Transition metal complexes and the mechanism of pentalene formation

The dehydrogenative transannular ring closure of cyclooctateraenes to pentalenes is rationalised on the basis of a disrotatory electrocyclisation of a cyclo-octadienyl moiety; the bridgehead hydorgens, now syn to a coordinatively unsaturated metal atom, are lost by a series of oxidative addition and reductive elimination processes.