Hydrogenation mechanisms routes

Figure 9.5 The mechanism of homogeneous hydrogenation unsaturate route versus hydride route .

Next, we examine the two reactions to determine whether both are expected to give a good yield of the target compound. Because route A combines a strongly basic nucleophile and a secondary alkyl halide, we expect the major product to result from elimination by the E2 mechanism. Route B, on the other hand, employs a primary alkyl halide that cannot give elimination (it has no hydrogen on the /3-carbon) and that is an excellent substrate for an SN2 substitution because it is benzylic. Route B is the obvious choice. 

Complex formation with substrate (S) can proceed directly, by route A, to yield a relaxed a-cyclodextrin with all six 0(2) -0(3 ) hydrogen bonds engaged (as in the a-cyclodextrin methanol complex, Fig. 18.8), or the macrocycle can first open up to a relaxed form, route B, with the enclosed water molecules disordered over several sites so as to fill, statistically, the 5 A diameter a-cydodextrin cavity (as observed in the a-cyclodextrin 7.57H20 crystal struc- ture, Fig. 18.6 b). The water is now in an activated form and can be replaced directly by the j substrate. In a third possible mechanism, route C, the substrate aggregates first at the periphery of tense a-cyclodextrin, and in a second step replaces the two enclosed water molecules. 

Scheme 14 Possible mechanism of A -alkylation reactions via the direct hydrogen transfer route... 

As in the alcohol activation step, like the TM-catalyzed direct hydrogen transfer route (Schemes 5, 14), the MPV-O process cannot provide carbonyl compounds directly according to the reaction mechanism (Scheme 39). This may make the alcohol activation reaction the rate-limiting step of the whole process. Most probably for this reason, the early TM-free iV-alkylation reactions [4—9] were performed under harsh conditions to facilitate the initial formation of aldehydes by high temperature dehydrogenation of the alcohols. Otherwise, alternative ways for alcohol activation should be adopted (vide infra). 

Structure and Mechanism of Formation. Thermal dimerization of unsaturated fatty acids has been explaiaed both by a Diels-Alder mechanism and by a free-radical route involving hydrogen transfer. The Diels-Alder reaction appears to apply to starting materials high ia linoleic acid content satisfactorily, but oleic acid oligomerization seems better rationalized by a free-radical reaction (8—10). 

RhCl(PPhi)i as a homogenous hydrogenation catalyst [44, 45, 52]. The mechanism of this reaction has been the source of controversy for many years. One interpretation of the catalytic cycle is shown in Figure 2.15 this concentrates on a route where hydride coordination occurs first, rather than alkene coordination, and in which dimeric species are unimportant. (Recent NMR study indicates the presence of binuclear dihydrides in low amount in the catalyst system [47].)... 

The evidence is that the thermolytic route does not involve radicals but the photochemical one does. A dissociative mechanism for the thermolytic route is indicated by its inhibition by added phosphine it is likely that once a phosphine group has dissociated, a metal-hydrogen bond is formed, with generation of a coordinated alkene (Figure 3.58). 

Under the present reaction conditions, we observed the formation of succinic anhydride almost simultaneously together with the formation of GBL. The hydrogaiation of maleic anhydride yields succinic anhydride, and the subsequent hydrogenation of succinic anhydride produces GBL. The rate of hydrogenation of maleic anhydride to succinic anhydride was very fast compare to that of succinic anhydride to GBL. When the reaction was CEuried out wifliout solvent, tetrahydrofiiran was not producal. The above results indicate that the Pd-Mo-Ni/SiOz catalyst under our experimental conditions played an important role for the selective formation of GBL. Therefore, it is inferred that the catalyst composition may influence the route by which tetrahydrofiiran was formed, probably due to the different absorption mechanism of maleic anhydride, succinic anhydride, and GBL. 

Recently, Moskaleva et al. have proposed a new mechanism based on electronic structure calculations." Earlier experimental studies by Kasdan et al. determined that methyne (HC) has a doublet ground state and with a doublet-quartet energy splitting (AEdq) of 71.5 + O.SkJ/mol." Moskaleva et al. noted that the initially proposed mechanism (for HCN and N(" S) atom formation) is therefore spin-forbidden, and they also proposed a more favorable and spin-allowed reaction on the doublet surface. This new route on the doublet energy surface proceeds through the formation of an NCN intermediate, with concomitant formation of (doublet) hydrogen atom. 

Addition reaction of peroxide-generated macroalkyl radicals with the reactive unsaturation in MA is shown in reaction scheme 4. The functionalised maleic-polymer adduct (II, scheme 4) is the product of hydrogen abstraction reaction of the adduct radical (I, scheme 4) with another PP chain. Concomitantly, a new macroalkyl radical is regenerated which feeds back into the cycle. The frequency of this feedback determines the efficiency of the cyclical mechanism, hence the degree of binding. Cross-linking reaction of I occurs by route c ( scheme 4). 

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