What is the reason for the anomalous C-substituent effects in the Lewis acid catalyzed thermal decomposition of [Me 2 Al(μ-OR)] 2 ?

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.