Nickel-catalyzed [4 + 2 + 2]-type annulation reaction of cyclobutanones with diynes and enynes

Article abstract of DOI:10.1246/bcsj.81.885

In the presence of a nickel(0) catalyst, cyclobutanones reacted with diynes to produce bicyclic eight-membered ring ketones. Cyclobutanones acted as a C4 unit in the formal [4 + 2 + 2]-type annulation reaction, which proceeded through a ring-expansion of a spirocyclic seven-membered oxanickelacycle to a nine-membered nickelacycle via β-carbon elimination. A similar annulation reaction was also examined with enynes.

Full text of DOI:10.1246/bcsj.81.885

Ó 2008 The Chemical Society of Japan
Bull. Chem. Soc. Jpn. Vol. 81, No. 7, 885–893 (2008)
885
Nickel-Catalyzed [4 þ 2 þ 2]-Type Annulation Reaction
of Cyclobutanones with Diynes and Enynes
ꢀ
Shinji Ashida and Masahiro Murakami
Department of Synthetic Chemistry and Biological Chemistry, Graduate School of Engineering, Kyoto University,
Katsura, Kyoto 615-8510
Received January 28, 2008; E-mail: murakami@sbchem.kyoto-u.ac.jp
In the presence of a nickel(0) catalyst, cyclobutanones reacted with diynes to produce bicyclic eight-membered
ring ketones. Cyclobutanones acted as a C4 unit in the formal [4 þ 2 þ 2]-type annulation reaction, which proceeded
through a ring-expansion of a spirocyclic seven-membered oxanickelacycle to a nine-membered nickelacycle via ꢀ-car-
bon elimination. A similar annulation reaction was also examined with enynes.
Transition-metal-catalyzed multi-component annulation re-
actions1 have been one of the most attractive research topics
Results and Discussion
in synthetic organic chemistry. Multiple carbon–carbon bond
formations occur in a single chemical operation with good
atom economy, providing a powerful method for the construc-
tion of cyclic carbon frameworks. A number of carbon units
and reaction modes have been developed to expand the range
of accessible carbocycles. For example, [5 þ 2 þ 1]2 and
[4 þ 2 þ 2]3 annulation reactions have enabled a direct synthe-
sis of eight-membered carbocycles, which are an important
structural feature often found in biologically active com-
pounds. Further exploration of new reaction modes is desired
to expand the utility of multi-component annulation reactions.
We have reported a nickel-catalyzed intermolecular alkyne
insertion reaction of cyclobutanones which forms six-mem-
bered carbocycles (Scheme 1).4 An alkyne was formally in-
serted between the carbonyl carbon and the ꢁ-carbon through
the initial formation of an oxanickelacycle intermediate by
oxidative cyclization of an alkyne and the carbonyl group of
a cyclobutanone with nickel(0), which was followed by ꢀ-
carbon elimination and reductive elimination. This result
presented an example of a [4 þ 2] annulation reaction, in
which cyclobutanones act as a C4 unit. We next envisaged
that incorporation of another unsaturated carbon–carbon bond
into the initially formed oxanickelacycle could extend this
protocol even further to a three-component annulation reaction
to construct medium-sized carbocycles. In this article, we
describe the details of our study on the nickel-catalyzed
[4 þ 2 þ 2] annulation reaction of cyclobutanones with diynes
and enynes.5
A solution of dimethyl 2,2-bis(but-2-ynyl)malonate (1a, 1.5
molar equivalents) in toluene was added dropwise to a mix-
ture of 3-methyl-3-phenylcyclobutanone (2a), bis(1,5-cyclo-
octadiene)nickel(0) (10 mol %), and tricyclohexylphosphine
(20 mol %) in toluene at 100 ꢁC. The reaction mixture was stir-
red at that temperature for 3 h. A formal [4 þ 2 þ 2] annulation
reaction took place to give bicyclo[6.3.0]undecadienone 3a in
83% yield (Table 1, Entry 1). The structure of 3a was unam-
biguously assigned by NMR spectroscopy (1H, 13C, NOESY,
HMQC, and HMBC). No formation of a six-membered ketone
that might have arisen from a [4 þ 2] annulation reaction of 2a
with the alkyne moiety was observed. Several phosphine
ligands were examined under otherwise identical conditions
(Table 1). Whereas triphenylphosphine and tri-t-butylphos-
phine showed low reactivities, tri-n-butylphosphine worked
efficiently at 100 ꢁC to give 3a in 92% (Entries 3–5). Self-oligo-
merization of the diyne 1a was a major side-reaction, which
was partially suppressed by slow addition of 1a. Ligands other
than phosphines were also examined to reveal that the use of
N-heterocyclic carbene ligands6 improved the catalyst activity
of nickel(0) so that the [4 þ 2 þ 2] annulation occurred even at
room temperature. In particular IPr (10 mol %) gave 3a in the
best yield of 91% with the use of 1.2 mol equiv of 2a (Entry 8).
The structure of alkyne 4 is given by substraction of one of
two alkyne moieties from diyne 1a with the sterically demand-
ing malonate moiety being retained. For comparison with
diyne 1a, alkyne 4 was subjected to reaction with cyclobuta-
none 2a (Scheme 2). The formal insertion reaction failed to
occur, suggesting that simultaneous coordination of the two al-
kyne moieties is required for the initial oxidative cyclization to
proceed.
O
O
R
We postulate the mechanism shown in Scheme 3. The diyne
and cyclobutanone initially bind to nickel(0) to form 5. Subse-
quent oxidative cyclization leads to the formation of spiro-
cyclic oxanickelacycloheptadiene 7. There are two species 6
and 60 conceivable as the intermediate between 5 and 7. Nick-
elacyclopentadiene 67 can occur through oxidative cyclization
R
cat. Ni(0)
+
R1
R2
R'
R1 R2
R'
Scheme 1.
Published on the web July 10, 2008; doi:10.1246/bcsj.81.885

886 Bull. Chem. Soc. Jpn. Vol. 81, No. 7 (2008)
Table 1. Optimization of Reaction ConditionsaÞ
Nickel-Catalyzed [4 þ 2 þ 2]-Type Annulation
Me
Me
O
O
10 mol % Ni(cod)2
ligand
Me
Me
E
E
E
E
+
Me
Ph
toluene
Ph Me
1a (E = CO2Me)
2a/mol equiv
2a
3a
Entry
Ligand
Temp/ꢁC
Time/h
Yield/%bÞ
1
2
3
4
5
6
7
8
9
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.2
1.2
1.2
1.2
20 mol % P(c-Hex)3
none
100
100
100
100
100
100
rt
rt
rt
rt
rt
3
3
3
3
3
1
1
1
83
0
8
92
0
93
94
91
30
76
0
20 mol % PPh3
20 mol % PBu3
20 mol % P(t-Bu)3
10 mol % IPr
10 mol % IPr
10 mol % IPr
10 mol % IMes
10 mol % SIPr
10 mol % ItBu
12
12
1
10
11
a) A solution of diyne 1a in toluene was added dropwise to the reaction mixture (Entries 1–5).
b) Isolated yield.
O
10 mol % Ni(cod)2
Me
10 mol % IPr
E
E
no reaction
+
Me
toluene, r.t., 2 h
Ph Me
4 (E = CO2Me)
2a
Scheme 2.
3
O
R
R2
R1
Ni(0)
Z
1 + 2
R2
Ni
R'
R1
8
R
O
Ni
R2
R2 R1
Z
R1
R
R
R'
6'
Ni
Z
Z
O
O
Ni
R2 R1
R'
R'
R
7
5
Z
Ni
O
R'
6
Scheme 3.

S. Ashida et al.
Bull. Chem. Soc. Jpn. Vol. 81, No. 7 (2008)
887
Table 2. Screening of Diynes 1aÞ
Entry
Diyne 1
Cyclobutanone 2
Condition
Product 3, Yield/%bÞ
R
R
O
R
O
O
O
E
E
E
E
Me
Ph
Ph Me
(E = CO2Me)
2a
R
1
2
1b (R = H)
1c (R = Et)
Ac),d)
A
3b, 68
3c, 85
Me
MeO
MeO
Me
Me
MeO
MeO
Me
Ph
Me
3
4
1d
1d
A
B
3d, 88
3d, 91
Me
Me
Me
Z
Z
Me
Ph
Me
5
6
7
8
9
1e (Z = CH2)
1e (Z = CH2)
1f (Z = O)
1g (Z = NTs)
1g (Z = NTs)
A
B
A
A
B
3e, 78
3e, 87
3f, 61
3g, 32
3g, 71
O
O
Ph
Ph
Ph Ph
10
1h
2b
Ad),e)
3h, 46
a) Condition A: Cyclobutanone 2, diyne 1 (1.5 mol equiv to 2), Ni(cod)2 (10 mol %), and PBu3
(20 mol %) in toluene at 100 ꢁC for 3 h. Condition B: Cyclobutanone 2, diyne 1 (1.2 mol equiv to
2), Ni(cod)2 (10 mol %), and IPr (10 mol %) in toluene at room temperature for 1–3 h. b) Isolated
yield. c) P(c-Hex)3 was used. d) 3.0 mol equiv of diyne was used. e) 20 mol % Ni(cod)2 and
40 mol % P(c-Hex)3 were used.
of the two alkyne moieties on nickel(0), whereas hetero-type
oxidative cyclization of the carbonyl group and one of the al-
kyne moieties would form another possible five-membered cy-
clic intermediate 60.8 Subsequent incorporation of the third un-
saturated functionality into the Ni–C bond of either five-mem-
bered nickelacycle 6 or 60 leads to the formation of spirocyclic
oxanickelacycloheptadiene 7.9,10 Then, the four-membered
ring of the spiro nickelacycle 7 is opened by ꢀ-carbon elimi-
nation11 to expand the seven-membered nickelacycle to the
nine-membered nickelacycle 8. This ring-expanding process
is promoted presumably by release of the ring strain of the
four-membered carbocycle of the nickel(II) tertiary cyclobuta-
nolate. Finally, reductive elimination gives the product 3 with
nickel(0) regenerated.
Next, various diynes 1 were subjected to the annulation re-
action under the reaction conditions A (Ni–phosphine) and B
(Ni–IPr) (Table 2). The reaction of diyne 1b, possessing termi-
nal alkyne moieties, suffered from its rapid self-oligomeriza-
tion, and required the use of tricyclohexylphosphine as ligand
and 3.0 molar equivalents of 1b to attain 68% yield of 3b,
whereas diethyl-substituted diyne 1c showed reactivity similar
to 1a and gave product 3c in 85% yield (Entries 1 and 2).
However, diynes having either isopropyl or phenyl substituents
at the alkyne terminus failed to undergo the reaction, presum-
ably due to steric reasons. The reaction worked well with
diynes having various tethers under appropriate conditions.
Diyne 1e linked by a trimethylene tether produced 3e in
87% yield under condition B (Entry 6). The reaction of oxa-
1,6-diyne 1f with 2a afforded 3f containing an ether linkage
in its framework in 61% yield under condition A (Entry 7).
Whereas aza-1,6-diyne 1g afforded product 3g in 32% yield
under condition A, condition B improved the yield to 71%
(Entries 8 and 9). Although octa-1,7-diyne (1h) whose linker
was longer by one carbon could also join the [4 þ 2 þ 2] annu-
lation reaction, it was less reactive than 1b and 1d to produce
3h in 46% yield under the fortified condition A (Entry 10).
The reaction of unsymmetrical diyne 1i was examined in
terms of the regiochemical selectivity (Scheme 4). Whereas
the tri-n-butylphosphine ligand showed no selectivity, moder-
ate selectivity was observed with the sterically bulkier tricyclo-
hexylphosphine ligand to give a 4:1 mixture of regioisomers
3i and 3j. Thus, the regioselectivity varied depending on the

888 Bull. Chem. Soc. Jpn. Vol. 81, No. 7 (2008)
Nickel-Catalyzed [4 þ 2 þ 2]-Type Annulation
H
O
Me
H
O
O
10 mol % Ni(cod)2
ligand
E
E
E
E
E
Me
H
+
Me
Ph
Me
Ph
+
E
toluene
Ph Me
Me
1i (E = CO2Me)
2a
3i
3j
20 mol % PBu3, 100 °C, 3 h
51% (51:49)
84% (79:21)
43% (>20:1)
20 mol % P(c-Hex)3, 100 °C, 3 h
10 mol % IPr, r.t., 1 h
Me
Me
H
Ph
Me
Ph
E
E
E
E
O
O
Ni
Ni
L
L
Me
H
A
B
Scheme 4.
Table 3. Screening of Cyclobutanones 2aÞ
Entry
Diyne 1a
Cyclobutanone 2
Condition
Product 3, Yield/%bÞ
Me
Me
O
O
E
E
E
E
Me
R2
R1
R1 R2
1a (E = CO2Me)
Me
1
2
3
4
5
6
2b (R1 = R2 = Ph)
A
A
A
B
A
B
3k, 84
3l, 70
3m, 83
3m, 84
3n, 84
3n, 91
2c (R1 = R2 = Et)
2d (R1 = Ph, R2 = H)
2d (R1 = Ph, R2 = H)
2e (R1 = Oct, R2 = H)
2e (R1 = Oct, R2 = H)
a) Condition A: Cyclobutanone 2, diyne 1 (1.5 mol equiv to 2), Ni(cod)2 (10 mol %), and PBu3
(20 mol %) in toluene at 100 ꢁC for 3 h. Condition B: Cyclobutanone 2, diyne 1 (1.2 mol equiv to
2), Ni(cod)2 (10 mol %), and IPr (10 mol %) in toluene at room temperature for 1–3 h. b) Isolated
yield.
ligands, which were accounted for in terms of bulkiness. There
The use of unsymmetrical 2-substituted cyclobutanones 2f
and 2g was also examined under the catalysis of Ni–IPr
(Scheme 5). A high regioselectivity of >20:1 was observed
for the ꢀ-carbon elimination step. We assume that, with inter-
mediate 70, migration of the methylene carbon (C40) is prefer-
red over that of the methyne carbon (C20), probably due to
steric reasons. In the case of the enantiomerically enriched
substrate 2g (48% ee), the enantiopurity decreased to 37% with
the product 3p, which might be a result of enolization partially
occurring with the carbonyl-containing compounds involved,
e.g. 2g, 3p, and/or 8.
We next examined an analogous nickel-catalyzed [4 þ
2 þ 2] annulation of enynes3c with cyclobutanones. Good re-
gioselectivity could be expected for the three-component as-
sembly because the alkene and alkyne moieties would possess
different reactivities. The reaction of 2c with dimethyl 2-allyl-
2-(but-2-ynyl)malonate (9a, 1.5 molar equivalents) was carried
out using the three typical ligands, tri-n-butylphosphine, tri-
cyclohexylphophine, and IPr (Table 4). A slow addition proce-
are two regioisomeric intermediates A and B conceivable for 7
in Scheme 3. When a ligand is bulky enough, intermediate A
would be favored over B because intermediate B would suffer
from unfavorable steric repulsion between the methyl group at
the alkyne terminus and the ligand (L). The IPr ligand would
be bulkier than tricyclohexylphosphine, and therefore, pro-
duced 3i in a more selective manner, although the chemical
yield was moderate.
Cyclobutanones 2b and 2c possessing two substituents at
the 3-position reacted with 1a to give bicyclo[6.3.0]undecane
derivatives 3k and 3l, respectively, in good yield (Table 3, En-
tries 1 and 2). With cyclobutanones 2d and 2e, the hydrogen at
the 3-position of the cyclobutanone might have caused ꢀ-hy-
dride elimination with intermediate 8. The following reductive
elimination could form a monocyclic product.4 To our delight,
however, cyclobutanones 2d and 2e afforded the correspond-
ing eight-membered ring products selectively without forma-
tion of such a side product (Entries 3–6).

S. Ashida et al.
Bull. Chem. Soc. Jpn. Vol. 81, No. 7 (2008)
889
Me
O
10 mol % Ni(cod)2
10 mol % IPr
O
R
E
E
Me
Me
E
E
+
R
toluene, r.t., 1 h
Me
2f (R = Ph)
2g (R = Oct)
(48%ee)
1a (E = CO2Me)
3o(R = Ph) 91%
3p(R = Oct) 91%
(37%ee)
Me
4'
2'
R
E
E
O
Ni
Me
7'
Scheme 5.
Table 4. Optimization of Reaction ConditionsaÞ
Me
O
O
10 mol % Ni(cod)2
Me
E
E
ligand
E
E
+
toluene
Et
Et
Et
2c
Et
9a (E = CO2Me)
1.5 mol equiv
10a
Entry
Ligand
20 mol % PBu3
20 mol % P(c-Hex)3
10 mol % IPr
10 mol % IPr
Temp/ꢁC
Time/h
Yield/%bÞ
1
2
3
4
5
100
100
rt
60
100
3
3
3
3
3
63
81
14
59
63
10 mol % IPr
a) A solution of enyne 9a in toluene was added dropwise to the reaction mixture (Entries1 and 2).
b) Isolated yield.
dure using tricyclohexylphosphine as ligand worked well
at 100 ꢁC to give the bicyclo[6.3.0]undecenone 10a in 81%
yield (Entry 2). The structure of 10a was unambiguously
assigned by NMR spectroscopy (1H, 13C, NOESY, HMQC,
and HMBC), and only one regioisomer in which the external
alkyne carbon of enyne 9a was connected to the carbonyl car-
bon of 2c was detected. The enyne 9a was less reactive toward
the [4 þ 2 þ 2] annulation reaction than the diyne 1a, and the
use of the IPr ligand gave a lower yield at both 60 and 100 ꢁC
due to concomitant occurrence of self-oligomerization of 9a
(Entries 3–5).
group failed to undergo the reaction, presumably due to steric
reasons. A tosylamide linker as well as an ether linker could be
included in the skeleton, although the yield was less than those
with malonate based linkers (Entries 5 and 6).
The mechanism of the annulation reaction of enynes would
be similar to that of diynes. We assume that the intermediate
12 is more likely to occur in order to explain the good regio-
selectivity observed. The higher reactivity of the alkyne moie-
ty relative to the alkene moiety for the initial hetero-type oxi-
dative cyclization could be the reason of the selective forma-
tion of the intermediate 12 (Scheme 6).
Under the optimized conditions, various bicyclo[6.3.0]-
undecenones were synthesized by [4 þ 2 þ 2] annulation of
enynes 9 with cyclobutanones 2 (Table 5). Cyclobutanone
2b possessing two substituents at the 3-position reacted with
enyne 9a to give the product in good yield (Entry 1). The
enyne 9b having an ethyl substituent at the alkyne terminus
afforded the product in 86% yield, whereas an enyne with a
terminal alkyne moiety failed to participate in the annulation
reaction due to rapid oligomerization at the terminal alkyne
moiety (Entry 2). The enyne having a 2-methyl-2-propenyl
Conclusion
In summary, we have developed
a nickel-catalyzed
[4 þ 2 þ 2]-type annulation reaction of cyclobutanones with
diynes/enynes, in which a spirocyclic intermediate was ex-
panded through ꢀ-carbon elimination. In the multi-component
annulation reaction, cyclobutanones acted as a useful C4 unit
in combination with alkyne and alkene units. This method
provides a new direct access to bicyclic eight-membered ring
ketones.

890 Bull. Chem. Soc. Jpn. Vol. 81, No. 7 (2008)
Nickel-Catalyzed [4 þ 2 þ 2]-Type Annulation
Table 5. Reaction of Cyclobutanones 2 and Enynes 9 Forming Bicyclo[6.3.0]undecenone 10aÞ
Entry
Enyne 9
Cyclobutanone 2
Product 10, Yield/%bÞ
R
R
O
O
E
E
E
E
Ph
Ph
Ph Ph
(E = CO2Me)
1
2
9a (R = Me)
9b (R = Et)
2b
2b
O
10b (R = Me), 87
10c (R = Et), 86
Me
O
MeO
Me
MeO
MeO
MeO
R
R
R
R
3
4
9c
9c
2b (R = Ph)
2c (R = Et)
O
10d (R = Ph), 87
10e (R = Et), 74
Me
O
Me
Me
O
O
Ph
Ph Ph
Ph
5
6
9d
9e
2b
10f, 32
Me
O
O
TsN
TsN
Et
Et
Et Et
2c
10g, 41
a) Cyclobutanone 2, enyne 9 (1.5 mol equiv to 2), Ni(cod)2 (10 mol %), and P(c-Hex)3
(20 mol %) in toluene at 100 ꢁC for 3 h. b) Isolated yield.
R2
R2
R1
R2 R1
R
R
R1
R
O
10
Ni
Ni
Z
Z
O
Ni
O
Z
11
12
13
Scheme 6.
acid.25 Cyclobutanones 2f and 2g were prepared from cyclopro-
pylidenation of benzaldehyde26 and subsequent mCPBA oxida-
tion.27 Toluene was distilled over sodium/benzophenone ketyl
prior to use. All other commercially available chemical resources
were used without further purifications.
Experimental
All reactions were carried out with standard
General.
Schlenk and glove box techniques under a nitrogen atmosphere.
Preparative thin-layer chromatography was performed with silica
gel 60 PF254 (Merck). 1H and 13C NMR spectra were recorded on
a Varian Gemini 2000 (1H at 300.07 MHz and 13C at 75.46 MHz)
spectrometer or a JEOL JNM-A400 (1H at 399.65 MHz and 13C at
100.40 MHz) spectrometer. Proton chemical shifts were refer-
enced to residual solvent signals in CDCl3 (ꢂ 7.26). Carbon chem-
ical shifts were referenced to the deuterated solvent signals in
CDCl3 (ꢂ 77.00) and C6D6 (ꢂ 128.00). High-resolution mass spec-
tra were recorded on a JOEL JMS-SX102A spectrometer.
General Procedure A.
A toluene solution (0.80 mL) of
Ni(cod)2 (5.5 mg, 0.020 mmol), PBu3 (8.1 mg, 0.040 mmol), and
cyclobutanone 2 (0.20 mmol) was stirred at 100 ꢁC for a few
minutes. To the solution was added dropwise a toluene solution
(0.20 mL) of diyne 1 (0.30 mmol) via syringe over 2.5 h. After
being stirred for a further 0.5 h, the reaction mixture was concen-
trated. The residue was purified by preparative thin-layer chroma-
tography on silica gel to afford product 3.
Materials. Diynes 1a,12 1b,12 1c,13 1d,13 1e,14 1f,15 1g,16 1i,17
and enynes 9a,18 9b,19 9c,20 9d,21 9e,22 and N-heterocyclic car-
benes IPr,23 SIPr,23 IMes,23 ItBu24 were prepared according to
the literature methods. Cyclobutanones 2a–2e were prepared by
[2 þ 2] cycloaddition of the corresponding olefins with dichloro-
ketene and subsequent dechlorination with zinc dust in acetic
General Procedure B. A toluene solution (0.50 mL) of Ni-
(cod)2 (5.5 mg, 0.020 mmol) and IPr (7.8 mg, 0.020 mmol) was
stirred for at least 6 h and added to a mixture of cyclobutanone
2 (0.20 mmol) and diyne 1 (0.24 mmol) in toluene (1.50 mL). Af-
ter being stirred for 1–3 h at room temperature, the reaction mix-
ture was filtered through a pad of FlorisilÒ. The filtrate was con-

S. Ashida et al.
Bull. Chem. Soc. Jpn. Vol. 81, No. 7 (2008)
891
centrated, and the residue was purified by preparative thin-layer
chromatography on silica gel to afford product 3.
1H), 3.21 (dd, J ¼ 12:3, 9.9 Hz, 1H), 3.79 (d, J ¼ 10:7 Hz, 1H),
5.56 (ddd, J ¼ 9:9, 6.5, 1.5 Hz, 1H), 5.91 (s, 1H), 7.15–7.32 (m,
10H); 13C NMR (CDCl3) ꢂ 26.9, 27.2, 36.7, 37.7, 38.6, 49.5,
54.9, 126.1, 126.3, 126.9, 127.0, 127.2, 128.0, 128.2, 129.1,
141.4, 146.3, 146.7, 152.9, 200.7; HRMS (EI) calcd for C24H24O
(Mþ) 328.1827, found 328.1828.
Dimethyl 2,5-Dimethyl-5-phenylbicyclo[6.3.0]undeca-1,7-
dien-3-one-10,10-dicarboxylate (3i): 1H NMR ꢂ 1.37 (s, 3H),
1.92 (s, 3H), 2.17 (br s, 1H), 2.62 (dd, J ¼ 12:1, 8.0 Hz, 2H),
2.98–3.33 (m, 5H), 3.72 (s, 3H), 3.74 (s, 3H), 5.97 (s, 1H),
7.19–7.35 (m, 5H); 13C NMR ꢂ 15.9, 30.0, 39.8, 41.6, 43.0,
48.7, 52.3, 53.0 [two carbons], 55.7, 125.3, 126.1, 128.2, 131.0,
133.1, 140.8, 146.0, 146.5, 171.3, 171.5, 202.3; HRMS (EI) calcd
for C23H26O5 (Mþ) 382.1780, found 382.1780.
Dimethyl 5,7-Dimethyl-5-phenylbicyclo[6.3.0]undeca-1,7-
dien-3-one-10,10-dicarboxylate (3j): 1H NMR ꢂ 1.19 (br s,
3H), 1.42 (s, 3H), 2.34–2.60 (m, 3H), 2.90–3.30 (m, 4H), 3.46
(br s, 1H), 3.73 (s, 3H), 3.76 (s, 3H), 5.99 (s, 1H), 7.20–7.36
(m, 5H); 13C NMR ꢂ 23.1, 29.0, 38.8, 43.6, 48.1, 50.1, 50.3,
53.0, 53.1, 56.1, 125.3, 126.1, 126.4, 128.3, 134.2, 144.1, 146.8,
152.8, 171.3, 171.4, 202.5; HRMS (EI) calcd for C23H26O5
(Mþ) 382.1780, found 382.1784.
Dimethyl 2,5,7-Trimethyl-5-phenylbicyclo[6.3.0]undeca-1,7-
dien-3-one-10,10-dicarboxylate (3a): 1H NMR ꢂ 1.18 (s, 3H),
1.41 (s, 3H), 1.96 (s, 3H), 2.31–2.42 (m, 3H), 2.96 (d, J ¼
15:3 Hz, 1H), 3.11–3.17 (m, 3H), 3.54 (d, J ¼ 9:9 Hz, 1H), 3.72
(s, 3H), 3.75 (s, 3H), 7.20–7.34 (m, 5H); 13C NMR (CDCl3) ꢂ
16.0, 23.0, 29.0, 38.9, 42.0, 47.4, 49.6, 50.4, 52.9, 53.0, 55.7,
125.4, 126.2, 128.2, 131.8, 135.3, 141.3, 147.1, 148.8, 171.6,
171.7, 201.7; HRMS (EI) calcd for C24H28O5 (Mþ) 396.1937,
found 396.1938.
Dimethyl 5-Methyl-5-phenylbicyclo[6.3.0]undeca-1,7-dien-
1H NMR ꢂ 1.38 (s, 3H),
3-one-10,10-dicarboxylate (3b):
2.26–2.33 (m, 1H), 2.61 (br s, 1H), 2.71 (dd, J ¼ 12:8, 8.0 Hz,
1H), 2.98–3.36 (m, 5H), 3.73 (s, 3H), 3.74 (s, 3H), 6.00 (s, 1H),
6.08 (br t, 1H), 7.19–7.26 (m, 1H), 7.31–7.34 (m, 4H); 13C NMR
(CDCl3) ꢂ 30.0, 40.8, 41.7, 42.2, 49.4, 51.6, 53.0 [two carbons],
56.2, 125.2, 126.3, 126.8, 128.4, 133.4, 140.2, 146.4, 150.7,
171.1, 171.2, 202.2; HRMS (EI) calcd for C22H24O5 (Mþ)
368.1624, found 368.1628.
Dimethyl 2,7-Diethyl-5-methyl-5-phenylbicyclo[6.3.0]un-
deca-1,7-dien-3-one-10,10-dicarboxylate (3c):
1H NMR ꢂ
0.50–0.75 (m, 3H), 0.80–1.20 (m, 3H), 1.41 (s, 3H), 1.74–1.93
(m, 1H), 2.18–2.56 (m, 6H), 3.06–3.29 (m, 4H), 3.52 (d, J ¼
10:8 Hz, 1H), 3.72 (s, 3H), 3.75 (s, 3H), 7.17–7.33 (m, 5H);
13C NMR (C6D6) ꢂ 11.5, 13.7, 24.2, 29.0, 29.6, 38.7, 41.5, 46.4,
48.0, 50.0, 52.4, 52.5, 55.9, 125.7, 126.4, 128.5, 135.5, 138.9,
146.7, 147.6, 147.7, 171.5, 171.6, 200.1; HRMS (EI) calcd for
C26H32O5 (Mþ) 424.2250, found 424.2253.
10,10-Bis(methoxymethyl)-2,5,7-trimethyl-5-phenylbicyclo-
[6.3.0]undeca-1,7-dien-3-one (3d): 1H NMR ꢂ 1.14 (s, 3H), 1.42
(s, 3H), 1.93 (s, 3H), 2.20–2.57 (m, 7H), 3.20–3.35 (m, 10H), 3.54
(d, J ¼ 9:9 Hz, 1H), 7.18–7.34 (m, 5H); 13C NMR (CDCl3) ꢂ 16.1,
23.2, 29.0, 37.3, 41.0, 43.0, 47.6, 49.7, 50.5, 59.26, 59.32, 75.7,
76.1, 125.4, 126.1, 128.2, 131.8, 137.8, 140.6, 147.4, 152.6,
201.9; HRMS (EI) calcd for C24H32O3 (Mþ) 368.2351, found
368.2354.
2,5,7-Trimethyl-5-phenylbicyclo[6.3.0]undeca-1,7-dien-3-one
(3e): 1H NMR ꢂ 1.17 (s, 3H), 1.43 (s, 3H), 1.70–1.78 (m, 2H),
1.97 (s, 3H), 2.31–2.70 (m, 7H), 3.59 (d, J ¼ 9:6 Hz, 1H), 7.19–
7.23 (m, 1H), 7.30–7.35 (m, 4H); 13C NMR (C6D6) ꢂ 16.7, 22.0,
23.3, 29.4, 31.9, 35.4, 47.8, 49.9, 50.2, 125.8, 126.3, 128.4,
131.4, 138.9, 139.5, 148.1, 152.6, 200.8; HRMS (EI) calcd for
C20H24O (Mþ) 280.1827, found 280.1827.
2,5,7-Trimethyl-5-phenyl-10-oxabicyclo[6.3.0]undeca-1,7-
dien-3-one (3f): 1H NMR ꢂ 1.18 (br s, 3H), 1.47 (s, 3H), 1.86 (s,
3H), 2.30–2.49 (m, 3H), 3.61 (br s, 1H), 4.51–4.66 (m, 4H), 7.20–
7.37 (m, 5H); 13C NMR (CDCl3) ꢂ 15.5, 23.4, 29.2, 47.5, 49.9,
51.7, 71.9, 73.4, 125.3, 126.4, 128.4, 129.4, 134.9, 141.0, 146.7,
147.3, 202.2; HRMS (EI) calcd for C19H22O2 (Mþ) 282.1620,
found 282.1617.
Dimethyl 2,7-Dimethyl-5,5-diphenylbicyclo[6.3.0]undeca-
1,7-dien-3-one-10,10-dicarboxylate (3k): 1H NMR ꢂ 1.16 (s,
3H), 1.85 (s, 3H), 2.71–2.77 (m, 2H), 3.03 (d, J ¼ 15:6 Hz, 1H),
3.16–3.23 (m, 4H), 3.75 (s, 3H), 3.77 (s, 3H), 3.92 (d, J ¼
9:9 Hz, 1H), 7.10–7.30 (m, 10H); 13C NMR (CDCl3) ꢂ 15.9,
23.0, 39.1, 41.9, 46.6, 47.8, 53.0, 55.7, 58.2, 126.3, 127.1,
127.9, 128.2, 132.1, 136.0, 140.6, 147.0, 147.2, 148.1, 171.6,
171.7, 199.9 [Some sp2 carbon chemical shifts are observed as
nonequivalent signals due to barrier to inversion of the cycloocta-
dienone ring]; HRMS (EI) calcd for C29H30O5 (Mþ) 458.2093,
found 458.2097. Anal. Calcd for C29H30O5: C, 75.96; H, 6.59.
Found: C, 75.72; H, 6.71.
Dimethyl 5,5-Diethyl-2,7-dimethylbicyclo[6.3.0]undeca-1,7-
dien-3-one-10,10-dicarboxylate (3l): 1H NMR ꢂ 0.82 (t, J ¼
7:1 Hz, 6H), 1.27 (br s, 4H), 1.77–2.08 (m, 3H), 1.87 (s, 3H),
1.95 (s, 3H), 2.54 (br s, 1H), 3.05–3.21 (m, 4H), 3.73 (s, 6H);
13C NMR (CDCl3) ꢂ 7.4, 16.0, 24.4, 27.9, 39.2, 42.1, 44.7, 47.5,
51.3, 53.0, 55.7, 131.8, 135.4, 140.8, 148.4, 171.6, 201.7; HRMS
(EI) calcd for C21H30O5 (Mþ) 362.2093, found 362.2094.
Dimethyl 2,7-Dimethyl-5-phenylbicyclo[6.3.0]undeca-1,7-
dien-3-one-10,10-dicarboxylate (3m): 1H NMR ꢂ 1.51 (br s,
3H), 1.93 (s, 3H), 2.38 (br s, 1H), 2.56 (br s, 2H), 3.10–3.24
(m, 5H), 3.65 (br s, 1H), 3.72 (s, 3H), 3.75 (s, 3H), 7.12–7.30
(m, 5H); 13C NMR (CDCl3) ꢂ 16.0, 23.5, 39.0, 41.9 [two carbons],
42.4, 50.4, 52.98, 53.04, 55.7, 126.6, 126.8, 128.3, 131.1, 135.1,
139.5, 143.4, 148.4, 171.6, 171.7, 203.5; HRMS (EI) calcd for
C23H26O5 (Mþ) 382.1780, found 382.1773.
Dimethyl
2,7-Dimethyl-5-octylbicyclo[6.3.0]undeca-1,7-
dien-3-one-10,10-dicarboxylate (3n): 1H NMR ꢂ 0.84 (t, J ¼
6:6 Hz, 3H), 1.23–1.40 (m, 14H), 1.85 (s, 3H), 1.91 (s, 3H),
1.96–2.85 (m, 5H), 3.11 (dd, J ¼ 21:9, 17.4 Hz, 4H), 3.71 (s,
6H); 13C NMR (CDCl3) ꢂ 14.0, 15.8, 22.6, 23.2, 27.4, 29.2,
29.5, 29.6, 31.8, 34.7, 38.9, 40.3, 41.8, 43.2, 46.2, 52.9 [two car-
bons], 55.7, 131.3, 134.4, 140.2, 148.2, 171.6 [two carbons],
203.5; HRMS (EI) calcd for C25H38O5 (Mþ) 418.2719, found
418.2721.
10-(4-Tolylsulfonyl)-2,5,7-trimethyl-5-phenyl-10-azabicyclo-
[6.3.0]undeca-1,7-dien-3-one (3g): 1H NMR ꢂ 1.11 (s, 3H), 1.44
(s, 3H), 1.86 (s, 3H), 2.15–2.42 (m, 3H), 2.43 (s, 3H), 3.27 (br s,
1H), 3.99–4.28 (m, 4H), 7.20–7.35 (m, 7H), 7.72 (d, J ¼ 8:4 Hz,
2H); 13C NMR (CDCl3) ꢂ 16.0, 21.5, 23.4, 29.0, 47.2, 49.7,
51.1, 52.0, 54.1, 125.2, 126.5, 127.7, 128.4, 129.8, 131.4, 132.0,
133.1, 143.0, 144.0, 144.1, 146.4, 201.2; HRMS (EI) calcd for
C26H29NO3S (Mþ) 435.1868, found 435.1868.
Dimethyl 2,7-Dimethyl-4-phenylbicyclo[6.3.0]undeca-1,7-
dien-3-one-10,10-dicarboxylate (3o): 1H NMR ꢂ 1.89 (s, 3H),
1.96 (s, 3H), 2.06–2.19 (m, 2H), 2.45–2.63 (m, 2H), 3.14 (d, J ¼
15:6 Hz, 1H), 3.19 (d, J ¼ 1:2 Hz, 2H), 3.29 (d, J ¼ 15:3 Hz, 1H),
5,5-Diphenylbicyclo[6.4.0]dodeca-1,7-dien-3-one
(3h):
1H NMR ꢂ 1.45–1.71 (m, 2H), 1.86–1.99 (m, 2H), 2.25–2.46
(m, 4H), 2.79 (dd, J ¼ 12:3, 6.5 Hz, 1H), 2.91 (d, J ¼ 10:7 Hz,

892 Bull. Chem. Soc. Jpn. Vol. 81, No. 7 (2008)
Nickel-Catalyzed [4 þ 2 þ 2]-Type Annulation
3.77 (s, 3H), 3.79 (s, 3H), 4.46 (dd, J ¼ 13:1, 4.1 Hz, 1H), 7.19–
7.29 (m, 5H); 13C NMR (CDCl3) ꢂ 16.3, 21.8, 34.5, 39.0, 41.0,
41.8, 51.5, 52.98, 53.04, 56.0, 126.7, 127.9, 129.1, 131.9, 134.0,
139.6, 140.4, 147.3, 171.5, 171.7, 206.1; HRMS calcd (EI) for
C23H26O5 (Mþ) 382.1780, found 382.1779.
(m, 1H), 4.11 (d, J ¼ 11:7 Hz, 1H), 7.06–7.38 (m, 10H); 13C NMR
(CDCl3) ꢂ 15.9, 30.6, 33.5, 40.8, 41.1, 42.0, 46.0, 47.0, 51.3, 59.2,
59.3, 75.5, 76.3, 125.9, 126.0, 127.2, 127.8, 128.3, 134.9, 146.4,
148.6, 158.4, 198.8 [one carbon missing]; HRMS (CI) calcd for
C28H34O3 (Mþ) 418.2508, found 418.2503.
Dimethyl
2,7-Dimethyl-4-octylbicyclo[6.3.0]undeca-1,7-
10,10-Bis(methoxymethyl)-5,5-diethyl-2-methylbicyclo-
[6.3.0]undeca-1-en-3-one (10e): 1H NMR ꢂ 0.77 (q, J ¼ 7:4 Hz,
6H), 1.06–1.40 (m, 8H), 1.80 (s, 3H), 1.81–1.94 (m, 1H), 2.09
(ddd, J ¼ 12:8, 8.9, 1.1 Hz, 1H), 2.30–2.43 (m, 2H), 2.53 (d,
J ¼ 17:7 Hz, 1H), 2.73 (d, J ¼ 11:7 Hz, 1H), 3.11–3.18 (m, 2H),
3.27–3.31 (m, 5H), 3.33 (s, 3H), 3.41–3.54 (m, 1H); 13C NMR
(CDCl3) ꢂ 7.3, 7.4, 15.9, 26.6, 29.0, 30.9, 31.9, 36.8, 40.6, 40.9,
41.7, 45.9, 49.9, 59.2, 59.3, 75.4, 76.6, 134.4, 158.4, 201.2;
HRMS (CI) calcd for C20H34O3 (Mþ) 322.2508, found 322.2502.
2-Methyl-5,5-diphenyl-10-oxabicyclo[6.3.0]undeca-1-en-3-
one (10f): 1H NMR ꢂ 1.41–1.63 (m, 2H), 1.78 (d, J ¼ 1:2 Hz,
3H), 2.23–2.33 (m, 1H), 2.26–2.47 (m, 1H), 3.02 (dd, J ¼ 12:0,
1.5 Hz, 1H), 3.47–3.55 (m, 1H), 3.69 (d, J ¼ 8:7 Hz, 1H), 3.95
(dd, J ¼ 8:7, 6.0 Hz, 1H), 4.17 (d, J ¼ 11:4 Hz, 1H), 4.40 (d,
J ¼ 15:6 Hz, 1H), 4.65 (d, J ¼ 15:9 Hz, 1H), 7.03–7.09 (m, 2H),
7.12–7.18 (m, 1H), 7.19–7.30 (m, 5H), 7.31–7.38 (m, 2H);
13C NMR (CDCl3) ꢂ 14.6, 26.7, 33.2, 42.3, 47.4, 51.4, 73.1,
76.3, 126.1, 126.2, 127.2, 127.3, 127.9, 128.4, 132.1, 146.1,
148.4, 153.3, 198.2; HRMS (CI) calcd for C23H24O2 (Mþ)
332.1776, found 332.1779.
dien-3-one-10,10-dicarboxylate (3p): 1H NMR ꢂ 0.83 (t, J ¼
6:6 Hz, 3H), 1.16–1.30 (m, 14H), 1.63–1.80 (m, 1H), 1.80–1.94
(m, 2H), 1.85 (s, 3H), 1.89 (s, 3H), 2.22–2.36 (m, 1H), 2.99–
3.12 (m, 1H), 3.00 (d, J ¼ 15:3 Hz, 1H), 3.10 (d, J ¼ 0:6 Hz,
2H), 3.20 (d, J ¼ 15:0 Hz, 1H), 3.71 (s, 3H), 3.72 (s, 3H);
13C NMR (CDCl3) ꢂ 14.1, 16.0, 21.6, 22.6, 28.1, 29.2, 29.4,
29.8, 31.3, 31.8, 34.4, 38.9, 41.1, 41.6, 46.5, 52.9 [two carbons],
55.9, 131.5, 133.3, 140.2, 146.5, 171.6, 171.7, 209.3; HRMS
(EI) calcd for C25H38O5 (Mþ) 418.2719, found 418.2720.
General Procedure for Enynes. A toluene solution (0.80 mL)
of Ni(cod)2 (5.5 mg, 0.020 mmol), P(c-Hex)3 (11.2 mg, 0.040
mmol), and cyclobutanone 2 (0.20 mmol) was stirred at 100 ꢁC
for a few minutes. To the solution was added dropwise a toluene
solution (0.20 mL) of enyne 9 (0.30 mmol) via syringe over 2.5 h.
After being stirred for a further 0.5 h, the reaction mixture was
concentrated. The residue was purified by preparative thin-layer
chromatography on silica gel to afford product 10.
Dimethyl 5,5-Diethyl-2-methylbicyclo[6.3.0]undeca-1-en-3-
one-10,10-dicarboxylate (10a):
1H NMR ꢂ 0.75 (q, J ¼
6:9 Hz, 6H), 1.01–1.38 (m, 7H), 1.78–1.90 (m, 1H) 1.82 (s, 3H),
2.00 (dd, J ¼ 16:5, 5.4 Hz, 1H), 2.32 (d, J ¼ 12:0 Hz, 1H), 2.68
(d, J ¼ 12:0 Hz, 1H), 2.81 (dd, J ¼ 13:1, 8.6 Hz, 1H), 3.05 (d,
J ¼ 18:0 Hz, 1H), 3.22 (d, J ¼ 18:0 Hz, 1H), 3.50–3.66 (m, 1H),
3.71 (s, 3H), 3.72 (s, 3H); 13C NMR (CDCl3) ꢂ 7.2, 7.3, 15.7,
26.6, 28.9, 29.5, 31.7, 36.8, 40.9, 42.57, 42.63, 49.8, 52.88,
52.95, 58.0, 134.6, 154.0, 171.7, 171.8, 200.7; HRMS (CI) calcd
for C20H30O5 (Mþ) 350.2093, found 350.2089.
10-(4-Tolylsulfonyl)-5,5-diethyl-2-methyl-10-azabicyclo[6.3.0]-
undeca-1-en-3-one (10g): 1H NMR ꢂ 0.73–0.80 (m, 6H), 1.01–
1.55 (m, 7H), 1.72 (s, 3H), 1.78–1.95 (m, 1H), 2.39 (d, J ¼
13:2 Hz, 1H), 2.43 (s, 3H), 2.61 (d, J ¼ 12:6 Hz, 1H), 3.11 (dd,
J ¼ 9:0, 6.6 Hz, 1H), 3.34 (d, J ¼ 9:0 Hz, 1H), 3.46–3.59 (m,
2H), 4.17 (d, J ¼ 16:2 Hz, 1H), 7.35 (d, J ¼ 8:4 Hz, 2H), 7.71
(d, J ¼ 8:4 Hz, 2H); 13C NMR (CDCl3) ꢂ 7.26, 7.33, 14.9, 21.6,
27.0, 27.1, 29.4, 31.1, 37.3, 41.1, 50.4, 53.7, 55.4, 128.0, 129.7,
131.3, 133.9, 144.0, 149.0, 199.4; HRMS (CI) calcd for
C22H31NO3S (Mþ) 389.2025, found 389.2028.
Dimethyl 2-Methyl-5,5-diphenylbicyclo[6.3.0]undeca-1-en-
1H NMR ꢂ 1.14–1.27 (m,
3-one-10,10-dicarboxylate (10b):
1H), 1.49–1.63 (m, 1H), 1.87 (d, J ¼ 1:2 Hz, 3H), 1.97 (dd,
J ¼ 13:5, 5.1 Hz, 1H), 2.23 (d, J ¼ 14:4 Hz, 1H), 2.35–2.46 (m,
1H), 2.78 (dd, J ¼ 13:4, 8.3 Hz, 1H), 2.99 (dd, J ¼ 11:4, 1.2 Hz,
1H), 3.10 (d, J ¼ 18:0 Hz, 1H), 3.32 (d, J ¼ 18:0 Hz, 1H),
3.50–3.65 (m, 1H), 3.74 (s, 6H), 4.06 (d, J ¼ 11:7 Hz, 1H),
7.04–7.17 (m, 3H), 7.18–7.27 (m, 5H), 7.29–7.37 (m, 2H);
13C NMR (CDCl3) ꢂ 15.8, 29.3, 33.4, 41.0, 42.80, 42.83, 47.0,
51.2, 52.9, 53.0, 58.0, 125.9, 126.1, 127.2, 127.8, 128.3, 135.2,
146.1, 148.4, 154.0, 171.6, 171.8, 198.4 [one carbon missing];
HRMS (CI) calcd for C28H30O5 (Mþ) 446.2093, found 446.2093.
Dimethyl 2-Ethyl-5,5-diphenylbicyclo[6.3.0]undeca-1-en-3-
This work was supported by The Asahi Glass Foundation
and Grants-in-Aid for Scientific Research (A) (No. 19205013)
from the Ministry of Education, Culture, Sports, Science and
Technology, Japan. S. A. is grateful for Research Fellowships
of the Japan Society for the Promotion of Science for Young
Scientists.
References
1
For reviews, see: a) M. Lautens, W. Klute, W. Tam, Chem.
one-10,10-dicarboxylate (10c):
1H NMR ꢂ 0.94 (t, J ¼
Rev. 1996, 96, 49. b) S. Saito, Y. Yamamoto, Chem. Rev. 2000,
100, 2901. c) M. Murakami, Angew. Chem., Int. Ed. 2003, 42,
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7:4 Hz, 3H), 1.15–1.27 (m, 1H), 1.51–1.65 (m, 1H), 1.91 (dd,
J ¼ 13:5, 5.7 Hz, 1H), 2.19–2.50 (m, 4H), 2.79 (dd, J ¼ 12:5,
8.3 Hz, 1H), 2.99 (d, J ¼ 11:1 Hz, 1H), 3.15 (d, J ¼ 17:7 Hz,
1H), 3.33 (d, J ¼ 18:0 Hz, 1H), 3.50–3.65 (m, 1H), 3.75 (s, 6H),
4.03 (d, J ¼ 11:4 Hz, 1H), 7.05–7.17 (m, 3H), 7.18–7.26 (m,
5H), 7.30–7.36 (m, 2H); 13C NMR (CDCl3) ꢂ 13.2, 23.4, 29.4,
33.4, 40.8, 41.9, 42.7, 47.0, 51.4, 52.90, 52.93, 58.0, 126.0,
126.1, 127.2, 127.8, 128.3, 141.7, 146.2, 148.2, 153.4, 171.5,
171.7, 197.8; HRMS (CI) calcd for C29H32O5 (Mþ) 460.2250,
found 460.2254.
10,10-Bis(methoxymethyl)-2-methyl-5,5-diphenylbicyclo-
[6.3.0]undeca-1-en-3-one (10d): 1H NMR ꢂ 1.26–1.40 (m, 2H),
1.54–1.68 (m, 1H), 1.85 (s, 3H), 2.03 (dd, J ¼ 13:1, 8.6 Hz, 1H),
2.25 (d, J ¼ 14:4 Hz, 1H), 2.38–2.65 (m, 3H), 3.00 (d, J ¼
11:1 Hz, 1H), 3.14–3.23 (m, 2H), 3.28–3.40 (m, 8H), 3.43–3.56
2
H. A. Wegner, A. de Meijere, P. A. Wender, J. Am. Chem.
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A preliminary communication of the nickel-catalyzed
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S. Ashida et al.
Bull. Chem. Soc. Jpn. Vol. 81, No. 7 (2008)
893
6
IPr = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene.
5666.
IMes = 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene. SIPr =
1,3-bis(2,6-diisopropylphenyl)imidazolin-2-ylidene. ItBu = 1,3-
di-t-butylimidazol-2-ylidene.
14 Y. Han, C. J. Harlan, P. Stoessel, B. J. Frost, J. R. Norton,
S. Miller, B. Bridgewater, Q. Xu, Inorg. Chem. 2001, 40, 2942.
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