Lithium diisopropylamide-mediated reactions of imines, unsaturated esters, epoxides, and aryl carbamates: influence of hexamethylphosphoramide and ethereal cosolvents on reaction mechanisms.
Several reactions mediated by lithium diisopropylamide (LDA) with added hexamethylphosphoramide (HMPA) are described. The N-isopropylimine of cyclohexanone lithiates via an ensemble of monomer-based pathways. Conjugate addition of LDA/HMPA to an unsaturated ester proceeds via di- and tetra-HMPA-solvated dimers. Deprotonation of norbornene epoxide by LDA/HMPA proceeds via an intermediate metalated epoxide as a mixed dimer with LDA. Ortholithiation of an aryl carbamate proceeds via a mono-HMPA-solvated monomer-based pathway. Dependencies on THF and other ethereal cosolvents suggest that secondary-shell solvation effects are important in some instances. The origins of the inordinate mechanistic complexity are discussed.
Ma
Yun
Y
Department of Chemistry and Chemical Biology, Baker Laboratory, Cornell University, Ithaca, New York 14853-1301, USA.
Collum
David B
DB
D000577:Amides / Q000737:chemistry*; D002219:Carbamates / Q000737:chemistry*; D004852:Epoxy Compounds / Q000737:chemistry*; D004952:Esters / Q000737:chemistry*; D007097:Imines / Q000737:chemistry*; D018020:Lithium Compounds / Q000737:chemistry*; D009682:Magnetic Resonance Spectroscopy; D008745:Methylation; D015394:Molecular Structure; D010758:Phosphorus / Q000737:chemistry; D012997:Solvents / Q000737:chemistry*
D000577:Amides; D002219:Carbamates; D004852:Epoxy Compounds; D004952:Esters; D007097:Imines; D018020:Lithium Compounds; D012997:Solvents; D010758:Phosphorus