Dynamic Behavior of Cycloheptadienyl di--propyl borane

CYCLOHEPTATRIENYL(DI- -PROPYL)BORANE EQUILIBRIUM WITH7-(DI- -PROPYLBORYL)NORCARADIENE 

1 Syndiesis of Cyclopentatrienyl Derivatives of Boron. Analysis of the Eqnilibriunt Cycloheptatriene-Norcaradiene 

Ab initio DFT calculations for all isomers of 21 (dimethylboryl derivatives) and the transition states for their interconversions have been used to clarify the reasons of the relative stabihty of 21b-exo and 2lb-endo. In the former compound the empty 2p-AO of the boron atom is ideally oriented for overlapping with the Walsh orbital of the cyclopropane ring. On the other hand, in 2lh-endo this interaction is elfectively switched off due to the perpendicular orientation of these orbitals emerging when the steric interactions of the alkyl groups and the cyclohexadiene ring are minimized. The thermodynamic parameters for the equilibrium between 21a and 21b were determined by NMR data from the linear dependence of In K versus /T AH =10.1 0.5 kJ mol L AS= -23 1 J mol K . These values are reasonably reproduced in the calculations (AH = 8.4 kJ mol AS = —15.5 J mol K ). 

Cycloheptatrienylborane 21 cannot be observed separately in the NMR spectra due to the fast equilibrium with its valence tautomer 21b. Nevertheless, at temperatures above 300 K this equilibrium is fast on the NMR timescale and the concentration of 21a is about 30%. If in 21a a sigmatropic migration of the di-H-propylboryl group takes place, this rearrangement should lead to the mixing of the chemical shifts in the NMR spectra of 21a, which inevitably 

The conclusive evidence for the [1,7]-B shifts in 21a has been obtained by ab initio DFT calculations. The calculated relative energies of the transition states for dilferent borotropic migrations show that the [1,7]-B shift is the most favorable energetically = 77 kJ mol ). For the [1,5]-B shift the calculations give = 108 kJmol , and for the [1,3]-B shift A = 115 kJ mol .  

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