Stereoelectronic Effects in Ring-Chain Tautomerism of 1,3-Diarylnaphth[1,2-e][1,3]oxazines and 3-Alkyl-1-arylnaphth[1,2-e][1,3]oxazines^†

The disubstitution effects of X and Y in 1-(Y-phenyl)-3-(X-phenyl)-2,3-dihydro-1H-naphth[1,2-e][1,3]oxazines on the ring-chain tautomerism, the delocalization of the nitrogen lone pair (anomeric effect), and the ¹³C NMR chemical shifts were analyzed by using multiple linear regression analysis. Study of the three-component equilibrium B ⇌ A ⇌ C revealed that the chain ⇌ trans (A ⇌ B) equilibrium constants are significantly influenced by the inductive effect (σ_F) of substituent Y on the 1-phenyl ring. In contrast, no significant substituent dependence on Y was observed for the chain ⇌ cis (A ⇌ C) equilibrium. There was an analogous dependence for the epimerization (C ⇌ B) constants of 1-(Y-phenyl)-3-alkyl-2,3-dihydro-1H-naphth[1,2-e][1,3]oxazines. With these model compounds, significant overlapping energies of the nitrogen lone pair was observed by NBO analysis in the trans forms B (to σ*_C1-C1‘, σ*_C1-C10b, and σ*_C3-O4) and in the cis forms C (to σ*_C1-H, σ*_C1-C10b, and σ*_C3-O4). The effects of disubstitution revealed some characteristic differences between the cis and trans isomers. However, the results do not suggest that the anomeric effect predominates in the preponderance of the trans over the cis isomer. When the ¹³C chemical shift changes induced by substituents X and Y (SCS) were subjected to multiple linear regression analysis, negative and values were observed at C-1 and C-3 for both the cis and trans isomers. In contrast, the positive values at C-1 and the negative values at C-3 observed indicated the contribution of resonance structures f (ρ_R > 0) and g (ρ_R < 0), respectively. The classical double bond−no-bond resonance structures proved useful in explaining the substituent sensitivities of the donation energies and the behavior of the SCS values.