PUB - Publikationen an der Universität Bielefeld

The reaction of the radical cation of 1,1-dichloroethene 1 with methanol has been investigated by FT-ICR spectrometry and compared with that of the radical cations of 1,2-dichloroethene 2 and of the 1,1- and 1,2-dibromoethenes 3 and 4 with methanol and ethanol, respectively. The 1,1-dichloro derivative 1(circle+) is special because it exhibits besides a slow hydride transfer also a slow isomerization into an unreactive isomer, which was proven by separate experiments to be the 1,2-disubstituted isomer 2(circle+). This unprecedented isomerization of a radical cation by a reciprocal 1,2-shift of Cl and H is catalyzed by methanol. The mechanism of this isomerization and the competing reactions of 1(circle+) with methanol have been modeled by ab initio calculation of the corresponding minimum energy reaction pathway (MERP). The results show that the isomerization takes place by a 1,2-Cl shift in a beta-distonic onium ion 1distb(circle+) which is formed by a fast addition of methanol to the ionized C=C double bond. The resulting alpha-distonic ion 1alphadistb(circle+) displays the structure of the addition product of methanol to the carbene center of chloromethyl chlorocarbene 5(circle+). However, the 1,2-H shift within 1alphadistb(circle+), which completes the exothermic isomerization 1(circle+)-->2(circle+), requires a very large critical energy and blocks this reaction pathway. This large barrier is encompassed by a reaction route on which 1alphadistb(circle+) starts to dissociate into methanol and 5(circle+), and the incipient chloromethyl chlorocarbene radical cation 5(circle+) isomerizes with the aid of methanol by some kind of an internal proton shuttle. Thus, the catalyst methanol acts in this system in two different modes. Firstly, it is covalently bonded to the 1,1-dichloroethene radical cation and enables an easy 1,2-Cl shift within the resulting beta-distonic ion. Secondly; during the process of reconstituting methanol it lowers the energy of the transition state of the 1,2-H shift in the incipient chloromethyl chlorocarbene radical cation 5(circle+) to the 1,2-dichloroethene radical cation 2(circle+).