Alberto Ripa

α-t-butyl- and α-i-propyl-ortho-hydroxybenzylamines: Racemic synthesis/resolution and asymmetric synthesis

Efficient routes to α-tert-butyl- and α-iso-propyl-ortho-hydroxybenzylamines 1a and 1b are described. Highly enantioenriched 1a and 1b were obtained by resolution of the methoxy derivatives 2 by recrystallization of the salts formed with mandelic acid followed by Lewis acid mediated demethylation. The chiral 1,3-amino alcohol 1a has also been obtained in an asymmetric synthesis with the key step a diastereoselective alkylation of the imine obtained by condensation of o-anisaldehyde with phenyl glycinol. The absolute stereochemistry of these 1,3-aminophenols was determined by CD spectroscopy of the salicylideneamines 12 and by an X-ray structure analysis of the salt formed between (R)-mandelic acid and (S)-α-tert-butyl-ortho-methoxybenzylamine ((S)-2a).

Chiral CO-emulating ligands: From arene chromium chemistry to enantioselective catalysis

A one pot nucleophile/electrophile additiodhydrogenation sequence was applied to ((benzene)Cr(CO)j) to give predominantly the 4,5-trans-disubstituted cy- clohexene. Several asymmemc modifications of the sequential addition of C-nu- cleophiles and C-elecuophiles to (arene)Cr(CO), complexes are discussed. A mechanistically intriguing route involves the use of chiral phosphorous ligands to control the diastereoselectivity in the migratory CO insertion step and/or the reductive elimination step in the sequence. Ephedrine and norephedrine derived ligands gave product ee of up to 69 9%. New C2-chiral bidentate ligands (L*) which emulate some of the bonding characteristics of CO were synthesized and briefly considered for this application. The main interest in these ligands concerns their potential in catalytic C-C bond forming reactions; a first application to the Lewis acid (CpFeL*)+ catalyzed Diels- Alder reaction between enals and dienes was successfully realized.

1,2‐Disubstituted [(η6‐Arene)Cr(CO)3] Complexes by Sequential Nucleophilic Addition/endo‐Hydride Abstraction

The surprising answer to the question Are the structural characteristics of the product of the nucleophilic addition to complex 1 those of a cyclohexadienyl complex (2 a) or those of a diene aza-enolate complex (2 b)? is: both. This is indicated by the X-ray structure determination of the complex with R=naphthyl. An unprecedented endo-hydride abstraction concludes the sequence that provides a new route to 1,2-substituted planar chiral [(n6-arene)Cr(CO)3] compounds.