Stereocontrol in intermolecular dirhodium(II)-catalyzed carbonyl ylide formation and reactions. Dioxolanes and dihydrofurans

Ethyl diazoacetate undergoes dirhodium(II)-catalyzed reactions with aryl aldehydes to form 1,3-dioxolanes as mixtures of diastereoisomers in good yields. Carbonyl ylides are reaction intermediates. Catalyst dependent diastereocontrol is observed for reactions with p-nitrobenzaldehyde, but not for those with p-anisaldehyde or benzaldehyde, so that at least with transformations involving p-nitrobenzaldehyde a metal-stabilized ylide is responsible for product formation, Higher yields are obtained with catalysis by dirhodium(II) carboxamidates than with the carboxylates. Diastereoselectivity in 1,3-dioxolane formation that occurs through the ''free'' ylide can be effectively controlled so that only one diastereomer is produced through the use of 2,6-di-tert-butyl-4-methylphenyl diazoacetate (BDA) or dicyclohexylmethyl diazoacetate (DCM). The thermodynamically least stable all-cia trisubstituted 1,3-dioxolane is the primary product from p-nitrobenzaldehyde ''cycloaddition'' to the metal-stabilized ylide. Reactions that take place in the presence of p-anisaldehyde and dimethyl acetylenedicarboxylate (DMAD) result in the formation of one 2,5-dihydrofuran-2-carboxylate stereoisomer in good yield. In contrast, with p-nitrobenzaldehyde and DMAD both dihydrofuran stereoisomers are produced along with, mainly, the dioxolane derived from the metal-stabilized ylide; there is in this case competition between addition reactions of the ''free'' ylide and the metal-associated ylide.