Transmission of substituent effects through the nitrogen-silicon-nitrogen system

Article abstract of DOI:10.1021/ic50047a023

Transmission of substituent effects as determined by nmr chemical shifts is used to test the hypothesis of (p → d)-π bonding in the N-Si-N system. Unsymmetrically substituted N,N′-p- and -m-diphenylimidazolidines and -diphenylsilaimidazolidines were chosen for this study as such molecules satisfy the requirement that the substituents be held relatively far from one another on the scale of molecular dimensions, thus minimizing inductive and direct electrostatic field effects as well as magnetic anisotropy which might alter the chemical shift of the protons under examination. Studies were carried out in mixtures of the compound with the N-p-nitrophenyl,N′-p-dimethylaminophenylimidazolidine as a reference in each case (Chemical Equation Presented) [M = C, Si; X= p-NO₂, p-OCH₃, p-Cl, m-NO₂, and p-N(CH₃)₂] and chemical shifts extrapolated to infinite dilution in chloroform to eliminate bulk magnetic susceptibility effects. Chemical shift differences in the dimethylamino group could be measured to better than ± 0.1 cps in this way. Transmission across analogous N,N′-diphenylethylenediamines is negligible. Shifts for a variety of organic N,N′-substituted diphenylimidazolidines are shown to correlate with the Hammett σ constant for each substituent. Shifts for the analogous silicon series are found to fall on the same linear plot against the Hammett σ constant for each substituent. Consequences of this finding on the possibility for (p → d)-π interactions in the N-Si-N system are discussed.