Dynamic behavior and rearrangements of group 14 organometallic substituted alkenylboranes

Article abstract of DOI:10.1021/om00156a030

Alkenylboranes 7 and 8 with a group 14 organometallic group (R²₃M; M = Sn, Pb, R² = Me, Et, Ph) at the C=C double bond, in a cis position with respect to the boryl group, are obtained via organoboration of the corresponding alkynylstannanes (5) and -plumbanes (6) (R²₃M-C≡C-R¹; R¹ = H, Me, ^tBu, Ph) with trialkylboranes (R₃B, R = Et (2a), ⁱPr (2b)). Many of the lead compounds are thermally and photolytically unstable. The dialkylboryl group in 7 and 8 prefers the conformation 10, where the C₂B plane is perpendicular to the C=C-B plane as shown by dynamic ¹H and ¹³C NMR spectra. In the alkene derivatives 7 and 8 cleavage of the =C-R bond occurs readily, re-forming the alkynylmetal compound (5,6) and the trialkylboranes 2 (deorganoboration). Direct proof is given for this by two-dimensional ¹H/¹H exchange NMR spectra. The preferred structure 10 makes CB (σπ) interactions possible, weakening the =C-R bond, and together with a weak M-C= bond the organoboration becomes readily reversible. The structure 10 is also favorable for R²/R exchange, which has been found to take place between lead and boron in 8, leading to alkenes 13 and 13′. Only one example for 8 has been found for cis/trans isomerization into 15. All products are characterized by their ¹¹B, ¹³C, ¹¹⁹Sn, and ²⁰⁷Pb NMR data.