THE 1,5-SIGMATROPIC SHIFT OF A BR2 UNIT (R = H, F, OH, NH2, CL, OR SH) OVER A CYCLOPENTADIENE SYSTEM

A molecular-orbital model supported by energy-optimized MNDO calculations is presented for the substituent effects on the rates of the 1,5-sigmatropic reaction of cyclopentadienylborane compounds. The η2-structure represents the transition state and the σ-structure the ground state. The degenerate rearrangement is retarded with electron-donating ligands (e.g. amino-groups) at the boron atom and/or electron-withdrawing substituents at the C5H5 unit. Electron-donating substituents at the C5H5 unit facilitate the reaction. In the transition-state geometry, inversion over retention is favoured at the boron atom.