Bicyclopropylidene — A Unique Tetrasubstituted Alkene and a Versatile C6-Building Block

Tandem Wittig Reaction-Ring Contraction of Cyclobutanes: A Route to Functionalized Cyclopropanecarbaldehydes

An original tandem reaction consisting of a Wittig reaction-ring contraction process between α-hydroxycyclobutanone and phosphonium ylides has been developed. Highly functionalized cyclopropanecarbaldehydes are obtained in good to high yield.

Superbase-promoted selective carbon-carbon bond cleavage driven by aromatization

A novel selective carbon-carbon single bond cleavage has been disclosed through the copper-catalyzed reaction of 1-alkyl-3-alkylindolin-2-imine hydrochlorides with substituted 1-(bromomethyl)-2-iodobenzenes leading to fused N-heterocycles. Mechanistic studies showed that the intrinsic drive of aromatization and the action of the superbase derived from sodium tert-butoxide and dimethylsulfoxide were the key factors leading to the carbon-carbon single bond cleavage. Furthermore, the obtained N-heterocycles are indoloquinoline derivatives with wide biological activities.

Thermally-induced intramolecular [2 + 2] cycloaddition of acrylamide-tethered alkylidenecyclopropanes

An efficient and highly regio- and diastereoselective synthetic method to cyclobutane-containing spiro[2.3]hexane fused with six-membered hetero-cycles has been disclosed via a thermally-induced intramolecular [2 + 2] cycloaddition of acrylamide-tethered alkylidenecyclopropanes. The DFT calculations indicate that this intramolecular cycloaddition proceeds in a concerted manner and account for the product selectivity. These reactions exhibited excellent yields and functional group tolerance under metal free conditions.

Regio- and Stereoselective Synthesis of Functionalized Dihydropyridines, Pyridines, and 2H-Pyrans: Heck Coupling of Monocyclopropanated Heterocycles

A palladium-catalyzed coupling between aryl halides and monocyclopropanated pyrroles or furans has been developed, leading to valuable six-membered N- and O-heterocycles. As the key step, a selective cleavage of the non-activated endocyclic C-C bond of the 2-heterobicyclo-[3.1.0]hexane framework is achieved. The developed method offers access to highly functionalized piperidines, pyridines, and pyrans that are challenging to access by traditional methods.

Highly Stereoselective Synthesis of Tetrasubstituted Acyclic All-Carbon Olefins via Enol Tosylation and Suzuki-Miyaura Coupling

A highly stereocontrolled synthesis of tetrasubstituted acyclic all-carbon olefins has been developed via a stereoselective enolization and tosylate formation, followed by a palladium-catalyzed Suzuki-Miyaura cross-coupling of the tosylates and pinacol boronic esters in the presence of a Pd(OAc)2/RuPhos catalytic system. Both the enol tosylation and Suzuki-Miyaura coupling reactions tolerate an array of electronically and sterically diverse substituents and generate high yield and stereoselectivity of the olefin products. Judicious choice of substrate and coupling partner provides access to either the E- or Z-olefin with excellent yield and stereochemical fidelity. Olefin isomerization was observed during the Suzuki-Miyaura coupling. However, under the optimized cross-coupling reaction conditions, the isomerization was suppressed to <5% in most cases. Mechanistic probes indicate that the olefin isomerization occurs via an intermediate, possibly a zwitterionic palladium carbenoid species.

Rhodium(II)-catalyzed intramolecular cycloisomerizations of methylenecyclopropanes with N-sulfonyl 1,2,3-triazoles

A novel rhodium(II)-catalyzed tandem cycloisomerization of methylenecyclopropanes (MCPs) with N-sulfonyl 1,2,3-triazoles is disclosed. The reaction produces a series of highly functionalized polycyclic N heterocycles via a rhodium imino carbene intermediate. A distinct feature of this divergent synthesis is that different types of substrates control the reaction pathways. Moreover, several interesting transformations of these products to construct diazabicyclo[3.2.1]octane derivatives are also reported.

Coordination chemistry at carbon: the patchwork family comprising (Ph3P)2C, (Ph3P)C(C2H4), and (C2H4)2C

The revitalized concept of "coordination at carbon" allows relationships between seemingly unrelated families of carbon-centered compounds to be discovered generating fascinating patchwork families of compounds. It is shown how olefins and cyclopropanes can be regarded as donors for carbon acceptors C(1), C(2), and C(3). Through this approach, hydrocarbons such as spiropentane and dicyclopropylidene are found to be counterparts of the bis-ylidic carbodiphosphoranes and the corresponding mixed mono-ylidic systems.

Use of sulfur fragments for the synthesis of nitrogen heterocycles

This contribution contains a representative sampling from the Padwa laboratory of the conjugate addition of oximes with 2,3-bis(phenylsulfonyl)-1,3-butadiene followed by a subsequent dipolar cycloaddition cascade to produce a variety of alkaloids. The resulting cycloadducts are cleaved reductively to provide azapolycyclic scaffolds with strategically placed functionality for further manipulation of the target compounds.

Strained small rings in gold-catalyzed rapid chemical transformations

Gold-catalyzed reactions, which have been widely explored over the past several years, are powerful tools in organic synthesis to access complex molecular frameworks, and some corresponding excellent reviews have been reported. However, little attention has been paid to summarize the reactions of strained small-ring-containing molecules catalyzed by gold. This critical review mainly puts its emphasis on the recent progress in the field of gold-catalyzed transformations of cyclopropyl-, cyclopropenyl-, epoxy- and aziridinyl-containing molecules. The rapid construction of interesting building blocks in organic synthesis from strained small rings catalyzed by gold has been summarized in this review (106 references).

Selectivity in metal-catalyzed carbon-carbon bond cleavage of alkylidenecyclopropanes

When more complex system leads to simpler reactivity profile; the ring-opening of strained three-membered rings such as methylene- and alkylidenecyclopropanes generally lead to several products. If one starts with more functionalized carbon skeletons, selective reactions are now observed and rationalization as well as synthetic applications are described in this concept article. This methodology could be used to the preparation of challenging structural motifs possessing quaternary carbon stereocenters in acyclic systems.