1,3 enynes/1,3 diynes hydrogenation

Another reaction is reductive cyclization. 1,6-Diynes and 1.6-enynes undergo reductive cyclization using hydrosilanes as a hydrogen source in AcOH. The 1,6-diynes 91 and 95 are converted into the 1,2-dialkylidenecyclopentane derivatives (1,3-dienes) 94 and 96. Triethylsilane is used as a hydrogen donor for the reaction[48]. The reaction involves the formation of a vinylpalladium bond in 92 via the insertion of an alkyne into the Pd—H bond, followed by the alkyne insertion to give 93, which is hydrogenolyzed with Si—H to give the 1,3-dienes 94 and 96. 

Concerning chemical selectivities in multiple unsaturated acetylenic compounds, conjugated enynes are reduced with modest selectivity. The most difficult case involves the selective reduction of an enyne with a terminal double bond and an internal triple bond, because the difference in the rates of their hydrogenation is minimal. The hydrogenation of nonterminal, unconjugated diynes to (Z,Z)-dienes can be achieved in good yield. 

Another reaction is reductive cyclization of 1,6-diynes and 1,6-enynes using hydrosilanes as a hydrogen source in AcOH. Triethylsilane is used as the hydrogen donor for this reaction [142], The reaction can be understood by the formation of vinylpalladium bond 350 via the insertion of alkyne 349 to the Pd—H bond in 328. Then intramolecular alkyne insertion gives vinylpalladium 351, which is hydro-genolysed with Si—H to give the 1,3-diene 352 by transmetallation and reductive... 

Regio- and stereoselective reduction of the non-silylated triple bond, either by partial catalytic hydrogenation,13,1 15 or by lithium aluminum hydride reduction of the propargylic alcohols,11,15,1 afford (after desilylation), respectively, terminal (2)- and (E)-enynes. Furthermore, the remaining trimethylsilyl group in both silylated diynes and enynes may be replaced by another electrophile in a second Friedel-Crafts reaction.18... 

Hydrogen adds exclusively to the terminal triple bond of monosubstituted conjugated diynes but the initially formed enynes react rapidly with hydrogen, and even from the beginning, products of over-hydrogenation appear - . After the absorption of one mole of hydrogen by 1,3-pentadiyne in the presence of Pd(CaCOs), the product contains 67% l-penten-3-yne. No products have been detected which correspond to the initial addition of hydrogen to the internal triple bond. 

A v ety of reactions are catalyzed by electrochemically generated Ni(0) (62). Electrochemical reduction of Ni(bipy)3Br2 affords a reagent that couples acid chlorides and alkyl or aryl halides to form unsymmetrical ketones (63). Symmetrical ketones are formed from alkyl halides and carbon dioxide (64). Reductive electrochemical carboxylation of terminal alkynes, enynes and diynes can be accomplished with 10% Ni(bipy)3(Bp4)2 in DMF (65-68). Terminal allies lead selectively to a-substituted acrylic acids. Electrocatalytic hydrogenation on hydrogen-active electrodes has been reviewed (69). Radical cyclizations of vinyl, alkyl and aryl radicals can be carried out by indirect electrochemical reduction with a Ni(II) complex as a mediator (70). 

Terminal Z-enynes (140) are produced by selective hydrogenation of the unsily-lated triple bond of the corresponding diynes (141) prepared by oxidative coupling of an acetylene with trimethylsilylacetylene (Scheme 82).Enynes are also obtained from a-cyclopropylacetylenic carbinols (128), and by Cope rearrangement of dienynols (137). 

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