The side-chain-ring coupling constants3 in
the three isomeric tetrachlorotoluenes are Jortho
0.63�0.02 c/s, Jmeta 0.36�0.02 c/s, and Jpara 0.63�0.01 c/s. The magnitudes of
side-chain coupling constants in benzene derivatives are discussed. In symmetrical
six-membered aromatic and heterocyclic compounds the magnitude of the ortho
side-chain coupling constants is approximately 0.6 c/s but varies from less
than 0.3 cis to 1.1 c/s in molecules where some bond localization occurs. Ortho
side-chain coupling constants in five-membered heterocyclic compounds also
appear to be affected by electron localization. It is therefore likely that the
magnitude of the ortho side-chain coupling constants may be a direct measure of
bond order. Long-range coupling data on indole derivatives are reported. In two
β,β-dimethylstyrenes the stronger side-chain to ring coupling is to
the ortho position. The relation between allylic coupling and electron
delocalization is explored and a new criterion for electron delocalization
proposed.
The preparation of some new β,β-dimethylstyrenes
and α-methylstilbenes is described and their p.m.r. spectra are reported.
The long-range shielding effects in β,β-dimethylstyrenes and other
styrene derivatives are discussed. The differences between the cisoid and
transoid allylic coupling constants in β,β-dimethylstyrenes are much
smaller than the corresponding differences in simple propenes. No appreciable
(J ≥ 0.3 c/s) coupling between the non-equivalent methyl protons in β,β-dimethylstyrenes
has been observed. Long-range coupling in the compounds investigated appears to
be independent of ring substitution. Some general features of allylic coupling
are discussed.
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