Stereochemical and Regiochemical Analyses

There is not much to say about the mechanism of nucleophilic addition to ligands. The nucleophile typically adds in the rate-determining step, and in some cases a leaving group 

CHAPTER 12 ORGANOTRANSITION METAL REACTION MECHANISMS AND CATALYSIS 

The reaction shown in Eq. 12.67 gives complete inversion at the stereogenic center attached to Fe. This requires a backside attack, and the most logical way for this to occur is to start with an oxidative addition of the Br2, and then nucleophilic attack by bromide with Fe as the leaving group. The sequence of electrophilic addition of X2 to the metal followed by nucleophilic attack on the ligand is common for middle-to-late transition metals. Interestingly, when phenyl is in the 3-position, the reaction proceeds with retention. Retention is best explained by a double inversion, and the phenonium ion has been substantiated as the intermediate formed (Eq. 12.68). 

Nucleophilic attack on alkenes proceeds almost always by an anti addition pathway. Eq. 12.69 shows an example. The nucleophile has added to the face of the it system opposite to the metal. This makes good sen.se, in that sterics would preclude attack on the alkene from the face where the metal is coordinated. However, there are some examples of the syn addition of a nucleophile, and these reactions presumably occur via coordination of the nucleophile first to the metal, followed by addition to the alkene. Coordination followed by addition formally constitutes a migratory insertion, and such reactions were covered in Section 12.2.5. 

The regiochemistry of addition to conjugated dienes normally occurs at the ends of the extended -it system. One of the most well studied examples involves the T -dienyl cationic iron complexes shown in Eqs. 12.70 and 12.71. Addition to either end of the extended it system gives regioisomeric products, which are difficult compounds to obtain using other synthetic routes. 

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