Proton-induced demetalation

Scheme 12. Adopted mechanism of catalysis. The rate constant kd describes the proton-induced demetalation of 1 (13,27) characterizes the buffer ion-induced demetalation of 1 as described above (27) / , and 2i refer to catalysticidal intra- or intermolecular inactivation (52). The catalysis is commonly run using very low concentrations of 1 and under these circumstances the / 2i-driven pathway can be negligible (52).

The rate of acid-induced demetalation depends only slightly on the nature of the head substituents X (Table I). In contrast, the tail-R groups dramatically affect k and, for the most part, k3, suggesting that tail amide O-atoms are sites of peripheral protonation. Thus, the acid tolerant Fem-TAML catalysts with tail electron-withdrawing groups should be more acid resistant and replacement of R = Me with R = F results in a remarkable stabilization. The rate constants (Table I) show that under weakly acidic conditions (pH 2-3), when the k pathway dominates over k3, fluorinated lk is 105-fold more H +-tolerant than la. 

The acid demetallation mechanism is simpler than the metallation mechanism.18 In general, the rate law is expressed as d[H4Por2+]/df = k[M(Por)][H+]" [n = 1-3 depending on the stability of M(Por)] where aggregation is negligible, and N protonation induces the metal dissociation. 

As far as the external action is concerned, several external stimuli can be conveniently used to achieve rotation of one ring into the other some success has been achieved by chemical means like addition of protons [60, 89, 90], demetalation of catenates [90, 91] or change of solvent [85], but by far the most convenient system for its ease of control is a redox action which can be either chemically, electro-chemically, or photochemically induced. The latter method, based on light-induced electron transfer processes, could take advantage of the development of laser technology and grant a complete control both in the space and in the time domain. 

Figure 11 Fast demetallation in acidic solution of [M"(dioxocyclamato(2 )] complexes (M = Cu, Ni). Two protons attack the very accessible and partially negative oxygen atoms of the carbonyl groups, which induces a drastic reduction of the coordinating tendencies of the adjacent nitrogen atoms and promotes metal extrusion.

A catalytic addition of acidic alcohols or phenols to hexafluoropropene is induced by the complex Pd(PPh3)4 [110]. Catalytic activity is increased in the presence of cocatalytic l,4-bis-(diphenylphosphino)butane (dppb) (Scheme 19). The authors propose a mechanism involving external protonation of a Pd(0)-coordinated olefin and reductive elimination to the ether product, but both steps appear improbable. There is literature precedence for insertion of tetrafluoroethylene into the Pt-O bond of (dppe)PtMe(OMe) to give (dppe)PtMe(CF2CF20Me), but proto-demetallation of the resulting complex has not been reported [111, 112]. 

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See also in sourсe #XX -- [ ]

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