On the origin of geminal regioselectivity in the ene reaction of singlet oxygen with substituted alkenes

The geminal regioselectivity observed in the ene reaction between singlet oxygen and alkenes with anion-stabilizing groups is rationalized on the basis of a perepoxide intermediate, in which in analogy to the nucleophilic attack on protonated epoxides, the perepoxide is opened preferentially at the C---O bond weakened by the substituent.     

An unusual reaction between coumarin and diethylmalonate,catalyzed in the heterogeneous phase

In the reaction of diethylmalonate and coumarin catalyzed by an activated barium hydroxide catalyst (C-200) in a solid liquid system an unusual nucleophilic addition-elimination to the lactonic C=O bond occurs.

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Eine ungewöhnliche in heterogener Phase katalysierte Reaktion zwischen Cumarin und Diethylmalonat (Kurze Mitteilung)

Zusammenfassung Bei der Reaktion von Diethylmalonat und Cumarin, die mit einem aktivierten Bariumhydroxid-Katalysator (C-200) in einem Fest-Flüssig-System katalysiert wird, ergab sich eine ungewöhnliche Addition-Eliminierung an der laktonischen C=O-Bindung.

     

Unexpected regioselectivity in the palladium‐catalyzed reaction of silacyclobutanes with aryl iodides

The reaction of aryl iodides with 1,1‐diphenyl‐silacyclobutane in the presence of a catalytic amount of Pd(PPh₃)₄ affords unexpected ring‐opening adducts, 1‐ and 2‐propenyl(triaryl)‐silanes, in good yields. On the other hand, the PdCl₂(PhCN)₂‐catalyzed reaction of 1,1‐diphenylsilacyclobutanes with aryl halides gives ­unexpected products, triarylsilanols, after ­hydrolysis in moderate yields. The catalysis involves the reaction of aryl–palladium intermediates with silacyclobutanes along with ­regioselective aryl–silicon bond formation. Copyright © 2001 John Wiley & Sons, Ltd.     

An experimental and computational study on the reactivity and regioselectivity for the nitrosoarene ene reaction: comparison with triazolinedione and singlet oxygen

The regioselectivities and the reactivities (relative rates) for the ene reaction of the enophile 4-nitronitrosobenzene (ArNO) with an extensive set of regiochemically defined acyclic and cyclic olefins have been determined. These experimental data establish that the ArNO enophile attacks the olefinic substrate along the novel skew trajectory, with preferred hydrogen abstraction at the corner (twix regioselectivity). This is in contrast to the isoelectronic species singlet oxygen ((1)O(2)), which abstracts at the higher substituted side of the double-bond (cis effect), and triazolindione (TAD), which undergoes the ene reaction at the more crowded end (gem effect). Ab initio computations (B3LYP/6-31+g) for the ene reaction of the ArNO with 2-methyl-2-butene reveal that the steric effects between the aryl group of the enophile and the substituents of the olefin dictate the skew trajectory. These computations identify the aziridine N-oxide (AI) as a bona fide intermediate in this ene reaction, whose formation is usually rate-determining and, thus, irreversible along the skew trajectory (twix selectivity). The reversible generation of the AI becomes feasible when conformational constraints outweigh steric effects, as manifested by enhanced twin regioselectivity.     

Applications of the thermal ene reaction of aldehyde t-butyl- and phenyl- hydrazones

The Thermal Ene reaction of aldehyde $\text{t}$-butyl- and phenyl- hydrazones with enophiles gave $\text{C}$-trapped azo-adducts which can be diverted into synthetically useful γ-keto-esters, γ-keto-nitriles, γ-alkyl-2-pyrrolidones, and γ-amino-esters.     

Pd-NHC catalyzed conjugate addition versus the Mizoroki-Heck reaction

Ace of base: A catalytic system is presented that, solely by choice of the base, selectively switches between conjugate addition and the Mizoroki-Heck reaction of aryl halides with Michael acceptors (see scheme; R, R' = alkyl, aryl). For conjugate addition reactions, this avoids the preparation and use of organometallics.     

Control of regioselectivity by the lone substituent through steric and electronic effects in the nitrosoarene ene reaction of deuterium-labeled trisubstituted alkenes

For the ene reaction of 4-nitronitrosobenzene (ArNO) with a variety of primary and secondary lone alkyl-substituted substrates, the twix/twin regioselectivity is constant at about 85:15. In contrast, for the lone tert-butyl group and for lone aryl substituents, the twix regioisomer is obtained exclusively. These regioselectivities have been rationalized in terms of steric interactions and coordination between the enophile and the substrates in the transition states of the first reaction step.     

Intramolecular ene reaction of a chiral bicyclic lactam

The preparation of bicylic lactam 2 is described, employing an acetoacetate-based synthesis of 1,4-ketoacid 3. The lactam 2 was shown to undergo a thermal ene reaction at 280-300 degrees C to afford tricycle 7. Reduction of the ene product followed by removal of the chiral auxiliary fragment provided the unusual lactam system 9 in highly enantioenriched form.     

An unusual thermal decomposition of a formal oxirene-diene adduct

Flash thermolysis of benzo-3,8-dioxatricyclo[3.2.1.0^2,4]-6-octene gave a dialdehyde and indene.     

Hydrogen-bond-mediated enantio- and regioselectivity in a Ru-catalyzed epoxidation reaction

A chiral epoxidation catalyst based on a tricyclic octahydro-1H-4,7-methanoisoindol-1-one scaffold, in which a hydrogen bonding site and the catalytically active ruthenium center are spatially separated, was synthesized. It was shown that epoxidation reactions in such a supramolecular catalyst occur with high enantio- and regioselectivity because the hydrogen bonds expose the substrate to the ruthenium porphyrin complex with a clear conformational preference. The epoxidation of 3-vinylquinolone proceeded in up to 95% ee (71% yield).     

Stereochemistry of the glyoxylate N-sulfonylimine ene reaction

Ene reactions of sulfonylimine $\text{1b}$ with cyclohexane and $\text{trans}$-2-butene are highly stereoselective, affording products $\text{4}$ and $\text{10}$, respectively.     

Solvent-dependent changes in the triazolinedione-alkene ene reaction mechanism

The influence of the solvent on the triazolinedione-alkene ene reaction mechanism has been investigated. Both inter- and intramolecular kinetic isotope effects with tetramethylethylenes and 2,2,2-(trideuterio)methyl-7-methyl-2,6-octadiene-[D3]-1,1,1 provide, for the first time, strong evidence for changes in the mechanism of the reaction on going from non-protic to polar protic solvents. In non-protic polar or apolar solvents, an aziridinium imide that equilibrates to an insignificant extent with an open intermediate (a dipolar or a polarized biradical) is formed irreversibly in the first, rate-determining step of the reaction, which is followed by fast hydrogen abstraction. On the contrary, in polar protic solvents, hydrogen abstraction is rate limiting, allowing the main dipolar intermediate to equilibrate with its open intermediate(s) as well as with the starting reagents.     

Effect of the heteroatomic substituent on the pi-facial diastereoselectivity in Lewis acid catalyzed carbonyl ene reaction: A theoretical study

Quantum chemical investigation on the optimized transition structures of intermolecular ene reaction containing various heteroatomic substituents at the -carbon atom reveal that, the electrostatic effect produced by the ene moiety plays a crucial role in controlling the conformation of the transition structure. The stereoselectivity calculated from the proposed model agreed nicely with the reported experimental results.     

Unusual regioselectivity in a Diels-Alder reaction of isoprene with levoglucosenone

The ZnCl₂-catalyzed [4+2]-cycloaddition of isoprene with levoglucosenone is highly regio- and stereoselective.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 8, pp. 2047–2049, August, 1996.     

Kishner's reduction of 2-furylhydrazone gives 2-methylene-2,3-dihydrofuran, a highly reactive ene in the ene reaction

[reaction: see text] The Kishner reduction of 2-furylhydrazone gives 2-methylene-2,3-dihydrofuran as the major abnormal reduction product. 2-Methylene-2,3-dihydrofuran is an excellent ene in the carbonyl-ene reaction, reacting with a variety of aldehydes. Most notable was the asymmetric carbonyl-ene reaction of 2-methylene-2,3-dihydrofuran and decanal using Ti(OCH(CH3)2)4/(S)-BINOL to give the corresponding alcohol in 66% yield and 94% ee. The reaction of 2-methylene-2,3-dihydrofuran with 2 equiv of 1,4-benzoquinone unexpectedly gave a monoalkylated 1,4-hydroquinone/1,4-benzoquinone electron donor-acceptor complex.