Markovnikov selectivity, metal-alkyne

In an effort to apply the cooperative principles of metalloenzyme reactivity, involving a combination of metal-ligand and hydrogen bonding, we have reported a ruthenium catalyst incorporating imidazolyl phosphine ligands that efficiently and selectively hydrates terminal alkynes (5). We subsequently found that application of pyridyl phosphines to the reaction resulted in a >10-fold rate enhancement and complete anti-Markovnikov selectivity, even in the... 

Another approach toward C-O bond formation using alkynes that has been pursued involves the intermediacy of transition metal vinylidenes that can arise from the corresponding y2-alkyne complexes (Scheme 13). Due to the electrophilicity of the cr-carbon directly bound to the metal center, a nucleophilic addition can readily occur to form a vinyl metal species. Subsequent protonation of the resulting metal-carbon cr-bond yields the product with anti-Markovnikov selectivity and regenerates the catalyst. 

Markovnikov or anti-Markovnikov selectivity in hydrophosphination of alkynes (Scheme 1) depends on the metal catalyst and reaction conditions it is thought to be controlled by the regioselectivity of insertion of an alkyne into an M-H or M-P bond. 

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. 

Catalytic transformations of alkynes have recently led to tremendous developments of synthetic methods with useful applications in the synthesis of natural products and molecular materials. Among them, the selective activations of terminal alkynes and propargylic alcohols via vinylidene- and allenylidene-metal intermediates play an important role, and have opened new catalytic routes toward anti-Markovnikov additions to terminal alkynes, carbocyclizations or propargylations, in parallel to the production of new types of molecular catalysts. 

In this section we describe the available literature on the addition reaction of thiols and selenols RZH (Z = S, Se). We do not discuss non-catalytic addition reactions carried out without transition metal catalysts as this topic has already been addressed in several publications (see [100-103,139-142] and references therein). It was shown that the non-catalytic reactions led to a different outcome the anti-Markovnikov products are formed in the addition of RZ H to alkynes. Our goal is to concentrate on the selective formation of the scarcely available Markovnikov isomer by RZH addition to the triple bond of alkynes. 

In addition to ruthenium-catalyzed reactions, a range of other transition metal catalysts have shown activity toward the addition reaction. A series of air-stable gold compounds promoted the addition of carboxylic acids to alkynes (Scheme 2.93) [138]. A variety of gold and silver compounds were screened as catalysts for the reaction, and the most effective pair under the mildest conditions was (Ph3P)AuCl and AgPF. Under the reactions conditions, the reaction was highly selective for the formation of the Markovnikov addition product, and minimal or none of the anti-Markovnikov products were observed. 

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]. 

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). 

In the cases of aromatic aUcynes, the radical addition of PhSeH induced by oxygen (or air) is a very fast process (the addition is finished immediately after mixing the alkynes and PhSeH), giving a n-Markovnikov adduct regioselectively. The key species, PhSe-, adds to alkynes to generate vinyl radicals, a-Aryl-substituted vinyl radicals (formed by the addition of PhSe- to aromatic alkynes) are among 7t-radicals and more stable than a-alkyl-substituted vinyl radicals as a-radicals (formed by the addition of PhSe- to aliphatic alkynes) [86]. Therefore, it is relatively difficult, compared with the hydrothiolation, to control the selectivity in transition-metal-catalyzed hydroselenation of aromatic alkynes. 

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