Markovnikov selectivity, metal-alkyne complexes reactions

Abstract Progress in the field of metal-catalyzed redox-neutral additions of oxygen nucleophiles (water, alcohols, carboxylic acids, and others) to alkenes, alkynes, and allenes between 2001 and 2009 is critically reviewed. Major advances in reaction chemistry include development of chiral Lewis acid catalyzed asymmetric oxa-Michael additions and Lewis-acid catalyzed hydro-alkoxylations of nonacti-vated olefins, as well as further development of Markovnikov-selective cationic gold complex-catalyzed additions of alcohols or water to alkynes and allenes. 

In the transition-metal-catalyzed addition reactions of thiols to terminal alkynes, several addition products, i.e., Markovnikov-type adduct 1, Markovnikov addition and then double-bond-isomerization product 2, a n -Markovnikov adduct 3, double hydrothiolation product 4, and bisthiolation product 5, may be formed (Scheme 2). Controlling the product selectivity can be attained by the selection of transition metal complexes as catalysts, the use of additives, and/or the optimization of the reaction conditions (solvent, temperature, molar ratios of the starting materials, and so on). 

The most efficient catalyst precursors for simple alkynes were found in the RuCl2(arene)(phosphine) series. These complexes are known to produce ruthenium vinylidene species upon reaction with terminal alkynes under stoichiometric conditions, and thus are able to generate potential catalysts for a f -Markovnikov addition [27]. In 1986, the possibility of the involvement of an active metal vinylidene in a catalytic cycle was suggested for the first time to rationalize the formation of these regioisomers [23]. Dienylcarbamates could be selectively prepared from conjugated enynes and secondary aliphatic amines but in this case, the best catalyst precursor was Ru(methallyl)2(diphenylphosphinoethane) [26]. 

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