Study of Reactions Leading to Sulfine Formation. 3. Competition of Reaction Pathways in the Reaction of Methoxide Ion with Methyl 1-Naphthylmethanesulfinates

Article abstract of DOI:10.1021/jo00276a019

In CD3O^1-/CD3OD methyl 1-naphthylmethanesulfinates, NpCH2S(O)OCH3 (2), undergo both exchange of CH3O by CD3O by substitution at the sulfinyl group and elimination to form the sulfine, NpCH=S=O.With use of methyl (2-methoxy-1-naphthyl)methanesulfinate (2a) it has been shown that formation of the sulfine takes place by an (E1cB)_irrev mechanism.The rates of substitution (k_s) and elimination (k_e) of a series of 2 have been determined in CD3O^1-/CD3OD by 1H NMR spectroscopy, and the effect of several reaction variables on the competition between substitution and elimination has been examined.Salient results are as follows: (1) the rate of elimination is markedly increased by the presence of electron-withdrawing substituents on the aromatic ring, but the rate of substitution is increased only modestly by the same substituents; (2) substituents at the 2-position of the naphthyl group cause a large decrease in k_s (steric hindrance to substitution at S=O) but have little effect on k_e (elimination rate not sensitive to steric requirements of ortho substituents); (3) the activation energy for elimination is almost 9 kcal/mol greater than the activation energy for substitution.This large difference in activation energy contrasts with the 1-2 kcal/mol difference for elimination vs substitution found¹⁴ with alkyl halides.