Formal anti-carbopalladation reactions of non-activated alkynes: requirements, mechanistic insights, and applications.

Formal anti-carbopalladation reactions of CC triple bonds are uncommon, but highly useful transformations. Alkynes can be designed to give anti-carbopalladation products. Prerequisite is the exclusion of other reaction pathways to provoke the cis-trans isomerization of the syn-carbopalladation intermediate. Detailed mechanistic studies of this crucial step by experimental and computational means were performed. Application of an intramolecular version for the synthesis of oligocyclic compounds and substituted dibenzofurans is also described.

A new golden age for donor-acceptor cyclopropanes.

The effective use of ring strain has been applied to considerable advantage for the construction of complex systems. The focus here is directed towards cyclopropanes as building blocks for organic synthesis. Although thermodynamics should take the side of synthetic chemists, only a specific substitution pattern at the cyclopropane ring allows for particularly mild, efficient, and selective transformations. The required decrease in the activation barrier is achieved by the combined effects of vicinal electron-donating and electron-accepting moieties. This Review highlights the appropriate tools for successfully employing donor-acceptor cyclopropanes in ring-opening reactions, cycloadditions, and rearrangements.

Donor-substituted nitrocyclopropanes: immediate ring-enlargement to cyclic nitronates.

The reaction of donor-substituted alkenes with α-diazo-α-nitro ethyl acetate under Rh catalysis was investigated; respective nitrocyclopropanes with a geminal ester functionality were generated in situ. Strong electron donors immediately led to ring-enlargement. In all cases, the nitro group was inserted forming cyclic nitronates whereas the ester moiety was not incorporated into the ring system. DFT studies revealed that the formation of cyclic nitronates is kinetically as well as thermodynamically favored over the formation of cyclic ketene acetals.

Symmetric and unsymmetric 3,3'-linked bispyrroles via ring-enlargement reactions of furan-derived donor-acceptor cyclopropanes.

A short and concise sequence for the synthesis of symmetrically and unsymmetrically substituted 3,3'-linked bispyrroles is described. Furan as a starting material is subjected to a twofold cyclopropanation by a diazo ester. Conversion of the ester functionalities to the respective ketones is achieved via Weinreb amide formation and the attack of a Grignard reagent. In the presence of amines the ketone moieties form imines that rearrange by ring-enlargement of the three-membered to the five-membered rings. In situ generated dihydropyrrole moieties eliminate water and afford aromatic pyrrole units. Unsymmetric bispyrroles are obtained by using different cyclopropanating agents or varying Grignard reagents for ketone formation before the addition of amines initiates the cascade to the respective bispyrroles.

Domino reactions of donor-acceptor-substituted cyclopropanes for the synthesis of 3,3'-linked oligopyrroles and pyrrolo[3,2-e]indoles.

Multiple displacement of oxygen: Electron-rich oligopyrroles and pyrrolo[3,2-e]indoles are generated by a domino process induced by donor-acceptor-substituted cyclopropanes. Up to seven molecules of water are eliminated, thus allowing the introduction of nitrogen and aromaticity.

One pot, two phases: iron-catalyzed cyclopropanation with in†situ generated diazomethane.

Tamed! The safe handling of diazomethane can be accomplished by a two-phase reaction. After being generated in aqueous media, the highly reactive species transfers to the organic phase and directly converts alkenes into cyclopropanes (see scheme). An air-stable iron(III) porphyrin complex serves as the catalyst.

Ring-enlargement reactions of donor-acceptor-substituted cyclopropanes: which combinations are most efficient?

A detailed theoretical study of ring-enlargement reactions of 72 differently substituted donor-acceptor-substituted cyclopropanes is presented. Transition states, activation barriers, and, for representative examples, the behavior in solution were additionally determined using the B3LYP/6-311G(d) level of theory.

The quest for tetracoordinated halonium ions: a theoretical investigation.

Di-, tri-, and tetravalent halonium ions have been investigated by theoretical means with respect to their geometries and their electronic structure using DFT as well as MP2 calculations. Even tricationic species where halides (F, Cl, Br, and I) are surrounded by an appropriate tetracationic hydrocarbon cage are calculated to be stable species. All halides show bonding interactions with all four cationic carbon centers.

From furan to molecular stairs: syntheses, structural properties, and theoretical investigations of oligocyclic oligoacetals.

The synthesis of oligocyclic oligoacetals using five-membered rings as repetitive unit is described. Furan was used as the starting material, which is converted by a three-step procedure consisting of twofold cyclopropanation, reduction, and oxidative ring enlargement into a tricyclic bis(enol ether). A repetition of this synthetic procedure leads to the formation of extended oligoacetal systems. Insights into the structures were gained by X-ray crystallographic investigations and revealed helical arrangements of the subunits in the solid-state. DFT (B3LYP) calculations have been carried out to elucidate the transition state of the ring enlargement and the flexibility of the annelated oligocyclic systems. Strain energies and topologies of potential cyclically condensed oligoacetals are predicted.

Caged chalcogens: theoretical studies on a tetracoordinated oxonium dication and its higher homologues.

Tetravalent chalcogenium ions utilizing a hydrocarbon cage have been investigated by theoretical means with respect to their geometries and their electronic structure using DFT as well as MP2 calculations. In all cases-even for oxygen-we predict thermodynamically stable molecular entities with chalcogens coordinated by four carbon atoms.

anti-Oligoannelated THF moieties: synthesis via push-pull-substituted cyclopropanes.

The first synthesis of anti-fused oligoannelated THF moieties is reported. The key transformation of the synthetic sequence, consisting of cyclopropanation, reduction and oxidation, is the expansion of a push-pull-substituted three-membered ring into a five-membered enol ether system. A repetition of the sequence allows the creation of oligoacetals up to a nonacyclic system.