Highly Enantioselective Synthesis of 1,3-Oxazolidin-2-imine Derivatives by Asymmetric Cycloaddition Reactions of Vinyloxiranes with Unsymmetrical Carbodiimides Catalyzed by Palladium(0) Complexes

Abstract

4-Vinyl-1,3-oxazoilidin-2-imine derivatives have been synthesized by cycloaddition reactions of 2-vinyloxiranes with unsymmetrical carbodiimides catalyzed by palladium(0) complexes in excellent total isolated yields. After reaction two compounds were always formed, one of which was isolated as the major product. A bulky alkyl group on one of the nitrogen atoms of the carbodiimide enhanced the product ratio in favor of the N-aryl-3-alkyl-1,3-oxazolidin-2-imine. Highly enantioselective cycloadducts (up to >99% ee) were formed by using TolBINAP as the chiral phosphine ligand, in THF at ambient temperatures. The enantiodetermination is believed to be dependent on nucleophilic attack of the anionic nitrogen of the carbodiimide due to the steric interaction of the carbodiimide substituents with the chiral phosphine ligand.

Enantioselective Palladium-Catalyzed Transformations

“…Recently, Alper and co-workers 163,164 have studied the catalytic asymmetric [3+2] cycloaddition of vinyloxiranes 405a with heterocumulenes such as N,N-disubstituted symmetrical and unsymmetrical carbodiimides, yielding 4-vinyl-1,3- oxazolidin-2-imines in high yields and enantioselectivity (eqs [126][127][128].…”

Palladium‐Catalyzed Diastereo‐ and Enantioselective Synthesis of Substituted Cyclopentanes through a Dynamic Kinetic Asymmetric Formal [3+2]‐Cycloaddition of Vinyl Cyclopropanes and Alkylidene Azlactones

“…The resulting Pd(II) complexes add across olefins, [3] isocyanates, [4,5] carbodiimides, [6] and aldehydes [7] to afford five-membered rings. We hypothesized that we could use 1,3-dipoles generated from vinyl cyclopropanes as a novel three carbon fragment to generate cyclopentanes in an asymmetric fashion via palladium catalysis.…”

Preparation and evaluation of bipyridyl-tagged reagents and scavengers

“…The expected compound was isolated as a white solid with 72% yield. Many different methods were attempted for the dehydration of the urea or thiourea (Scheme 6): TsCl under solid -liquid phase-transfer catalytic conditions, 7 TsCl/ pyridine, MsCl/Et 3 N/DMAP, SOCl 2 /Et 3 N, Ph 3 PBr 2 /Et 3 N, 8 phosgene iminium chloride/Et 3 N. 9 None of these methods gave the expected compound.…”