Substrate-oriented selectivity in the Mg-mediated conjugate addition of bromoform to electron-deficient alkenes

The Mg-mediated conjugate addition of bromoform to a variety of electron-deficient alkenes has been investigated. In the case of nitrodienes and dibenzylideneacetones, tribromomethylated products were isolated, whereas spiro-cyclopropanated products were obtained with cyclic dibenzylideneketones and 3-olefinic oxindoles. The spiro-cyclopropyl ketones derived from cyclic dibenzylideneketones were successfully transformed into fused furans via the Cloke-Wilson rearrangement.

Ruthenium Catalyzed Rearrangement of Ene-cyclopropenes. Divergent Reaction Pathways

The reaction of ene-cyclopropenes with Cp*RuCl(cod) leads to alkenyl bicyclo[3.1.0]hexanes, bicyclo[4.1.0]heptanes, and bicyclo[5.1.0]octanes. This reaction involves a reverse regioselectivity in the cyclopropene opening than with gold chlorides. With gem-disubstituted cyclopropenes, a novel cycloisomerization based on ring-opening nucleophilic attack and rearrangement is observed. Alternatively, some gem-disubstituted cyclopropenes give dimerizations of the intermediate carbene.

Synthesis of fused cyanopyrroles and spirocyclopropanes via addition of N-ylides to chalconimines

DABCO-ylides react as a one-carbon source with chalconimines to afford fused cyanopyrrolesvia[4 + 1] annulation and spirocyclopropanesvia[2 + 1] annulation.

Pd-Catalyzed Carboamination of Alkylidenecyclopropanes: Synthesis of Congested Pyrrolidines and Piperidines

The synthesis of densely functionalized N-heterocycles is an ongoing challenge in chemical synthesis. Herein, we report an efficient method for the construction of pyrrolidine and piperidine scaffolds using a palladium-catalyzed carboamination of alkylidenecyclopropanes.

Mechanistic Understanding of the Divergent Reactivity of Cyclopropenes in Rh(III)-Catalyzed C-H Activation/Cycloaddition Reactions of N-Phenoxyacetamide and N-Pivaloxybenzamide

Density functional theory calculations were conducted to develop a mechanistic understanding of the Rh(III)-catalyzed C-H activation/cycloaddition reactions of N-phenoxyacetamide and N-pivaloxybenzamide with cyclopropenes, and insights into the substrate-dependent chemoselectivity were provided. The results showed that the divergence originated from the different reactivity of the seven-membered rhodacycles from the insertion of cyclopropene into the Rh-C bond. In reactions of N-pivaloxybenzamide, such an intermediate undergoes the pivalate migration to form a cyclic Rh(V)-nitrenoid intermediate in a reaction that is easier than the opening of the three-membered ring by β-carbon elimination, leading finally to a tricyclic product with retention of the cyclopropane moiety by facile reductive elimination. While similar Rh(V)-nitrenoid species could also be possibly formed in Cp*Rh(III)-catalyzed reactions of N-phenoxyacetamide, the β-carbon elimination occurs more easily from the corresponding seven-membered rhodacycle intermediate and the subsequent O-N bond cleavage gives rise to an unexpected dearomatized (E)-6-alkenylcyclohexa-2,4-dienone intermediate. The E/Z isomerization of this intermediate is required for the final cyclization to 2H-chromene, and interesting metal-ligand cooperative catalysis with Rh(III) carboxylate was disclosed in the C═C double bond rotation process.

Reaction of cyclopropenes with a trichloromethyl radical: unprecedented ring-opening reaction of cyclopropanes with migration

The triethylborane-mediated direct addition of chloroform to cyclopropenes afforded trichloromethylcyclopropanes. In contrast, the use of dimethylzinc gave the unconjugated diene via ring-opening reactions.

Gold-catalyzed intermolecular anti-markovnikov hydroamination of alkylidenecyclopropanes

The cationic gold phosphine complex [{PCy2 (o-biphenyl)}Au(NCMe)](+) SbF6 (-) (Cy=cyclohexyl) catalyzes the intermolecular, anti-Markovnikov hydroamination reaction of monosubstituted and cis- and trans-disubstituted alkylidenecyclopropanes (ACPs) with imidazolidin-2-ones and other nucleophiles. This reaction forms 1-cyclopropyl alkylamine derivatives in high yield and with high regio- and diastereoselectivity. NMR spectroscopic analysis of gold π-ACP complexes and control experiments point to the sp hybridization of the ACP internal alkene carbon atom as controlling the regiochemistry of the ACP hydroamination reaction.

Electrophile-induced C-C bond activation of vinylcyclopropanes for the synthesis of Z-alkylidenetetrahydrofurans

We present a detailed study on the behavior of vinylcyclopropanes as masked donor-acceptor system toward the stereoselective synthesis of Z-alkylidenetetrahydrofurans. Results of bromenium catalyzed indirect activation of C-C bond of vinylcyclopropanes and concomitant cyclization to alkylidenetetrahydrofuran and other heterocycles have been discussed. The stereoselective formation of the Z-isomer is strongly controlled by the extent of destabilization of one of the gauche conformers of the vinylcyclopropane. The ring-opening/cyclization step was found to be stereospecific as in the case of DA cyclopropanes. The activation of the C-C bond leads to a tight-carbocation intermediate, which is evident from the complete retention of the stereochemistry. The retention of configuration has been established by a necessary control experiment that rules out the possibility of a double inversion pathway. The present results serve as direct stereochemical evidence in support of a tight ion-pair intermediate versus the controversial S(N)2 pathway. A 2D potential energy scan has been carried out at B3LYP/6-31G(d) level theory to obtain the relative energies of the conformers. The Z-selectivity observed has been explained on the basis of the relative population of the conformers and modeling the intermediate and transition state involved in the reaction at M06-2x/6-31+G(d) level. Energy profile for the cyclization step was modeled considering various possible pathways through which cyclization can happen. The methodology has been successfully demonstrated on vinylcyclobutanes as well.

Recent developments of cyclopropene chemistry

This critical review discusses recent developments in the field of cyclopropene chemistry. Although several excellent reviews that mainly focused on the thermolysis and pyrolysis as well as metal-mediated reactions of cyclopropenes have been published, significant new developments have also been achieved in recent years. This brand new review provides an overview of the progress from 2007 to 2011 on the syntheses and transformations of cyclopropenes as well as their related mechanistic studies (238 references).