Role of ligands in controlling the regioselectivity in ruthenium-catalysed addition of carboxylic acids to terminal alkynes : A DFT study / created by Bholanath Maity, Totan Mondal, Kaustav Dey, Sankarsan Biswas and Debasis Koley

Density functional studies are performed to understand the role of chelating bi-phosphine ligands [(Ph2P(CH2)mPPh2); m=1–4] in modulating the regio-selectivity of benzoic acid addition to 1-hexyne, in presence of ruthenium(II) catalyst [(Ph2P(CH2)mPPh2)Ru(methallyl)2]. The Markovnikov addition to1-hexyne isobserved when catalyst1a[(Ph2P(CH2)PPh2)Ru(methallyl)2] is employed, whereas a reverseregio-selectivityis witnessed in presence of1d[(Ph2P(CH2)4PPh2)Ru(methallyl)2]. Anti-Markovnikovaddition occurs via the neutral vinylidene intermediates (5a/d)formed after 1,2-hydrogen shift in hexyne coordinated ruthenium(II)complexes3a/d. The energy profile shows clear preference forMarkovnikov addition by 15.0 kcal/mol (�GSL)in case of catalyst system1a. In contrast, anti-Markovnikov pathway following neutral vinylidenes are mor efavourable by 9.1 kcal/mol (�GSL)for catalyst system1d. The Z-enol ester formation is more predominant in the anti-Markovnikov pathway since the activation barrier for this step requires less energy (5.9 kcal/mol,�GSL)than the one furnishing the E-product. The calculated results are in good agreement with the reported experimental findings.