Gold catalyzed enantioselective intermolecular [3+2] dipolar cycloaddition of N-allenyl amides with nitrones

Enantioselective gold catalyzed dearomative [2+2]-cycloaddition between indoles and allenamides

The highly enantioselective synthesis of densely functionalized 2,3-indoline-cyclobutanes by means of chiral gold catalysis is presented. Intermolecular formal [2+2]-cycloaddition reactions between substituted indoles and allenamides enabled direct access to methylenecyclobutane-fused indolines, featuring two consecutive quaternary stereogenic centers with excellent stereochemical control (dr > 20 : 1, ee up to 99%).

Enantioselective gold-catalyzed intermolecular [2+2] versus [4+2]-cycloadditions of 3-styrylindoles with N-allenamides: observation of interesting substituent effects

The cycloaddition mode ([2+2] vs. [4+2]) can be unexpectedly switched by the simple modification of the N-substituent of the 3-styrylindoles.

A highly enantioselective [2+2] versus a [4+2]-cycloaddition of 3-styrylindoles to N-allenamides catalyzed by identical gold(i)/chiral phosphoramidite complexes is presented, which provides facile access to synthetically valuable, optically active substituted cyclobutanes and tetrahydrocarbazoles. The cycloaddition mode unexpectedly depends on the electronic nature of the N-substituent 3-styrylindoles, the origin of which could be well rationalized using DFT calculations and experimental results. To the best of our knowledge, the present work represents the first example of such an impressive substituent effect in tuning the reaction mode with high chemo-, regio- and enantioselectivity in asymmetric gold catalysis.

Regio- and stereoselective synthesis of ensulfonamides/enamides via catalyst-free intermolecular addition of indoles/pyrroles/imidazole to allenamides

A catalyst-free intermolecular addition of indoles, pyrroles, and imidazole to allenamides is reported. The reaction proceeds smoothly and provides a series of (E)-ensulfonamide/enamide derivatives in high yields with excellent regioselectivity.

Gold(i)-catalyzed asymmetric [3 + 2]-cycloadditions of γ-1-ethoxyethoxy-propiolates and aldehydes

A gold-catalyzed asymmetric intermolecular [3 + 2]-cycloaddition of γ-1-ethoxyethoxy-propiolate and aldehydes by using the chiral ligand 5a or 5b is developed, which provides facile access to highly substituted 2,5-dihydrofurans in up to 84% yield with up to 97% ee.

Gold(i)-catalyzed hydroindolylation of allenyl ethers

Nucleophilicity game: the gold(i)-catalyzed reaction/rearrangement of allenyl ethers has been investigated in the presence of indoles. The reaction outcome seems to be decided mainly by the nature of the pendant group of allenyl ether. Control experiments are indicative of an inner sphere mechanism for the hydroindolylation reaction.

Gold(I)-catalyzed intermolecular cycloaddition of allenamides with α,β-unsaturated hydrazones: efficient access to highly substituted cyclobutanes

α,β-Unsaturated N,N-dialkyl hydrazones undergo a mild [2 + 2] cycloaddition to allenamides when treated with a suitable gold catalyst. The method, which represents the first application of N,N-dialkyl hydrazones in gold catalysis, is compatible with a wide variety of substituents at the alkenyl moiety of the hydrazone component, proceeds with excellent levels of regio- and diastereoselectivity, and provides densely substituted cyclobutanes with good to excellent yields.

Meeting the challenge of intermolecular gold(I)-catalyzed cycloadditions of alkynes and allenes

The development of gold(I)-catalyzed intermolecular carbo- and hetero-cycloadditions of alkynes and allenes has been more challenging than their intramolecular counterparts. Here we review, with a mechanistic perspective, the most fundamental intermolecular cycloadditions of alkynes and allenes with alkenes.

Gold-catalyzed intermolecular synthesis of alkylidenecyclopropanes through catalytic allene activation

A stereoselective gold(I)-catalyzed intermolecular cyclopropanation of allenamides with stabilized sulfonium ylides is reported. This transformation enables the direct synthesis of diacceptor alkylidenecyclopropanes and proceeds under very mild conditions through allene activation.

Synthesis of 1,4-enamino ketones by [3,3]-rearrangements of dialkenylhydroxylamines

The synthesis of 1,4-enamino ketones has been achieved through the [3,3]-rearrangement of dialkenylhydroxylamines generated from the addition of N-alkenylnitrones to electron-deficient allenes. The mild conditions required for this reaction, and the simultaneous installation of a fluorenyl imine N-protecting group as a consequence of the rearrangement, avoid spontaneous cyclization of the 1,4-enamino ketones to form the corresponding pyrroles and allow for the isolation and controlled divergent functionalization of these reactive intermediates. The optimization, scope, and tolerance of the new method are discussed with demonstrations of the utility of the products for the synthesis of pyrroles, 1,4-diones, and furans.

Gold(I)-catalyzed enantioselective cycloaddition reactions

In recent years there have been extraordinary developments of gold(I)-catalyzed enantioselective processes. This includes progress in the area of cycloaddition reactions, which are of particular interest due to their potential for the rapid construction of optically active cyclic products. In this article we will summarize some of the most remarkable examples, emphasizing reaction mechanisms and key intermediates involved in the processes.

Mechanistic intricacies of gold-catalyzed intermolecular cycloadditions between allenamides and dienes

The mechanism of the gold-catalyzed intermolecular cycloaddition between allenamides and 1,3-dienes has been explored by means of a combined experimental and computational approach. The formation of the major [4+2] cycloaddition products can be explained by invoking different pathways, the preferred ones being determined by the nature of the diene (electron neutral vs. electron rich) and the type of the gold catalyst (AuCl vs. [IPrAu](+), IPr=1,3-bis(2,6-diisopropylphenyl)imidazole-2-ylidene). Therefore, in reactions catalyzed by AuCl, electron-neutral dienes favor a concerted [4+3] cycloaddition followed by a ring contraction event, whereas electron-rich dienes prefer a stepwise cationic pathway to give the same type of formal [4+2] products. On the other hand, the theoretical data suggest that by using a cationic gold catalyst, such as [IPrAuCl]/AgSbF6, the mechanism involves a direct [4+2] cycloaddition between the diene and the gold-activated allenamide. The theoretical data are also consistent with the observed regioselectivity as well as with the high selectivity towards the formation of the enamide products with a Z configuration. Finally, our data also explain the formation of the minor [2+2] products that are obtained in certain cases.