Carbon‐13 NMR studies of substituted naphthalenes. II—rotational barriers and conformations in some naphthaldehydes and azulenaldehydes

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Abstract
Proton‐coupled and noise‐decoupled 13C NMR spectra of 1‐azulenecarbaldehyde, 1‐acetylazulene and 1,3‐azulenedicarbaldehyde have been studied, and complete assignments have been made based on the 13C1H coupling constants, additivity of substituent effects (SIS), and previous assignment for the parent hydrocarbon. The barriers to rotation of the aldehyde group in the above azulenecarbaldehydes and in some naphthaldehydes have been determined by 13C dynamic NMR, (DNMR), resulting in free energies of activation of 42.7, 26.8 and 34.4 kJ mol−1 for 1‐azulenecarbaldehyde, 1‐naphthaldehyde and 2‐naphthaldehyde respectively. The same order of barriers is obtained by CNDO/2 calculations. A 4‐methoxy substituent in 1‐naphthaldehyde and a 6‐methoxy substituent in 2‐naphthaldehyde increases the rotational barrier by 4.6 and 2.9 kJ mol−1, respectively, whereas a 3‐methoxy substituent in 2‐naphthaldehyde reduces the barrier by 6.7 kJ mol−1. The conformations of the dominant rotamers are deduced from 13C chemical shifts to be Z for 1‐azulenecarbaldehyde and 1‐naphthaldehyde, and E for 2‐naphthaldehyde.

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