Patrick Romanens

Highly diastereoselective coupling reactions between chiral benzaldehyde tricarbonylchromium complexes and activated double bonds

The amine catalyzed coupling reaction of acrylic derivatives with benzaldehydes (Baylis-Hillman reaction) is more efficient when the arene is complexed to the electrophilic Cr(CO)3 group. Reactions with planar chiral o-substituted benzaldehyde complexes are highly diastereoselective. 3-Hydroxy-2-methylenealkanoic acid derivatives can be obtained efficiently and with very high enantiomeric excess from readily accessible enantiomerically pure complexes. The configuration of the benzylic stereogenic center relative to the planar chirality of the arene complex was determined in one case by an X-ray structural analysis. The reaction has been extended to an o-anisaldehydeimine complex.

1,2-Asymmetric induction in radical reactions of β-amino acid derivatives

Carbon centered radicals adjacent to a chiral amine and generated by the addition of alkyl radicals to racemic or enantiomerically pure acrylates 1 react with tributyltin hydride in high yields and moderate to good diastereoselectivity. 1,2-Asymmetric induction depends on the nature of the groups at the stereogenic and the prochiral center and is remarkably sensitive to changes in the aryl-o-substituent.

Acrylate-Assisted Arene-Chromium Bond Cleavage: Generation of a [Cr(CO)>2>] Fragment under Mild Conditions

Displacement of the benzene ligand in 1 surprisingly occurs more readily than that of the methyl acrylate ligand. This paves the way for 1 to undergo arene exchange and arene substitution reactions, which may be triggered by a η2 →η4 haptotropic rearrangement of the acrylate. Complex 1 is thus a mild precursor of the highly unsaturated fragments [Cr(CO)2 ] (see scheme) and [Cr(CO)2 (η2 -acrylate)].

Stereoselective chromium- and molybdenum- mediated transformations of arenes*

Tricarbonylchromium-mediated dearomatization provides a rapid access to substituted cyclohexadienes. Efficient asymmetric routes to planar chiral arene complexes and to substituted cyclohexadienes have been developed. The article sums up the main features of this chemistry. Highly enantiomerically enriched ortho-substituted benzaldehyde complexes are accessible via asymmetric lithiation followed by trapping with electrophiles. In different solvents, the trimethylsilyl complex exhibits [alpha] values ranging from −174 to +108 for the same enantiomer. Details of two asymmetric syntheses of natural products are given: the alkaloid lasubine I starting from a highly enantiomerically enriched planar chiral arene complex and the marine furanosesquiterpene acetoxytubipofuran. The latter is assembled via asymmetric dearomatization of a benzaldehyde imine complex. Other key steps include an Eschenmoser–Claisen rearrangement and a regio- and diastereoselective Pd-catalyzed allylic substitution. The final section deals with labile arene metal complexes. For the first time, dearomatization reactions mediated by the Mo(CO)3 group have been realized. The reactions show strong analogies to the Cr(CO)3-mediated reactions, but exhibit also marked differences: the arene–Mo bond is stronger, but more labile, and the sequential double additions show different selectivities compared to the chromium analogs.

Acrylat‐assistierter Aren‐Chrom‐Bindungsbruch: Bildung eines [Cr(CO)2]‐Fragments unter milden Reaktionsbedingungen

Leichter verdrangen last sich der Benzolligand in 1 im Vergleich zum Methylacrylatligand, die beide bei Raumtemperatur labil sind. Damit sind Aren-Substitutions- sowie -Austauschreaktionen moglich, die vermutlich durch eine η2→η4-Umlagerung eingeleitet werden. Der Komplex 1 ist daher eine unter milden Bedingungen zugangliche Vorstufe fur die in hohem Mase ungesattigten Fragmente [Cr(CO)2] (siehe Schema) und [Cr(CO)2(η2-acrylat)].