Gold(I)-Catalyzed Intermolecular [2+2] Cycloadditions between Allenamides and Alkenes

Gold(i) operational in synergistic catalysis for the intermolecular α-addition reaction of aldehydes across allenamides

A synergistic approach to the catalytic intermolecular α-addition of aldehydes across allenamides using gold(i) and proline derivatives.

Synergistic gold and enamine catalysis: intermolecular α-alkylation of aldehydes with allenamides

Aldehydes can be α-alkylated with allenamides by the combined action of an organocatalyst and a gold complex.

Enantioselective gold-catalyzed intermolecular [2 + 2]-cycloadditions of 3-styrylindoles with N-allenyl oxazolidinone

A series of novel chiral monophosphine ligands Xiang-Phoses were designed and have been applied to the enantioselective gold-catalyzed intermolecular [2 + 2]-cycloaddition of 3-styrylindoles 1 with N-allenyl oxazolidinone 2.

Gold-catalyzed intermolecular formal (3+2) cycloaddition of stabilized vinyldiazo derivatives and electronically unbiased allenes

The gold-catalyzed reaction of electronically unbiased allenes with alkenyldiazo compounds provided alkylidenecyclopentene derivatives resulting from a formal intermolecular (3+2) carbocyclization.

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%).

Cyclobutane and cyclobutene synthesis: catalytic enantioselective [2+2] cycloadditions

Cyclobutanes and cyclobutenes are important structural motifs found in numerous biologically significant molecules, and they are useful intermediates for chemical synthesis. Consequently, [2+2] cycloadditions to access cyclobutanes and cyclobutenes have been established to be particularly useful transformations. Within the last 10 years, an increase in the frequency of publications for catalytic enantioselective [2+2] cycloadditions has occurred. These reactions provide access to a wide array of enantiomerically enriched chemical diversity that was not previously attainable. Described in this review are the advances made in catalytic enantioselective [2+2] cycloadditions to access cyclobutanes and cyclobutenes.

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.

Gold(i)-catalyzed [2 + 2 + 2] cycloaddition of allenamides, alkenes and aldehydes: a straightforward approach to tetrahydropyrans

Allenamides participate as two-carbon components in an intermolecular [2 + 2 + 2] cycloaddition with alkenes and aldehydes when treated with catalytic amounts of a phosphite gold complex. The reaction is highly regio- and chemoselective, and works with different types of alkenes, including styrenes, enol ethers or enamides, as well as with aromatic and aliphatic aldehydes. Accordingly, different types of 2,6-disubstituted tetrahydropyrans can be stereoselectively assembled in a single step from commercial or very accessible starting materials.

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 cycloisomerization of 1,6-diyne esters to 1H-cyclopenta[b]naphthalenes, cis-cyclopenten-2-yl δ-diketones, and bicyclo[3.2.0]hepta-1,5-dienes

A synthetic method to chemoselectively prepare 1H-cyclopenta[b]naphthalenes, cis-cyclopenten-2-yl δ-diketones, and bicyclo[3.2.0]hepta-1,5-dienes efficiently by gold-catalyzed cycloisomerization of 1,6-diyne esters is described. These three product classes were accessed divergently by taking advantage of the electronic and steric differences between a phosphine and NHC (NHC = N-heterocyclic carbene) ligand in the respective gold(I) complexes and that of gold(III) complex combined with substrate substitution patterns and optimized reaction conditions. In the presence of [PhCNAuIPr](+)SbF6(-) (IPr = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidine) as the catalyst, substrates with a pendant aryl group at the acetate alkynyl position were found to undergo preferential 1,3-acyloxy migration/5-exo-dig cyclization/Friedel-Crafts reaction to give 1H-cyclopenta[b]naphthalenes. In contrast, the analogous reactions with PicAuCl2 (Pic =2-picolinate) were found to proceed by selective 1,3-acyloxy migration/5-exo-dig cyclization/1,5-acyl migration to afford cis-cyclopenten-2-yl δ-diketones. Changing the catalyst to [MeCNAu(JohnPhos)](+)SbF6(-) (JohnPhos = (1,1'-biphenyl-2-yl)-di-tert-butylphosphine) and the acetate alkynyl position from an aryl to vinyl substituent in the starting ester led to 1,3-acyloxy migration/5-exo-dig cyclization/Prins-type [2 + 2]-cycloaddition to provide bicyclo[3.2.0]hepta-1,5-dienes.

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.

[4+2] and [4+3] catalytic cycloadditions of allenes

This review summarizes the different catalytic [4+2] and [4+3] cycloadditions that have been developed by using allenes as 2C, 3C or even 4C atom partners.

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.

Gold-catalyzed synthesis of tetrahydrocarbazole derivatives through an intermolecular cycloaddition of vinyl indoles and N-allenamides

A gold-catalyzed formal [4+2] cycloaddition of vinyl indoles and N-allenamides leading to tetrahydrocarbazoles is described. Moreover, new multicomponent reactions of vinyl indoles with two allene molecules are reported.

Gold catalyzed [3+2] cycloaddition of N-allenyl amides with azomethine imines

Gold(I) catalyzed [3+2] cycloaddition of azomethine imines with N-allenyl amides was developed to provide two types of pyrazolyl based bicyclic heterocycles. Both pyrazolidin-3-one derived and dihydroisoquinoline derived azomethine imines reacted smoothly with N-allenyl amides to give 6-methylene bipyrazolidin-1-ones and 1-methylene hexahydropyrazolo[5,1-a] isoquinolines in moderate to good yields.

Synthesis of functionalized allenamides from ynamides by enolate Claisen rearrangement

The Claisen rearrangement of N-Boc glycinates derived from ynamido-alcohols affords an efficient and stereoselective access to highly functionalized allenamides. These compounds undergo silver-catalyzed cyclization to 3-pyrrolines which are useful precursors for the synthesis of substituted pyrrolidines.

Electrophilic activation of unsaturated systems: Applications to selective organic synthesis

Selected examples from previous work on iodonium-triggered approaches for the functionalization of unsaturated systems, which summarize innovative transformations, are presented. This section is mostly focused on C–C bond-forming processes from alkynes that are directly bonded to relevant heteroatoms, such as iodine, silicon, or sulfur. Besides, recent advances related to iodonium-promoted C–H functionalization reactions are briefly outlined. A second section shows representative examples of our current research activity on electrophilic reactions aimed at the activation of unsaturated systems, which now are built upon the potential offered by the so-called carbophilic catalysis. More specifically, a new catalytic cyclopentannulation sequence from N-tosylimines and propargyl tosylates and novel C–H functionalization processes from related tosylates are studied. Likewise, representative examples for the intermolecular Au(I)-catalyzed [2 + 2] cycloaddition reaction of sulfonylallenamides with activated alkenes are given, including the first enantioselective reaction of this type, which is also among the first examples of an intermolecular asymmetric gold-catalyzed reaction. The discussion of the reported iodonium and gold chemistry emphasizes a search for new intermolecular processes, although intramolecular reactions are also pursued and developed.

α-Aryl-substituted allenamides in an imino-Nazarov cyclization cascade catalyzed by Au(I)

An imino-Nazarov cyclization using α-aryl-substituted allenamides is described. This gold(I)-catalyzed cascade is efficient and regioselective in constructing a diverse array of synthetically useful aromatic-ring fused cyclopentenamides. The success in this transformation represents a solution to the challenge in establishing an imino-Nazarov cyclization process.

Axially chiral triazoloisoquinolin-3-ylidene ligands in gold(I)-catalyzed asymmetric intermolecular (4 + 2) cycloadditions of allenamides and dienes

The first highly enantioselective intermolecular (4 + 2) cycloaddition between allenes and dienes is reported. The reaction provides good yields of optically active cyclohexenes featuring diverse substitution patterns and up to three stereocenters. Key to the success of the process is the use of newly designed axially chiral N-heterocyclic carbene-gold catalysts.