First observation of a [1,7] boron shift - Three independent fluxional processes in the tricarbonyliron complex of cycloheptatrienyl(dipropyl)borane

The fluxional behavior of [1-4-η⁴ - exo - 7 - dipropylborylcyclohepta -1,3,5-triene]tricarbonyliron (14) has been studied by dynamic NMR. The 2D ¹H and ¹³C EXSY NMR experiments carried out at various temperatures revealed that three independent fluxional processes occur in 14. The fastest rearrangement is a [1,7] B sigmatropic shift accompanied by [1,2] Fe migration (E_A = 71.2 ± 2.3 kJ mol ^-1, ln A = 32±1). Two other rearrangements also take place: [1,3] B sigmatropic and [1,3] Fe haptotropic migrations. The analysis of the observed selectivity towards [1,j] B sigmatropic shifts ([1,3] and [1,7] B shifts are observed, but [1,5] B shifts are not) in terms of theoretical predictions gives reason to assume that the [1,j] boron migrations occur with inversion of configuration at the migrating atom.