Angewandte
Chemie
Metal–benzyne complexes have found many applications in
organic synthesis, mechanistic studies, and the synthesis of
functional materials.[1] In contrast, the isolobally analogous
metal–carboryne complexes[2] are largely unexplored,
although the reactivity pattern of carboryne (generated
in situ) has been actively investigated.[3–5] We have recently
demonstrated the complementary reactivities of [Cp2Zr(h2-
C2B10H10)] (produced in situ, Cp = C5H5)[6] and [(h2-
C2B10H10)Ni(PPh3)2] (1).[7] The former reacts readily with
polar unsaturated organic substrates, such as those containing
isonitrile, nitrile, and azide groups to give monoinsertion
products; however, it does not show any activity toward
internal alkynes.[6] The latter (1) undergoes regioselective
[2+2+2] cycloaddition with alkynes to afford benzocarbor-
anes, but it does not react with the polar unsaturated
molecules listed above.[7] These results indicate that the
nature of the transition metals plays a crucial role in these
reactions. We have extended our research to include alkenes,
and report herein their reaction with nickelcarboryne to
afford alkenylcarboranes.
In a typical procedure, the alkene (2 equiv) was added to a
solution of nickel–carboryne 1, prepared in situ by the
reaction of Li2C2B10H10 with [NiCl2(PPh3)2] in THF, and the
reaction mixture was heated at 908C in a closed vessel
overnight. Standard workup procedures afforded the coupled
products in excellent regio- and steroselectivity for most
alkenes (Table 1). The temperature is crucial for this reaction;
no reaction occurred at less than 608C. In contrast, higher
reaction temperatures (> 908C) led to the decomposition of 1,
as indicated by 11B NMR spectroscopy. Toluene and diethyl
ether were not suitable for this reaction because of the poor
solubility of 1 in these solvents. Reactions that employed
other phosphine compounds such as PEt3, P(OEt)3, and
1,2-bis(diphenylphosphino)ethane (dppe) gave very similar
results to those obtained using PPh3. The use of the isolated
pure complex 1 as the starting material gave the same results
as the complex used in situ.
good yields for aliphatic alkenes and a-methylstyrene (2 f,
entries 6, 9–11). For example, 4k was isolated in 67% yield,
which is much higher than the 10–20% yield from the direct
reaction of carboryne with cyclohexene.[3] Vinyl ethers 2n and
2o also reacted with nickel carboryne 1, but the coupled
products were formed in low yields, probably because of the
coordination of the oxygen atom that occupies the vacant site
of the nickel atom (entries 14 and 15). Such interactions may
alter the regioselectivity of the olefin insertion and stabilize
the inserted product, which leads to the formation of 5n after
hydrolysis. In the case of norbornene (2l), the corresponding
inserted product was thermodynamically very stable,[8] and
afforded only hydrolysis product 5l in 60% isolated yield
(entry 12). No double-insertion product was observed. For
indene (2m), both hydrolysis product 5m and “ene-reaction-
type” product 3m were isolated in 27% and 31% yield,
respectively (entry 13). No reaction was observed with cis-
and trans-stilbene, 6,6-dimethylfulvene, 1,1-dimethylallene,
1-phenylallene, 2-propenenitrile, diphenylvinylphosphine,
ethylvinylsulfide, anthracene, furan, and thiophene. All new
products were fully characterized by various spectroscopic
techniques and high-resolution mass spectrometry.[9a] The
molecular structure of 3e was further confirmed by single-
crystal X-ray analyses (Figure 1).[9b]
As shown in Table 1, a variety of alkenes are compatible
with this nickel-mediated cross-coupling reaction. Substituted
styrene derivatives reacted efficiently to give “Heck-type”
products 3 as single regioisomers with excellent stereoselec-
tivity and in very good yields. The nature of the substituents
on the phenyl ring has no apparant effect on the reactions
(entries 1–5). The yields were lower for 1,1-diphenylethene
and vinyltrimethylsilane because of steric effects (entries 7
and 8). The “ene-reaction-type” products were isolated in
Figure 1. Molecular structure of 3e. Selected bond lengths () and
angles (o): C1–C2 1.626(3), C1–C11 1.495(3), C11–C12 1.302(3), C12–
C13 1.475(3), C1-C11-C12 122.2(2), C11-C12-C13 129.1(2).
The reaction of carboryne (generated in situ) with an-
thracene, furan, or thiophene has been shown to yield [2+4]
cycloaddition products.[5] The nickel carboryne 1, however,
did not react with any of these molecules. This result suggests
that carboryne and nickel carboryne should undergo different
reaction pathways in reactions with alkenes.
[*] Z. Qiu, Prof. Dr. Z. Xie
Scheme 1 shows a plausible mechanism for the formation
of the coupled products. Dilithiocarborane reacts with [NiCl2-
(PPh3)2] to generate the nickel carboryne 1.[2a] Coordination
and insertion of the alkene gives a nickelacycle A.[10] The
regioselectivity observed in the reaction can be rationalized
by the large steric effect of the carborane moiety. b-H/b’-H
elimination prior to the insertion of the second molecule of
alkene produces the intermediate B/B’.[11] Reductive elimi-
nation affords the alkenylcarboranes 3 (“Heck-type” prod-
ucts) or 4 (“ene-reaction-type” products). In general, b-H
elimination of five-membered metallacycles is more difficult
Department of Chemistry and
Center of Novel Functional Molecules
The Chinese University of Hong Kong
Shatin, N.T., Hong Kong (China)
Fax: (+852)2603-5057
E-mail: zxie@cuhk.edu.hk
[**] This work was supported by grants from the Research Grants
Council of the Hong Kong Special Administration Region (Project
No. 403805) and a Strategic Investments Scheme administrated by
The Chinese University of Hong Kong.
Supporting information for this article is available on the WWW
Angew. Chem. Int. Ed. 2008, 47, 6572 –6575
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
6573
Qiu, Zaozao
