Nickel(0)-mediated [2+2+1] cyclization reaction of chromium carbene complexes and internal alkynes

Article abstract of DOI:10.1016/j.tet.2006.03.117

Alkyl, aryl, and heteroaryl chromium Fischer carbene complexes undergo Ni(0)-mediated [2+2+1] cyclization reaction with internal unactivated and electron-poor internal alkynes to yield highly substituted cyclopentadienes with complete regioselectivity in

Full text of DOI:10.1016/j.tet.2006.03.117

Tetrahedron 62 (2006) 7547–7551
Nickel(0)-mediated [2D2D1] cyclization reaction of
chromium carbene complexes and internal alkynes
*
Jose Barluenga, Pablo Barrio, Lorena Riesgo, Luis A. Lopez and Miguel Tomas
´
´
ꢀ
´
Instituto Universitario de Quımica Organometalica ‘Enrique Moles’, Unidad Asociada al C.S.I.C., Universidad de Oviedo,
ꢀ
´
Juliaꢀn Claverıa 8, Oviedo 33006, Spain
Received 24 November 2005; revised 2 March 2006; accepted 2 March 2006
Available online 5 June 2006
Dedicated to Professor Gunther Wilke for his contribution to the field of organonickel chemistry
¨
Abstract—Alkyl, aryl, and heteroaryl chromium Fischer carbene complexes undergo Ni(0)-mediated [2+2+1] cyclization reaction with
internal unactivated and electron-poor internal alkynes to yield highly substituted cyclopentadienes with complete regioselectivity in most
cases. The intramolecular version of this cyclization has been accomplished with 1,8-diphenyl-1,7-octadiyne to produce indene derivatives.
This three-component [2+2+1] cyclization represents a very uncommon process in the chemistry of Fischer carbene complexes.
Ó 2006 Elsevier Ltd. All rights reserved.
1. Introduction
More importantly, the different nature of chromium and
nickel allowed us to discover novel reaction patterns in the
transition metal carbene chemistry area.9,11 Specifically, we
found that complexes 1 undergo a very unusual [2+2+2+1]
cyclization reaction with terminal alkynes to give selectively
chromium cycloheptatriene complexes (Scheme 2).9
In the last years Fischer carbene complexes have become
useful reagents in synthetic organic chemistry.1 Although
these type of carbene complexes are well known for a number
of transition metals, only a few nickel(0) carbene complexes
have been hitherto reported. Wilke et al.2 and Pinhas and
Simunic3 described the preparation and characterization
of the complexes [L2Ni]C(NR2)Ph] and [(CO)3Ni]
C(OSiMe3)Bu], respectively.4 We and others have studied
recently the transmetalation reaction5 as a convenient route
to access new carbene complexes of late transition metals,
such as Cu(I),6 Pd(0),7 and Rh(I) .8 This methodology proved
to be also useful for the generation of nickel(0) alcoxycar-
bene complexes I from the corresponding chromium com-
plexes 1 and [Ni(cod)2]. The existence of the species I
is validated by (i) the room temperature thermal dimeriza-
tion of the carbene ligand, and (ii) its capability to cyclo-
propanate electron-poor alkenes at room temperature
(Scheme 1).9,10
R1
OMe
R2
OMe
R1
[Ni(cod)2]
CH3CN
R2
R2
Cr(CO)3
+
(CO)5Cr
R2
1
Scheme 2.
This result sharply contrasts with those found previously
after extensive studies with the group 6 carbene precursors
themselves. In this respect, Figure 1 summarizes the most
common cyclization modes of group 6 Fischer carbene
complexes toward terminal and internal alkynes. Thus,
metal aryl carbenes afford variable mixtures of naphthol,
indene, cyclobutenone, and furan derivatives,12 while chro-
mium alkyl carbenes have been reported to yield substituted
cyclopentenones.13
R1
OMe
R1
25°C
MeO
OMe
R1
OMe
R1
[Ni(cod)2]
CH3CN
(CO)5Cr
LnNi
EWG
OMe
R1
EWG
25°C
We report herein that the reaction of chromium carbene
complexes with internal alkynes (Fig. 2) in the presence of
[Ni(cod)2] neither proceeds as outlined in Figure 1 nor via
a [2+2+2+1] cyclization as for terminal alkynes (Scheme
2), but a new reaction pathway for Fischer carbene com-
plexes, namely the [2+2+1] cyclization, actually operates.14
It will be also noted that the mode of assembling of both
alkyne units into the resulting cyclopentadiene depends
primarily on the electronic demand of the alkyne.
1
I
Scheme 1.
Keywords: Carbene complexes; Chromium(0); Nickel(0); Carbene transfer;
Alkynes; Cyclopentadienes; Three-component process.
* Corresponding author. Tel./fax: +34 98 5103450; e-mail: barluenga@
uniovi.es
0040–4020/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2006.03.117

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J. Barluenga et al. / Tetrahedron 62 (2006) 7547–7551
OMe
OMe
R
OMe
Ph
O
OMe
Ph
[Ni(cod)2]
CH3CN
O
R
(CO)5Cr
+
Et
Et
+
Ph
Et
Et
R
R
OMe
Ph
OH
1a
2a
3
R
R
(CO)5M
OMe
80ºC
OMe
Scheme 3.
O
Ph
OMe
M = Cr, W
+
R
Interestingly, starting from appropriate carbene complex/
alkyne combinations resulted in the complete inhibition of
cyclobutenone formation in favor of a three-component
[2+2+1] cyclization reaction (Scheme 4). Thus, [Ni(cod)2]
was added to a solution of 3-hexyne and methylcarbene 1b
(R1¼Me) in acetonitrile at ꢀ10 ꢁC (molar ratio 1:2.5:1)
and the mixture allowed to reach room temperature.
Removal of solvent and alkyne excess followed by column
chromatographypurification afforded the denselysubstituted
cyclopentadiene 4 in 52% yield.
R
R
R
O
R2
R2
R2
(CO)5Cr
R1
R2
R1
OMe
Figure 1.
OMe
R1
R2
R3
(CO)5Cr
The regioselectivity of this rare cyclization was then tested
with 1-phenyl-1-propyne 2b. In this case, the reaction of
such an alkyne with carbene complexes 1a (R1¼Ph) and
1c (R1¼2-furyl) under the above reaction conditions led to
unsymmetrical cycloadducts 5a,b (48 and 51% yield) as
a sole isomer in each case.
1a, R1 = Ph
1b, R1 = Me
2a, R2 = R3 = Et
2b, R2 = Me, R3 = Ph
2c, R2 = Ph, R3 = COOMe
2d, R2 = Me, R3 = COOMe
(CH2)4
1c, R1 = 2-Furyl
1d, R1 = p-MeOC6H4
2e, R2 = Ph, R3
=
Ph
Figure 2.
It was delightful to find that alkynes with an electron-with-
drawing substituent are productive components in this reac-
tion to provide functionalized cycloadducts (Scheme 5). For
instance, the cyclization of the carbene complex 1a and
methyl phenylpropynoate 2c resulted in the formation of
a mixture of symmetrical and unsymmetrical cyclopenta-
dienes 6 and 7, respectively, in 62% yield (6/7 ratio ¼ 1:1).
In contrast, the cyclization between 1a,d and methyl 2-
butynoate 2d resulted in the regioselective formation of
the cycloadducts 8a,b in 51–56% yield. The structural
assignment of 1,4-diphenyl-1,3-cyclopentadiene 6 and
1,4-dimethyl-1,3-cyclopentadiene 8 over 2,3-diphenyl- and
2. Results and discussion
First we examined the reaction of chromium phenyl carbene
complex 1a with 3-hexyne 2a and [Ni(cod)2] (molar ratio
1:3:1) in acetonitrile. The reagents were mixed at ꢀ10 ꢁC
and allowed to reach room temperature during 2 h. The reac-
tion mixture was chromatographed giving rise to the
[2+1+1] cycloadduct 3 (25% yield)12 as the sole isolable
reaction product (Scheme 3).15
Et
Ph
Et
Et
Ph
[Ni(cod)2]
(R1 = Ph, 2-Furyl)
Me
Et
Et
Me
Ph
OMe
R1
OMe
Me
OMe
R1
2a
2b
(CO)5Cr
[Ni(cod)2]
(R1 = Me)
Et
4 (52%)
Me
5a (R1 = Ph, 48%)
1
5b (R1 = 2-Furyl, 51%)
Scheme 4.
R1
R1
MeO
Ph
MeO
Ph
Ph
COOMe
2c
Ph
COOMe
+
[Ni(cod)2]
(R1 = Ph)
COOMe
6 (30%)
Ph
7 (32%)
MeOOC
MeOOC
OMe
R1
(CO)5Cr
R1
1
MeO
Me
COOMe
Me
Me
2d
[Ni(cod)2]
COOMe
MeOOC
(R1 = Ph, p-MeOC6H4) 8a (R1 = Ph, 51%)
8b (R1 = p-MeOC6H4, 56%)
Scheme 5.

J. Barluenga et al. / Tetrahedron 62 (2006) 7547–7551
7549
2,3-dimethylcyclopentadiene regioisomers (not shown) be-
comes apparent from NMR experiments (including HMQC,
HMBC, and NOESY).
yielding 7 and 6, respectively.20,21 We have no further evi-
dences so as to decide the actual mechanism in the case of
the intramolecular cyclization. While the single, neutral
alkyne would suggest the insertion route, the entropic argu-
ment would facilitate the assembling of both reaction
components and thus would favor the nickelacyclopenta-
diene pathway.
The scope of this cyclization could be successfully extended
to the intramolecular version, thus allowing to access bicy-
clic cyclopentadienes (Scheme 6). This task was brought
about by reacting carbene complexes 1a,b and 1,8-di-
phenyl-1,7-octadiyne 2e under the above reaction conditions
to produce tetrahydroindenes 9a,b (52–56% yield).
3. Conclusion
Ph
In conclusion, we have developed a highly efficient inter-
and intramolecular [2+2+1] cyclization of chromium (meth-
oxy)carbene complexes with internal alkynes and diynes
in the presence of [Ni(cod)2]. Except for methyl phenylpro-
piolate, the reaction is completely regioselective in all cases
tested. Depending on the electronic nature of the alkyne dif-
ferent reaction pathways are proposed beginning with
a nickel(0) Fischer carbene complex, namely carbene/alkyne
insertion and alkyne/nickel cyclometalation. This approach
provides a convenient entry to highly substituted cyclo-
pentadiene and indene derivatives.22 Further studies aimed
at investigating the scope of this new reaction are currently
ongoing in our laboratories.
Ph
OMe
R1
[Ni(cod)2]
CH3CN
OMe
R1
+
(CO)5Cr
Ph
Ph
9a (R1 = Ph, 52%)
9b (R1 = Me, 56%)
1
2e
Scheme 6.
Concerning the reaction pathway, it should be noted that
a common mechanism cannot operate in all cases since a di-
vergent regiochemistry is observed depending on the nature
of the alkyne (compare cyclopentadienes 5 vs 8). A tentative
rationale for the alkynes 1-phenyl-1-propyne 2b and methyl
2-butynoate 2d is outlined in Scheme 7. The chromium–
nickel exchange to form the nickel carbene species I is
assumed to be the common initial step. Two consecutive
regioselective insertion reactions of 1-phenyl-1-propyne into
the nickel–carbon double bond would generate the 1-nickela-
1,3,6-hexatriene II, which would produce the final cycload-
ducts 5 upon cyclization to the nickelacycle III and Ni(0)
reductive elimination (via A).16 On the contrary, the symmet-
rical structure of cycloadducts 8 makes this mechanism
unsatisfactory. On the ground of a recent proposal by Ni
and Montgomery involving nonheteroatom stabilized car-
bene nickel(0) species and dienynes,17 we propose that the
formation of the symmetrical products 8 begins with the
oxidative co-cyclization of nickel carbene complex I with
two alkyne units to form regioselectively the nickelacyclo-
pentadiene IV (via B).18 Then, carbene insertion into the
nickel–carbon bond would allow production of nickelacyclo-
hexadiene V, which would provide 8 by reductive elimina-
tion.19 When phenylpropiolate, instead of methylpropiolate
is employed, we suspect that both mechanisms, double inser-
tion (via A) and cyclometalation (via B), are productive
4. Experimental
4.1. General methods
All reactions were carried out under N2 using standard
Schlenck techniques. Fischer carbene complexes 1a–d23
and 1,8-diphenyl-1,7-octadiyne24 were prepared according
to literature procedures. All common reagents were obtained
from commercial suppliers and used without further purifi-
cation unless otherwise indicated. Acetonitrile was distilled
from CaH2. TLC was performed on aluminum-backed
plates coated with silica gel 60 with F254 indicator. Flash
column chromatography was carried out on silica gel 60
(230–240 mesh). 1H NMR (300 MHz) and 13C NMR
(70.5 MHz) spectra were measured in CDCl3 at room tem-
perature on a Bruker AC-300 instrument, with tetramethyl-
1
silane (d¼0.0, H NMR) and CDCl3 (d¼77.0, 13C NMR)
as internal standard. Elemental analyses were carried out
on Perkin–Elmer 2400 and Carlo Erba 1108 microanalyzers.
Ph
Ph
Me
Ph
[Ni]
R1
Me
Ph
- [Ni]
[Ni]
R1
MeO
2b
5
via A
OMe
Me
III
MeO
Me
(CO)5Cr
II
R1
OMe
R1
+
[Ni]
[Ni(cod)2]
MeO
R1
OMe
R1
I
[Ni]
Me
MeOOC
- [Ni]
[Ni]
2d
Me
MeOOC
Me
COOMe
8
via B
Me
COOMe
V
IV
Scheme 7.

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J. Barluenga et al. / Tetrahedron 62 (2006) 7547–7551
4.2. General procedure for the nickel(0)-mediated
[2D2D1] reaction of Fischer carbene complexes 1
with internal alkynes
4.2.4.1. 5-Methoxy-2,3-bis(methoxycarbonyl)-1,4,5-
triphenyl-1,3-cyclopentadiene (6). The title compound 6
was isolated as a yellowish oil (66 mg, 30%); [Found: C,
76.48; H, 5.37. C28H24O5 requires C, 76.35; H, 5.49%]; dH
(300 MHz, CDCl3) 7.45–7.13 (15H, m, Ph), 3.78 (6H, s,
COOMe), 3.34 (3H, s, OMe); dC (70.5 MHz, CDCl3)
167.7, 152.3, 138.0, 136.9, 134.5, 128.7, 128.1, 127.9,
127.7, 96.3, 52.3, 51.7.
To a solution of complex 1 (1.0 equiv) and alkyne (2.5 equiv)
or diyne (1.1 equiv) in acetonitrile was added [Ni(cod)2]
(1.1 equiv) at ꢀ10 ꢁC. The reaction mixture was allowed to
reach room temperature during 2 h. The solvent was then
removed and the resulting residue was subjected to flash
chromatography (SiO2, mixtures of hexane/ethyl acetate)
to give pure cyclopentadiene and indene derivatives.
4.2.4.2. 5-Methoxy-1,3-bis(methoxycarbonyl)-2,4,5-
triphenyl-1,3-cyclopentadiene (7). The title compound 7
was isolated as a yellowish oil (70 mg, 32%); [Found: C,
76.24; H, 5.52. C28H24O5 requires C, 76.35; H, 5.49%]; dH
(300 MHz, CDCl3) 7.57–7.22 (15H, m, Ph), 3.56 (3H, s,
OMe), 3.44 (3H, s, OMe), 3.42 (3H, s, OMe); dC
(70.5 MHz, CDCl3) 166.2, 162.7, 154.3, 153.3, 136.6,
135.8, 135.5, 133.6, 131.8, 129.2, 128.6, 128.3, 128.2,
127.8, 127.7, 127.4, 125.1, 94.3, 52.1, 52.0, 51.0.
4.2.1. 1,2,3,4-Tetraethyl-5-methoxy-5-methyl-1,3-cyclo-
pentadiene (4). The general procedure was followed
using complex 1b (125 mg, 0.5 mmol), 3-hexyne (103 mg,
1.25 mmol), and [Ni(cod)2] (151 mg, 0.55 mmol) in aceto-
nitrile (7 ml). Final chromatographic purification using a
10:1 mixture of hexane/ethyl acetate as eluent afforded the
title compound 4 (58 mg, 52%) as a colorless oil; [Found:
C, 81.22; H, 11.70. C15H26O requires C, 81.02; H,
11.79%]; dH (300 MHz, CDCl3) 2.84 (3H, s, OMe), 2.29–
2.10 (8H, m, CH2–CH3), 1.24 (3H, s, Me), 1.10 (6H, t,
J 7.9 Hz, CH2–CH3), 1.06 (6H, t, J 7.9 Hz, CH2–CH3); dC
(70.5 MHz, CDCl3) 141.6, 141.4, 89.4, 51.1, 20.4, 18.6,
17.6, 14.8, 13.8.
4.2.5. 5-Methoxy-2,3-bis(methoxycarbonyl)-1,4-di-
methyl-5-phenyl-1,3-cyclopentadiene (8a). The general
procedure was followed using complex 1a (156 mg,
0.5 mmol), methyl butynoate (123 mg, 1.25 mmol), and
[Ni(cod)2] (151 mg, 0.55 mmol) in acetonitrile (7 ml). Final
chromatographic purification using a 5:1 mixture of hex-
ane/ethyl acetate as eluent afforded the title compound 8a
(81 mg, 51%) as a yellowish oil; [Found: C, 68.53; H, 6.51.
C18H20O5 requires C, 68.34; H, 6.37%]; dH (300 MHz,
CDCl3) 7.33–7.03 (5H, m, Ph), 3.60 (6H, s, COOCH3),
3.23 (3H, s, OCH3), 2.40 (6H, s, C]C–CH3); dC
(70.5 MHz, CDCl3) 163.5, 154.8, 138.2, 137.5, 127.7,
126.7, 124.9, 92.1, 51.6, 51.1, 13.2.
4.2.2. 5-Methoxy-1,3-dimethyl-2,4,5-triphenyl-1,3-cyclo-
pentadiene (5a). The general procedure was followed
using complex 1a (156 mg, 0.5 mmol), 1-phenyl-1-propyne
(145 mg, 1.25 mmol), and [Ni(cod)2] (151 mg, 0.55 mmol)
in acetonitrile (7 ml). Final chromatographic purification
using a 20:1 mixture of hexane/ethyl acetate as eluent
afforded the title compound 5a (84 mg, 48%) as a colorless
oil; [Found: C, 88.52; H, 6.94. C26H24O requires C, 88.60;
H, 6.86%]; dH (300 MHz, CDCl3) 7.37–7.02 (15H, m, Ph),
3.27 (3H, s, OMe), 2.11 (3H, s, Me), 1.58 (3H, s, Me); dC
(70.5 MHz, CDCl3) 144.2, 142.7, 141.4, 140.3, 140.1,
135.5, 135.3, 129.3, 128.2, 128.1, 128.0, 127.9, 127.0,
126.6, 126.2, 125.3, 94.4, 50.9, 14.3, 10.2.
4.2.6. 5-Methoxy-2,3-bis(methoxycarbonyl)-5-(4-
methoxyphenyl)-1,4-dimethyl-1,3-cyclopentadiene (8b).
The general procedure was followed using complex 1d
(171 mg, 0.5 mmol), methyl butynoate (123 mg, 1.25 mmol),
and [Ni(cod)2] (151 mg, 0.55 mmol) in acetonitrile (7 ml).
Final chromatographic purification using a 5:1 mixture of
hexane/ethyl acetate as eluent afforded the title compound
8b (97 mg, 56%) as a yellowish oil; [Found: C, 65.73;
H, 6.29. C19H22O6 requires C, 65.88; H, 6.40%]; dH
(300 MHz, CDCl3) 7.26 (2H, d, J 8.9 Hz, Ar), 6.77 (2H, d,
J 8.9 Hz, Ar), 3.77 (3H, s, ArOCH3), 3.62 (6H, s, COOCH3),
3.21 (3H, s, OCH3), 2.38 (6H, s, C]C–CH3); dC (70.5 MHz,
CDCl3) 163.5, 158.3, 154.4, 138.0, 129.2, 126.0, 113.1, 91.8,
55.0, 51.6, 51.1, 13.2. HRMS (EI) calcd for C19H22O6:
346.1416; found 346.1416 [M]+.
4.2.3. 5-(2-Furyl)-5-methoxy-1,3-dimethyl-2,4-diphenyl-
1,3-cyclopentadiene (5b). The general procedure was
followed using complex 1c (151 mg, 0.5 mmol), 1-phenyl-
1-propyne (145 mg, 1.25 mmol), and [Ni(cod)2] (151 mg,
0.55 mmol) in acetonitrile (7 ml). Final chromatographic
purification using a 40:1 mixture of hexane/ethyl acetate
as eluent afforded the title compound 5b (87 mg, 51%) as
a colorless oil; [Found: C, 84.25; H, 6.37. C24H22O2 requires
C, 84.18; H, 6.48%]; dH (300 MHz, CDCl3) 7.31 (1H, d,
J 1.7 Hz, Fu), 7.30–7.15 (10H, m, Ph), 6.46 (1H, dd, J 3.2
and 1.7 Hz, Fu), 6.34 (1H, dd, J 3.2 and 0.6 Hz, Fu), 3.56
(3H, s, OMe), 1.87 (3H, s, Me), 1.79 (3H, s, Me); dC
(70.5 MHz, CDCl3) 162.5, 153.6, 141.9, 141.4, 139.5,
135.9, 135.8, 129.3, 128.4, 127.9, 127.4, 126.7, 118.8,
110.1, 107.5, 63.7, 60.84, 12.0, 10.8. HRMS (EI) calcd for
C24H22O2: 342.1620; found 342.1635 [M]+.
4.2.7. 4,5,6,7-Tetrahydro-2-methoxy-1,2,3-triphenyl-2H-
indene (9a). The general procedure was followed using
complex 1a (156 mg, 0.5 mmol), 1,8-diphenyl-1,7-octadiyne
(142 mg, 0.55 mmol), and [Ni(cod)2] (151 mg, 0.55 mmol)
in acetonitrile (7 ml). Final purification using a 20:1 mixture
of hexane/ethyl acetate afforded the title compound 9a
(98 mg, 52%) as a colorless oil; [Found: C, 88.92; H, 6.83.
C28H26O requires C, 88.85; H, 6.92%]; dH (300 MHz,
CDCl3) 7.29–7.12 (15H, m, Ph), 3.21 (3H, s, OMe),
2.88–2.72 (4H, m, C]C–CH2–CH2), 1.84–1.73 (4H, m,
C]C–CH2–CH2); dC (70.5 MHz, CDCl3) 141.2, 141.1,
140.6, 134.7, 127.9, 126.2, 125.2, 95.3, 51.0, 26.9, 23.6.
4.2.4. Reaction of complex 1a and methyl phenylpro-
pynoate 2c. The general procedure was followed using com-
plex 1a (156 mg, 0.5 mmol), methyl phenylpropynoate
(200 mg, 1.25 mmol), and [Ni(cod)2] (151 mg, 0.55 mmol)
in acetonitrile (7 ml). Final chromatographic purification us-
ing a 5:1 mixture of hexane/ethyl acetate as eluent afforded
compounds 6 and 7.
4.2.8. 4,5,6,7-Tetrahydro-2-methoxy-2-methyl-1,3-
diphenyl-2H-indene (9b). The general procedure was

J. Barluenga et al. / Tetrahedron 62 (2006) 7547–7551
7551
´
11. (a) Barluenga, J.; Vicente, R.; Barrio, P.; Lopez, L. A.; Tomas,
M. J. Am. Chem. Soc. 2004, 126, 5974; (b) Barluenga, J.;
ꢀ
followed using complex 1b (125 mg, 0.5 mmol), 1,8-di-
phenyl-1,7-octadiyne (142 mg, 0.55 mmol), and [Ni(cod)2]
(151 mg, 0.55 mmol) in acetonitrile (7 ml). Final purifica-
tion using a 20:1 mixture of hexane/ethyl acetate afforded
the title compound 9b (88 mg, 56%) as a colorless oil;
[Found: C, 87.42; H, 7.71. C23H24O requires C, 87.30; H,
7.64%]; dH (300 MHz, CDCl3) 7.49–7.30 (10H, m, Ph),
3.20 (3H, s, OMe), 2.91–2.75 (4H, m, C]C–CH2–CH2),
1.84–1.71 (4H, m, C]C–CH2–CH2), 1.38 (3H, s, Me); dC
(70.5 MHz, CDCl3) 140.0, 138.6, 135.4, 128.1, 127.8,
126.3, 91.5, 51.2, 26.1, 23.3, 23.1.
´
Vicente, R.; Barrio, P.; Lopez, L. A.; Tomas, M.; Borge, J.
J. Am. Chem. Soc. 2004, 126, 14354.
ꢀ
12. Wulff, W. D.; Bax, B. M.; Brandvold, T. A.; Chan, K. S.;
Gilbert, A. M.; Hsung, R. P.; Mitchell, J.; Clardy, J.
Organometallics 1994, 13, 102.
13. Challener, C. A.; Wulff, W. D.; Anderson, B. A.; Chamberlin,
S.; Faron, K. L.; Kim, O. K.; Murray, C. K.; Xu, Y. C.; Yang,
D. C.; Darling, S. D. J. Am. Chem. Soc. 1993, 115, 1359.
14. (a) For an isolated example involving the [2+2+1] cyclization
of pentacarbonyl[(N,N-dimethylamino)methylene]chromium
with methyl acrylate and acrylonitrile, see: Sierra, M. A.;
Soderberg, B.; Lander, P. A.; Hegedus, L. S. Organometallics
1993, 12, 3769; (b) For the thermal [2+2+1] cyclization of phe-
nylallene and chromium carbene complexes, see: Aumann, R.;
Uphoff, J. Angew. Chem., Int. Ed. Engl. 1987, 26, 357.
15. In terms of comparison, the reaction of 1a and 3-hexyne
has been reported by Wulff et al. to take place at 80 ꢁC produc-
ing variable mixtures wherein cyclobutenone 3 constitutes a
minor component (0–23% yield, depending on the reaction
conditions).
Acknowledgements
´
We are grateful to the Ministerio de Educacion y Ciencia
(MEC, Project BQU2001-3853), the Principado de Asturias
´
´
(Project PR-01-GE-9), and the Fundacion Ramon Areces for
the support of this research. P.B. thanks the Principado de
Asturias for a predoctoral fellowship.
References and notes
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observed in the present case.
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