Nickel-mediated regioselective [2 + 2 + 2] cycloaddition of carboryne with alkynes

Article abstract of DOI:10.1021/ja061605j

Transition metal-benzyne complexes have found many applications in organic synthesis, mechanistic studies, and the synthesis of functional materials. In sharp contrast, the chemistry of transition metal-carboryne complexes, especially late transition metal complexes, is virtually unknown. This communication reports a novel nickel-mediated regioselective [2 + 2 + 2] cycloaddition reaction of carboryne with alkynes via the Ni-carboryne intermediate (η²-C₂B₁₀H₁₀)Ni(PPh₃)₂. Because of the bulkiness of the carborane moiety, a high regioselectivity was achieved in the reactions involving unsymmetrical alkynes. This work furnishes a novel method for the preparation of highly substituted benzocarboranes which are difficult to obtain by other methods. Copyright

Full text of DOI:10.1021/ja061605j

Published on Web 05/24/2006
Nickel-Mediated Regioselective [2 + 2 + 2] Cycloaddition of Carboryne with
Alkynes
Liang Deng, Hoi-Shan Chan, and Zuowei Xie*
Department of Chemistry, The Chinese UniVersity of Hong Kong, Shatin, New Territory, Hong Kong, China
Received March 8, 2006; E-mail: zxie@cuhk.edu.hk
Chart 1. Isolobal Analogue
Benzyne (and its transition metal complexes) has found many
applications in organic synthesis, mechanistic studies, and the
synthesis of functional materials since it was reported as an active
intermediate in the 1950s.1,2 In sharp contrast, carboryne (1,2-
dehydro-o-carborane), a three-dimensional relative of benzyne, came
to the scene at a much later stage, which was first reported in 1990
as a very reactive intermediate.3 Similar to benzyne, the in situ
generated carboryne can react with alkenes, dienes, and alkynes in
[2 + 2] cycloaddition and ene reaction patterns.3,4 On the other
hand, the chemistry of metal-carboryne complexes is largely
unexplored, although the first example of (η2-C2B10H10)Ni(PPh3)2
was reported in 1973,5 even earlier than the parent carboryne
intermediate.
Table 1. Ni-Mediated Coupling Reaction of Carboryne with
Alkynes
Recently, we prepared an early transition metal-carboryne
complex, [{η5:σ-Me2C(C9H6)(C2B10H10)}ZrCl(η3-C2B10H10)][Li-
(THF)4],6 and investigated the reactivity of Cp2Zr(µ-Cl)(µ-C2B10H10)-
Li(OEt2)2, a precursor of Cp2Zr(η2-C2B10H10).7 Our work revealed
that Cp2Zr(η2-C2B10H10) produced in situ has a similar reactivity
pattern to that of Cp2Zr(η2-C6H4) in reactions with polar unsaturated
organic substrates, such as isonitrile, nitrile, and azide. However,
significantly different from Zr-benzyne species, Zr-carboryne is
inert toward internal alkynes because of steric hindrance of the
carborane moiety and the π coordination requirement of alkynes.7
To overcome these problems, we thought about (η2-C2B10H10)Ni-
(PPh3)2 since the coordinated phosphines, which can create vacant
sites for incoming substrates, are often labile during the reactions.
With this in mind and in view of the reactions of nickel-benzynes
with alkynes to generate substituted naphthalenes8 and the analogy
between nickel-benzyne and nickel-carboryne complexes (Chart
1), we then examined the reactivity of (η2-C2B10H10)Ni(PPh3)2 (2)
toward alkynes. Herein, we report our preliminary results on the
reactions of 2 with 2 equiv of alkynes to afford [2 + 2 + 2]
cycloaddition product, benzocarborane 3, as shown in Table 1.
A typical procedure is as follows.9 To a THF solution (30 mL)
of Li2C2B10H10 (10 mmol), prepared in situ from the reaction of
nBuLi (20 mmol) with o-carborane (10 mmol), was added (PPh3)2-
NiCl2 (10 mmol) at room temperature. The reaction mixture was
further stirred for 0.5 h giving the Ni-carboryne intermediate 2.5
3-Hexyne (30 mmol) was then added, and the closed reaction vessel
was heated at 90 °C for 4 days. The reaction mixture was then
cooled to room temperature and quenched with NaHCO3 solution.
Normal workup afforded the coupling product 3a (R1 ) R2 ) Et)
in 67% isolated yield with a small amount of hexaethylbenzene as
the byproduct (8% based on 3-hexyne). The reaction temperature
is crucial for this reaction. Lower temperatures often gave lower
yields, and the reaction did not proceed at T < 60 °C. On the other
hand, higher reaction temperatures (>90 °C) led to the decomposi-
tion of 2 as indicated by the 11B NMR. The use of excess alkynes
was also necessary because of the side reaction of the trimeriza-
tion of alkynes by the extruded nickel(0) species.10 More than 3
equiv of alkyne was not necessary, as it did not offer higher yield
entry
R1/R2
product
yield (%)a,b
1
2
3
4
5
6
7
8
Et/Et
3a
3b
3c
3d
3e
3f
NR
NR
67 (8)
65 (7)
65 (7)
33 (0)
54 (0)
57 (4)
nPr/nPr
nBu/nBu
Ph/Ph
Me/Ph
nBu/CCBun
tBu/nBu
COOMe/COOMe
a Isolated yields. b Yields in parentheses are corresponding to those of
benzene derivatives.
of 3. It is noteworthy that other nickel-carboryne complexes,
(η2-C2B10H10)NiLn (L ) PEt3, P(OEt)3, n ) 2; L ) dppe, n ) 1),
gave very similar results.
Listed in Table 1 are representative results obtained from the
nickel-mediated coupling reactions of carboryne with alkynes. All
products were fully characterized by 1H, 13C, and 11B NMR as well
as high-resolution mass spectrometry.9 The molecular structures
n
of 3a (R1 ) R2 ) Et), 3b (R1 ) R2 ) Pr), 3d (R1 ) R2 ) Ph),
and 3e (R1 ) Ph, R2 ) Me) were further confirmed by single-
crystal X-ray analyses (see Supporting Information).9 Figure 1
shows the representative structure of 3e. The localized double bonds
C(3)-C(4)/C(5)-C(6) suggest there is no substantial π-delocal-
ization in the six-membered ring. As indicated in Table 1, both
aliphatic and aromatic alkynes underwent [2 + 2 + 2] cycloaddition
reactions. The electron-rich alkynes were more favorable as indi-
cated by the relatively high yields in entries 1-3. No reaction was
observed for the electron-deficient alkyne (entry 8). Steric factors
also played an important role in the reactions. Sterically less
demanding linear dialkylalkynes offered the best results in com-
parison to the phenyl and tert-butyl-substituted ones. No reaction
proceeded for tBuCtCBun (entry 7). Such a large steric effect made
the reactions highly regioselective. As a result, only head-to-tail
addition product was observed (entries 5 and 6). It is noted that
terminal alkynes are not suitable for the reactions because they can
protonate 2 to give o-carborane as monitored by the 11B NMR.
9
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C O M M U N I C A T I O N S
into the Ni-C(vinyl) bond of the intermediate I is supported by
the absence of III.11 In addition, the control experiments indicated
that (σ-MeC2B10H10)2Ni(PPh3)2 was inert toward 3-hexyne under
various reaction conditions.12 Many attempts to isolate the inter-
mediate I failed. NMR experiments were not able to detect it,
suggesting that the second insertion was very fast.
In conclusion, we have shown for the first time that a nickel-
carboryne complex can regioselectively react with internal alkynes
in a [2 + 2 + 2] cycloaddition manner to give highly substituted
benzocarboranes. The presence of the very bulky carborane moiety
dominates the regioselectivity.
Acknowledgment. The work described in this paper was
supported by a grant from the Research Grants Council of the Hong
Kong Special Administration Region (Project No. 403805).
Figure 1. Molecular structure of 3e. Selected bond lengths (Å): C1-C2
1.648(3), C1-C6 1.486(3), C2-C3 1.493(3), C3-C4 1.338(3), C4-C5
1.476(3), C5-C6 1.351(3).
Supporting Information Available: Detailed experimental pro-
cedures, characterization data, and X-ray data in cif format. This
material is available free of charge via the Internet at http://pubs.acs.org.
Scheme 1. Proposed Mechanism for the Formation of 3e
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The formation of products 3 can be rationalized by the sequential
insertion of alkynes into the Ni-C bond, as illustrated in Scheme
1. The first insertion gives a nickelacyclopentene intermediate (I).
The second equivalent of alkyne can possibly insert into either the
nickel-C(cage) bond giving a nickelacycloheptadiene (IIA) or the
nickel-C(vinyl) bond giving IIB. Both of them could undergo
reductive elimination to form the same product 3 and highly reactive
Ni(0) species that is capable of catalyzing the cyclotrimerization
of alkynes.10 The insertion of unsymmetrical alkynes in the two
consecutive steps clearly determines the substitution pattern in the
final product. The exclusive formation of the head-to-tail products
3e and 3f suggests that path b is preferred over the path a. As shown
in Scheme 1, the sterically controlled insertion of phenylpropyne
(9) Detailed experimental procedures and complete characterization data
including X-ray data for complexes 3a,b,d,e are provided in the Supporting
Information.
(10) Ni(0) species catalyzed coupling reactions of alkynes have been well
documented. For examples, see ref 8 and (a) Alphonse, P.; Moyen, F.;
Mazerolles, P. J. Organomet. Chem. 1988, 345, 209-216. (b) Sambaiah,
T.; Cheng, C.-H. Bull. Inst. Chem., Acad. Sin. 1999, 46, 41-51.
(11) The regioselectivity of the first insertion was similar to that observed in
the sterically controlled insertion of unsymmetrical alkyne into Ni(η2-
4,5-X2-C6H2)(PEt3)2; see ref 8c.
(12) The (σ-MeC2B10H10)2Ni(PPh3)2 was prepared according to literature
method. See: Bresadola, S.; Cecchin, G.; Turco, A. Gazz. Chim. Ital.
1970, 100, 682-683.
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