Molecular hydrogen, activation routes

Although the isolation and reactivity of acyl complexes strongly support the hydride mechanism, the other mechanism cannot be excluded. For example H20, the acid or molecular hydrogen, which can act as a hydride source, can promote the Pd - C splitting of the Pd-alkylcarboalkoxy intermediate in the alkoxy cycle as well. More convincing for the hydride route is the fact that the acid, which does not promote the formation of a Pd-OCH3+ species, has a promoting effect on the catalysis and can activate a Pd(0) complex, otherwise inactive, whilst a base, which not only promotes the formation of this species, but also deprotonates a Pd - H+ species to Pd(0), suppresses the catalysis. 

There seems no reason why any of the mechanisms discussed in Sections 3.4-3.6 cannot function in the conversion of alkynes to alkenes. The alkene route of hydrogenation is frequently encountered because alkynes complex more strongly to transition metals than alkenes and their complexes are formed preferentially in competition with the oxidative addition of dihydrogen. Internal alkynes coordinate to bis(arylimino)acenaphthene complexes of palladium and the fricoordinate species activate molecular hydrogen. Transfer of both atoms of hydrogen forms... 

At this time, the proposal of additional access channels is quite conjectural. It seems likely that there is a channel or access route to the proximal side of the heme in order to provide access for the hydrogen peroxide or water needed for heme oxidation and His-Tyr bond formation. Furthermore, the electron density of compoimd I from PMC (97) reveals the presence of an anionic species that is not present in the native enz5une. However, the rapid influx-efflux rates up to 10 per sec needed for such a species to be a component of compoimd I would pose interesting constraints on a channel, and there does not seem to be a likely candidate in the region. Similarly, the potential channel leading to the cavity at the molecular center is not an ideal candidate for substrate or product movement because of its relationship to the active site residues. However, if the lateral channel is truly blocked by NADPH in small-subunit enzymes, this route may provide an alternative access or exhaust route. Both of these latter two channels require further investigation before a clear role can be ascribed to them. 

The most common pathway for catalysis of autoxidations by transition metal complexes involves the decomposition of alkyl hydroperoxides. Another route that may be possible for chain initiation involves direct oxygen activation, whereby the complexation of molecular oxygen by a transition metal would lower the energy of activation for direct reaction with the substrate [reaction (9)]. For example, oxygen coordinated to a metal might be expected to possess properties similar to alkylperoxy radicals and undergo hydrogen transfer with a hydrocarbon ... 

There are, however, examples indicating that in ion molecule reactions between a protonated species (AH+) and benzene (B), two isomeric forms of the intermediate complex may exist (AH+)(B) and (A)(BH+) [74,286]. In the cases of water [287] and propene [74], quantum chemical calculations clearly indicate that the former corresponds to a n complex where A-H acts as a hydrogen bond donor towards the centre of the benzene ring, while the latter is a hydrogen bonded complex between the benzenium ion and A. In neither case has a barrier been located, but is probably rather low in both cases. The role of the n complex has still not been clarified, since direct downhill routes from the reactants to the a complex exist. It has been pointed out that n complex formation between a pro electrophile and the substrate may be important in solution and in biological systems for molecular recognition purposes. In such cases the proelectrophile is activated to form the actual electrophile subsequent to n complexation, thereupon giving rise to the a complex. This has been shown by quantum chemistry to provide a reasonable scenario for the reaction between HF and benzene, in which BF3 is ultimately required to promote ion formation of the HF/benzene tt complex [288]. 

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