Aluminum Alkoxides as Synthons for Methylalumoxane (MAO): Product-Catalyzed Thermal Decomposition of [Me 2 Al(μ-OCPh 3 )] 2 Stephen J. Obrey, 1a Simon G. Bott, 1b and Andrew R. Barron* ,1a Department of Chemistry and Center for Nanoscale Science and Technology, Rice University, Houston, Texas 77005, and Department of Chemistry, University of Houston, Houston, Texas 77204 Received July 9, 2001 The thermal decomposition of [Me 2 Al(µ-OCPh 3 )] 2 , to yield Ph 3 CMe and methylalumoxane ([MeAlO] n , MAO), is initially catalyzed by the addition of AlMe 3 ; however, the reaction is also catalyzed by the MAO product. The overall reaction rate takes the form: rate ) k TMA [{Me 2 Al(µ-OCPh 3 )} 2 ][AlMe 3 ] + k MAO [{Me 2 Al(µ-OCPh 3 )} 2 ][MAO], where k MAO . k TMA . The ∆H q for the AlMe 3 - and MAO-catalyzed reactions have been determined as 175 ( 8 and 190 ( 15 kJ‚mol -1 , respectively. Both reactions show a large positive value of ∆S q (41 ( 8 eu and <53 eu, respectively), indicative of a dissociative reaction. The thermal decomposition of [Me 2 Al(µ-OCPh 3 )] 2 is also catalyzed by Lewis acids, including AlCl 3 , AlCl 2 Me, and AlClMe 2 . On the basis of the relative rates of the AlMe 3 -catalyzed thermal decomposition of [Me 2 Al- (µ-OCPh 3 )] 2 , [Me 2 Al(9-Ph-fluoroxy)] 2 (1), and [Me 2 Al(9-Me-fluoroxy)] 2 (2) and the MAO- catalyzed C-methylation of [Me 2 Al(µ-OR)] 2 [R ) CMePh 2 (3), CMe 2 Ph (4), CH 2 Ph, C 6 H 11 , C 6 H 4 -4- t Bu (5)], it is proposed that the rate-determining step for C-methylation involves heterolytic cleavage of the O-C bond and the formation of a carbonium ion. The more stable the carbonium ion, the faster the reaction. Additionally, it is proposed that Lewis acid catalysis is due to the formation of an asymmetrically bridged hemi-alkoxide, whose formation is an equilibrium process such that the observed rate of the reaction will be dependent on the equilibrium for the reaction of [Me 2 Al(µ-OCPh 3 )] 2 with the Lewis acid. Introduction In 1974, Mole and co-workers published a series of papers describing the AlMe 3 C-methylation of tertiary alcohols, ketones, and carboxylic acids. 2-4 Ordinarily, the reaction of AlMe 3 with tertiary alcohols, R 3 COH, would be expected to yield the alkoxide, i.e., eq 1. 5 When AlMe 3 was reacted with a tertiary alcohol in toluene solution at room temperature, then heated in a sealed ampule at temperatures ranging from 80 to 300 °C for up to 36 h, the C-methylation products were isolated, eq 2. 2 This reaction was found to be generally applicable for tertiary alcohols as well as secondary and primary alcohols, where one of the substituents was an aryl group; however, as the number of aryl substituents was reduced, the reaction was found to require more heat and longer reaction times. Based on the isolation of similar products from the thermolysis of a mixture of R 3 COH and AlMe 3 as well as the previous isolation of [Me 2 Al(µ-OCR 3 )] 2 , it was proposed that the C-methylation reaction occurs via the aluminum alkoxide compound. 2 The thermolysis of the alkoxide compounds was found to be slow unless per- formed in the presence of an excess of AlMe 3 . It was noted by Mole and co-workers that the role of the AlMe 3 in increasing the rate of the reaction, and reducing the occurrence of side products, was due to the possible formation of the hemi-alkoxide, Me 2 Al(µ-OCR 3 )(µ-Me)- AlMe 2 . Despite this, the mechanism of C-methylation was proposed to involve protonation of the alkoxide oxygen since water was observed to further catalyze the C-methylation. 2,6 Although the specific role of the ad- dition of the Lewis acid was unclear, it was known at that time that addition of water to AlMe 3 yields the formation of methylalumoxane ([MeAlO] n , MAO). 7 * To whom correspondence should be addressed (url: www.rice.edu/ barron). (1) (a) Rice University. (b) University of Houston. (2) Harney, D. W.; Meisters, A.; Mole, T. Aust. J. Chem. 1974, 27, 1639. (3) Meisters, A.; Mole, T. Aust. J. Chem. 1974, 27, 1655. (4) Meisters, A.; Mole, T. Aust. J. Chem. 1974, 27, 1665. (5) (a) Mole, T. Aust. J. Chem. 1966, 19, 373. (b) Rogers, J. H.; Apblett, A. W.; Cleaver, W. M.; Tyler, A. N.; Barron, A. R. J. Chem. Soc., Dalton Trans. 1992, 3179. (6) The effect of water in increasing the rate of MAO formation has been confirmed, as well as the effect of solid MAO; see: Sangokoya, S. A. US Patent, 6,013,820 2000. (7) It should be noted that at this time it was known that addition of water to AlMe3 yields the formation of methylalumoxane (MAO); see for example: Manyik, R. M.; Walker, W. E.; Wilson, T. P. U.S. Patent 3,242,099, 1966. AlMe 3 + R 3 COH f 1 2 [Me 2 Al(µ-OCR 3 )] 2 + CH 4 (1) R 3 COH 9 8 AlMe 3 R 3 CMe (2) 5162 Organometallics 2001, 20, 5162-5170 10.1021/om0106128 CCC: $20.00 © 2001 American Chemical Society Publication on Web 10/26/2001