Role of ligands in controlling the regioselectivity in ruthenium-catalysed addition of carboxylic acids to terminal alkynes: A DFT study

Density functional studies are performed to understand the role of chelating bi-phosphine ligands [(Ph2P(CH2)mPPh(2)); m = 1-4] in modulating the regio-selectivity of benzoic acid addition to 1-hexyne, in presence of rutheniumn catalyst [(Ph2P(CH2)(m)PPh2)Ru(methallyl)(2)]. The Markovnikov addition to 1-hexyne is observed when catalyst 1(a) [(Ph2P(CH2)PPh2)Ru(methallyl)(2)] is employed, whereas a reverse regio-selectivity is witnessed in presence of 1(d) [Ph2P(CH2)(4)PPh2)Ru(methallyl)(2)]. Anti-Markovnikov addition occurs via the neutral vinylidene intermediates (5(a/d) formed after 1,2-hydrogen shift in hexyne coordinated ruthenium(II) complexes 3(a/d). The energy profile shows clear preference for Markovnikov addition by 15.0 kcal/mol (Delta G(L)(S)) in case of catalyst system 1(a). In contrast, anti-Markovnikov pathway following neutral vinylidenes are more favourable by 9.1 kcal/mol (Delta G(L)(S)) for catalyst system 1(d). The Z-enol ester formation is more predominant in the anti-Markovnikov pathway since the activation bather for this step requires less energy (5.9 kcal/mol, Delta G(L)(S)) than the one furnishing the E-product. The calculated results are in good agreement with the reported experimental findings.