Affiliations: [a] Department of Chemistry, Rice University, Houston, TX, USA | [b] Department of Materials Science and Nano Engineering, Rice University, Houston, TX, USA | [c] Energy Safety Research Institute, College of Engineering, Swansea University, Bay Campus, Swansea, Wales, UK
Correspondence:
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Corresponding author: Andrew R. Barron, Energy Safety Research Institute, College of Engineering, Swansea University, Bay Campus, Swansea, Wales, UK. Tel.: +1 713 348 5610; E-mail: arb@rice.edu
Abstract: The high temperature AlMe3 C-methylation of [Me2Al(μ-OR)]2 is reported to follow the order: 9-Ph-fluoroxy >OCPh3 >9-Me-fluoroxy. We suggested this to be due to the relative stability of the appropriate carbocation; however, experimental data, inconsistent with this explanation, caused us to reinvestigate the system. We have performed ab initio calculations on [Me2Al(μ-OR)]2 and Me2Al(μ-OR)(μ-Me)AlMe2 (R = CPh3, 9-Ph-fluoroxy, and 9-Me-fluoroxy) as well as the carbocations and hypothetical reaction products from the heterolytic cleavage of the O–C bond. Heterolytic O–C bond cleavage from [Me2Al(μ-OCR3)]2 and Me2Al(μ-OCR3)(μ-Me)AlMe2 all follow the trend CPh3 >9-phenylfluorene >9-methylfluorene (as does the C–C bond cleavage from R3CMe). The exchange reaction between (AlMe3)2 and [Me2Al(μ-OCR3)]2 is found to be exothermic for OCPh3, but endothermic for 9-Ph-fluoroxy and 9-Me-fluoroxy. It is proposed that the anomalous C-substituent effects are due to the temperature dependence of the exchange reaction, and hence the concentration of the intermediate species, Me2Al(μ-OR)(μ-Me)AlMe2, which is the pre-equilibrium species to the rate determining step of heterolytic O–C bond cleavage.
Keywords: Aluminum, methylation, ab initio, carbocation, equilibrium
