Michael Wölfle

Gold Catalysis: Tandem Reactions of Diyne–Diols and External Nucleophiles as an Easy Access to Tricyclic Cage‐Like Structures

Different diyne-diols composed of two terminal homopropargylic alcohol groups were prepared by bi-directional synthesis. Subjection of the syn diastereomers to NAC-gold catalysts (NAC=nitrogen acyclic carbene) in the presence of external nucleophiles such as water or anilines provided substituted and highly rigid heterocyclic cages. The corresponding anti disastereomers polymerised. An intermediate of the reactions of the syn diastereomers could be isolated and even be characterised by crystal structure analysis. Overall, eight new bonds are formed in the reaction, which proceeds by a multistep sequence of highly selective hydroalkoxylations and hydrohydroxylation or hydroaminations. For furyl substituents and for internal alkynes competing reaction pathways could be identified. By the cross-coupling of a product with an iodoaryl substituent, the use of these cage compounds as geometrically defined linking groups by using orthogonal transition-metal-catalysed methodology, namely, gold and palladium catalysis, could be demonstrated.

Gold Catalysis: Catalyst Oxidation State Dependent Dichotomy in the Cyclization of Furan–Yne Systems with Aromatic Tethers

Four different synthetic strategies led to a variety of furan-yne systems that contained an aryl system in the tether. Due to the short routes to these systems (four steps or less), a small library of substrates could easily be prepared. These were treated with AuCl(3) or with the Gagoszs catalyst Ph(3)PAuNTf(2) complex. The AuCl(3)-catalyzed reactions delivered highly substituted fluorene derivatives, a class of compounds of great importance as precursors for luminophores with extraordinary abilities. Conversely, a different mechanistic pathway was observed with the cationic gold(I) catalyst. In the latter case, a mechanistically interesting reaction cascade initiated a formal alkyne insertion into the furyl-sp(3)-C bond, which gave indene derivatives as the final products. This new reaction pathway depends on the aromatic moiety in the tether, which stabilizes a crucial cationic intermediate as a benzylic cation.

Gold catalysis: News from the homogeneous wing

Phenol synthesis using a homogeneous gold catalyst serves as an example of the tremendous progress in homogeneous catalysis by gold which has been made in the past few years. Various aspects of this reaction, including catalyst selectivity and activity and the simple conditions used are described; and the mechanism is discussed in this context.