σ-Correlations and steric effects

✍ Scribed by S. N. Balasubrahmanyam; I. M. Mallick

• Book ID: 102526400
• Publisher: John Wiley and Sons
• Year: 1982
• Tongue: English
• Weight: 431 KB
• Volume: 19
• Category: Article
• ISSN: 0749-1581
• DOI: 10.1002/mrc.1270190311

✦ Synopsis

Abstract

Carbon‐1 shifts in 1‐dimethylimino‐3‐dimethylamino‐1‐(4‐substituted)phenylprop‐2‐ene perchlorates show high correlation and negative dependence v. a σ set having high f character, similar to α and carbonyl carbon shifts in 4‐substituted styrenes and N,N‐dimethylbezamides, respectively. In sharp contrast with the β‐carbons of the styrenes and the ^15^N absorptions of benzamides, whose shifts show high positive correlations with specific ratios of f and R characters, the C‐2 shifts remain invariant. In the correlation of the barriers about the N‐CO bond in N,N‐dimethylbenzamides v. f/R, the R effect has been reported to increase dramatically for the meta‐series when values determined in acetonitrile are exchanged with those determined in chloroform and, in the para series, to be higher than (normal) in both solvents. α‐Carbon shifts in tris‐para‐substituted triphenylcarbonium ions correlate with a σ set having exceptionally high R character (c. 98 %). Explanations, based mainly on the likely action of steric factors, are suggested.