Dimerization of rhenium alkynyl carbene complexes by a process involving two [1,1.5] Rhenium shifts and coupling of the remote alkynyl carbons [1]

Full text of DOI:10.1021/ja9945010

J. Am. Chem. Soc. 2000, 122, 3771-3772
3771
Communications to the Editor
Dimerization of Rhenium Alkynyl Carbene
Complexes by a Process Involving Two [1,1.5]
Rhenium Shifts and Coupling of the Remote Alkynyl
Carbons
Charles P. Casey,* Stefan Kraft, and Douglas R. Powell
Department of Chemistry, UniVersity of Wisconsin
Madison, Wisconsin 53706
ReceiVed December 30, 1999
Alkynyl carbene metal complexes offer the possibility of
observing unprecedented [1,3] metal shifts along the π-system
to form an isomeric alkynyl carbene complex.1,2 By using a nearly
symmetrical alkynyl carbene complex 1, the [1,3]-shifted com-
pound 2 should have comparable stability making the [1,3] shift
thermodynamically feasible (Scheme 1).3 Here we report that in
addition to a small amount of [1,3] shift, 1 undergoes an unusual
dimerization that involves a [1,1.5] rhenium shift4 and carbene-
like coupling at the remote alkynyl carbon.
Figure 1. X-ray crystal structure of 4.
Scheme 1
Rhenium alkynyl carbene complexes were prepared in good
yield by addition of alkynylmetal reagents to metal carbyne
complexes.5 Addition of C6H5CtCZnBr to [Cp(CO)2RetC(C6H4-
p-CH3)]BCl4 gave Cp(CO)2RedC(C6H4-p-CH3)(CtCC6H5) (1)
as black crystals in 65% yield. A significantly higher yield was
obtained from the zinc acetylide than from the corresponding
lithium acetylide (<4%). The isomeric alkynyl carbene complex
Cp(CO)2RedC(C6H5)(CtCC6H4-p-CH3) (2) was prepared in 60%
yield by a similar route. 1 and 2 were readily distinguishable by
1H NMR spectroscopy; the tolyl methyl group of 1 appears at δ
1.78, while that of 2 appears at δ 2.11. The symmetric compound
Cp(CO)2RedC(C6H4-p-CH3)(CtCC6H4-p-CH3) (3) was also syn-
thesized and its structure confirmed by X-ray crystallography.6
When a 0.765 M solution of 1 in toluene-d8 was heated at 120
°C for 4 h, little isomerization to 2 via a [1,3] rhenium shift was
observed by 1H NMR spectroscopy.7 Instead, clean formation of
a single new product 4 was seen (Scheme 1).8 4 was isolated in
77% yield by preparative thin-layer chromatography. MS estab-
lished that 4 was a dimer (m/e ) 1024.1). The observation of
1
only one Cp and one methyl resonance in both the H and 13C
NMR spectra indicated that compound 1 had dimerized sym-
metrically.9
Single-crystal X-ray diffraction established the trans-enediyne
structure of 4 (Figure 1). In the solid state, the two Cp(CO)2Re
units are positioned above and below the plane of the ligand
π-system. Interestingly, the X-ray structure revealed that 4 is
formed by coupling of the remote alkynyl carbons.10 Since
formation of symmetric alkenes by coupling of the carbene
carbons of two metal carbene complexes is well known,11 it was
surprising to find that dimerization of 1 occurred by coupling at
the remote carbon atoms rather than the carbene carbons.
When the dimerization of 1 was carried out at 120 °C in an
NMR probe, no intermediates were detected by 1H NMR
spectroscopy. The conversion of 1 to 4 in toluene-d8 followed
clean second-order kinetics over the concentration range from
0.171 to 0.00124 M. The second-order rate constant for dimer-
ization was 1.45 ( 0.2 × 10-3 M-1 s-1 at 120 °C, which
corresponds to ∆Gq ) 28.3 ( 0.1 kcal mol-1. The rate constants
for dimerization showed little solvent dependence: 3.9 × 10-3
M-1 s-1 in methylcyclohexane-d14, 1.5 × 10-3 M-1 s-1 in
bromobenzene-d5, 1.1 × 10-3 M-1 s-1 in o-dichlorobenzene-d4.
This indicates that there is little polarity difference between the
moderately polar carbene complexes and the transition state for
dimerization.
(1) For heteroatom-substituted alkynyl carbene complexes, see: (a) Au-
mann, R.; Nienaber, H. AdV. Organomet. Chem. 1997, 41, 161. (b) Wulff,
W. D. In ComprehensiVe Organometallic Chemistry II; Abel, E. W., Stone,
F. G. A., Wilkinson, G., Eds.; Pergamon Press: Oxford, 1995; Vol. 12, Chapter
5.3. (c) Herndon, J. W. Coord. Chem. ReV. 1999, 181, 177.
(2) A [1,3] shift was postulated in a rhodium alkynyl carbene presumably
formed in a catalytic cycle: Padwa, A.; Austin, D. J.; Gareau, Y.; Kassir, J.
M.; Xu, S. L. J. Am. Chem. Soc. 1993, 115, 2637.
(3) Non-donor stabilized CpRe(CO)2 carbene complexes are generally less
prone to decomposition than the analogous Cr(CO)5 or W(CO)5 carbene
complexes. (a) Casey, C. P.; Vosejpka, P. C.; Askham, F. R. J. Am. Chem.
Soc. 1990, 112, 3713. (b) Casey, C. P.; Czerwinski, C. J.; Powell, D. R.;
Hayashi, R. K. J. Am. Chem. Soc. 1997, 119, 5750. (c) Iwasawa, N.; Maeyama,
K.; Saitou, M. J. Am. Chem. Soc. 1997, 119, 1486.
(4) We introduce the term “[1,1.5] shift” to describe the migration of
rhenium from carbon 1 of carbene complex 1 to the midpoint between carbons
1 and 2 of the resulting alkyne complex 4.
(5) For the first synthesis of a CpRe(CO)2 carbyne complex, see: Fischer,
E. O.; Clough, R. L.; Stu¨ckler, P. J. Organomet. Chem. 1976, 120, C6.
(6) 3 adopts a nearly Cs-symmetric geometry in which the plane of the
carbene ligand is nearly perpendicular to the plane of the Cp ring. The alkynyl
unit is syn to the Cp ligand. See Supporting Information.
(9) Addition of lithium or zinc reagents to alkynyl carbene complexes has
led to formation of bimetallic compounds. (a) Do¨tz, K. H.; Christoffers, C.;
Knochel, P. J. Organomet. Chem. 1995, 489, C84. (b) Fischer, H.; Meisner,
T.; Hofmann, J. Chem. Ber. 1990, 123, 1799.
(10) Prior to the X-ray structure determination, 13C NMR evidence for the
site of carbene ligand coupling and the trans geometry in the enediyne complex
4 was obtained from studies of the thermolysis of Cp(CO)2RedC(Tol)(Ct
13CPh) and Cp(CO)2RedC(Tol)(13Ct13CPh). See Supporting Information.
(11) (a) Casey, C. P.; Anderson, R. L. J. Chem. Soc., Chem. Commun.
1975, 895. (b) Fischer, H.; Schmid, J. J. Mol. Catal. 1988, 46, 277. (c)
Hohmann, F.; Siemoneit, S.; Nieger, M.; Kotila, S.; Do¨tz, K. H.; Chem. Eur.
J. 1997, 3, 853.
(7) Too little isomerization of 1 to 2 occurred to be observable by 1H NMR
spectroscopy. However, compelling evidence for a [1,3] shift was found when
Cp(CO)2RedC(Tol)(13Ct13CPh) was thermolyzed at 120 °C to give small
amounts of Cp(CO)2Re)13C(Ph)(13CtCTol). See Supporting Information.
(8) The use of high concentrations of 1 produced higher yields of dimer 4
since the rate of formation of 4 depends on [1]2 and thermal decomposition
of 4 is a first-order process.
10.1021/ja9945010 CCC: $19.00 © 2000 American Chemical Society
Published on Web 03/29/2000

3772 J. Am. Chem. Soc., Vol. 122, No. 15, 2000
Communications to the Editor
Scheme 2
Scheme 3
Scheme 4
moderately polar rhenium carbene complex 1 and the transition
state involving addition of zwitterion A to 1.
Pathway B involves generation and ring opening of neutral
cyclopropene intermediate C and is consistent with the small
solvent effects observed. To test for the rapid cyclopropene ring
opening required in pathway B, the synthesis of cyclopropene-
substituted carbene complex 6 was attempted. Addition of the
cyclopropenyl zinc reagent 7 to [Cp(CO)2RetC(C6H5-p-CH3)]-
BCl4 at -78 °C in THF-d8 was monitored by 1H NMR
spectroscopy at low temperature within 10 min of mixing (Scheme
3). None of the expected cyclopropenyl carbene complex 6 was
observed; instead, the cyclopropene ring-opening product 8 was
cleanly formed.16 The smooth transformation 6 to 8 at low
temperature supports the plausibility of a cyclopropene to vinyl
carbene rearrangement in pathway B.17
Evidence suggesting that the remote alkynyl carbon of an
alkynyl carbene can have carbenoid properties was obtained from
studies of alkynyl carbene complex 9 which has a pendant allyl
unit (Scheme 4). When 9 was heated at 120 °C for 30 min,
intramolecular cyclopropanation occurred by addition of C3 to
the allyl double bond to form 10 in 95% yield.18 Padwa has shown
that related free alkynyl carbenes undergo similar intramolecular
cyclopropanations.2
Two plausible mechanisms for dimerization are shown in
Scheme 2. Both involve a rapid and reversible “[1,1.5] shift” of
CpRe(CO)2 along the π-system of the alkynyl carbene ligand to
form A,12 which has a zwitterionic and a carbenoid resonance
structure. Pathway A uses the carbanionic character of C3 in A
to nucleophilically add to the remote alkynyl carbon of 1 to
produce the zwitterionic intermediate B. The negatively charged
rhenium center in the σ-allenyl moiety of B
can then undergo a “[1,1.5] shift” of Re with the formation of
the neutral product 4. Pathway B uses the carbenoid character of
A in reaction with 1 to produce cyclopropene C. Ring opening
of the cyclopropene would produce vinyl carbene D,13 which
rearranges by a "[1,1.5] shift" of Re to produce the enediyne
complex 4. The observed second-order kinetics are also consistent
with a concerted dimerization not involving any intermediate.
The electrophilic character of remote alkynyl carbon C3 in 1
required in pathway A was demonstrated by the rapid conjugate
addition of PMePh2 to 1 to give the σ-allenyl complex [Cp-
(CO)2Re-](C6H4-p-CH3)CdCdC(PCH3Ph2+)Ph (5).14 Cationic
rhenium metallacyclopropenes and anionic rhenium σ-vinyl
complexes similar to the part structures in B are known.3b,15
Nevertheless, the C-C bond formation between the two electro-
philic atoms as the key step in this mechanism is unprecedented.
The insensitivity of dimerization to solvent polarity would have
to be attributed to little difference in polarity between the
In conclusion, we have observed a fascinating dimerization of
alkynyl carbene complexes that produces enediyne complexes.
Further studies will be required to sort out the detailed mechanism
of this transformation.
Acknowledgment. Financial support from the National Science
Foundation is gratefully acknowledged. Grants from NSF (CHE-9629688)
for the purchase of the NMR spectrometers and a diffractometer (CHE-
9709005) are acknowledged.
Supporting Information Available: Preparation of compounds,
spectral data, description of kinetics, and X-ray crystallographic data for
3 and 4 (PDF). This material is available free of charge via the Internet
at http://pubs.acs.org.
(12) A similar [1,1.5] shift has been postulated in the rearrangement of
chromium alkynylcarbene complexes [(CO)5CrdC(O)(CtC-OEt)]- to a
cyclopropenylidene complex. Juneau, K. N.; Hegedus, L. S.; Roepke, F. W.
J. Am. Chem. Soc. 1989, 111, 4762.
JA9945010
(16) The in situ formation of 7 was independently confirmed in a separate
experiment by trapping it with trimethylsilyl chloride to form 1-(trimethylsilyl)-
2-phenyl-cyclopropene. See Supporting Information.
(17) Similar rearrangements are normally observed at elevated temperatures,
although sulfonyl-substituted cyclopropenes undergo ring opening at -25 °C.
Franck-Neumann, M.; Lohmann, J.-J. Angew. Chem., Int. Ed. Engl. 1977,
16, 323.
(18) For a related metal-induced intramolecular cyclization via carbenoid
intermediates, see: Shiu, Y.-T.; Madhushaw, R. J.; Li, W.-T.; Lin, Y.-C.;
Lee, G.-H.; Peng; S.-M.; Liao, F.-L.; Wang, S.-L.; Liu, R.-S. J. Am. Chem.
Soc. 1999, 121, 4066.
(13) Note that D is a resonance structure of B.
(14) Compound 1 (4.38 × 10-4 M-1 s-1 at -36.0 °C) is only 115 times
less reactive than methyl triflate (second-order rate constant: 5.93 × 10-2
M-1 s-1 at -36.0 °C) toward PMePh2. Since alkyl triflates are approximately
104-105 times more reactive toward nucleophiles than alkyl tosylates, the
electrophilicity of 1 lies between methyl triflate and methyl tosylate. (a) Stang,
P. J.; Hanack, M.; Subramanian, L. R. Synthesis 1982, 85. (b) Su, T. M.;
Sliwinski, W. F.; Schleyer, P. v. R. J. Am. Chem. Soc. 1969, 91, 5386.
(15) Casey, C. P.; Brady, J. T.; Boller, T. M.; Weinhold, F.; Hayashi, R.
K. J. Am. Chem. Soc. 1998, 120, 12500.