Regioselectivity alkynes

Arylthiols (but not alkylthiols) add to terminal alkynes regioselectively to afford a Markovnikov-type adduct 212 in good yield using Pd(OAc)2 as a catalyst[120]. This result is clearly different from the an/i-Markovnikov addition induced by a radical initiator. The hydroselenation of terminal alkynes with benzeneselenol catalyzed by Pd(OAc)2 affords the terminal alkene 213, which undergoes partial isomerization to the internal alkene 214[121]. 

A related cyclization of 2-(alkynyl)phenylisocyanates with terminal alkynes to oxindoles was also reported by the same group (Equation (115)).472 (E)-exo-olefinic oxoindoles are selectively obtained. It was proposed that a palladium acetylide generated by the C-H activation of terminal alkynes regioselectively inserts to the alkyne moiety and the resulting vinylpalladium intermediate adds to the C=0 part of the isocyanate to give a (Z)-oxindole. This (Z)-isomer is isomerized to the ( )-isomer under the reaction conditions through catalysis of the phosphine. 

The regiochemistry can be controlled by the nature of the substituents. With a tri-methylsilyl-substituted acetylene, the trimethylsilyl groups are placed in a positions of zirconacyclopentadienes with excellent selectivity (Eq. 2.8) [20]. With a phenyl-substituted alkyne, regioselective reactions are usually observed, although in some cases a mixture of two isomers may be formed. 

Addition of an alkene insertion step to the sequence above prior to reductive elimination of the final product gives an alkene-derived R group. Although for reliable results the reaction is restricted to ethylene, giving an ethyl substituent at C-4, yields and alkyne regioselectivity are reasonable (equation 14)P... 

In the formation of indoles from o-haloarylamines and alkynes, regioselectivity is attained to some degrees. The larger substituent of the aUcyne appears at C-2 of the product. ... 

Another route to alkenyl radicals is by addition of radicals to alkynes. An application of this procedure, which serves as a model for the synthesis of the CD ring system of cardiac aglycones was reported by Stork and co-workers (4.55). The initial alkyl radical, formed selectively from the bromide 61 attacks the alkyne regioselectively to give an intermediate alkenyl radical, which reacts further with the alkene of the cyclohexene to give the product 62. A mixture of alkene stereoisomers is produced owing to the ease of E-Z alkenyl radical isomerization. 

It is well-known that organic thiols add to alkynes regioselectively under radical conditions to afford anti-Markovnikov-type vinylic sulfides as a stereoisomeric mixture. In contrast, the addition of arenethiols to terminal alkynes in the presence of palladium acetate is revealed to proceed with a different regioselectivity to give the corresponding Markovnikov-type adducts in high yields (Scheme 12). ° ... 

For internal alkynes, regioselectivity seldom occurs. The Markovnikov addition of water to internal alkynes gives a mixture of ketone products that have to be separated. 

Han L-B, Zhao Ch-Q, Tanaka M (2001) Rhodium-catalyzed regio- and stereoselective addition of diphenylphosphine oxide to alkynes. J Org Chem 66 5929-5932 Hua R, Tanaka M (1998) Ruthenium-catalyzed addition reaction of diphenylphosphinic acid to terminal alkynes regioselective synthesis of alkenyl diphenylphosphinates. Chem Lett 27 431-432... 

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