J . Am. Chem. SOC.1992, 114, 1736-1740
1736
Phenol-Cyclohexadienone Equilibrium for v2-Coordinated Arenes Michael E. Kopach, William G . Hipple, and W. Dean Harman* Contribution from the Department of Chemistry, University of Virginia, Charlottesville, Virginia 22901. Received August 26, 1991
Abstract: The effect of T~ coordination on an arene is explored in the context of the phenol-ketodiene equilibrium. Whereas for the free ligand this equilibrium heavily favors the phenol tautomer, we find that for the complexes [O~(NH,),(2,3-q~-arene)]~+ (arene = phenol; 4-, 5-, 6-methylphenol; 4,5-dimethylphenol) the corresponding equilibrium constants approach unity (20 "C). Starting with the phenolic isomer in methanol, conversion to 2,4-cyclohexen-l-one is kinetically favored over the formation of the 2,5-analogue, although the latter is the thermodynamically favored product. All tautomerization processes are stereocontrolled, with protonation and deprotonation occurring selectively at the ring face opposite the metal. Electrochemical studies indicate that the phenol-2J-dienone equilibrium is not profoundly influenced by complexation of osmium(II1). In addition to the mononuclear species, where L = phenol, two binuclear complexes are characterized of the form [(Os(NH3)5)z(p-s2:qz-L)]4+ where L = 2,4- or 2,5-cyclohexadien-l-one.
The reactivity of aromatic molecules can be profoundly affected by their coordination t o a transition metal, and a great variety of systems have been investigated in which an arene is coordinated symmetrically(i.e., @).I Such coordination can activate the arene toward nucleophilic attack or deprotonation, and these processes have found synthetic applications.2 Although several examples of v2-arenecomplexes have been reported,) their ligand chemistry remains largely unexplored owing to the tendency of these species to undergo oxidative addition or dissociation. Pentaammineosmium(I1) forms $-arene complexes which are relatively stable in this regard4 and, as such, provides the opportunity to explore how q2 coordination affects t h e arene reactivity. In contrast t o $-arene complexes, q2 coordination partially localizes the ligand ?r electrons, an outcome which enhances t h e 1,3-diene nature of arenesS5 To probe the extent of this dearomatization, we have undertaken an investigation of t h e phenol-dienone equilibrium for several $-phenol complexes of pentaammineosmium(I1).
Experimental Section Infrared spectra were recorded on a Mattson Cygnus 100 FTIR spectrometer as KBr pellets. "C and IH NMR spectra were obtained on a General Electric QE300 (300 MHz), GN300 (300 MHz), or GN500 (500 MHz) spectrometer and are reported vs tetramethylsilane. Electrochemical experiments were performed under nitrogen using a PAR Model 362 potentiostat driven by a PAR Model 175 universal programmer. Cyclic voltammograms (CV) were recorded (Kipp & Zonen X-Y recorder) in a standard three-electrode cell6 from +1.5 to -1.5 V with a glassy carbon working electrode. All potentials are reported vs NHE and, unless otherwise noted, were determined in acetonitrile (-0.5 M tetrabutylammonium hexafluorophosphate (TBAH)) using ferrocene ( E l l 2= 0.55 V), decamethylferrocene ( E l j 2= 0.04 V), or cobaltocenium hexafluorophosphate ( E l l 2 = -0.78 V) in situ as a calibration standard. The peak to peak separation (Ep,,was between 60 and 80 mV for all reversible couples reported unless otherwise (1) Collman, J. P.; Hegedus, L. S.; Norton, J. R.; Finke, R. G. Principles and Applications of Organotransition Metal Chemistry, 2nd ed.; University Science Books: Mill Valley, CA, 1987;Chapter 20. (2)Davies, S.G.Organotransition Metal Chemistry Applications to Organic Synthesis; Pergamon Press: Oxford, 1982;Chapters 4 and 5. (3) Sweet, J. R.; Graham, W. A. G. J . Am. Chem. SOC.1983,105, 305. v.d. Heijden, H.; Orpen, A. G.; Pasman, P. J . Chem. SOC.,Chem. Commun. 1985, 1576. Jones, W. D.; Feher, F. J . Am. Chem. Soc. 1984,106, 1650. Jones, W. D.; Dong, L. J. Am. Chem. SOC.1989,111, 8722. Belt, S. T.; Duckett, S. B.; Jones, W. D.; Partridge, M. G.; Perutz, R. N. J . Chem. SOC., Chem. Commun. 1991,266.Brandt, L.;Green, M.; Parkins, A. W. Angew. Chem., Int. Ed. Engl. 1990,29, 1046. Shiu, K.;Chou, C.; Wang, S.; Wei, S. Organometallics 1990,9, 2632. (4)(a) Harman, W. D.; Sekine, M.; Taube, H. J. Am. Chem. SOC.1988, 110, 5725. (b) Harman, W. D.; Taube, H. J. Am. Chem. SOC.1990,112, 2682. (c) Hasegawa, T.; Sekine, M.; Schaefer, W. P.; Taube, H. Inorg. Chem. 1991, 30, 449. ( 5 ) Muetterties, E. L.; Bleeke, J. R.; Wucherer, E. J. Chem. Reo. 1982, 82, 499. (6) Bard, A. J. Faulkner, L. R.Electrochemical Methods; Wiley: New York, 1980.
noted. This work was carried out under nitrogen atmosphere in a Vacuum Atmospheres Co. glovebox. Reagents. [OS(NH,),(OT~)](OT~)~ (OTf = CF3SO
