3138
Organometallics 1995, 14, 3138-3140
Fragmentation of, and Oxygen Atom Abstraction by, {CP*~(O)12~'0)2 Kevin P. Gable,*J Jerrick J. J. Juliette, and Michael A. Gartman Department of Chemistry, Oregon State University, Corvallis, Oregon 97331 Received March 17, 1995@ Summary: The dimeric oxo complex (Cp*ReO)dp-O)2is found to be in equilibrium with a monomeric form, proposed to be Cp*ReOz. The mixture of monomer and dimer stereospecifically abstracts oxygen from epoxides to make alkene plus Cp*ReO3. Transition-metal oxo complexes have been the subject of intense scrutiny as reagents and catalysts for a variety of oxidations of organic molecules,2particularly epoxidation3 and bi~hydroxylation~-~ of alkenes. We have undertaken mechanistic investigations and have focused on the microscopic reverse of bishydroxylation, alkene extrusion from rhenium(V1 diolates.' In the course of this study, it became necessary to consider whether a coordinated epoxide was a viable intermediate in the process (eq 1). This led us to investigate
closely the chemistry of (Cp*ReO)z(p-0)~.~ We report here that fragmentation of this dimer is observable by both NMFl and W-visible spectroscopy and that the *Abstract published in Advance ACS Abstracts, June 1, 1995. (1)E-mail:
[email protected] (internet). (2)(a) Sheldon, R. A.; Kochi, J. K. Metal Catalyzed Oxidations of Organic Chemistry; Academic Press: New York, 1981. (b) Organic Synthesis by Oxidatwn with Metal Compounds; Mijs, W .J., ddonghe, C. R. H. I., Eds.; Plenum: New York, 1986. (3)(a)Palucki, M.; Pospisil, P. J.; Zhang, W.; Jacobsen, E. N. J.Am. Chem. Soc. 1994, 116,9333-9334.(b) Ostovic, D.;Bruice, T. C. Acc. Chem. Res. 1992,25,314-320. (c) Meunier, B. Chem. Rev. 1992,92, 1411-1456. (d)Schurig, U.;Betschinger, F.Chem. Reu. 1992,92,873888. (4)(a)Schrijder, M. Chem. Rev. 1980,80,187-213. (b) Jorgensen, K. A.; Schiott, B. Chem. Rev. 1990,90,1483-1506. (5)(a)Giibel, T.; Sharpless, K. B. Angew. Chem.,Znt. Ed. Engl.,1993, 32,1329-1331. (b) Kolb, H.C.; Andersson, P. G; Sharpless, K. B. J. Am. Chem. Soc. 1994,116,1278-1291. (c) Norrby, P.-0.; Kolb, H. C.; Sharpless, K. B. Organometallics 1994, 13,344-347. (6)Corey, E.J.; Jardine, P. D.; Virgil, S.; Yuen, P.-W.; Connell, R. D. J.Am. Chem. Soc. 1989,111,9243-9244.(b)Corey, E.J.; Noe, M. C.; Sarshar, S. J. Am. Chem. Soc. 1993,115,3828-3829. (7)Gable, K. P.; Phan, T. N. J. Am. Chem. Soc. 1994, 116,833839. (8)(a) Herrmann, W. A.; Floel, M.; Kulpe, J.; Felixberger, J . K.; Herdtweck, E. J . Organomet. Chem. 1988,355, 297-313. (b) Herrmann, W. A.; Serrano, R.; Kiisthardt, U.; Ziegler, M. L.; Guggolz, E.; Zahn, T. Angew. Chem.,Int. Ed. Engl. 1984,7,515-517.(c)Herrmann, W.A.; Serrano, R.; Kiisthardt, U.; Guggolz, E.; Nuber, B.; Ziegler, M. L. J. Organomet. Chem. 1985,287,329-344.
-6
25°C
1°C I
-13 1 -14 1 0.0026
. 0.0028
0.003
0.0032
0.0034
lnemperature, K-'
Figure 1. van't Hoff plot for dimer * monomer equilibrium: slope -8450, intercept 15.717, 1.2 = 0.9745.
monomeric Cp*ReOz is capable of several oxygen atom abstraction processes. At room temperature, solutions of (Cp*Re012(p-0)2 ( c a s , sealed under vacuum) exhibit a single 'H NMR signal at 6 1.96 ppm. However, where the temperature is raised, a new signal appears a t 6 1.66 ppm. This is clearly distinct from the signal for cp*Reo3, which occurs a t 6 1.60 ppm. When the solutions are cooled, this new peak slowly disappears, and that for the dimer regains its original i n t e n ~ i t y .If~ the initial concentration of dimer is varied, one observes that small concentrations and high temperatures favor the formation of this new compound. At 373 K, a solution initially 0.007 M in dimer showed a ratio of Cp* methyl peaks of 100: 17.6. The fragmentation shows a minor dependence on solvent; in THF-ds a t 373 K (initially 0.0106 M in dimer), the ratio of peaks is 100:22.5. The 13CNMR also shows two new signals a t 12.4 and 109.3 ppm. These are distinct from those of the dimer (11.4 and 108.5 ppm), as well as from cp*Reo3 (9.9 and 119.3 ppm). This concentration vs composition relationship fits an equilibrium constant expression for fragmentation of a dimer: K = [monomer12/[dimerl. At 366 K, the equilibrium constant is calculated to be (6.05 f 1.29) x M. Furthermore, evaluation of K a t various temperatures and concentrations yields the van't Hoff plot seen in Figure 1;from this, the thermodynamic parameters A?Io = 16.8 f 0.3 kcal/mol and ASo= f31.2 f 1.4 call (mol K)may be calculated. Electronic spectra of the mixture (toluene solution) were obtained over a similar temperature range. A decrease of the peak a t 418 nm is observed a t higher (9)It is possible to find a small peak (intensity 1,l-dimethyloxirane > (Z)-1,2-dimethyloxirane > (E)-1,2dimethyloxirane. Evaluation of the rate law for deoxygenation of styrene oxide was performed. When run with a large excess of epoxide (pseudo-first-order conditions), there was a half-order dependance on (Cp*Re0)2@-0)2. This is expected if monomeric Cp*Re02 is responsible for oxygen atom abstraction. There was also a first-order dependence on epoxide concentration between 0.3 and 2.0 M. This leads to a rate law of the form -d[(Cp*ReO),(p-O),I/ddt
*/ R
Cp'Re03
+
Me2S
temperatures; an isosbestic point is observed a t 349 nm (see Figure 2). This feature is likely due to the metalmetal bond that has been suggested for the dimer.1° On the basis of the NMR behavior, analysis of these spectra gave a calculatedll molar absorptivity of e = 1550 M-l cm-l for the dimer. The monomer has a calculated absorptivity of e = 100 M-l cm-' at 418 nm, probably due to the tailoff of the ligand-to-metal charge transfer band seen below 300 nm for this system. Subtraction of spectra revealed no new features unequivocally attributable to the monomer.12 Reaction of dimer (in equilibrium with monomer)with an excess of dimethyl sulfoxide at 65 "C gave Cp*ReO3 and dimethyl sulfide (Scheme l).13 The system also reacted with several epoxides at 50-105 "C, including styrene oxide, cyclohexene oxide, 1,l-dimethyloxirane, and (E)- and (2)-1,2-dimethyloxirane.Formation of Cp*ReO3 was quantitative by lH NMR (no other Cp* methyl signals were observed). The alkene was also observed, though loss of volatile alkene to the head (10)Hemann, W. A.; Kusthardt, U.; Fl&l, M.; Kulpe, J.; Herdtweck, E.; Voss, E. J . Organomet. Chem. 1986,324, 151-162. (11) A two-component Beer's law plot was generated using variable-
temperatureW - v i s data for different initial concentrationsof dimer. Concentrations of monomer and dimer were calculated using the van't Hoff plot shown in Figure 1. (12) In addition to LMCT bands, the expected feature of this d* species is a weak, long-wavelength d-d transition. Given the low concentrationsof monomer generated in this system, the inability to identify this feature is unremarkable. (13)Brown, S. M.; DuMez, D. D.; Mayer, J. M. Abstracts of Pupers, 207th National Meeting of the American Chemical Society, San Diego, CA, Spring 1994, American Chemical Society: Washington, DC, 1994; INOR 168.
=
M-'I2 s-l At 50.1 "C, k&s was found to be 6.4 x for deoxygenation of styrene oxide in C6D6. This corresponds to a second-order rate of 2.1 x lom5M-l s-l for conversion of monomer plus epoxide to trioxo plus alkene. Such a rate corresponds to AG* = 25.6 kcaV mol. In THF-ds at the same temperature, the rate is M-l12 s-l. marginally faster, kObs = 8.7 x These observations can be interpreted on the basis of an analysis performed by Holm and Donahue.14 The dimer (Cp*Re0)2@-0)2is formed from cp*Reo3 and PPh3.15 Since both DMSO and epoxides are thermodynamically better 0 atom donors than Ph3P0, and given that oxo transfer to DMSO from Cp*ReO3 to yield MenSO2 is not observed, we can estimate the average Re=O bond enthalpy in the trioxide at 120-141 kcdmol, consistent with other third-row metal oxo compounds.16 There are several possible mechanisms for these deoxygenations (Scheme 2). The simplest is a concerted fragmentation of the coordinated epoxide. This parallels Bercaw's investigation of a tantalum-based epoxide de~xygenationl~ and Mayer's studies on L4WC1218 and is also suggested by the facile reduction of DMSO we observe; however, one expects that a coordinating solvent such as THF should have a larger effect either on the rate of deoxygenation (by inhibiting coordination of the epoxide) or on the monomer-dimer equilibrium. One alternative would be a radical process, such as that observed by Nugent and co-workers.lg However, this appears incompatible with the high stereospecificity of the reaction. Finally, oxidative addition of the threemembered ring to form a metallaoxetane20offers a third possibility. This last possibility could,also include the ~~~~
~
(14) Holm, R. H.; Donahue, J. P. Polyhedron 1993,12, 571-589. (15) Reference 8c. (16)Glidewell, C. Znorg. Chin. Acta 1977,24, 149-157. (17)Whinnery, L. L.; Henling, L. M.; Bercaw, J. E. J. Am. Chem. SOC. 1991,113,7575-7582. (18)Atagi, L. M.; Over, D. E.; McAlister, D. R.; Mayer, J. M. J . Am. Chem. SOC.1991,113,870-874. (19)(a) RajanBabu, T. V.; Nugent, W. A.; Beattie, M. S. J. Am. Chem. SOC.1990, 222,6408-6409. (b) RajanBabu, T. V.; Nugent, W. A. J . Am. Chem. Soc. 1994,116,986-997. (20)Calhorda, M. J.; Galviio, A. M.; Unaleroglu, C.; Zlota, A. A.; Frolow, F.; Milstein, D. M. Organometallics 1993, 22, 3316-3325.
3140 Organometallics, Vol. 14, No. 7, 1995
Communications Scheme 2
Cp'Re03
coordinated epoxide as a precursor. Each of these has ramifications concerning the mechanism of fragmentation of diolates; we are currently evaluating the reaction energetics t o discern the relationship between the two processes.
+
Acknowledgment. Support from the National Science Foundation (Grant CHE-9312650) is gratefully acknowledged. OM950200X
