Metal—carbon triple bonds reduction reactions

Many anionic nucleophiles add to the carbyne carbon in cationic Fischer complexes, and these reactions can be used to synthesize carbene ligands unavailable by other routes. The stepwise reduction of a metal-carbon triple bond has been demonstrated (32) ... 

Reactions of carbyne complexes that maintain the integrity of the metal-carbon triple bond form the third route to new carbynes. Substituent modification, ligand exchange, oxidation, and reduction reactions have all been reported (see, e.g., Ref. 126). 

Reactions of carbon-carbon % bonds dissolving metal reductions, conversions to alcohols and enantiomerically pure alcohols, chemo- and enantioselective epoxidations, procedures for cleavage of carbon-carbon double bonds, and reactions of carbon-carbon triple bonds... 

Addition of 1 mol of hydrogen to the carbon-carbon triple bond can be accomplished stereospecifically. Catalytic reduction leads to the cis isomer. This is most often carried out using Lindlar catalyst, a lead-poisoned palladium-on-calcium carbonate preparation. Palladium on BaS04 is an alternative. Some examples are recorded in Scheme 3.10. Numerous other catalyst systems have been employed to effect the same reduction. Many specific cases are cited in reviews of catalytic hydrogenations. If the trans alkene is desired, the usual method is a dissolving-metal reduction in ammonia. This reaction is believed to involve two successive series of reduction by sodium and protonation ... 

Reduction of an alkyne occurs in two stages first, addition of one mole of H2 to form an al-kene and then addition of the second mole of H2 to the alkene to form the alkane. In most cases, it is not possible to stop the reaction at the alkene stage. However, by careful choice of catalyst, it is possible to stop the reaction at the addition of one mole of hydrogen. The catalyst most commonly used for this purpose consists of finely powdered palladium metal deposited on solid calcium carbonate that has been specially modified with lead salts. This combination is known as the Lindlar catalyst. Reduction (hydrogenation) of alkynes over a Lindlar catalyst is stereoselective syn addition of two hydrogen atoms to the carbon—carbon triple bond gives a cis alkene ... 

Reaction (2) is a catalytic reduction of the carbon-nitrogen triple bond to a 1 ° amine. Just as a carbon-carbon triple bond can be reduced to a carbon-carbon single bond by hydrogen in the presence of a transition metal catalyst, a carbon-nitrogen triple bond can be similarly reduced. 

The first point may be intimately connected with the notion that CO activation is achieved in this system by maximal reduction of the carbon-oxygen triple bond via interaction with more than one metal center, while the second is suggestive of the type of Lewis acid binding to carbonyl oxygens as seen by Shriver, Burlitch, and others (47-54). It is noteworthy that molten NaAlCl4, which lacks acidic character, is a relatively ineffective medium under the reaction conditions. 

Type 1. Consecutive reactions. The common feature of these examples (Scheme 2.68) is that the product formed in the first step is capable of reacting further under essentially the same reaction conditions. If the requirement for selectivity is to stop the process after the first step, a variety of approaches can be attempted. For example, in case (a) both consecutive steps belong to the same type of chemical process. Therefore to ensure the selective hydrogenation of the alkyne to the alkene, it is necessary to utilize a catalyst that permits the reduction of the triple bond but not the double bond. This requirement is met in Lindlar s catalyst, a palladium metal catalyst adsorbed on a carbonate that is partially deactivated with lead (Pd-CaC03-Pb0). 

The reduction of alkynes ° and electronegatively substituted alkenes by Cr in aqueous DMF was first reported by Castro and coworkers in 1964, who also carried out a detailed study of the mechanism of the reactions. It does not appear that reductions by other low-valent metal ions have been studied, or if so they are not capable of reducing carbon-carbon double and triple bonds. 

Trimethylsilyl groups at triple bonds are easily removed by mild bases such as alkali metal carbonates to yield 35 with a terminal triple bond. The following reaction is a Pd-catalyzed reductive addition of tributyl stannane, HSnBu3, to the triple bond, which forms the -configured vinyl stannane 19. One possible rationalization of the outcome is the mechanism shown below First, the catalytically active species 36 inserts into the tin-hydrogen bond to form 37. Then, cis addition takes place after coordination to the alkyne triple bond, generating species 39. Reductive elimination affords the vinyl stannane 19 and regenerates the catalyst 36. ... 

One of die most popular reactions in organic chemistry is dissolving metal reductions [1-3], Two systems are frequently used - sodium dissolved in ammonia with alcohol and lithium dissolved in alkylamines [4]. Although calcium is seldom used, it has been successfully applied to the reduction of a variety of compounds and functional groups [5], including aromatic hydrocarbons, carbon-carbon double and triple bonds, benzyl ethers, allyl ethers, epoxides, esters, aliphatic nitriles, dithianes, als well as thiophenyl and sulfonyl groups. 

You have already seen that hydrogen can be added to carbon-carbon double and triple bonds in the presence of a metal catalyst (Sections 4.11 and 6.8). These reactions, called catalytic hydrogenations, are reduction reactions because there are more C—H bonds in the products than in the reactants. AUcenes and alkynes are both reduced to alkanes. 

The question about the existence and the stability of pure carbon chain polymers has been repeatedly discussed in many papers. These polymers are called carbynes and are believed to consist of alternating triple and single bonds (polyynes) rather than of non alternating double bonds (polycumulenes) because of the lower ground state energy of the former. Thus, vibrational spectra of such materials are characterized by a more or less expressed structure around 2000 cm which is typical for the triple bond. A very efficient way to obtain such triple bonded carbon polymers nses an internal electrochemical reduction of fluorinated polymers such as poly(tetrafluoroethylene) (PTFE) or poly(tetrafluoroethylene/hexafluoropropylene) (FEP) by alkali metal amalgam according to the reaction... 

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