Electron transfer-catalyzed substitution

A better approach to substitution at (1) is the use of catalytic routes. Induction with Me3NO has proved successful for (1) in some cases, but of particular value for a clean reaction with (1) was electron-transfer catalyzed substitution using Ph2CONa in low concentration as a reductive activator. The likely intermediate is the 49-electron species [Ru3(CO)i2], which quickly reacts to form [Ru3(CO)iiL]". Electron-transfer occurs again to form Ru3(CO)iiL and to regenerate [Ru3(CO)i2] to repeat the cycle. The same type of reaction cycle can be induced with [PPN]X, where X = acetate, cyanide, or halides. The [PPN][Ru3(CO)i2-nX] (n = 1, 2, 3) species is the intermediate, and X is displaced by the appropriate ligand to obtain the substitution product. 

Note that the overall reaction for this scheme is simply A B, with no net transfer of electrons. Thus, at a suitable potential, the electrode accelerates a reaction that presumably would proceed slowly without the electrode. An interesting extension of this mechanism is the electron-transfer-catalyzed substitution reaction (equivalent to the organic chemist s SrnI mechanism) (7, 14) ... 

Table 2 illustrates and scheme (13) explains the very variable rates of (electron-transfer catalyzed) substitution by TCNE in seemingly related carbonylmetal compounds, the manganese systems reacting faster by a factor of at least 10 than the pentacarbonyltungsten complex. 

Electron Transfer-induced and Electron Transfer-catalyzed Ligand Substitution... 

The cluster does not undergo electron transfer-catalyzed CO substitution reactions because the lifetimes of the intermediate radical anions are too short (see below). 

The real utility of this electron-transfer catalyzed reaction lies in the enormous enhancement of reactivity toward substitution in the anion radicals. For example, thermal substitution of 51 (R = CF3) by phosphines gives 10-60% product yields after several hours in boiling benzene. By contrast, the monosubstituted complexes 52 (n = 1) were produced in greater than 90% yield, in less than 1 minute at room temperature, by adding a catalytic amount of the reductant [Ph2CO] to a 1 1 mixture of 51 and L (727). 

Transition to the final product, the cyclobutane LV, can be greatly facilitated by the use of electron-donating substituents R. In this case, the electron transfer from substituted ethylene to the orbital of the cation-radical LIV occupied by one electron switches on the chain mechanism represented on the above scheme, similar to that for the reactions of the SrnI type (Sect. 9.1). Such mechanisms are a characteristic feature for the class of reactions, catalyzed by electron transfer, in which the sequence 1) ionization (electron capture), 2) reaction in the ion-radical, 3) electron capture (ionization) - is a very convenient route for effecting the needed transformation [96]. 

Electron-transfer-catalyzed (ETC) nucleophilic substitution in polynuclear metal carbonyls promises to become a useful synthetic procedure for a wide variety of compounds. Scheme 6 shows the general reaction with a nucleophile L and Scheme 7 gives some mechanistic details (nonessential ligands omitted). For the ETC process to be efficient the radical-anion formation must be reversible, and the rate of electron transfer must be fast... 

It has been well known since the pioneering work of Bunnett59 that some nucleophilic aromatic substitutions can be catalyzed by single electron transfer. Electrochemistry was shown60,61 to be an efficient technique both for inducing reactions and for determining mechanisms and thermodynamic data concerning equilibria in the overall process. 

Iron(III)-catalyzed autoxidation of ascorbic acid has received considerably less attention than the comparable reactions with copper species. Anaerobic studies confirmed that Fe(III) can easily oxidize ascorbic acid to dehydroascorbic acid. Xu and Jordan reported two-stage kinetics for this system in the presence of an excess of the metal ion, and suggested the fast formation of iron(III) ascorbate complexes which undergo reversible electron transfer steps (21). However, Bansch and coworkers did not find spectral evidence for the formation of ascorbate complexes in excess ascorbic acid (22). On the basis of a combined pH, temperature and pressure dependence study these authors confirmed that the oxidation by Fe(H20)g+ proceeds via an outer-sphere mechanism, while the reaction with Fe(H20)50H2+ is substitution-controlled and follows an inner-sphere electron transfer path. To some extent, these results may contradict with the model proposed by Taqui Khan and Martell (6), because the oxidation by the metal ion may take place before the ternary oxygen complex is actually formed in Eq. (17). 

Many synthetically important substitutions of aromatic compounds are effected by nucleophilic reagents. There are several general mechanisms for substitution by nucleophiles. Unlike nucleophilic substitution at saturated carbon, aromatic nucleophilic substitution does not occur by a single-step mechanism. The broad mechanistic classes that can be recognized include addition-elimination, elimination-addition, metal-catalyzed, and radical or electron-transfer processes. (See Sections 10.5, 10.6, 12.8, and 12.9, Part A to review these mechanisms.)... 

Nevertheless, there are two highly efficient CL systems which are believed to involve the CIEEL mechanism in the chemiexcitation step, i.e. the peroxyoxalate reaction and the electron transfer initiated decomposition of properly substituted 1,2-dioxetanes (Table 1)17,26 We have recently confirmed the high quantum yields of the peroxyoxalate system and obtained experimental evidence for the validity of the CIEEL hypothesis as the excitation mechanism in this reaction. The catalyzed decomposition of protected phenoxyl-substituted 1,2-dioxetanes is believed to be initiated by an intramolecular electron transfer, analogously to the intermolecular CIEEL mechanism. Therefore, these two highly efficient systems demonstrate the feasibility of efficient excited-state formation by subsequent electron transfer, chemical transformation (cleavage) and back-electron transfer steps, as proposed in the CIEEL hypothesis. 

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