A Series of Sigmatropic Rearrangements in 2,4,6-Heptatrienyldipropylborane. Kinetic Study of a 1,7-Hydrogen Shift Facilitated by 1,3-Boron Shifts

The dynamic behavior of a novel polyunsaturated allylic borane, 2,4,6-heptatrienyldipropylborane, 5, was studied by NMR spectroscopy. It was found that four geometric isomers 5a–d interconvert reversibly via successive 1,3 sigmatropic shifts of boron (BPr₂ group). Due to the thermal 1,7-hydrogen shift occurring in the minor Z,Z-isomer, 5d, the allylic borane 5 is converted stereoselectively and reversibly into the vinylic (E,Z,Z)-1,3,5-heptatrienyldipropylborane, 8. The final equilibrium mixture contains 92% 8 and 8% 5. The effective activation energy of the conversion of 5 into 8 and the activation energy for the reverse reaction were found to be 79.8 and 87.1 kJ/mol, respectively; the preexponential factors proved to be the same, 4·10⁸ s⁻¹. Reaction of 5 with cyclopentanone gives the trans-homoallylic alcohol 15a, while 8 reacts with cyclopentanone yielding (Z,Z)-1,3,5-heptatriene, 18.