Formation of cis-enediyne complexes from rhenium alkynylcarbene complexes

Article abstract of DOI:10.1021/ja011962o

Dimerization of the alkynylcarbene complex Cp(CO)₂Re=C(Tol)C≡CCH₃ (8) occurs at 100 °C to give a 1.2:1 mixture of enediyne complexes [Cp(CO)₂Re]₂[η²,η ²-TolC≡CC(CH₃)=C(CH₃)C≡CTol] (10-E and 10-Z), showing no intrinsic bias toward trans-enediyne complexes. The cyclopropyl-substituted alkynylcarbene complex Cp(CO)₂Re=C(Tol)C≡CC₃H₅ (11) dimerizes at 120 °C to give a 5:1 ratio of enediyne complexes [Cp(CO)₂Re]₂[η²,η²- TolC≡C(C₃H₅)C=C (C₃H₅)C≡CTol] (12-E and 12-Z); no ring expansion product was observed. This suggests that if intermediate A formed by a [1,1.5] Re shift and having carbene character at the remote alkynyl carbon is involved, then interaction of the neighboring Re with the carbene center greatly diminishes the carbene character as compared with that of free cyclopropyl carbenes. The tethered bis-(alkynylcarbene) complex Cp(CO)₂Re=C(Tol)C≡CCH₂ CH₂CH₂C≡CC(Tol)=Re(CO)₂Cp (13) dimerizes rapidly at 12 °C to give the cyclic cis-enediyne complex [Cp(CO)₂Re]₂ [η²,η²-TolC≡CC (CH₂CH₂CH₂)=CC≡CTol] (15). Attempted synthesis of the 1,8-disubstituted naphthalene derivative 1,8-[Cp(CO)₂Re=C(Tol)C≡C] ₂C₁₀H₆ (16), in which the alkynylcarbene units are constrained to a parallel geometry, leads to dimerization to [Cp(CO)₂Re]₂ (η²,η²-1,2-(tolylethynyl)acenaphthylene] (17). The very rapid dimerizations of both 13 and 16 provide compelling evidence against mechanisms involving cyclopropene intermediates. A mechanism is proposed which involves rate-determining addition of the carbene center of A to the remote alkynyl carbon of a second alkynylcarbene complex to generate vinyl carbene intermediate C, and rearrangement of C to the enediyne complex by a [1,1.5] Re shift.

Full text of DOI:10.1021/ja011962o

Published on Web 02/23/2002
Formation of cis-Enediyne Complexes from Rhenium
Alkynylcarbene Complexes
Charles P. Casey,* Stefan Kraft, and Douglas R. Powell
Contribution from the Department of Chemistry, UniVersity of Wisconsin,
Madison, Wisconsin 53706
Received August 13, 2001
Abstract: Dimerization of the alkynylcarbene complex Cp(CO)2RedC(Tol)CtCCH3 (8) occurs at 100 °C
to give a 1.2:1 mixture of enediyne complexes [Cp(CO)2Re]2[η2,η2-TolCtCC(CH3)dC(CH3)CtCTol] (10-E
and 10-Z), showing no intrinsic bias toward trans-enediyne complexes. The cyclopropyl-substituted
alkynylcarbene complex Cp(CO)2RedC(Tol)CtCC3H5 (11) dimerizes at 120 °C to give a 5:1 ratio of
enediyne complexes [Cp(CO)2Re]2[η2,η2-TolCtC(C3H5)CdC(C3H5)CtCTol] (12-E and 12-Z); no ring
expansion product was observed. This suggests that if intermediate A formed by a [1,1.5] Re shift and
having carbene character at the remote alkynyl carbon is involved, then interaction of the neighboring Re
with the carbene center greatly diminishes the carbene character as compared with that of free cyclopropyl
carbenes. The tethered bis-(alkynylcarbene) complex Cp(CO)2RedC(Tol)CtCCH2CH2CH2CtCC(Tol)d
Re(CO)2Cp (13) dimerizes rapidly at 12 °C to give the cyclic cis-enediyne complex [Cp(CO)2Re]2[η2,η2-
TolCtCC(CH2CH2CH2)dCCtCTol] (15). Attempted synthesis of the 1,8-disubstituted naphthalene derivative
1,8-[Cp(CO)2RedC(Tol)CtC]2C10H6 (16), in which the alkynylcarbene units are constrained to a parallel
geometry, leads to dimerization to [Cp(CO)2Re]2(η2,η2-1,2-(tolylethynyl)acenaphthylene] (17). The very rapid
dimerizations of both 13 and 16 provide compelling evidence against mechanisms involving cyclopropene
intermediates. A mechanism is proposed which involves rate-determining addition of the carbene center of
A to the remote alkynyl carbon of a second alkynylcarbene complex to generate vinyl carbene intermediate
C, and rearrangement of C to the enediyne complex by a [1,1.5] Re shift.
Introduction
substituted rhenium alkynylcarbene complexes such as Cp-
(CO)2RedC(Tol)CtCPh (1) [Tol ) C6H4-p-CH3] and found
Heteroatom-substituted alkynylcarbene complexes such as
(OC)5M)C(OR)CtCR′ have a rich chemistry that combines
high reactivity, easy accessibility, and entry into a large variety
of products.1 In particular, nucleophilic additions to either the
carbene or the remote alkyne carbon and cycloadditions to the
triple bond have been exploited synthetically. Despite the large
number of donor-substituted alkynylcarbene complexes, [1,3]
metal migrations such as from (OC)5MdC(OR)CtCR′ to
(OC)5MdC(R′)CtCOR were virtually unknown either due to
a high kinetic barrier or due to an unfavorable equilibrium.2,3
Recently, we began a search for [1,3] metal shifts in nondonor-
small amounts of the [1,3] shifted isomer Cp(CO)2RedC(Ph)Ct
CTol (2) upon thermolysis at 120 °C (Scheme 1).4,5 The major
product, however, was the dimeric trans-enediyne complex [Cp-
(CO)2Re]2[η2-,η2-TolCtCC(Ph)dC(Ph)CtCTol] (3) formed by
regioselective coupling at the remote alkynyl carbons.6 When
the conversion of 1 f 3 was followed by 1H NMR spectroscopy,
no intermediates were observed, and a second-order rate
dependence on [1] was seen. This regioselectivity stands in
contrast to the reported ability of Fischer carbene complexes
(M ) CRR′) to couple the carbene carbons and form alkenes.7
Most notably, Sierra recently showed a selective “head-to-head”
coupling in the Pd-catalyzed dimerization of (CO)5CrdC(OEt)Ct
CPh to (E,Z)-PhCtCC(OEt)dC(OEt)CtCPh with no evidence
for “tail-to-tail” or “head-to-tail” couplings.8
(1) For reviews of heteroatom-substituted alkynylcarbene complexes, see: (a)
Aumann, R.; Nienaber, H. AdV. Organomet. Chem. 1997, 41, 163. (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. (d) de
Meijere, A.; Schirmer, H.; Duetsch, M. Angew. Chem., Int. Ed. 2000, 39,
3964.
(2) A [1.3]-shift of a rhodium alkynylcarbene complex presumed to be an
intermediate in a catalytic cycle was postulated: Padwa, A.; Austin, D. J.;
Gareau, Y.; Kassir, J. M.; Xu, S. L. J. Am. Chem. Soc. 1993, 115, 2637.
(3) [1.3]-Transition metal shifts are common in σ-propargyl to σ-allenyl
rearrangements: (a) Keng, R.-S.; Lin, Y.-C. Organometallics 1990, 9, 289.
(b) Hajela, S.; Schaefer, W. P.; Bercaw, J. E. J. Organomet. Chem. 1997,
532, 45. (c) Barluenga, J.; Trabanco, A. A.; Flo´rez, J.; Garc´ıa-Granda, S.;
LLorca, M.-A. J. Am. Chem. Soc. 1998, 120, 12129. (d) Blosser, P. W.;
Schimpff, D. G.; Gallucci, J. C.; Wojcicki, A. Organometallics 1993, 12,
1993. (e) Liang, K.-W.; Lee, G.-H.; Peng, S.-M.; Liu, R.-S. Organometallics
1995, 14, 2353. (f) Pu, J.; Peng, T.-S.; Arif, A.; Gladysz, J. A. Organo-
metallics 1992, 11, 3232. (g) Caro, B.; Le Poul, P.; Robin-Le Guen, F.;
Se´ne´chal-Tocquer, M.-C.; Saillard, J.-Y.; Kahlal, S.; Ouahab, L.; Golhen,
S. Eur. J. Org. Chem. 2000, 577.
Three types of mechanism are under consideration for dimeri-
zation of alkynylcarbene complexes. The first involves an initial
(4) Casey, C. P.; Kraft, S.; Powell, D. R. J. Am. Chem. Soc. 2000, 122, 3771.
(5) Nondonor stabilized CpRe(CO)2 carbene complexes are generally less prone
to decomposition than the analogous Cr(CO)5 or W(CO)5 carbene com-
plexes. (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.
(6) Addition of dilithium or dizinc reagents to alkynylcarbene 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.
9
2584 VOL. 124, NO. 11, 2002 J. AM. CHEM. SOC.
10.1021/ja011962o CCC: $22.00 © 2002 American Chemical Society

Formation of cis-Enediyne Complexes
A R T I C L E S
Scheme 1
Scheme 2
Scheme 3
[1,1.5] shift9,10 of rhenium to give an intermediate A with
enhanced electrophilic carbene character at the remote alkynyl
carbon. Intermediate A is suggested to attack a second molecule
of 1 to produce the cyclopropene intermediate B. Ring opening
of B produces vinyl carbene C, which then rearranges via a
[1,1.5] Re shift to produce enediyne complex 3 (Scheme 2).
A number of observations are consistent with this mechanistic
hypothesis. The remote alkynyl carbon of 1 has electrophilic
character as demonstrated by the addition of phosphines to this
site.11 The carbene character of the remote alkynyl carbon in
intermediate A explains the intramolecular cyclopropanation of
4 and the insertion into a methyl CH bond of the Cp* complex
5 (Scheme 3).12 As required by the mechanism shown in Scheme
2, attempted synthesis of cyclopropenyl carbene complex 6 led
to immediate ring opening and formation of alkyne complex 7
(Scheme 4).
(7) (a) Casey, C. P.; Anderson, R. L. J. Chem. Soc., Chem. Commun. 1975,
895. (b) Casey, C. P.; Burkhardt, T. J.; Bunnell, C. A.; Calabrese, J. C. J.
Am. Chem. Soc. 1977, 99, 2127. (c) Casey, C. P.; Polichnowski, S. W. J.
Am. Chem. Soc. 1977, 99, 6097. (d) Fischer, H.; Schmid, J. J. Mol. Catal.
1988, 46, 277. (e) Fischer, H.; Zeuner, S.; Ackermann, K.; Schmid, J. Chem.
Ber. 1986, 119, 1546. (f) Fischer, H.; Zeuner, S.; Ackermann, K. Chem.
Commun. 1984, 684. (g) Hohmann, F.; Siemoneit, S.; Nieger, M.; Kotila,
S.; Do¨tz, K. H. Chem.-Eur. J. 1997, 3, 853.
(8) Sierra, M. A.; del Amo, J. C.; Manchen˜o, M. J.; Go´mez-Gallego, M. J.
Am. Chem. Soc. 2001, 123, 851.
(9) We introduced the term “[1,1.5] shift” to describe the migration of rhenium
from carbon 1 of carbene complex l to the midpoint between carbons 1
and 2 of the resulting alkyne complex 3. See ref 4.
(10) (a) A similar [1,1.5] shift has been postulated in the rearrangement of the
chromium acyl complex [(CO)5CrdC(O)(CtCOEt)]- to a cycloprope-
nylidene complex. Juneau, K. N.; Hegedus, L. S.; Roepke, F. W. J. Am.
Chem. Soc. 1989, 111, 4762. (b) This rearrangement was recently observed
in a related system. de Meijere, A.; Mu¨ller, S.; Labahn, T. J. Organomet.
Chem. 2001, 617-618, 318.
A second mechanism also involves an initial [1,1.5] shift of
rhenium to give intermediate A′, but in this mechanism the
(11) Casey, C. P.; Kraft, S.; Powell, D. R.; Kavana, M. J. Organomet. Chem.
2001, 617-618, 723.
(12) Casey, C. P.; Kraft, S.; Powell, D. R.; Kavana, M. Organometallics 2001,
20, 3795.
9
J. AM. CHEM. SOC. VOL. 124, NO. 11, 2002 2585

A R T I C L E S
Casey et al.
Scheme 4
Scheme 5
Scheme 6
remote alkynyl carbon is suggested to have nucleophilic
character. A′ is similar to isolated 1-metallacyclopropenes.13,14
Nucleophilic attack on the remote alkynyl carbon of a second
complex would give the zwitterionic intermediate C′,15 which
can then collapse to product with a second [1,1.5] Re shift
(Scheme 5). It should be pointed out that A and A′, and C and
C′, are resonance structures that have the same geometry but
differ in their depicted polarizations and in the reactivity patterns
they suggest. Protonation of alkynylcarbene complexes occurs
primarily at the carbene carbon, but some protonation also
occurs at the remote alkynyl carbon indicating that this center
can be nucleophilic.16 The observed insensitivity of the rate of
dimerization to solvent polarity would require that the zwitte-
rionic intermediate C′ be similar in polarity to the starting
carbene complex 1.
A third mechanism involves no intermediate but is simply a
concerted conversion of two alkynylcarbene complexes to a
dimer with CdC bond formation occurring simultaneously with
two [1,1.5] rhenium migrations (Scheme 6). At one extreme,
where the transition state has very advanced [1,1.5] Re shifts
and little C-C bond formation, this mechanism approaches
dimerization of carbene-like intermediate A. However, a much
earlier transition state would appear more likely than prior
formation of two high-energy intermediates.
One distinction between these three mechanisms involves the
geometry of approach of the two three-carbon units of the
alkynylcarbene. In the first mechanism, the two units must
approach orthogonally to produce the cyclopropene intermediate
B. In the second mechanism, there is no geometric constraint
(13) For exo-vinylidene metallacyclopropene complexes, see: (a) Gambel, A.
S.; Birdwhistell, K. R.; Templeton, J. L. J. Am. Chem. Soc. 1990, 112,
1818. (b) Feher, F. J.; Green, M.; Rodrigues, R. A. J. Chem. Soc., Chem.
Commun. 1987, 1206.
(14) For cationic rhenium metallacyclopropenes, see: Casey, C. P.; Brady, J.
T.; Boller, T. M.; Weinhold, F.; Hayashi, R. K. J. Am Chem. Soc. 1998,
120, 12500.
(15) For σ-allenyl complexes, see: Doherty, S.; Corrigan, J. F.; Carty, A. J.;
Sappa, E. AdV. Organomet. Chem. 1995, 37, 39.
(16) Kraft, S. Ph.D. Thesis, University of Wisconsin, 2001.
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2586 J. AM. CHEM. SOC. VOL. 124, NO. 11, 2002

Formation of cis-Enediyne Complexes
A R T I C L E S
Scheme 7
Scheme 8
on the first C-C bond formation to generate zwitterionic
intermediate C′. In the third mechanism, if the transition state
has some resemblance to that of carbene dimerization, then a
least motion coplanar approach of the carbenes to form the new
CdC bond would be symmetry forbidden;17 geometries involv-
ing a parallel but not coplanar approach of the developing
carbene centers appear favorable.
In an effort to distinguish between these mechanisms, we have
synthesized bis carbene complexes in which the approach of
the alkynylcarbene units is restricted by the linking unit. Here
we report that intramolecular coupling of linked alkynylcarbene
complexes occurs remarkably rapidly to give cyclic enediyne
complexes.
layer chromatography gave partial separation of the two
products, and the major isomer was isolated in pure form after
exposure of the mixture in CH2Cl2 to air.18 No evidence was
found for isomerization via a [1,3] rhenium shift to Cp(CO)2Red
C(CH3)CtCTol (9). A [1,1.5] shift of 8 would generate a
carbene-like intermediate similar to A in Scheme 2. Such an
intermediate might undergo a 1,2 hydrogen shift to form Cp-
(CO)2Re(η2-TolCtCCHdCH2). However, no evidence for
formation of such an eneyne complex was obtained.
The structures of the products were assigned as the trans-
and cis-enediyne complexes [Cp(CO)2Re]2[η2,η2-TolCtCC-
(CH3)dC(CH3)CtCTol] (10-E and 10-Z) on the basis of
spectroscopy (Scheme 8). Mass spectrometry established that
both complexes were dimers. There are six possible dimers:
cis- and trans-isomers arising from tail-to-tail, head-to-head, and
Results
1
head-to-tail combinations. Both H and 13C NMR established
cis- and trans-Enediyne Complexes from Thermolysis of
Cp(CO)2RedC(Tol)CtCCH3 (8). Only the trans-enediyne
complex 3 was observed in the coupling of 1. To determine
whether this was intrinsic to the coupling mechanism or merely
due to the steric effect of large phenyl substituents on the
forming double bond, we investigated the reactions of less
crowded substrates. Cp(CO)2RedC(Tol)CtCCH3 (8) was pre-
pared by a route similar to that used for the synthesis of Cp-
(CO)2RedC(Tol)CtCPh (1). Addition of the zinc acetylide
BrZnCtCCH3 to the rhenium carbyne complex [Cp(CO)2Ret
CTol]BC14 in THF at -35 °C gave the black alkynylcarbene
complex Cp(CO)2RedC(Tol)CtCCH3 (8), which was isolated
in 22% yield following column chromatography (Scheme 7).
the symmetric nature of the products, ruling out head-to-tail
isomers. The formation of a cis head-to-head isomer with tolyl
groups on the CdC bond seems unlikely on steric grounds and
such cis isomers were not observed from coupling of 1. The
13C NMR chemical shifts of the methyl groups of 10-E and
10-Z, both at δ 21.73, are consistent with their assignment as
vinyl methyl groups of the tail-to-tail isomers. If head-to-head
isomers were formed, the chemical shifts of the complexed
CH3CtCR unit would have been expected to be in the δ 10
region; for example, the chemical shift of (CH3CtCCH3)Re-
(CO)2Cp is δ 14.2.19 The major isomer was assigned as 10-E
on the basis of the observation of a 1.0% nOe signal enhance-
ment of the resonance at δ 7.59 (assigned to aromatic CH ortho
to the alkynyl substituent) upon irradiation of the allylic methyl
resonance at δ 1.96. For the minor isomer assigned as 10-Z,
only a 0.1% nOe enhancement of the resonance at δ 7.23
(assigned to aromatic CH ortho to the alkynyl substituent) was
seen upon irradiation of the allylic methyl resonance at δ 2.24.
The structure of 8 was established spectroscopically; the 13
C
NMR spectrum established the presence of a carbene ligand (δ
258.1). Evidence for an aryl group bound to the carbene carbon
comes from a high-frequency aromatic resonance at δ 157.2
assigned to the ipso-carbon bound to the carbene carbon. This
assignment is based on 13C NMR spectra of 1 and the labeled
derivative Cp(CO)2RedC(Tol)(Ct13CPh) (1-13C). Therefore,
no [1,3] rhenium shift to give Cp(CO)2RedC(CH3)CtCTol (9)
occurred during synthesis or isolation of 8.
The kinetics of dimerization of 8 in toluene-d8 were followed
by 1H NMR spectroscopy between 80 and 120 °C. NMR yields
of >95% based on conversion of 8 were determined using 1,4-
bis-trimethylsilylbenzene as an internal NMR standard. Clean
second-order kinetics for disappearance of starting material were
When a solution of 8 was heated at 100 °C in toluene-d8, the
1
Cp H NMR resonance of 8 at δ 5.7 decreased, and a 1.2:1
ratio of two new Cp resonances at δ 5.5 and 5.3 grew in. Thin-
(18) Isomerization in air at room temperature was unexpected since the
(17) Hoffmann, R.; Gleiter, R.; Mallory, F. B. J. Am. Chem. Soc. 1970, 92,
1460. Ohta, K.; Davidson, E. R.; Morokuma, K. J. Am. Chem. Soc. 1985,
107, 3466. Alder, R. W.; Blake, M. E.; Oliva, J. M. J. Phys. Chem. A
1999, 103, 11200.
compounds are stable at 120 °C in the absence of air. We speculate that
isomerization occurs via a reversibly formed radical cation 10-Z+ O2 in
-
which the bonding character of the central CdC bond is weakened.
(19) Alt, H. G.; Engelhardt, H. E. J. Organomet. Chem. 1988, 342, 235.
9
J. AM. CHEM. SOC. VOL. 124, NO. 11, 2002 2587

A R T I C L E S
Casey et al.
Scheme 9
observed. Activation parameters (∆Hq ) 18.3 ( 0.4 kcal mol-1
,
∆Sq ) -19.8 ( 1.2 eu) were determined from an Eyring plot.
The rate of dimerization of 8 (25.4 ( 0.2 × 10-3 M-1 s-1
)
which has an alkynyl methyl group was about 17 times faster
at 120 °C than was the rate of dimerization of 1 (1.8 ( 0.05 ×
10-3 M-1 s-1) which has an alkynyl phenyl group.4 Thus, the
effect of replacing the phenyl group of 1 by a methyl group in
8 is to increase the rate of dimerization and to allow the
formation of both cis- and trans-enediynes.
The observation of cis-enediyne product 10-Z indicates that
the coupling mechanism has no intrinsic bias toward trans-
enediyne complexes. The previously observed selective forma-
tion of only trans-enediyne 3 from dimerization of phenyl-
substituted alkynylcarbene complex 1 is more likely related to
steric effects. While we cannot rigorously exclude the possibility
that [1,3] isomerization of 8 to 9 is rapid and reversible and
favors 8, this seems highly unlikely. In the case of aryl-
substituted alkynylcarbene complexes, we found that electron-
withdrawing substituents sped up interconversion of isomers
but did not greatly affect the equilibrium constants.20 For
example, Cp(CO)2RedC(Tol)CtCC6H4-p-SO2CF3 equilibrates
with the [1,3] shift isomer Cp(CO)2RedC(C6H4-p-SO2CF3)Ct
CTol 88 times more rapidly than equilibration of 1 with 2, but
the equilibrium constants for the two systems are similar, 4 and
1, respectively. We are trying to devise a synthesis of Cp-
(CO)2RedC(CH3)CtCTol (9) to test the hypothesis that
isomerization is slow.
Figure 1. X-ray crystal structure of cyclopropyl-substituted alkynylcarbene
complex Cp(CO)2RedC(Tol)CtCC3H5 (11).
The cyclopropyl-substituted alkynylcarbene complex Cp-
(CO)2RedC(Tol)CtCC3H5 (11) was synthesized in 64% yield
by addition of the zinc cyclopropylacetylide BrZnCtCC3H5 to
the rhenium carbyne complex [Cp(CO)2RetCTol]BCl4 in THF
at -35 °C. Complex 11 was characterized spectroscopically;
the 13C NMR spectrum established the presence of a carbene
ligand (δ 256.67) and of an aryl group bound to the carbene (δ
156.81, t, 3JCH ) 7.9 Hz). X-ray crystallography confirmed the
structural assignment (Figure 1).
Cyclopropyl-Substituted Alkynylcarbene Complexes
Dimerize without Ring Expansion. One mechanism for
dimerization of alkynylcarbene complexes involves an initial
[1,1.5] shift to generate intermediate A which is suggested to
have carbene character at the remote alkynylcarbene carbon.
Such a carbon center might undergo ring expansion to a
cyclobutene since free cyclopropyl carbenes undergo such ring
expansions (Scheme 9).21
When the cyclopropyl-substituted alkynylcarbene complex
Cp(CO)2RedC(Tol)CtCC3H5 (11) was heated at 120 °C, only
dimerization to a 5:1 ratio of trans-:cis-enediyne complexes [Cp-
(CO)2Re]2[η2,η2-TolCtC(C3H5)CdC(C3H5)CtCTol] [12-E and
12-Z] was seen (Scheme 9). When the thermolysis was followed
1
by H NMR spectroscopy, a 58% yield of 12 was observed at
(21) Cyclopropyl carbene undergoes ring expansion to cyclobutene, ring
fragmentation to acetylene and ethylene, and ring opening to butadiene.
Arct, J.; Brinker, U. H. In Methoden der organischen Chemie (Houben-
Weyl); Regitz, M., Ed.; Thieme Verlag: Stuttgart, 1989; Vol. E19b, p 337.
(20) Casey, C. P.; Kraft, S.; Powell, D. R. Organometallics 2001, 20, 2651.
9
2588 J. AM. CHEM. SOC. VOL. 124, NO. 11, 2002

Formation of cis-Enediyne Complexes
A R T I C L E S
Scheme 10
Table 1. Selected 13C NMR Chemical Shifts of Enediyne
Complexes 3, 10-E, 10-Z, 12-E, 15, and 17
compound
TolCtC
TolCtC
CdC
3a
79.69
79.60
79.87
77.50
72.21
72.52
85.19
80.53
82.29
85.06
85.24
93.45
134.98
129.15
128.92
133.73
136.15
135.19
10-Eb
10-Zb
12-Ec
15b
17d
a In C6D6 at 60 °C. b In CD2Cl2 at 24 °C. c In CDC13 at 60 °C. d In THF-
d8 at 60 °C.
allow a distinction to be made between the three dimerization
mechanisms presented in the Introduction.
The tethered bis-(alkynylcarbene) complex Cp(CO)2Red
C(Tol)CtCCH2CH2CH2CtCC(Tol)dRe(CO)2Cp (13) was ob-
tained from reaction of the bis-alkynylzinc compound BrZnCt
CCH2CH2CH2CtCZnBr with the carbyne complex [Cp(CO)2-
RetCTol]BCl4. The yields of isolated 13 were significantly
lower (only 8%) than for other alkynylcarbene complexes due
to its temperature sensitivity which required a tedious low-
temperature thin-layer chromatography workup. A major side
product was the monofunctionalized complex Cp(CO)2Red
C(Tol)CtCCH2CH2CH2CtCH (14) which was formed in 20-
30% isolated yield. The structure of 13 was established
spectroscopically. The 13C NMR spectrum established the
presence of a carbene ligand (δ 254.5) and of an aryl group
bound to the carbene carbon (δ 154.1 assigned to the ipso-carbon
bound to the carbene carbon).
The bis-alkynylcarbene complex 13 rearranged to the cyclic
cis-enediyne complex [Cp(CO)2Re]2[η2,η2-TolCtCC(CH2CH2-
CH2)dCCtCTol] (15) below room temperature (Scheme 10).
An NMR yield of >97% was determined using 1,4-bis-
trimethylsilylbenzene as an internal NMR standard. The struc-
ture of 15 was determined spectroscopically (Table 1) and
confirmed by X-ray crystallography (Figure 3). The X-ray
crystal structure of 15 shows near C2 symmetry broken only
by the envelope conformation of the cyclopentene.22 The
cyclopentene CdC double bond is slightly twisted (the C(2)-
C(3)-C(2A)-C(3A) dihedral angle is 4.4°), the complexed
alkynes are bent [the C(1)-C(2)-C(3) angle is 156.8°, and the
C(2)-C(3)-C(aryl) angle is 149.8°], and the two rheniums are
Figure 2. X-ray crystal structure of cyclopropyl-substituted enediyne
complex [Cp(CO)2Re]2[η2,η2-(E)-TolCtC(C3H5)CdC(C3H5)CtCTol]
(12-E).
68% conversion of 11 to 12. No ring expansion product was
1
observed by H or 13C NMR spectroscopy. Dimer 12-E was
isolated as a yellow crystalline compound following thin-layer
chromatography and recrystallization. The structure of 12-E was
established by X-ray crystallography which shows a trans-
1
enediyne structure (Figure 2). The low frequency H NMR
chemical shifts (δ 0.31, 0.47, 1.51), low frequency 13C NMR
1
chemical shifts [δ 7.25 (t, JCH ) 162.6 Hz, CH2), 15.84 (d,
1JCH ) 162.6 Hz, CdCCH)], and large CH coupling constants
seen for 12-E are typical of cyclopropanes. The second-order
rate constant for disappearance of 8 at 120 °C was k2 ) 5.33 ×
10-3 M-1 s-1. The rate of dimerization of 11 is about 3 times
faster than that of the dimerization of 1 which has a slightly
larger phenyl substituent on the alkynyl unit and is about 5 times
slower than the dimerization of 8 which has a smaller methyl
substituent.
Cyclic cis-Enediynes from Tethered Bis-(alkynylcarbene)
Complexes. The accessibility of alkyl-substituted cis-enediynes
encouraged us to attempt to prepare cyclic enediynes from
tethered bis-(alkynylcarbene) complexes. These substrates also
(22) A 50:50 disorder in the structure involves the two different envelope
conformations of the cyclopentene ring.
9
J. AM. CHEM. SOC. VOL. 124, NO. 11, 2002 2589

A R T I C L E S
Casey et al.
Scheme 11
bracket ∆Hq between 21.4 and 18.0 kcal mol-1 23
.
This range is
close to the activation enthalpy of the intermolecular reaction
8 f 10 (∆Hq ) 18.3 ( 0.4 kcal mol-1 and ∆Sq ) -19.8 (
1.2 eu), suggesting that in both cases the same principle
mechanism is operative. Essentially, the intermolecular reaction
of 8 f 10 is slowed due to an unfavorable entropy of activation
as compared with that of the intramolecular conversion of 13
f 15.
In an effort to further constrain the geometry of the linked
alkynylcarbene complexes to a parallel orientation, we set out
to prepare the 1,8-disubstituted naphthalene derivative 1,8-[Cp-
(CO)2RedC(Tol)CtC]2C10H6 (16). Addition of 1,8-di-(bro-
mozincethynyl)naphthalene to [Cp(CO)2RetCTol]BCl4 at -78
°C gave a deep yellow-black solution characteristic of carbene
complexes. This solution underwent a subtle color change to
dark purple at about -60 °C and showed no further color change
upon warming to room temperature. No attempt was made to
spectroscopically observe the likely intermediate 1,8-[Cp-
(CO)2RedC(Tol)CtC]2C10H6 (16) at low temperature. When
a sample taken from the -60 °C reaction mixture was
immediately analyzed by thin-layer chromatography, only the
coupled product [Cp(CO)2Re]2[(η2,η2-1,2-(tolylethynyl)acenaph-
thylene] (17) was seen; there was no evidence for a slower
moving carbene complex such as 16 (Scheme 11). For com-
parison, when a solution of bis-alkynylcarbene complex 13 was
prepared at low temperature and immediately analyzed by TLC,
spots for both 13 and the cis-enediyne complex [Cp(CO)2Re]2-
(η2,η2-TolCtCC(CH2CH2CH2)dCCtCTol) (15) were seen.
Therefore, the rate of CdC bond formation leading to 17 must
be at least as fast as that leading to 15.
Figure 3. X-ray crystal structure of the cyclic cis-enediyne complex [Cp-
(CO)2Re]2[η2,η2-TolCtCC(CH2CH2CH2)dCCtCTol] (15).
complexed on opposite faces of the enediyne. In the room
1
temperature H NMR spectrum of 15, the allylic methylene
hydrogens in the five-membered ring are diastereotopic (δ 2.83,
3.11) due to the neighboring CpRe(CO)2 group on one face of
the enediyne. The carbonyls on Re (δ 205.15, 205.44) are
diastereotopic in the 13C NMR spectrum; interconversion of their
environments requires both rotation of the alkyne about rhenium
and passing the two alkynyl groups past one another.
The kinetics of the rearrangement of the bis-alkynylcarbene
complex 13 to the cis-enediyne complex [Cp(CO)2Re]2[η2,η2-
TolCtCC(CH2CH2CH2)dCCtCTol] (15) were followed by 1H
NMR spectroscopy in toluene at 11.9 °C. The intramolecular
conversion of 13 to 15 was remarkably fast as compared to the
intermolecular coupling of 8. The half-life was 52 min at 11.9
°C as compared with the extrapolated half-life for a 0.02 M
solution of 8 of 154 days. Thus at the same 0.02 M concentra-
tion, intramolecular coupling of 13 occurs 4270 times faster
than does intermolecular coupling of 8. The first-order rate
constant for conversion of 13 to 15 was k ) 2.2 ( 0.1 × 10-3
The coupled product [Cp(CO)2Re]2(η2,η2-1,2-(tolylethynyl)-
acenaphthylene] (17) was isolated in 32% yield by column
chromatography. The structure of 17 was determined by X-ray
crystallography (Figure 4).
(23) If we assume that ∆Hq is the same as the 18.3 ( 0.4 kcal mol measured
for the intermolecular dimerization of 3, then ∆Sq ) -10.9 eu for the
intramolecular coupling of 13. This is a reasonable value for a first-order
process that must involve an ordered intramolecular configuration for
coupling.
s-1, corresponding to ∆Gq ) 21.43 ( 0.03 kcal mol-1
.
If we assume that the activation entropy of this intramolecular
first-order process lies in the range from 0 to -12 eu, we can
9
2590 J. AM. CHEM. SOC. VOL. 124, NO. 11, 2002

Formation of cis-Enediyne Complexes
A R T I C L E S
this twisted system.24-26 The cyclopropene intermediate E from
the 1,8-bis(alkynyl)naphthalene system would be even more
strained than cyclopropene D derived from the trimethylene-
linked complex 13.
However, coupling of both 13 and 16 not only occurred, but
did so at much lower temperatures than intermolecular dimer-
izations. A fair comparison of first-order intramolecular dimer-
izations with second-order intermolecular dimerizations should
center on ∆Hq of the two reactions and not simply on rates.
Our estimates suggest similar ∆Hq for intermolecular dimer-
ization of 8 (∆Hq ) 18.3 kcal mol-1) and for intramolecular
coupling of 13 (∆Hq ) 18-21 kcal mol-1). The rate of
formation of acenaphthylene 17 from the presumed 1,8-bis-
(alkynyl)naphthalene intermediate 16 was too fast to measure
but was apparently even more rapid than conversion of 13 to
15. The extreme ease of this intramolecular coupling may be
due to release of strain in the 1,8-dialkynylnaphthalene system.27
The very rapid dimerizations of both 13 and 16 provide
compelling evidence against mechanisms involving cyclopro-
pene intermediates.
Figure 4. X-ray crystal structure of Cp(CO)2Re]2 [η2,η2-1,2-(tolylethynyl)-
acenaphthylene] (17).
Sander recently observed that carbenes can react with alkynes
to generate a new CdC double bond between the carbene carbon
and one alkyne carbon and a new carbene center at the other
alkyne carbon.28 This suggests a fourth mechanism for alky-
nylcarbene complex dimerization that involves three steps: (1)
an initial reversible [1,1.5] Re shift to give “carbene” intermedi-
ate A, (2) rate-determining addition of the carbene center of A
to the remote alkynyl carbon of a second alkynylcarbene
complex to generate vinyl carbene intermediate C, and (3)
rearrangement of C to the enediyne complex by a [1,1.5] Re
shift (Scheme 12). In the transition state, the carbene lone pair
is suggested to act as a nucleophile and conjugately add to the
remote alkynyl carbon, while the empty carbene orbital accepts
electrons from an alkyne π-bond.29 This mechanism avoids
impossibly strained cyclopropene intermediates, makes use of
the carbene-like reactivity of the remote alkynyl carbon of the
Discussion
Mechanism of Dimerization. One of our suggested mech-
anisms for alkynylcarbene complex dimerization involves an
initial [1,1.5] Re shift to give a species A with carbene character
at the remote alkynyl carbon. The ability of Re to undergo a
[1,3] shift [at somewhat slower rates than dimerization for 1
and 8, and at faster rates than dimerization of Cp(CO)2Red
C(Tol)CtCC6H4-p-SO2CF3] suggested that the more modest
[1,1.5] shift needed to be seriously considered for the dimer-
ization of 1. The carbene-like reactivity of the remote alkynyl
carbon seen in the cyclopropanation of 4 and in the insertion
into a CH bond of the Cp* complex 5 (Scheme 3) is readily
accounted for by initial [1,1.5] shifts to give carbene intermedi-
ates related to A.
The cyclopropyl-substituted alkynylcarbene complex 11 was
prepared as a test for carbene-like reactivity of the remote
alkynyl carbon. The cyclopropyl substituent should be able to
intercept a free carbene via ring expansion. Cyclopropyl
carbenes are known to quickly rearrange to cyclobutenes often
accompanied by fragmentation reactions forming alkenes and
alkynes.21 The failure of complex 11 to undergo ring expansion
to a cyclobutene suggests that if intermediate A is involved,
then interaction of the neighboring Re(alkyne) complex with
the carbene center greatly diminishes the carbene character as
compared with that of the free cyclopropyl carbenes. Similarly,
the failure of methyl-substituted alkynylcarbene complex 8 to
undergo a 1,2 hydrogen shift to form Cp(CO)2Re(η2-TolCt
CCHdCH2) suggests that if intermediate A is involved it has
diminished carbene character.
(24) Domnin, I. N.; Ponomarev, D. A.; Takhistov, V. V.; Pihlaja, K. Russ. J.
Org. Chem. 1999, 35, 28.
(25) Nevertheless, bicyclo[3.1.0]hex-1(16)-ene derivatives have been generated
in situ, and their structures have been deduced from trapping experiments.
(a) Weber, J.; Brinker, U. H. Tetrahedron 1996, 52, 14641. (b) Halton, B.;
Diggins, M. D.; Kay, A. J. J. Org. Chem. 1992, 57, 4080. (c) Baird, M. S.;
Netherscott, W. Tetrahedron Lett. 1983, 24, 605. (d) Coleman, B.; Jones,
M., Jr. J. Organomet. Chem. 1979, 168, 393. (e) Wolff, S.; Agosta, W. C.
J. Am. Chem. Soc. 1984, 106, 2363. (f) Billups, W. E.; Haley, M. M.; Lee,
G.-A. Chem. ReV. 1989, 89, 1147. (g) Baird, M. S. Functionalized
Cyclopropenes as Synthetic Intermediates. In Topics in Current Chemistry;
de Meijere, A., Ed.; Springer-Verlag: Berlin, 1988; Vol. 144, p 137.
(26) Bicyclo[3.1.0]hex-1(6),3-diene-2-one has been isolated in an argon matrix
at 10 K, and the cyclopropenyl moiety was identified by IR spectroscopy.
Bucher, G.; Sander, W. J. Org. Chem. 1992, 57, 1346.
(27) 1,8-Dialkynylnaphthalenes are known to undergo cycloadditions, bromi-
nations, and hydrogenations accompanied by intramolecular CdC bond
formation of the internal alkyne carbons. Release of strain is supposedly a
driving force for this unique reactivity. (a) Staab, H. A.; Nissen, A.;
Ipaktschi, J. Angew. Chem., Int. Ed. Engl. 1968, 7, 226. (b) Mitchell, R.
H.; Sondheimer, F. Tetrahedron 1970, 26, 2141. (c) Staab, H. A.; Ipaktschi,
J.; Nissen, A. Chem. Ber. 1971, 104, 1182. (d) Mu¨ller, E.; Thomas, R.;
Zountsas, G. Justus Liebigs Ann. Chem. 1972, 758, 16. (e) Bossenbroek,
B.; Sanders, D. C.; Curry, H. M.; Shechter, H. J. Am. Chem. Soc. 1969,
91, 371. (f) Zhou, Z.; Costa, M.; Chiusoli, G. P. J. Chem. Soc., Perkin
Trans. 1 1992, 1407. (g) Blum, J.; Badrieh, Y.; Shaaya, O.; Meltser, L.;
Schumann, H. Phosphorus, Sulfur Silicon Relat. Elem. 1993, 79, 87. (h)
Blum, J.; Baidossi, W.; Badrieh, Y.; Hoffman, R. E.; Schumann, H. J.
Org. Chem. 1995, 60, 4738. (i) Eckert, T.; Ipaktschi, J. Monatsh. Chem.
1998, 129, 1035. (j) Cobas, A.; Guitia´n, E.; Castedo, L. J. Org. Chem.
1997, 62, 4896.
The tethered bis(alkynylcarbene) complexes 13 and 16 were
studied to probe the geometric requirements for CdC bond
formation. In particular, the mechanism shown in Scheme 2
requires an orthogonal approach of A to the alkyne unit of a
second alkynylcarbene complex to produce cyclopropene in-
termediate B. Formation of cyclopropenes from either 13 or 16
would lead to the very strained intermediates D and E and would
be expected to greatly retard or prevent dimerization by such a
mechanism. Calculations suggest the strain energy in cyclopro-
penes (∼54 kcal mol-1) is increased by 24 kcal mol-1 in
bicyclo[3.1.0]hex-1(6)-ene due to the extra bicyclic strain in
(28) Sander found that the reactive singlet vinylidene CF2dC: adds to
difluoroacetylene to form the allenylcarbene CF2dCdCF-CF: (observed
by IR in matrix isolation) without the intermediacy of a methylene
cyclopropene (more stable than the allenylcarbene by 18.1 kcal mol-1).
Ko¨tting, C.; Sander, W.; Senzlober, M. Chem.-Eur. J. 1998, 4, 2360.
9
J. AM. CHEM. SOC. VOL. 124, NO. 11, 2002 2591

A R T I C L E S
Casey et al.
Scheme 12
Scheme 13
alkynylcarbene complex, and is consistent with the insensitivity
of the rate of dimerization to solvent polarity. This transition
state for dimerization is similar to the transition states proposed
for cyclopropane formation from 4, and for CH insertion of 5;
all involve carbene-like reactivity at the remote alkynyl carbon
of intermediates similar to A, formed by an initial reversible
[1,1.5] Re shift of an alkynylcarbene complex.30
a rate of heterodimerization intermediate between the rates of
the two homodimerizations.
Synthetic Potential of Intramolecular Coupling Reactions.
In light of the antitumor activity of cyclic cis-enediynes that
are able to cleave double-stranded DNA via Bergman cycliza-
tion,32-34 we became interested in finding conditions to obtain
cis-enediyne systems via coupling of rhenium alkynylcarbene
complexes. Surprisingly, few procedures for the preparation of
cyclic cis-enediynes are currently available and often involve
multistep syntheses.35 To enforce a cis-geometry at the central
double bond, we geometrically constrained the coupling step
Our observations cannot exclude the dimerization mechanism
(Scheme 6) which involves C-C bond formation between the
remote alkynyl carbons concerted with two [1,1.5] Re shifts.
In as much as this mechanism resembles a carbene dimerization
for which a coplanar approach of the carbene units is symmetry
forbidden,17 a geometry of approach and orbital interactions as
shown in Scheme 13 should be considered.
(32) Jones, R. R.; Bergman, R. G. J. Am. Chem. Soc. 1972, 94, 660.
(33) Cyclic enediynes can be extremely potent anticancer antibiotics. Ko¨nig,
B.; Pitsch, W.; Klein, M.; Vasold, R.; Prall, M.; Schreiner, P. R. J. Org.
Chem. 2001, 66, 1742.
(34) For reviews of cyclic enediynes, see: (a) Grissom, J. W.; Gunawardena,
G. U.; Klingberg, D.; Huang, D. Tetrahedron 1996, 52, 6453. (b) Nicolaou,
K. C.; Smith, A. L. In The Enediyne Antibiotics in Modern Acetylene
Chemistry; Stang, P. D., Diederich, F., Eds.; VCH: Weinheim, 1995;
Chapter 7, p 203. (c) Maier, M. E. Synlett 1995, 13. (d) Smith, A. L.;
Nicolaou, K. C. J. Med. Chem. 1996, 39, 2103. (e) Suffert, J.; Abraham,
E.; Raeppel, S.; Bru¨ckner, R. Liebigs Ann. 1996, 447. (f) Sander, W. Acc.
Chem. Res. 1999, 32, 669. (g) Nicolaou, K. C.; Dai, W.-M. Angew. Chem.,
Int Ed. Engl. 1991, 30, 1387.
(35) (a) Nicolaou, K. C. Angew. Chem., Int. Ed. Engl. 1993, 32, 1377. (b)
Brandstetter, T.; Maier, M. E. Tetrahedron 1994, 50, 1435. (c) Iida, K.-I.;
Hirama, M. J. Am. Chem. Soc. 1994, 116, 10310. (d) Wender, P. A.;
Beckham, S.; Mohler, D. L. Tetrahedron Lett. 1995, 36, 209. (e) Magnus,
P.; Fortt, S. M. J. Chem. Soc., Chem. Commun. 1991, 544. (f) Nicolaou,
K. C.; Zuccarello, G.; Ogawa, Y.; Schweiger, E. J.; Kumazawa, T. J. Am.
Chem. Soc. 1988, 110, 4866. (g) Wender, P. A.; McKinney, J. A.; Mukai,
C. J. Am. Chem. Soc. 1990, 112, 5369. (h) Shair, M. D.; Yoon, T. Y.;
Mosny, K. K.; Chou, T. C.; Danishefsky, S. J. J. Am. Chem. Soc. 1996,
118, 9509. (i) Funk, R. L.; Young, E. R. R.; Williams, R. M.; Flanagan,
M. F.; Cecil, T. L. J. Am. Chem. Soc. 1996, 118, 3291. (j) Jones, G. B.;
Huber, R. S.; Mathews, J. E.; Li, A. Tetrahedron Lett. 1996, 37, 3643. (k)
Wright, J. M.; Jones, G. B. Tetrahedron Lett. 1999, 40, 7605. (l) Jones, G.
B.; Mathews, J. E. Tetrahedron 1997, 53, 14599.
One possible way to distinguish between these two mecha-
nisms would be to compare the rates of homo- and heterodimer-
ization of two different alkynylcarbene complexes. The mech-
anism shown in Scheme 12 suggests that rates of heterodimeri-
zation of differently substituted compounds might be faster than
either homodimerization since the two alkynylcarbene com-
plexes play different roles in the rate-determining step.31 In
contrast, the symmetric mechanism shown in Scheme 6 predicts
(29) While this mechanism is similar to our second mechanism shown in Scheme
5, intermediate C in Scheme 12 differs markedly from intermediate C′ in
Scheme 5 in terms of charge separations and the bond order of the newly
formed C-C bond.
(30) Intermediate A can be considered an extreme depiction of the reactive center
in the reactions.
(31) In particular, the carbene-like intermediate A is an electrophile and might
be more reactive with electron-withdrawing groups attached, while the
alkynylcarbene complex undergoing attack acts as a nucleophile and might
be more reactive with electron-donor substituents attached.
9
2592 J. AM. CHEM. SOC. VOL. 124, NO. 11, 2002

Formation of cis-Enediyne Complexes
A R T I C L E S
1
3
aromatic), 132.74 (dd, JCH ) 160.2 Hz, JCH ) 4.7 Hz, aromatic),
137.46 (sextet, 2JCH ) 3JCH ) 6.2 Hz, aromatic), 204.81 (broad s, CO),
205.93 (broad s, CO).
by tethering two alkynylcarbene units. The successful conversion
of 13 f 14 illustrates the viability of this concept.
The transformation of our initial observation of the synthesis
of cyclic cis-enediyne rhenium complexes into a viable synthetic
procedure faces many challenges. Rhenium is too expensive
for a stoichiometric reaction, and the high kinetic stability of
rhenium alkyne complexes poses a problem for metal release.
The synthesis of alkynylcarbene complexes is limited by the
functional group compatibility of the precursor metal acetylides
and metal carbyne complexes. Extension to carbyne precursors
other than aryl carbyne complexes, extensions to allow use of
two different alkyne precursors, and extensions to unsymmetric
diyne precursors are all needed. Our initial efforts to improve
this new route to cyclic cis-enediynes will center on analogous
manganese compounds36 since their use would be more eco-
nomical and metal decomplexation from π-complexes is ex-
pected to be facile.
Cp(CO)2RedC(Tol)CtCC3H5 (11). Addition of BrZnCtCC3H5
[from LiCtCC3H5) (22 mg, 0.31 mmol) and ZnBr2 (70 mg, 0.31 mmol)
in 1 mL of THF] to an orange solution of [Cp(CO)2RetCTol]BCl4
(162 mg, 0.307 mmol) in 5 mL of THF at -35 °C produced a black
color immediately. After 10 min, the cold reaction mixture was poured
onto a silica gel column (30 × 2 cm), and a black fraction was eluted
with 4:1 hexane:diethyl ether. Evaporation of solvent under reduced
pressure produced a solid material which was redissolved in 2 mL of
3:1 hexane:CH2Cl2. Slow evaporation gave black needles of 11 (93
mg, 0.20 mmol, 64%) suitable for X-ray crystal structure analysis. 1H
NMR (CD2Cl2, 500 MHz): δ 0.98 (AA′ part of AA′BB′X pattern,
CHHCHH), 1.10 (BB′ part of AA′BB′X pattern, CHHCHH), 1.70 (tt,
3
3Jcis ) 8.3 Hz, Jtrans ) 4.9 Hz, CtCCH), 2.15 (s, ArCH3), 5.67 (s,
C5H5), 7.11 (d, 3J ) 8.3 Hz, aromatic CH), 7.91 (d, 3J ) 8.3 Hz,
1
aromatic CH). 13C NMR (CD2Cl2, 125 MHz): δ 3.65 (dpent, JCH
)
2
2′
1
170.5 Hz, JCH
)
J
) 2.8 Hz, CtCCH), 10.51 (dm, JCH ) 164.9
CH
1
3
Hz, CH2), 21.84 (qt, JCH ) 126.5 Hz, JCH ) 4.5 Hz, ArCH3), 94.72
Experimental Section
(dpent, 1JCH ) 179.6 Hz, 2JCH ) 3JCH ) 6.7 Hz, C5H5), 99.73 (d, 3JCH
Cp(CO)2RedC(Tol)CtCCH3 (8). Addition of BrZnCtCCH3 [pre-
pared from LiCtCCH3 (14 mg, 0.30 mmol) and ZnBr2 (66 mg, 0.29
mmol) in 1 mL of THF] to an orange solution of [Cp(CO)2RetCTol]-
BCl4 (151 mg, 0.287 mmol) in 5 mL of THF at -35 °C produced a
black color immediately. After 10 min, the cold reaction mixture was
poured onto a silica gel column (30 × 2 cm), and a black fraction was
eluted with 4:1 hexane:diethyl ether. Evaporation of solvent under
reduced pressure produced a solid material which was redissolved in 2
mL of 3:1 hexane:CH2Cl2. Slow evaporation gave black needles of 8
(28 mg, 0.0623 mmol, 22%). 1H NMR (CD2Cl2, 500 MHz): δ 1.90 (s,
CtCH3), 2.14 (s, ArCH3), 5.73 (s, C5H5), 7.11 (d, 3J ) 8.4 Hz, aromatic
CH), 7.97 (d, 3J ) 8.4 Hz, aromatic CH). 13C{1H} NMR (CD2Cl2, 125
MHz): δ 7.3 (CtCCH3); 21.9 (ArCH3); 94.9 (C5H5); 103.1 (CtCCH3);
120.5 (CtCCH3); 127.6, 129.6, 140.7, 157.2 (aromatic); 205.4 (CO);
258.1 (RedC). IR (CH2Cl2): 1961, 1885 cm1. HRMS(El) m/z: calcd
for C18H15O2Re (M+), 456.0630; found, 456.0628.
1
3
) 3.5 Hz, CtCCH), 127.45 (dd, JCH ) 159.2 Hz, JCH ) 5.7 Hz,
1
2
3
aromatic), 129.59 (dpent, JCH ) 159.1 Hz, JCH ) JCH ) 5.6 Hz,
aromatic), 131.17 (m, CtCCH), 140.69 (q, 2JCH ) 6.8 Hz, aromatic),
3
156.81 (t, JCH ) 7.9 Hz, aromatic), 205.40 (s, CO), 256.67 (s, Red
C). IR (CH2Cl2): 1959, 1884 cm-1. HRMS(EI) m/z: calcd for
C20H17O2185Re (M+), 474.0758; found, 474.0745.
[Cp(CO)2Re]2[η2,η2-(E)-TolCtC(C3H5)CdC(C3H5)CtCTol] (12-
E). Thermolysis of Cp(CO)2RedC(Tol)CtC(C3H5) (11) (19.8 mg,
0.0416 mmol) and 1,4-bis-trimethylsilylbenzene (2 mg, internal NMR
standard) in 0.58 mL of toluene-d8 at 120 °C for 281 min produced a
yellow precipitate. The conversion of starting material as determined
by 1H NMR integration was 82% (with 95% mass balance). A 1H NMR
spectrum taken after 91 min (with no precipitate formed at this point)
indicated a mixture of 45% starting material and 55% of a 5:1 ratio of
3
12-E:12-Z. [1H NMR (toluene-d8, 500 MHz) 12-E: δ 0.49 (d, Jcis
)
3Jtrans ) 6.0 Hz, CH2), 1.74 (pent, Jcis ) Jtrans ) 6.0 Hz, CtCCH),
2.15 (s, ArCH3), 4.93 (s, C5H5), 7.07 (d, J ) 8.0 Hz, aromatic CH),
3
3
[Cp(CO)2Re]2[η2,η2-(E)-TolCtC(CH3)CdC(CH3)CtCTol] (10-
E). Thermolysis of Cp(CO)2RedC(Tol)CtCCH3 (8) (80.8 mg, 0.180
mmol) in 0.75 mL of toluene-d8 at 100 °C for 55 min and subsequent
preparative thin-layer chromatography (silica, 9:1 hexane:diethyl ether,
Rf ) 0.3, yellow band) gave a yellow powder consisting of a 1.2:1
mixture of 10-E:10-Z. The mixture was dissolved in CH2Cl2:hexane;
slow evaporation of solvent under air exposure gave yellow needles of
10-E (40.1 mg, 0.0446 mmol, 50%). For 10-E, 1H NMR (CD2Cl2, 500
MHz): δ 1.95 (s, CdCCH3), 2.40 (s, ArCH3), 5.49 (s, C5H5), 7.24 (d,
3
7.77 (d, 3J ) 8.0 Hz, aromatic CH). 12-Z: δ 4.78 (s, C5H5), 6.95 (d, 3J
) 8.0 Hz, aromatic CH), all other peaks overlap with peaks from 12-E
and solvent.] Preparative thin-layer chromatography (silica, 3:1 hexane:
diethyl ether, Rf ) 0.3, yellow band) gave a yellow powder. Recrys-
tallization from CHCl3 afforded yellow crystals of (12-E)2•CHCl3 (10.1
mg, 0.00499 mmol, 48%) suitable for X-ray crystal structure analysis.
1H NMR (CDCl3, 500 MHz, 60 °C): δ 0.31 (AA′ part of AA′BB′X
pattern, CHHCHH), 0.47 (BB′ part of AA′BB′X pattern, CHHCHH),
3
3J ) 8.1 Hz, aromatic CH), 7.54 (d, J ) 8.1 Hz, aromatic CH). 13C
3
3
1.51 (tt, Jcis ) 7.6 Hz, Jtrans ) 5.2 Hz, CtCCH), 2.38 (s, ArCH3),
1
3
NMR (CDCl3, 125 MHz): δ 21.40 (qt, JCH ) 125.6 Hz, JCH ) 4.6
Hz, ArCH3), 21.73 (q, 1JCH ) 127.7 Hz, CdCCH3), 79.59 (s, CtCAr),
80.50 (s, CtCAr), 88.76 (dp, 1JCH ) 179.0 Hz, 2JCH ) 3JCH ) 6.7 Hz,
C5H5), 128.14 (t, 3JCH ) 6.7 Hz, aromatic), 129.16 (q, 3JCH ) 7.1 Hz,
CdC), 129.44 (d, 1JCH ) 157.8 Hz, aromatic), 132.46 (dd, 1JCH ) 159.1
3
3
5.45 (s, C5H5), 7.18 (d, J ) 8.0 Hz, aromatic CH), 7.50 (d, J ) 8.0
Hz, aromatic CH). 13C NMR (CDCl3, 125 MHz, 60 °C): δ 7.25 (t,
1JCH ) 162.6 Hz, CH2), 15.84 (d, JCH ) 162.6 Hz, CdCCH), 21.41
1
(qt, 1JCH ) 126.5 Hz, 3JCH ) 4.6 Hz, ArCH3), 77.50 (d, 3JCH ) 5.6 Hz,
CtCAr), 85.06 (s, CtCAr), 86.68 (d, 1JCH ) 179.5 Hz, C5H5), 129.38
2
3
Hz, JCH ) 4.7 Hz, aromatic), 138.06 (sext, JCH ) JCH ) 6.2 Hz,
aromatic), 205.38 (s, CO). IR (CH2Cl2): 1966, 1885 cm-1. MS(FAB)
m/z: calcd for C36H30O4Re2 (M+), 898.1; found, 898.1.
3
1
3
(t, JCH ) 3.1 Hz, aromatic), 129.49 (dt, JCH ) 157.5 Hz, JCH ) 5.6
Hz, aromatic), 131.75 (dd, 1JCH ) 159.3 Hz, 3JCH ) 6.6 Hz, aromatic),
133.73 (m, CdC), 138.03 (q, 2JCH ) 5.5 Hz, aromatic), 206.28 (s, CO).
IR (CH2Cl2): 1966, 1878 cm-1. MS(EI) m/z: calcd for C40H34O4Re2
(M+ + 1), 951.2; found, 951.2.
For 10-Z, 1H NMR (CD2Cl2, 500 MHz): δ 2.19 (s, CdCCH3), 2.28
(s, ArCH3), 5.28 (s, C5H5), 6.92 (d, 3J ) 7.8 Hz, aromatic), 7.18 (d, 3J
1
) 8.0 Hz, aromatic). 13C NMR (CDCl3, 125 MHz): δ 21.36 (qt, JCH
Cp(CO)2RedC(Tol)CtCCH2CH2CH2CtCC(Tol)dRe-
(CO)2Cp (13). Addition of BrZnCtCCH2CH2CH2CtCZnBr [from
LiCtCCH2CH2CH2CtCLi (5 mg, 0.05 mmol) and ZnBr2 (22 mg,
0.098 mmol, sublimed) in 0.2 mL of THF] to [Cp(CO)2RetCTol]BCl4
(78 mg, 0.15 mmol) in 0.2 mL of THF at -35 °C produced a black
solution. The cold reaction mixture was poured onto a silica thin-layer
chromatography plate which was precooled to -40 °C. While maintain-
ing this temperature with solid dry ice underneath the plate, the solvent
was evaporated under a gentle stream of cold N2. The plate was placed
3
1
) 125.6 Hz, JCH ) 4.6 Hz, ArCH3), 21.73 (q, JCH ) 127.6 Hz, Cd
CCH3), 79.87 (s, CtCAr), 82.29 (s, CtCAr), 88.35 (dp, 1JCH ) 179.0
Hz, 2JCH ) 3JCH ) 6.3 Hz, C5H5), 128.92 (broad s, CdC), 128.99 (dt,
1JCH ) 158.5 Hz, JCH ) 6.3 Hz, aromatic), 129.38 (t, JCH ) 7.5 Hz,
3
3
(36) The synthesis of an analogous Mn alkynylcarbene complex Cp(CO)2Mnd
CPhCtCTol has recently been reported. The interconversion of diman-
ganese compounds Cp(CO)2MndCPh{η2-CtCTol[Mn(CO)2Cp]} by two
[1, 1.5] shifts was also observed. Ortin, Y.; Coppel, Y.; Lugan, N.; Mathieu,
R.; McGlinchey, M. J. Chem. Commun. 2001, 1690.
9
J. AM. CHEM. SOC. VOL. 124, NO. 11, 2002 2593

A R T I C L E S
Casey et al.
into a precooled chamber, and a black band was eluted (4:1 pentane:
diethyl ether, Rf ) 0.3). After extraction with CH2Cl2 and evaporation
of the solvent under reduced pressure at 0 °C, 13 (4 mg, 8%) was
obtained as a black solid. A faster moving band containing Cp-
(CO)2RedC(Tol)CtCCH2CH2CH2CtCH (14) was also observed. For
[Cp(CO)2Re]2[η2,η2-1,2-(TolCtC)2C12H6] (17). Sequential addition
of n-butyllithium (0.48 mL, 1.6 M, 0.30 mmol) and ZnBr2 (68 mg,
0.30 mmol) to 1,8-(HCtC)2C10H6 (25 mg, 0.14 mmol) in 3 mL of
THF at -78 °C produced a solution of 1,8-(BrZnCtC)2C10H6. Upon
addition of a solution of [Cp(CO)2RetCTol]BCl4 (147 mg, 0.280
mmol) in 1 mL of THF, the solution turned black immediately. After
warming to 24 °C, the reaction mixture was poured onto a silica gel
column (30 × 2 cm), and a purple black fraction was eluted with 2:1
hexane:diethyl ether. Evaporation of solvent under reduced pressure
produced a solid material which was redissolved in 2 mL of 2:1 CH2-
Cl2:hexane. Slow evaporation gave fine black crystals of 17 (45 mg,
32%) suitable for X-ray crystallography. 1H NMR (CD2Cl2, 300
MHz): δ 2.29 (s, ArCH3), 5.44 (s, C5H5), 6.93 (d, 3J ) 8.0 Hz, aromatic
CH), 7.22 (d, 3J ) 7.8 Hz, aromatic CH), 7.64 (dd, 3J ) 8.2 Hz, 3J )
7.0 Hz, aromatic CH), 7.85 (d, 3J ) 7.0 Hz, aromatic CH), 7.90 (d, 3J
) 8.2 Hz, aromatic CH). 13C{1H} NMR (THF-d8, 125 MHz, 60 °C):
δ 21.30 (ArCH3); 72.52 (CtCTol); 89.65 (C5H5); 93.45 (CtCTol);
123.33, 127.78, 128.66, 129.29, 129.85, 130.36, 131.15, 132.97, 135.19,
137.82, 141.36 (aromatic); 205.06 (broad, ω1/2 ) 48 Hz, CO). IR (CH2-
Cl2): 1970, 1885 cm-1. MS(FAB) m/z: calcd for C44H30O4Re2 (M+),
994.1; found, 994.1.
1
13, H NMR (CD2Cl2, 500 MHz, -80 °C): δ 2.05 (s, CH3), 2.42 (m,
CH2CH2CH2), 5.70 (s, C5H5), 7.08 (d, 3J ) 8.2 Hz, aromatic CH), 8.03
(d, 3J ) 8.3 Hz, aromatic CH). 13C NMR (CD2Cl2, 125 MHz): δ 21.3
(t, 1JCH ) 131.1 Hz, CH2CH2CH2), 21.45 (q, 1JCH ) 126.7 Hz, ArCH3),
1
1
26.6 (t, JCH ) 129.1 Hz, CH2CH2CH2), 94.9 (d, JCH ) 179.5 Hz,
1
C5H5), 101.9 (s, CtCAr), 122.2 (s, CtCAr), 127.8 (d, JCH ) 159.9
Hz, aromatic), 129.2 (d, 1JCH ) 159.1 Hz, aromatic), 140.8 (s, aromatic),
154.1 (s, aromatic), 205.6 (s, CO), 254.5 (s, RedC).
[Cp(CO)2Re]2[η2,η2-TolCtCC(CH2CH2CH2)dCCtCTol] (15).
The bis-zincacetylide BrZnCtCCH2CH2CH2CtCZnBr [from
LiCtCCH2CH2CH2CtCLi (12 mg, 0.12 mmol) and ZnBr2 (52 mg,
0.23 mmol, sublimed) in 1 mL of THF] was added to [Cp(CO)2Ret
CTol]BCl4 (125 mg, 0.237 mmol) in 2 mL of THF at -35 °C. The
cold black reaction mixture was poured onto a silica gel column (30 ×
2 cm), and a black band was eluted with 4:1 hexane:diethyl ether.
Evaporation of solvent under reduced pressure gave a dark solid which
was redissolved in 1 mL of CH2Cl2 and stirred for 2 h. Preparative
thin-layer chromatography (silica, 4:1 hexane:diethyl ether) gave 15
(36 mg, 35%) as an orange band (Rf ) 0.4) and 14 (17 mg, 0.034
mmol, 29%) as a black band (Rf ) 0.5).
Acknowledgment. Financial support from the National Sci-
ence Foundation is gratefully acknowledged. Grants from NSF
(CHE-9629688) for the purchase of the NMR spectrometers and
a diffractometer (CHE-9709005) are acknowledged.
1
3
For 15, H NMR (CD2Cl2, 500 MHz): δ 2.08 (pent, J ) 7.5 Hz,
CH2CH2CH2), 2.23 (s, CH3), 2.83 [broad, ω1/2 ) 35 Hz, CHHCH2-
CHH], 3.11 (broad, ω1/2 ) 35 Hz, CHHCH2CHH), 5.44 (s, C5H5), 6.89
(d, 3J ) 7.9 Hz, aromatic), 7.00 (d, 3J ) 7.9 Hz, aromatic). 13C NMR
Supporting Information Available: General experimental
procedures, description of kinetics of the conversion of 8 to
10-E/l0-Z and of 13 to 15, spectra of 14, and X-ray crystal
structures of 11, (12-E)2•CHCl3, 15, and 17 (PDF). This
material is available free of charge via the Internet at
http://pubs.acs.org.
1
(CD2Cl2, 125 MHz): δ 21.29 (q, JCH ) 125.3 Hz, ArCH3), 23.24 (t,
1JCH ) 132.2 Hz, CH2CH2CH2), 40.06 (t, JCH ) 133.8 Hz, CH2-
1
CH2CH2), 72.21 (br s, CtCAr), 85.24 (s, CtCAr), 88.56 (dpent, 1JCH
2
3
1
) 179.5 Hz, JCH ) JCH ) 6.5 Hz, C5H5), 128.83 (d, JCH ) 159.2
Hz, aromatic), 128.87 (s, aromatic), 136.15 (s, CdC), 137.34 (q, 2JCH
) 6.8 Hz, aromatic), 205.15 (s, CO), 205.44 (s, CO). IR (CH2Cl2):
1964, 1881 cm-1. MS(FAB) m/z: calcd for C37H30O4Re2 (M+), 910.1;
found, 910.1.
JA011962O
9
2594 J. AM. CHEM. SOC. VOL. 124, NO. 11, 2002