New Fischer Carbene Complexes of Rhodium(I): Preparation and 2-Cyclopentenone Ring Synthesis by Annelation to Alkynes

Article abstract of DOI:10.1021/ja038817q

A new type of metal carbene complexes of group 9, specifically a cationic Fischer carbene of rhodium(I), has been synthesized from chromium carbene complexes via double transfer of carbene and CO ligands. These complexes reveal a different reactivity than other transition metal carbenes, including their chromium precursors, toward neutral and electron-poor alkynes, giving selectively polysubstituted cyclopentenones. Copyright

Full text of DOI:10.1021/ja038817q

Published on Web 12/23/2003
New Fischer Carbene Complexes of Rhodium(I): Preparation and
2-Cyclopentenone Ring Synthesis by Annelation to Alkynes
Jose´ Barluenga,* Rube´n Vicente, Luis A. Lo´pez, Eduardo Rubio, Miguel Toma´s, and
Carmen AÄ lvarez-Ru´a
Instituto UniVersitario de Qu´ımica Organometa´lica “Enrique Moles”sUnidad Asociada al C.S.I.C.,
UniVersidad de OViedo, Julia´n ClaVeria, 8, 33006 OViedo, Spain
Received October 1, 2003; E-mail: barluenga@sauron.quimica.uniovi.es
Scheme 2. [3 + 2] Cyclization of Carbenes 3a-c with 1-Pentyne
Heteroatom-stabilized transition metal carbene complexes, par-
ticularly those derived from group 6, are recognized as versatile
organometallic intermediates in organic synthesis.1 Surprisingly,
late transition metal carbene complexes have been scarcely studied
despite the special features of these metals in carbon-carbon bond-
forming reactions. A few examples have been reported wherein
the chromium-palladium(0),2 -rhodium(I),3 and -copper(I)3c,4
exchange from chromium alkoxycarbene complexes results in
increased reactivity. A major goal consisting in qualitative modi-
fication of the reactivity of chromium carbenes has been recently
achieved by us via chromium-nickel exchange, specifically new
[2 + 2 + 2 + 1] and [3 + 2 + 2] carbocyclization reactions.5
Because of our interest in this area, as well as the importance of
rhodium carbenes as intermediate species for fundamental organic
processes,6 we now report the preparation of novel cationic
rhodium(I) alkoxycarbene complexes7 and their potential for
cyclopentenone synthesis.
this case, the rhodium-catalyzed reaction was found very effective.
Thus, stirring a CH2Cl2 solution of chromium carbene complexes
1, terminal electron-poor alkynes 5 (R3 ) H), and 2 (10 mol %)
yielded cyclopentenone derivatives 6a-g with complete selectivity
(64-89%, Table 1, entries 1-7).8 Carbene complexes with different
substitution patterns were successfully used toward alkynyl esters
and alkynyl ketones.
Table 1. Rh-Catalyzed [3 + 2] Cyclization of Alkenyl Chromium
Carbene Complexes with Electron-Poor Alkynesa
Carbene complexes of rhodium(I) were prepared by stirring at
room termperature a CH2Cl2 solution of chromium methoxycarbene
complexes 1 and readily available rhodium complex 2 (Scheme
1). The reaction went to completion after 1 h, affording cationic
rhodium(I) alkenyl(methoxy)carbene complexes 3 in yields higher
than 60%. The solid-state structure of 3b could be determined by
a single-crystal analysis.8
Scheme 1. Synthesis of Cationic Carbonyl Rhodium(I) Carbene
Complexes 3a-d
compd
(yield, %)b
1
2
3
entry
R
R
R
EWG
1
2
3
4
5
6
7
8
9
Ph
H
H
H
H
H
H
H
H
H
H
H
H
Ph
COOMe
COOMe
COOMe
COOMe
COOMe
COOMe
COMe
COOEt
COOMe
COOMe
6a (75)
6b (81)
6c (88)
6d (71)
6e (70)
6f (89)
6g (64)
7a (75)
7b (85)
8 (81)
p-MeOC6H4
2-furyl
ferrocenyl
n-Bu
-CH2CH2CH2O-
2-furyl
2-furyl
2-furyl
2-furyl
H
H
H
H
The simultaneous transfer of two ligands (carbene and CO)
observed in this case represents a rare process.9 On other hand,
cationic rhodium(I) carbene complexes are scarcely found in the
literature.10
The treatment of complexes 3a-c with 1-pentyne in CH2Cl2 at
room temperature for 16 h resulted in the regioselective formation
of cyclopentenones 4a-c (Scheme 2).8 Therefore, a new and
specific reactivity toward alkynes is encountered for rhodium(I)
carbene complexes which is different from that found for transition
metal carbene complexes.11
1-cyclohexenyl
Me
10
a Reagents and conditions: [(naphthalene)Rh(cod)][SbF6] (10 mol %),
CH2Cl2, 25 °C, 12-36 h, and then chromatographic workup. b Yields of
isolated products.
Representative electron-poor internal alkynes 5 (R3 ) Ph,
1-cyclohexenyl, Me) also undergo efficiently the [3 + 2] cyclization,
affording 7 or 8 depending on the alkyne nature (Table 1, entries
8-10).8 It should be noted in these cases that the cyclization occurs
with reversal of regiochemistry (6 vs 7, 8).
Next, we decided to check the reactivity of rhodium complexes
toward electrophilic alkynes, since the reaction of these systems
with group 6 carbene complexes presents severe limitations.12 In
A mechanistic proposal to explain the behavior of these new
rhodium systems seems at this point rather speculative.13 On the
9
470
J. AM. CHEM. SOC. 2004, 126, 470-471
10.1021/ja038817q CCC: $27.50 © 2004 American Chemical Society

C O M M U N I C A T I O N S
stabilized palladium(0) carbene complex, see: Albe´niz, A. C.; Espinet,
P.; Manrique, R.; Pe´rez-Mateo, A. Angew. Chem., Int. Ed. 2002, 41, 2363.
(3) For specific cyclization reactions of Cr(0) carbenes which are accelerated
by Rh(I) and Cu(I), see: (a) Aumann, R.; Go¨ttker-Schnetmann, I.;
Fro¨hlich, R.; Meyer, O. Eur. J. Org. Chem. 1999, 2545. (b) Go¨ttker-
Schnetmann, I.; Aumann, R. Organometallics 2001, 20, 346. (c) Go¨ttker-
Schnetmann, I.; Aumann, R.; Bergander, K. Organometallics 2001, 20,
3574.
(4) Barluenga, J.; Lo´pez, L. A.; Lo¨ber, O.; Toma´s, M.; Garc´ıa-Granda, S.;
Alvarez-Ru´a, C.; Borge, J. Angew. Chem., Int. Ed. 2001, 40, 3392.
(5) 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.
(6) (a) Padwa, A.; Weingarten, M. D. Chem. ReV. 1996, 96, 223. (b) Doyle,
M. P.; Forbes, D. C. Chem. ReV. 1998, 98, 911. (c) Davies, H. M. L.;
Panaro, S. A. Tetrahedron 2000, 56, 4871. (d) Davis, H. M. L.;
Antoulinakis, E. G. J. Organomet. Chem. 2001, 617-618, 47.
(7) For the synthesis of rhodium(I) zirconoxycarbene complexes, see: (a)
Barger, P. T.; Bercaw, J. E. Organometallics 1984, 3, 278. (b) Erker,
G.; Lecht, R.; Tsay, Y.-H.; Kru¨ger, C. Chem. Ber. 1987, 120, 1763. (c)
Erker, G.; Mena, M.; Hoffmann, U.; Menjo´n, B. Organometallics 1991,
10, 291.
basis of previous models where metalladienes are involved, we think
that a metalla-Diels-Alder reaction would account well for the
observed results (Scheme 3).14,15 Thus, the regiochemistry observed
in the case of the terminal alkynes (cycloadducts 4, 6) would be
dictated simply by steric discrimination, the larger alkyne substituent
being placed away from rhodium. In the case of internal electron-
poor alkynes (cycloadducts 7, 8) the steric differentiation is not so
large and the electronic factors, which favor the interaction between
the electrophilic metal center and the nucleophilic CR-alkyne, would
dominate, leading to cycloadducts with opposite regiochemistry.
Scheme 3. Proposed Mechanism for the [3 + 2] Cyclization
(8) Compound 3b shows a square planar coordination at the rhodium(I) center
with a rhodium-carbene carbon and Rh-CO bond lengths of 1.993(9)
and 1.901(10) Å, respectively. Details are given in the Supporting
Information. Compounds 3, 4, and 6-8 were characterized spectrospically
and by elemental analysis. The structures of 4 and 6-8 were ascertained
by NMR experiments (including HMQC, HMBC, COSY, and NOESY).
The enol ethers, precursors of the final cyclopentenones, are detected by
NMR in the reaction crude before chromatographic purification.
(9) For the transfer of carbene and carbonyl ligands from a tungsten
diaminocarbene complex to platinum, see: (a) Liu, S.-T.; Hsieh, T.-Y.;
Lee, G.-H.; Peng, S.-M. Organometallics 1998, 17, 993. (b) Ku, R.-Z.;
Huang, J.-C.; Cho, J.-Y.; Kiang, F.-M.; Reddy, K. R.; Chen, Y.-C.; Lee,
K.-J.; Lee, J.-H.; Lee, G.-H.; Peng, S.-M.; Liu, S.-T. Organometallics
1999, 18, 2145.
(10) Cationic complexes of Rh(I) with diaminocarbene ligands: (a) Herrmann,
W. A.; Goosen, L. J.; Spiegler, M. Organometallics 1998, 17, 2162. (b)
Werner, H.; Ho¨rlin, G.; Mahr, M. J. Organomet. Chem. 1998, 551, 367.
(c) Buron, C.; Stelzig, L.; Gerret, O.; Gortnitzka, H.; Romanenko, V.;
Bertrand, G. J. Organomet. Chem. 2002, 664, 70. For a cationic Rh(I)
diarylcarbene complex, see: (d) Bleuel, E.; Weberndo¨rfer, B.; Werner,
H. J. Organomet. Chem. 2001, 617-618, 502. For cationic Rh(I)
vinylidene and allenylidene complexes, see: (e) Windmu¨ller, B.; Nu¨rnberg,
O.; Wolf, J.; Werner, H. Eur. J. Inorg. Chem. 1999, 613.
(11) The [3 + 2] cyclization of alkynes to metal carbenes leading to substituted
cyclopentadienes has been reported for the particular case of group 6
â-aminoalkenyl(alkoxy)carbene complexes. See: (a) de Meijere, A.;
Shirmer, H.; Duetsch, M. Angew. Chem., Int. Ed. 2000, 39, 3964. (b)
Aumann, R. Eur. J. Org. Chem. 2000, 17. (c) Reference 3.
(12) Electron-poor alkynes react with alkenyl(alkoxy)carbenes of chromium
yielding mixtures of bicyclic lactones (Halban-White cyclization)
and phenols depending on the nature of the activating group: (a) Wulff,
W. D.; Chan, K.-S.; Tang, P.-C. J. Org. Chem. 1984, 49, 2293.
(b) Brandvold, T. A.; Wulff, W. D.; Rheingold, A. L. J. Am. Chem.
Soc. 1991, 113, 5459. (c) Brandvold, T. A.; Wulff, W. D.; Rheingold, A.
L. J. Am. Chem. Soc. 1990, 112, 1645. (d) Waters, M. L.; Brandvold, T.
A.; Isaacs, L.; Wulff, W. D.; Rheingold, A. L. Organometallics 1998,
17, 4298.
In summary, a new type of metal carbene complexes of group
9, specifically a cationic Fischer carbene of rhodium(I), has been
synthesized from chromium carbene complexes via double transfer
of carbene and CO ligands and characterized in solution and in the
solid state. These complexes reveal a different reactivity than other
transition metal carbenes, including their chromium precursors,
toward neutral and electron-poor alkynes. Synthetically, polysub-
stituted cyclopentenones are readily synthesized from chromium
Fischer carbene complexes and alkynes by a [3 + 2] cyclization
mediated (for neutral alkynes) or catalyzed (for activated alkynes)
by rhodium(I).16 Most of these types of cyclopentenones are not at
all straightforwardly accessible by the intermolecular Pauson-
Khand reaction because of a lack of either reactivity (for unstrained
alkenes and electron-poor alkenes) or regioselectivity (2,5-disub-
stituted cyclopentenones are generally formed).17
(13) For an interesting discussion on the intramolecular reaction of diazo
derivatives and unactivated alkynes catalyzed by rhodium(II), see: Padwa,
A. J. Organomet. Chem. 2001, 617-618, 3.
Acknowledgment. Financial support for this work is acknowl-
edged (BQU-2001-3853 and PR-01-GE-09). R.V. thanks the
Ministerio de Ciencia y Tecnolog´ıa for a predoctoral fellowship.
We are also grateful to Dr. Ce´sar J. Pastor (Unversidad Autonoma,
Madrid) for his assistance in the collection of the X-ray data.
(14) For previous examples of 1-metalla-1,3-dienes in [4 + 2] cycloadditions,
see: (a) Trost, B. M.; Hashmi, S. K. J. Am. Chem. Soc. 1994, 116, 2183.
(b) Hoffmann, M.; Buchert, M.; Reissig, H.-U. Chem. Eur. J. 1999, 5,
876. (c) Kagoshima, H.; Okamura, T.; Akiyama, T. J. Am. Chem. Soc.
2001, 123, 7182. (d) Barluenga, J.; Lo´pez, S.; Flo´rez, J. Angew. Chem.,
Int. Ed. 2003, 42, 231. For an excellent theoretical study on 1,2- and 1,4-
cycloaddition reactions of rhodiabenzene complexes, see: (e) Iron, M.
A.; Martin, J. M. L.; van der Boom, M. E. J. Am. Chem. Soc. 2003, 125,
11702.
Supporting Information Available: Detailed experimental pro-
cedures and characterization data for all compounds (PDF); X-ray
crystallographic data for compound 3b (CIF). This material is available
free of charge via the Internet at http://pubs.acs.org.
(15) The vinyl rhodiacyclobutene formation/1,3-rearrangement sequence might
be also acceptable. See: Padwa, A.; Kassir, J. M.; Xu, S. L. J. Org. Chem.
1997, 62, 1642. A carbene/alkyne insertion mechanism apparently would
not explain the observed regiochemistry.
(16) In a different approach, 3-methoxycyclopentenones result from neutral
alkynes and cyclopropyl(methoxy)carbenes of chromium. See: Herndon,
J. W.; Zu, J. Org. Lett. 1999, 1, 15 and references therein.
(17) Reviews: (a) Gibson, S. E.; Stevenazzi, A. Angew. Chem., Int. Ed. 2003,
42, 1800. (b) Schore, N. E. In ComprehensiVe Organometallic Chemistry
II; Abel, E. W., Stone, F. G. A., Wilkinson, G., Eds.; Pergamon: New
York, 1995; Vol. 12, p 703. For an intermolecular reaction involving
electron-poor and strained alkenes, see, respectively: (c) Ahmar, M.;
Antras, F.; Cazes, B. Tetrahedron Lett. 1999, 40, 5503. (d) Marchueta,
I.; Verdaguer, X.; Moyano, A.; Perica`s, M. A.; Riera, A. Org. Lett. 2001,
3, 3193.
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