Propargyl compounds alkynes

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

Acylzirconocene chlorides 78, which are easily available through the hydrozirco-nation of alkenes or alkynes with Cp2Zr(H)Cl and subsequent CO insertion, can be used as acyl anion equivalents Cu(I)-catalyzed reactions with propargyl compounds 77 afford allenyl ketones 79 (Scheme 3.40) [86]. The use of an excess of 77 (2 equiv. to 78) is important for the selective preparation of 79, which prevents an undesirable side reaction of the allenic products 79 with 78. 

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. 

Reduction of propargylic compounds with Sml2 is possible in the presence of Pd catalysts. Propargylic acetates 233 are converted mainly to the allene 236 by Pd-catalysed reaction with Sml2 in the presence of a proton source [51]. In this reaction, the allenylpalladium 234 is reduced with Sm(II) to the allenyl anion 235, which is protonated to give allene 236. The alkyne 237 is a byproduct. 2,3-Naphthoquinodi-methane (240) as a reactive intermediate, can be generated by applying this reaction. 

Scheme 11.45 Cross-coupling of propargylic compounds with terminal alkynes.

Scheme 11.44 Mechanism and an example of the cross-coupling of propargylic compounds with terminal alkynes (X = 0C02Me, OAc, Cl).

Catalytic hydrogenation of alkynes takes place in a stepwise manner, and both the alkene and the alkane can he isolated. Complete reduction of alkynes to the saturated compound is easily accomplished over platinum, palladium or Raney nickel. A complication which sometimes arises, particularly with platinum catalysts, is the hydrogenolysis of hydroxyl groups a- to the alkyne (propargylic hydroxyl groups) (7.15). 

Allenes 169 and alkynes 170 are prepared by hydrogenolysis of propargyl compounds with several hydrides. Triethylammonium formate is used most conveniently under mild conditions [45]. Chromium tricarbonyl-complexed phenylallene 172 was prepared from the carbonate 171 [46]. The alkyne 174 was obtained selectively from the propargyl formate 173 having an amino group [47]. 

Soon afterwards, the same group developed methodologies for the oxidative carbonylation of 4-yn-l-ones and propargylic esters (Scheme 8.22). While 2-cyclopentenone carboxylates were obtained from appropriate carbonyl-substituted alkynes [96], cyclic orthoesters and furanones were successfully synthesized starting from corresponding propargylic compounds [97]. More recently, they also realized the asymmetric version of this reaction [98], which has also been applied in the total synthesis of (—)-AL-2 by Mukai and Miyakoshi [99]. 

Pd-catalyzed hydrogenolysis of allylic compounds with formates is an efficient and mild method. The hydride generated from the palladium formate attacks the more substimted side of the allylic system to give less substituted olefins in contrast to the case with other hydride sources. Pd-catalyzed hydrogenolysis of propargylic compounds affords either aUenes or alkynes depending on the structure of the propargylic compounds. 

This concept was refined by the research group of Li, who employed the thiol-yne reaction instead of the olefin cross-metathesis reaction as key step [60]. In this case, the carboxylic acid component served as anchor, whereby terminal alkynes were introduced by the remaining components (5-hexyn-l-al and propargyl isocyanoacetamide). Interestingly, thiol-yne addition of 3-mercaptopropionic acid to the pendant alkynes enabled not only the incorporation of further carboxylic acids, but also resulted in additional branching. Therefore, the second generation dendrimer, synthesized in three steps, exhibited 16 peripheral triple bonds. Moreover, this concept offers the opportunity to introduce structural diversity into the dendrimer architecture because the use of only one alkyne-functionalized compound in the Passerini-3CR still results in branching due to the thiol-yne reaction. Here, a structural sequence of employed phenylacetaldehyde and 2-nitrobenzaldehyde was demonstrated. 

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