Demethylation of coordinated hexamethylbenzene. Carbon-carbon bond activation during ruthenium-mediated [3 + 2] allyl alkyne cycloaddition reactions

Article abstract of DOI:10.1021/om980823l

The demethylation of coordinated hexamethylbenzene is integrated into [3 + 2] allyl/alkyne cycloaddition reactions mediated by (η⁶-hexamethylbenzene)-Ru(η³-allyl)OTf. The reaction proceeds in high yield at or below room temperature and yields methane and (η ³-pentamethylbenzene)ruthenium(1,2-dialkylcyclopentadienyl)+OTf ^- complexes exclusively. Cycloaddition with carbon-carbon bond activation is general for a range of disubstituted alkynes.

Full text of DOI:10.1021/om980823l

5596
Organometallics 1998, 17, 5596-5598
Dem eth yla tion of Coor d in a ted Hexa m eth ylben zen e.
Ca r bon -Ca r bon Bon d Activa tion d u r in g
Ru th en iu m -Med ia ted [3 + 2] Allyl Alk yn e Cycloa d d ition
Rea ction s
C. M. Older and J . M. Stryker*
Department of Chemistry, University of Alberta, Edmonton, Alberta T6G 2G2, Canada
Received October 1, 1998
Summary: The demethylation of coordinated hexameth-
ylbenzene is integrated into [3 + 2] allyl/ alkyne cyclo-
addition reactions mediated by (η6-hexamethylbenzene)-
Ru(η3-allyl)OTf. The reaction proceeds in high yield at
or below room temperature and yields methane and
(η6-pentamethylbenzene)ruthenium(1,2-dialkylcyclopenta-
dienyl)+OTf- complexes exclusively. Cycloaddition with
carbon-carbon bond activation is general for a range
of disubstituted alkynes.
F igu r e 1.
benzene into pentamethylbenzene in high yield under
exceptionally mild conditions.7 The demethylation proc-
ess is integrated into an overall [3 + 2] cycloaddition
reaction, which converts cationic (η6-hexamethylben-
zene)ruthenium η3-allyl complexes and disubstituted
alkynes into methane and (η6-pentamethylbenzene)-
ruthenium η5-cyclopentadienyl complexes. This “un-
forced” carbon-carbon bond activation appears to be
driven by relief of the cumulative steric strain experi-
enced by the six mutually buttressed methyl substitu-
ents in hexamethylbenzene and by a surprisingly low
kinetic barrier to the carbon-carbon bond scission.
Metal-mediated “oxidative” allyl/alkyne [3 + 2] cyclo-
addition reactions have been previously reported for
half-sandwich η5-cyclopentadienyl,8 η5-pentamethyl-
cyclopentadienyl,9,10 and η6-arene8 complexes of the late
transition metals (Figure 1). The reaction represents an
unexploited convergent synthesis of differentially sub-
stituted cyclopentadienyl ligands within the coordina-
tion sphere of a metal and is potentially attractive for
the development of catalyst libraries and derivatizing
alkyne-rich materials and polymers. The reaction pro-
ceeds via initial migratory coupling of allyl and alkyne
ligands (a )11a and is presumably followed by migratory
cyclization (b) and â-hydride elimination (c). Cyclopen-
tadiene hydride complex I then transforms into the
cyclopentadienyl complex by dehydrogenation or, as we
demonstrate for the hexamethylbenzene ruthenium
system, by an unexpected alternative pathway involving
hexamethylbenzene dealkylation.
Transition metals mediate the activation of carbon-
carbon bonds in a range of contexts.1 Both oxidative and
nonoxidative (viz., â-alkyl elimination) processes are
common, driven by various kinetic and thermodynamic
effects: the relief of ring strain,2 coordination-induced
proximity,3,4 ligand aromatization,5 or in many cases, a
combination of factors.6
In this communication, we report a general demethyl-
ation reaction that converts coordinated hexamethyl-
(1) Crabtree, R. H. Chem. Rev. 1985, 85, 245. Herrmann, W. A.;
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2843.
(3) â-Alkyl elimination. Selected recent references: (a) McNeill, K.;
Andersen, R. A.; Bergman, R. G. J . Am. Chem. Soc. 1997, 119, 11244,
and references therein. (b) Nicholls, J . C.; Spencer, J . L. Organo-
metallics 1994, 13, 1781. Cracknell, R. B.; Nicholls, J . C.; Spencer, J .
L. Organometallics 1996, 15, 446. (c) Takemori, T.; Suzuki, H.; Tanaka,
M. Organometallics 1996, 15, 4346. Suzuki, H.; Takaya, Y.; Takemori,
T. J . Am. Chem. Soc. 1994, 116, 10779, and references therein.
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Chem. Soc. 1986, 108, 4679, and references therein. J un, C.-H.
Organometallics 1996, 15, 895. (b) van der Boom, M. E.; Ben-David,
Y.; Milstein, D. J . Chem. Soc., Chem. Commun. 1998, 917. Gandelman,
M.; Vigalok, A.; Ben-David, Y.; Milstein, D. Organometallics 1997, 16,
3981. Rybtchinski, B.; Vigalok, A.; Ben-David, Y.; Milstein, D. J . Am.
Chem. Soc. 1996, 118, 12406, and references therein.
(5) (a) Eilbracht, P. Chem Ber. 1976, 109, 1429, 3136. Eilbracht, P.;
Dahler, P. Chem. Ber. 1980, 113, 542, 1033, 1420, 2211. (b) Benfield,
F. W. S.; Green, M. L. H. J . Chem. Soc., Dalton Trans. 1974, 1324. (c)
Crabtree, R. H.; Dion, R. P.; Gibboni, D. J .; McGrath, D. V.; Holt, E.
H. J . Am. Chem. Soc. 1986, 108, 7222. (d) Hemond, R. C.; Hughes, R.
P.; Locker, M. B. Organometallics 1986, 5, 2392. (e) J ones, W. D.;
Maguire, Organometallics 1987, 6, 1301. (f) J . A. Rondon, D.; Chaudret
B.; He, X.-D.; Labroue, D. J . Am. Chem. Soc. 1991, 113, 5671.
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1, 149.
(7) The dealkylation of hexaalkylbenzenes is also observed in AlCl3-
mediated exchange reactions of (cyclopentadienyl)metal complexes
under very harsh conditions: Fischer, E. O.; Elschenbroich, C. Chem.
Ber. 1970, 103, 162. Roman, E.; Astruc, D. Inorg. Chim. Acta 1979,
11, L465. Astruc, D. Tetrahedron 1983, 39, 4027.
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J . Organomet. Chem. 1985, 281, 349. (b) Lutsenko, Z. L.; Petrovskii,
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1996, 118, 8285, and references within. (b) Kondo, T.; Kodoi, K.;
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Am. Chem. Soc. 1998, 120, 5587.
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M. J . Am. Chem. Soc. 1998, 120, 9702.
10.1021/om980823l CCC: $15.00 © 1998 American Chemical Society
Publication on Web 12/01/1998

Communications
Organometallics, Vol. 17, No. 26, 1998 5597
Sch em e 1
To support our investigation of allyl/alkyne [3 + 2 +
2] cycloaddition reactions,11 the η6-hexamethylbenzene
ruthenium allyl complex 1 was prepared by modification
of a literature procedure.12 Thus, the reaction of Ben-
nett’s dimer13 and tetraallyltin (1-2 equiv) in aceto-
nitrile affords (η6-C6Me6)Ru(allyl)Cl14,15 (1) in near
quantitative yield (eq 1).16 Treatment with silver triflate
provides inner-sphere triflate complex 2, isolated from
the reaction mixture as an analytically pure, thermally
stable, mildly air-sensitive orange powder.15
ylbenzene ligand is broken and a downfield singlet (1H
NMR: δ 5.9-6.1, 1H) is observed, consistent with the
presence of a η6-pentamethylbenzene ligand.
Either chloride complex 1 or triflate complex 2 can
be used for subsequent allyl/alkyne cycloaddition reac-
tions, the former in conjunction with in situ ionization
using silver salts or highly polar solvents (e.g., CF3CH2-
OH). The reactions of triflate complex 2, however,
generally proceed more cleanly and provide [3 + 2]
cycloadducts in higher isolated yields. Treatment of the
triflate complex at room temperature with 1 equiv17 of
a range of disubstituted alkynes provides the disubsti-
tuted cyclopentadienyl complexes 3a -d in yields of 72-
96% after purification by recrystallization (eq 2).15,18 The
To confirm the dealkylation and determine the ulti-
mate fate of the methyl group, the reaction with
2-butyne was carried out in a sealed NMR tube and
1
monitored by H NMR spectroscopy. The formation of
methane was detected (0.15 ppm in acetone-d6) and
confirmed by comparison with an authentic sample.15
In addition, photolysis of dimethylcyclopentadienyl
complex 3a (450W Hanovia lamp, Pyrex filter) in the
presence of 3 equiv of trimethylphosphine leads to the
liberation of pentamethylbenzene, identified by com-
parison to an authentic sample, and formation of the
tentatively identified tris(phosphine) complex, [(η5-
Me2C5H3)Ru(PMe3)3]+OTf- (4).15,19
-
f
Exclusive dealkylation is also observed in the reac-
tions of 2,8-decadiyne with triflate complex 3, which can
be converted with high selectivity into either mono-
(cyclopentadienyl) complex 3e or tethered bis(cyclopen-
tadienyl) complex 3f, depending on reaction stoichiom-
etry and conditions (Scheme 1).15 The latter complex is
formed as the expected inseparable 1:1 diastereomeric
mixture.
new cyclopentadienyl ligands were identified by char-
acteristic signatures in both the 1H and 13C NMR
spectra: very small vicinal coupling constants (3J HH
)
The mechanism of the dealkylation process remains
under investigation, but it is reasonable to posit the
intermediacy of a diene hydride complex analogous to I
in Figure 1 (Scheme 2). After that, we suggest that
(reversible) migration of the hydride to the arene
ligand20 (Scheme 2) is more kinetically facile than
dehydrogenation, providing hexamethylcyclohexadienyl
intermediate II bearing an exo-methyl substituent. This
species may, in principle, suffer direct radical scission
(path a ), as postulated by Chaudret to explain related
ruthenium-mediated exo-methyl dealkylations.5f The
significantly milder reaction conditions required for the
present demethylation (below room temperature vs
>100 °C), however, render such a radical mechanism
2-3 Hz) for the methine hydrogen atoms and, where
determined, large one-bond carbon-hydrogen coupling
constants (1J CH ) 170-180 Hz) for the methine carbon
atoms. NMR spectroscopic analysis, however, also re-
veals that in each case the symmetry of the hexameth-
(12) Allylation of [(η6-C6H6)RuCl2)]2 using tetraallyltin in aceto-
nitrile: Baird, M. C. Zelonka, R. A. J . Organomet. Chem. 1972, 44,
383. The reported use of a large excess of the tin reagent unnecessarily
complicates isolation and purification of the allylated product.
(13) Bennett, M. A.; Huang, T.-N.; Matheson, T. W.; Smith, A. K.
Inorg. Synth. 1982, 21, 74.
(14) Bennett, M. A. Huang, T.-N.; Turney, T. W. J . Chem. Soc.,
Chem. Commun. 1979, 312.
(15) Complete experimental, spectroscopic, and analytical data are
provided as Supporting Information.
(16) The use of the more convenient allyltriphenyltin reagent is also
effective, but the yield is somewhat lower (70-80%) and separation of
the byproduct Ph3SnCl requires chromatography.
(17) For most disubstituted alkynes, the presence of excess alkyne
has no significant effect on the course of the reaction. In the presence
of excess 2-butyne, however, a more complicated reaction manifold is
accessed. The reactions of terminal acetylenes and dimethylacetylene
dicarboxylate also proceed via alternative reactivity patterns. These
reactions will be discussed separately: Older, C. M.; Stryker, J . M.
Manuscript in preparation.
(19) Tris(phosphine) complex 4 was not isolated from the photolysis
mixture, but is spectroscopically very similar15 to the known complex,
[(η5-C5H5)Ru(PMe3)3]+PF6-: Bruce, M. I.; Wong, F. S.; Skelton, B. W.;
White, A. H. J . Chem. Soc., Dalton Trans. 1981, 1398.
(20) Alkyl or hydride migration to a coordinated arene ring is rare,
but documented: (a) Beard, L. K.; Silvon, M. P.; Skell, P. S. J .
Organomet. Chem. 1981, 209, 245. (b) Brookhart, M.; Pinhas, A. R.;
Lukacs, A. Organometallics 1982, 1, 1730. (c) Rush, P. K.; Noh, S. K.;
Brookhart, M. Organometallics 1986, 5, 1745. (d) Kowalski, A. S.;
Ashby, M. T. J . Am. Chem. Soc. 1995, 117, 12639.
(18) Demethylation occurs cleanly using either acetone or dichlo-
romethane as reaction solvent.

5598 Organometallics, Vol. 17, No. 26, 1998
Communications
Sch em e 2
is suggested that the endo-methyl minor isomer of a
ruthenium η5-1-methylcyclohexadienyl intermediate un-
dergoes carbon-carbon bond activation under condi-
tions where the concomitantly formed exo-isomer does
not.5g While it is not yet possible to rigorously exclude
bimolecular dealkylation mechanisms, we note that the
reaction of triflate complex 2 with diphenylacetylene
proceeds slowly but exclusively to demethylated product
3b even at high dilution (<0.0003 M in ruthenium). No
intermediates were detected in a reaction monitored at
1
room temperature by H NMR spectroscopy.
The steric origin of the driving force for this general
demethylation process is supported by allyl/alkyne
cycloaddition in the corresponding η6-pentamethylben-
zene series: the reaction of (η6-pentamethylbenzene)-
Ru(η3-allyl)OTf15,24 and diphenylacetylene provides
mainly η6-pentamethylbenzene product 3b (>80%), ac-
companied by trace amounts of several unidentified, but
presumably demethylated byproducts.15 Further mecha-
nistic investigations are in progress.
less attractive.21 A more interesting and, perhaps,
reasonable alternative involves 1,2-migration22 of the
exo-methyl group to form geminal dimethyl complex III
(path b), which now projects an endo-methyl group into
the coordination sphere of the unsaturated metal. The
carbon-carbon bond activation then belongs to a rela-
tively common class of â-carbon elimination reactions,3,5
although such transformations also often require sig-
nificantly harsher reaction conditions.23 Itoh et al.,
however, have demonstrated that a similarly mild
demethylation pathway exists in (C5Me5)Ru-mediated
[4 + 2] diene/alkyne cycloaddition reactions, where it
Ackn owledgm en t. Financial support from the Natu-
ral Sciences and Engineering Research Council of
Canada and the University of Alberta is gratefully
acknowledged. C.M.O. acknowledges the support of an
NSERC Post-graduate Scholarship. We also thank
Professor N. Branda for helpful discussions.
(21) The Chaudret mechanism was developed to rationalize ruthe-
nium-mediated angular methyl dealkylations observed in stereochemi-
cally defined unsaturated steroidal substrates, reactions for which
various alternative mechanisms can be discounted.5f
(22) A sigmatropic shift on the exo-face of a coordinated ligand was
proposed by Crabtree5c to account for rearranged products observed
from the reactions of 1,1-dialkylcyclopentanes with an unsaturated
iridium complex.
(23) The closely related complex [(dppe)Ru(1,1-dimethylcyclohexa-
dienyl)(CH2Cl2)]+PF6-, bearing a labile ligand and both endo- and exo-
methyl substituents, nonetheless requires 12 h at 90 °C to induce
methyl migration to the metal center.5h
Su p p or tin g In for m a tion Ava ila ble: Experimental pro-
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OM980823L
(24) This complex was prepared by the same methodology used to
synthesize triflate complex 2.15