Redox Catalyzed Substitution

Figure 3.6 Redox-catalyzed substitution in transition metal chemistry.

Key reactions include (a) substitution (b) electron transfer (c) activation of ligands (d) redox-catalyzed substitution (e) oxidative-addition reaction (f) insertion (g) redox-catalyzed insertion. 

Os(NH3)6] condenses with acetone in a redox-catalyzed coordinated ligand reaction, impelled by the Os(II)-imine intermediate being strongly stabilized by TT-bonding. Indeed one of the most distinctive features of the chemistry of osmium(II) is its unusually high tendency to 7r-bond. This has a large effect on reactivity, manifested in the various substitution and tautomerization reactions of [Os(NH3)5L] complexes discussed in this section. Kinetic parameters for reactions of the L = 17 -arene series have been conveniently assembled. ... 

The same result can be achieved by adding a salt of chromium(II) rather than by forming it in situ. Table VII gives some similar redox-catalyzed substitutions. Note that, as in Table VII, whereas the first-row metal ions... 

In basic aqueous media, a kinetic study of the reaction between stannate(II) ions and alkyl halide shows that mono- and disubstituted organotin compounds are formed (Eq. 6.12a).27 The monosubstituted organotin compound is obtained after a nucleophilic substitution catalyzed by a complexation between the tin(II) and the halide atom. The disubstituted compound results from an electrophilic substitution coupled with a redox reaction on a complex between the monosubstituted organotin compound and the stannate(II) ion. Stannate(IV) ions prevent the synthesis of the disubstituted compound by complexation. Similarly, when allyl bromide and tin were stirred in D2O at 60° C, allyltin(II) bromide was formed first. This was followed by further reaction with another molecule of allyl bromide to give diallyltin(IV) dibromide (Eq. 6.12b).28... 

The low specificity of electron-donating substrates is remarkable for laccases. These enzymes have high redox potential, making them able to oxidize a broad range of aromatic compounds (e.g. phenols, polyphenols, methoxy-substituted phenols, aromatic amines, benzenethiols) through the use of oxygen as electron acceptor. Other enzymatic reactions they catalyze include decarboxylations and demethylations [66]. 

Since the oxidative polymerization of phenols is the industrial process used to produce poly(phenyleneoxide)s (Scheme 4), the application of polymer catalysts may well be of interest. Furthermore, enzymic, oxidative polymerization of phenols is an important pathway in biosynthesis. For example, black pigment of animal kingdom "melanin" is the polymeric product of 2,6-dihydroxyindole which is the oxidative product of tyrosine, catalyzed by copper enzyme "tyrosinase". In plants "lignin" is the natural polymer of phenols, such as coniferyl alcohol 2 and sinapyl alcohol 3. Tyrosinase contains four Cu ions in cataly-tically active site which are considered to act cooperatively. These Cu ions are presumed to be surrounded by the non-polar apoprotein, and their reactivities in substitution and redox reactions are controlled by the environmental protein. 

The second-order redox reaction, giving rise to the rate constant k2, is accompanied also by loss of the iron(II) complex by hydrolysis, which leads to the /tj term. The latter can be more accurately measured in the absence of Tl(III). The kinetics of substitution of many square-planar complexes conform to behavior (c), see Sec. 4.6. It is important to note that an intercept might be accurately defined and conclusive only if low concentrations of B are used. In the base catalyzed conversion... 

Certain substitutions can be catalyzed by the operation of a redox process. It is most easily detected with inert Cr(III), Co(III) and Pt(IV). Hydrolysis, anation and anion interchange all have been accelerated In complexes of these metals by the presence of the lower oxidation state (which is more labile). 

The V(IV)-H20 exchange is catalyzed by V(V) (Prob. 5(a)). The enhanced reactivity for the base form is observed in dimerization, substitution and redox reactions (below). The mechanism of substitution of remains uncertain. One of the problems is to assess the contribution of the highly reactive VO(OH)+. Rate constants for complexing by VO + are all = lO M s, Ref. 35, consistent with an/ mechanism. By using chelating ligands to tie up... 

The reactions of aldehydes at 313 K [69] or 323 K [70] in CoAlPO-5 in the presence of oxygen results in formation of an oxidant capable of converting olefins to epoxides and ketones to lactones (Fig. 23). This reaction is a zeolite-catalyzed variant of metal [71-73] and non-metal-catalyzed oxidations [73,74], which utilize a sacrificial aldehyde. Jarboe and Beak [75] have suggested that these reactions proceed via the intermediacy of an acyl radical that is converted either to an acyl peroxy radical or peroxy acid which acts as the oxygen-transfer agent. Although the detailed intrazeolite mechanism has not been elucidated a similar type IIaRH reaction is likely to be operative in the interior of the redox catalysts. The catalytically active sites have been demonstrated to be framework-substituted Co° or Mn ions [70]. In addition, a sufficient pore size to allow access to these centers by the aldehyde is required for oxidation [70]. 

Minisci-type substitution is one of the most useful reactions for the synthesis of alkyl- and acyl-substituted heteroaromatics. The acyl radicals are formed by the redox decomposition from aldehyde and /-butyl hydroperoxide or by silver-catalyzed decarboxylation of a a-keto acid with persulfate. Synthesis of acylpyrazines 70 as ant pheromones are achieved by this methodology using trialkyl-substituted pyrazines 69 with the acyl radicals generated from aldehydes or a-keto acids (Equation 10) <1996J(P1)2345>. The latter radicals are highly effective for the acylation. Homolytic alkylation of 6-chloro-2-cyanopyrazine 71 is performed by silver-catalyzed decarboxylation of alkanoic acids to provide 5-alkyl-substituted pyrazines 72 (Scheme 18) <1996CCC1109>. 

Goals and five limitations in conjunction with the development of selective catalytic homogeneous oxidation systems are evaluated. Systems are presented that address several of the problems or goals. One involves oxidation of alkenes by hypochlorite catalyzed by oxidatively resistant d-electron-transition-metal-substituted (TMSP) complexes. A second involves oxidation of alkenes by H2O2 catalyzed by specific TMSP complexes, and a third addresses functionalization of redox active polyoxometalate complexes with organic groups. 

Platinum(IV) is kinetically inert, but substitution reactions are observed. Deceptively simple substitution reactions such as that in equation (554) do not proceed by a simple SN1 or 5 2 process. In almost all cases the reaction mechanism involves redox steps. The platinum(II)-catalyzed substitution of platinum(IV) is the common kind of redox reaction which leads to formal nucleophilic substitution of platinum(IV) complexes. In such cases substitution results from an atom-transfer redox reaction between the platinum(IV) complex and a five-coordinate adduct of the platinum(II) compound (Scheme 22). The platinum(II) complex can be added to the solution, or it may be present as an impurity, possibly being formed by a reductive elimination step. These reactions show characteristic third-order kinetics, first order each in the platinum(IV) complex, the entering ligand Y, and the platinum(II) complex. The pathway is catalytic in PtnL4, but a consequence of such a mechanism is the transfer of platinum between the catalyst and the substrate. 10 This premise has been verified using a 195Pt tracer.2011... 

Redox substitution reactions can be photoinitiated. Taube first proposed that the photo-catalyzed substitution of PtCll- occurs by an electron-transfer process (equation 560) to give a kinetically labile platinum(III) intermediate.2040 Further work on this system has shown that the exchange occurs with quantum yields up to 1000,2041-2043 and the intermediate has beer assigned a lifetime in the fis range.2044 Recently the binuclear platinum(III) complexes Pt2(P2OsH2)4Xr (X = Cl, Br, I) have been found to show similar behavior and both photoreduction and complementary redox reactions are again proposed to explain the substitution behavior.1500... 

Such regioselectivities are unique and suggest that redox pillared clays may have broad scope and utility as selective, heterogeneous catalysts for liquid phase oxidations. Indeed, V-PILC also catalyzes the oxidation of benzyl alcohol (to a mixture of benzoic acid and benzylbenzoate) whilst a-methyl benzylalcohol is left completely untouched.71 Similarly, p-substituted benzyl alcohols are oxidized whilst o-substituted benzyl alcohols are inert.71... 

Metalloenzymes or metal ion-activated enzymes catalyze an enormous variety of organic reactions that are not restricted to any particular reaction class, but appear as catalysts for all types of reactions. Thus neither the presence of the metal ion nor the reaction type seems to be restrictive as far as metal-assisted enzyme catalysis is concerned. In some cases the metal ion appears to function as an electron acceptor or donor, but flavin cofactors have substituted as redox centers during evolution in some enzymes. 

---