Nitroxide, radicals asymmetric

Despite the usual loss of optical activity noted above, asymmetric radicals can be prepared in some cases. For example, asymmetric nitroxide radicals are known. An anomeric effect was observed in alkoxy radical (31), where the ratio of 31a/31b was 1 1.78. ... 

Finally, chiral nitroxyl radicals can be synthesized by the intramolecular coupling of enones with nitro compounds under reductive conditions [109]. Thus reaction of appropriately substituted nitro enones with Sml2, followed by a quench with reactive acyl chloride electrophiles, provides a-asymmetric nitroxide radicals in excellent yields (Eq. 97). 

Using novel asymmetric difunctional initiators containing TEMPO and 2-bromopropanoate or 2-bromo-2-methylpropanoate groups, block copolymers can be prepared via combination of ATRP and STRP [147,148]. For example, asymmetric difunctional initiator, CLI-25 was used in the ATRP of MM A with CuCl/N, N, N , N ,N -pentamethyldiethylene-triamine (PMDETA) as catalyst. The low initiator efficiency (i.e. 0.8) may be related to the side reactions that occur in the initiation step. Subsequently, the TEMPO-terminated PMMA was used as the macroinitiator in the nitroxide-mediated radical polymerization of St at 125 °C. A series of block copolymers, PMMA-6-PSt [147], PrBA-6-PSt [147] and PtBA-6-PMMA-Zi-PSt [148] have been obtained, an example of which is shown in Scheme 3.36. 

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