Substituent effects in reductions of heteroaromatic cations

Abstract

A set of 11 each of 2,4,6-triphenylpyrylium, -thiopyrylium and -N-methylpyridinium tetrafluoroborates carrying a range of substituents in the phenyl rings were prepared. First and second wave reduction potentials were determined. For the thiopyrylium series there are linear correlations between scaled potentials (E°/0.05915) and summed Hammett constants for substituents in the pendant phenyl groups ( = 2.29 and 3.38 for first and second waves respectively). For the pyrylium series, a good linear relationship ( = 2.79) is obtained for all substituent patterns for the first wave reduction potentials, but for the second wave there are separate correlations for salts carrying substituents in the 4-phenyl and for those carrying substituents in 2-and 6-phenyls. For the pyridinium series, the first wave potentials show separate correlations for salts carrying substituents in the 4-phenyl and for those carrying substituents in 2-and 6-phenyls, but a single linear relationship for the second wave potentials. These are related to particular structural features in the cations, radicals and anions in each series. Rates and products were determined for reductions of the pyrylium and thiopyrylium cations by sodium cyanoborohydride and of all cations by sodium borohydride in acetonitrile solution. Reactions are first order in reducing agent and cation. Primary kinetic isotope effects were determined for borohydride reduction of the least reactive of each of the series of cations. Plots of logarithms of second-order rate constants against summed Hammett constants for substituents in the pendant phenyl groups are linear for all combinations of reagent and cation with 0.91 < < 1.50 across all substituent patterns. For parent pyrylium and thiopyryliums, kBH 4 /kCNBH 3 = 8.4 Â 10 4 and 1.5 Â 10 4 , respectively, and for reductions by borohydride the reactivities of the pyrylium, thiopyrylium and pyridinium, series decrease in the order 1.4 Â 10 5 :8.8 Â 10 3 :1. Constant selectivities are not observed. Comparison of the correlations for electrochemical reduction and for hydride addition leads to the conclusion that charge neutralization in the hydride addition transition states runs ahead of bonding changes at the originating B-H bond.

3,5-Bis(trifluoromethyl)acetophenone

“…The yield of the reaction can be improved by just refluxing the mixture of reagents in the absence of solvent. 2 Bis(trifluoromethyl)phenyl]-3-phenylprop-2-en-1-one, which has been also used as chalcone partner, is prepared in a 94% yield by condensation between benzaldehyde and 3,5-bis(trifluoromethyl) acetophenone in ethanolic sodium hydroxide solution. 2 When a mixture of tris(trimethylsilyl)phosphane (2-2.5 equiv) and the corresponding pyrylium salt is refluxed in acetonitrile for 5 h, the phosphabenzene derivatives are obtained in moderate yields (eq 3).…”

“…The reaction between triethoxyvinylsilane with 3,5-bis(trifluoromethyl)acetophenone in the presence of catalytic amounts of [Ru(H) 2 (CO)(PPh 3 ) 3 ] in toluene as solvent gives the corresponding adduct in a 69% yield (eq 13). 35 [Ru(H) 2 The stability 36 as well as several 13 C NMR parameters such as 13 C NMR shifts 37−39 and 13 C-13 C NMR coupling constants 40 of 3,5-bis(trifluoromethyl)acetophenone and its O-protonated carboxonium ion have been well studied.…”

Additional effect of para‐hydroxyl on the reduction potentials of theN‐benzylidenebenzenamines

“…Dosa et al studied the relationship between the reduction rate and the E Red of substituted azobenzene. Liu et al studied the relationship between the reduction activity and the E Red of para‐substituted bromobenzene. The works of Dosa and Liu show that there is a linear relationship between the reduction activity and the E Red .…”

A Novel Solution‐Processable n‐Dopant Based on 1,4‐Dihydropyridine Motif for High Electrical Conductivity of Organic Semiconductors

“…[27] An aldol reaction between commercially available acetophenone and p-anisaldehyde afforded chalcone 1 in 92% yield. After an acidcatalyzed condensation between chalcone 1 and acetophenone, pyrylium salt 2 was prepared in 74% yield.…”

“…Scheme 2 illustrates the synthetic route to 1,4-dihydropyridine derivative, DPDHP, which was developed following a reported procedure with some slight modifications. [27] An aldol reaction between commercially available acetophenone and p-anisaldehyde afforded chalcone 1 in 92% yield. After an acidcatalyzed condensation between chalcone 1 and acetophenone, pyrylium salt 2 was prepared in 74% yield.…”