What is the reason for the anomalous C-substituent effects in the Lewis acid catalyzed thermal decomposition of [Me2Al(mu-OR)](2)?

摘要

The high temperature AlMe3 C-methylation of [Me2Al(mu-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(mu-OR)](2) and Me2Al(mu-OR)(mu-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(mu-OCR3)](2) and Me2Al(mu-OCR3)(mu-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(mu-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(mu-OR)(mu-Me)AlMe2, which is the pre-equilibrium species to the rate determining step of heterolytic O-C bond cleavage.