Alkenes propargyl compounds

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. 

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. 

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). 

Although superficially similar, propargyl compounds do not form t -complex intermediates, but give t -allenic complexes. As part of a catalytic cycle, these can undergo typical reactions, such as coupling (Schemes 9.82 and 9.83), ° ° reduction by formate, alkene insertion and carbonylation (Scheme 9.84). 

Among several propargylic derivatives, the propargylic carbonates 3 were found to be the most reactive and they have been used most extensively because of their high reactivity[2,2a]. The allenylpalladium methoxide 4, formed as an intermediate in catalytic reactions of the methyl propargylic carbonate 3, undergoes two types of transformations. One is substitution of cr-bonded Pd. which proceeds by either insertion or transmetallation. The insertion of an alkene, for example, into the Pd—C cr-bond and elimination of/i-hydrogen affords the allenyl compound 5 (1.2,4-triene). Alkene and CO insertions are typical. The substitution of Pd methoxide with hard carbon nucleophiles or terminal alkynes in the presence of Cul takes place via transmetallation to yield the allenyl compound 6. By these reactions, various allenyl derivatives can be prepared. 

Organogermanium compounds can be prepared by transmetallation reactions with tin reagents. Examples include Me2PhGeCl (Equation (66)),89 the alkene-functionalized species 26-28, (Equations (67) and (68)),90 and the allenic (Equation (69)) and propargylic (Equation (70)) species 29 and 30.91 A series of aryltrichlorogermanes was prepared from the corresponding tin reagents (Equation (71), Table 9).92 Transmetallation with zirconium species can also be used (Equation (72), Table 10).93... 

As catalytic semihydrogenation of alkynes to Cis-alkenes is not only a very important synthetic operation (ref. 1) but also of industrial interest, it is a challenging task for both synthetic and catalytic chemists. For instance, the importance of the problem is illustrated by numerous recent publications on different aspects of the selective hydrogenation of many compounds related to the propargyl alcohol structure (refs. 2-7). In this respect, 1,4-butenediol, obtained by the liquid-phase semihydrogenation of 1,4-butynediol, is a raw material for insecticides and Vitamin Bg (refs. 2,8,9). Furthermore, the total and selective liquid-phase hydrogenation of this compound is one of the procedure for making butanediol, the top 95 chemical produced in the United States (refs. 10,11), whose major use is in the manufacture of polyesters. 

The first issue confronted by Myers was preparation of homochiral epoxide 7, the key intermediate needed for his intended nucleophilic addition reaction to enone 6. Its synthesis began with the addition of lithium trimethylsilylacetylide to (R)-glyceraldehyde acetonide (Scheme 8.6).8 This afforded a mixture of propargylic alcohols that underwent oxidation to alkynone 10 with pyridinium dichromate (PDC). A Wittig reaction next ensued to complete installation of the enediyne unit within 11. A 3 1 level of selectivity was observed in favour of the desired olefin isomer. After selective desilylation of the more labile trimethylsilyl group from the product mixture, deacetalation with IN HC1 in tetrahydrofuran (THF) enabled both alkene components to be separated, and compound 12 isolated pure. 

Thiolate-bridged diruthenium complexes such as Cp RuCl(p2-SR)2RuCp Cl catalyze the propargylic substitution reaction of propargylic alcohol derivatives with various carbon-centered nucleophiles [118-120]. Ketones [119] (Eq. 88), aromatic compounds [120] (Eq. 89), or alkenes thus selectively afford the corresponding propargylated products with C-C bond formation. An allenylidene intermediate is proposed in these reactions. They are detailed in the chapter Ruthenium Vinylidenes and Allenylidenes in Catalysis of this volume. 

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