Henri Brunner

Optically Active Organometallic Compounds of Transition Elements with Chiral Metal Atoms

Chiral transition metal atoms are not only present in tris-chelate complexes [M(LL)3 ]n+ , which were already resolved into the enantiomers by Alfred Werner, but also in organometallic half-sandwich complexes such as 1 with three- or four-legged piano-stool structure. These complexes have been tools in the elucidation of the spatial course of reactions and in organic syntheses. Applications in enantioselective catalysis are beginning to show up.

Dendrizymes: Expanded ligands for enantioselective catalysis

Abstract Dendrizymes are a new class of expanded ligands, designed for enantioselective catalysis with transition metal complexes. These expanded ligands consist of a strongly binding chelate core, which is surrounded by space-filling dendrimer substituents, built-up by branching units and optically active groups. In a complex of such a dendrimer ligand a reaction is supposed to take place in the same way as in the pocket of an enzyme.

Enantioselective catalysis 98. Preparation of 9-amino(9-deoxy)cinchona alkaloids

Abstract Cinchonine and quinine were allowed to react with hydrazoic acid in a Mitsunobu reaction to yield the corresponding azides with inversion of configuration at C9. In situ reduction of these azides provided 9-amino(9-deoxy)epicinchonine and 9-amino(9-deoxy)epiquinine, respectively. The structures were confirmed by CD-spectroscopy and X-ray crystallography.

Chiral Metal Atoms in Optically Active Organo-Transition-Metal Compounds

Publisher Summary This chapter discusses chiral metal atoms in optically active organo-transition-metal compounds. An important approach to stereochemical problems is to make use of the concept of chirality. The chiral center most frequently encountered is the asymmetric carbon atom––a tetrahedral C atom––bonded to four different substituents. Chiral molecules may be studied by a great many techniques. Occasionally, optically active ligands have been employed in organo-transition-metal chemistry to demonstrate the stereospecificity of reactions at metal–ligand bonds with respect to the α-carbon atom of the ligand. The chapter focuses on organo-transition-metal compounds having chiral metal atoms whose optical activity have been demonstrated. The principle of introducing a diastereoisomer relationship into a pair of mirror-image isomers is the basis for each optical resolution. After the preparation of diastereoisomers by the introduction of an optically active resolving agent, the next problem in an optical resolution is to separate the diastereoisomers. The optical purity of enantiomers has been determined by nuclear magnetic resonance (NMR) spectroscopy with the help of optically active shift-reagents. Thus, for racemization, the steric effect seems to be most important. Metal–alkyl bonds can be carbonylated, and metal–acyl bonds can be decarbonylated. Optically active organo-transition-metal compounds exhibit extremely large specific rotations, usually exceeding the specific rotations encountered in organic chemistry by a factor of one or two powers of ten.

Stability of the Metal Configuration in Chiral-at-Metal Half-Sandwich Compounds

Half-sandwich compounds with a three-legged piano-stool geometry are prominent examples of optically active chiral-at-metal complexes. In these compounds, the configuration at the metal atom may be stable or labile in solution. Configurationally stable compounds can be used for the elucidation of the stereochemical course of substitution reactions and for organic synthesis in ligand transformation reactions. With configurationally labile compounds, the rate of change of the metal configuration can be measured, which sets an upper limit with regard to the handling of the compounds in solution. Surprisingly, in a number of recent papers the lability of the metal configuration has been overlooked resulting in misinterpretations and wrong conclusions.

Asymmetrische katalysen: XLVI. Enantioselektive Hydrosilylierung von Ketonen mit [Rh(COD)Cl]2 und optisch aktiven Stickstoff-Liganden☆

The synthesis and characterisation of 16 optically active nitrogen ligands, namely, pyridinethiazolidones, pyridinethiazolines, pyridinimidazolines, Schiff bases and bipyridines, are described. These ligands are used as cocatalysts together with the procatalyst [Rh(COD)Cl]2 in the catalytic hydrosilylation of prochiral ketones with diphenylsilane. With these homogeneous in situ catalysts, optically active 1-phenylethanol is produced from acetophenone after hydrolysis of the silyl ether. The diastereomers of N-(1-phenylethyl)-2-(2-pyridinyl)-thiazolidine-4-one give opposite optical inductions. The best optical purity of 71.6% ee is obtained with a pinanyl-substituted bipyridine.

OPTISCH AKTIVE METALLORGANISCHE VERBINDUNGEN DER UBERGANGSELEMENTE MIT CHIRALEN METALLATOMEN

Chirale Ubergangsmetallatome treten nicht nur in Tris(chelat)komplexen [M(LL)3]n+ auf, die bereits von Alfred Werner in die Enantiomere gespalten wurden, sondern auch in metallorganischen Halbsandwichkomplexen wie 1 mit Dreibein- oder Vierbein-Klavierstuhl-Struktur. Die Verbindungen haben sich bei der Aufklarung des raumlichen Ablaufs von Reaktionen und in der organischen Synthese bewahrt. Anwendungen in der enantioselektiven Katalyse beginnen sich abzuzeichnen.

Asymmetric catalysis. XL: Enantioselective hydrosilylation of ketones by diphenylsilane with [Rh(cod)Cl]2/pyridinethiazolidine catalysts

Abstract Fiftyeight prochiral ketones have been used inenantioselective hydrosilylation with diphenylsilane promoted by in-situ catalysts consisting of [Rh(cod)Cl] 2 and the chiral ligands (4 S -2-methyl-2-(2-pyridyl)-4-carbomethoxy-1,3-thiazolidine ( A ) and (4 S )-2-(2-pyridyl)-4-carbethoxy-1,3-thiazolidine ( B ). Hydrolysis of the silyl ethers gave the corresponding secondary alcohols. Aryl methyl ketones were reduced with ees better than 80% irrespective of whether the substituents Me, Cl, F, OMe were in o -, m -,. or p -position of the phenyl ring. The only exceptions were ketones containing the p -OMe substituent, for which a “ p -methoxy effect” diminished the optical yields. Heterocyclic ketones were also hydrosilylated with high optical inductions, e.g. 2-acetylpyridine with 88.5% ee. Linear alkyl ketones with the CO group in the 2-position (methyl ketones) gave up to 50% ee R , in contrast to the corresponding ethyl ketones with the CO group in 3-position, which gave predominantely S -configurated products. In 35 cases the asymmetric inductions were higher with ligand B than with ligand A .

Asymmetrische katalysen: XXII. Enantioselektive hydrosilylierung von ketonen mit CuI-katalysatoren

Abstract Mixtures of Cu I compounds (CuOC(CH 3 ) 3 , CuO 2 CC 6 H 5 ) optically active chelate phosphines ((−)Diop, (+)Norphos, (−)BPPFA) are catalysts for the quantitative hydrosilylation of acetophenone with diphenylsilane, optical yields ranging between 10 and 40% ee.

Optisch aktive Übergangsmetall-Komplexe: XCX. Optisch reines 3a,4,5,6,7,7a-Hexahydro-2-phenyl-4,7-methano-1H-inden-1-on durch asymmetrische Khand-Reaktion

Abstract The prochiral complex Co2(CO)6(HC2Ph) (I) obtained from the reaction of Co2(CO)8 with phenylacetylene, after reaction with the optical active phosphane Glyphos (II) gives the complex Co2(CO)5(HC2Ph)(Glyphos) which consists of two diastereomers (IIIa and IIIb) differing only in the chirality of the Co2C2 cluster. IIIa and IIIb can be separated by preparative liquid chromatography. They epimerize at higher temperatures, with the half life of the approach to the equilibrium IIIa/IIIb = 60 40 , at 60°C in toluene, being approximately 170 min. IN the Khand reaction of III with norbornene (IV) the cyclopentenone 3a,4,5,6,7,7a-hexahydro-2-phenyl-4,7-methano-1H-inden-1-one (V) is formed. The optical purity of V can be determined with the optishift reagent Pr(tfc)3. The equilibrium mixture IIIa/IIIb = 60 40 gives the (−)-cyclopentenone V in an enantiomeric purity of 36% ee. The optically pure (−)589-diasstereomer IIIb yields the enantiomerically pure (+)-cyclopentenone V, provided the epimerization is slow under the reaction conditions used.