Acetylene binding

Clasalcally, acetylene binds to metal centers by using its K and ir orbitals to form, respectively, a-donor and jT-acceptor bonds to the metal. This situation can hold when the metal is in a sulfur coordination 64-65) environment such as In Mo0(S2CNR2)2(Rinteracts directly with the metal center a mode of binding that must be considered for nitrogenase substrates. 

The work of M. Rakowski DuBols and coworkers(66.67) points to a totally different mode of acetylene binding. For example, (Cp )2Mo2S4 reacts with acetylene to produce... 

Figure 3. Possible modes of acetylene binding to transition metal sulfur sites.

These generally bind to enzyme that produces them and destroy or inhibit it. For example, olefins and acetylenes bind to nitrogen atoms in the pyrrole part of cytochrome P-450. This may lead to the release of the Fe and destruction of the enzyme. The intermediate can be detected as bound to the enzyme protein or active site. 

Recent experiments by Wolkow43 and by Mezhenny and coworkers44 yielded another adsorption configuration an end-bridge structure with an acetylene molecule binding to two adjacent dimers and oriented perpendicular to the dimer rows (Figure 4a) a tetra-coordinated structure with the acetylene binding to the four Si-atoms of two adjacent... 

Lowe et al. (7) specified two groups of sites (rather than single sites) for ethylene or acetylene binding. 

The oxidation of terminal acetylenes, like that of monosubstituted olefins, often results in inactivation of the P450 enzyme involved in the oxidation. In some instances, this inactivation involves reaction of the ketene metabolite with nucleophilic residues on the protein [196, 197], but in other instances it involves alkylation of the prosthetic heme group (Fig. 4.31). Again, as found for heme alkylation in the oxidation of olefins, the terminal carbon of the acetylene binds to a pyrrole nitrogen of the heme and a hydroxyl is attached to the internal carbon of the triple bond. Of course, as one of the two m-bonds of the acetylene remains in the adduct, keto-enol equilibration yields a final adduct structure with a carbonyl on the original internal carbon of the triple bond [182, 198]. It is to be noted that the oxidation of terminal triple bonds that produces ketene metabohtes requires addition of the ferryl oxygen to the imsubstituted, terminal carbon, whereas the oxidation that results in heme alkylation requires its addition to the internal carbon. As a rale, the ratios of metabolite formation to heme alkylation are much smaller for terminal acetylenes than for olefins. 

A selective poison is one that binds to the catalyst surface in such a way that it blocks the catalytic sites for one kind of reaction but not those for another. Selective poisons are used to control the selectivity of a catalyst. For example, nickel catalysts supported on alumina are used for selective removal of acetjiene impurities in olefin streams (58). The catalyst is treated with a continuous feed stream containing sulfur to poison it to an exacdy controlled degree that does not affect the activity for conversion of acetylene to ethylene but does poison the activity for ethylene hydrogenation to ethane. Thus the acetylene is removed and the valuable olefin is not converted. 

However, milder conditions are necessary for obtaining the less stable 1-methyl-3-butadiynyl- and l-methyl-5-butadiynylpyrazole. The acetylenic alcohols were stirred with KOH at room temperature in acetylene atmosphere (to bind the isolated carbonyl compound). The product yields were 68% and 65% (69IZV2546), respectively. 

Tazarotene (Table 1) is an acetylenic third generation retinoid derivative. It is a poorly absorbed, nonisome-risable arotinoid, which is rapidly metabolized to its free carboxylic acid, tazarotenic acid, binding with high affinity to RARs, with the rank order of affinity being... 

The following is a comprehensive smwey of the chemistry of macrocycles comprised entirely of phenyl and acetylenic moieties. Although over fom" decades old, this area of research has come into its own just in the last few years. Widespread interest in the field has been spurred by recent discoveries utilizing these compoimds as ligands for organometallic chemistry, hosts for binding guest molecules, models of synthetic carbon allotropes, and precursors to fullerenes and other carbon-rich materials. This review will discuss the preparation of a tremendous variety of novel structm-es and detail the development of versatile synthetic methods for macro cycle construction. 

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