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Radical Addition to C=N Bonds Mediated by Chiral Ammonium Salts
D. O. Jang*, S. Y. Kim
Yonsei University, Wonju, Korea
22 January 2009 (online)
A protonated cinchona alkaloid derivative (PC)-mediated enantioselective radical addition of alkyl radicals to the C=N bond of N-benzoyl hydrazones is reported (for a Lewis acid mediated version, see: G. K. Friestad et al. Angew. Chem. Int. Ed. 2003, 42, 5061). In the presence of PC, Ph2SiH2, Et3B and air in dichloromethane, the addition of alkyl radicals generated from alkyl iodides to N-benzoyl hydrazones furnished the adducts in good yields and good to excellent enantioselectivities (90:10 to 99.5:0.5) at -30 ˚C. The employment of diphenylsilane instead of organotin hydrides as the radical chain carrier can avoid such disadvantages as high toxicity and difficulty in purifying the product. Triethylborane is necessary for initiating the reaction at low temperatures.