Catalysis of diazoalkane-carbonyl homologation. How new developments in hydrazone oxidation enable the carbon insertion strategy for synthesis

Asymmetric formal C-C bond insertion into aldehydes via copper-catalyzed diyne cyclization

The formal C-C bond insertion into aldehydes is an attractive methodology for the assembly of homologated carbonyl compounds. However, the homologation of aldehydes has been limited to diazo approach and the enantioselective reaction was rarely developed. Herein, we report an asymmetric formal C-C bond insertion into aldehydes through diyne cyclization strategy. In the presence of Cu(I)/SaBOX catalyst, this method leads to the efficient construction of versatile axially chiral naphthylpyrroles in moderate to excellent yields with good to excellent enantioselectivities. This protocol represents a rare example of asymmetric formal C-C bond insertion into aldehydes using non-diazo approach. The combined experimental and computational mechanistic studies reveal the reaction mechanism, origin of regioselectivity and stereoselectivity. Notably, the chiral phosphine ligand derived from synthesized axially chiral skeleton was proven to be applicable to asymmetric catalysis.

Copper-catalyzed intermolecular formal (5 + 1) annulation of 1,5-diynes with 1,2,5-oxadiazoles

One-carbon homologation reactions based on one-carbon insertion into the N-O bond of heterocycles have received tremendous interest over the past decades. However, these protocols have to rely on the use of hazardous and not easily accessible diazo compounds as precursors, and examples of the relevant asymmetric catalysis have not been reported. Here we show that a copper-catalyzed intermolecular formal (5 + 1) annulation of 1,5-diynes with 1,2,5-oxadiazoles involving one-carbon insertion into the heterocyclic N-O bond via non-diazo approach. This method enables practical and atom-economic synthesis of valuable pyrrole-substituted oxadiazines in generally moderate to good yields under mild reaction conditions. In addition, the possibility of such an asymmetric formal (5 + 1) annulation also emerges.

Rhodium-Catalyzed (4+1) Cycloaddition between Benzocyclobutenones and Styrene-Type Alkenes

Herein, we describe a unique one-carbon ring-expansion strategy to access multi-substituted 2-indanones from benzocyclobutenones and styrene-type olefins. The use of a cationic "ligandless" rhodium catalyst was the key for both high reactivity and selectivity towards the (4+1) product. Broad functional group tolerance, a good substrate scope, and scalability have been demonstrated. Computation studies reveal that the origin of the (4+1) selectivity is due to a facile β-H elimination pathway that reduces the overall barrier of the turnover-limiting C-C reductive elimination step.

Intramolecular One-Carbon Homologation of Unstrained Ketones via C-C Activation-Enabled 1,1-Insertion of Alkenes

Here, we describe the development of a Rh-catalyzed intramolecular one-carbon homologation of unstrained aryl ketones through a formal 1,1-insertion process of olefins, enabled by temporary directing group (TDG)-aided C-C activation. The reaction provides a distinct approach to access various substituted 1-indanones. Computational mechanistic studies reveal that the formal 1,1-insertion is realized by a selective C(sp²)-C(sp³) activation and turnover limiting 2,1-insertion into the alkene, followed by a facile β-H elimination and reinsertion process.

Asymmetric Catalytic Rearrangements with α-Diazocarbonyl Compounds

α-Diazocarbonyl compounds serve as nucleophiles, dipoles, carbene precursors, and rare electrophiles, enabling a vast array of organic transformations under the influence of metal catalysts. Among them, rearrangement processes are attractive and provide straightforward and efficient accesses to one-carbon extension adducts or heteroatom-containing molecules. The reactions occur upon the release of dinitrogen after nucleophilic addition or before ylide formation. Although significant progress has been made for these two types of rearrangement reactions, the issue of enantiocontrol is challenging because the final optically enriched products are generated via multistep transformations and the inherent spacial arrangement of the intermediates has more or less influence on the regio- and enantioselectivity.In this Account, we collected several rearrangements of α-diazocarbonyl compounds, showcasing the efficient catalysts and tailored strategies for tackling enantioselective varieties of these two types of rearrangement reactions. Our research group initiated the catalytic asymmetric reactions of α-diazocarbonyl compounds during the development of chiral Feng N,N'-dioxide-metal complex catalysts and others. As a kind of useful chiral Lewis acid catalyst chiral N,N'-dioxide-metal complexes are favorable for the activation of various carbonyl compounds, accelerating the diastereo- and enantioselective nucleophilic addition of α-diazoesters and the sequential rearrangements in either an intermolecular or intramolecular manner. Aldehydes, acyclic and cyclic ketone derivatives, and α,β-unsaturated ketones could participate in efficient asymmetric homologation reactions, and an obvious ligand-acceleration effect is observed in these processes. For example, the Roskamp-Feng reaction of aldehydes gives optically active β-ketoesters through a H-shift, overwhelming the aryl group shift or oxygen attack. The shift preference and enantiocontrol in the homologation of acyclic and cyclic ketone derivatives could be under excellent control of the chiral catalysts. An unusual electrophilic α-amination of aryl/alkyl ketones and even a complicated homologation/dyotropic rearrangement/interconversion/[3 + 2] cycloaddition cascade used to construct dimeric polycyclic compounds were discovered as a result of the selection of chiral ligands and additives. On the basis of the understanding of the interaction of the functional group with N,N'-dioxide-metal complexes in catalysis and the key enantio-determining issues in ylide-based rearrangements, we designed new α-diazocarbonyl compounds by introducing a pyrazole-1-carboxyl group as the acceptor unit, which could benefit the formation of both carbenoid species and the chiral catalyst-bound ylides to deliver stereoselectivity. Taking advantage of Ni(II) or Co(II) complexes of Feng N,N'-dioxide ligands, we realized several kinds of enantioselective [2,3]-sigmatropic rearrangements, such as the Doyle-Kirmse reaction with allylic sulfides or selenides, [2,3]-Stevens rearrangements of vinyl-substituted α-diazo pyrazoleamides with thioacetates, Sommelet-Hauser rearrangements of aryl-substituted α-diazo pyrazoleamides with thioamides, and thio-Claisen rearrangements of 2-thio-indoles as well. Moreover, this strategy was shown to be applicable to highly γ-selective and enantioselective insertion into N-H bonds of secondary amines with vinyl-substituted α-diazo pyrazoleamides.

Ag-Catalyzed Insertion of Alkynyl Carbenes into C-C Bonds of β-Ketocarbonyls: A Formal C(sp2) Insertion

Here we report a silver-catalyzed alkynyl carbene insertion into β-ketocarbonyls using alkynyl N-nosylhydrazones as alkynyl carbene precursors, which provides access to trisubstituted allenyl ketones. This reaction represents the first example of an alkynyl carbene insertion into a C-C σ bond, affording products homologated with an sp² carbon center. The products are useful substrates for further transformations. Experimental investigations and theoretical calculations suggest the reaction proceeds through a stepwise enol cyclopropanation/retro-aldol pathway.

Catalytic Asymmetric Homologation of Ketones with α-Alkyl α-Diazo Esters

The homologation of ketones with diazo compounds is a useful strategy to synthesize one-carbon chain-extended acyclic ketones or ring-expanded cyclic ketones. However, the asymmetric homologation of acyclic ketones with α-diazo esters remains a challenge due to the lower reactivity and complicated selectivity. Herein, we report the enantioselective catalytic homologation of acetophenone and related derivatives with α-alkyl α-diazo esters utilizing a chiral scandium(III) N,N'-dioxide as the Lewis acid catalyst. This reaction supplies a highly chemo-, regio-, and enantioselective pathway for the synthesis of optically active β-keto esters with an all-carbon quaternary center through highly selective alkyl-group migration of the ketones. Moreover, the ring expansion of cyclic ketones was accomplished under slightly modified conditions, affording a series of enantioenriched cyclic β-keto esters. Density functional theory calculations have been carried out to elucidate the reaction pathway and possible working models that can explain the observed regio- and enantioselectivity.

Chiral Lewis acid-catalyzed enantioselective cyclopropanation and C-H insertion reactions of vinyl ketones with α-diazoesters

Enantioselective cyclopropanation and C-H insertion reactions of α-substituted vinyl ketones with α-diazoesters have been accomplished using a N,N'-dioxide-scandium(iii) complex catalyst. Various tetrasubstituted cyclopropanes and E-enone derivatives bearing a chiral ester substituent were obtained simultaneously with good yields and excellent enantioselectivities.

General Methodologies Toward cis-Fused Quinone Sesquiterpenoids. Enantiospecific Synthesis of the epi-Ilimaquinone Core Featuring Sc-Catalyzed Ring Expansion

A stereocontrolled approach to the cis-decalin framework of clerodane diterpenes and biologically active quinone sesquiterpenes is reported. Starting from an inexpensive optically pure tetrahydroindanone, Birch reductive alkylation builds two new contiguous chiral centers—one of which is quaternary and all-carbon-substituted. Also featured is a highly regioselective diazoalkane—carbonyl homologation reaction to prepare the 6,6-bicyclic skeleton. Therein, the utility of Sc(OTf)₃ as a mild catalyst for formal 1C insertion in complex settings is demonstrated.

First Insertions of Carbene Ligands into Ge-N and Si-N Bonds

The insertion of carbene ligands into Ge-N (three examples) and Si-N (one example) bonds has been achieved for the first time by treating Fischer carbene complexes (M=W, Cr) with bulky amidinatotetrylenes (E=Ge, Si). These reactions, which start with a nucleophilic attack of the amidinatotetrylene heavier group 14 atom to the carbene C atom, proceed through a stereoselective insertion of the carbene fragment into an E-N bond of the amidinatotetrylene ENCN four-membered ring, leading to [M(CO)₅ L] derivatives in which L belongs to a novel family of tetrylene ligands comprising an ECNCN five-membered ring.

Diverse Natural Products from Dichlorocyclobutanones: An Evolutionary Tale

11-Nor PGE2 was prepared in our laboratory several years ago and used to obtain the corresponding ring-expanded γ-butyrolactam, γ-butyrolactone, and cyclopentanone derivatives. The conversion of a cyclobutanone into a cyclopentanone had relatively little precedent and merited further study. It was soon found that the presence of a single chlorine adjacent to the carbonyl not only greatly accelerated the reaction with ethereal diazomethane, but also substantially enhanced its regioselectivity; not surprisingly, a second chlorine further increased both. The confluence of this finding and the discovery by Krepski and Hassner that the presence of phosphorus oxychloride significantly improved the Zn-mediated dehalogenation procedure for the preparation of α,α-dichlorocyclobutanones from olefins provided the starting point for decades' worth of exciting adventures in natural product synthesis. A wide variety of naturally occurring 5-membered carbocycles (e.g., hirsutanes, cuparenones, bakkanes, guaianolides, azulenes) could thus be prepared by using dichloroketene-olefin cycloaddition, followed by regioselective one-carbon ring expansion with diazomethane. Importantly, it was also found that natural γ-butyrolactones (e.g., β-oxygenated γ-butyrolactones, lactone fatty acids) could be secured through regioselective Baeyer-Villiger oxidation of cycloadducts with m-CPBA and that naturally occurring γ-butyrolactam derivatives (e.g., amino acids, pyrrolidines, pyrrolizidines, indolizidines) could be efficiently obtained by regioselective Beckmann ring expansion of the adducts with O-(mesitylenesulfonyl)hydroxylamine (Tamura's reagent). These 5-membered carbocycles, γ-butyrolactones, and γ-butyrolactam derivatives were generally secured in enantiopure form through the use of either intrinsically chiral olefins or olefins bearing Stericol, a highly effective chiral auxiliary developed specifically for this "three-atom olefin annelation" approach. In addition, considerable useful chemistry has been developed in the context of this synthesis program. This includes new methods for olefin vicinal dicarboxylation, β-methylene-γ-butyrolactonization, γ-butyrolactone and δ-valerolactone α-methylenations, transesterification, angelic ester synthesis, chiral enol and ynol ether preparations, dichloroacetylene synthesis, and trans, trans hydroxy triad introduction. This versatile dichlorocyclobutanone-centered approach to natural product synthesis, together with the attendant new methods that have been developed, forms the basis of this Account, which is presented as an evolutionary tale. It is hoped that the Account will stimulate other research groups to seek to exploit the rich chemistry of dichlorocyclobutanones for possible solutions to problems in organic synthesis.

Rh-catalyzed Reagent-Free Ring Expansion of Cyclobutenones and Benzocyclobutenones

Here we report a reagent-free rhodium-catalyzed ring-expansion reaction via C-C cleavage of cyclobutenones. A variety of poly-substituted cyclopentenones and 1-indanones can be synthesized from simple cyclobutenones and benzocyclobutenones. The reaction condition is near pH neutral without additional oxidants or reductants. The potential for developing a dynamic kinetic asymmetric transformation of this reaction has also been demonstrated. Further study supports the proposed pathway involving Rh-insertion into the cyclobutenone C-C bond, followed by β-hydrogen elimination, olefin insertion and reductive elimination.

Direct activation of relatively unstrained carbon-carbon bonds in homogeneous systems

New modes of chemical reactivity are of high value to synthetic organic chemistry. In this vein, carbon-carbon (C-C) activation is an emerging field that offers new possibilities for synthesizing valuable complex molecules. This review discusses the pioneering stoichiometric discoveries in this field up to the most recent synthetic applications that apply catalytic transformations. Specifically, the review focuses on C-C activation in relatively unstrained systems, including stoichiometric reactions, chelation-directed and chelation-free catalytic reactions. While the field of C-C activation of relatively unstrained systems is underdeveloped, we expect that this review will provide insight into new developments and pave the path for robust, practical applications.