Isotope‐induced chemical shifts ²Δ10/11B(²⁹Si) in alkenylsilanes containing three‐ and four‐coordinate boron. A new tool for studying weak intramolecular boron–element bonds

Numerous alkenylsilanes with silyl and boryl groups in cis ‐positions at the CC bond were studied for the first time by 29 Si NMR with regard to isotope‐induced chemical shifts Δ 10/11 B( 29 Si). Such effects are transmitted across an Si—X—B bridge (X = H, OR, SR, Cl) most efficiently if the interactions in this bridge are weak. In the absence of an Si—X—B bridge, 3 Δ 10/11 B( 29 Si) effects were not observed. In such cases, where, depending on the substituents, both weak and strong B—X bonding are possible, the 2 Δ 10/11 B( 29 Si) effects were readily observed only for weak B—X interactions (except for X = Cl), whereas for strong B—X bonding the effects were small or not detectable. Thus, the 2 Δ 10/11 B( 29 Si) values provide another tool for probing weak interactions which are not clearly reflected by other NMR data.