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 ²⁹Si NMR with regard to isotope‐induced chemical shifts Δ^10/11B(²⁹Si). 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, ³Δ^10/11B(²⁹Si) effects were not observed. In such cases, where, depending on the substituents, both weak and strong B—X bonding are possible, the ²Δ^10/11B(²⁹Si) 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 ²Δ^10/11B(²⁹Si) values provide another tool for probing weak interactions which are not clearly reflected by other NMR data. Copyright © 2002 John Wiley & Sons, Ltd.