Chromium-copper exchange of Fischer carbene complexes: X-ray crystal structure of a [Cu{=CR1(OR2)}(MeCN)(Et2O)][PF6] complex

Article abstract of DOI:10.1002/1521-3773(20010917)40:18<3392::AID-ANIE3392>3.0.CO;2-Y

The chromium-copper interaction is evidenced for chromium-carbene complexes and Cu+ in the CuBr-catalyzed cross-coupling reaction of ethyl diazoacetate (EDA) and Fischer alkoxycarbene - chromium complexes (see scheme). In contrast, with [Cu(MeCN)₄][PF₆] (0.5 equiv) instead of CuBr a carbene ligand exchange between a chiral alkenylchromium carbene complex (R¹ = (E) CH=CH-2-furyl, R² = (1R, 2S, 5R)-menthyl) and this Cu compound occurs at room temperature. The resulting novel trigonal-planar copper(I) - carbene complex, which contains MeCN and Et₂O as the remaining ligands, has been isolated and structurally characterized by X-ray diffraction.

Full text of DOI:10.1002/1521-3773(20010917)40:18<3392::AID-ANIE3392>3.0.CO;2-Y

COMMUNICATIONS
[
4] All compounds 2 were characterized by IR, 1H, and 13C NMR
spectroscopy, mass spectrometry, and elemental analysis and/or high-
resolution mass spectrometry data.
5] Overall, it represents a formal and selective remote activation of
aldehydes under mild conditions. For alternative syntheses from a
bifuntional system, see: a) C. F. Sturino, A. G. Fallis, J. Am. Chem.
Soc. 1994, 116, 7447 ± 7448; b) G. D. Cuny, S. L. Buchwald, J. Am.
Chem. Soc. 1993, 115, 2066 ± 2068.
Chromium ± Copper Exchange of Fischer
Carbene Complexes: X-Ray Crystal Structure
1
2
of a [Cu{ˆCR (OR )}(MeCN)(Et O)][PF ]
[
2
6
Complex**
Jos e Barluenga,* Luis A. L o pez, Oliver Löber,
Miguel Tom a s, Santiago García-Granda,
Carmen Alvarez-R u a, and Javier Borge
[
2
6] There are scant reports on this transformation. HgO/I leads to related
chemistry in steroids; however, it is not efficient on simple secondary
cycloalkanols, see: a) H. Suginome, S. Yamada, J. Org. Chem. 1984, 49,
3753 ± 3762; b) H. Suginome, H. Senboku, Tetrahedron 1994, 50,
13101 ± 13112; c) M. Akhtar, D. H. R. Barton, J. Am. Chem. Soc.
1964, 86, 1528 ± 1536; N-iodosuccinimide (NIS) gave poor results in
Over the last two decades Fischer-type carbene complexes
have become valuable tools in stoichiometric transition metal
mediated organic synthesis. The carbene nature of these
[1]
this chemistry: d) T. R. Beebe, A. L. Lin, R. D. Miller, J. Org. Chem.
974, 39, 722 ± 724; for sparse reports on the application of the
PhI(OAc) /I system to promote the opening of cyclopentanol
derivatives see: e) U. P. Spitz, P. E. Eaton, Angew. Chem. 1994, 106,
263 ± 2265; Angew. Chem. Int. Ed. Engl. 1994, 33, 2220 ± 2222; f) J. L.
complexes is demonstrated by their ability to transfer the
carbene ligand not only to alkenes (cyclopropanation),[ but
also to oxygen, nitrogen, and carbon through the correspond-
ing ylides (amine oxides and dimethyl sulfoxide; sulfilimines;
1
2]
2
2
2
Courtneidge, Tetrahedron Lett. 1992, 33, 3053 ± 3056.
[3]
P�C, S�C, and N �C ylides). Although transmetalation is
2
[
[
7] The process is efficient not only in terms of the yield of the iodo
derivative with respect to the starting cycloalkanol but also in terms of
the number of iodine atoms required to incorporate just one in the
final product and the ease of the isolation step.
probably the most important single process in organometallic
chemistry, particularly for transition metal catalyzed reac-
tions,[ the simple transfer of a carbene ligand from a metal ±
4]
8] For skeletal reorganizations based on C�C bond-breaking reactions
[5]
carbene complex to another metallic center is rare. Carbene
see: a) P. M. Wovkulich in Comprehensive Organic Synthesis, Vol. 1
complexes of Rh, Pd, Pt, Cu, and Ag have been formed by
transfer of diaminocarbene ligands (imidazolinylidene or
imidazolidinylidene ligands, which are found to be readily
(Eds.: B. M. Trost, I. Fleming, S. L. Schreiber), Pergamon, Oxford,
1991, pp. 843 ± 899; b) P. Weyerstahl, H. Marschall in Comprehensive
Organic Synthesis, Vol. 6 (Eds.: B. M. Trost, I. Fleming, E. Winter-
feldt), Pergamon, Oxford, 1991, pp. 1041 ± 1070.
transferred between metal centers) from Group 6 metal
[
9] 4-Iodobutanal (2b) is a key compound in the synthesis of 2-pyrrolin-
odoxorubicin (an anthracycline antibiotic): A. Nagy, P. Armatis, A. V.
Schally, Proc. Natl. Acad. Sci. USA 1996, 93, 2464 ± 2469.
10] a) D. P. Curran in Comprehensive Organic Synthesis, Vol. 4 (Eds.:
B. M. Trost, I. Fleming, M. F. Semmelhack), Pergamon, Oxford, 1991,
pp. 812 ± 814 and 817; b) P. Gray, A. Williams, Chem. Rev. 1959, 59,
[5±7]
carbene complexes.
However, the transfer of alkoxycar-
bene ligands has been reported only for [Mo{ˆCPh(OMe)}-
[
8]
(CO)(Cp)(NO)] to [Fe(CO) ] and [W{ˆCPh(OMe)}(CO) ]
[
5
5
[9]
to H[AuCl ]. Very recently Sierra et al. have proposed
4
palladium biscarbene complexes as the active intermediates in
the palladium-catalyzed dimerization of alkoxycarbene ±
chromium complexes.[
239 ± 328.
[
[
11] J. I. Concepci o n, C. G. Francisco, R. Hern a ndez, J. A. Salazar, E.
Su a rez, Tetrahedron Lett. 1984, 25, 1953 ± 1956.
10, 11]
12] This C�H activation product is also formed in absence of cyclo-
In the context of our studies on copper-catalyzed coupling
reactions of Fischer carbene complexes we report here on 1)
the cross-coupling of Fischer chromium ± carbene complexes
with ethyl diazoacetate (EDA), and 2) the isolation and full
characterization of the first alkoxycarbene complex of
hexanol. The extension of this observation to related alkanes is being
currently analyzed. For related intramolecular C�H activation using
this reagent, see reference [6e] and H. Togo, T. Muraki, Y. Hoshina, K.
Yamaguchi, M. Yokoyama, J. Chem. Soc. Perkin Trans. 1 1997, 787 ±
7
93; R. L. Dorta, A. Martín, E. Su a rez, T. Prang e , Tetrahedron:
Asymmetry 1996, 7, 1907 ± 1910; A. F. Barrero, J. E. Oltra, A.
Barrag a n, Tetrahedron Lett. 1995, 36, 311 ± 314; K. Furuta, T. Nagata,
H. Yamamoto, Tetrahedron Lett. 1988, 29, 2215 ± 2218.
copper(i). A recent, excellent report by Hofmann and
[12]
Straub
on the characterization of an alkoxycarbonylcar-
bene ± copper complex has prompted us to release our results.
When chromium ± carbene complexes 1 were treated with
EDA (2.5 equiv) and CuBr (15% mol) in THF at room
temperature the alkenes 2 were isolated with high yields
[
[
13] By GC-MS and NMR analysis of crude reaction mixtures.
14] Prepared from (� )-2-carene according to H. C. Brown, J. V. N. Vara-
Prasad, M. Zaislewicz, J. Org. Chem. 1988, 53, 2911 ± 2916.
15] An epimerization reaction of the hydrogen at the a-position to the
carbonyl group was ruled out by a 2D-NOESY experiment. An
intense cross peak for the cyclopropyl hydrogen atoms is observed,
which confirms the cis relationship between them, as in the starting
material 1i.
16] We thank the referees for suggestions and in particular to one of them
for drawing our attention to the mechanism proposed in Scheme 3.
17] For recent evidence on the participation of these intermediates in
related transformations, see: a) J. Madsen, C. Viuf, M. Bols, Chem.
Eur. J. 2000, 6, 1140 ± 1146; b) J. L. Courtneidge, J. Lusztyk, D. Pag e ,
Tetrahedron Lett. 1994, 35, 1003 ± 1006.
[
(
80 ± 95%) along with ethyl maleate and ethyl fumarate
[*] Prof. Dr. J. Barluenga, Dr. L. A. L o pez, Dr. O. Löber,
Prof. Dr. M. Tom a s
[
Instituto Universitario de Química Organomet a lica ªEnrique Molesº
Unidad Asociada al CSIC, Universidad de Oviedo
Juli a n Clavería 8, 33071-Oviedo (Spain)
Fax : (‡34)98-510-34-50
[
E-mail: barluenga@sauron.quimica.uniovi.es
[
18] For b-scission reactions from alkoxy radical intermediates, see: a) S.
Wilsey, P. Dowd, K. N. Houk, J. Org. Chem. 1999, 64, 8801 ± 8811;
b) A. L. Beckwith, B. P. Hay, J. Am. Chem. Soc. 1989, 111, 2674 ± 2681;
c) A. L. Beckwith, B. P. Hay, J. Am. Chem. Soc. 1989, 111, 230 ± 234,
and references therein.
[‡]
[‡]
[‡]
Dr. S. García-Granda, C. Alvarez-R u a, Dr. J. Borge
Departamento de Química Física y Analítica Universidad de Oviedo
Juli a n Clavería 8, 33071-Oviedo (Spain)
‡
[
] X-ray crystal structure analysis.
[
**] This work was supported by DGICYT (PB97-1271) and CICYT
[
19] For an early related observation see: C. Walling, R. T. Clark, J. Am.
Chem. Soc. 1974, 96, 4530 ± 4534.
(
BQU2000-0219). O.L. thanks the Fonds der Chemischen Industrie for
a postdoctoral fellowship.
Supporting information for this article is available on the WWW under
http://www.angewandte.com or from the author.
3392
ꢀ WILEY-VCH Verlag GmbH, D-69451 Weinheim, 2001
1433-7851/01/4018-3392 $ 17.50+.50/0
Angew. Chem. Int. Ed. 2001, 40, No. 18

COMMUNICATIONS
(
Scheme 1). This heterocoupling reaction worked well for
purification, 70% based on the conversion). This process
implies two unprecedented events on the coordination sphere
of copper, 1) exchange of the carbene ligand for a diethyl
ether ligand, and 2) transfer of the carbene ligand back to the
chromium atom.
phenyl- (1a), butyl- (1b) and alkenyl(methoxy)carbene (1c)
complexes, as well as for the nonracemic alkenyl(menthoxy)-
carbene 1d, giving rise to alkenes and dienes with a high
[
13]
degree of functionality.
The methoxy complexes 1a ± c
afforded a 1:1 E/Z mixture, whereas only the E stereoisomer
was formed in the case of the menthoxy derivative 1d.
The structure of 5 was confirmed by a single-crystal X-ray
structure analysis (Figure 1), [ which shows that the copper
atom is three-coordinate, bonded to the carbene carbon atom
15]
OR2
R2O
R1
a
+
N2CHCO2Et
(CO)5Cr
8
0-95%
R1
2
CO Et
1a-d
2a-d
Scheme 1. CuBr-catalyzed cross-coupling reaction of alkoxychromium
1
2
carbene complexes
1
with ethyl diazoacetate (a: R ˆ Ph, R ˆ Me;
1
2
1
2
1
b: R ˆ Bu, R ˆ Me; c: R ˆ (E)-CHˆCH-2-furyl, R ˆ Me; d: R ˆ (E)-
2
CHˆCH-2-furyl, R ˆ (1R,2S,5R)-menthyl): a) 15 mol% CuBr, THF, 258C,
2
2
8
0 ± 95%; E:Z ˆ 1:1 (R ˆ Me), >30:1 (R ˆ (1R,2S,5R)-menthyl).
On the other hand, when the (methoxy)carbene 1c was
stirred with 15 mol% [Cu(MeCN) ][PF ] in the absence, or
4
6
even in the presence, of EDA and under homogeneous
conditions (CH Cl , 258C) a clean dimerization to triene 3a
2
2
took place (95% yield; E/Z ˆ 10:1; Scheme 2). On the basis of
OR2
R2O
R1
R1
a
(
CO)5Cr
R1
for 1c
OR2
9
5%
1c,d
3a
(E:Z = 10:1)
Figure 1. The structure of 5 (ORTEP view). For clarity, the hexafluoro-
phosphate ion and hydrogen atoms are omitted. Thermal ellipsoids are
drawn at 30% probability. Selected distances [Š] and angles [8]: Cu-C1
b
for 1d
1
.882(3), Cu-N 1.891(3), Cu-O2 2.345(2), C1-Cu-N 150.88(14), C1-Cu-O2
[
Cr(CO)5(MeCN)]
+
OR2
R1
MeCN
Et2O
114.22(12), N-Cu-O2 94.90(11), N-Cu-C1-C2 -69.4(4), O2-Cu-C1-C2
112.5(3), Cu-C1-C2-C3 4.5(5).
c
Cu
[PF6]
1
2
- 1d
[
Cu{=CR (OR )}2L2] [PF6]
(L = MeCN)
Scheme 2. Formation of the copper± carbene complex 5 from 1d and
4
5
(
C1), the diethyl ether oxygen atom (O2), and the acetonitrile
1
2
1
nitrogen atom (N) in an approximately trigonal fashion. The
planes defined by O2, N, C1, Cu (mean deviation: 0.0050 Š)
and Cu, O1, C2, C1 (mean deviation: 0.0003 Š) deviates from
an orthogonal arrangement; the N-Cu-C1-C2 and O2-Cu-C1-
C2 torsion angles are � 69.48 and 112.58, respectively. The
alkenyl and copper carbene moieties are coplanar (Cu-C1-C2-
C3 torsion angle: 4.58). The Cu ± C(carbene) distance
[
(
(
Cu(MeCN)
4
][PF
E)-CHˆCH-2-furyl,
MeCN) ][PF ], CH
Cl , 258C; c) crystallization from CH
0% (35% recovery of 1d).
6
] (3a: R ˆ (E)-CHˆCH-2-furyl, R ˆ Me; 3b, 4, 5: R ˆ
2
R ˆ (1R,2S,5R)-menthyl):
a) 15 mol%
[Cu-
][PF ],
O (1/5, v/v), � 208C,
4
6
2
Cl
2
, 258C, 95%; b) 50 mol% [Cu(MeCN)
Cl /Et
4
6
CH
2
2
2
2
2
5
previous work[5, 10] we assume that a copper biscarbene species
1
of the type [Cu{ˆCR (OMe)} L ][PF ] might be the real metal
(1.882 Š) is in agreement with that reported for X�CuˆCR
2
n
6
2
16]
intermediate. At this point, it was gratifying to find that the
complexes.[
The Cu�C2 (2.96 Š) and Cu�C3 (3.24 Š)
[17]
homochiral (menthoxy)carbene complex 1d consumed
distances rule out a possible copper± alkene coordination.
0
.5 equivalents of [Cu(MeCN) ][PF ] (CH Cl , 258C) yielding
Complex 5 is stable in the solid state or in solution (CH Cl ,
2 2
4
6
2
2
1
a clean crude mixture consisting of [Cr(CO) (MeCN)]
toluene) for several days at � 208C under nitrogen. The H,
5
13 31 19
(
identified by comparison with an authentic sample) and a
copper± carbene complex, presumably of structure
Scheme 2). The structure 4 is proposed on the basis of the
C, P, and F NMR spectra were recorded in CD Cl at
2 2
4
ambient temperature without decomposition. The CuˆC
[18]
(
carbon atom resonates at d ˆ 276.5. Despite the creation
1
3
single set of signals in the C NMR spectrum (d ˆ 280
of a chirality axis, a single set of signals was observed in the
[
14]
1
(
CuˆC), 4.1 (Cu�NC�CH )). Surprisingly, when the crude
variable-temperature H NMR spectra between � 808C and
3
mixture was dissolved in CH Cl and Et O (1/5, v/v) was
258C. The UV spectrum of 5 (l ˆ 383 nm, e ˆ 42300, in
2
2
2
max
allowed to diffuse into the brown solution the color slowly
changed to reddish and orange crystals of the new complex 5
deposited at � 208C (50% yield with respect to the starting
copper complex). The starting carbene complex 1d could be
recovered from this solution (ca. 35% after chromatographic
CH Cl ) suggests a p ± p* transition of the copper± carbon
2
2
double bond with predominant metal-to-ligand charge-trans-
fer character.
The carbene ligand of both complexes 4 and 5 dimerized
stereoselectively in the presence of tributylphosphane
Angew. Chem. Int. Ed. 2001, 40, No. 18
ꢀ WILEY-VCH Verlag GmbH, D-69451 Weinheim, 2001
1433-7851/01/4018-3393 $ 17.50+.50/0
3393

COMMUNICATIONS
(
3
3 equiv, CH Cl , 258C) affording the (1E,3E,5E)-1,3,5-triene
b (75% for 4; 82% for 5). Moreover, we found that the
HMBC experiments clearly indicate correlation between the olefinic
hydrogen atom and the carbene carbon atom.
15] Crystal structure data for 5. C23 NO P: M
2
2
C
b
[
H
37CuF
6
3
r
ˆ 584.05, orange
copper complex 5 catalyzed the dimerization of EDA
CH Cl , 258C) to a mixture of ethyl maleate and ethyl
plates, dimensions ˆ 0.45  0.20  0.10 mm, orthorhombic, space
(
2
2
group P2 2 2 (determined from the systematic absences), a ˆ
1
1 1
3
fumarate, with retention of the structural integrity of the
catalyst.
7.3757(1), b ˆ 14.2948(2), c ˆ 26.6661(3) Š, Vˆ 2811.52(6) Š , l ˆ
�
3
1
2
.5418 Š, Z ˆ 4,
1
calcd ˆ 1.380 Mgm
,
F(000) ˆ 1216, m(CuKa) ˆ
�
1
.207 mm . The crystals are extremely air-sensitive. The sample was
In summary, chromium ± carbene complexes efficiently
transfer the carbene ligand to a copper(i) center allowing
the preparation of functionalized alkenes, dienes, and trienes
through homo- and heterocoupling reactions. A new cop-
per(i) ± carbene complex has been isolated and structurally
characterized by X-ray diffraction that features, 1) an
uncommon tricoordinate pattern, and 2) the presence of two
other ligands with fair (MeCN) and very low (Et O)
coordination capability. It should be emphasized that no
X-ray structural determination has been hitherto achieved for
copper(i) carbene complexes of the type [ML ], rather only
the spectroscopic characterization has been reported for a few
d
carbene ligands (tert-butyl-substituted imidazoline and imid-
azolidine carbenes)
ing iminophosphanamide ligands. Further work directed to
study the potential of these chiral copper(i) ± carbene com-
plexes in asymmetric catalysis is in progress.
mounted in an appropriately sized cryoloop, and it was completely
covered with mineral oil to prevent it from being in contact with
oxygen. The crystal was held at 120(2) K with an Oxford Cryosystems
Cryostream Cooler. Data collection was performed on a Nonius
KappaCCD single-crystal diffractometer (fine-focus sealed tube,
horizontally mounted graphite crystal, 95 mm CCD camera). The
crystal ± detector distance was fixed at 29 mm, and a total of 1018
images were collected by using the oscillation method (f and w scans),
with 18 oscillation and 30 s exposure time per image. The data
collection strategy was calculated with the program COLLECT. Data
reduction and cell refinement were performed with the programs
HKL DENZO and SCALEPACK. Unit cell dimensions were
determined from 30381 reflections between q ˆ 1.4738 and 70.0768.
A total of 20320 reflections were measured (q ˆ 3.518, q ˆ 69.798;
2
[
17]
3
min
max
1
0
copper carbene complexes containing either highly donor
� 8 ꢀ h ꢀ 8, 0 ꢀ k ꢀ 17, 0 ꢀ l ꢀ 32). Multiple observations were aver-
aged (Rint ˆ 0.059), resulting in 5197 unique reflections of which 4910
were observed with I > 2s(I). Final mosaicity was 0.616(1)8. All data
completeness was 98.7%. Intensity ± error ratio for all reflections was
[
5±7]
or extremely basic, sterically demand-
[
12]
4
13.0:17.6. The crystal structure was solved by Patterson methods and
phase expansion using the program DIRDIF-99.2. An empirical
absorption correction was applied (Tmin ˆ 0.450; Tmax ˆ 1.000) using
XABS2. Anisotropic least-squares refinement was carried out with
SHELXL-97. All non-hydrogen atoms were anisotropically refined.
Hydrogen atoms were geometrically placed riding on their parent
atoms with isotropic displacement parameters set to 1.2 times the Ueq
value of the atoms to which they are attached (1.5 for methyl groups).
The final cycle of full-matrix least-squares refinement based on 5197
Received: May 10, 2001 [Z17084]
[
1] W. D. Wulff in Comprehensive Organometallic Chemistry II, Vol. 12
Eds.: E. W. Abel, F. G. A. Stone, G. Wilkinson), Pergamon, Oxford,
995, p. 469.
(
1
reflections and 317 parameters converged to a final value of
[
[
2] M. Brookhart, W. B. Studebaker, Chem. Rev. 1987, 87, 411.
3] B. Alcaide, L. Casarrubios, G. Domínguez, M. A. Sierra, Curr. Org.
Chem. 1998, 2, 551.
4] a) L. S. Hegedus, Transition Metals in the Synthesis of Complex
Organic Molecules, University Science Books, Sausalito, CA, 1999;
b) A. Boudier, L. O. Bromm, M. Lotz, P. Knochel, Angew. Chem.
R1(observed) ˆ 0.0405, R1(all data) ˆ 0.0445, wR2(all data) ˆ 0.1024,
2
2
S ˆ 1.083, (D/s)max ˆ 0.003, weighting scheme w ˆ 1/[s (F
o
† ‡
2
2
2
(0.0000P) ‡ 2.6320P], where P ˆ (F
o
‡ 2F †/3. Final difference Four-
� 3
c
[
ier maps showed no peaks higher than 0.366 eŠ nor deeper than
�
3
� 0.288 eŠ . Flack parameter ˆ 0.00(3). Geometrical calculations
were made with PARST97. The crystallographic plots were made with
PLATON. Crystallographic data (excluding structure factors) for the
structure reported in this paper have been deposited with the
Cambridge Crystallographic Data Centre as supplementary publica-
tion no. CCDC-163238. Copies of the data can be obtained free of
charge on application to CCDC, 12 Union Road, Cambridge
CB21EZ, UK (fax: (‡44)1223-336-033; e-mail: deposit@ccdc.cam.
ac.uk).
2000, 112, 4584; Angew. Chem. Int. Ed. 2000, 39, 4414.
[
[
5] Review, see: S.-T. Liu, K. R. Reddy, Chem. Soc. Rev. 1999, 28, 315.
6] R.-Z. Ku, J.-C. Huang, J.-Y. Cho, F.-M. Kiang, K. R. Reddy, Y.-C.
Chen, K.-J. Lee, J.-H. Lee, G.-H. Lee, S.-M. Peng, S.-T. Liu, Organo-
metallics 1999, 18, 2145.
I
I
[
7] For the first synthesis of Ag ± and Cu ± carbene complexes from the
free 1,3-dimesitylimidazol-2-ylidene and the corresponding metal
triflate, see: A. J. Arduengo III, H. V. R. Dias, J. C. Calabrese, F.
Davidson, Organometallics 1993, 12, 3405.
[16] The X-ray structure of two chloro-(1,3-thiazolin-2-ylidene)copper
complexes has been reported. See: a) H. G. Raubenheimer, S. Cronje,
P. H. van Rooyen, P. J. Olivier, J. G. Toerien, Angew. Chem. 1994, 106,
687; Angew. Chem. Int. Ed. Engl. 1994, 33, 672; b) H. G. Rauben-
heimer, S. Cronje, P. J. Olivier, J. Chem. Soc. Dalton Trans. 1995, 313.
[17] For this type of interaction in the case of chromium complexes, see: J.
Barluenga, F. Aznar, A. Martín, S. García-Granda, E. P e rez-Carre nÄ o,
J. Am. Chem. Soc. 1994, 116, 11191.
[
8] E. O. Fischer, H.-J. Beck, C. G. Kreiter, J. Lynch, J. Müller, E. Winkler,
Chem. Ber. 1972, 105, 162.
9] R. Aumann, E. O. Fischer, Chem. Ber. 1981, 114, 1853.
[
[
[
10] a) M. A. Sierra, J. C. del Amo, M. J. Manche nÄ o, M. G o mez-Gallego, J.
Am. Chem. Soc. 2001, 123, 851; b) M. A. Sierra, M. J. Manche nÄ o, E.
S a ez, J. C. del Amo, J. Am. Chem. Soc. 1998, 120, 6812; c) the
participation of chromium biscarbene complexes for the thermal
dimerization was proposed a long time ago: C. P. Casey, R. L.
Anderson, J. Chem. Soc. Chem. Commun. 1975, 895.
11] Rhodium ± carbene species have been proposed as intermediates: a) I.
Göttker-Schnetmann, R. Aumann, Organometallics 2001, 20, 346;
b) R. Aumann, I. Göttker-Schnetmann, R. Fröhlich, O. Meyer, Eur. J.
Org. Chem. 1999, 2545.
[18] Selected NMR signals of 5 (numbering according to Figure 1).
1
H NMR: d ˆ 8.15 (1H, d, J ˆ 15 Hz; C3-H), 7.1 (1H, d, J ˆ 15 Hz;
C2-H), 3.4 (4H, q, J ˆ 7 Hz; C20-H, C21-H), 2.35 (3H, s; C19-H), 1.1
13
(6H, t, J ˆ 7 Hz; C22-H, C23-H); C NMR: d ˆ 276.5 (C1), 152.4,
152.2, 150.5, 133.3, 126.1, 115.6 (C1 ± C7), 119.6 (C18), 66.5 (C20, C21),
3.1 (C19).
[
[
12] B. F. Straub, P. Hofmann, Angew. Chem. 2001, 113, 1328; Angew.
Chem. Int. Ed. 2001, 40, 1288.
13] The uncatalyzed cross-coupling reaction requires that the chromium
complexes are heated at 608C in the presence of more than five
equivalents of EDA. This type of diene is also available from
phosphorus ylides.[
3]
[
14] The vinylic hydrogen atoms of the carbene ligand of 4 are low-field
shifted (around 1 ppm) compared with those of complex 1d. The
3394
ꢀ WILEY-VCH Verlag GmbH, D-69451 Weinheim, 2001
1433-7851/01/4018-3394 $ 17.50+.50/0
Angew. Chem. Int. Ed. 2001, 40, No. 18