Catalytic and asymmetric variations, enones, enynes, and allene as other starting materials, and numerous applications in natural product synthesis are all evidence of the diversity of the Pauson-Khand reaction [Eq. (a)], which now definitely counts among the standard tools of organic synthesis.

A modular approach to a new class of structurally diverse bidentate P/N, P/P, P/S, and P/Se chelate ligands has been developed. Starting from hydroquinone, various ligands were synthesized in a divergent manner via orthogonally bis-protected bromohydroquinones as the central building block. The first donor functionality (L1) is introduced to the aromatic (hydroquinone) ligand backbone either by Pd-catalyzed cross-coupling (Suzuki coupling) with hetero-aryl bromides, by Pd-catalyzed amination, or by lithiation and subsequent treatment with electrophiles (e.g., chlorophosphanes, disulfides, diselenides, or carbamoyl chlorides). After selective deprotection, the second ligand tooth (L2) is attached by reaction of the phenolic OH functionality with a chlorophosphane, a chlorophosphite, or a related reagent. Some of the resulting chelate ligands were converted into the respective PdX2 complexes (X = Cl, I), two of which were characterized by X-ray crystallography. The methodology developed opens an access to a broad variety of new chiral and achiral transition metal complexes and is generally suited for the solid-phase synthesis of combinatorial libraries, as will be reported separately.

The synthesis of unsymmetrical alkyldiarylamines from a primary amine and two aryl bromides is described. A catalyst system composed of Pd(OAc)2/(rac)-BINAP is used to prepare an alkylarylamine (1) from a primary amine and aryl bromide. The palladium-catalyzed arylation of 1, by means of a different catalyst system, affords an alkyldiarylamine. The efficiency of each catalyst for the second step depends on the electronic nature of the substrates. This method has reasonable generality and compatiblity with base-sensitive functional groups.

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Ethylen/CO (310-360 bar) 0.22 Mol-% [Co 2 (CO) 8 ] 150 °C, 16 h Schema 3. Katalytische Pauson-Khand-Reaktion nach Rautenstrauch.