Carbon dioxide reductive coupling with alkynes

Carbon dioxide instead of aldehydes can be involved in Ni(0)-promoted reductive coupling reactions (Equations (76) and (77) Scheme 90).434,434a 434c A stoichiometric amount of Ni(COD)2/DBU reacts with C02 and dienes, alkynes, or allenes to afford a metallacycle intermediate. This metallacycle reacts with organozinc compounds or aldehydes in one-pot to give carboxylic acid derivatives. As shown in Scheme 90, double carboxylation occurs in the presence of dimethylzinc, where the stereochemical outcome is opposite to that of the reaction with diphenylzinc. 

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). 

Cohen SA, Bercaw JE (1985) Titanacycles derived from reductive coupling of nitriles, alkynes, acetaldehyde, and carbon dioxide with bis(pentamethylcyclopentadienyl)(ethyl-ene)-titanium(II). Organometallics 4 1006-1014... 

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