Migratory step

The insertion of CO into metal-carbon a bonds has been reviewed.585-590 Carbonylation of alkyl platinum(II) complexes usually requires elevated temperatures, although at higher temperatures the reaction is reversible (equation 211).591 With PtMe2(dppe) insertion occurs into only one of the Pt—Me bonds. For complexes PtX(Ar)L2, carbonylation follows pseudo first-order kinetics. Rates are decreased by addition of L to a maximum value where the carbonylation rate is independent of L. The pathway involves formation of a five-coordinate intermediate PtX(Ar)(CO)L2, followed by dissociation to form PtX(Ar)(CO)L. The migratory step to yield PtX(COAr)L is unaffected by added L. This pathway is outlined in Scheme 6.502... 

For the more general case, a coordinatively unsaturated intermediate of type A resulting from migration of the hydrocarbyl group may be stabilized in a number of ways . It has been pointed out that for CO insertion into the methyl-molybdenum bond of Me—Mo( -C5H5)(CO)3 [reaction (b)], which has been studied in tetrahydrofuran (THF) and in methyl-substituted THFs, the rate constant of the migratory step /ci significantly increases with the donicity of the solvent (the methyl-substituted THFs differ in donicity but have similar dielectric constants). 

The kinetics are reminiscent of dissociative substitution (Section 4.4) except that the 2e site is formed at the metal in the migratory step, not by loss of a hgand. Using the usual steady-state method, the rate is given by Eq. 7.8. 

With unsymmetrical cyclic ketones, however, mixtures due to similar migratory aptitudes of the different groups are obtained (Table 13, Nos. 2, 3). The rearrangement has also been used as key step in a d,/-muscone synthesis (Table 13, No. 4). 

A catalyst used for the u-regioselective hydroformylation of internal olefins has to combine a set of properties, which include high olefin isomerization activity, see reaction b in Scheme 1 outlined for 4-octene. Thus the olefin migratory insertion step into the rhodium hydride bond must be highly reversible, a feature which is undesired in the hydroformylation of 1-alkenes. Additionally, p-hydride elimination should be favoured over migratory insertion of carbon monoxide of the secondary alkyl rhodium, otherwise Ao-aldehydes are formed (reactions a, c). Then, the fast regioselective terminal hydroformylation of the 1-olefin present in a low equilibrium concentration only, will lead to enhanced formation of n-aldehyde (reaction d) as result of a dynamic kinetic control. 

In the absence of an excessive driving force derived, as in the case of CO, from the oxophilicity of the metal, the reaction with Bu NC allowed one to better single out almost all of the steps of the migratory insertion of the isocyanide into the Zr-C bonds, as shown in Scheme 19.47... 

According to the general migratory-insertion mechanism proposed by Cossee,58 chain propagation (4.105) is a two-step process in which the precursor metal reagent (I) forms an intermediate alkene complex (II) that subsequently rearranges to the insertion product (III),... 

Chain Migratory Insertion Mechanism. For a given catalytic model, the stereoselectivity of each insertion step does not assure its stereospecificity (i.e., to lead to a stereoregular polymer). In fact, the possible presence as well as the kind of stereospecificity depends on possible differences between stereostructures of transition states of two successive insertion steps. 

In the framework of the chain migratory insertion mechanism (Scheme 1.3), the stereospecific behavior of the model sites depends on the relationship between the two situations obtained by exchanging, in the coordination step, the relative positions of the growing chain and the incoming monomer. Depending on the local symmetry of the coordinated bridged n-ligand, these two situations can be as follows ... 

Extensive computational calculations have been performed by using molecular mechanics (MM) [79], quantum mechanics (QM) [80], or combined MM/QM methods [81]. As major contributions, these theoretical studies predict the greater stability of the major isomer, explain the higher reactivity of the minor diastereomer, introduce the formation of a dihydrogen adduct as intermediate in the oxidative addition of H2 to the catalyst-substrate complexes, and propose the migratory insertion, instead of the oxidative addition, as a turnover-limiting step. 

One pervasive mechanistic feature of many of the hydrogenations described in other chapters of this handbook concerns the bonding of the unsaturated substrate to a metal center. As illustrated in generalized form in Eq. (1) for the hydrogenation of a ketone, a key step in the traditional mechanism of hydrogenation is migratory insertion of the bound substrate into a metal hydride bond (M-H). 

Fig. 31.14 The present state of knowledge on rhodium-complex-catalyzed enantioselective hydrogenation. The H2 addition stage in the main cycle is predicted to be two-step in DFT calculations whether that or migratory insertion is turnover-limiting remains debatable.

---