Cyclopropenes in the 1,3-dipolar cycloaddition with carbonyl ylides: experimental and theoretical evidence for the enhancement of sigma-withdrawal in 3-substituted-cyclopropenes

The carbonyl ylide dipoles generated by the dirhodium tetra-acetate-catalyzed decomposition of diazocarbonyl precursors 1, 5, and 8 cycloadd to 3-substituted 1,2-diphenylcyclopropenes 3a-e and 3,3-disubstituted cyclopropenes 13, 14, 19, and 20 to give polycyclic compounds with 8-oxatricyclo[3.2.1.0(2,4)]octane and 9-oxatricyclo[3.3.1.0(2,4)]nonane frameworks. Generally, reactions proceed stereoselectively to give adducts of exo stereochemistry with the approach of the carbonyl ylide dipoles from the less-hindered face of cyclopropenes. The electronic properties of the substituent at the C3 position of cyclopropenes play an important role in governing the reactivity of cyclopropenes: when the C3 position is substituted by electron-acceptors such as the methoxycarbonyl or cyano groups, the yields of adducts are decreased significantly or no adducts can be detected at all. Relative reactivities of cyclopropenes were quantified by competition experiments to give the best correlation with sigmaF-Taft constants. Both measured photoelectron spectra and ground-state calculations of a series of 1,2-diphenylcyclopropenes indicate considerable lowering of cyclopropene pi-HOMO energies by substitution with an acceptor group. Such changes in electronic structures of cyclopropenes may cause the inversion of frontier molecular orbital (FMO) interactions from HOMO(cyclopropene)-LUMO(ylide) to LUMO(cyclopropene)-HOMO(ylide) type. In terms of philicity, nucleophilic properties of acceptor-substituted cyclopropenes are diminished to such an extent that these species are no longer good nucleophiles in the reaction with carbonyl ylides, and neither are they good electrophiles, being unreactive. This was shown by the B3LYP calculations of addends.     

Silver‐Catalyzed Allylation of Ketones and Intramolecular Cyclization through Carbene Intermediates from Cyclopropenes Under Ambient Conditions

Tandem C?C bond formation was achieved through silver‐catalyzed ring‐opening of cyclopropenes via carbene intermediates. The reaction of cyclopropenes in the presence of a silver catalyst gave indene derivatives under ambient conditions. In contrast, the insertion of organozinc reagents to silver carbene or allylic cation intermediates afforded allylmetal intermediates for the tandem allylation of carbonyl compounds.     

Lipidated cyclopropenes via a stable 3-N spirocyclopropene scaffold

Lipidated cyclopropenes serve as useful bioorthogonal reagents for imaging cell membranes due to the cyclopropene’s small size and ability to ligate with pro-fluorescent tetrazines. Previously, the lipidation of cyclopropenes required modification at the C3 position because methods to append lipids at C1/C2 were not available. Herein, we describe C1/C2 lipidation with the biologically active lipid ceramide and a common phospholipid using a cyclopropene scaffold whose reactivity with 1,2,4,5-tetrazines has been caged.     

Rhodium(III)‐Catalyzed Transannulation of Cyclopropenes with N‐Phenoxyacetamides through C?H Activation

An efficient rhodium(III)‐catalyzed synthesis of 2H‐chromene from N‐phenoxyacetamides and cyclopropenes has been developed. The reaction represents the first example of using cyclopropenes as a three‐carbon unit in rhodium(III)‐catalyzed C(sp²) H activations.     

Rhodium(III)‐Catalyzed Transannulation of Cyclopropenes with N‐Phenoxyacetamides through CH Activation

An efficient rhodium(III)‐catalyzed synthesis of 2H‐chromene from N‐phenoxyacetamides and cyclopropenes has been developed. The reaction represents the first example of using cyclopropenes as a three‐carbon unit in rhodium(III)‐catalyzed C(sp²)? H activations.     

Main fragmentation routes of functionally substituted cyclopropenes under electron impact

The behaviour of substituted cyclopropenes under electron impact is to a large extent determined by the presence of the unsaturated three-membered ring which is capable of efficient delocalization of the positive charge. The loss of one of the substituents at the C(3) position of the small ring is characteristic for the fragmentation of cyclopropenes; the loss of the substituent which is less electron donating occurs preferentially. The presence of substituents with heteroatoms on the three-membered ring may lead to changes in the fragmentation scheme characteristic for the specific set of substituents.     

Nitrone cycloadditions to 1,2-diphenylcyclopropenes and subsequent transformations of the isoxazolidine cycloadducts

1,3-Dipolar cycloaddition of C-aryl,N-aryl (or N-methyl) nitrones with a number of 1,2-diphenylcyclopropenes substituted at the C(3) position occurs with the formation of expected "normal" cycloadducts (with N-methylnitrones) and products of their subsequent transformations. Among them are corresponding alpha-acetophenyl aziridines and tetra (or penta) -arylpyrroles. Aziridines and the normal cycloadducts can be also thermally converted to such arylpyrroles with moderate to good yields. Substitution at the C(3( position of cyclopropenes by an electron acceptor group decreases the reactivity of cyclopropenes.     

Catalytic Synthesis of Trifluoromethyl Cyclopropenes and Oligo-Cyclopropenes

The synthesis of trifluoromethylated cyclopropenes is often associated with important applications in drug discovery and functional materials. In this report, we describe the use of readily available chiral rhodium(II) catalysts for a highly efficient asymmetric cyclopropenation reaction of fluorinated donor–acceptor diazoalkanes with a broad variety of aliphatic and aromatic alkynes. Further studies highlight the unique reactivity of fluorinated donor–acceptor diazoalkanes in the synthesis of oligo-cyclopropenes. Subsequent C−H functionalization of trifluoromethyl cyclopropenes furnishes densely substituted cyclopropene frameworks and also allows the alternative synthesis of bis-cyclopropenes.     

Dinuclear gold-catalyzed C-H bond functionalization of cyclopropenes

Science China Chemistry - We report an unprecedented C–H bond functionalization of cyclopropenes enabled by dinuclear gold catalysis. Highly selective C–H allylation, alkynylation and...     

Direct palladium-catalyzed arylation of cyclopropenes

The first examples of direct palladium-catalyzed arylation and heteroarylation of cyclopropenes have been demonstrated. This method allows for efficient synthesis of various tetrasubstituted cyclopropenes, incuding nonracemic cyclopropenes, which are not available via known asymmetric cyclopropenation methods. Mechanistic studies strongly suggest an electrophilic path for this Heck-type transformation.     

Nucleophilic substitution reactions of 1-sulfonyl substituted cyclopropenes with alkyl lithium reagents

Sulfonyl substituted cyclopropenes react with a variety of alkyl lithium reagents to give disubstituted cyclopropenes in good yield.     

Copper-catalyzed silylation of cyclopropenes using (trifluoromethyl)trimethylsilane

A variety of cyclopropenes undergo direct silylation using (trifluoromethyl)trimethylsilane in the presence of a copper-bisphosphine catalyst; under these conditions, cyclopropenes that might otherwise undergo ring-opening are silylated efficiently.     

Synthesis and application of alkenylstannanes derived from base-sensitive cyclopropenes

In the presence of stoichiometric potassium fluoride, a range of base-sensitive cyclopropenes undergo direct stannylation using (pentafluoroethyl)tributylstannane. The resulting stannylcyclopropenes serve as precursors to a variety of tetrasubstituted cyclopropenes that might otherwise be difficult to access using alternative methods.     

Integral intensities and polarity of vinylic CH bonds of 3,3-disubstituted cyclopropenes

The influence of substituents on the polarity of vinylic CH bonds of 3,3-disubstituted cyclopropenes was studied by analysis of the integral absorption intensities of infrared bands. The influence of intermolecular interactions in solutions of cyclopropenes in non-polar solvents on integral intensities has been determined. The dipole moments of the vinylic bonds were determined starting from infrared band intensities. The experimental data were compared with CNDO/S3 calculations. High sensitivity of intensities and frequencies of the absorption bands to the changes in electron acceptor character of substituent has been found. Correlations were established between dipole moment values of vinylic bonds, determined from experiment and on the basis of CNDO/S3 calculations, as well as between stretching vibrational frequencies on the one hand and values of substituent constants on the other.     

Controllable Si−C Bond Activation Enables Stereocontrol in the Palladium-Catalyzed [4+2] Annulation of Cyclopropenes with Benzosilacyclobutanes

A novel and unusual palladium-catalyzed [4+2] annulation of cyclopropenes with benzosilacyclobutanes is reported. This reaction occurred through chemoselective Si−C(sp²) bond activation in synergy with ring expansion/insertion of cyclopropenes to form new C(sp²)−C(sp³) and Si−C(sp³) bonds. An array of previously elusive bicyclic skeleton with high strain, silabicyclo[4.1.0]heptanes, were formed in good yields with excellent diastereoselectivity under mild conditions. An asymmetric version of the reaction with a chiral phosphoramidite ligand furnished a variety of chiral bicyclic silaheterocycle derivatives with good enantioselectivity (up to 95.5:4.5 er). Owing to the mild reaction conditions, the good stereoselectivity profile, and the ready availability of the functionalized precursors, this process constitutes a useful and straightforward strategy for the synthesis of densely functionalized silacycles.     

Enantioselective palladium-catalyzed carbozincation of cyclopropenes

A highly enantioselective palladium-catalyzed carbozincation of cyclopropenes has been developed. The intermediate cyclopropylzinc species, after transmetalation with copper, were trapped with various electrophiles. This one-pot procedure furnished functionalizied cyclopropenes with excellent diastereo- and enantioselectivity.     

Controllable Si−C Bond Activation Enables Stereocontrol in the Palladium‐Catalyzed [4+2] Annulation of Cyclopropenes with Benzosilacyclobutanes

A novel and unusual palladium‐catalyzed [4+2] annulation of cyclopropenes with benzosilacyclobutanes is reported. This reaction occurred through chemoselective Si?C(sp²) bond activation in synergy with ring expansion/insertion of cyclopropenes to form new C(sp²)?C(sp³) and Si?C(sp³) bonds. An array of previously elusive bicyclic skeleton with high strain, silabicyclo[4.1.0]heptanes, were formed in good yields with excellent diastereoselectivity under mild conditions. An asymmetric version of the reaction with a chiral phosphoramidite ligand furnished a variety of chiral bicyclic silaheterocycle derivatives with good enantioselectivity (up to 95.5:4.5 er). Owing to the mild reaction conditions, the good stereoselectivity profile, and the ready availability of the functionalized precursors, this process constitutes a useful and straightforward strategy for the synthesis of densely functionalized silacycles.     

Rhodium-catalyzed hydroformylation of cyclopropenes

The first catalytic diastereo- and enantioselective hydroformylation of cyclopropenes was demonstrated. The reaction proceeds efficiently under very mild conditions and low catalyst loadings providing high yields of cyclopropylcarboxaldehydes. This novel methodology represents a convenient, atom-economic approach toward optically active cyclopropylcarboxaldehydes from readily available prochiral cyclopropenes.     

Copper-Catalyzed Enantio- and Diastereoselective Addition of Silicon Nucleophiles to 3,3-Disubstituted Cyclopropenes

A highly stereocontrolled syn-addition of silicon nucleophiles across cyclopropenes with two different geminal substituents at C3 is reported. Diastereomeric ratios are excellent throughout (d.r.≥98:2) and enantiomeric excesses usually higher than 90 %, even reaching 99 %. This copper-catalyzed C−Si bond formation closes the gap of the direct synthesis of α-chiral cyclopropylsilanes.     

Synthesis of polyfunctionalized vinyl cyclopropanes via the NaI-catalyzed ring-opening cyclization of doubly activated cyclopropenes with 1,1-bis(phenylsulfonyl)ethylene

A NaI-catalyzed reaction of 3,3-bis(alkoxycarbonyl)cyclopropenes in the presence of 1,1-bis(phenylsulfonyl)ethylene providing an efficient route to a series of polyfunctionalized vinyl cyclopropanes is described. The reaction is general for a range of different 3,3-bis(alkoxycarbonyl)cyclopropenes affording the products in moderate to high yields. A plausible rationale for this transformation is discussed.