Alkynes, chemical reduction

As was discussed in Sections 11.1.5 and 11.2.3, the stereoselective partial hydrogenation of alkynes to either cis or trans alkenes is of key importance. Chemical reductions can also be applied to achieve both selective transformations. 

Besides heterogeneous and homogeneous catalytic hydrogenations, chemical reductions can also transform alkynes to cis alkenes. Interestingly, activated zinc in the presence of a proton donor (alcohol), although a dissolving-metal reagent, reduces disubstituted alkynes to cis alkenes 199... 

The molybdenum dimer 547 can be converted, by alkylation, to a cation (548), which can be transformed into an anion (549) by two-electron chemical reduction. Treatment of 547 with excess methyllithium leads, after work-up in air, to the bis (/z-methylthiolate) 550, which can also be prepared by addition of methyllithium to 548. The addition of vinylmag-nesium bromide to 548 yields 551. The anion reacts with alkynes, activated olefines, and an allene to form 552, 553, and 554, respectively (305). 

At ambient temperatures, the primary CV processes observed for Co2(CO)6 2 ifi-T] ijL-ri -RC2C2R) (R = Ph, Fc), which contain two chemically equivalent C02 (/z-alkyne)(CO)6 redox centers, are an apparent irreversible 2-e reduction, with an... 

Bisalkyne bisdithiocarbamate derivatives are both harder to reduce and harder to oxidize than their carbonyl analogs (53). Factors discussed in the Section VI and used to rationalize visible absorption spectra are also applicable here. The potentials required for oxidation and reduction reflect the strength of the alkyne-metal dn interactions. Their properties, as well as chemical behavior, no doubt reflect the complementary nature of the complete set of a and 77 metal-ligand bonds in these happy d4 M(RC=CR)2(S2CNEt2)2 complexes. 

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). 

For the electrochemical oxidation and reduction of alkynes and alkenes an analogy may be drawn with their relative reactivities towards electrophilic and nucleophilic attack. Alkynes are the more easily attacked by nucleophiles and are slightly easier to reduce. Alkynes are, however, much less prone to electrophilic attack than alkenes and are correspondingly more difficult to oxidize electro-chemically. 

Chemical, as opposed to cathodic, delivery of electrons to the triple bond has been accomplished in various ways. The conventional fl/7//-selective reductions of alkynes by dissolved metals is still important--", but new reagents and solvents have widened the scope of nucleophilic reductions (Table 9). Indeed, an understanding of the mechanistic options has made for greater flexibility different initiation and entry/ departure of participants is now possible. 

The reduction of intermediately formed, isolable unsaturated nitriles opens up an easy route to regio- and stereo-chemically pure unsaturated aldehydes. The overall reaction is a regio- and stereo-selective hydroformylation of alkynes [61]. 

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