Heterocycles from gold catalysis

The article reviews the most important developments in the synthesis of heterocycles by gold catalysts for reactions directly involving the heteroatoms in the reacting groups. Reactions without direct participation of the hetero atoms, for example in substrates with the heteroatom in a tether between the reacting groups, are not included. In the last decade homogeneous gold catalysis has emerged from a handful of scattered publications to a hot topic, an area with now hundreds of publications per year. Among the many different products accessible by the new methods developed, heterocycles are probably the most important class of compounds with significant impact on the synthesis of agrochemicals and pharmaceuticals.     

A Theoretical DFT‐Based and Experimental Study of the Transmetalation Step in Au/Pd‐Mediated Cross‐Coupling Reactions

In this work a combined theoretical and experimental investigation of the cross‐coupling reaction involving two metallic reaction centers, namely gold and palladium, is described. One metal center (Au) hereby is rather inert towards change in its oxidation state, whereas Pd undergoes oxidative insertion and reductive elimination steps. Detailed mechanistic and energetic studies of each individual step, with the focus on the key transmetalation step are presented and compared for different substrates and ligands on the catalytic Pd center. Different aryl halides (Cl, Br, I) and aryl triflates were investigated. Hereby the nature of the counteranion X turned out to be crucial. In the case of X=Cl and L=PMe₃ the oxidative addition is rate‐determining, whereas in the case of X=I the transmetalation step becomes rate‐determining in the Au/Pd‐cross‐coupling mechanism. A variety of Au–Pd transmetalation reaction scenarios are discussed in detail, favoring a transition state with short intermetallic Au–Pd contacts. Furthermore, without a halide counteranion the transmetalation from gold(I) to palladium(II) is highly endothermic, which confirms our experimental findings that the coupling does not occur with aryl triflates and similar weakly coordinating counteranions—a conclusion that is essential in designing new Au–Pd catalytic cycles. In combination with experimental work, this corrects a previous report in the literature claiming a successful coupling potentially catalytic in both metals with weakly coordinating counteranions.     

Gold catalysis: domino reaction of en-diynes to highly substituted phenols

By Sonogashira coupling of 1,7-heptadiynes and 1,8-octadiynes with 2-iodoallyl alcohols, various substrates that bear a 2-alkynylallyl alcohol moiety tethered to an additional alkyne were prepared in one step. Subjection to nitrogen acyclic carbene (NAC)/gold(I) catalysts delivered highly substituted phenols in an efficient domino reaction. Furan derivatives were formed as intermediates; this was proven by in situ NMR spectroscopy. The uncommon substitution pattern of these furans opens the way for a selective formation of phenols that contain the hydroxyl group in the meta position to the ring junction, which previously was not possible by gold-catalyzed furan-yne cyclization. Furthermore, interesting mechanistic insights were obtained by products derived from secondary allyl alcohols. In this case, in addition to the phenolic compounds, a ketone is formed by 1,2-alkyl shift.     

Gold catalysis: 1,3-oxazines by cyclisation of allene amides

A series of allene amides was prepared and their gold-catalysed cyclisation was investigated. The formation of six-membered rings, 1,3-oxazines, was observed. Dihydropyrroles originating from intramolecular hydroamination of the distal C=C double bonds of the allenes were minor side products. Mechanistic studies by in situ (31)P NMR spectroscopy showed only one additional species during the conversion in each case; a computational study of the different allyl gold(I) species involved allowed this to be assigned as the σ-allyl gold species bearing the gold catalyst at the sterically less hindered methylene end. The regiospecific deuterodeauration of this intermediate confirmed a S(E) '-type mechanism for this last step of the catalytic cycle.     

Light-Induced Metal-Free Generation of Cyanocarbenes from Alkynyl Triazenes for the Synthesis of Nitrile Derivatives

The chemistry of alkynyl triazenes is an emerging field for organic chemists and especially acid-induced nucleophilic functionalizations, either directly, or after a prior reaction towards aromatic triazenes under extrusion of nitrogen, paved the way for fruitful strategies. In contrast, the chemical behavior of alkynyl triazenes upon irradiation with light is still unknown. Herein we present the first photoactivation of alkynyl triazenes that triggers an uncommon reactivity pattern involving the cleavage of the N1−N2 bond of the triazene moiety resulting in a unique approach to cyanocarbenes from a readily available, stable, and insensitive precursor. This allows to access various nitrile compounds without the use of a toxic cyanating agent by exploiting the reactivity pattern of carbenes. By variation of the reaction conditions and light sources, different substitution patterns can be obtained selectively in good yields under mild and metal-free conditions, thus introducing the alkynyl triazene unit as a photo accessible methylene nitrile synthon. Using this synthon, subclasses like α-alkoxynitriles, α-aminonitriles and α-cyanohydrazones become easily available. These exhibit synthetically valuable substitution patterns for the synthesis of pharmaceuticals, intermediates for total synthesis and amino acid synthesis.     

Gold-Catalyzed Formal [4+2] Cycloaddition as Access to Antitumor-Active Spirocyclic Oxindoles from Alkynes and Isatin-Derived Ketimines

Due to its excellent bioactivity profile, which is increasingly utilized in pharmaceutical and synthetic chemistry, spirooxindole is an important core scaffold. We herein describe an efficient method for the construction of highly functionalized new spirooxindolocarbamates via a gold-catalyzed cycloaddition reaction of terminal alkynes or ynamides with isatin-derived ketimines. This protocol has a good functional group compatibility, uses readily available starting materials, mild reaction conditions, low catalyst loadings and no additives. It enables the transformation of various functionalized alkyne groups into cyclic carbamates. Gram-scale synthesis was achieved and DFT calculations verify the feasibility of the mechanistic proposal. Some of the target products exhibit good to excellent antiproliferative activity on human tumor cell lines. In addition, one of the most active compounds displayed a remarkable selectivity towards tumor cells over normal ones.