An insight into regioselectivity in the transformation through a ruthenacycle

Ru(0)-catalysed cross-dimerisation of unsymmetrically substituted internal alkynes with conjugated dienes yields two conjugated triene products depending on the regioselectivity of the C-C bond formation reaction via a ruthenacycle intermediate. The electronic and steric effects of alkynes are comprehensively evaluated based on Hammett's (sigma(p)) and Taft's (sigma*, E-s) substituent constants. An electron-withdrawing substituent favours the external position of the conjugated triene products. With unsymmetrically 4,4 '-disubstitued diaryl acetylenes, the logarithm plot for the regioisomer ratios of the products and differential Hammet's value Delta sigma(p) between the substituents shows a linear relationship with a positive slope. This trend suggests that the electron-rich alpha-carbon in a ruthenacycle favours an electron-withdrawing group. This system is less sensitive to steric effects on the regioselectivity, but the sterically less bulky groups tend to prefer the external position.