Elimination reactions propargylic compounds

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. 

Umpolung of propargyl compounds occurs in the presence of excess Et2Zn, and homopropargyl alcohols 187 are obtained by the reaction of propargyl benzoates (186) with benzaldehyde, although the exact mechanism is not known. Ethylallene, which is expected to be formed by transmetallation of the allenylpalladium with Et2Zn and reductive elimination, is not formed [41],... 

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. 

The reaction of alkynyllithium compounds with alkoxycarbene tungsten complexes leads to anionic propargyl tungsten complexes (Figure 2.33 see also Figure 3.9). These intermediates are stable at low temperatures and react upon Lewis acid catalysis with aldehydes or A -sulfonyl imines to yield five-membered heterocycles [280]. Oxidative methoxycarbonylation [375] of the intermediate vinyl tungsten complex, followed by elimination of methanol leads to pyrroles or furanes (Figure 2.33 Entry 6, Table 2.22). 

Common reactions of the ylide include (i) [2,3]-sigmatropic rearrangement of allylic, propargylic, and allenic ylides (ii) [l,2]-shift (Stevens rearrangement) (iii) 1,3-dipolar cycloaddition of the ylide generated from carbonyl compounds or imines with dipolarophiles, usually G=G or C=C bonds and (iv) nucleophilic addition/elimination, leading to the formation of epoxides or cyclopropanes (Figure 2). 

The reaction of primary or secondary alcohols with thionyl chloride is a general method for preparing the corresponding chloro compounds. In the first step a chlorosulfne ROSOC) is formed from which S02 is eliminated in a relatively slow step. This decomposition is facilitated by a tertiary amine, e.g. pyridine. The ammonium salt RO-SON+.Cl— formed from the chlorosulftte is subsequently attacked on carbon (in R) by CF. Since nucleophilic substitutions on propargylic carbon proceed more easily than on carbon in saturated compounds, it may be expected that the conversion of propargylic chlorosulfites into the chlorides will take place under relatively mild conditions. 

Ethynyl carbinols (propargylic alcohols) such as 134 (Scheme 2.58) represent another important group of oxidation level 3 compounds. Their preparation involves nucleophilic addition of acetylides to the carbonyl group, a reaction that is nearly universal in its scope. Elimination of water from 134 followed by hydration of the triple bond is used as a convenient protocol for the preparation of various conjugated enones 135. Easily prepared O-acylated derivatives are extremely useful electrophiles in reactions with organocuprates, which proceed with propargyl-allenyl rearrangements to furnish allene derivatives 136. 

Isoxazole-supported selenium resins, produced via 1,3-dipolar cycloaddition of nitrile oxides with propargyl selenium resin, were subjected to a-alkylation reactions with various electrophiles, leading to 3-aryl-5-i4-substituted ethenylisoxazoles in satisfactory yields (62-78%) and purity (90-99%). Compound 238 gave olefin 240, through selenoxide elimination from the a-alkylation product 239 (Scheme 56) <2003OL4649>. 

Acetals prepared from chiral diols and carbonyl compounds serve as a chiral synthetic equivalent of aldehydes or ketones. 1,3-Dioxanes synthesized from chiral 2,4-pentanediols are especially useful, and high asymmetric inductions are observed in the Lewis acid promoted reactions of a variety of organometallic compounds. After the removal of the chiral auxiliary by the oxidation and -elimination procedures, optically active alcohols are obtained. Optically active propargylic alcohols and cyanohydrins are synthesized from organosilane compounds, TMS-C CR or TMS-CN in the presence of TiCU (Scheme 24). 1 6-138 Reactive wganometals such as alkyl-lithiums, -magnesiums or -coppers also react with chiral... 

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