Ruthenium(IV)-Catalyzed Markovnikov Addition of Carboxylic Acids to Terminal Alkynes in Aqueous Medium

Abstract

The dimeric bis(allyl)ruthenium(IV) complex [{RuCl(μ-Cl)(η 3 :η 3 -C 10 H 16 )} 2 ] (C 10 H 16 = 2,7-dimethylocta-2,6-diene-1,8-diyl) (5) and several mononuclear species trans-[RuCl 2 (η 3 :η 3 -C 10 H 16 )(L)] (L = two-electron-donor ligand) (6) derived from 5 have been checked as catalysts for the addition of carboxylic acids onto terminal alkynes using water as a green reaction medium. The best results in terms of activity and regioselectivity were obtained with the mononuclear derivative trans-[RuCl 2 -(η 3 :η 3 -C 10 H 16 )(PPh 3 )] (6a), which was able to promote the selective Markovnikov addition of both aromatic and aliphatic carboxylic acids to a large variety of terminal alkynes, enynes, and diynes as well as propargylic alcohols. In this way, a wide number of enol esters and β-oxo esters could be synthesized in moderate to good yields under mild conditions (60 °C) in an aqueous medium.

Metal-Catalyzed Intra- and Intermolecular Addition of Carboxylic Acids to Alkynes in Aqueous Media: A Review

“…In this regard, Cadierno, Gimeno and co-workers evaluated in 2011 the catalytic potential of a diverse family of ruthenium(IV) complexes, of general composition trans-[RuCl 2 (η 3 :η 3 -C 10 H 16 )(L)] (31a-s in Figure 6), for the hydro-oxycarbonylation of terminal alkynes in aqueous medium [68].…”

“…In this regard, Cadierno, Gimeno and co-workers evaluated in 2011 the catalytic potential of a diverse family of ruthenium(IV) complexes, of general composition trans-[RuCl2(η 3 :η 3 -C10H16)(L)] (31a-s in Figure 6), for the hydro-oxycarbonylation of terminal alkynes in aqueous medium [68]. All these compounds were found to be active catalysts in the addition of benzoic acid to 1-hexyne in pure water, providing the corresponding enol esters n BuC(OBz)=CH2 (Markovnikov addition product) and (E/Z)-n BuCH=CH(OBz) (anti-Markovnikov addition products) in moderate to good yields after 3-24 h of heating at 60 °C (with a Ru loading of 2 mol %).…”

“…The most active catalyst of the series, i.e., [RuCl 2 (η 3 :η 3 -C 10 H 16 )(PPh 3 )] (31a), showed also a wide scope allowing the preparation of a large variety of enol esters 32 by selective Markovnikov addition of different carboxylic acids to both aliphatic and aromatic terminal alkynes, as well as to 1,3-enynes (Scheme 19) [68,72,73]. As a general trend, the reactions proceeded faster, and with higher yields, when aliphatic terminal alkynes were employed as substrates.…”

“…Further evidence of the versatility of [RuCl2(η 3 :η 3 -C10H16)(PPh3)] (31a) was gained in the reactions of the terminal diynes 33 with benzoic acid. Thus, as shown in Scheme 20, the corresponding enynes 34 or the diesters 35 could be selectively synthesized with 31a just by adjusting the diyne/benzoic acid ratio employed [68,72,73]. The only limitation encountered with the ruthenium catalyst 31a is that it is completely inactive with internal alkynes; substrates which, as commented in the introduction of this article, show a much lower reactivity in these intermolecular hydrocarboxylation reactions.…”

“…Further evidence of the versatility of [RuCl 2 (η 3 :η 3 -C 10 H 16 )(PPh 3 )] (31a) was gained in the reactions of the terminal diynes 33 with benzoic acid. Thus, as shown in Scheme 20, the corresponding enynes 34 or the diesters 35 could be selectively synthesized with 31a just by adjusting the diyne/benzoic acid ratio employed [68,72,73]. Further evidence of the versatility of [RuCl2(η 3 :η 3 -C10H16)(PPh3)] (31a) was gained in the reactions of the terminal diynes 33 with benzoic acid.…”