Multinuclear NMR spectroscopic studies of aryltrimethylsilanes and aryldimethylphosphaneboranes

Abstract

Proton, boron-11, carbon-13, silicon-29, and phosphorus-31 NMR chemical shifts and coupling constants are reported for nine ortho- and 2,6-disubstituted aryltrimethylsilanes and five similarly substituted aryldimethylphosphaneboranes. Resonances in the natural-abundance carbon-13 NMR spectra for both sets of derivatives are assigned on the basis of additivity relationships, proton-coupled spectra, and relative magnitudes of | J( 31P 13C) | coupling constants. Carbon-13 chemical shifts and | 1 J( 13C 1H) | coupling constants indicate that the P(BH 3)(CH 3) 2 group is electron-withdrawing. The 13C chemical shifts of aryl C(5) carbons can be attributed to steric inhibition of resonance of about the same magnitude as that produced by ortho-Si(CH 3) 3. Chemical shift and coupling constant data from previous work are expanded in terms of Taft's dual substituent constants σ 1 and σ R 0. Least squares solutions of these equations for aryldimethylphosphaneborane derivatives provide values of 0.41 for σ 1 and 0.04 for σ R 0 for the P(BH 3)(CH 3) 2 group. These constants produce reasonable agreement with observed 13C chemical shifts and coupling constants in the ortho derivatives.