An Unusual Access to Medium Sized Cycloalkynes by a New Gold(I)-Catalysed Cycloisomerisation of Diynes

Dual gold catalysis

For more than a decade the innovative field of homogeneous catalysis by gold was dominated by the interaction of the substrate molecule with one gold center, in most cases in mononuclear gold complexes. The initial interaction was typically a π-coordination of a carbon-carbon double bond to the gold, an activation of the unsaturated substrate molecule by a π-acidic metal center. Only recently clear evidence for reactions that involve the activation of organic substrates by two gold centers was obtained. In that new class of gold-catalyzed reactions the two gold centers interact with the substrate in a very different way. One gold complex is σ-bonded to a terminal alkynyl group in the substrate, the other one is π-coordinated. Only in a few cases, a combination π-coordination and σ-coordination to the same alkyne, which is the energetically preferred mode of interaction with two gold centers, initiates the reaction. In most of the cases, the reaction proceeds through an intermediate with one alkyne σ-bonded to one gold complex and a different alkyne π-coordinated to the second gold complex. Experimental and computational results for many new reactions provide a clear picture of the overall sequence of elemental steps of these conversions; some of the steps are unprecedented in organometallic catalysis and chemistry. For example, the reaction of diynes can involve gold vinylidene-like species as very reactive substructures formed by a 5-exo-dig cyclization. NBO analysis indicates no gold-carbon double bond character in these "vinylidenes". In other reactions, a 6-endo-dig cyclization is energetically preferred; after that gold aryne complexes are not local minima but transition states of a 1,2-shift of gold. Computational studies showed a good correlation of the cyclization mode with the aromaticity of the intermediate. For both the 5-exo-dig and the 6-endo-dig cyclization modes, the intermediates are able to react even with unactivated alkyl-C,H bonds, in low yields even in intermolecular reactions. The final step of the catalytic cycles is also remarkable, because the protodeauration has to occur with the next alkyne substrate molecule. Only then the next gold acetylide is formed directly and a loss of selectivity can be avoided. A computational study suggests that two gold complexes are on the substrate throughout the catalytic cycle. The most recent results indicate that analogous intermediates can be accessible by the reaction of other electrophiles with gold acetylides. With regard to organic synthesis, the overall catalytic conversions open up a universe of new possibilities. Selective C,H-activations now allow to one use usually innocent alkyl side chains for additional anellation reactions by an sp(3)-C,H activation. The C,H activation can even be combined with halogen transfer reactions, directly providing vinyl iodides as versatile building blocks. Short and efficient routes to different carbo- and heterocycles including benzocyclobutenes, fulvenes, and pentalenes demonstrate the synthetic potential not only for total synthesis but also for material science.

Strategic innovation in the total synthesis of complex natural products using gold catalysis

This review has been organized from the perspective of synthetic target families, with emphasis on the use of gold-catalyzed transformations and cascade reactions that significantly increase molecular complexity.

Synthesis of spiroaminals by bimetallic Au/Sc relay catalysis: TMS as a traceless controlling group

An efficient synthesis of spiroaminals over fused aminals has been successfully developed by bimetallic Au/Sc catalysis by using TMS as a traceless controlling group.

Gold-catalyzed cyclizations of cis-enediynes: insights into the nature of gold-aryne interactions

Golden aryne? Gold aryne complexes are inferred as transition states in dual gold-catalyzed cyclizations of cis-enediynes (see scheme; DCE = 1,2-dichloroethane). They are better described as ortho-aurophenyl cations, which react with weak nucleophiles and undergo facile intramolecular insertions into C(sp(3))-H bonds. Indanes, fused heteroarenes, and phenol derivatives are readily prepared using this method.

Gold-catalyzed cycloisomerization of 1,6-diyne carbonates and esters to 2,4a-dihydro-1H-fluorenes

A synthetic method to prepare 2,4a-dihydro-1H-fluorenes efficiently from gold(I)-catalyzed 1,2-acyloxy migration/cyclopropenation/Nazarov cyclization of 1,6-diyne carbonates and esters is described. The suggested reaction pathway provides rare examples of [2,3]-sigmatropic rearrangement in this class of compounds as well as the involvement of an in situ formed cyclopropene intermediate in gold catalysis. Experimental and ONIOM(QM:QM') [our own n-layered integrated molecular orbital and molecular mechanics(quantum mechanics:quantum mechanics')] computational studies based on the proposed Au carbenoid species provide insight into this unique selectivity.

Dual gold catalysis: σ,π-propyne acetylide and hydroxyl-bridged digold complexes as easy-to-prepare and easy-to-handle precatalysts

A series of dinuclear gold σ,π-propyne acetylide complexes were prepared and tested for their catalytic ability in dual gold catalysis that was based on the reaction of an electrophilic π-complex of gold with a gold acetylide. The air-stable and storable catalysts can be isolated as silver-free catalysts in their activated form. These dual catalysts allow a fast initiation phase for the dual catalytic cycles without the need for additional additives for acetylide formation. Because propyne serves as a throw-away ligand, no traces of the precatalyst are generated. Based on the fast initiation process, side products are minimized and reaction rates are higher for these catalysts. A series of test reactions were used to demonstrate the general applicability of these catalysts. Lower catalyst loadings, faster reaction rates, and better selectivity, combined with the practicability of these catalysts, make them ideal catalysts for dual gold catalysis.

Gold vinylidene complexes: intermolecular C(sp3)-H insertions and cyclopropanations pathways

Highly reactive gold vinylidene species are used for intermolecular C(sp(3))-H insertions into unactivated alkanes (see scheme). In addition, they can be regarded as synthons for alkylidene carbenes. Initiated by cyclopropanation of the vinylidene species/alkylidene carbenoide, cyclobutene derivatives are formed in a diastereoselective fashion by a ring-enlargement cascade in only one step.

Gold-catalyzed cycloisomerization of 1,7-diyne benzoates to indeno[1,2-c]azepines and azabicyclo[4.2.0]octa-1(8),5-dines

A synthetic method that relies on Au(I)-catalyzed cycloisomerization reactions of 1,7-diyne benzoates to prepare indeno[1,2-c]azepines and azabicyclo[4.2.0]octa-1(8),5-dines is described.