[1,3]-metal shifts in rhenium alkynyl carbene complexes

Article abstract of DOI:10.1021/om0103299

The electron-withdrawing SO₂CF₃ substituent of Cp(CO)₂Re=C(Tol)(C≡CC₆H₄-p-SO₂ CF₃) (1d) greatly accelerates rearrangement via a [1,3]-rhenium shift. The [1,3]-rhenium shift is proposed to proceed via an 18-electron dehydrometallacyclobutadiene intermediate formed by alkyne coordination concerted with an η⁵-η³ Cp ring slip.

Full text of DOI:10.1021/om0103299

© Copyright 2001
Volume 20, Number 13, June 25, 2001
American Chemical Society
Communications
[1,3]-Meta l Sh ifts in Rh en iu m Alk yn yl Ca r ben e
Com p lexes
Charles P. Casey,* Stefan Kraft, and Douglas R. Powell
Department of Chemistry, University of WisconsinsMadison, Madison, Wisconsin 53706
Received April 20, 2001
Summary: The electron-withdrawing SO2CF3 substitu-
ent of Cp(CO)2RedC(Tol)(CtCC6H4-p-SO2CF3) (1d )
greatly accelerates rearrangement via a [1,3]-rhenium
shift. The [1,3]-rhenium shift is proposed to proceed via
an 18-electron dehydrometallacyclobutadiene intermedi-
ate formed by alkyne coordination concerted with an η5-
η3 Cp ring slip.
were observed (Scheme 1).4 Surprisingly, dimerization
to the trans enediyne complex 3 was the major reaction
observed.
Sch em e 1
Heteroatom-substituted alkynyl carbene complexes
such as (CO)5MdC(OR)CtCR′ are highly electrophilic
and synthetically versatile.1 While many such complexes
have been studied, [1,3]-shifts have not been observed
because of either unfavorable equilibria or high kinetic
barriers. Recently, we began a search for an unprec-
edented [1,3]-metal shift in a non-donor-substituted,
nearly symmetric alkynyl carbene complex;2 analogous
rearrangements of σ-propargyl to σ-allenyl compounds
are well-known.3 When Cp(CO)2RedC(Tol)(13Ct13CPh)
(1a -13C2; Tol ) C6H4-p-CH3) was heated at 120 °C, only
small amounts of its [1,3]-metal-shifted isomer 2a -13C2
Sch em e 2
(1) For reviews of heteroatom-substituted alkynyl carbene com-
plexes, see: (a) Aumann, 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, U.K., 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 was postulated but not directly observed in
a
rhodium alkynyl carbene complex that had 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.
10.1021/om0103299 CCC: $20.00 © 2001 American Chemical Society
Publication on Web 05/25/2001

2652 Organometallics, Vol. 20, No. 13, 2001
Communications
Sch em e 3
Ta ble 1. Equ ilibr iu m a n d Ra te Con sta n ts for
[1,3]-Rh en iu m Sh ifts in Tolu en e-d 8 a t 120 °C
In an effort to increase the ratio of [1,3]-metal shift
to dimerization, we explored effects of electron-with-
drawing para substituents on the alkynyl carbene
complexes 1a -d (Scheme 2). Here we report that
electron-withdrawing substituents substantially in-
crease the rate of the [1,3]-metal shift and that related
rhenium indenyl complexes undergo the [1,3]-metal
shift at dramatically higher rates. A mechanism involv-
ing a dehydrometallacyclobutadiene intermediate or
transition state stabilized by ring slippage is proposed
for the [1,3]-metal shift.
L
X
Keq
kobsd (10-6 s-1
)
k1(rel)
1a
1b
1c
1d
4a
Cp
Cp
Cp
Cp
Ind
H
1.0
1.7
3.3
4.0
1.0
4.3
3.2
21
240
2400
1.0
0.9
7.4
88
550
N(CH3)2
CF3
SO2CF3
H
ated isomerization by a [1,3]-metal shift (k1(rel) ) 88).
The electron-donating NMe2 substituent of 1b slightly
decreased the rate of isomerization to 2b (k1(rel) ) 0.9).
Only small solvent effects were seen on rates of isomer-
ization of 1d to 2d (k1 ) 1.9 × 10-4 s-1 in toluene, 1.1
× 10-4 s-1 in methylcyclohexane, 4.4 × 10-4 s-1 in tert-
butyl alcohol, and 9.7 × 10-4 s-1 in nitrobenzene),
indicating similar polarities of the ground and transition
states. 11
A net [1,3]-metal shift requires a transition structure
resembling a dehydrometallacyclobutadiene with car-
bons 1 and 3 symmetrically bound to the metal. Alkyne
coordination to rhenium would generate a 20-electron
intermediate unless CO dissociation or Cp ring slippage
occurred.
A mechanism involving CO dissociation was excluded
by the absence of 13CO exchange during isomerization
of 1d . When the partial isomerization of 1d under 0.1
atm of 13CO was followed by 13C NMR spectroscopy, the
60:40 mixture of 1d and 2d showed no Re13CO signal
enhancement.
Reactions involving ring slippage occur much more
rapidly for indenyl than cyclopentadienyl complexes
because the six-membered ring becomes more aromatic
upon ring slippage.12 To test whether ring slippage is
an important component in the [1,3]-metal shifts, we
measured rates of isomerization of indenyl complexes
4a -d . Indenyl complex 4a underwent a [1,3]-metal shift
550 times faster than the Cp analogue 1a (Table 1).13
Interestingly, the rate accelerations induced by electron-
Substituted Cp alkynyl carbene complexes 1a -d and
related indenyl complexes 4a -d were synthesized in
good yields (60-70%) by addition of metal acetylides
MCtCAr (M ) Li, ZnBr, Cu) to the cationic rhenium
5
carbyne complexes [Cp(CO)2RetCAr′]BCl4 and [(η5-
indenyl)(CO)2RetCAr′]BCl4.6 When a dilute solution7
of the SO2CF3-substituted complex 1d was heated at 120
°C, rapid isomerization to an 80:20 equilibrium mixture
of the [1,3]-shift isomer 2d to the starting material 1d
occurred and no dimerization was seen. 2d was isolated
by preparative thin-layer chromatography, and its
structure was unambiguously determined by X-ray
crystallography.8
Rate constants for approach to equilibrium (kobsd
)
k1 + k-1) and equilibrium constants (Keq ) k1/k-1) were
measured for Cp complexes 1a -d at 120 °C in toluene-
d8 by 1H NMR spectroscopy (∆δ g 0.2 ppm for tolyl
methyl groups) (Scheme 2, Table 1).9,10 The electron-
withdrawing SO2CF3 substituent of 1d strongly acceler-
(3) (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. Orga-
nometallics 1995, 14, 2353. (f) Pu, J .; Peng, T.-S.; Arif, A.; Gladysz, J .
A. Organometallics 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.
(4) Casey, C. P.; Kraft, S.; Powell, D. R. J . Am. Chem. Soc. 2000,
122, 3771.
(5) Fischer, E. O.; Clough, R. L.; Stu¨ckler, P. J . Organomet. Chem.
1976, 120, C6.
(6) Miguel, D.; Steffan, U.; Stone, F. G. A. Polyhedron 1988, 7, 443.
(7) Since dimerization is a second-order process and isomerization
is a first-order process, low concentrations maximize the [1,3]-metal
shift to dimerization ratio.
(11) The 20:80 equilibrium ratio of 1d to 2d was solvent-indepen-
dent.
(12) (a) Rerek, M. E.; J i, L.-N.; Basolo, F. J . Chem. Soc., Chem.
Commun. 1983, 1208. (b) Basolo, F. Inorg. Chim. Acta 1984, 100, 33.
(c) Hart-Davis, A. J .; Mawby, R. J . J . Chem. Soc. A 1969, 2403. (d)
Hart-Davis, A. J .; White, C.; Mawby, R. J . Inorg. Chim. Acta 1970, 4,
441. (e) White, C.; Mawby, R. J . Inorg. Chim. Acta 1970, 4, 261. (f)
J ones, D. J .; Mawby, R. J . Inorg. Chim. Acta 1972, 6, 157. (f) O’Conner,
J . M.; Casey, C. P. Chem. Rev. 1987, 87, 307.
(8) See the Supporting Information.
(9) Wilkins, R. G. The Study of Kinetics and Mechanism of Reactions
of Transition Metal Complexes; Allyn and Bacon: Boston, MA, 1974.
(10) The narrow range of equilibrium constants (from Keq ) 1.0 for
[2a ]/[1a ] to Keq ) 4.0 for [2d ]/[1d ]) is probably related to the fact that
both arene rings are conjugated with the electron-deficient carbene
carbon.
(13) 13C NMR evidence for a [1,3]-rhenium shift with an indenyl-
substituted complex was found when (η5-indenyl)(CO)2RedC(Tol)13Ct
13CTol (4e-13C2) was heated to 120 °C. New signals at δ 110.39 and δ
251.50 (1J
C ) 64.5 Hz) indicated the formation of (η5-indenyl)(CO)2-
13C13
Red13C(Tol)13CtCTol (5e-13C2).

Communications
Organometallics, Vol. 20, No. 13, 2001 2653
Ta ble 2. Equ ilibr iu m a n d Ra te Con sta n ts for
[1,3]-Rh en iu m Sh ifts of In d en yl Com p lexes in
Tolu en e-d 8 a t 100 °C
The acceleration of [1,3]-rhenium shifts by electron-
withdrawing substituents can be explained in terms of
resonance structures A1 and A4, which emphasize the
similarity of dehydrometallacyclobutadienes and alkyne
complexes. Electron-withdrawing substituents stabilize
alkyne complexes by enhancing d-π* back-bonding. A
similar stabilization of the transition state leading to
A by electron-withdrawing substituents can be expected.
L
X
Keq
kobsd (10-4 s-1
)
k1(rel)
4a
4b
4c
4d
Ind
Ind
Ind
Ind
H
1.0
2.2
2.3
4.3
5.5
2.1
14
1.0
0.5
3.4
20
N(CH3)2
CF3
SO2CF3
69
withdrawing substituents in the indenyl series 4a -d
(Table 2) were similar to those in the Cp series, implying
that Cp and Ind compounds react by the same basic
mechanism.
Ack n ow led gm en t. Financial support from the Na-
tional Science Foundation is gratefully acknowledged.
Grants from NSF (Grant No. CHE-9629688) for the
purchase of the NMR spectrometers and a diffractome-
ter (Grant No. CHE-9709005) are acknowledged.
A plausible mechanism for the [1,3]-rhenium shifts
involves coordination of the alkyne triple bond to Re
parallel to the Cp plane,14 concerted with an η5-η3 Cp
ring slip to give an 18-electron dehydrometallacyclob-
utadiene intermediate A (Scheme 3). Ring slippage is
strongly supported by the 550-fold indenyl rate ac-
celeration and is required to avoid a 20-electron inter-
mediate. While the four dehydrometallacyclobutadiene
resonance structures (A1-4) all look bizarre, Schrock has
reported the isolation of stable Mo- and W-dehydro-
metallacyclobutadiene complexes and showed that the
C3 ligands are symmetrically bonded to the metal.15,16
Su p p or tin g In for m a tion Ava ila ble: Text giving a de-
scription of the preparation of compounds, spectral data, and
a description of kinetics and text and tables giving details of
the X-ray crystal structure determinations. This material is
available free of charge via the Internet at http://pubs.acs.org.
OM0103299
(15) (a) McCullough, L. G.; Schrock, R. R.; Dewan, J . C.; Murdzek,
J . C. J . Am. Chem. Soc. 1985, 107, 5987. (b) Strutz, H.; Dewan, J . C.;
Schrock, R. R. J . Am. Chem. Soc. 1985, 107, 5999. (c) Freudenberger,
J . H.; Schrock, R. R. Organometallics 1986, 5, 1411. (d) McCullough,
L. G.; Listemann, M. L.; Schrock, R. R.; Churchill, M. R.; Ziller, J . W.
J . Am. Chem. Soc. 1983, 105, 6729.
(16) Dehydrometallacyclobutadienes were identified as key inter-
mediates in the decomposition of active alkyne metathesis catalysts:
(a) References 14c,d. (b) Bray, A.; Mortreux, A.; Petit, F.; Petit, M.;
Szymanska-Buzar, T. J . Chem. Soc., Chem. Commun. 1993, 197.
(14) This assumes fairly unhindered rotation around the RedC bond.
The MdC rotational barrier of Cp(CO)2MndC(CH3)2 was calculated
to be 9 kcal mol-1 (Kostic´, N. M.; Fenske, R. F. J . Am. Chem. Soc.
1982, 104, 3879). The 1H NMR spectrum of this compound is consistent
with a low barrier: Fischer, E. O.; Besl, G. Z. Z. Naturforsch., B: Anorg.
Chem., Org. Chem. 1979, 34, 1186.