A series of borylcarbonyl compounds, 60a, 60b, 61, 62, and
63a, are the first examples of fully characterized acylborane,54
base-free borylcarboxylate ester, and base-free borylcarboxylic
acid.55 Table 3 shows the νCdO wavenumbers in the IR spectra
and CdO bond lengths of the obtained borylcarbonyl com-
pounds in the solid state along with those of the corresponding
benzophenone,56 benzoic acid,57 and its phenyl58 or t-butyl
esters.56b The νCdO wavenumbers of all the boryl derivatives
are smaller, and the CdO lengths are longer than those of the
corresponding phenyl derivatives, indicating weaker CdO bond
strengths in borylcarbonyls. A similar tendency was also
observed for silylcarbonyl derivatives (1618 cm-1 for PhCO-
SiMe3, 1646 cm-1 for HOCOSiMe3).59,60 In CHCl3 solution,
borylcarboxylic acid 63a showed two types of CdO vibration,
which indicates that 63a has an equilibrium between monomeric
and dimeric structures in solution. Table 4 shows the 13C NMR
chemical shift for carbonyl carbons of borylcarbonyl compounds
Figure 11. Crystal structure of 60a (50% thermal ellipsoids; hydrogen
atoms are omitted for clarity).
byproduct, hydroxyborane 64a (Vide infra). Boryllithium 35a
also reacted with fluoroarenes. The reaction with PhF yielded
phenylborane 65 in a low yield because the reaction is slower
than the decomposition of boryllithium. However, the reaction
with C6F6 afforded a pentafluorophenylborane 66 in a moderate
yield within a shorter reaction time.53
Some reactions of saturated boryllithium 35b with electro-
philes were also examined (Scheme 13). The reaction with
benzoyl chloride generated benzoylborane 60b in a good yield.
However, the reaction with CO2 did not give any borylcarboxy-
lic acid 63b, and hydroxyborane 64b was found as the sole
product instead, which is in contrast to the reaction of 35a with
CO2, giving hydroxyborane 64a as a minor product. A plausible
mechanism for the formation of these unexpected products,
hydroxyboranes 64a and 64b, is shown in Scheme 14. The
reaction of the boryl anion with CO2 formed the corresponding
borylcarboxylate anion 67, the negatively charged carboxylate
oxygen directly interacted with a vacant p-orbital of the boron
(54) Preparation of n-Bu2BCOPh and n-Bu2BCO2Et has been proposed;
however, there was no modern spectroscopic characterization. See ref
52.
(55) (a) Base-stabilized esters have been widely explored, mainly as a boron
analogue of R-amino acid derivatives with hydroborate structure
instead of R-carbon. See: Gabel, D.; El-Zaria, M. B. Product subclass
19: Carboxyboranes and Related Derivatives. In Science of Synthesis;
Kaufmann, D. E., Matteson, D. S., Eds.; Georg Thieme Verlag:
Stuttgart-New York, 2005; Vol. 6, pp 563-583.
(56) (a) The structure was deposited to Cambridge Crystallographic Data
Centre (CCDC-245188) as a private communication (2004) by
Coppens, P., and Moncol, J. (b) SDBSWeb: http://www.aist.go.jp/
RIODB/SDBS/ (National Institute of Advanced Industrial Science and
Technology, 2007/03/06).
(57) Bruno, G.; Randaccio, L. Acta Crystallogr., Sect. B 1980, 36, 1711–
1712.
(52) Diaminoboryllithium [(H2N)2BLi] was calculated to react with form-
aldehyde to form a three-membered ring structure consisting of B, C,
and O atoms. See ref 44. Acylborane [R2BC(dO)R′] has been
postulated to dimerize to form a six-membered ring structure consisting
of B, C, and O atoms. See: (a) Schmid, G.; No¨th, H. Chem. Ber. 1968,
101, 2502–2505.
(58) Adams, J. M.; Morsi, S. E. Acta Crystallogr., Sect. B 1976, 32, 1345–
1347.
(59) Picard, J. P.; Calas, R.; Dunogues, J.; Duffaut, N.; Gerval, J.;
Lapouyade, P. J. Org. Chem. 1979, 44, 420–424.
(60) Steward, O. W.; Dziedzic, J. E.; Johnson, J. S. J. Org. Chem. 1971,
36, 3475–3480.
(53) Sun, H.; DiMagno, S. G. J. Am. Chem. Soc. 2005, 127, 2050–2051.
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Chemistry of Boryllithium
A R T I C L E S
Table 3. Carbonyl Vibrations (cm-1) and the CdO Bond Distances
(Å) of Borylcarbonyl Compounds Compared with Those of
Reference Compounds
bond is polarized where the boron atom is anionic in all
(35a·DME)2, 35a·(THF)2, 35b·(THF)2, and 35c·(THF)2
when compared to the structures of hydroborane 38a-c and
optimized free boryl anion opt-46a-c. Dissolution of the
isolated single crystals of (35a · DME)2, 35a · (THF)2,
35b·(THF)2, and 35c·(THF)2 in THF-d8 showed that the
boron-lithium bond remained in solution and free DME or THF
molecules were observed. Temperature-dependent 11B NMR
chemical shift changes of 35a were observed in THF-d8 or
methylcyclohexane-d14, suggesting a change of chemical shift
anisotropy around the boron center. The HOMO of opt-
35a·(THF)2 had a lone pair character on the boron atom, as
observed for phenyllithium, whereas the HOMO of hydroborane
38a corresponds to the π-orbital of the boron-containing five-
membered heterocycle. The polarity of the BsLi bond, esti-
mated by AIM analysis, was similar to that of alkyllithium.
Boryllithiums 35a and 35b behave as a base or a boron
nucleophile in reaction with organic electrophiles via deproto-
nation, SN2-type substitution, halogen-metal exchange or
electron-transfer, 1,2-addition to a carbonyl group, and SNAr
reaction. In the case of the reaction with CO2, intramolecular
cyclization followed by CO elimination from borylcarboxylate
anion took place to afford the corresponding boronate, which
could be protonated to give hydroxyboranes 64a and 64b. The
characters of a carbonyl group in the borylcarbonyl compounds
60a, 60b, 61, 62, and 63a, which were obtained from the
reaction of boryllithiums 35a and 35b, were investigated by
X-ray crystallography, IR, and 13C NMR spectroscopy to show
that the boryl substituent weakened the CdO bond, when
compared to their carbon substituted analogues.
compd KBr disk
CHCl3 soln
CdO
compd
KBr disk
CdO
60a
60b
61
62
63a
1618 1628
1638 1638
1690 1680
1719 1713
1666 1672, 1709
1.241(2) PhCOPh
1655 1.223(3)
1.237(2)
1.213(2) PhCO2 Bu 1713
t
1.204(2) PhCO2Ph
PhCO2H
1729 1.194(6)
1689 1.252(2)
Table 4. 13C NMR Chemical Shifts of the Borylcarbonyl
Compounds with Those of Reference Compounds
compd
δC (C6D6)
compd
δC (C6D6)
compd
δC (C6D6)
60a
60b
61
62
63a
218.7
221.0
174.4
172.5
181.6
PhCOPh
195.2
PhCOCH3
196.6
t
PhCO2 Bu
165.0
164.4
172.7
PhCONMe2
PhCOOMe
PhCOF
170.7
166.7
157.4
PhCO2Ph
PhCO2H
with the same set of reference compounds and p-block element
substituted benzoyl compounds.61 In the 13C NMR spectra,
carbonyl peaks of 60a, 60b, 61, 62, and 63a appeared in a lower
field than those of the corresponding benzophenone and benzoic
acid derivatives. The carbonyl group of 60a attached to a boron
atom resonates in the lowest field among the p-block element-
substituted benzoyl derivatives (B > C > N > O > F). Although
the reason why 60a has the lowest field signal is not clear, the
boron atom may significantly affect the paramagnetic term to
determine the carbonyl 13C chemical shift. A low-field shift of
the carbonyl signal has also been reported for benzoylsilane
derivatives,59,62 in which the silicon atom has a lower elec-
tronegativity, similar to that of the boron atom, than that of the
carbon atom (C, 2.50; B, 2.04; Si, 1.90).3
Acknowledgment. The authors thank Professors Norihiro
Tokitoh and Takahiro Sasamori for data processing of (35a·DME)2.
This work was supported by KAKENHI (No. 17065005 for
“Advanced Molecular Transformations of Carbon Resources”, No.
19027015 for “Synergy of Elements”, Nos. 18750027 and 20-9103)
from MEXT, Japan, and by the Global COE Program for
“Chemistry Innovation”. Computer resources for theoretical cal-
culation were provided by the Research Center for Computational
Science in the National Institutes of Natural Sciences. Y. Segawa
acknowledges a JSPS fellowship for young scientists.
Conclusion
A series of lithium salts of boryl anion, boryllithiums, were
synthesized and characterized by NMR spectroscopy and
crystallographic analysis. Reduction of bromoborane 34a-Br by
lithium naphthalenide provided boryllithium 35a. Both chlo-
roborane (34a-Cl) and iodoborane (34a-I) were also usable as
precursors. Structural modification of boryllithium, using satu-
rated CsC and benzannulated CdC backbones in the five-
membered ring and mesityl groups on the nitrogen atoms, also
allowed generation of the corresponding boryllithium. The solid
state structures of boryllithium showed that the boron-lithium
Supporting Information Available: Details for all experi-
mental procedures, X-ray crystallography, and computational
studies. This material is available free of charge via the Internet
at http://pubs.acs.org.
(61) The 13C chemical shifts in C6D6 were measured with commercially
available chemicals.
(62) Brook, A. G.; Abdesaken, F.; Gutekunst, G.; Plavac, N. Organome-
tallics 1982, 1, 994–998.
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