REACTIONS ON TRIPLE-BONDS

The addition of a nucleophile Y to a triple-bond as in 2 can take place to give a product anion where the entering nucleophile is trans (2) or cis (3) to the non-bonded electron pair. Stereoelectronic effects should therefore affect product formation. 

Ingold (1) has mentioned that the older literature contains several observations indicating the existence of stereochemically favorable and unfavorable situations for elimination reactions leading to acetylenic compounds. For instance, Michael (2) found that chlorofumaric acid ( ) is converted by alkali about 60 times faster than in chloromaleic acid into acetylene dicarboxylic acid (6). Chovanne (3) has observed that cis-dichloroethylene (H is transformed by alkali about 20 times faster than is the trans-isomer 8 into chloroacetylene ( ). 

Cristol and co-workers (4) found that cis-p-nitrostyryl bromide 10. the presence of ethanolic sodium ethoxide, undergoes elimination (yielding 2300 times faster than the trans-isomer 1 which undergoes an alternative reaction of addition (-.13). 

Modena and co-workers (5) have also reported that cis-B-arenesulfonylvinyl chlorides (14) undergo nucleophilic substitution of the halogen by methoxide ion by way of an E2 trans-elimination to an acetylene derivative ( S) followed by a trans-addition to yield the cis-product 16. 

Miller (6) has reported that the raethoxide-catalyzed addition of methanol to phenyl acetylene (27) gave stereospecifically ci s-B-methoxystyrene (19) via the intermediate IB. 

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Reactions on Triple-bonds 295 acylisoamide 5 id quantitative yield. On heating, the Z isomer 35 yields the isomer 36 which is rapidly transformed into the corresponding amide 37 (11). 

The most comprehensive modern works on the subject are the relevant volumes of Patai s series The Chemistry of Functional Groups, namely the two volumes on diazonium and diazo groups (Patai, 1978), the two volumes on hydrazo, azo, and azoxy groups (Patai, 1975) and the two Supplement C volumes on triple-bonded groups (Patai and Rappoport, 1983). Supplement C contains chapters on arene- and alkene-diazonium ions and on dediazoniation reactions. 

Similarly, halo-acetoxylation is known. This reaction has also been carried out on triple bonds, to give the unsaturated analogs of 85 (syn addition). ... 

This strategy chapter is rather different. We shall look at one class of starting material—alkynes or acetylenes—and see what special jobs they can do in synthesis. In particular, we shall see how they can solve some problems we have already met. Acetylene itself 3 is readily available and its first important property is that protons on triple bonds are much more acidic than most CH protons. Acetylene forms a genuine anion 4 with sodium in liquid ammonia, a lithium derivative 1 with BuLi and a Grignard reagent 2 by reaction with a simple alkyl Grignard such as EtMgX. 

An acyl and an acyloxy group can be added to a double bond by treatment with an acyl fluoroborate and acetic anhydride. As expected, the addition follows Markovnikov s rule, with the electrophile Ac going to the carbon with more hydrogens. In an analogous reaction, an acyl and an amido group can be added to give 183, if a nitrile is used in place of the anhydride. Similarly, halo acetoxyla-tion is known.This reaction has also been carried out on triple bonds, to give the... 

Principle was studied in depth for both reactions whereas the MHP (for its DFT based foundation see [72, 73]) was considered for the addition reaction to triple bonds only. At first sight the HSAB principle, serving as a guide for regioselectivity (preferential attack on X or Y in (46) and on Cj or C2 in (47) seems to be... 

Heteroatom Wittig chemistry also includes reactions of N-sulfonyl imines. It was demostrated that these compounds underwent olefination reactions with nonstabilized phosphonium ylides under mild conditions to afford an array of both Z- and E-isomers of 1,2-disubstituted alkenes, allylic alcohols, and allylic amines.Additionally, studies of the reactions of 5-bromo-4,6-dimethyl-2-thioxo-l,2-dihydropyridine-3-carboni-trile and thiazolidinone with phosphorus ylides have proved the formation of new phosphonium ylides. Annulations via P-ylides are a common occurrence in the literature. For example, on photochemical irradiation, phosphonium-iodonium ylides were shown to undergo 1,3-dipolar cycloaddition reactions with triple bonds, via a carbene intermediate, to yield furans. " Even more common are the reactions of Morita-Baylis-Hillman (MBH) acetates and carbonates. Zhou et al. demostrated that these substrates were able to generate very reactive 1,3-dipoles in the presence of tertiary phosphines the dipoles then underwent cycloaddition reactions to yield annulation products (Scheme 16). ... 

The intramolecular carbopalladation (or insertion) of the triple bond in dimethyl 4-pentynylmalonate (215) with Pd—H species and malonate anion as shown by 216 proceeds in the presence of f-BuOK and 18-crown ether, affording the methylenecyclopentane derivatives 217 and 218, the amounts of which depend on the reaction conditions. The Pd—H species may be formed... 

Butyrolactones are prepared by intramolecular reaction of haloallylic 2-alkynoates. The a-chloromethylenebutyrolactone 301 is prepared by the intramolecular reaction of300[150,151]. 4 -Hydroxy-2 -alkenyl 2-alkynoates can be used instead of haloallylic 2-alkynoates, and in this reaction, Pd(II) is regenerated by elimination of the hydroxy group[152]. As a related reaction, the q-(chloromethylene)-7-butyrolactone 304 is obtained from the cinnamyl 2-alkynoate 302 in the presence of LiCl and CuCbflSS]. Isohinokinin (305) has been synthesized by this reaction[l 54]. The reaction is explained by chloro-palladation of the triple bond, followed by intramolecular alkene insertion to generate the alkylpalladium chloride 303. Then PdCb is regenerated by attack of CuCb on the alkylpalladium bond as a key step in the catalytic reaction. 

Hydration. Water adds to the triple bond to yield acetaldehyde via the formation of the unstable enol (see Acetaldehyde). The reaction has been carried out on a commercial scale using a solution process with HgS04/H2S04 catalyst (27,28). The vapor-phase reaction has been reported at... 

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