Theoretical studies on the ene reaction mechanisms of propene and cyclopropene with ethylene and cyclopropene: concerted or stepwise

The potential energy surfaces of the ene reactions of propene and cyclopropene with ethylene and cyclopropene were studied by ab initio molecular orbital (MO) methods. The reaction mechanisms were analyzed by CiLC method on the basis of CASSCF MOs. The concerted and stepwise reaction pathways of the ene reaction of propene with ethylene as the parent reaction were located. The energy barrier of the stepwise process is about 4 kcal/mol lower than that of the concerted one. The other reactions can be found only the stepwise mechanism. Although the endo-type reaction of propene with cyclopropene, where cyclopropene is the enophile, probably occurs through a one-step process, the mechanism is divided into the CC bond formations and the hydrogen migration as a stepwise reaction. The CiLC-IRC analysis of the concerted process of propene with ethylene shows the different patterns of the electronic state variation for the CC bond formation/breaking and the hydrogen migration.     

Rhodium‐Catalyzed Arylation of Cyclopropenes Based on Asymmetric Direct Functionalization of Three‐Membered Carbocycles

A variety of highly diastereo‐ and enantiomerically enriched arylcyclopropanes is obtained through the asymmetric rhodium‐catalyzed arylation reaction of achiral nonfunctionalized cyclopropene derivatives with commercially available aryl boronic acids in the presence of (R,S)‐Josiphos.     

Dianion approach to chiral cyclopropene carboxylic acids

[reaction: see text] In this Letter, we describe a general method for preparing the dianions of cyclopropene carboxylic acids, and we show that their subsequent reactions with electrophiles provide a general means for selectively introducing diverse types of functional groups. This provides a general method for the synthesis of chiral 1,2-disubstituted cyclopropenes, and opens new avenues for the enantioselective preparation of cyclopropenes.     

Evidence for cyclopropene formation in the reactions of 4,4-dimethyl-4-silacyclohexadienylidene

In the reactions of 4,4-dimethylcyclohexadienylidene no evidence for cyclopropene formation has been found. By contrast, 4,4-dimethyl-4-silacyclohexadienylidene yields a product on reaction with butadiene that is best rationalized by intramolecular cyclopropene formation.     

Cobalt-Catalyzed Facial-Selective Hydroalkylation of Cyclopropenes

Cyclopropene hydrofunctionalization has been a promising strategy for accessing multi-substituted cyclopropanes; however, cyclopropene hydroalkylation remains underdeveloped. Herein, we report a low-valent CoH-catalyzed facial-selective cyclopropene hydroalkylation to access multi-substituted cyclopropanes. This reaction exhibits a broad substrate scope of alkyl halides and cyclopropenes and tolerates many functional groups. Moderate-to-good facial-selectivity is obtained without any directing groups. Mechanism studies provide evidence that alkyl radicals are generated from alkyl halides and irreversible CoH insertion is responsible for the facial-selectivity. Our preliminary exploration demonstrates that asymmetric cyclopropene hydroalkylation can be realized without conspicuous auxiliary groups.     

A computational study of chlorocarbene additions to cyclooctyne

Dichloro- and phenylchlorocarbene (CCl2 and PhCCl) add to cyclooctyne via a barrierless process (MP2/6-311+G*, B3LYP/6-311+G*, B3LYP/6-31G*) to yield the expected corresponding cyclopropene adducts. A three-dimensional potential energy surface (PES) for CCl2 addition to cyclooctyne (B3LYP/6-31G*) shows the formation of the cyclopropene product and also possible formation of a vinylcarbene. Residing in a shallow energy well, the vinylcarbene easily rearranges to the cyclopropene product, or to an exocyclic vinyl bicyclo[3.3.0]octane. Although the calculated three-dimensional PES indicates possible dynamic control of the cyclooctyne-chlorocarbene system through the putative formation of a vinylcarbene (in addition to the expected cyclopropene), additional calculations and preliminary experimental work show paths through the vinylcarbene to be unlikely. If the additions of chlorocarbenes to cyclooctyne are controlled by reaction dynamics, we predict that the vast majority of the reactions proceed via traditional carbene cycloaddition with only a very minor amount of products formed from the alternative pathway.     

[(NHC)NiIIH]‐Catalyzed Cross‐Hydroalkenylation of Cyclopropenes with Alkynes: Cyclopentadiene Synthesis by [(NHC)NiII]‐Assisted C−C Rearrangement

A cross‐hydroalkenylation/rearrangement cascade (HARC), using a cyclopropene and alkyne as substrate pairs, was achieved for the first time by using new [(NHC)Ni(allyl)]BAr^F catalysts (NHC=N‐heterocyclic carbenes). By controlling the (NHC)Ni^IIH relative insertion reactivity with cyclopropene and alkyne, a broad scope of cyclopentadienes was obtained with highly selectively. The structural features of the new (NHC)Ni^II catalyst were important for the success of the reaction. The mild reaction conditions employed may serve as an entry for exploring (NHC)Ni^II‐assisted vinylcyclopropane rearrangement reactivity.     

Solid phase synthesis of cyclopropenes

Cyclopropene derivatives of high purity were prepared by reaction of acetylenes with a rhodium carbenoid bound to a polystyrene resin. This solid phase method avoids the formation of undesired byproducts obtained in the corresponding solution reaction and an eventual extension to combinatorial synthesis of cyclopropene derivatives could be achieved.     

Chemistry of organosilicon compounds XCIV. Preparations of 8-silatricyclo[3.2.1.02,4]oct-6-enes,new photochemical silylene generators

The reaction of siloles with cyclopropene afforded products representative of a new organosilicon ring system, 8-silatricyclo[3.2.1.0^2,4]oct-6-ene, which proved to be photolytic silylene generators.     

Comparison of the reactivity of thiophene,thiophene 1-oxide,and thiophene 1,1-dioxide as diene for diels-alder reactions. An ami semiempirical study

A semiempirical AMI theoretical study was carried out to examine the very low reactivity of thiophene; for example, the high reactivity of thiophene 1-oxide as a diene in the Diels Alder reactions. The reactivities of cyclopentadiene, thiophene, thiophene 1-oxide, and thiophene 1,1-dioxide were studied as dienes in the reaction with ethylene, cyclopropene, and benzoquinone. Ethylene was chosen as a standard, while cyclopropene, with its high strain energy was released in the course of a reaction making it relatively reactive. The benzoquinone has a lower LUMO energy, making it a very reactive dienophile for the Diels-Alder reaction. Frontier molecular orbital energy gap between the reactants was considered, and the disadvantage of this approach in studying the reactivity was demonstrated. For all combinations, the corresponding transition structures are generated and the activation energies are estimated. The estimated activation barrier for sulfur dioxide elimination from the adduct was used to explain the failure to accumulate the cycloadduct in the reaction mixture. The obtained results are compared with experimental data when available. An excellent agreement of theory and experiment was obtained.     

A highly regioselective tandem 1,3-dipolar cycloaddition of cyclopropene 1,1-diesters and nitrile oxides: synthesis of highly functionalized isoxazoles

A highly regioselective tandem 1,3-dipolar cycloaddition of cyclopropene 1,1-diesters with nitrile oxides was described. This reaction supplied a new method for synthesis of polyfunctionalized isoxazole derivatives in moderate to excellent yields under mild conditions.     

Preparation of substituted gem-dimethylcyclopropanecarboxylic acids

Conclusions A method, based on the addition of Grignard reagent to cyclopropene hydrocarbons, was proposed for the synthesis of alkyl-, phenyl-, and alkenylcyclopropanecarboxylic acids that bear a geminal dimethyl grouping in the ring.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 3, pp. 669–671, March, 1979.     

Gold-Catalyzed Divergent Ring-Opening Rearrangement of Cyclopropenes Enabled by Dichotomous Gold−Carbenes

The gold-catalyzed ring-opening rearrangement of cyclopropenes affords an efficient route to either polysubstituted naphthols or aryl-substituted furans. Owing to the unique dichotomy of gold−carbenes, this protocol provides a switchable reaction selectivity between naphthols and furans enabled by the use of TFP−Au(MeCN)SbF₆ (tri(2-furyl) phosphine) or PNP(AuNTf₂)₂ (bis(diphenylphosphino)(isopropyl) amine) as catalysts respectively. It is proposed that the gold−carbene intermediate might be involved in the cyclopropene→naphthol rearrangement while the gold-carbocation is more likely to be involved in the cyclopropene→furan rearrangement.     

Photoinduced Electron Transfer Reactions of 3,3‐Dialkylated 4,5‐Diphenyl‐3H‐Pyrazoles: A New Route to the Formation of the Solvent Adducts

3,3‐Dialkyl‐4,5‐diphenyl‐3H‐pyrazoles undergo readily photoinduced electron transfer (PET) reaction with 2,4,6‐triphenylpyrylium tetrafluoroborate (TPP⁺) in acetonitrile to produce cyclopropenes and 2H‐pyrroles. During prolonged irradiation, the new ring‐closure products derived from 2H‐pyrroles as the secondary photoproducts are also produced. However, the corresponding ester analog exhibits different behavior to obtain the cyclopropene as the primary photoproduct and a [2+2] dimer of the cyclopropene as the secondary photoproduct. A rationale for the different behavior is offered.     

Asymmetric Copper-Catalyzed Carbomagnesiation of Cyclopropenes

The highly diastereo- and enantioselective formation of polysubstituted cyclopropanes was easily achieved through the asymmetric copper-catalyzed carbomagnesiation reaction of nonfunctionalized cyclopropene derivatives. The carbometalated species generated in situ readily undergo C−C and C−X bond-forming reactions with various electrophiles with complete retention of configuration.     

Formal cycloaddition of ethyl 2-aroyl-1-chlorocyclopropanecarboxylates:facile synthesis of diversified tetrahydrocyclopropa[b]chromenes

Tandem reaction of ortho-hydroxy chalcone with ethyl 2-aroyl-1-chlorocyclopropanecarboxylates has been disclosed, affording facile synthesis of diversified tetrahydrocyclopropa[b]chromenes via electron-deficient cyclopropene intermediate.     

Mechanism of abstraction reactions of heavy cyclopropenes with carbon tetrachloride

The potential energy surfaces for the abstraction reactions of heavy cyclopropenes (X-Y-Z) with carbon tetrachloride have been characterized in detail using density functional theory (B3LYP), including zero-point corrections. Five cyclopropene analogues including A(C-C-C), B(Ge-Si-Si), C(Si-Si-Si), D(Si-Si-Ge), and E(Ge-Ge-Ge), have been chosen in this work as model reactants. Two reaction paths, the Cl abstraction I and the CCl3 abstraction II, have been considered in the present study. Our theoretical findings strongly suggest that the former is more favorable, with a very low activation energy and a large exothermicity. This is in accordance with available experimental observations. Moreover, our theoretical investigations also indicate that the more electropositive the elements making up the double bond of a heavy cyclopropene, the lower its activation barrier and the more exothermic the haloalkane abstraction. That is, electronic factors play a dominant role in determining the chemical reactivity of the heavy cyclopropene species kinetically as well as thermodynamically. Furthermore, a configuration mixing model based on the work of Pross and Shaik is used to rationalize the computational results. The results obtained allow a number of predictions to be made.     

Intermolecular Pauson-Khand reactions of cyclopropene: a general synthesis of cyclopentanones

[reaction: see text] The Pauson-Khand reaction of cyclopropene with a variety of terminal alkynes has been studied. The best reaction conditions involve NMO activation in CH(2)Cl(2) at -35 degrees C. In this way, 3-substituted-bicyclo[3.1.0]hex-3-en-2-ones have been obtained in good to excellent yields. As a synthetic application, several types of substituted cyclopentenones have been prepared from these cycloadducts by protocols involving conjugate addition and reductive ring opening.     

Stereospecific cyclopropenic synthesis of cis-chrysanthemic methyl ester.2. The by-passing diazoalkane way (1).

A stereospecific synthesis of cis-chrysanthemate by catalytic hydrogenation of a cyclopropene is described. The required precursor, a 3H-pyrazole, is obtained here via a novel dianion reaction and MnO₂ dehydrogenation of a Δ²-pyrazoline, instead of the more cumbersome diazoaddition method.     

Asymmetric Copper‐Catalyzed Carbomagnesiation of Cyclopropenes

The highly diastereo‐ and enantioselective formation of polysubstituted cyclopropanes was easily achieved through the asymmetric copper‐catalyzed carbomagnesiation reaction of nonfunctionalized cyclopropene derivatives. The carbometalated species generated in situ readily undergo C−C and C−X bond‐forming reactions with various electrophiles with complete retention of configuration.