Carbon-hydrogen and carbon-carbon bond activation with highly electrophilic transition metal complexes

Highly electron deficient scandocene derivatives of the types (q5-CsMe&ScR, {(q5-CsMe4)2SiMe2}ScR and {(q5-C5H3CMe3)2SiMe2}ScR catalyze the polymerization of ethylene, the head-to-tail dimerization of a olefins, the cyclization of a,w dienes to meth lene cycloalkanes, and the branching of 1,4 pentadienes to isoprenes. The mechanisms oythe individual steps have been studied. Key steps involve sequential and reversible olefin insertionlb H eliminationlb alkyl elimination, the last of which is particularly facile in these sys!ems. [((q5-C5Me4)Me2Si(qi-NCMe3)(PMe3)Sc(p-H)]2, catalyzes the polymerization of a olefins. Evidence is presented in support of a well defined, one component catalyst system with all scandium centers functioning alike. INTRODUCTION Organometallic compounds of scandium resemble those of aluminum, particularly in their tendency to form dimeric or oligomeric structures with bridging alkyl or hydride groups. Recently our research group has reported the synthesis and some features of the reactivity of alkyl and hydride derivatives of bis(pentamethylcyclopentadienyl)scandium, Cp*2ScR (Cp* = (q5-C5Mes)) (ref.1). The bulky (q5-CsMe5) ligands prevent dimerization and restrict the types of ligands which can coordinate to the formally 14 electron, do scandium center. We have suggested the name "sigma bond metathesis" for a dominant reaction type for these highly electrophilic compounds: Cp*2Sc-R + R'-H CP*~SC-R' + R-H (1) (R, R' = hydride, alkyl, alkenyl, aryl, alkynyl) More recently we have turned our attention to the reactivity of these compounds with olefins. Early indications were that they readily polymerize ethylene without coordinating bases or co-catalysts, common complications for Ziegler-Natta polymerizations using group 4 transition metal catalysts. We have therefore undertaken a study of this model system (ref. 2). Cp*2Sc-R + n CH2=CH2 C P * ~ S C ( C H ~ C H ~ ) ~ R (2) Due to unfavorable steric interactions with the (q5-C5Me5) ligands, a olefins do not undergo insertion into the scandium-carbon bonds of Cp*2ScR; rather, they react by sigma bond metathesis (eq. 1). Less crowded scandocene derivatives were prepared, and, indeed, these proved to be very efficient a olefin dimerization catalysts. Moreover, with 1,4-pentadienes, skeletal rearrangement to isoprenes are observed. A related system with linked cyclopentadienyl and amide ligands {(q5-C5Me4)SiMe2(q1-NCMe3)}ScR catalyzes the polymerization of a olefins in a quasi-living manner. ETHYLENE INSERTION AND PH ELIMINATION FOR PERMETHYLSCANDOCENE ALKYLS The rates of ethylene insertion into the Sc-C bond for Cp*2ScR (R = H, CH3, CH2CH3, CH2CH2CH3) have been examined at -80% by I3C NMR spectroscopy. k l CH2=CH2 Cp*2Sc-R +CH2=CH2 Cp"2ScCH2CH2R -etc. (3) The following second order rate constants (kl; M-lsec-i, -8OOC) have been measured: R = H R = CH3,(8.1(2)x The fast rate for the hydride derivative is expected, and almost certainly is due to the greater overlap, hence better bonding in the transition state for H (nondirectional Is orbital) vs. alkyl (more directional sp3 orbital). On the other hand, the greater insertion rate for the scandium propyl relative to the scandium methyl is likely due to ground state differences. Equilibrium measurements indicate that the relative ordering of Sc-R bond dissociation ener ies (BDEs) mirrors that for the corresponding H-R BDEs (ref 3); hence, complex is slowest to insert ethylene cannot be rationalized by these types of metal-carbon bond R = CH2CH3 (4.4(2) x R = CH2CH2CH3 (6.1 (2) x BDE(Sc-CH3) BDE(Sc-CH2 e H2CH3) t ca. 6 kcal.mol-I. The observation that the scandium ethyl