Pentadienyl di--propyl borane

1 Syndiesis and Dynamic Behavior of 2,4-Pentadienyl(di-n-propyl)Borane 

The lower rate of the sigmatropic 1,3-B migrations in the Z-isomer 16b compared to 16a is probably caused by the greater steric hindrance by the c -allylic substituent for the formation of the transition state in 16b. 

The equilibrium concentration of the Z-isomer 16b is approximately 20%. Nevertheless, thermolysis of 16 gives 3-borinene 25 with 40% yield. Owing to geometric considerations only 16b can participate in this transformation. Since the yield of 25 is greater than the equilibrium content of 16b, it is clear that, when 

---

Pentadienyl(di- -propyl)borane 16 was prepared by the reaction of penta-dienyllithium with chloro(di-/i-propyl)borane in a mixture of hexane and THE at -30 °C (Scheme 2.5) [26],... 

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). 

---