Reactions of Propargylic Compounds

Propargylic compounds (2-alkynyl compounds) are derivatives of alkynes and they undergo several types of transformations in the presence of transition metal catalysts. However, catalytic reactions of propargylic compounds, particularly their esters and halides, clearly differ mechanistically from those of simple alkynes, except in a few cases. Therefore, the catalytic reactions of propargylic compounds are treated independently from those of simple alkynes. The most extensive studies have been carried out using Pd catalysts, and mainly Pd-catalysed reactions are treated in this chapter [1], 

From a mechanistic viewpoint, the Pd(0)-eatalysed reactions of propargylic compounds so far discovered can be classified into four types I IV The allenyl complexes 5 undergo three types of transformations depending on reactants. Type I reactions proceed by insertion of unsaturated bonds to the n-bond between Pd and the sp2 carbon in 5. Type la is the insertion of alkenes to the palladium-carbon n-bond, and the 1,2,4-alkatrienes are formed by /f-elimination. Alkynes insert to form the alkenylpalladium 6, which undergoes various transformations such as insertion of unsaturated bonds and anion captures. 

Insertion of CO also generates the acylpalladium intermediate 7, and 2,3-alkadienoates are obtained by alcoholysis (Type lb). 

The reactions of type II proceed by transmetallation of the complex 5. The transmetallation of 5 with hard carbon nucleophiles M R (M = main group metals) such as Grignard reagents and metal hydrides MH generates 8. Subsequent reductive elimination gives rise to an allene derivative as the final product. Coupling reactions of terminal alkynes in the presence of Cul belong to Type II. 

Type III reactions proceed by attack of a nucleophile at the central sp carbon of the allenyl system of the complexes 5. Reactions of soft carbon nucleophiles derived from active methylene compounds, such as /i-kcto esters or malonates, and oxygen nucleophiles belong to this type. The attack of the nucleophile generates the intermediates 9, which are regarded as the palladium-carbene complexes 10. The intermediates 9 pick up a proton from the active methylene compound and n-allylpalladium complexes 11 are formed, which undergo further reaction with the nucleophile, as expected, and hence the alkenes 12 are formed by the introduction of two nucleophiles. 

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Reactions of Propargylic Compounds Catalyzed by Pd(Q) 5.2 Reactions with Alkenes and Terminal Alkynes... 

The rhodium(II)-catalyzed reaction of propargyl compounds 169 and diazo compounds 170 gave corresponding functionalized allenes 171 together with cydopro-penes 172 (Scheme 3.87) [126]. Rh2(pfb)4, where pfb represents perfluorobutyrate, was found to be an excellent catalyst for preparing the allenes 171. An analogous rhodium(II) complex, Rh2(OAc)4, afforded mainly 172 with only a trace amount of 171 (<5%). 

Most of the synthetic routes to allenes utilize the reaction of propargylic compounds as electrophiles. In contrast, if the propargylic compounds serve as nucleophiles, a wide variety of substituted allenes, which are not easily accessible by the reaction of propargylic compounds with nucleophiles, are available. However, in order to synthesize enantioenriched allenes by this method, it is necessary to generate configurationally stable propargyl or allenylmetal reagents (cf. Chapter 9). 

Some reactions of propargylic compounds which do not belong to the above-mentioned types are known. These reactions are surveyed in this section, although their mechanisms are not always clear. 

Tsuji, J. and Mandai, T., Palladium-catalyzed Coupling Reactions of Propargylic Compounds , in Metal-catalyzed Cross-coupling Reactions, Wiley-VCH, Weinheim, 1998, pp. 455-489. 

The Pd(0)-catalyzed reactions of propargylic compounds so far discovered can be classified into four types, I, II, III, and TV, from a mechanistic viewpoint. The allenyl intermediate complex 8 undergoes three types of transformation, depending on reactants. The reactions of Type I proceed by insertion of unsaturated bonds into the a-bond between palladium and sp carbon in 8. This a-bond has a reactivity similar to the a-bond formed by the oxidative addition of alkenyl halides to Pd(0) in the Heck reaction [3]. Therefore, reactions similar to those observed in the Heck reaction are expected to occur witli the intermediate 8. Alkenes and carbon monoxide are known to insert into the palladium-carbon a-bond. The allene derivatives 9 are formed by these reactions (Scheme 11.3). 

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