Enantioselective Synthesis of Biscyclopropanes Using Alkynes as Dicarbene Equivalents

Chiral biscyclopropanes are an important skeleton in many bioactive molecules. However, there are few routes to synthesize these molecules with high stereoselectivity due to the nature of multiple stereocenters. Herein, we report the first example of Rh2(II)-catalyzed enantioselective synthesis of bicyclopropanes with alkynes as dicarbene equivalents. The bicyclopropanes with 4-5 vicinal stereocenters and 2-3 all-carbon quaternary centers were constructed in excellent stereoselectivity. This protocol features high efficiency and excellent functional group tolerance. Moreover, the protocol was also extended to the cascaded cyclopropanation/cyclopropenation with excellent stereoselectivities. In these processes, both sp-carbons of alkyne were converted into stereogenic sp3-carbons. Experimental and density functional theory (DFT) calculations revealed that the cooperative weak hydrogen bonds between the substrates and the dirhodium catalyst may play key roles in this reaction.

I2 -Catalyzed Cycloisomerization of Ynamides: Chemoselective and Divergent Access to Indole Derivatives

Herein, an I₂ -catalyzed unprecedented cycloisomerization of ynamides is developed, furnishing various functionalized bis(indole) derivatives in generally good to excellent yields with wide substrate scope and excellent atom-economy. This protocol not only represents the first molecular-iodine-catalyzed tandem complex alkyne cycloisomerizations, but also constitutes the first chemoselective cycloisomerization of tryptamine-ynamides involving distinctively different C(sp³ )-C(sp³ ) bond cleavage and rearrangement. Moreover, chiral tetrahydropyridine frameworks containing two stereocenters are obtained with moderate to excellent diastereoselectivities and excellent enantioselectivities. Meanwhile, cycloisomerization and aromatization of ynamides produce pyrrolyl indoles with high efficiency enabled by I₂ . Additionally, control experiments and theoretical calculations reveal that this reaction probably undergoes a tandem 5-exo-dig cyclization/rearrangement process.

Gold-Catalyzed Transformation of Ynamides

Ynamide, a unique species with inherited polarization of nitrogen lone pair electron to triple bond, has been largely used for the developement of novel synthetic methods and the construction of unusual N-bearing heterocycles. The reaction versatility of ynamide on umpolung reactivity, radical reactions and asymmetric synthesis have been recently reviewed. This review provides an overall scenic view into the gold catalyzed transformation of ynamides. The ynamides reactivity towards nitrogen-transfer reagents, such as azides, nitrogen ylides, isoxazoles, and anthranils; oxygen atom-transfer reagents, like nitrones, sulfoxides, and pyridine N-oxides; and carbon nucleophiles under gold catalysis are herein uncovered. The scope as well the mechanistic insights of each reaction is also briefed.

Cycloisomerization of π-Coupled Heteroatom Nucleophiles by Gold Catalysis: En Route to Regiochemically Defined Heterocycles

Since the dawn of millennium, catalytic gold chemistry is at the forefront to set off diverse organic reactions via unique activation of π-bonded molecules. Within this purview, cycloisomerization of heteroatom nucleophiles linked to a π-system is one of the well recognized chemistry for the construction of numerous heterocyclic cores. Though the rudimentary aspects of this transformation are reviewed by several groups in different timeline, a holistic view on regiochemistry of such reactions went largely overlooked. Hence, this account emphasizes the gold catalyzed regioselective cycloisomerization of structurally distinctive π-connected hetero-nucleophiles leading to different heterocycles documented in the last two decades. From an application perspective, this account also highlights those methodologies which find a role in the total synthesis of natural products. Wherever appropriate, mechanistic details and contributing factors for selectivity are also discussed.

Hypothesis-Driven Palladium-Catalyzed Transformations for the Construction of New Molecular Architectures

The development of new synthetic protocols to access diverse molecular scaffolds from readily available starting compounds is of significance in both academia and industry. Towards this, the catalysis by transition metals has been employed as a powerful tool to access molecules with broad structural and functional diversity. An overview of the recent literature manifested the tremendous potential of transition metal-catalyzed processes in advancing organic synthesis in a new direction. This account compiles new conceptual advancements in the palladium-catalyzed Alder-ene type cycloisomerization reactions, C-H functionalizations, and one-pot multicatalytic processes, which have become essential tools to access new classes of molecules.

The mechanisms of gallium-catalysed skeletal rearrangement of 1,6-enynes - Insights from quantum mechanical computations

The transition metal-catalysed skeletal reorganization of 1,6-enynes can lead to three types of products - a type I product occurring via the cleavage of the alkene C-C bonds and the migration of the terminal alkene carbon to the terminus of the alkyne; a type II product arising from cleavage of both the double and the triple bonds followed by insertion of the terminal alkene carbon into the alkyne C-C triple bond; and a type III product which is obtained when there is a cleavage of the olefinic bond followed by formation of two new bonds from each carbon to each of the acetylenic carbons. The course of these reactions is highly dependent on the metal catalyst used and type of substitution at the alkene and alkyne moieties of the enyne. In this mechanistic study of the re-organization of 1,6-enynes catalysed by GaCl₃, performed at the DFT M06/6-311G(d,p) level of theory, the parent reaction selectively leads to the formation of the type I product through the formation of the open cyclopropane ring. The presence of substituents at the acetylenic moiety governs the preferred position of the metal along the alkyne bond within the pi-complex: with electron-withdrawing groups (EWGs), the metal prefers the terminal carbon while electron-donating groups (EDGs) lead to the metal preferring the internal carbon. EWGs at the alkyne moiety efficiently favour the formation of the type I product. Substituents at the olefin moiety alter the mechanism of the reaction which may favour the selective formation of the type I or III product depending on the type of substituent. EWGs at the olefinic moiety favour formation of the type III product when the alkyne moiety is unsubstituted whiles EDGs forms the type I product selectively. Solvent and temperature have no substantial effects on the energetic trends and product distribution. Hence, gas-phase calculations are deemed adequate for the problem at hand.

Computational study of Ru-catalyzed cycloisomerization of 2-alkynylanilides

The reaction mechanism of Ru-catalyzed cycloisomerization of 2-alkynylanilides to 3-substituted indole or 2-substituted indole was analyzed at the B3LYP level of density functional theory. The solvation effect of the system was also considered by SMD model. The calculation results show that the reaction system first forms a ruthenium π-alkyne complex. On the one hand, the cyclization reaction of the amino group and the alkyne carbon in the reaction precursor complex directly forms the 2-substituted indole product. On the other hand, the benzene ring in the reaction complex undergoes 1,2-carbon migration on the C≡C triple bond to form the vinylidene complex. After cyclization and hydrogen transfer, the catalyst is regenerated to obtain 3-substituted indole product. The latter reaction process has relatively lower activation free energy and is also the main reaction channel for this reaction, which is consistent with published experimental results. This study provides a reasonable reaction mechanism and effective experimental supplement.

Synthesis of furans and pyrroles via migratory and double migratory cycloisomerization reactions of homopropargylic aldehydes and imines

A novel gold-catalyzed divergent sysnthesis of furans and pyrroles employing readily available homopropargylic aldehydes and imines have been developed. The regiochemical outcome of this reaction is dependent on the substituent on the terminal alkyne of substrate. Thus, substrates possessing alkyl and aryl substituent at the alkyne moiety produce 2,3,5-substituted furans and pyrroles via a migratory cycloisomerizaton reaction. Whereas, their silicon analogues are capable to undergo a double migratory process leading to 2,3,4-substituted heterocycles.

A mild and efficient AgSbF6-catalyzed synthesis of fully substituted pyrroles through a sequential propargylation/amination/cycloisomerization reaction

Development of an efficient synthesis of fully substituted pyrroles via a sequential as a catalyst. The propargylation/amination/cycloisomerization was accomplished using AgSbF₆ one-pot three-component reaction of propargylic alcohols, 1,3-dicarbonyl compounds, and primary amines proceeds at a mild temperature, which prevents the formation of furan by-product. The reaction was also successfully applied to the more basic aliphatic amines with the addition of 1.1 eq. of acetic acid.