Nickel-catalyzed reactions involving strained bonds. 17. Nickel(0)-catalyzed reactions of bicyclo[1.1.0]butanes. Geminal two-bond cleavage reaction and the stereospecific olefin trapping of the carbenoid intermediate

Gold-Catalyzed Synthesis of Small Rings

Three- and four-membered rings, widespread motifs in nature and medicinal chemistry, have fascinated chemists ever since their discovery. However, due to energetic considerations, small rings are often difficult to assemble. In this regard, homogeneous gold catalysis has emerged as a powerful tool to construct these highly strained carbocycles. This review aims to provide a comprehensive summary of all the major advances and discoveries made in the gold-catalyzed synthesis of cyclopropanes, cyclopropenes, cyclobutanes, cyclobutenes, and their corresponding heterocyclic or heterosubstituted analogs.

Cobalt- and Silver-Promoted Methylenecyclopropane Rearrangements

The rate of the methylenecyclopropane rearrangement is enhanced by an alkyne-Co₂(CO)₆ complex bonded to the para position of a benzene ring. This is explained by a stabilizing effect on the transition state leading to the biradical intermediate. Computational studies indicate that the benzylic-type biradical intermediate is stabilized by a delocalization mechanism, where spin is delocalized onto the two cobalt atoms. Silver cation also enhances the rate of the methylenecyclopropane rearrangement. Computational studies suggest that silver cation can also stabilize a benzylic radical by spin delocalization involving silver. In the case of the silver-promoted reactions, the rate enhancements in a series of aryl-substituted methylenecyclopropanes correlate with σ⁺ values. The question remains open as to whether the silver-catalyzed methylenecyclopropane rearrangement proceeds via an argento-stabilized biradical or whether the reaction involves an argento-substituted allylic cation.

Ring-strain-enabled reaction discovery: new heterocycles from bicyclo[1.1.0]butanes

Mechanistically as well as synthetically, bicyclo[1.1.0]butanes represent one of the most fascinating classes of organic compounds. They offer a unique blend of compact size (four carbon atoms), high reactivity (strain energy of 66 kcal/mol), and mechanistic pathway diversity that can be harvested for the rapid assembly of complex scaffolds. The C(1)-C(3) bond combines the electronic features of both σ and π bonds with facile homolytic and heterolytic bond dissociation properties and thereby readily engages pericyclic, transition-metal-mediated, nucleophilic, and electrophilic pathways as well as radical acceptor and donor substrates. Despite this multifaceted reaction profile and recent advances in the preparation of bicylo[1.1.0]butanes, the current portfolio of synthetic applications is still limited compared with those of cyclopropanes and cyclobutanes. In this Account, we describe our work over the past decade on the exploration of substituent effects on the ring strain and the reactivity of bicyclo[1.1.0]butanes, particularly in the context of metal-mediated processes. We first describe Rh(I)-catalyzed cycloisomerization reactions of N-allyl amines to give pyrrolidine and azepine heterocycles. The regioselectivity of the C,C-bond insertion/ring-opening step in these reactions is controlled by the phosphine ligand. After metal carbene formation, an intramolecular cyclopropanation adds a second fused ring system. A proposed mechanism rationalizes why rhodium(I) complexes with monodentate ligands favor five-membered heterocycles, as opposed to Rh(I)-bidentate ligand catalysts, which rearrange N-allyl amines to seven-membered heterocycles. The scope of Rh(I)-catalyzed cycloisomerization reactions was extended to allyl ethers, which provide a mixture of five- and seven-membered cyclic ethers regardless of the nature of the phosphine additive and Rh(I) precatalyst. The chemical diversity of these cycloisomerization products was further expanded by a consecutive one-pot metathesis reaction. Rh(I)-catalyzed cycloisomerizations of propargyl amides, ethers, and electron-deficient bicyclo[1.1.0]butanes diverged mechanistically and often led to a significant number of decomposition products. In these cases, Pt(II) emerged as a superior, more alkynophilic late transition metal with its own mechanistic peculiarities. While monosubstituted bicyclo[1.1.0]butanes led to the formation of tetrahydropyridines, 1,3-disubstituted and electron-deficient bicyclo[1.1.0]butanes reacted distinctly differently with Pt(II) and ultimately provided a complementary set of nitrogen- and oxygen-containing cyclic scaffolds. The metal-catalyzed ring transformations of bicyclo[1.1.0]butanes presented herein suggest additional strategies for new reaction discoveries that can access a wide variety of novel cyclic frameworks from relatively simple starting materials. In addition, these case studies highlight the considerable potential for future applications in natural products, medicinal, and diversity-oriented synthesis based on the wealth of mechanistic pathways available to these strained small-ring carbocycles.

Gold(I) carbenes by retro-Buchner reaction: generation and fate

The fate of the aryl gold(I) carbenes generated by retro-Buchner reaction of ortho-substituted 7-aryl-1,3,5-cycloheptatrienes is dependent on the constitution of the ortho substituent. Indenes and fluorenes are obtained by intramolecular reaction of highly electrophilic gold(I) carbenes with alkenes and arenes. According to density functional theory calculations, the gold-catalyzed retro-Buchner process occurs stepwise, although the two carbon–carbon cleavages occur on a rather flat potential energy surface.

Cyclopropanation with gold(I) carbenes by retro-Buchner reaction from cycloheptatrienes

Cationic gold(I) promotes the retro-Buchner reaction of 7-substituted 1,3,5-cycloheptatrienes, leading to gold(I) carbenes that cyclopropanate alkenes.

Gold-catalyzed annulation/fragmentation: formation of free gold carbenes by retro-cyclopropanation

The gold(I)-catalyzed cyclization of 1-(prop-2-yn-1-yl)-2-alkenylbenzenes substituted at the benzylic position with OR groups gives 1,3-disubstituted naphthalenes with concomitant fragmentation of the alkene. One of these annulations proceeds by a retro-cyclopropanation that leads to free gold(I) carbenes.

Rhodium(I)-catalyzed cycloisomerizations of bicyclobutanes

The selection of rhodium precatalyst and phosphine ligand determines the course of the cycloisomerization of N-allylated bicyclo[1.1.0]butylalkylamines. Cyclopropane-fused pyrrolidines and azepines are obtained with high levels of stereo- and regiocontrol. Novel azatricyclo[6.1.0.0(1,5)]nonanes are the result of a tandem cycloisomerization-ring closing metathesis sequence. Allylic ethers lead to furans and oxepanes.