Synthesis and reactivity of triethylborane adduct of N-heterocyclic carbene: Versatile synthons for synthesis of N-heterocyclic carbene complexes

Article abstract of DOI:10.1039/b405459h

The reaction of an imidazolium salt with LiBEt₃H afforded triethylborane adduct of imidazol-2-ylidene, which can act as a carbene precursor for the synthesis of a transition metal complex as well as a main group element complex.

Full text of DOI:10.1039/b405459h

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Synthesis and reactivity of triethylborane adduct of N-heterocyclic
carbene: versatile synthons for synthesis of N-heterocyclic carbene
complexes{
Yoshitaka Yamaguchi,*a Taigo Kashiwabara,a Kenichi Ogata,a Yumiko Miura,a Yoshiyuki Nakamura,b
Kimiko Kobayashic and Takashi Ito*a
a Department of Materials Chemistry, Graduate School of Engineering, Yokohama National University,
Tokiwadai, Hodogaya-ku, Yokohama 240-8501, Japan
b Chemical Resources Laboratory, Tokyo Institute of Technology, Nagatsuda, Midori-ku, Yokohama
226-8503, Japan
c The Institute of Physical and Chemical Research (RIKEN), Hirosawa, Wako, Saitama 351-0198, Japan
Received (in Cambridge, UK) 14th April 2004, Accepted 5th July 2004
First published as an Advance Article on the web 20th August 2004
The reaction of an imidazolium salt with LiBEt3H afforded
BEt3 was observed at 212.6 ppm for 2a and at 213.3 ppm for 2b,
respectively, in the 11B NMR spectra. These chemical shifts are
consistent with that of the reported carbene–BEt3 compound.7
The X-ray analyses of 2a and 2b were undertaken.{ The
structures of 2a and 2b are shown in Fig. 1 and are consistent with
their spectroscopic data. The B–C(carbene) bond distances are
triethylborane adduct of imidazol-2-ylidene, which can act as
a carbene precursor for the synthesis of a transition metal
complex as well as a main group element complex.
N-Heterocyclic carbenes (NHCs) are an increasingly useful class of
ligands for not only transition metals but also main group
elements.1 One of the most widely used methods is the thermal
cleavage of the enetetramines, so-called electron-rich olefins, in the
presence of metal species.2 However the metal precursor and/or
product of the NHC are occasionally decomposed due to the high
temperature requirement for enetetramine cleavage. The successful
isolation of the free NHCs by Arduengo and co-workers in 19913
afforded the most common method of direct complexation of the
free NHC, either isolated or generated in situ, produced by depro-
tonation of the corresponding salt. However manipulation of the
free carbenes was often difficult due to their highly reactive nature.
Recently the employment of NHC adducts as ‘‘protected’’ forms of
the free carbene attracted much attention for synthesis of the metal
complexes bearing these ligands. Grubbs and co-workers reported
the utilization of NHC–alcohol or –chloroform adducts for
saturated NHCs (imidazolin-2-ylidenes),4 and Lin et al. reported
the carbene transmetallation of silver complexes for unsaturated
NHCs (imidazol-2-ylidenes).5 Recent studies have revealed that the
latter method is reliable for the preparation of the unsaturated
NHC complexes.6
˚
˚
1.683(5) A for 2a and 1.678(6) A for 2b. The B–C bond distances in
2 are remarkably longer than those in the BH3-adduct of imidazol-
˚
˚
2-ylidene compounds (1.596(4) A for IMes?BH3 and 1.603(3) A for
1,3-diethyl-4,5-dimethylimidazol-2-ylidene?BH3),8 and are slightly
longer than those for the BF3-adduct of imidazol-2-ylidene
˚
˚
(1.635(5) A for IMes?BF3 and 1.669(6) A for 1,3-dimesityl-4,5-
dichloroimidazol-2-ylidene?BF3).9
Recent study on the carbene adducts with Group 13 elements
such as B, Al, Ga, and In revealed that these adducts have the
remarkable thermal stability because of the formation of the stable
acid–base adduct.10 However, we focused attention on the
elongated B–C(carbene) bond distance in the compounds 2,
because, if the bond cleavage would occur under mild conditions,
compounds 2 could be utilized as the precursor for the HNCs. So,
we investigated the reaction of compounds 2 with some metal
compounds (Scheme 2).
On treating 2a with BH3?THF complex, a clean substitution
During the course of our research on NHC complexes, a BEt3
adduct of NHC was obtained unexpectedly by the reaction of an
imidazolium salt with LiBEt3H; the latter acts not only as a base for
the generation of a carbene but also the protecting group for it. We
herein report the preparation, characterization, and utilization of
NHC?BEt3 compounds.
Scheme 1
Treatment of 1,3-diisopropylimidazolium chloride (IiPr?HCl, 1a)
with one equivalent of LiBEt3H in THF at 278 uC yielded a
homogeneous reaction mixture from which pale yellow crystals of
2a were isolated in 80% yield. In the case of 1b (IMes?HCl), the
2,4,6-trimethylphenyl (mesityl) analogue of 1a, a similar reaction
took place to give a pale yellow solid 2b (46%). Elemental analyses,
1
and H, 13C, and 11B NMR spectra established the formation of
BEt3-adduct NHC compounds (2a and 2b) (Scheme 1). In the 1H
NMR spectra, the BEt3 coordinated carbene carbon in both
compounds displayed a triplet and a quartet for the methyl and
methylene groups, respectively, in a region of high magnetic field
(ca. 20.1 to 0.6 ppm). The 13C NMR spectra showed the broad
signals assignable to the carbene carbons at 174.0 ppm for 2a and
at 180.4 ppm for 2b, respectively. The broadening may be caused
by the influence of the binding boron. A singlet signal assignable to
Fig. 1 Molecular structures of 2a (left) and 2b (right). ORTEP diagrams
drawn at 30% probability (upper) and space filling diagrams (bottom).
{ Electronic supplementary information (ESI) available: full experimental
data. See http://www.rsc.org/suppdata/cc/b4/b405459h/
2 1 6 0
C h e m . C o m m u n . , 2 0 0 4 , 2 1 6 0 – 2 1 6 1
T h i s j o u r n a l i s ß T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 4

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to their stability toward air and moisture. We also demonstrated
the utilization of these adducts for the preparation of metal
complexes.
The authors are grateful to Professor Kohtaro Osakada and
Dr Yasushi Nishihara (Tokyo Institute of Technology), and
Dr Noriyuki Suzuki (RIKEN) for the kind help in the elemental
analyses.
Notes and references
{ Crystal data for 2a: C15H31BN2, M ~ 250.23, orthorhombic, a ~
3
˚
˚
16.033(8), b ~ 9.904(8), c ~ 10.58(1) A, U ~ 1680(2) A , T ~ 223 K,
space group Pnma (no. 62), Z ~ 4, m(Mo–Ka) ~ 0.56 cm 21, 2788
measured reflections, 2575 unique reflections. R1 ~ 0.082, wR2 ~ 0.212,
for 1754 reflections. For 2b: C27H39BN2, M ~ 402.43, orthorhombic, a ~
Scheme 2
3
˚
˚
16.929(10), b ~ 17.29(1), c ~ 8.511(8) A, U ~ 2490(3) A , T ~ 173 K,
space group P212121 (no. 19), Z ~ 4, m(Mo–Ka) ~ 0.61 cm21, 4085
measured reflections, 4058 unique reflections. R1 ~ 0.062, wR2 ~ 0.173,
for 3219 reflections. CCDC numbers: 2a: 236673; 2b: 236674. See http://
www.rsc.org/suppdata/cc/b4/b405459h/ for crystallographic data in .cif
format.
reaction took place to give the BH3-adduct of IiPr (3a). However,
3a was so highly soluble toward hydrocarbons that a low isolated
yield (31%) resulted. The similar reaction employing 2b in the place
of 2a also afforded the BH3-adduct 3b as a white powder in a good
yield (93%).11 11B NMR spectra of 3a and 3b displayed quartet
signals at 236.4 ppm (1JBH ~ 88.0 Hz) and at 236.8 ppm (1JBH
~
1 For a review of N-heterocyclic carbene complexes, see: W. A. Herrmann
and C. Ko¨cher, Angew. Chem., Int. Ed. Engl., 1997, 36, 2162–2187;
D. Bourissou, O. Guerret, F. P. Gabba¨ı and G. Bertrand, Chem. Rev.,
2000, 100, 39–91; T. Weskamp, V. P. W. Bo¨hm and W. A. Herrmann,
J. Organomet. Chem., 2000, 600, 12–22; W. A. Herrmann, Angew.
Chem., Int. Ed., 2002, 41, 1290–1309; A. H. Cowley, J. Organomet.
Chem., 2001, 617–618, 105–109.
2 M. F. Lappert, J. Organomet. Chem., 1988, 358, 185–214; M. F. Lappert,
P. L. Pye and G. M. McLaughlin, J. Chem. Soc., Dalton Trans., 1977,
1272–1282; M. F. Lappert and P. L. Pye, J. Chem. Soc., Dalton Trans.,
1977, 1283–1291; M. F. Lappert, P. L. Pye, A. J. Rogers and
G. M. McLaughlin, J. Chem. Soc., Dalton Trans., 1981, 701–704;
M. F. Lappert, J. Organomet. Chem., 1975, 100, 139–159.
3 A. J. Arduengo, III, R. L. Harlow and M. Kline, J. Am. Chem. Soc.,
1991, 113, 361–363; A. J. Arduengo, III, Acc. Chem. Res., 1999, 32,
913–921.
88.0 Hz), respectively, whose data are consistent with the reported
data for the related compound.8 Other spectroscopic data
supported the formation of BH3-adducts 3. Secondly the reaction
of BF3?OEt2 with 2b was examined, and thus the BF3 adduct of
IMes (4b) was formed as a colorless solid. The characterization of
4b was done by 1H and 11B NMR measurements and these spectral
data are consistent with the reported data by Arduengo.9 Treat-
ment of 2a with Mo(CO)6 in toluene under reflux conditions
afforded a carbene complex of molybdenum 5a as a pale yellow
solid in moderate yield (67%). In the case of 2b, Mo(CO)5(IMes)
(5b) was also obtained as a pale yellow solid in 67% yield. These
1
complexes were characterized by elemental analyses, IR, and H
and 13C NMR spectra.12
It is of interest to compare the behavior of BH3- and/or BF3-
adducts toward Mo(CO)6 to that of the BEt3-adduct with a view to
find possible reagents capable of carbene transfer to transition
metals. Treatment of BH3 adduct 3b with Mo(CO)6 under the same
conditions, i.e., refluxing in toluene, afforded the carbene complex
5b, accompanied by the formation of Mo(g-toluene)(CO)3 (6) (the
ratio 5b/6 being ca. 1/1). On the other hand, from the reaction of
Mo(CO)6 with BF3 adduct 4b, the carbene complex 5b was not
formed and 82% of 4b was recovered unreacted. These results
clearly show that the BEt3 adduct of NHC (2) effectively acts as a
carbene precursor for a metal complex, which might be due to the
weak Lewis acidity13 and the steric bulk of BEt3 compared with
BF3 and BH3.
In order to investigate whether this method is applicable to the
bidentate system or not, we examined the reaction of xylyl bridged
bis(imidazolium) salts (7a: ortho-form, 7b: meta-form) with two
equivalents of LiBEt3H. In both reactions, BEt3-adducts of bis-
(imidazol-2-ylidene) (8a and 8b) were obtained as white powders in
good yield (Scheme 3).
4 M. Scholl, S. Ding, C. W. Lee and R. H. Grubbs, Org. Lett., 1999, 1,
953–956; T. M. Trnka, J. P. Morgan, M. S. Sanford, T. E. Wilhelm,
M. Scholl, T.-L. Choi, S. Ding, M. W. Day and R. H. Grubbs, J. Am.
Chem. Soc., 2003, 125, 2546–2558.
5 H. M. J. Wang and I. J. B. Lin, Organometallics, 1998, 17, 972–975.
6 A. R. Chianese, X. Li, M. C. Janzen, J. W. Faller and R. H. Crabtree,
Organometallics, 2003, 22, 1663–1667; M. Frøseth, A. Dhindsa, H. Røise
and M. Tilset, Dalton Trans., 2003, 4516–4524; A. M. Magill,
D. S. McGuinness, K. J. Cavell, G. J. P. Britovsek, V. C. Gibson,
A. J. P. White, D. J. Williams, A. H. White and B. W. Skelton,
J. Organomet. Chem., 2001, 617–618, 546–560.
7 A. Wacker, H. Pritzkow and W. Siebert, Eur. J. Inorg. Chem., 1998,
843–849.
8 T. Ramnial, H. Jong, I. D. McKenzie, M. Jennings and J. A. C. Clyburne,
Chem. Commun., 2003, 1722–1723; N. Kuhn, G. Henkel, T. Kratz,
J. Kreutzberg, R. Boese and A. H. Maulitz, Chem. Ber., 1993, 126,
2041–2045.
9 A. J. Arduhengo, III, F. Davidson, R. Krafczyk, W. J. Marshall and
R. Schmutzler, Monatsh. Chem., 2000, 131, 251–265.
10 R. J. Baker, M. L. Cole, C. Jones and M. F. Mahon, J. Chem. Soc.,
Dalton Trans., 2002, 1992; R. J. Baker, A. J. Davies, C. Jones and
M. Kloth, J. Organmet. Chem., 2002, 656, 203; M. D. Francis,
D. E. Hibbs, M. B. Hursthoues, C. Jones and N. A. Smithies, J. Chem.
Soc., Dalton Trans., 1998, 3249.
In summary, we showed the one-pot synthesis of BEt3 adduct
of NHC (2) by the reaction of imidazolium salt (1) with LiBEt3H.
The manipulation of BEt3 adducts of NHC (2) is very easy due
11 Jennings and Clyburne also reported that the direct reaction of 1b with
NaBH4 resulted in the formation of 3b. See reference 8.
¨
12 A related compound was reported, see: K. Ofele, W. A. Herrmann,
D. Mihalios, M. Elison, E. Herdtweck, W. Scherer and J. Mink,
J. Organomet. Chem., 1993, 459, 177–184; H. G. Raubenheimer,
Y. Stander, E. K. Marais, C. Thompson, G. J. Kruger, S. Cronje and
M. Deetlefs, J. Organomet. Chem., 1999, 590, 158–168.
13 N. Frafa´n and R. Contreras, J. Chem. Soc., Perkin Trans. 2, 1987,
771–773.
Scheme 3
C h e m . C o m m u n . , 2 0 0 4 , 2 1 6 0 – 2 1 6 1
2 1 6 1