Neutral hyperconjugation challenged: No support from one‐bond CC coupling constants

Abstract

AbstractA significant decrease in 1J(Cipso, CH2) coupling constants from a value typical of C(sp2)C(sp3) bonds (43.5 Hz or smaller) to the unexpectedly low value of 36 Hz typical of C(sp3)C(sp3) bonds was reported by Lambert and Singer on passing from XC6H4CH2Sn(CH3)3 to XC6H4CH2C(CH3)3 (X = H, CH3, OCH3, NO2). This trend was invoked by these authors as the first evidence for neutral hyperconjugation in terms of coupling constants. However, the present measurements reveal that also in C6H5CH2C(CH3)3 and p‐O2NC6H4CH2C(CH3)3 the 1J(Cipso, CH2) coupling constant bears a value typical of this type of bonding (43.8 and 42.6 Hz, respectively). Thus, at variance with Lambert and Singer's observation, a slight increase in 1J(CC) coupling constants occurs on passing from Sn to C derivatives. This contradicts not only their experimental data but also conclusions concerning hyperconjugation in the neutral ground state.