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

Abstract

Density functional studies are performed to understand the role of chelating bi-phosphine ligands [(Ph 2 P(CH 2) m PPh 2); m = 1-4] in modulating the regio-selectivity of benzoic acid addition to 1-hexyne, in presence of ruthenium(II) catalyst [(Ph 2 P(CH 2) m PPh 2)Ru(methallyl) 2 ]. The Markovnikov addition to 1-hexyne is observed when catalyst 1 a [(Ph 2 P(CH 2)PPh 2)Ru(methallyl) 2 ] is employed, whereas a reverse regio-selectivity is witnessed in presence of 1 d [(Ph 2 P(CH 2) 4 PPh 2)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 (G S L) in case of catalyst system 1 a. In contrast, anti-Markovnikov pathway following neutral vinylidenes are more favourable by 9.1 kcal/mol (G S L) for catalyst system 1 d .T h eZ-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, G S L) than the one furnishing the E-product. The calculated results are in good agreement with the reported experimental findings.

Solvent‐Promoted Regio‐ and Stereoselectivity in Ru‐Catalyzed Hydrocarboxylation of Terminal Alkynes: A DFT Study

“…proposed an alternative mechanism that is initiated by the oxidative addition of the alkyne C(sp)−H bond instead of Nu−H. On the contrary, if the ruthenium center is electron deficient (that is, Ru II ), then the alkyne coordination precedes the nucleophilic attack step . It is worthwhile to mention that the active species 8 is a 16‐ e Ru II complex with square‐pyramidal geometry.…”

“…The above reaction flow will provide the regioselective Markovnikov addition product. However, to get the anti‐Markovnikov product, the reaction includes one additional step in which alkyne to vinylidene tautomerization occurs prior to the nucleophilic attack . In the subsequent sections we will first discuss the pathways involving Markovnikov addition followed by the anti‐Markovnikov addition route.…”

“…In 9 III , the C β with formal positive charge ( =−0.213/0.105 e , Table S4 in the Supporting Information) can invoke the adjacent carboxylate group towards nucleophilic attack, initiating the Markovnikov addition step . Out of two oxygen atoms of the carboxylate group, O 1 is more suitable for nucleophilic attack as it is more loosely bound to the metal center than O 2 ( =2.236/2.229 Å, Figure , Figure S4 in the Supporting Information).…”

“…In the vinylidene intermediate, the C α represents a strong electrophilic center whereas in the Ru‐alkyne intermediate, the C β center is more electropositive. Recently, we have successfully addressed the ligand‐controlled regio‐ and stereoselectivity in hydroamidation and hydrocarboxylation of terminal alkynes (Schemes S1 and S2 in the Supporting Information). Our results further revealed that the regioselectivity is decided in the alkyne to vinylidene transformation step, whereas the stereoselectivity is controlled in the step of nucleophilic addition to vinylidene …”

Ruthenium‐Catalyzed Hydrocarboxylation of Internal Alkynes

“…[1,2] Such esters can be applied as monomers in numerous polymerization reactions [3,4] and are important intermediates in organic synthesis. [1,2] Such esters can be applied as monomers in numerous polymerization reactions [3,4] and are important intermediates in organic synthesis.…”