2,2,6,6-Tetramethylpiperidinyl bases

A selective magnesiation or zincation of highly functionalized alkenes and cycloalkenes using 2,2,6,6-tetramethylpiperidinyl bases has been reported. The kinetically very active tetramethylpiperidinyl bases allow the smooth metalation of various substituted olefins under practical reaction conditions to give unsaturated organometallic compounds. 

This type of chemoselectivity has been observed in the oxidations of alcohols using copper(II) chloride and a catalytic quantity of 2,2,6,6-tetramethylpiperidinyl-l-oxyl (11 equation 11). The oxidizing species which is generated, in fw example the oxidation of (12) to (13), is the cation (14), which may also be produced using electrochemicai oxidation (in the wesence of the weak base 2,6-lutidine). Allylic and ben lic alccAols are easily oxidized by this reagent, whereas secondary alcdiols react slowly. 

Alternative bimolecular methods have been reported that involve mixing appropriate ratios of monomer with free-radical initiators (such as benzoyl peroxide) and an excess of the nitroxide stable free-radical moiety. Such bimolecular methods do not afford the same degree of control of molecular weight and polydispersity since the stoichiometry of the mediating system cannot be accurately dehned, which is a crucial factor in these controlled polymerization systems. A wide variety of unimolecular nitroxide based initiator systems have been described in the literature with those based upon the 2,2,6,6-tetramethylpiperidinyl-l-oxy (TEMPO) group proving to be the most commonly used. 

Ananchenko, G. S., and Fischer, H. (2001). Decomposition of model alkoxyamines in simple and polymerizing systems. I. 2,2,6,6-tetramethylpiperidinyl-N-oxyl-based compounds. J. Polym. Sci., Part A Polym. Chem., 59(20) 3604-3621. 

Simple amides and esters 116 were conveniently deprotonated by Zn(tmp)2 (tmp = 2,2,6,6-tetramethylpiperidinyl anion) to generate zinc enolates 117. The zinc enolates 117 were readily coupled with aryl bromides using typical palladium-catalyzed Negishi cross-coupling reactions to give arylketones 118. Enolates formed by this method were suitable for use in aldol reactions that tolerate base-sensitive functional groups. 

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