Organometallics 2004, 23, 2015-2019
2015
General Synthesis of Cyclopentadienylchromium(II) η3-Allyl Dicarbonyl Complexes David W. Norman, Michael J. Ferguson,† and Jeffrey M. Stryker* Department of Chemistry, University of Alberta, Edmonton, Alberta, T6G 2G2 Canada Received December 22, 2003
Thermally stable chromium(II) dicarbonyl complexes bearing substituted η3-allyl and η5cyclopentadienyl ligands have been prepared via oxidative addition of the allyl bromide to tris(acetonitrile)tricarbonylchromium at -30 °C in acetonitrile, followed by in situ addition of the ancillary cyclopentadienyl ligand anion at low temperature. Spectroscopic data for allyl complexes 1, 2, and 4-7 indicate that the solution configuration of the allyl ligand is preferentially exo, while the data for the 2-methylallyl derivative 3 suggest the formation of a 65:35 mixture of endo and exo isomers. The cyclohexenyl and indenyl complexes 5 and 7, respectively, exhibit interesting spectroscopic and structural characteristics. While most complexes were prepared in good yield, the addition of sterically larger ancillary ligands was problematic: the addition of indenyllithium led to low yields of the allylchromium complex, while treatment with (pentamethylcyclopentadienyl)lithium led to the isolation of only organic products. These diminished returns are attributed to attack of the ancillary ligand anion directly on the coordinated allyl ligand of the intermediate formed by oxidative addition. Solid-state structures of the η3-allyl, crotyl, and cyclohexenyl complexes, all in the exo configuration, have been determined by X-ray crystallography. Permethyltitanocene(III) η3-allyl and substituted bis(indenyl)titanium(III) η3-allyl complexes undergo regioselective central carbon alkylation upon treatment with organic free radicals.1,2 The resulting titanacyclobutane complexes can be converted inter alia to cyclic organic compounds via carbonylation2b,c and isonitrile insertion,2e providing a basis for developing applications of this reactivity pattern in organic synthesis. To extend this methodology to other transition-metal systems, we have initiated investigations of the corresponding chromium η3-allyl complexes. The literature, however, reveals that many chromium η3-allyl complexes are thermally unstable and existing synthetic strategies are not particularly attractive for development in an explicitly organic context.3-12 Thus, we turned to developing general synthetic methodology for preparing low-valent allylchromium complexes. * To whom correspondence should be addressed. E-mail: jeff.stryker@ ualberta.ca. † Department of Chemistry Structure Determination Laboratory. (1) Casty, G. L.; Stryker, J. M. J. Am. Chem. Soc. 1995, 117, 7814. (2) (a) Carter, C. A. G.; McDonald, R.; Stryker, J. M. Organometallics 1999, 18, 820. (b) Carter, C. A. G.; Casty, G. L.; Stryker, J. M. Synlett 2001, 1046. (c) Carter, C. A. G.; Greidanus, G.; Chen, J.-X.; Stryker, J. M. J. Am. Chem. Soc. 2001, 123, 8872. (d) Greidanus, G.; McDonald, R.; Stryker, J. M. Organometallics 2001, 20, 2492. (e) Greidanus-Strom, G.; Carter, C. A. G.; Stryker, J. M. Organometallics 2002, 21, 1011. (3) Trofimenko, S. J. Am. Chem. Soc. 1967, 89, 3904. Trofimenko, S. J. Am. Chem. Soc. 1969, 91, 588. (4) Nesmeyanov, A. N.; Krivykh, V. V.; Il’Minskaya, E. S.; Rybinskaya, M. I. J. Organomet. Chem. 1981, 209, 309. Krivykh, V. V.; Gusev, O. V.; Rybinskaya, M. I. J. Organomet. Chem. 1989, 362, 351. (5) Wilke, G.; Bogdanovic, B.; Hardt, P.; Heimbach, P.; Keim, W.; Kro¨ner, M.; Oberkirch, W.; Tanaka, K.; Steinru¨cke, E.; Walter, D.; Zimmermann, H. Angew. Chem., Int. Ed. Engl. 1966, 5, 151. O’Brien, S.; Fishwick, M.; McDermott, B.; Wallbridge, M. G. H.; Wright, G. A. Inorg. Synth. 1972, 13, 73. (6) Aoki, T.; Furusaki, A.; Tomiie, Y.; Ono, K.; Tanaka, K. Bull. Chem. Soc. Jpn. 1969, 42, 545. (7) Nieman, J.; Pattiasina, J. W.; Teuben, J. H. J. Organomet. Chem. 1984, 262, 157. (8) Brisdon, B. J.; Griffin, G. F. J. Organomet. Chem. 1974, 76, C47.
Although the corresponding cyclopentadienylmolybdenum dicarbonyl η3-allyl complexes can be prepared directly from the reaction of Na[CpMo(CO)3] with allyl bromide13 or by oxidative addition of allyl bromide to (CH3CN)3Mo(CO)3 followed by treatment with LiCp,14 neither method can be extended directly to the preparation of the chromium analogues. The oxidative addition of allyl bromide to (CH3CN)3Cr(CO)3, for example, reportedly returns only chromous halide complexes, providing no evidence for the formation of even transient allyl intermediates.14 Here we report that conditions for the latter process can be modified to provide a general new synthesis of substituted cyclopentadienylchromium dicarbonyl η3-allyl complexes.15 Initially, modification of the anionic alkylation procedure was investigated. When allyl tosylate is substituted for allyl halide and trimethylamine N-oxide is introduced to induce decarbonylation at low temperature, Na[CpCr(CO)3] can indeed be converted to the η3-allyl complex 1, albeit in unacceptably low yields (eq (9) (a) Angermund, K.; Do¨hring, A.; Jolly, P. W.; Kru¨ger, C.; Roma˜o, C. C. Organometallics 1986, 5, 1268. (b) Iczek, F.; Jolly, P. W.; Kru¨ger, C. J. Organomet. Chem. 1990, 382, C11. (c) Betz, P.; Jolly, P. W.; Kru¨ger, C.; Zakrzewski, U. Organometallics 1991, 10, 3520. (d) Jolly, P. W.; Kru¨ger, C.; Zakrzewski, U. J. Organomet. Chem. 1991, 412, 371. (e) Betz, P.; Do¨hring, A.; Emrich, R.; Goddard, R.; Jolly, P. W.; Kru¨ger, C.; Roma˜o, C.; Scho¨nfelder, K. U.; Tsay, Y.-H. Polyhedron 1993, 12, 2651. (f) Jolly, P. W. Acc. Chem. Res. 1996, 29, 544. (g) Do¨hring, A.; Go¨hre, J.; Jolly, P. W.; Kryger, B.; Rust, J.; Verhovnik, G. P. J. Organometallics 2000, 19, 388. (10) Wink, D. J.; Wang, N.-F.; Springer, J. P. Organometallics 1989, 8, 259. (11) Shiu, K.-B.; Liou, K.-S.; Cheng, C. P.; Fang, B.-R.; Wang, Y.; Lee, G.-H.; Vong, W.-J. Organometallics 1989, 8, 1219. (12) (a) Smith, J. D.; Hanusa, T. P.; Young, V. G., Jr. J. Am. Chem. Soc. 2001, 123, 6455. (b) Carlson, C. N.; Smith, J. D.; Hanusa, T. P.; Brennessel, W. W.; Young, V. G., Jr. J. Organomet. Chem. 2003, 683, 191. (13) Cousins, M.; Green, M. L. H. J. Chem. Soc. 1963, 889. Luh, T.-Y.; Wong, C. S. J. Organomet. Chem. 1985, 287, 231. (14) Hayter, R. G. J. Organomet. Chem. 1968, 13, P1.
10.1021/om030690q CCC: $27.50 © 2004 American Chemical Society Publication on Web 03/27/2004
2016
Organometallics, Vol. 23, No. 9, 2004
Norman et al.
Table 1. Synthesis of η3-Allyl Complexes 1-7 from (CH3CN)3Cr(CO)3 (Eq 2)
a The time refers to the period allowed for the oxidative addition of allyl substrate to (CH CN) Cr(CO) . b Infrared spectra recorded in 3 3 3 THF solution, in units of cm-1.
1). The modest success of this reaction is presumably
dependent on stabilizing the proposed seven-coordinate σ-allyl intermediate I to avoid chromium-carbon bond homolysis.16 Consistent with this hypothesis, warming the solution to room temperature prior to addition of
trimethylamine N-oxide provides only intractable decomposition products. Fortunately, modification of the oxidative addition procedure was more successful, providing the desired chromium(II) allyl complexes in much higher yields (eq 2, Table 1). Treatment of (CH3CN)3Cr(CO)3 with allyl bromide in acetonitrile at -30 °C, for example, results in a rapid color change from yellow to red, attributed to the formation of the thermally unstable intermediate II (eq 2). Subsequent addition of NaCp in acetonitrile to intermediate II at low temperature yields allyl complex 1 (entry 1) in good yield after chromatography (15) Only two members of this compound class have been previously reported. (a) CpCr(η3-cyclopentenyl)dicarbonyl, prepared in
