6812
J. Am. Chem. Soc. 1998, 120, 6812-6813
Chromium(0)-Carbene Complexes as Carbene
Sources: Self-Dimerization and Inter- and
Intramolecular C-H Insertion Reactions Catalyzed
by Pd(OAc)2
Scheme 1
Miguel A. Sierra,* Mar ´ı a J. Manche n˜ o, Elena S a´ ez, and
Juan C. del Amo
Departamento de Qu ´ı mica Org a´ nica I, Facultad de Qu ´ı mica
UniVersidad Complutense, 28040-Madrid, Spain
ReceiVed March 16, 1998
7
dimer 2a (2:1 E/Z mixture), and a crystalline white solid
8,9
identified as Cr(CO)
the reaction was carried out at 0 °C, in other solvents (hexane,
Cl CH , and Et O), or when lower catalyst loads (2% molar ratio)
were used. The reaction of pentacarbonyl[(p-bromophenyl)-
methoxy)carbene]chromium(0) (1b) gave similar results (Scheme
). Complex 1b also dimerized at room temperature in the
presence of Pd(PPh (3% and 0.6% molar ratios) to yield olefin
b. The olefin E/Z ratios were identical to those obtained with
Pd(OAc) /Et N, although a longer reaction time was needed when
.6% of catalyst was employed. Even when carbene complex
b was reacted with Pd(OAc) under the above conditions but in
6
.
Analogous results were obtained when
The role of heteroatom-stabilized group 6 metal-carbene
complexes as stable carbene sources has been extensively
investigated.1 In fact, the cyclopropanation of olefins was one
of the earliest reactions reported for Fischer-type carbene
complexes.2 On the other hand, thermal decomposition (T > 130
2
2
2
(
1
°
C) of carbene complexes leads to carbene dimers. The E/Z ratio
3 4
)
of the olefinic products is metal dependent, which excludes the
participation of free carbenes in at least some of these processes.3
C-H insertion processes have also been reported for group 6
2
2
3
0
4
Fischer-carbene complexes. Apart from intramolecular cyclo-
1
2
5
propanation reactions, which take place at room temperature or
the presence of methyl acrylate, styrene, and 1-hexene, dimer 2b
was obtained exclusively and no traces of cyclopropane products
were observed.10
below, the carbene transfer from heteroatom-stabilized group 6
metal-carbene complexes usually needs temperatures over 80
C to occur. We report here that dimerization and intramolecular
°
C-H insertion reactions of alkoxychromium(0) carbenes can be
effected at room temperature or below, in the presence of catalytic
Scheme 2
amounts of Pd(OAc)
mium(0) carbenes into olefinic C-H bonds is also catalyzed by
Pd(OAc)
2
, and that the formal insertion of aminochro-
2
.
The first assays to determine the viability of metal-catalyzed
carbene transfer from group 6 stabilized metal-carbene com-
plexes were carried out by reacting pentacarbonyl[(methoxy)-
(
phenyl)carbene]chromium(0) (1a) and catalytic amounts of
Rh (OAc)
at different temperatures.6 We observed that self-
dimerization of complex 1a occurred at temperatures around 100
C, high enough to not be clearly advantageous over the
uncatalyzed process. We then turned our attention to Pd catalysts.
Addition of Pd(OAc) (10% molar ratio) to a THF solution of
carbene complex 1a and Et N, at room temperature, resulted
within 1 h in a reaction mixture containing exclusively carbene
2
4
°
2
The reactivity of Cr(0) carbenes 3a,b was tested next. Both
metal carbenes 3a,b reacted at room temperature in the presence
3
2 3 6
of 10% Pd(OAc) /Et N to give vinyl ethers 4a,b and Cr(CO) ,
(
1) For general reviews on the chemistry and synthetic applications of group
11
as the exclusive reaction products. Dimers analogous to 2a,b
were not observed. Compound 4a was unstable and could not
be obtained analytically pure, while product 4b could be isolated
in 65% yield (Scheme 2). Clearly, the nature of the substituent
at the carbene carbon set the nature of the reaction products. To
determine the feasibility of intramolecular self-dimerization, we
6
metal carbenes, see: (a) D o¨ tz, K. H.; Fischer, H.; Hofmann, P.; Kreissel,
R.; Schubert, U.; Weiss, K. Transition Metal Carbene Complexes; Verlag
Chemie: Deerfield Beach, FL, 1983. (b) D o¨ tz, K. H. Angew. Chem., Int. Ed.
Engl. 1984, 23, 587-608. (c) Wulff, W. D. In ComprehensiVe Organic
Synthesis; Trost, B. M., Fleming, I., Eds.; Pergamon: Oxford, U.K., 1991;
Vol. 5, pp 1065-1113. (d) Schwindt, M. A.; Miller, J. R.; Hegedus, L. S. J.
Organomet. Chem. 1991, 413, 143-153. (e) Wulff, W. D. In ComprehensiVe
Organometallic Chemistry II; Abel, E. W., Stone, F. G. A., Wilkinson, G.,
Eds.; Pergamon: Oxford, U.K., 1995; Vol. 12, pp 470-547. (f) Hegedus, L.
S. In ref 1e, pp 549-576. (g) Harvey, D. F.; Sigano, D. M. Chem. ReV. 1996,
(7) The E/Z or product ratios were determined through this work by
1
integration of well-resolved signals in the H NMR spectra of the crude reaction
9
6, 271-288. (h) Hegedus, L. S. Tetrahedron 1997, 53, 4105-4127. (i)
Aumann, R.; Nienaber, H. AdV. Organomet. Chem. 1997, 41, 163-241.
2) (a) Fischer, E. O.; D o¨ tz, K. H. Chem. Ber. 1970, 103, 1273. (b) D o¨ tz,
mixtures prior to purification.
1
3
(8) The obtained white crystalline solid was identical ( C NMR, IR, and
(
6
EI mass spectra) to an authentic sample of Cr(CO) .
K. H.; Fischer, E. O. Chem. Ber. 1972, 105, 1356-1367. (c) Fischer, E. O.;
D o¨ tz, K. H. Chem. Ber. 1972, 105, 3966-3973. Reviews: (a) Brookhart,
M.; Studabaker, W. B. Chem. ReV. 1987, 87, 411-432. (b) Doyle, M. P. In
ref 1e, pp 387-420. See also ref 1g.
(9) All of the yields reported through this paper are for pure compounds,
except for compound 4a, which was unstable and could not be purified. See
the Supporting Information for full experimental procedures and spectroscopic
and analytical data. The following procedure for the self-dimerization of
complex 1b is representative: A THF solution (4 mL) of complex 1b (312
mg, 1 mmol) was placed in a flame-dried airless flask containing a magnetic
stirring bar and was degassed by evacuation/back-fill with argon (3×). Then,
(
3) Unpublished results by K. H. D o¨ tz cited in ref 1a, pp 196-197.
(4) Examples: (a) Takeda, K.; Okamoto, Y.; Nakajima, A.; Yoshii, E.;
Koizumi, T. Synlett 1997, 1181-1183. (b) Barluenga, J.; Aznar, F.; Fern a´ ndez,
M. Chem. Eur. J. 1997, 3, 1629-1637. (c) Barluenga, J.; Rodr ´ı guez, F.;
Vadecard, J.; Bendix, M.; Fa n˜ anas, F. J. J. Am. Chem. Soc. 1996, 118, 6090-
2 3
Pd(OAc) (22 mg, 0.1 mmol) and Et N (0.15 mL, 1.1 mmol) were added at
room temperature and the mixture was stirred for 1.5 h. The solvent was
distilled under reduced pressure, and the residue was stirred in pentane and
filtered through Celite. The solvent was removed in vacuo, and the residue
6
091. (d) Takeda, K.; Takeda, M.; Nakajima, A.; Yoshii, E. J. Am. Chem.
Soc. 1995, 117, 6400-6401. (e) Wang, S. L. B.; Su, J.; Wulff, W. D. J. Am.
Chem. Soc. 1992, 114, 10665-10666. (f) Fischer, H.; Schmid, J. Chem.
Commun. 1985, 572-573.
6
(170 mg) containing an E/Z mixture (2:1) of olefin 2b and Cr(CO) was
purified by molecular distillation to yield 120 mg (62%) of an E/Z mixture
(2:1) of pure olefin 2b as a colorless solid. The isomers [E-2b, 72 mg (37%);
Z-2b, 32 mg (16%)] could be separated by flash chromatography (hexane/
EtOAc, 10:1).
(5) See, for example: (a) Toledano, C. A.; Rudler, H.; Daran, J.-C.; Jeannin,
Y. Chem. Commun. 1984, 574-576. (b) Casey, C. P.; Vollendorf, N. W.;
Haller, K. J. J. Am. Chem. Soc. 1984, 106, 3754-3764. (c) S o¨ derberg, B.;
Hegedus, L. S. Organometallics 1990, 9, 3113-3121. (e) Barluenga, J.;
Monserrat, J. M.; Fl o´ rez, J. Chem. Commun. 1993, 1068-1070.
(10) Complex 1a reacts with methyl acrylate in boiling cyclohexane to
yield the corresponding cyclopropane. Wienand, A.; Reissig, H.-U. Tetrahe-
dron Lett. 1988, 29, 2315-2318.
(6) Sierra, M. A.; Casarrubios, L.; Dom ´ı nguez, G. Unpublished results.
S0002-7863(98)00849-X CCC: $15.00 © 1998 American Chemical Society
Published on Web 06/23/1998