Kinetics boronate assemblies

Unless there is a chiral ligand on boron, assemblies a and b of Scheme 5.2 are enantiomeric and the product will be racemic. If the ligand is chiral, then the transition structures are diastereomeric and the products will be formed in unequal amounts under conditions of kinetic control (Chapter 1). Figure 5.1 illustrates several chiral boron reagents that have been tested in the allyl boration reaction, with typical enantioselectivities for each. 

Despite the fact that the first representatives of clathrochelates based on dioxi-mates have been assembled in the presence of kinetically inert cobalt(III), the kinetically labile iron(II) is the most suitable template with which to control this kind of encapsulation reaction. The cage-like, and now low-spin, boron-containing iron(II) coordination compounds [Fe(L892)] to [Fe(L900)] are prepared in good yield from alcoholic or aqueous solution on reaction of the iron(II) salt, dimethyl-... 

Matteson et al have studied the homologation of boronic esters with dichloromethyl lithium. The stereoselectivity of the reaction involving pinanediol boronic esters is decreased due to epimerization of the homologated product by co-produced lithium chloride. Assembling the proposed intermediate by the reaction of the dichloromethyl boronic ester with an alkyl lithium does not give the same enantioselectivity as with the previous procedure. This was thought to be associated with kinetic selectivity in the attack by the nucleophiles R and CHClJ. 

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