Chiral controller group, 14, Chart

Spatial and/or coordinative bias can be introduced into a reaction substrate by coupling it to an auxiliary or controller group, which may be achiral or chiral. The use of chiral controller groups is often used to generate enantioselectively the initial stereocenters in a multistep synthetic sequence leading to a stereochemically complex molecule. Some examples of the application of controller groups to achieve stereoselectivity are shown retrosynthetically in Chart 19. 

There are a number of powerful synthetic reactions which join two trigonal carbons to form a CC single bond in a stereocontrolled way under proper reaction conditions. Included in this group are the aldol, Michael, Claisen rearrangement, ene and metalloallyl-carbonyl addition reactions. The corresponding transforms are powerfully stereosimplifying, especially when rendered enantioselective as well as diastereoselective by the use of chiral controller groups. Some examples are listed in Chart 20. 

It was demonstrated that a stereocenter positioned far away from the reactive isocyanide group as in monomer 82b (Chart 14) can still induce chirality in the main chain of polyisocyanides, resulting in the formation of an excess of one particular helix.224-226 Kinetic control over the helix sense in the polymerization of 82b was confirmed by the noncooperative transfer of chirality from the monomer to the macromolecule e.g., a linear relation was found between the ee present in the isocyanide and the optical activity in the polymers formed.227 The kinetic inhibition of the growth of one particular handedness using (5)-2-isocyanovaleric acid184 (vide supra) is used to force 82b and 82c into a macromolecular helicity with a screw sense opposite that of the one preferred... 

To control the molecular weight (MW) and MWD, Sakurai and coworkers developed an anionic polymerization method with masked disilene, a highly strained precursor with biphenyl (Chart 13.4). Although the substituents are limited to alkyl and certain dialkylamino groups, this method made it possible to design well-defined Si Si sequences, leading to diblock polymers with organic polymer blocks such as poly(triphenylmethylmethacrylate) by means of chiral anionic catalysts. 

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