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Hydrogen Elimination for Permethylscandocene Alkyl

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Hydrogen Elimination for Permethylscandocene Alkyl 1566 J. Am. Chem. SOC.1990,112, 1566-1577 Ethylene Insertion and @-HydrogenElimination for Permethylscandocene Alkyl Complexes. A Study of the Chain Propagation and Termination Steps in Ziegler-Natta Polymerization of Ethylene Barbara J. Burger,’ Mark E. Thompson,* W. Donald Cotter, and John E. Bercaw* Contribution NO.7948 from the Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, California 91 125. Received May 5, 1989 Abstract: The rates of ethylene insertion into the Sc-C bond for Cp*&R (Cp* = ($-C5Me5), R = CH3, CH2CH3,CH2CH2CH3) have been measured at -80 OC by 13C NMR; the second order rate constants (M-k’) are as follows: R = CH3, 8.1 (2) X lo4: R = CH2CH3,4.4 (2) X IO4: R = CH2CH2CH3,6.1 (2) X 10”. The slow rate for the ScCH2CH3complex is attributed to a ground-state stabilization by a 8-C-H ‘agostic” interaction. The distributions of molecular weights for ethylene oligomers (CH3(CH2)nCH3,n = 11-47) produced from known amounts of ethylene and Cp*,ScCH2CH2CH3 at -80 OC satisfactorily fit a Poisson distribution, indicative of a “living” Ziegler-Natta polymerization system. From the measured, slower initiation rates of insertion for Cp*wH3and Cp*&CH2CH3and propagation rates equal to that for Cp*#cCH2CH2CH3, the molecular weight distributions of ethylene oligomers are also accurately predicted. CP*~SCCH~undergoes a single insertion with 2-butyne with a moderate enthalpy of activation and a large, negative entropy of activation. The second-order rate constants for the insertion of 3-phenyl-2-propyne, 2-pentyne, and 4-methyl-2-pentyne have been measured. The rates for &hydrogen elimination for members of the series of permethylscandocene alkyl complexes Cp*wH2CH2R(R = H, CH3, CH2CH3,C6HS, C&-pCH3, CsH,-pCF3, C6H4-pNMq) have been obtained by rapidly trapping Cp*&H with 2-butyne. A transition state for the ,%hydrogen elimination is indicated with partial charge on the &carbon. Hydrogen is thus transferred to the scandium center as hydride in the 8-H elimination process. Olefin insertion into metal-carbon bonds and &hydrogen undergo multiple insertions to form oligomers and polymers, thus elimination are fundamental transformations that occur in a variety complicating the kinetic scheme. Moreover, in most cases the rates of organometallic ~ystems.”~Indeed, at least one of these reactions are exceedingly fast, approaching diffusion controlled, and the is an essential step in virtually all catalytic cycles involving olefins. catalytically active species is ill-defined so that its identity and For olefin polymerization, chain propagation occurs by olefin concentration are unknown.I2 Although previous studies have insertion into a metal-carbon bond while &hydrogen elimination shown that the rate of insertion varies markedly with the nature serves as a chain-termination (“chain-transfer”) step. Normally, of the olefin (the more substituted the olefin, the less reactive: it is the relative rates of these two steps that determine the chain i.e., ethylene >> propene > 1-butene > 2-butene),I3 the stereoe- length of the polymer.$ lectronic factors dictating this order are not yet understood. The recent work of Watson,6 Eisch, Jordan,8 Marks: and Furthermore, little is known about the effect that the alkyl group Grubbs’O provides compelling support for olefin insertion into the (the migrating group) has on the rate of olefin insertion. metal-alkyl bond as the mode of propagation in Ziegler-Natta The factors governing the rate of 8-H elimination, “chain polymerization of ethylene and other olefins (the Cossee-Arlman transfer”, in these catalytic systems are also poorly understood. mechanism), at least for catalysts based on do transition and dof“ In addition to being a termination path for olefin polymerization, lanthanide metals.” Efforts to gain further insight into the &hydrogen elimination is a common decomposition route for metal mechanism and the factors affecting the rate of the actual olefin alkyls: indeed, the facility of this pathway often precludes isolation insertion step have been met with limited success, however. Unlike of stable alkyl complexes with 8-hydrogens. As with the olefin carbon monoxide, which undergoes a single insertion, most olefins insertion reaction, &hydrogen elimination often is not rate limiting and, as such, is difficult to study mechanistically, since other reactions complicate the kinetic scheme.I4 As shown by (1) Present address: Chevron Research Co., P.O. Box 1627, Richmond, white side^,'^ thermal extrusion of a ligand from a coordinatively CA 94802-0627. (2) Present address: Department of Chemistry, Princeton University, saturated complex generally requires higher temperatures than Princeton, NJ 08544. 8-hydrogen transfer. In some cases, rate-limiting formation of (3) Parshall, G. W. Homogeneous Catalysis; John Wiley & Sons: New the coordinatively unsaturated species can be circumvented by York, 1980; Chapter 3. photochemically dissociating a ligand.I6 Due to the difficulties (4) For a general review of &elimination reactions, see: Cross, R. J. In The Chemistry of the Metal-Carbon Bond; Hartley, F. R., Patia, S., Eds.; described with isolating the @-hydrogenelimination reaction, there John Wiley and Sons: New York; 1985; Vol. 2, Chapter 8. has been no systematic study done to date that has addressed steric (5) Collman, J. P.; Hegedus, L. S.;Norton, J. R.; Finke, R. G. Principles and electronic effects. A better understanding of these processes and Applications of Organometallic Chemistry; University Science Books: could aid in the design of new catalysts capable of producing Mill Valley, CA, 1987; p 567. (6) (a) Watson, P. L. J. Am. Chem. Soc. 1982,104,337-339. (b) Watson, polymers with controlled molecular weight distributions as well P. L.; Roe, D. C. J. Am. Chem. Soc. 1982, 107,6471-6473. as in developing new “living” and block polymerization systems. (7) Eisch, J. J.; Piotrowski, A. M.; Brownstein, S. K.; Gabe, E. J.; Lee, F. L. J. Am. Chem. Soc. 1985, 107, 7219-7221. (8) (a) Jordan, R. F.; Dasher, W. E.; Echols, S. F. J. Am. Chem. Soc. (12) Gates, B. C.; Katzer, J. R.; Schuit, G. C. A. Chemistry of Catalytic 1986, 108, 1718-1719. (b) Jordan, R. F.; Bajgur, C. S.; Willett, R.; Scott, Processes; McGraw-Hill: New York, 1979; p 150. B. J. Am. Chem. Soc. 1986, 108, 7410-7411. (13) Reference 3, p 578. (9) Jake, G.;Lauke, H.; Mauermann, H.; Swepston, P. N.; Schwann, (14) (a) Doherty, N. M.; Bercaw, J. E. J. Am. Chem. SOC.1985, 107, H.; Marks, T. J. J. Am. Chem. Soc. 1985, 107, 8091. 2670. (b) Burger, B. J.; Santarsiero, B. D.; Trimmer, M. S.;Bercaw, J. E. (IO) (a) Soto, J.; Steigerwald, M. L.; Grubbs, R. H. J. Am. Chem. SOC. J. Am. Chem. SOC.1988, 110, 3134. 1982, 104,4479. (b) Clawson, L.; Soto, J.; Buchwald, S. L.; Steigenvald, M. (IS) Whitesida, G. M.; Gaasch, J. F.; Stedronsky, E. R. J. Am. Chem. L.; Grubbs, R. H. J. Am. Chem. Soc. 1985, 107, 3377-3378. SOC.1972, 94, 5258-5270. (1 1) (a) Cossee, P. J. Catal. 1964, 3, 80. (b) Arlman, E. J.; Cossee, P. (16) Kazlauskas, R. J.; Wrighton, M. S. J. Am. Chem. Soc. 1982, 104, J. Catal. 1964, 3, 99. 6005-6015. 0002-7863190115 12-1566%02.50/0 0 1990 American Chemical Society Permethylscandocene Alkyl Complexes J. Am. Chem. SOC.,Vol. 112, No. 4, 1990 1567 Recent efforts in our research group have focused on the Table I. ‘H NMR Data” synthesis and reactivity of alkyl complexes of permethylscandocene, compound assignment 6, ppm coupling, Hz Cp*,ScR [Cp* = ($-C,MeS)] .I7 Permethylscandocene alkyl complexes with @-hydrogens have been prepared and appear to Cp*,ScCH2CH2CH2CH? [Cs(CHj)s] 1.88 (s) (u-CH, 0.90 (t) Juu.... = 7.44 be moderately stable in solution.’7c They are, however, coordi- &y,6-kH2(,, 0.56&61 (m) natively unsaturated and, as such, could decompose by formation Cp*2SCCH2CH2C6HsC [Cs(CH3)5] 1.84 (S) of a scandium hydride complex and liberation of olefin (olefin CH~CH~C~HS1.01 (m) AA’BB’ adducts for a do metal center are not expected to be stable and CH~CH~C~HS2.38 (m) AA’BB’ have not been directly observed). Although @-hydrogen elimi- CH2CH2CJIs 7.48 (d) ~JHH= 7.0 nation has been predicted to be thermodynamically unfavorable CH2CH2CJIS 7.30 (d) 3J~~= 8.0 CH2CH2CJIs 7.124 (t) ’JHHe: 7.5 in do and dof“systems’* (eq I), it could, nonetheless, be kinetically CP*2SCCH2CH2C6H4- [Cs(CHj)s] 1.89 (S) p-NMet Cff2CH2C6H4 1.1 1 AA’BB’ Cp*,ScCHZCHZR F, Cp*ZScH CH,=CHR (m) + (1) CH2CH2C6H4 2.27 (m) AA’BB’ significant. Indeed, evidence from our studies of ethylene po- CHzCH2C6H4 7.04 (d) 3J~~= 8.5 CH2CH2CJf4 6.57 (d) ~JHH= 8.5 lymerization (vide infra) suggests that @-Helimination is a major N(CH3)2 2.82 (s) termination pathway for this rea~ti0n.I~ Cp*2SCCH,CH2C,H,- [Cs(CH,)sJ 1.82 (S) These permethylscandocene alkyl complexes have been shown PChC CH2CH2C6H4 0.84 (m) AA’BB’ to react with olefins by two distinct routes (eq 2 and 3). Insertion CH2CH2C6H4 2.38 (m) AA’BB’ CH2CH2CJI4 7.48 (d) ~JHH= 8.0 C~*~SC-R+ CH*=CHR’ -+ CH~CHZC~H~7.35 (d) ’JHH = 8.0 Cp*,Sc-CH,CHRR’ (insertion) (2) Cp*,SCCH2CH2C6H4- [CI(CH~)S] I .85 (S) P-CH, CH~CHZC~H,1.15 (m) AA’BB’ CP*~SC-R + CHZzCHR’ - CHZCH~C~H~2.42 (m) AA’BB’ Cp*,Sc-CH=CHR’ H-R (u-bond metathesis) (3) CH2CH2CJId 7.19 (d) ~JHH= 7.8 + CH2CH2CJf4 7.51 (d) ’JHH = 7.8 of olefin into the Sc-C bond (eq 2) occurs for all scandium alkyl CH3 2.25 (s) and aryl complexes when the olefin is ethylene, and for the ‘H(90- and 400-MHz)NMR spectra were taken in benzene-d6 at am- bient temperature, unless otherwise stated.
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