Substituent effects in cyanoborohydride reductions of heterocyclic aromatic cations

Abstract

First and second wave E_½^redn potentials have been measured by cyclic voltammetry for a set of fifteen 2,6-diphenyl-4-(X-phenyl)-pyrylium, -thiopyrylium and -N-methylpyridinium tetrafluoroborates, with X =p-CH₃, -H, m-Cl, p-CF₃ and m,m˙-(CF₃)₂. There are linear correlations between Hammett substituent constants and scaled potentials (E/0.059 15) giving slopes of ρ= 2.59 ± 0.06 (r²= 0.9984), 2.38 ± 0.07 (r²= 0.9973) and 3.10 ± 0.16 (r²= 0.9918), respectively, for the first wave potentials.

Products, rates and kinetic hydrogen isotope effects for reductions of the cations with sodium and tetrabutylammonium cyanoborohydride in acetonitrile solution have been determined. Reaction of the thiopyrylium 2 (X = H) yields kinetically controlled 54 : 46 mixture of the 4H- and 2H-thiopyrans. With the pyrylium ion 1 (X = H) the 4H- to 2H- ratio is 24 : 76 with the 2H-pyran suffering electrocyclic ring opening to 1,3,5-triphenylpentadienal. Product ratios are weakly sensitive to substituents on the 4-phenyl, with electron withdrawing groups increasing the amount of 2H-product for both thiopyryliums and pyrylium cations. Reductions are first order in cation and cyanoborohydride. For sodium cyanoborohydride and 2,4,6-triphenylpyrylium, k(25 °C)= 65.6 dm³ mol^–1 s^–1 and for 2,4,6-triphenylthiopyrylium, k(25 °C)= 16.7 dm³ mol^–1 s^–1, with kinetic isotope effects (k_BH3CN/k_BD3CN) of 1.17 and 1.68, respectively. Logarithms of rate constants correlate with Hammett constants giving ρ= 1.46 ± 0.06 (r²= 0.9951) for the pyrylium series and ρ= 1.12 ± 0.08 (r²= 0.9831) for the thiopyryliums. The N-methylpyridinium ions do not react with cyanoborohydride.

The X-ray crystal structure of 2,4,6-triphenyl-N-methylpyridinium tetrafluoroborate has been determined. Dihedral angles about bonds from the pyridinium ring to the 2- and 6-phenyls are larger than corresponding angles in 2,4,6-triphenylpyrylium and 2,4,6-triphenylthiopyryliums and are ascribed to relief of non-bonded interactions between the 2- and 6-phenyls and the N-methyl group.