Calculation of the relative basicity of three α,ω‐diphenylpolyenes with trifluoroacetic acid

Abstract

AbstractLinear polyenes are an important class of compounds containing two or more alternating carbon‐carbon double and single bonds that are soluble primarily in organic non‐polar solvents. However, determining the relative basicity of unsubstituted polyenes experimentally has proven to be challenging in practice because such studies require mixing non‐polar polyene‐organic solvent mixtures with high concentrations of moderately polar organic acids. In this study, we used both computational and experimental approaches to calculate potential sites of protonation of trans‐1,4‐diphenyl‐1,3‐butadiene (DPB), all trans‐1,6‐diphenyl‐1,3,5‐hexatriene (DPH), and all trans‐1,8‐diphenyl‐1,3,5,7‐octatetraene (DPO) with trifluoroacetic acid (TFAH) in both n‐hexane or benzene solvents. Density functional theory (DFT) calculations with a 6‐311 + G (d,p) basis set and the B3LYP exchange correlation functional predict that the carbon atoms α to either phenyl ring are the most likely sites of protonation. Our calculations indicate that the basicities of the DPPs increase with increasing length of the polyene moiety (DPO > DPH >> DPB) in the gas phase and in both benzene and n‐heptane solvents. Consistent with these computational predictions, the experimental rates of protonation of DPB, DPH, and DPO in benzene and hexane were consistent with the calculated basicity trends. Dynamic light scattering data confirmed that these reactions were phase separated resulting in emulsions between TFAH and both solvents; these phase separations complicated specification of the actual reaction rates. Finally, GC‐MS and NMR data confirm that the crude products from the protonation of DPB and DPH were mixtures of DPB and DPH dimers. In significant contrast, DPO did not form dimers but rather an unidentified monomeric trifluoroacetate addition product.