Asymmetric Propargylation/Allylation/Pauson−Khand Cyclization of a Planar Chiral Anisole Tricarbonylchromium Complex

Chiral Lithium Amides: Tuning Asymmetric Synthesis on the Basis of Structural Parameters

An overview on the structural arrangements adopted by Chiral Lithium Amides (CLAs), alone or in mixed complexes, is presented. These species are important reagents for asymmetric synthesis and understanding their organization is essential to improve their design and the reaction conditions.

Stereoselective Synthesis of Both Enantiomers ofN-Aryl Indoles with Axially Chiral N−C Bonds¶

N-Aryl indoles with axially chiral N-C bonds were synthesized by stereoselective nucleophilic aromatic substitution reactions of planar chiral tricarbonyl(2,6-disubstituted-1-fluorobenzene)chromium complexes. The stereochemistry of the products is highly dependent on the position of the substituent in the indole. When indoles devoid of a substituent at the 2-position were used, N-aryl indole chromium complexes having anti orientation with respect to the tricarbonylchromium fragment were obtained diastereoselectively. In contrast, 2-substituted indoles gave the N-aryl indoles with syn orientation between the tricarbonylchromium fragment and the benzene ring of the indole. These results demonstrate that we have succeeded in synthesizing both enantiomers of N-aryl indoles utilizing an identical planar chiral arene chromium complex.

Arene−Chromium Tricarbonyl Complexes in the Pauson−Khand Reaction

[reactions: see text] We show the use of arene-chromium tricarbonyl complexes in intra- and intermolecular Pauson-Khand reactions. Both styrene and ethynylbenzene complexes react with alkynes and olefins. The synthesis of enynes connected through chromium-complexed aromatic rings is developed. The intramolecular Pauson-Khand reaction occurs in a totally diastereoselective manner.

Ligand-modulated stereo- and regioselective tandem addition reactions of rhenium-bound naphthalene

A series of complexes of the form TpRe(CO)(L)(eta(2)-naphthalene) (Tp = hydridotris(pyrazolyl)borate) undergoes tandem electrophile/nucleophile addition reactions with a high degree of regiocontrol depending on the auxiliary ligand, L. When L = PMe(3), the reaction of the eta(2)-naphthalene complex with triflic acid followed by a silyl ketene acetal favors the 1,4-addition product, whereas when L = pyridine, N,N-dimethylaminopyridine, N-methylimidazole, or NH(3) the 1,2-addition product is favored. These reactions proceed with excellent stereocontrol: both electrophile (H(+), D(+)) and nucleophile (silyl ketene acetal) add anti to the face of metal coordination, and a single coordination diastereomer can be isolated for each reaction. One-electron oxidation of the Re complex affords the corresponding free dihydronaphthalene in good yield.

Asymmetric Conversion of Arenechromium Complexes to Functionalized Cyclohexenones: Progress toward Defining an Optimum Chiral Auxiliary

An investigation into the asymmetric synthesis of 5-substituted cyclohexenones via nucleophile addition to (alkoxyarene)chromium tricarbonyl complexes is described. Diastereoselectivity during the nucleophile addition step was achieved using alkoxy substituents derived from terpenoid substrates as chiral auxiliaries. Selectivities as high as 24:1 were obtained when 2-phenylisoborneol was used as the chiral auxiliary and as high as 17:1 using 3,3-(ethylenedioxy)isoborneol. The absolute stereochemistry of the major products was assigned by Mosher's method, after their conversion to the corresponding cyclohexenol. A study of the temperature dependence of the nucleophile addition to alkoxytoluene complexes revealed a thermodynamic preference for addition ortho to the ether substituent.