Metal-Controlled Selective [3+2] Cyclization Reactions of Alkenyl Fischer Carbene Complexes and Allenes

Article abstract of DOI:10.1021/ja049200r

Chromium alkenyl Fischer carbene complexes undergo selective [3+2] cyclization reactions to allenes in the presence of nickel(0) or rhodium(I) catalysts. Interestingly, the chemo- and regioselectivity of the process are entirely dependent on the nature of the metal. Copyright

Full text of DOI:10.1021/ja049200r

Published on Web 04/27/2004
Metal-Controlled Selective [3+2] Cyclization Reactions of Alkenyl Fischer
Carbene Complexes and Allenes
Jose´ Barluenga,* Rube´n Vicente, Pablo Barrio, Luis A. Lo´pez, and Miguel Toma´s
Instituto UniVersitario de Qu´ımica Organometa´lica “Enrique Moles”, Unidad Asociada al CSIC,
UniVersidad de OViedo, Julia´n ClaVer´ıa 8, 33071 OViedo, Spain
Received February 13, 2004; E-mail: barluenga@uniovi.es
Scheme 2. Ni(0)-Mediated [3+2] Cyclization of Chromium Alkenyl
Carbene Complexes 1 with Allenes 2
Allenes are versatile building blocks with growing importance
in organic synthesis and in transition metal-mediated carbon-carbon
bond forming reactions.1 The major synthetic limitation is concerned
with the increasing selectivity problems derived of the presence of
two reactive orthogonal double bonds, so that the control of the
allene constitutional group selectivity represents currently an
important challenge.2 On the other hand, Fischer-type carbene
complexes have become valuable tools in stoichiometric transition
metal-mediated organic synthesis.3 Whereas the benzannulation
reaction with alkynes represents probably the most relevant process
of Fischer carbene complexes,4 studies of their reactivity toward
allenes are restricted to the simplest systems.5,6 In 1987, Aumann
found that pentacarbonyl[(methoxy)benzylidene]chromium(0) forms
metal trimethylenemethane complexes with allenes,5a while Hwu
et al. described some years later the insertion reaction of pentac-
arbonyl[(methoxy)ethylidene]chromium(0) and -tungsten(0) into the
C-H bond of vinylidene cyclopropanes.6 In both cases, the authors
suggest the occurrence of an alkenylidenemetallacyclobutane
intermediate which undergoes reductive demetalation or Cr-Hâ
elimination/reductive demetalation, respectively. We report herein
that pentacarbonyl[alkenyl(methoxy)carbene]chromium(0) com-
plexes undergo selective [3+2] cycloaddition to allenes in the
presence of nickel(0) or rhodium(I) species, the nature of the metal
being responsible for the chemoselectivity.
removal of the solvent, and column chromatography purification
allowed one to isolate 4-alkylidenecyclopentenes 4 in 60-78% yield
(Scheme 2, Table 1).10,11 This [3+2] cyclization is chemo- and
regioselective, the unsubstituted allene CdC being solely involved.
The formation of cycloadducts 4 can be explained by chromium-
nickel exchange9,12 followed by [2+2] cycloaddition through the
less substituted CdC to generate the intermediates II and III, and
reductive elimination of Ni(0).
The thermal reaction (1,2-dichloroethane, 60 °C) of pentacar-
bonyl[E-styryl(methoxy)carbene]chromium(0) 1a and 1,1-diphenyl-
or 1,1-dimethyl allenes 2 leads, after silica gel treatment, to
4-phenyl-3-buten-2-one 3 in 70-80% yield. This metathesis
reaction, a very unusual process for alkoxy carbenes of chromium,7
is in agreement with the participation of a metallacyclobutane
species I that evolves via the retro [2+2] cycloaddition, rather than
through reductive metal elimination, as found for simple carbenes
(vide supra) (Scheme 1).
The particular nature of rhodium carbene complexes13 prompted
us at this point to examine whether a new reaction course might
operate. Gratifyingly, when carbene complexes 1 were mixed with
1,1-disubstituted allenes 2 (1.5-3 equiv) (CH2Cl2, 25 °C, 18-36
h) in the presence of 10 mol % of [(naphthalene)(cod)Rh][SbF6],
cyclopentene derivatives 5a-h were obtained in good yields, after
column chromatography (Scheme 3, Table 1). The reaction of
carbene complexes 1 with monosubstituted allenes under the same
reaction conditions afforded exclusively the trans-configurated
Table 1. Cyclization of Carbene Complexes 1 and Allenes 2 to
Cyclopentenes 4 and 5 in the Presence of Ni(0) and Rh(I)
Scheme 1. Thermal Reaction of Alkenyl Carbene Complex 1a
and 1,1-Disubstituted Allenes 2
1
2
3
entry
R
R
R
4 (%)a
5 (%)a
1
2
3
p-MeOC6H4
Ph
2-furyl
Ph
Ph
Ph
Ph
Ph
Ph
4a (70)
5a (75)
5b (84)
5c (92)
4
5
6
7
8
9
10
11
12
13
14
Ph
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
4b (78)
4c (68)
4d (69)
p-MeOC6H4
2-furyl
ferrocenyl
1-cyclohexenyl
p-MeOC6H4
2-furyl
p-MeOC6H4
2-furyl
p-MeOC6H4
p-MeOC6H4
5d (82)
5e (65)
5f (42)
5g (93)
-(CH2)5-
-(CH2)5-
Considering that Ni(0) is a well-known catalyst in carbon-carbon
bond formation with allenes8 and our own experience on the change
of reactivity of chromium carbene complexes toward alkynes when
Ni(cod)2 is present,9 the reaction of alkenyl carbenes 1 and allenes
2 (3 equiv) was carried out in the presence of 1 equiv of Ni(cod)2
(toluene, 25 °C, 3-4 h). Exposure of the reaction crude to light,
4e (72)
Ph
Ph
Ph
Me
H
H
5h (68)b
5i (78)
5j (81)
5k (77)
CH2CH2OH
H
a Yields of isolated products. b Isolated as a 1.5:1 trans/cis mixture.
9
5974
J. AM. CHEM. SOC. 2004, 126, 5974-5975
10.1021/ja049200r CCC: $27.50 © 2004 American Chemical Society

C O M M U N I C A T I O N S
Scheme 3. Rh(I)-Catalyzed [3+2] Cyclization of Chromium
Alkenyl Carbene Complexes 1 with Allenes 2
Acknowledgment. This work is respectfully dedicated to the
memory of Dr. Juan C. Del Amo (1976-2004) who was a victim
of the March-11 tragedy in Madrid. Financial support for this work
is acknowledged (BQU2001-3853 and PR-01-GE-9). R.V. and P.B.
thank the Ministerio de Ciencia y Tecnolog´ıa and the Principado
de Asturias for predoctoral fellowships.
Supporting Information Available: Experimental procedures and
spectral and analytical data for all products. This material is available
free of charge via the Internet at http://pubs.acs.org.
References
(1) For reviews, see: (a) The Chemistry of Ketenes, Allenes and Related
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A. Chem. ReV. 2000, 100, 3067.
(2) Hashmi, A. S. K. Angew. Chem., Int. Ed. 2000, 39, 3590.
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cyclopentene derivatives 5i-k. Different types of bicyclic structures,
like 5g (from vinylidenecyclohexane), 5l (from 1,2-nonadiene), and
5m (from pentacarbonyl [methoxy(4H-5,6-dihydropyran-2-yl)-
carbene]chromium), are readily accessible. This unprecedented
rhodium-catalyzed [3+2] cycloaddition reaction of chromium
alkenyl Fischer carbene complexes and allenes takes place with
complete chemo-, regio-, and stereoselectivity.10,11 Interestingly,
only the more substituted CdC of the allene is now involved.14
The mechanistic proposal (Scheme 4) is based on our previous
report and takes into consideration the less carbene character of
the rhodium carbene.13 The process would be initiated by chromium-
rhodium exchange, followed by a nonconcerted metalla-[4+2]
cycloaddition to form V and reductive metal elimination. Thus, the
observed chemo- and regioselectivity would be rationalized by
assuming a charge-developing transition state of type IV. The exo-
approach (trans-cycloadducts 5i-k, 5l) would be conceivable in
terms of minimizing steric interactions.
(4) Do¨tz, K. H. Angew. Chem., Int. Ed. Engl. 1984, 23, 587.
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(b) Aumann, R.; Melchers, H.-D. J. Organomet. Chem. 1988, 355, 351.
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methoxy(phenyl)carbenes, see: Barluenga, J.; Aznar, F.; Mart´ın, A.
Organometallics 1995, 14, 1429. (c) For the synthesis of chromium
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W. C.; Nieger, M.; Do¨tz, K. H. Chem.-Eur. J. 1999, 5, 2014. (d) For the
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Scheme 4. Proposed Mechanism for the Rh(I)-Catalyzed [3+2]
Cyclization of Chromium Alkenyl Carbene Complexes 1 with
Allenes 2
(9) Barluenga, J.; Barrio, P.; Lo´pez, L. A.; Toma´s, M.; Garc´ıa-Granda, S.;
Alvarez-Ru´a, C. Angew. Chem., Int. Ed. 2003, 42, 3008.
(10) The chemo-, regio-, and stereochemistry of compounds 4 and 5 were
ascertained by NMR experiments.
(11) Compounds 4d and 5d were hydrolized to 2- and 3-alkylidenecyclopen-
tanones 6 (80%) and 7 (90%), respectively.
In conclusion, we have discovered a new [3+2] carbocyclization
of alkenyl carbene complexes toward neutral allenes which shows
that expanding the synthetic potential of Fischer carbene complexes
is feasible. While chromium carbenes simply undergo thermal
metathesis through the terminal CdC bond,15 nickel and rhodium
carbenes cleanly cycloadd to monosubstituted, 1,1-disubstituted, and
1,3-disubstituted allenes with complete and complementary chemo-
and regioselectivity. This may be regarded as an unusual way for
selectivity tuning of allenes. From a synthetic point of view, it can
be envisioned that either 2-alkylidene and particularly 3-alkylidene
cyclopentanone derivatives can be readily synthesized starting from
a single series of alkenylcarbene complexes. Although no mecha-
nistic studies have been performed as yet, the specific behavior of
either metal carbene stimulates us to pursue further studies in this
field.
(12) (a) Review of group 6 metal exchange: Liu, S.-T.; Reddy, K. R. Chem.
Soc. ReV. 1999, 28, 315. (b) For Cr/Pd exchange, see: Sierra, M. A.; del
Amo, J. C.; Manchen˜o, M. J.; Go´mez-Gallego, M. J. Am. Chem. Soc.
2001, 123, 851. (c) For Cr/Rh exchange, see: Go¨ttker-Schnetmann, I.;
Aumann, R. Organometallics 2001, 20, 346.
(13) Previous work of alkoxy carbene complexes of rhodium: Barluenga, J.;
Vicente, R.; Lo´pez, L. A.; Rubio, E.; Toma´s, M.; Alvarez-Ru´a, C. J. Am.
Chem. Soc. 2004, 126, 470.
(14) This unusual selectivity of allenes has been reported in the cyclopropa-
nation reaction of the nonheteroatom stabilized benzylidenepentacarbonyl
tungsten: Fischer, H.; Didell, W.; Hofmann, J. J. Chem. Soc., Chem.
Commun. 1990, 858.
(15) This is certainly an unusual reaction of alkoxy-stabilized carbene
complexes (see ref 7). Therefore, further work to determine the synthetic
importance of this process will be undertaken.
JA049200R
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