Published on Web 06/04/2003 Homogeneous Catalysis with Methane. A Strategy for the Hydromethylation of Olefins Based on the Nondegenerate Exchange of Alkyl Groups and σ-Bond Metathesis at Scandium Aaron D. Sadow and T. Don Tilley* Contribution from the Department of Chemistry and Center for New Directions in Organic Synthesis (CNDOS), UniVersity of California, Berkeley, Berkeley, California 94720-1460 Received November 7, 2002 Abstract: The scandium alkyl Cp*2ScCH2CMe3 (2) was synthesized by the addition of a pentane solution of LiCH2CMe3 to Cp*2ScCl at low temperature. Compound 2 reacts with the C-H bonds of hydrocarbons including methane, benzene, and cyclopropane to yield the corresponding hydrocarbyl complex and CMe4. Kinetic studies revealed that the metalation of methane proceeds exclusively via a second-order pathway -4 -1 -1 described by the rate law: rate ) k[2][CH4](k ) 4.1(3) × 10 M s at 26 °C). The primary inter- and intramolecular kinetic isotope effects (kH/kD ) 10.2 (CH4 vs CD4) and kH/kD ) 5.2(1) (CH2D2), respectively) are consistent with a linear transfer of hydrogen from methane to the neopentyl ligand in the transition state. Activation parameters indicate that the transformation involves a highly ordered transition state (∆S‡ )-36(1) eu) and a modest enthalpic barrier (∆H‡ ) 11.4(1) kcal/mol). High selectivity toward methane activation suggested the participation of this chemistry in a catalytic hydromethylation, which was observed in the slow, Cp*2ScMe-catalyzed addition of methane across the double bond of propene to form isobutane. Introduction a number of interesting stoichiometric transformations, there have been significantly fewer reports describing selective The selective conversion of saturated hydrocarbons to func- conversions of alkanes via homogeneous catalysis.3-5 Methane tionalized and more valuable products remains an important goal is a particularly attractive substrate for such conversions since in chemical research.1-7 Intense interest in this topic has led to it is cheap and readily available, and represents a potentially many important advances including the discovery of several useful reagent for the incorporation of methyl groups into mechanisms by which transition metal species react with molecular structures. Research on homogeneous methane con- unactivated C-H bonds.2 Although these studies have revealed version has focused on selective oxidations via activations with V V electrophilic late metal complexes in acidic media, or with (1) (a) Selecti e Hydrocarbon Acti ation; Davies, J. 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SOC. 2003, 125, 7971-7977 9 7971 ARTICLES Sadow and Tilley Si-C bonds via σ-bond metathesis.12-15 This rich reaction chemistry suggested that similar activation steps might be used in catalytic hydrocarbon functionalizations, given appropriately active and selective catalysts. We recently reported an initial step in this direction with the description of a catalytic methane dehydrosilylation, which appears to occur via σ-bond meta- thesis.12 This discovery prompted further reactivity studies on 10a compounds of the type Cp*2ScR and a search for transforma- tions that might be incorporated into a catalytic cycle. Useful catalytic processes that might utilize σ-bond metathesis steps involve the formation and cleavage of C-C bonds (e.g., hydrocarbon homologation and hydrocracking, respectively). This possibility has seemed rather remote given some of the apparent limitations associated with such steps. For example, it seems that carbon is disfavored in the â-position of the four- centered transition state for σ-bond metathesis, which should prevent the direct formation (and cleavage) of C-C bonds.16,17 However, a potentially useful product-forming step could involve the nondegenerate exchange of hydrocarbyl groups at Figure 1. ORTEP diagram of Cp*2ScCH2CMe3 (1). the metal center (eq 1) that Cp*2ScMe (1) undergoes the thermoneutral exchange of - + ′- f - ′ + - 10a Cp*2Sc R R H Cp*2Sc R R H (1) its methyl ligand with methane. Slow addition of a freshly prepared pentane solution of LiCH2CMe3 (0.149 M, 1.01 20 - ° Very few reactions of this type have been reported, and the equiv) to a pentane solution of Cp*2ScCl at 78 Cinthe majority of these form products that exhibit low reactivities dark, followed by extraction and repeated fractional crystal- - ° toward further bond activations (e.g., M-Ph, M-CtR, M-OR, lization at 78 C afforded yellow crystals of Cp*2ScCH2CMe3 etc).9,10 Nevertheless, the possibility that highly active metal (2) in 50% yield. All manipulations were performed in the dark, centers may promote carbon-carbon interactions is suggested because workup under ambient room lighting did not provide by the work of Basset and co-workers on silica-supported 2 and led to formation of deep red solutions and oily catalysts,18 and by the fact that alkene polymerization occurs decomposition products. The formation of 2 was quantitative by an insertion process that passes through a 4-center transition in benzene-d6 but preparative-scale reactions in benzene yielded state with carbon in the â-position.19 Here, we describe a mixtures contaminated with Cp*2ScC6H5 (3) nondegenerate alkyl exchange reaction involving scandium, and pentane Cp* ScCl + LiCH CMe 98 the apparent participation of this reaction type in a catalytic 2 2 3 -78 °C, dark C-C bond formation, the hydromethylation of propene. Cp*2ScCH2CMe3 + LiCl (2) Results and Discussion 2 Synthesis of Cp*2ScCH2CMe3 (2). A search for new Compound 2 thermally decomposes at room temperature and catalytic methane activation chemistry began with attempts to its solutions are sensitive to ambient light; however, it can be observe a nondegenerative alkyl-exchange reaction (eq 1).