Alkynes halogen effect

Kuniyasu H, Yamashita F, Terao J, Kambe N. Definitive evidence for the insertion of terminal alkynes into arylS—Pt bonds o-halogen effect in stoichimetric and catalytic reactions. Angew. Chem. Int. Ed. 2007 46 5929-5933. 

Kuniyasu H, Takekawa K, Yamashita F, Miyafuji K, Asano S, Takai Y, Ohtaka A, Tanaka A, Sugoh K, Kurosawa H, Kambe N (2008) Insertion of alkynes into an ArS-Pt bond regio- and Stereoselective thermal reactions, facilitation by o-halogen effect and photoirradiation, different alkyne preferences depending on the ancillary ligand, and application to a catalytic reaction. Organometallics 27 4788-4802... 

The reaction time between 4-iodopyrazoles and 1-alkynes varies from 5 to 25 h and the yield of products is 55-95%. It is noteworthy that the nature of the terminal acetylene has a greater effect on the rate of halogen atom substitution for low-reactive 4-iodopyrazoles. Thus, the reaction time for ethynylarenes is 5-6 h, and for less acidic aliphatic 1-alkynes is 10-25 h (Table XTT). 

Both reactions involve nucleophilic attack of tricoordinated phosphorus on tetrahedral carbon and show all the characteristics of non-polar reactants combining through polar transition states although the solvent effects are sometimes quite modest.— Subsequent studies have demonstrated nucleophilic attack on activated alkenes,Z activated alkynes,Z the carbonyl groupZ-Z and halogen , whilst in the Perkow reaction (eqn. 1) all four possible sites in... 

The reactivity of halogen atoms in position 2 of oxazoles is considerably enhanced by quaternization. This is best illustrated by the synthetic utility of the salt (141) which, like other Ar-alkyl-2-halogeno-azolium and -azinium salts, effects a number of condensation reactions but it far surpasses these other salts in activity (79AG(E)707). Thus it activates carboxylic acids and it dehydrates formamides to isocyanides (Scheme 6). Amides are similarly converted into cyanides, ketones (RCH2COAr) into alkynes (RC=CAr), and cyanohydrins (RCH(OH)CN) are transformed into the corresponding chloro compounds <79CL1117>. 

The effect of fluorinated systems on the reaction process shown in Figure 10.67 is of interest [139] it is to be noted that insertion of the palladium catalyst into the carbon-halogen bond may be considered as a nucleophihc attack by the palladium centre, albeit a soft nucleophile, which prefers to attack C—Br over C—F [138]. This process is, of course, aided by the presence of electron-withdrawing groups (EWG) in the organic system. It is likely, however, that co-ordination of the other reactant, e.g. alkyne, to the palladium is the rate-determining step [137], but this will be aided by EWGs attached to the metal. 

Et3B is an effective tool for halogen atom transfer radical reactions (see also Chap. 1.5). Perfluoroalkyl iodide [29], a-halo nitrile and a-halo ester [30] added to alkenes and alkynes at low temperature. Not only terminal alkenes but also internal alkenes can be employed to furnish iodine atom transfer adducts (Scheme 23). Furthermore, addition of perfluoroalkyl iodide to silyl and germyl enolate provided a-perfluoroalkyl ketones [31]. The reaction would involve the elimination of a tri-... 

Possibly the measurement of the chlorine isotope effect for a series of suitably substituted substrates may help to decide whether or not greater stereospecificity is associated with decreased carbanion character of the transition state for elimination from alkenes. Of course, such studies must be accompanied by labelling experiments to exclude the possibility that alkynes arise by an apparent alpha-elimination (Section 5.1) involving formation of a carbanion at the alpha carbon, which subsequently eliminates alpha halogen simultaneously with migration of a substituent from the beta carbon. Results with alcoholic media, however, tend to indicate this alternative mechanism is unimportant if a normal beta-elimination is possible. 

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