Dissociative substitutions stereochemistry

In five coordinate complexes, both associative and dissociative substitution pathways are possible. Five coordination leads to two limiting stereochemistries, trigonal bipyramidal (4.7) and square pyramidal (4.8) which normally have similar energies. Many square... 

Studies of the stereochemical course of rmcleophilic substitution reactions are a powerful tool for investigation of the mechanisms of these reactions. Bimolecular direct displacement reactions by the limSj.j2 meohanism are expected to result in 100% inversion of configuration. The stereochemical outcome of the lirnSj l ionization mechanism is less predictable because it depends on whether reaction occurs via one of the ion-pair intermediates or through a completely dissociated ion. Borderline mechanisms may also show variable stereochemistry, depending upon the lifetime of the intermediates and the extent of internal return. It is important to dissect the overall stereochemical outcome into the various steps of such reactions. 

How- does this reaction take place Although it appears superficially similar to the SN1 and S 2 nucleophilic substitution reactions of alkyl halides discussed in Chapter 11, it must be different because aryl halides are inert to both SN1 and Sj 2 conditions. S l reactions don t occur wdth aryl halides because dissociation of the halide is energetically unfavorable due to tire instability of the potential aryl cation product. S]sj2 reactions don t occur with aryl halides because the halo-substituted carbon of the aromatic ring is sterically shielded from backside approach. For a nucleophile to react with an aryl halide, it would have to approach directly through the aromatic ring and invert the stereochemistry of the aromatic ring carbon—a geometric impossibility. 

For nucleophilic substitution at tetrahedral carbon, the relationships between stereochemistry and mechanism are well understood. Direct displacement on a carbon turns the molecule inside out (inversion of configuration) if the site of reaction is an asymmetric carbon, a molecule in the D series will be converted to one in the L series (or vice versa). Substitution by the dissociation mechanism at an asymmetric carbon... 

The transmetallation step (iii) is certainly the most enigmatic part of the catalytic cycle. Generally, it is assumed to be rate limiting, and several mechanisms are proposed depending on the solvent. An open transition state with inversion of the stereochemistry would arise with polar solvents which are able to stabilize the transient partial charges , whereas a cyclic transition state with retention of the stereochemistry would arise in less polar solvents. It should be noted that the nature of the ligands on the palladium may influence dramatically the kinetics of the transmetallation step. A 1000-fold rate enhancement was observed when replacing triphenylphosphine by tri(2-furyl)phosphine . However, the dissociative or associative nature of the substitution on the palladium is stiU under discussion . ... 

Stereochemistry of CO Dissociation. If a complex has nonequivalent CO groups, the stereochemistry of the dissociative process can be defined. This has prompted many studies of exchange reactions with Mn(CO)5Br. Substitution reactions always lead to m-Mn(CO)4LBr, but substitution products do not necessarily provide information on the site of CO dissociation. In an exchange reaction, the products must reflect the site of dissociation by the principle of microscopic reversibility. One would predict that the site of CO dissociation would be cis to the bromide in Mn(CO)sBr since each cis-CO (equatorial) is trans to CO and should not be as strongly jt bonded as the axial CO, which is trans to Br. However, both the axial and equatorial positions were emiched when reacted withC Oor CO ... 

Casey and co-workers 178, 179) also studied the photochemistry of olefin-substituted carbene complexes and observed CO dissociation. For example, irradiation of a 1.38 1 mixture of the Z and E isomers of 145 led to CO substitution by the allyl-carbene group to give the isolable carbene-olefin complex 146 [Eq. (138)], a reaction which demonstrates CO dissociation. (Z)-145 has the proper stereochemistry to yield 146 upon photoinduced CO loss. However, the E isomer does not. Yet, the authors observed simultaneous disappearance of the E isomer upon irradiation of... 

Table 4.1 shows the stereochemistry of substitution for five representative systems. Displacement at the primary 1-butyl system occurs mainly by inversion (Entry 1). However, there is also extensive formation of a rearranged product, 2-butanol (not shown in the table). Similarly, the 2-butyl diazonium ion gives 28% inversion in the unrearranged product, but the main product is r-butanol (Entry 2). These results indicate competition between concerted rearrangement and dissociation. Several secondary diazonium ions were observed to give alcohol with predominant... 

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