Syntheses, structures, and reactivities of borylcopper and -zinc compounds: 1,4-Silaboration of an α,β-unsaturated ketone to form a γ-siloxyallylborane

Article abstract of DOI:10.1002/anie.200801728

(Chemical Equation Presented) New borylcopper and borylzinc compounds were synthesized by transmetalation of boryllithium with metal halides. The 1,4-addition of lithium borylbromocuprate [BCu(μ²-Br)Li(thf) ₃] to 2-cyclohexen-1-one as a boryl anion, followed by trapping of the resulting boron-substituted copper enolate with Me₃SiCl, afforded the γ-siloxyallylborane.

Full text of DOI:10.1002/anie.200801728

Communications
DOI: 10.1002/anie.200801728
Boryl Anions
Syntheses, Structures, and Reactivities of Borylcopper and -zinc
Compounds: 1,4-Silaboration of an a,b-Unsaturated Ketone to Form a
g-Siloxyallylborane**
Takashi Kajiwara, Tomomi Terabayashi, Makoto Yamashita,* and Kyoko Nozaki*
Carbanions are one of the most important reagents in
synthetic organic chemistry.[1] They act as a carbon nucleo-
phile to form a carbon–carbon bond as a consequence of the
À
nucleophilic borylcopper intermediate that reacts with
organic electrophiles, such as a,b-unsaturated carbonyls[6] or
allylic esters.[7] Recently, a borylcopper species (5)was
isolated[8] and its reactivity as a nucleophile toward carbonyl
compounds has been studied experimentally[8,9] and theoret-
ically.[10] Our recent study on a nucleophilic borylation using
boryllithium to form a series of boryl–Group 11 metal
complexes also resulted in the isolation of borylcopper
species 6a,b.[11] Although borylcopper species 5 and 6a,b
having an N-heterocyclic carbene ligand were isolated, there
have been no reports with copper having boryl ligands instead
of carbyl ligands as in a conventional organocopper, [(RCu)n],
or Gilman-type cuprates, [R2CuLi]. Borylzinc species pos-
sessing boron–zinc two-center, two-electron bonds have also
not yet been synthesized.[12] To develop the chemistry of boryl
anions, it is also important to estimate the properties of
borylcopper and borylzinc compounds which have a conven-
tional formula as with carbanionic species, because they may
show different reactivities with organic electrophiles com-
pared with those observed for boryllithium and borylmagne-
sium compounds.
highly polarized carbon–metal bonds (CdÀ
M
d+). The proper-
ties of polar carbon–metal bonds in a carbanionic species
depend on the metallic counterpart, such as lithium, magne-
sium, copper, and zinc. The first two organometallic com-
pounds directly attack the carbonyl group of a,b-unsaturated
ketones, whereas the latter two[2] react by 1,4-addition
(conjugate addition).[3]
In contrast to the rich chemistry of carbanions, there is a
limited number of examples of boryl anions. We recently
reported the first synthesis of boryllithium 2a as a boron
Herein, we report the syntheses and structures of bor-
ylcopper and borylzinc compounds, and their reactivity with
a,b-unsaturated ketones as boryl anions, where trapping of
the enolate intermediate with silyl electrophile gives the
corresponding 1,4-silaboration product, g-siloxyallylborane.
Transmetalation of boryllithium 2b, which was prepared
from bromoborane 1b,[11] with one equivalent of CuBr and
ZnBr2, respectively, followed by recrystallization of the
products from pentane, gave crystals of lithium borylbromo-
cuprate 7b and lithium boryldibromozincate 8b (Scheme 1).
The two “ate” complexes 7b and 8b were spectroscopically
nucleophile.[4] Borylmagnesium species 3a and 4a could be
synthesized by transmetalation reactions of 2a with magne-
sium bromide, and behave as nucleophiles.[5] Prior to our
recent studies on boryllithium and borylmagnesium com-
pounds, there have been reports proposing a formation of a
[*] Dr. T. Kajiwara, T. Terabayashi, Dr. M. Yamashita, Prof. Dr. K. Nozaki
Department of Chemistry and Biotechnology
Graduate School of Engineering, The University of Tokyo
7-3-1 Hongo, Bunkyo-ku, 113-8656 Tokyo (Japan)
Fax: (+81)3-5841-7263
E-mail: makotoy@chembio.t.u-tokyo.ac.jp
nozaki@chembio.t.u-tokyo.ac.jp
[**] This work was supported by a Grant-in-Aid for Scientific Research on
Priority Areas (No. 17065005, “Advanced Molecular Transforma-
tions of Carbon Resources” and No. 19027015, “Synergy of
Elements”) and for Young Scientists (B) (18750027) from MEXT
(Japan) and by Kurata Memorial Hitachi Science and Technology
Foundation. T.K. thanks AGC Corporation for financial support.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200801728.
Scheme 1. Syntheses and yields of 7b and 8b by transmetalation of
boryllithium 2b. Dip=2,6-diisopropylphenyl.
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and structurally characterized. The 11B NMR spectrum has a
broad signal at 45 ppm for 7b, and at 41 ppm for 8b. These
values are comparable to that of boryllithium 2b (52 ppm).
The molecular structures of 7b and 8b were determined by X-
ray structural analysis (Figure 1 and 2).[13] The first example of
a borylcuprate, 7b, contains a B-Cu-Br-Li chain with three
slightly bent, as was observed for the B-Cu-C angle of 5
(168.07(16)8),[8] whereas that of 6b was almost linear
(179.41(15)8).[11] This difference may arise from the less-
bulky bridging bromide ligand on the central copper atom in
7b compared to carbene ligand in 6b. Lithium boryldibro-
mozincate 8b[15] is the first example containing a two-center,
À
À
two-electron B Zn bond, and the B Zn bond length
[16]
(2.075(5)Š) is slightly shorter
than the sum of covalent
radii (2.13 Š)[17] of boron and zinc atoms, and than the
shortest B Zn bond length (2.15(2)Š)
viously reported multicenter, multielectron B Zn bonds.
Tetranuclear copper(I)complex 10b was formed by
changing the stoichiometry of CuBr to boryllithium 2b to
2:1 (Scheme 2). The 11B NMR spectrum of 10b has a broad
[12d]
À
among the pre-
À
[12]
Scheme 2. Synthesis and yield of tetranuclear copper complex 10b.
Figure 1. ORTEP drawing one of two independent molecules of 7b,
with ellipsoids set at 50% probability. Hydrogen atoms and minor
contributors to disordered THF molecules are omitted for clarity.
signal at 38 ppm, which is shifted slightly upfield compared to
that of 7b (45 ppm). The structure of 10b obtained from
crystallographic analysis is shown in Figure 3.[18] Each of the
two bromine atoms and two boron atoms bridges two copper
atoms in an alternating fashion.[19] The B Cu bond lengths in
À
10b (2.093(4)and 2.073(5)Š)are longer than the two-center,
two-electron bonds in 7b, which may reflect a bridging
situation of the boryl ligand, as was observed for previously
reported bridging boryl complexes.[20]
Figure 2. ORTEP drawing of 8b, with ellipsoids set at 50% probability.
Hydrogen atoms and minor contributors to disordered THF molecules
are omitted for clarity.
tetrahydrofuran molecules coordinating to the lithium
atom.[14] The average B Cu bond length of 7b (1.983 Š)is
À
Figure 3. ORTEP drawing of 10b, with ellipsoids set at 50% proba-
bility. Hydrogen atoms and minor contributors to disordered bromine
atoms are omitted for clarity. One half of the whole molecule is an
asymmetric unit.
similar to that of carbene borylcopper complex 6b
[11]
(1.983(3)Š),
(2.002(3)Š).
and somewhat shorter than that of
The average B-Cu-Br angle of 7b (172.08)is
5
[8]
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Solvent-free diborylzinc species 11a and 11b were
reaction of 10b or 8b with 2-cyclohexen-1-one gave the
corresponding conjugate addition product, 3-borylcyclo-
hexan-1-one 12b, in 74 and 41% yield after hydrolysis
(Scheme 4). In the case of 8b, the lithium atom is expected
obtained as colorless, crystalline solids by addition of
0.5 equivalents of zinc halide to boryllithium compounds 2a
and 2b, respectively (Scheme 3). The 11B NMR spectrum has
Scheme 3. Syntheses and yields of diborylzinc complexes 11a and
11b.
Scheme 4. Reaction of 8b or 10b with 2-cyclohexen-1-one. M denotes
{CuLn} (from 10b) or {ZnLn} (from 8b) where L=Br, LiBr, or thf.
a broad signal at 32 ppm for 11a, and at 38 ppm for 11b. The
1H and 13C NMR signals of 11 were slightly shifted from those
of recently reported diborylmagnesium species.[5] X-ray
crystallographic analysis (Figure 4 for 11a; see the Support-
to act as a Lewis acid to achieve the conjugate addition.[25] In
contrast, the reaction of boryllithium 2b with 2-cyclohexen-1-
1
one led only to protonation of 2b (as detected by H NMR
spectroscopy)to form the corresponding hydroborane
[HB(NDipCH2)2] (14)instead of the 1,2- or 1,4-addition
product. The b-borylketone 12b was characterized by X-ray
crystallography (see the Supporting Information), and the
formation of the C B bond was also confirmed by 13C NMR
À
spectroscopy, which showed a broad signal (d = 25.23)arising
from coupling to the quadrupolar 11B nucleus. The regiose-
lectivity of the addition is the same as those of organo-
cuprates,[2a,b] organozincates,[3] and transient borylcopper
species.[6] Trapping the copper enolate intermediate gener-
ated from 7b and 2-cyclohexen-1-one with Me3SiCl afforded
g-siloxyallylborane 13b in 80% yield (Scheme 5). The g-
Scheme 5. Synthesis of g-siloxyallylborane 13b from 7b.
Figure 4. ORTEP drawing of 11a, with ellipsoids set at 50% proba-
bility. Hydrogen atoms and minor contributors to disordered isopropyl
groups are omitted for clarity.
siloxyallylborane 13b is both a silicon enolate and an
allylborane. Multistep syntheses of such g-siloxyallylboranes
and their application to organic synthesis have been
reported.[26] It should be noted that transition-metal-catalyzed
silaboration of an a,b-unsaturated ketone affords comple-
mentary products, presumably arising from the g-boroxyal-
lylsilane.[27] Thus, it is not completely clear whether the g-
boroxyallylsilane is actually formed prior to hydrolysis in
Oestereichꢀs system.
In summary, a series of new borylmetal species, namely
the borylcopper compounds 7b and 10b and borylzinc
compounds 8b and 11a,b, were synthesized by a transmeta-
lation reaction of boryllithium with metal halides, and were
structurally characterized by X-ray crystallography. Tetranu-
clear borylcopper 10b and borylzincate 8b reacted with
2-cyclohexen-1-one to form the corresponding conjugate
addition product 12b. Trapping the borylated copper enolate
ing Information for 11b)revealed that diborylzinc com-
pounds 11a and 11b have a two-coordinate linear structure[21]
in which the B-Zn-B angles are 178.50(11)8 for 11a and
177.41(11)8 for 11b, as reported for most dialkylzinc and
diarylzinc species.[22] It should be noted that 11a and 11b are
the first isolated examples of a homoleptic borylmetal
species.[23] The B Zn bond distances (2.052(3)and
À
2.053(3)Š for 11a)are slightly shorter than that in boryldi-
À
bromozincate 8b (2.075(5)Š). Saturation of the C C bond in
the boron-containing five-membered ring leads to a longer
B Zn bond (2.088(3)and 2.087(3)Š for 11b).[24]
À
To investigate the reactivity of the boryl compounds as
boron nucleophiles, borylcopper 10b and borylzinc 8b were
allowed to react with an a,b-unsaturated ketone. Thus, the
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Zhao, Z. Lin, T. B. Marder, Organometallics 2007, 26, 2824 –
generated from 7b and 2-cyclohexen-1-one with Me3SiCl
afforded g-siloxyallylborane 13b. Further investigations on
the reactivity of these boryl metal compounds and the
application of 12b and 13b to synthetic organic chemistry
are currently in progress.
2832; e)T. B. Marder, Organomet. Chem. 2008, 34, 46 – 57.
[11] Y. Segawa, M. Yamashita, K. Nozaki, Angew. Chem. 2007, 119,
6830 – 6833; Angew. Chem. Int. Ed. 2007, 46, 6710 – 6713.
[12] There have been several reports on zinc borane complexes
possessing electron-deficient multicenter–multielectron bond(s)
À
between boron and zinc atoms. Among those, the shortest B Zn
Received: April 14, 2008
Published online: July 21, 2008
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Keywords: allylic compounds · anions · boron · copper · zinc
.
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[13] Crystal data and a summary of the intensity data collection
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diffractometer with graphite-monochromated MoKa radiation
(l = 0.71070 Š)to 2 qmax = 558. The structures were solved by
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(8b), CCDC-668898 (10b), CCDC-675171 (11a), CCDC-680348
(11b), and CCDC-668899 (12b)contain the supplementary
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Centre via www.ccdc.cam.ac.uk/data_request/cif.
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[14] Isolation of related organo(halo)cuprates has been reported that
have
a similar structure to 7b: for [Li([12]crown-4)2]
[Cu(Br){CH(SiMe3)2}], see a)H. Hope, M. M. Olmstead, P. P.
Power, J. Sandell, X. Xu, J. Am. Chem. Soc. 1985, 107, 4337 –
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[15] A related lithium alkyldibromozincate, [{(PhN(Me)Me2Si)-
(Me3Si)2C}-Zn(m2-Br)2Li(thf)2] (9), has also been structurally
À
characterized, and has a C Zn bond length of 2.014(5)Š, which
is similar to the sum of covalent radii (2.02 Š)of the carbon and
zinc atoms. See D. Azarifar, M. P. Coles, S. M. El-Hamruni, C.
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2
À
Zn (m -Br)bonds in 8b (2.504 Š)are longer than those in
9
[7] a)P. V. Ramachandran, D. Pratihar, D. Biswas, A. Srivastava,
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[16] This is in contrast to boryllithium 2a[4] and borylmagnesium
compounds 3 and 4,[5] in which the B M bond (M = Li or Mg)is
À
longer than the sum of covalent radii, probably owing to the
[8] D. S. Laitar, P. Müller, J. P. Sadighi, J. Am. Chem. Soc. 2005, 127,
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À
difference in polarity of boron metal bond.
[17] J. Emsley, The Elements, 3rd ed., Oxford University Press, New
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À
[18] The Cu Cu distances of 10b are 2.3120(11)Š for boron-bridged
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(thf)4 (2.414(2)and 2.432(2)Š; See, H. Eriksson, M. Håkansson,
Organometallics 1997, 16, 4243 – 4244), probably owing to the
existence of an electronegative bromine atom, which makes the
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À
À
B Cu interaction stronger than the C Cu interaction in
tetranuclear mesitylcopper species, even though the boron
atom is larger than carbon.
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[23] This absence of THF ligands in the boryl metal compound is
similar to the dicarbylzinc species.[22] There has been only one
report on the structural characterization of dialkylzinc coordi-
nated by THF molecules. For difluorenylzinc(thf)2 species, see B.
Fischer, J. Boersma, G. van Koten, W. J. J. Smeets, A. L. Spek,
Organometallics 1989, 8, 667 – 672. All the other examples of
dicarbylzinc species having a coordination number of three or
higher in the Cambridge Crystallographic Database contain
either stronger monodentate ligands than THF or a multidentate
ligand.
[24] This elongation is most likely due to the slightly higher donor
ability of a saturated boryl ligand than that of an unsaturated
boryl ligand, as was studied for boryl–Group 10 or 11 metal
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[25] A lithium-bridging bimetallic transition state was calculated for
a reaction of LiZnMe3 with methyl vinyl ketone. See Ref. [3f].
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