Alkyne substrate catalysts

The kinetic investigation of this reaction reveals the reaction is first-order in substrate, catalyst and hydrogen concentration, and thus yields the rate law r=kCat[Os][alkyne][H2]. The proposed mechanism as given in Scheme 14.6 is based on the rate law and the coordination chemistry observed with these osmium complexes. 

In a subsequent study, they used ethylene for a dual purpose, as a substrate as well as a supercritical fluid solvent. This notoriously unreactive olefin to PKR served nicely to give 2-substituted cyclopentenones. Reaction efficiency of each alkyne substrate can be tuned by changing catalyst precursors. Not only Co2(CO)8 but also the two cobalt clusters [Co4(CO)i2] and [Co4(GO)n P(OPh)3 ] work well for some substrates (Equation (8)). The comparison with Rautenstrauch s result clearly shows the beneficial effect of this approach. 

The application of immobilized heterobimetallic cobalt-rhodium in nanoparticles has also been reported. In the presence of water, CO, and amine, internal acetylenes 119 were converted to 3,4-disubstituted furan-2(5H)-ones 120 and 121 in high yields, in which an amine was necessary for the formation of furanone and a higher CO pressure was required for good yield (Equation (8)). It is important to notice that the catalyst has been easily recovered without loss of activity or formation of hydrogenated side-products. The reaction proceeded in good yield for the symmetric substrates (entries 1 and 2) while it always gave two regioisomers for asymmetric alkyne substrates (entries 3-8). The isomer ratio was dependent on the steric and electronic nature of the substituents. 

After compiling many results obtained in similar studies of different substrates (alkenes, dienes, alkynes and so on), the results cannot be correlated to draw definitive conclusions due to the wide variety of parameters that can influence the reaction (substrates, catalyst precursors, supports, pressure, temperature and so on) [9, 208-214]. This is maybe the main reason why there are no clear mechanistic explanations for this simple reaction, unlike homogeneous gold-catalyzed processes. 

Low-loaded, organophilic Pd-montmorillonites were shown to exhibit high cis-selectivity in the hydrogenation of 1-phenyl-1-alkynes working with high (=5000) substrate catalyst ratio.402 Studies with respect to the use palladium membrane catalysts403 105 and polymeric hollow fiber reactors406"408 were reported. 

Some catalysts suffer a different type of alkyne poisoning. Chlorotris(triphenylphosphine)rhodium(I) is an effective terminal alkyne polymerization catalyst. When this complex is used in the reduction of these alkynes, it gradually loses its activity because of the competing polymerization reaction. Even initially the rate of alkyne hydrogenation is much slower than that of the corresponding alkene because of the greater binding constant of the former substrate. 

In recent years, attention has been focused on alkyne carbonylation catalysts based on the metals nickel, palladium, and platinum, modified with a variety of tertiary (bi)phosphines [5]. TTie main goal has been to develop chemo- and regio-selective carbonylation catalysts for application to higher alkyne substrates for the synthesis of certain fine chemicals. Many of these catalysts do allow the carbonylation to proceed under milder conditions than those applied in the catalytic Reppe process, and some of these catalysts do provide the branched regioisomer product from higher alkynes with good selectivity. However, in all cases reaction rates are very low, i.e., below 100 (and in most cases even below 10) mol/mol metal per h, as are the product yields in mol/mol metal (< 100). These catalyst productivities are far too low for large-scale industrial application in the production of commodity-type products, such as (meth)acrylates. 

Alkynes are readily hydrocyanated in the presence of a homogeneous catalyst, especially a nickel-based catalyst system. However, zerovalent palladium compounds are reported to catalyze the reaction as well, but are less efficient [60], The reaction gives an easy access to the synthetically valuable a,P-un-saturated nitriles. The use of acetone cyanohydrin as a synthetic equivalent for the difficult-to-handle HCN provides an efficient alternative, but the substrate/ catalyst ratio has to be increased in comparison with the reaction with HCN. The regioselectivity of the reaction is controlled by steric, electronic, and chelative effects. Investigations were predominantly performed by changing the substituent pattern on the acetylenic substrate [61]. 

The mechanism of the Sonogashira cross-coupling follows the expected oxidative addition-reductive elimination pathway. However, the structure of the catalytically active species and the precise role of the Cul catalyst is unknown. The reaction commences with the generation of a coordinatively unsaturated Pd species from a Pd " complex by reduction with the alkyne substrate or with an added phosphine ligand. The Pd " then undergoes oxidative addition with the aryl or vinyl halide followed by transmetallation by the copper(l)-acetylide. Reductive elimination affords the coupled product and the regeneration of the catalyst completes the catalytic cycle. 

Hydroamination reactions involving alkynes and enantiomerically pure chiral amines can produce novel chiral amine moieties after single pot reduction of the Schiffbase intermediate 82 (Scheme 11.27) [123]. Unfortunately, partial racemiza tion ofthe amine stereocenter was observed with many titanium based hydroamina tion catalysts, even in the absence of an alkyne substrate. No racemization was observed when the sterically hindered Cp 2TiMe2 or the constrained geometry catalyst Me2Si(C5Me4)(tBuN)Ti(NMe2)2 was used in the catalytic reaction. Also, the addition of pyridine suppressed the racemization mostly. 

Alkynes have been reported to participate in cocyclization with arynes under the same reaction conditions, affording a mixture of phenanthrenes 218, naphthalenes 219 and triphenylenes (Scheme 12.62) [115]. The chemoselectivity depends on both the palladium catalyst and the alkyne substrate used. For example, when dimethyl acetylenedicarboxylate (DMAD) is employed as the alkyne, a remarkable chemoselectivity is achieved by appropriate selection of the... 

Allene hydroamination is less commonly explored, even though the thermodynamic profile of the reaction is comparable to alkyne hydroamination [40]. Intermolecular allene hydroamination has been established using group 4 catalysts in combination with reactive arylamine substrates [8, 41]. The more reactive aforementioned alkyne hydroamination catalyst 7 has been shown to be usefiil for allene hydroamination catalysis in an intermolecular manner, even with less reactive, sterically less demanding alkylallene substrates. In this case, only the branched product is observed (Table 15.5). These results show good selectivity for the branched product, and recent results show that even heteroatom-substituted allenes can be tolerated with this precatalyst [42]. 

To explain the preceding results, the authors proposed the mechanism depicted in Scheme 4.15. First, both the enoate and alkyne substrates coordinate to the Ni(0) catalyst, which bears the chiral N-heterocyclic carbene ligand. Next, the enantioselectivity-determining step of the process is the oxidative cyclisation, giving metallocyclic intermediate E. Then, cyclisation and subsequent (3-alkoxide elimination releases the enone. Transmetallation with BEtj, followed by p-hydride elimination, gives a nickel hydride. Reductive elimination then closes the catalytic cycle. 

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