Substituent effects in fluoromethylnaphthalenes by 19F nuclear magnetic resonance

Abstract

19F substituent chemical shifts (SCS) are reported for a series of twenty-one 3- and 4-substituted 1-fluoromethylnaphthalenes. The fluoromethylnaphthalenes exhibit an inverse SCS dependence: electron-withdrawing substituents produce upfield shifts. The results correlate well with SCS values previously reported for substituted benzyl fluorides. Hammett correlations are poor with conjugatively electron-withdrawing substituents exhibiting weaker than expected effects in the 3-position and stronger than expected effects in the 4-position. Dual substituent parameter analysis confirms the enhanced substituent–aromatic ring resonance interaction when the substituent is in the 4-position (ρR/ρI = 2). There is no evidence for enhanced resonance interaction between fluoromethyl side-chain and aromatic ring. The 19F chemical shift of 1-fluoromethylnaphthalene is markedly temperature dependent.