Electrophiles alkynylation

For oxidative addition of Pd to alkenyl, aryl and alkynyl electrophiles, most of the leaving groups listed in Scheme 23 are ineffective. Primarily for economic reasons, intensive investigations have been made to successfully employ bromides and chlorides over the past 10-15 years. This topic is discussed throughout those sections discussing recent developments. Therefore, only a brief discussion of some fundamentally important points will be presented here. In the oxidative addition of Pd(0) complexes, the Pd complexes serve primarily as nucleophiles for their Pd(0)-to-Pd(II) oxidation. This predicts that those... 

In general, allylic and propargyllic electrophiles are much more reactive than alkenyl and alkynyl electrophiles. Thus, a wide variety of electrophiles containing halogens, e.g. I, Br, and Cl, and oxygen groups, e.g. sulfonates, phosphates, carboxylates, carbonates, alkyl and aryl ethers, and even silyl... 

Negishi, E.-i., Xu, C. Palladium-catalyzed alkynylation with alkynylmetals and alkynyl electrophiles. Handbook of Organopalladlum Chemistry for Organic Synthesis 2002, 1, 531-549. 

III.2.8.2 Palladium-Catalyzed Alkynylation with Alkynylmetals and Alkynyl Electrophiles... 

Pale et al. have reported that the stereoselective electrophile trapping of alkynyl-stabilized lithiated epoxides 189, generated from the parent epoxide 188 and n-BuLi, gives substituted epoxides such as 190, in good yield and de (Scheme 5.44) [67]. 

Once again, a large amount of diverse evidence indicates the intermediacy of a vinyl cation in electrophilic additions to arylacetylenes. As in the case of the hydration of alkynyl ethers and thioethers, the vinyl cation formed is especially stable because of resonance interaction and charge delocalization with the adjacent rr center of the aromatic system. 

C. Reactions not involving P=0 or P=S Groups.—Enamine phosphine oxides (45) have been prepared by the addition of amines to 1-alkynyl-phosphine oxides, and the reactions of their anions with various electrophiles have been reported. - With ketones a Wittig-type reaction leads to the formation of a/3-unsaturated ketones, in 53—70% yield, while with epoxides cyclopropyl ketimines are formed. A Diels-Alder reaction of l-phenyl-A -phospholen-l-oxide (46) with 1,4-diacetoxybutadiene has been used in the preparation of l-phenyl-benzo[/>]phosphole (47), as... 

This reaction works with aryl, allylic and 1-alkynyl halides, but not vinylic halides. A vinylic group can be introduced into the 4 position of an isoquinoline by using PdBr2 as the electrophile and an alkene (Scheme 9).10 This process... 

In favor of a dihydrogen intermediate, the following should be noted (i) the addition of HX molecules to OsHCl(CO)(P Pr3)2 is a useful synthetic route for dihydrogen compounds (vide infra), (ii) the electrophilic character of the dihydrogen complexes has been demonstrated,38 and (iii) the addition of electrophiles to metal alkynyl complexes is a general method of preparing vinylidene compounds.39... 

The hydride-methyl complex OsH(Me)(CO)2(P Pr3)2 reacts with electrophilic reagents. The reaction products depend on the nature of the reagent (Scheme 39). Whereas the reaction with iodine gives almost quantitatively the diiodide OsI2(CO)2(P,Pr3)2, the reaction with a five-fold excess of phenylacetylene does not lead to the formation of the previously mentioned bis-alkynyl complex... 

At present, the chemisty of selenophenes and tellurophenes is a relatively scantily studied area. Nevertheless, a number of new valuable contributions dealing with their chemistry have emerged. Electrophilic cyclization of l-(l-alkynyl)-2-(methylseleno)arenes provides a route to a variety of 2,3-disubstituted benzo[fe]selenophenes, as illustrated by the preparation of the system 88. Other useful electrophiles for similar reactions are E or NBS <06JOC2307>. Similar chemistry has also been employed in preparation of 2,3-disubstituted benzo[f>]selenophenes on solid phase <06JCC163>. In addition, syntheses of 2,3-dihydroselenolo[2,3- >]pyridines have been achieved using radical chemistry <06OBC466>. 

In particular, the reactions of electrophilically activated benzonitrile A-oxides with 3-methylenephthalimidines with formation of 2-isoxazolines and oximes and the cycloaddition between alkynyl metal(O) Fischer carbenes and nitrones leading to 4-isoxazolines have been investigated by density functional theory methods <06JOC9319 06JOC6178>. 

In most cases, it has been proposed that the cationic intermediate resulting from electrophilic attack by the mercuric ion is, in fact, a mercurinium species 109 in which the mercury cation is unsymmetrically coordinated to the two alkynyl carbon atoms. Hence, these reactions typically afford Markovnikov products. In the presence of excess Hg(OAc)2, nucleophilic attack of the mercurinium intermediate is intermolecular and occurs in an anti-fashion leading to the (ft)-isomer. When no excess of Hg(OAc)2 is present, nucleophilic attack of the mercurinium intermediate is intramolecular and occurs in a ry -fashion leading to the (Z)-isomer.1... 

Alkynyl enals cyclize on treatment with a stoichiometric amount of Ni(COD)2/TMEDA complex to give nickel enolates such as 193,436>436a These metallacycles react with electrophiles including methyl iodide and benzaldehyde to yield cyclopentenol derivatives (Scheme 91). 

Scheme 4.8. Hydrozirconation and electrophilic quenching of alkynyl sulfoxides.

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