Outer Sphere Insertions

Of the possible erroneous insertions, double CO insertion is forbidden for the thermodynamic reasons discussed in Section 7.2, and double alkene insertion is rare because of its much slower intrinsic rate and the high affinity of the catalyst for CO, together amounting to a rate enhancement of 2000 versus CO insertion into M-R. 

In some cases, the A=B bond does not need to coordinate to the metal prior to insertion and can undergo the reaction with an 18e complex. The weakly binding ligand, CO2, can insert into an M-H bond in this way. The nucleophilic hydride first attacks the carbon of free CO2 to give a 16e M+ unit and free HCOO. The formate then binds to the metal to give the 1,2-insertion product, M-OCHO. 

Sulfur dioxide is a much stronger electrophile than CO2 and also needs no vacant site. If SO2 electrophilically attacks the a carbon of an 18e alkyl from the side opposite the metal, an alkyl sulfinate ion is formed with inversion at carbon. Since the anion has much of its negative charge on the oxygens, it is not surprising that the kinetic product of ion recombination is the O-bound sulfinato complex. On the other hand, the thermodynamic product is usually the S-bound sulfinate, as is appropriate for a soft metal binding. This sequence constitutes a 1,2 (O bound sulfinate) or a 1,1 insertion of SO2 (S bound). 

As expected for this mechanism, the reactivity falls off for bulky alkyls and electron attracting substituents. A crossover reaction of a mixture of RS and SR isomers of [CpFe (CO)L CH2C H(Me)Ph ], chiral at both Fe and the (3-carbon, forms very little of the crossover products, the R,R and S,S isomers of the sulfinate complex. This shows both that the intermediate must stay ion-paired, and that the intermediate iron cation must have stereochemical stability. Ion pairing is very common in organic solvents of relatively low polarity, such as CH2CI2, and ion pairs can have a well-defined solution structure, and such pairing can affect reaction outcomes. O2 can insert into M-H to give M-O-O-H in some cases, an H atom abstraction mechanism by O2 via M and O-O-H has been identified. Insertions of CO2 are discussed in Section 12.3. 

In 16e complexes, a 2e site is usually available for 3 elimination. For example, 16e (f tra 5-[PdL2Et2] complexes (L = PR3), can decompose by 3 elimination via an 18e transition state, but PR3 dissociation is still required for elimination in trans- il i2, where the preference for 16e over 18e structures is more marked than for Pd(II). The related metalacycle 7.9 3-eliminates 10 times more slowly than [PtL2Bu2], 

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As indicated under section 2.2. the overall result is the same as that of an insertion reaction, the difference being that insertion gives rise to a yw-addition and nucleophilic attack to an anri-addition. Sometimes the two reaction types are called inner sphere and outer sphere attack. There is ample proof for the anti fashion the organic fragment can be freed from the complex by treatment with protic acids and the organic product can be analysed [19], Appropriately substituted alkenes will show the syn or anti fashion of the addition. The addition reaction of this type is the key-step in the Wacker-type processes catalysed by palladium. 

The formation of ester via reaction (11) of Figure 12.10 deserves some further attention as it is not one of the elementary steps discussed in Chapter 2. One possible mechanism is the direct, outer-sphere attack of an alcohol or alkoxide at the acyl carbon atom, similar to the reaction of acid halides and alcohols (17-18 in Figure 12.13). This reaction is accessible for both cis and trans diphosphine complexes 12 and 13. Since monophosphines give mainly trans acyl complexes 13, not suited for insertion reactions, they were thought to have a preference for making esters or low molecular weight products. Trans complexes do form esters in reactions with alcohols or alkoxides, but this does not give direct information about the mechanism [42,43,44],... 

The insertion steps are probably preceded by deformation of H2P and/or outer sphere complexing. 35-137 jigse are incomplete they will introduce rapid preequilibria constants A, and K2 into the rate expression, and by reducing the... 

Electrochemical reductions of CO2 at a number of metal electrodes have been reported [12, 65, 66]. CO has been identified as the principal product for Ag and Au electrodes in aqueous bicarbonate solutions at current densities of 5.5 mA cm [67]. Different mechanisms for the formation of CO on metal electrodes have been proposed. It has been demonstrated for Au electrodes that the rate of CO production is proportional to the partial pressure of CO2. This is similar to the results observed for the formation of CO2 adducts of homogeneous catalysts discussed earlier. There are also a number of spectroscopic studies of CO2 bound to metal surfaces [68-70], and the formation of strongly bound CO from CO2 on Pt electrodes [71]. These results are consistent with the mechanism proposed for the reduction of CO2 to CO by homogeneous complexes described earlier and shown in Sch. 2. Alternative mechanistic pathways for the formation of CO on metal electrodes have proposed the formation of M—COOH species by (1) insertion of CO2 into M—H bonds on the surface or (2) by outer-sphere electron transfer to CO2 followed by protonation to form a COOH radical and then adsorption of the neutral radical [12]. Certainly, protonation of adsorbed CO2 by a proton on the surface or in solution would be reasonable. However, insertion of CO2 into a surface hydride would seem unlikely based on precedents in homogeneous catalysis. CO2 insertion into transition metal hydrides complexes invariably leads to formation of formate complexes in which C—H bonds rather than O—H bonds have been formed, as discussed in the next section. 

The previous section described a two-step mechanism. The first step is the coordination of the substrate into the metal coordination sphere. The second is the most characteristic step within the inner-sphere mechanism the insertion of the substrate into the M-H bond. Nevertheless there are other mechanistic options that include neither substrate coordination nor M—H insertion. They are outer-sphere mechanisms and in turn can be classified as bifunctional and ionic mechanisms. 

Thus, in hydrogen-transfer reactions, most of the catalysts do prefer the outer-sphere mechanism instead of the MPV or the insertion mechanisms. For instance, the high stability of the intermediate formed, alkoxide in the case of carbonyl hydrogenation, is a major drawback for the inner-sphere mechanism. Nevertheless, in some particular cases, the inner-sphere mechanism may be competitive with the outer-sphere one. In these cases, some requirements must be accomplished, such as the high lability of one of the metal ligands in order to allow easily the substrate coordination or the formation of not very stable intermediates. 

In the present chapter, a classification of the hydrogenation reaction mechanisms according to the necessity (or not) of the coordination of the substrate to the catalyst is presented. These mechanisms are mainly classified between inner-sphere and outer-sphere mechanisms. In turns, the inner-sphere mechanisms can be divided in insertion and Meerweein-Ponndorf-Verley (MPV) mechanisms. Most of the hydrogenation reactions are classified within the insertion mechanism. The outer-sphere mechanisms are divided in bifunctional and ionic mechanisms. Their common characteristic is that the hydrogenation takes place by the addition of H+ and H- counterparts. The main difference is that for the former the transfer takes place simultaneously, whereas for the latter the hydrogen transfer is stepwise. 

Until now, only a few theoretical studies of porphyrin metalation by divalent metal ions in solution have been reported (72,94,95). In the first theoretical work (94,96) on this topic, insertion of Fe2+ and Mg2+ into the porphyrin ring was studied by DFT methods. The authors followed the reaction from the outer-sphere complex formation via stepwise displacement of the solvent molecules until... 

The reduction of [(NH3)5Co02CH2/o N]2+ apparently has not been studied. The general experience with reduction of Co (III) by Cr2+ however is that when CH2 is inserted into a conjugated system, the reaction by remote attack is much less rapid than it is by the outer-sphere path, and this latter path is usually much slower than that involving remote attack, when this can occur.) Our limited capacity to predict the results of the experiments on intramolecular electron transfers is a strong motivation for continuing the work. 

The stereochemical outcome was in agreement with a mechanism for the palladium-catalyzed cyclization/carboalkoxylation of a substituted alkene (Scheme 47) that involves outer-sphere attack of the indole on the palladium-olefin complex I which, coupled with loss of HCI, would form the alkylpalladium intermediate II. 1,1-Migratory insertion of CO into the Pd-C bond of II with retention of stereochemistry would form the acyl-palladium complex III, which could undergo methanolysis to release c/.v-product and form a palladium(0) complex. Oxidation with Cu(II) would then regenerate the active Pd(II) catalyst. 

Fig. 3. Configuration of outer-sphere complex (MA, H2P) preceding the insertion of the metal. Space angle indicated about 4yt/50...

The mono- and diprotonated forms of the porphyrin are unreactive. The insertion steps are probably preceded by deformation of PH2 and/or outer sphere complex formation. Ligand dissociation and formation of the first metal-nitrogen bond then occurs... 

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