Hydrogen bonding alkynes

The conditions for hydrogenation of alkynes are similar to those employed for alkenes In the presence of finely divided platinum palladium nickel or rhodium two molar equivalents of hydrogen add to the triple bond of an alkyne to yield an alkane... 

Hydrogenation of alkynes with internal triple bonds gives cis alkenes... 

Hydrogenation of alkynes to alkenes using the Lindlai catalyst is attractive because it sidesteps the regioselectivity and stereoselectivity issues that accompany the dehydration of alcohols and dehydrohalogenation of alkyl halides. In tenns of regioselectivity, the position of the double bond is never in doubt—it appears in the carbon chain at exactly the sane place where the triple bond was. In tenns of stereoselectivity, only the cis alkene forms. Recall that dehydration and dehydrohalogenation normally give a cis-trans mixture in which the cis isomer is the minor product. 

Concerning consecutive reactions, a typical example is the hydrogenation of alkynes through alkenes to alkanes. Alkenes are more reactive alkynes, however, are much more strongly adsorbed, particularly on some group VIII noble metal catalysts. This situation is illustrated in Fig. 2 for a platinum catalyst, which was taken from the studies by Bond and Wells (45, 46) on hydrogenation of acetylene. The figure shows the decrease of... 

In an effort to apply the cooperative principles of metalloenzyme reactivity, involving a combination of metal-ligand and hydrogen bonding, we have reported a ruthenium catalyst incorporating imidazolyl phosphine ligands that efficiently and selectively hydrates terminal alkynes (5). We subsequently found that application of pyridyl phosphines to the reaction resulted in a >10-fold rate enhancement and complete anti-Markovnikov selectivity, even in the... 

This study supports rate-determining H-OH bond breaking, which constrasts with previous reports that identified vinylidene isomerization as the key step in catalytic alkyne activation. The results indicate an enzyme-like mechanism is operative involving cooperative substrate activation by a metal center and proximal hydrogen bond donor/acceptors. In the future we will apply these principles to the activation of additional species. 

Hydrogenation of alkynes in the presence of P-2 catalyst causes syn addition of hydrogen to take place, and the alkene that is formed from an alkyne with an internal triple bond has the (Z) or cis configuration. 

The hydrogenation of alkynes is a very interesting reaction, since the selectivity toward the partially or the fully reduced product allows the in-situ comparison of the ability of a catalyst to reduce C=C versus C=C bonds. This is perhaps the area in which duster catalysis has been most extensively developed, as recently reviewed by Cabeza [27], Adams and Captain [4], and Dyson [28]. A good number of metal clusters have been employed as catalyst precursors in alkyne hydrogenation, the majority of them containing ruthenium. 

The stereoselective hydrogenation of alkynes to alkenes can be effected by a wide variety of homogeneous catalysts. The appropriate choice of catalyst and reaction conditions allows the selective formation of either the (Z)- or the (l )-a1-kene. Most of the catalysts display a very high chemoselectivity, as they are not reactive towards reducible functional groups such as carbonyl, ester, and double bonds. Many of the details related to catalyst behavior and intricate mechanistic details concerning semihydrogenation of alkynes have often not been unraveled, and will remain a topic of research for the coming years. 

There are not many drugs that are alkynes however, one good example is ethinyl estradiol (Fig. 4.6). Even though ethinyl estradiol is not an aryl alkyne, the acetylenic group is attached to a tertiary carbon and not adjacent to an a-carbon-hydrogen bond. 

The hydrostannation of alkenes and alkynes can be catalyzed by a number of transition metals (Ni, Pd, Pt, Mo, etc.), but most studies have involved palladium.106 The mechanism that is most commonly accepted is shown for an alkyne in Scheme 1 the model for an alkene is similar. This mechanism accounts for the observations that the reactions normally involve cir-addition, that the R3Sn group enters so as to avoid steric hindrance by the largest substituent group on the alkene or alkyne, and that the regioselectivity depends on the polarization of the palladium-hydrogen bond in the sense Pd -H15-. 

The expansions, Eqs. (5.36) and (5.37), of IT(p) can be used to reduce dimensionality by focusing on the radial coefficients [157]. Thakkar and coworkers [157] formulated empirical mles to help understand the smaU-p behavior of II( p) for linear molecules using only the first four n p) terms. This technique was subsequently used to analyze bonding in 14-electron diatomics [337], strong hydrogen bonding [338], substiment effects in alkynes and cyanides [339], and bonding in alkaline-earth oxides [340,341]. 

Recently, a new class of supramolecular CpRu-containing catalysts for hydration of alkynes has emerged. These catalysts are based on the supramolecular self-assembly of monodentate ligands through hydrogen bonding association, as shown in Scheme 10.9 [41-43]. The remarkable activity of catalytic systems such as 15-17... 

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