Migrations borotropic

Formally this corresponds to the [1,5] sigmatropic migration of the di-n-propylboryl group. However, it has been supposed that this dynamic behavior is instead caused by two consequent [1,3]-B migrations with the intermediacy of relatively unstable isomer 20a (Scheme 2.19). This conclusion was supported by the close values of the activation barriers of borotropic migrations in 20 (found from 2D EXSY NMR) and structurally similar Z-2,4-pentadienyl(di-n-propyl)borane 16b (see Table 2.1). 

The same conclusion follows from the phase-sensitive 2D H EXSY spectra of the borane 21 taken at 323 K. Since the interconversion of 21a and 21b does not change the relative disposition of the protons (Scheme 2.21), the cross-peaks between the averaged signals directly indicate the mechanism of the borotropic migrations in 21a. Thus, the proton H gives a cross-peak with the proton H - that is indicative of a [1, 7]-B shift. 

Synthesis of Cyclononatetraenyl(di-R-propyl)borane 22 and Mechanism of Borotropic Migrations in this Borane. Selection Rules for the Sigmatropic Migrations of Dialkylboryl Groups... 

Experimentally found selection rules for the borotropic migrations correspond to the sigmatropic shifts with inversion on the migrating atom according to the orbital symmetry principles [43]. This in turn corresponds to the electronic configuration of the boron atom ideal for the formation of a Moebius type transition state. 

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