Reduction acetylene bond

Reduction of enynones to dienones is structure sensitive and is often unsatisfactory if the acetylenic bond is attached directly to the carbonyl 30J 1,52). Selectivity is improved if the acetylenic bond is terminal 52,70,71). 

The precipitate shows an acetylene bond at the end of a chain with an acidic H. With —the other three carbons must be present, as a (CHjljCH— group, because of reduction of (CH,)2CH—C=CH to (CHOjCHCHXH,. 

The electron transfer to the acetylenic bond forms the frans-sodiovinyl radical 20 that, after protonation, produces tram radical 21. At low temperature (—33°C) in the presence of excess sodium, the conversion of the trans radical to sodiovinyl intermediate 22 is slightly more rapid than the conversion of the tram radical to the cis radical 23 (21 —> 22 > 22 —> 23). As a result, protonation yields predominantly the trans alkene. However, low sodium concentration and increased temperature lead to increasing proportion of the cis alkene. Although other dissolving-metal reductions are less thoroughly studied, a similar mechanism is believed to be operative.34 Another synthetically useful method for conversion of alkynes to trans alkenes in excellent yields is the reduction with CrS04 in aqueous dimethylforma-mide.198... 

The preparation of Lindlar s catalyst (palladium on calcium carbonate moderated by treating with lead acetate and quinoline)31 has been described in detail it is used for effecting the partial reduction of an acetylenic bond to an olefin. 

The a-acetylene complex undergoes subsequent reductive cleavage to methane (equation 20). The photoinduced o-ir acetylene bond rearrangement clearly explains the lack of reductive cleavage of dimethylacetylene since this transformation is not possible. The... 

Other Raney catalysts have been prepared. Raney cobalt has been described by several authors (28,29). The active cobalt has been claimed to be especially suitable for the reduction of nitriles. The preparation of an active copper has been described by Faucounau (30). Paul and Hilly (31) have described the preparation of Raney iron. It is claimed that Raney iron reduces acetylenic bonds to ethylenic bonds with no further hydrogenation occurring. 

The addition of hydrogen to olefinic or acetylenic bonds is symmetry-forbidden [87, 88]. However, the participation of a catalyst subdivides the addition of H2 to an unsaturated system into a series of successive steps which do not suffer from these symmetry restrictions. These successive steps are oxidative addition of hydrogen, insertion of the coordinated unsaturated system into a metal-hydrogen bond, and reductive elimination of the hydrogenation product. Irrespective of the individual mechanism there is overwhelming evidence from D2 addition experiments that the catalytic addition of H2 to carbon-carbon double and triple bonds is a cA-addition [20]. 

A mechanism is proposed to accommodate the observed stereoselectivity. The mechanism includes a first silicon shift to an acetylenic bond and a carbene-type zwit-terionic rhodium complex (7) as the key intermediate, which undergoes isomerization from a higher energy form (Z-complex, 6) to a lower energy form ( -complex, 8) followed by reductive elimination to cis-isomer (3) as the kinetic product. 

Moreover, presence of neighboring hydroxyl groups259-263 or conjugation to double bonds260 permits LiAlH4 to be used for partial reduction of the acetylenic bond if a conjugated double bond and a neighboring hydroxyl... 

Although the rates of hydrogenation of acetylenes and olefins barely differ or, as work with homogeneous catalysts has shown, are actually greater for C=C double bonds,124 partial reduction of the acetylenic bond is relatively easy. The reason for the selectivity is the preferential adsorption of acetylene derivatives on the catalyst surface it is therefore recommended that relatively small amounts of catalyst (sometimes 1-2% of the weight of the substrate)... 

A combination of catalytic and electrocatalytic mechanisms in the reduction of acetylenic compounds has been reported. The selectivity of hydrogeneration of 3-butyne-l-ol was achieved by controlled potential of the catalyst at 100 to 200 mV and it was found that the acetylenic bond was selectively hydrogenated to the ethylenic bond and that hydrogenation and isomerization of the ethylenic product only took place after the entire butynol had reacted. 

The hydrogenation of dimethylethynylcarbinol (DMEC), an alcohol containing an acetylene bond was conducted with the most active catalyst, P4VP-Pd(NaBH4) [70]. The rate of reduction of the triple bond of DMEC is lower than that of the double bond. This is clearly seen from the kinetic curve in Fig. 26a i . the reaction rate sharply increases after the absorption of a half of the calculated quantity of hydrogen. The chromatographic analysis of reaction products shows relatively selective hydrogenation of the acetylenic bond (Fig. 26b). 

Barton and coworkers used free radical cyclization in the synthesis of tetracyclines (Scheme 105). Photolysis of 254 (X,Y = SR or OR) gives the corresponding radical, which cyclizes to the (Cy6) compound 255 in 80% yield when X,Y = SCH2CH2O. Quite remarkably, 255 is formed only in the cis form. Another completely stereoselective reaction toward the cis compound involving intramolecular addition to an acetylenic bond has been described by Pradhan and was discussed in Section IX.2 (Scheme 70). An analogous reductive cyclization (K, NH3) of ethynyl ketones has been used by Stork in the construction of a tricyclic intermediate for the synthesis of gibberellic acid. ... 

The regioselective reduction of the acetylenic bond nearest to the hydroxyl function in the diynol (109) with LiAlH4 is the key step in producing the E-enyne unit of ( )-helenynolic acid (110) (Scheme 20). A convenient procedure has... 

Polyacetylenes and vinyl acetylenes are known to show some selectivity in their reactions [109, 110]. Raphael [111] showed that terminal acetylenes could be protected as their sodium salts during reduction of disubstituted acetylenes using sodium in liquid ammonia. This procedure enabled him to reduce a disubstituted acetylenic bond in the presence of a terminal acetylene in the stereospecific synthesis of trans-undeca-7-en-l-yne (18) from undeca-l,7-diyne (19). This method of protection of monosubstituted acetylenes was used during reduction of the enone system (20) with lithium in liquid ammonia [112]. Raphael made a study of other possible methods of protecting terminal acetylenes and found that the best of these was the bromoacetylene [113]. 

Arens [114], using a finding of Petrov [115], found that silylation protected terminal acetylenes against catalytic reduction under conditions which, in unprotected molecules, would reduce monosubstituted acetylenes preferentially. He was able to reduce the central acetylenic bond in the triyne (21) in this... 

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