Unprecedented coordination modes and demetalation pathways for unbridged polyenyl ligands. Ruthenium η1η4-cycloheptadienyl complexes from allyl/alkyne cycloaddition

Article abstract of DOI:10.1021/ja0556023

Cationic (η⁶-hexamethylbenzene)ruthenium(II) mediates the [3 + 2 + 2] cycloaddition of allyl and alkyne ligands, leading to the unexpected isolation of η¹,η⁴-cycloheptadienyl complexes, an unprecedented coordination mode for transition metal complexes of simple organic rings. The nonconjugated, η¹,η⁴-coordinated complex is obtained as the kinetic reaction product from treatment of the unsubstituted allyl complex with excess ethyne; this complex rearranges slowly at 80 °C to the thermodynamically more stable conjugated η⁵-cycloheptadienyl isomer. The η¹,η⁴-coordinated isomer is fluxional at room temperature, undergoing rapid and reversible equilibration with a cycloheptatriene hydride intermediate via facile β-hydride elimination/reinsertion. The reinsertion process is remarkably regioselective, returning the nonconjugated η¹,η⁴-cycloheptadienyl isomer exclusively at room temperature. For reactions incorporating dimethylacetylene dicarboxylate (DMAD) as one or both of the alkyne components, η¹,η⁴-coordination appears to be both kinetically and thermodynamically favored, despite undergoing equilibration among all possible η¹,η⁴-cycloheptadienyl and cycloheptatriene hydride isomers prior to arriving at one observed η¹,η⁴-isomer. For this series, no isomerization to η⁵-coordination is observed even upon prolonged heating. In contrast, the cyclization incorporating both DMAD and phenylacetylene proceeds directly to the η⁵-cycloheptadienyl isomer at or below room temperature, indicating that η⁵-coordination remains energetically accessible to this system. The DMAD-based cyclization reactions produce structurally diverse minor byproducts, including both η¹,η⁴-methanocyclohexadiene and acyclic η³,η²-heptadienyl isomers, which have been isolated and rigorously characterized. The unusual η¹,η⁴-coordination of the seven-membered ring leads to unique new organic products upon oxidative demetalation by iodinolysis. Thus, reactions with excess iodine afford bridged tricyclic cyclopropane-containing lactones or substituted cycloheptatrienes in good but sometimes variable yields, depending on the substrate and specific reaction conditions. The ruthenium in these reactions is returned in high yield as the interesting cationic μ-triiodo pseudodimer of (η⁶-hexamethylbenzene)ruthenium, which is obtained as a triiodide salt. This Ru(III) complex, along with several representative Ru(II) cyclization products, has been characterized in the solid state by X-ray crystallography. Copyright

Full text of DOI:10.1021/ja0556023

Published on Web 09/23/2005
Unprecedented Coordination Modes and Demetalation Pathways for
Unbridged Polyenyl Ligands. Ruthenium η1,η4-Cycloheptadienyl Complexes
from Allyl/Alkyne Cycloaddition
Christina M. Older, Robert McDonald, and Jeffrey M. Stryker*
Department of Chemistry, UniVersity of Alberta, Edmonton, Alberta, Canada T6G 2G2
Received August 16, 2005; E-mail: jeff.stryker@ualberta.ca
Metal-mediated cycloaddition reactions provide a range of cyclic
polyenyl ligands coordinated to transition metals. The coordination
mode adopted by such ligands determines to a considerable extent
the strategies available for subsequent elaboration and demetalation
of the organic moiety. For the most part, cyclic polyenyl ligands
unconstrained by bridged structures adopt fully conjugated coor-
dination modes, typically as a direct consequence of thermodynamic
preferences. New coordination modes and, in particular, less con-
jugated coordination modes raise new possibilities for post-cycli-
zation functionalization and demetalation pathways, both essential
to the development of synthetically valuable transformations.
All unbridged cycloheptadienyl complexes of the transition
metals exhibit fully conjugated η5-coordination.1-5 Transient forma-
tion of η1,η4-coordinated intermediates has been proposed for
cobalt- and iridium-mediated allyl/alkyne [3 + 2 + 2] cycloaddition
reactions,4-6 but no evidence to support this conjecture has been
obtained. Here, however, we report the selective formation of
unprecedented η1,η4-cycloheptadienyl complexes from ruthenium-
mediated allyl/alkyne cycloaddition. The unique coordination mode
promotes unique oxidative demetalation pathways, leading to the
efficient construction of synthetically valuable seven-membered
carbocycles.
To induce ruthenium to mediate seven-membered ring formation
over cyclization to the kinetically favored six-membered ring, the
ancillary η6-benzene ligand used in previous investigations7 was
replaced with the sterically larger η6-hexamethylbenzene ligand.
In the event, treatment of allyl complex 1a8 with excess ethyne
leads to the clean formation of two inseparable cycloaddition
products, strongly favoring seven- over six-membered ring forma-
tion (Scheme 1).9 The major product was identified as the
unexpected symmetric η1,η4-cycloheptadienyl complex 2 by two-
dimensional NMR spectroscopy.10 The resonance for the unique
σ-bonded methine appears characteristically shifted upfield in both
1H NMR (δ -0.09) and 13C NMR (δ -38.1) spectra.
Surprisingly, complex 2 is both indefinitely stable and fluxional
at room temperature, as established by double irradiation experi-
ments,9 rapidly equilibrating with the otherwise undetected cyclo-
heptatriene hydride complex 2′. At 80 °C, however, slow isomer-
ization occurs to give the more thermodynamically stable η5-
coordinated isomer 4.9,11 The formation and subsequent isomerization
of the η1,η4-complex 2 strongly supports prior mechanistic proposals
rationalizing η5-cycloheptadienyl ring formation in the cobalt and
iridium series.4a,b
A lower activation barrier for η1,η4- to η5-isomerization is
apparent in the reaction of tert-butylacetylene, which proceeds
directly to η5-cycloheptadienyl complex 59,12 even under mild
reaction conditions. No η1,η4-coordinated isomer is detected in the
crude reaction mixture. Among several minor products, the simple
acyclic 1-tert-butyl-η5-pentadienyl complex 69 (∼4%, not shown)
can be isolated by chromatography.
Although some reactions of complex 1a with alkynes are
complicated,13 treatment with dimethylacetylene dicarboxylate
(DMAD) provides η1,η4-cycloheptadienyl complex 7 free from
significant byproducts.9,14 The substituent array in complex 7 can
arise only by extensive isomerization of the initial η1,η4-cyclohep-
tadienyl intermediate, presumably via iterative â-hydride elimination
and reinsertion (Scheme 2). Notably, the system reacts with perfect
fidelity for migration to η1,η4-coordination rather than fully
conjugated η5-hapticity. No further reaction is induced upon heating,
strongly suggesting that the η1,η4-coordination mode is thermo-
dynamically favored.
The electron-deficient alkyne also allows for cycloaddition
reactions incorporating two different alkynes. Unique among
alkynes,8 treatment of complex 1a with 1 equiv of DMAD leads to
exclusive formation of the acyclic adduct, [(C6Me6)Ru(1,2-syn-
dicarbomethoxypentadienyl)]OTf 8 (90%, not shown).9 In wet
benzene, however, the reaction stops at the η1,η2-vinyl olefin
intermediate, providing cationic aquo complex 9 (Scheme 1).9 This
Scheme 1
9
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10.1021/ja0556023 CCC: $30.25 © 2005 American Chemical Society

C O M M U N I C A T I O N S
Scheme 2
Acknowledgment. Financial support from NSERC and the
University of Alberta is gratefully acknowledged. R.M. is affiliated
with the Department of Chemistry X-ray Crystallography Labora-
tory.
Supporting Information Available: Experimental procedures and
complete characterization data for all new compounds; details of the
X-ray crystallography for complexes 12-14 and 18. This material is
available free of charge via the Internet at http://pubs.acs.org.
References
(1) See, for example: (a) Salzer, A.; Werner, H. J. Organomet. Chem. 1975,
87, 101-108. (b) Bennett, M. A.; Matheson, T. W. J. Organomet. Chem.
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Lalancette, R. A. Organometallics 1996, 15, 2700-2706 and references
therein.
Scheme 3
(2) Nonconjugated pentahapticity (invariably as η2,η3-coordination) occurs
more commonly in larger rings with greater coordination strain and/or
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nomet. Chem. 1984, 272, 179-188 and references therein.
(3) An early claim2a that protonation of (C5H5)Co(η4-cycloheptatriene)
provides (C5H5)Co(η2,η3-cycloheptadienyl)+ is not supported by spectro-
scopic data and remains unsubstantiated. The isolated product of this
reaction was subsequently identified as the expected η5-cycloheptadienyl
complex.1c
(4) (a) Schwiebert, K. E.; Stryker, J. M. J. Am. Chem. Soc. 1995, 117, 8275-
8276. (b) Etkin, N.; Dzwiniel, T. L.; Schwiebert, K. E.; Stryker, J. M. J.
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cyclopentenyl ring expansion: Dzwiniel, T. L.; Stryker, J. M. J. Am. Chem.
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complex converts slowly and quantitatively at room temperature
to η5-pentadienyl complex 8. Similarly, solvolysis of allyl chloride
complex 1b15 in trifluoroethanol containing DMAD gives the
analogous but neutral vinyl olefin complex 10 in near quantitative
yield.9
Complexes 9 and 10 transform selectively to η1,η4-cycloadducts
on treatment with a second alkyne. Optimal yields and selectivity
are obtained from neutral complex 10 under conditions of assisted
ionization, providing η1,η4-cycloheptadienyl complexes 11 and 13
from the reactions of ethyne and 2-butyne, respectively (Scheme
1).9,16 Structurally distinct minor products are isolated from each
reaction, including the unusual acyclic η3,η2-heptadienyl complex
12, which presumably arises from allylic C-H bond activation
following migratory insertion of ethyne. Structural assignments for
complexes 12, 13, and 14 have been confirmed by X-ray crystal-
lography.9
(5) Mn/η3-allyl/R,ω-diyne: (a) Tang, J.; Shinokubo, H.; Oshima, K. Orga-
nometallics 1998, 17, 290-292. (b) Nishikawa, T.; Kakiya, H.; Shinokubo,
H.; Oshima, K. J. Am. Chem. Soc. 2001, 123, 4629-4630.
(6) Other recent [3 + 2 + 2] cycloaddition reactions: (a) Baluenga, J.; Barrio,
P.; Lo´pez, L. A.; Toma´s, M.; Garc´ıa-Granda, S.; Alvarez-Ru´a, C. Angew.
Chem., Int. Ed. 2003, 42, 3008-3011. (b) Saito, S.; Masudo, M.;
Komegawa, S. J. Am. Chem. Soc. 2004, 126, 10540-10541.
(7) (a) Lutsenko, Z. L.; Aleksandrov, G. G.; Petrovskii, P. V.; Shubina, E.
S.; Andrianov, V. G.; Struchkov, Y. T.; Rubezhov, A. Z. J. Organomet.
Chem. 1985, 281, 349-364. (b) Lutsenko, Z. L.; Petrovskii, P.; Bezrukova,
A.; Rubezhov, A. Z. Bull. Acad. Sci. USSR 1988, 735-738.
(8) Older, C. M.; Stryker, J. M. Organometallics 1998, 17, 5596-5598.
(9) Complete experimental details are provided as Supporting Information.
(10) The minor product 3 is spectroscopically very similar to Rubezhov’s
[(C6H6)Ru(2,3,4,5-tetramethyl-1-methanocyclohexadiene)]+PF6- complex.7b
(11) Complex 4 is spectroscopically identical to the known PF6 salt.2c
-
(12) The substitution pattern for η5-cycloheptadienyl complex 5 is identical to
that obtained from (C5Me5)Co(allyl)OTf and 3,3-dimethyl-1-butyne.4b
(13) Phenylacetylene undergoes oligomerization rather than cyclization. Di-
substituted alkynes undergo exclusive [3 + 2] cycloaddition with
concomitant demethylation of the hexamethylbenzene ligand.8 Reactions
involving 2-butyne (g2 equiv) diverge substantially and will be discussed
elsewhere.
The unique coordination of the η1,η4-cycloheptadienyl ring
suggested the use of oxidative bond heterolysis as a novel
demetalation strategy, simultaneously effecting both scission of the
metal-carbon σ-bond and minimization of metal-diene back-
bonding.17 In the event, exhaustive iodinolysis indeed mediates the
decomplexation of cycloadducts 7 and 11, returning the unusual
tricyclic lactones 17 in moderate to good yields (Scheme 3).9,18
The Ru(III) coproduct was identified crystallographically as triiodide
salt 18,9 accounting for the unexpected stoichiometry in iodine.
Direct chromatography of the crude reaction mixture without
thiosulfate workup unmasks the latent seven-membered ring,
providing cycloheptatriene 19 exclusively.9,19 Iodinolysis of complex
13 leads directly to cycloheptatriene anhydride 20 in as yet variable
yields,9 a bicyclic ring system of considerable synthetic interest.20
The use of stoichiometric (but recyclable) ruthenium obviously
limits the potential use of this cycloaddition reaction in organic
synthesis. Nonetheless, the strong preference for η1,η4-coordination
of the seven-membered ring is unique in transition metal chemistry,
offering mechanistic insight and raising new opportunities for
developing synthetically valuable metal-mediated cycloaddition and
demetalation reactions.
(14) Neither the cobalt nor the iridium system reacts with DMAD. No tractable
product was obtained from the reaction of DMAD with (C6H6)Ru(allyl)+.7
(15) Bennett, M. A.; Huang, T.-N.; Turney, T. W. J. Chem. Soc., Chem.
Commun. 1979, 312-313.
(16) That η1,η4-complexes 7, 11, and 13 face no significant kinetic barrier to
conjugation is strongly suggested by the isolation of η5-cycloheptadienyl
complex 15 in modest yield (42%) as the major among many products
obtained from the reaction of complex 10 with phenylacetylene:9
(17) This demetalation strategy fails in the cobalt η5-cycloheptadienyl series.
(18) This extensive transformation presumably involves intramolecular iodo-
lactonization/iodide elimination, with concomitant iodide-mediated ester
demethylation. Related, completely organic, transformations are known:
Garratt,D.G.;Ryan,M.D.;Beaulieu,P.L. J.Org.Chem.1980,45,839-845.
(19) This process nominally involves lactone elimination followed, presumably,
by cycloreversion driven by norcaradiene/cycloheptatriene equilibration.
(20) See, for example: (a) Tanino, K.; Shimizu, T.; Miyama, M.; Kuwajima,
I. J. Am. Chem. Soc. 2000, 122, 6116-6117. (b) Wockenfufl, B.; Wolff,
C.; Tochtermann, W. Tetrahedron 1997, 53, 13703-13708. (c) Bowden,
S. L.; Harris, S. A.; Heller, H. G.; Hewlins, M. J. E. J. Chem. Soc., Perkin
Trans. 1 1992, 725-728. (d) Rainer, H. Ugi, I. Angew. Chem., Int. Ed.
Engl. 1985, 24, 594-596.
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