Intramolecular activation of strong Si-O bonds by gold(I): regioselective synthesis of 3-bromo-2-silylbenzofurans

The high reactivity of gold-vinylidene complexes, generated in situ by [1,2]-bromine shift from the corresponding 1-bromoalkynes, allows the activation of one of the strongest bonds in organic chemistry (Si-O), strategically placed at the ortho-position. In this way, the synthesis of 3-bromo-2-silylbenzofuranes is achieved in good yields. Several substituents with different electronic properties and substitution patterns are well tolerated on the tethering aromatic ring as well as a number of silyl groups on the O-atom. A preliminary mechanistic study is compatible with the participation of gold vinylidene intermediate species. The synthetic applicability of the obtained scaffolds was preliminarily showcased by orthogonal C-C bond forming transformations.

A mild tetradehydro-Diels-Alder reaction of aryldiyne compounds affords exclusively linear products

The thermal tetradehydro-Diels-Alder (TDDA) reaction for the synthesis of polysubstituted aromatic compounds remains underestimated probably due to the harsh conditions and multiproduct results. Herein, a mild intramolecular TDDA reaction of aryldiyne compounds is presented with linear naphthalenes only, exhibiting good functional group tolerance. The reaction is easy to operate and amenable to multigram-scale synthesis. From the preliminary work, it was found that the mild conditions may be the key to the completely linear product in the reactions.

Computational exploration for possible reaction pathways, regioselectivity, and influence of substrate in gold-catalyzed cycloaddition of cyanamides with enynamides

The current work focuses on the DFT calculation of the rational mechanism and catalytic activity of the gold(i)-catalyzed isotetradehydro-Diels–Alder cycloaddition of cyanamides and enamides to substituted 2,6-diaminopyridines. IPrAuCl is used as a model catalyst to catalyze cyanamide and enynamide reactants with different substituents in DCM as a research system. DFT data indicates that the catalytic cycle starts from the triple bond coordination between the catalyst's gold cation and the enamide to obtain the gold π-complex, and the cyanamide attacks the alkynyl carbon atom from different directions to generate two reaction channels of five-membered and six-membered heterocycles, respectively. The calculation results show that the 2,6-diaminopyridine compounds produced by this catalytic reaction have lower activation energy and higher reactivity, that is, the pyridine skeleton structure can be easily obtained under mild reaction conditions. At the same time, electron-withdrawing substituents in the reactants are more helpful for the reaction. In addition to being in good agreement with the experimental data, the calculated results also provide an important contribution to the further understanding of the mechanism of such reactions.

The current work focuses on the DFT calculation of the rational mechanism and catalytic activity of the gold(i)-catalyzed isotetradehydro-Diels–Alder cycloaddition of cyanamides and enamides to substituted 2,6-diaminopyridines.

Gold-Catalyzed Formal Hexadehydro-Diels-Alder/Carboalkoxylation Reaction Cascades

A dual gold-catalyzed hexadehydro-Diels-Alder/carboalkoxylation cascade reaction is reported. In this transformation, the gold catalyst participated in the hexadehydro-Diels-Alder step, switching the mechanism from a radical type to a cationic one, and then the catalyst activated the resulting aryne to form an ortho-Au phenyl cation species, which underwent a carboalkoxylation rearrangement rather than the expected aryne-ene reaction.

Isoindolinone Synthesis via One-Pot Type Transition Metal Catalyzed C-C Bond Forming Reactions

Isoindolinone structure is an important privileged scaffold found in a large variety of naturally occurring as well as synthetic, biologically and pharmaceutically active compounds. Owing to its crucial role in a number of applications, the synthetic methodologies for accessing this heterocyclic skeleton have received significant attention during the past decade. In general, the synthetic strategies can be divided into two categories: First, direct utilization of phthalimides or phthalimidines as starting materials for the synthesis of isoindolinones; and second, construction of the lactam and/or aromatic rings by different catalytic methods, including C-H activation, cross-coupling, carbonylation, condensation, addition and formal cycloaddition reactions. Especially in the last mentioned, utilization of transition metal catalysts provides access to a broad range of substituted isoindolinones. Herein, the recent advances (2010-2020) in transition metal catalyzed synthetic methodologies via formation of new C-C bonds for isoindolinones are reviewed.

Catalyst-controlled site-selective N-H and C3-arylation of carbazole via carbene transfer reactions

A site-selective direct arylation reaction of carbazole and other N-heterocycles with diazo-naphthalen-2(1H)-ones has been developed. While Au(i)-NHC catalysts lead to selective C3-arylation, palladium acetate allows for selective N-H arylation, displaying complete site-selectivity each. To show the applicability of these arylation reactions, one-pot, two-fold diarylation reactions of carbazole were demonstrated.

Rhodium-catalysed tetradehydro-Diels-Alder reactions of enediynes via a rhodium-stabilized cyclic allene

Efficient methods for the synthesis of fused-aromatic rings is a critical endeavour in the creation of new pharmaceuticals and materials. A direct method for preparing these systems is the tetradehydro-Diels-Alder reaction, however this is limited by the need for harsh reaction conditions. A potential, but underdeveloped, route to these systems is via transition metal-catalysed cycloaromatisation of ene-diynes. Herein, tethered unconjugated enediynes have been shown to undergo a facile room-temperature Rh^I-catalysed intramolecular tetradehydro-Diels-Alder reaction to produce highly substituted isobenzofurans, isoindolines and an indane. Furthermore, experimental and computational studies suggest a novel mechanism involving an unprecedented and complex Rh^I/Rh^III/Rh^I/Rh^III redox cycle involving the formation of an unusual strained 7-membered rhodacyclic allene intermediate and a Rh^III-stabilized 6-membered ring allene complex.

Divergent Pd-catalyzed cross-coupling of allenyloxazolidinones to give chiral 1,3-dienes and vinyloxazolidinones

The divergent reactivity of 5-allenyloxazolidinones has been explored. This novel building block undergoes Pd(0)-catalyzed cross-coupling with boronic acids to form a wide range of chiral 1,3-dienes and pharmaceutically useful vinyloxazolidinones, the chemoselectivity being tightly controlled by a simple switch in additive.

Generation of Donor/Donor Copper Carbenes through Copper-Catalyzed Diyne Cyclization: Enantioselective and Divergent Synthesis of Chiral Polycyclic Pyrroles

The generation of metal carbenes from readily available alkynes represents a significant advance in metal carbene chemistry. However, most of these transformations are based on the use of noble-metal catalysts and successful examples of such an asymmetric version are still very scarce. Here a copper-catalyzed enantioselective cascade cyclization of N-propargyl ynamides is reported, enabling the practical and atom-economical construction of diverse chiral polycyclic pyrroles in generally good to excellent yields with wide substrate scope and excellent enantioselectivities (up to 97:3 e.r.). Importantly, this protocol represents the first copper-catalyzed asymmetric diyne cyclization. Moreover, mechanistic studies revealed that the generation of donor/donor copper carbenes is presumably involved in this 1,5-diyne cyclization, which is distinctively different from the related gold catalysis, and thus it constitutes a novel way for the generation of donor/donor metal carbenes.

Oxyacylation of Iodoalkynes: Gold(I)-Catalyzed Expeditious Access to Benzofurans

(Acetonitrile)[1,3-bis(2,6-diisopropylphenyl)-imidazole-2-ylidene] gold(I) catalyzes the cycloisomerization of 2-(iodoethynyl)aryl esters to give 3-iodo-2-acyl benzofurans. This catalytic transformation is the result of an unprecedented selective synthetic event, which comprises a [1,2]-iodine shift, a C-O ring-closure step, and a C-C bond-formation that installs the ketone functionality into the new ring. Experimental evidence supports the involvement of a β-iodo-substituted gold vinylidene as the intermediate species. The reaction tolerates different substitution patterns at the phenol moiety and a wide diversity of groups at the carboxylic fragment, including not only alkyl but also alkenyl, aryl, and heteroaryl groups.

Gold-Catalyzed Homogeneous (Cyclo)Isomerization Reactions

Gold is currently one of the most used metals in organometallic catalysis. The ability of gold to activate unsaturated groups in different modes, together with its tolerance to a wide range of functional groups and reaction conditions, turns gold-based complexes into efficient and highly sought after catalysts. Natural products and relevant compounds with biological and pharmaceutical activity are often characterized by complex molecular structures. (Cyclo)isomerization reactions are often a useful strategy for the generation of this molecular complexity from synthetically accessible reactants. In this review, we collect the most recent contributions in which gold(I)- and/or gold(III)-catalysts mediate intramolecular (cyclo)isomerization transformations of unsaturated species, which commonly feature allene or alkyne motifs, and organize them depending on the substrate and the reaction type.

Theoretical study on the mechanism and chemoselectivity in gold(i)-catalyzed cycloisomerization of β,β-disubstituted ortho-(alkynyl)styrenes

The mechanism and chemoselectivity of gold(i)-catalyzed cycloadditions of β,β-disubstituted ortho-(alkynyl)styrenes were explored by DFT calculations.