Bonding proton-assisted

The catalytic cycle in Fig. 18.20 also rationalizes the potential-dependent av of series 2 catalysts (Fig. 18.19). The primary partially reduced oxygen species was determined to be superoxide, 02, by using 02 scavengers incorporated in catalytic films. Superoxide is produced by autoxidation, i.e., heterolysis of the Fe-O bond in the ferric-superoxo intermediate [Shikama, 1998], probably induced by protonation of the terminal O atom in bound O2. The hypothesis of protonation-assisted autoxidation was supported by the observation that av at the rising part of catalytic curves was smaller in acidic media (more superoxide was produced), whereas no partially reduced oxygen species were detected at any potentials in basic (pH > 8) electrolytes. The autoxidation rate constant at pH 7 was estimated to be 0.03 s (for the Fe-only forms of series 2 catalysts) and <0.01 s for the FeCu forms. 

The acid-catalyzed aquation of iron(III)-(substituted)oxinate complexes involves iron oxygen bond breaking and concomitant proton transfer in transition state formation. The latter aspect contrasts with the much slower acid-catalyzed aquation of hydroxamates, where proton transfer seems not to take place in the transition state. Reactivities, with and without proton assistance, for various stages in dissociation of a selection of bidentate and hexadentate hydroxamates, oxinates, and salicylates are compared and discussed—the overall theme is of dissociative activation. ... 

The pKa for dissociation of its proton is 12.3 and the hydrogen-bonded proton is probably located in the center of the bond with both amino groups sharing the charge.160 Enols can also form unusually strong "resonance-assisted" hydrogen bonds ... 

This process can be contrasted directly with the oxo transfer scheme (Reaction 16) discussed above. In either case, the cleavage of the N-O bond is assisted by the binding of oxygen to an electrophile (to molybdenum itself in the oxo transfer mechanism or to proton(s) in the coupled proton-electron transfer scheme). Although the coupled proton-electron transfer mechanism would possibly have the advantage of leaving an open site on molybdenum to restart the cycle, there is no strong data to support either of these mechanisms at present. 

Figure 13 Reaction scheme for water sind proton assisted 0-0 bond cleavage in cytochrome oxidase.

Scheme 14 Proposed catalytic cycle for hydroamination using zirconium precatalyst 10 in which the key step is a proton-assisted C—N bond formation...

The nitrosylation of [Fe (CN)5(N02)] led to a particularly interesting result a notoriously fast conversion to nitroprusside was observed in the stopped-flow time scale. As E° for the [Fe" (CN)5(N02)] couple is also 0.4 V (83), we can still anticipate similar rates for the encounter-complex formation and the electron transfer reaction steps (analogs of 4-5). However, fe Kio2- cannot be high enough to account for the fast conversion to final products (its rate constant should be comparable to fe py, s ). Instead, the final step might involve a fast proton-assisted N02 /N0 interconversion (cf Section 2.2.1), which would yield the product without rupture of the initial Fe —N02 bond ... 

Recently, o-alkynyl biaryls have been shown to undergo intramolecular 5-exo-dig hydroarylation by a mechanism that proceeds by a C—H bond activation assisted by the alkynyl substituent [41]. This reaction also proceeds with a palladium complex bearing a dibentate phosphine (l,l -bis(diisopropylphosphinoferrocene)) and shows a sigruficant kinetic isotope effect (ku/ko = 3.5 for the intramolecular process), which is consistent with a mechanism involving a proton-abstraction mechanism. 

Interestingly, the Schafer group [34] has established that group 4 complexes can indeed be used for the intermolecular hydroamination of alkynes with secondary amines consistent with a shift in mechanism from the established [2+2] cycloaddition pathway. Specifically, a Zr ureate catalyst 5 has been developed to realize this shift in mechanism. For example, entry 20 shows that morpholine can be used at elevated temperatures to give enamine products that can be characterized in situ. Using the ureate catalyst 5, mechanistic analyses [35] and subsequent computational investigations [36] support a proton-assisted C-N bond formation (see later discussion). 

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