Ene Reaction Of Singlet Oxygen Research Articles

Photooxidations in zeolites. Part 2: A new mechanistic model for reaction selectivity in singlet oxygen ene reactions in zeolitic media

Abstract The singlet oxygen ene reaction in Methylene Blue doped NaY was examined. A new mechanism is proposed which involves complexation of the sodium cation to the pendant oxygen in the perepoxide intermediate. It is argued that this complexation leads to greater positive charge on the carbon framework and to steric interactions between the sodium and the groups on the proximal side of the perepoxide.

Stereochemistry in the Ene Reactions of Singlet Oxygen and Triazolinediones with Allylic Alcohols. A Mechanistic Comparison

The ene reaction of singlet oxygen with (Z)-4-methylpent-3-en-2-ol-2,5,5,5-d4 (1-OH-d4) in nonpolar solvents exhibits a 90% threo diastereoselectivity in the adduct derived from the major syn perepoxide intermediate, but also a moderate threo diastereoselectivity in the adduct derived from the minor anti perepoxide. Photooxygenation of 2,4-dimethylpent-3-en-2-ol (2) exhibits a significant solvent dependence in the syn/anti methyl stereoselectivity, with nonpolar solvents promoting syn methyl reactivity, while polar solvents promote anti methyl reactivity. These results are in agreement with a steering effect between hydroxyl and singlet oxygen in the rate-determining step of the reaction. N-Phenyltriazolinedione addition to the chiral allylic alcohol 4-methylpent-3-en-2-ol (1-OH) is highly threo diastereoselective in nonpolar solvents, with a solvent dependent variation in the threo/erythro ene products. On the other hand, the nonfunctionalized chiral alkene 2,4-dimethyl-2-hexene (1-Et) exhibits poor diastereoselectivity. Reaction of PTAD with 1-OH-d4 in nonpolar solvents, exhibits a significant threo diastereoselectivity from the syn aziridinium imide intermediate, and a moderate threo diastereoselectivity from the anti intermediate. These results are consonant with a steering effect between the hydroxyl and the electrophile, as proposed in the case of singlet oxygen addition to allylic alcohols 1-OH and 2. In contrast to the analogous 1O2 ene reaction, a solvent independent ratio syn/anti ∼ 50/50 was found in the addition of MTAD to 2. The intermolecular kinetic isotope effect in the reaction of 2 with MTAD (kH/kD = 1.15 ± 0.02), is consistent with formation of the intermediate in fast step, indicative that the steering effect during the formation of aziridium imide is not important in the reaction kinetics. This energetic profile is in contrast to triazolinedione addition to the secondary allylic alcohol 1-OH, where the high threo selectivity and the slight inverse kinetic isotope effect of kH/kD = 0.98 ± 0.02 are consonant with the formation of the intermediate in the rate-determining step. An explanation for the increased reactivity of the syn methyl in the addition of MTAD to 2 (∼50%) is offered.

Reaction of Highly Methylated 2-Methylenecycloalkyl Hydroperoxides with FeSO4/CuCl2. Remarkably Efficient 5-endo-trigor 6-endo-trigCyclization of the Intermediate Carbon Radicals

Treatment of 1,3,3,4,4,5,5-heptamethyl-2-methylenecyclopentyl hydroperoxide, derived from a singlet oxygen ene reaction of 1,2,3,3,4,4,5,5-octamethylcyclopentene, with FeSO4/CuCl2 gave 1-chloro-2,2,3,3,4,4-hexamethylcyclopentyl methyl ketone in high yield, suggesting that the consecutive O−O and C−C bond fission is followed by a novel 5-endo-trig cyclization of the intermediate carbon radical to the activated C−C double bond. In the case of 1,3,3,6,6-pentamethyl-2-methylene-1-cyclohexyl hydroperoxide also, an efficient 6-endo-trig cyclization of the corresponding carbon radical was realized giving 1-chloro-2,2,5,5-tetramethylcyclohexyl methyl ketone in high yield.

Halogenated oxo- and peroxotitanium porphyrinates as sensitizers for the photooxygenation of olefinic compounds

A series of porphines, oxotitanium(IV) and peroxotitanium(IV) porphyrinates were tested as photosensitizers of the singlet oxygen ene reaction with cyclohexene and cis-cyclooctene. The relative order of activity is: H 2(P)>OTi(P)≥Ti(O 2)P. Best stability is obtained for porphines or porphyrinate complexes with fluorine substituents. Oxotitanium(IV) porphyrinates are active in the decomposition of allylic hydroperoxides. The (photo)catalytic epoxidation occurs less readily than the thermal reaction at 82°C. Attempts of photocatalytic hydrogen peroxide activations lead to fast destruction of OTi(P).

Hydroxy-directed diastereoselective ene reaction of triazolinediones with chiral allylic alcohols

Abstract The ene reaction of the N -methyltriazolidinone (MTAD) with the allylic alcohols 1a-e afforded the corresponding urazoles 2 in high threo diastereoselectivity (Scheme 1). These results are mechanistically explained in terms of an attractive hydrogen-bonding interaction between the enophile and the allylic hydroxy functionality in the threo transition state, which is favored on account of minimal 1,3-allylic strain. The threo diastereoselectivity is completely lost when the allylic hydroxy-functionality is masked. The results parallel the hydroxy-directing effect previously established in the singlet-oxygen ene reaction.

Dramatic Diastereoselectivity Differences in the Asymmetric Ene Reactions of Triazolinediones and Singlet Oxygen with Chiral 2,2-Dimethyloxazolidine Derivatives of Tiglic Acid

The diastereoselectivity in the ene reactions of triazolinedinone and singlet oxygen with chiral oxazolidine or oxazolidinone derivatives of tiglic acid depends both on the chiral auxiliary used and the size of the attacking enophile. While chiral 2,2-dimethyloxazolidine amides of tiglic acid give with triazolinediones TAD only one single ene adduct, the corresponding chiral oxazolidinone derivatives (Evans′ auxiliaries) display a very low diastereoselectivity. No selectivity is observed in the singlet-oxygen ene reaction with dimethyloxazolidine amides of tiglic acid. These stereochemical results are rationalized in terms of the differences in the steric demand of the singlet oxygen and TAD enophiles rather than electronic factors.

Dramatic Diastereoselectivity Differences in the Asymmetric Ene Reactions of Triazolinediones and Singlet Oxygen with Chiral 2,2-Dimethyloxazolidine Derivatives of Tiglic Acid

Dramatic Diastereoselectivity Differences in the Asymmetric Ene Reactions of Triazolinediones and Singlet Oxygen with Chiral 2,2-Dimethyloxazolidine Derivatives of Tiglic Acid

An Interplay in the Regioselectivity Induced by Non Bonding Interactions, in the ene Reactions of Singlet Oxygen and Triazolinediones with Tetrasubstituted Alkenes

The ene reactions of N-phenyl-1,2,4-triazoline-3,5-dione (PTAD) and singlet oxygen ( 1O 2) with tetrasubstituted alkenes, follow regioselectivity trends governed mainly by 1,3-non bonded interactions (type I) for the case of PTAD, and mainly by the steric hindrance caused during the formation of the new double bond (type II) for the case of 1O 2. The results are explained in terms of a “late” product forming transition state of hydrogen abstraction for triazolinediones and an “early” transition state for the analogous singlet oxygen reaction. © 1997 Elsevier Science Ltd.

ChemInform Abstract: Regioselectivity in the Ene Reaction of Singlet Oxygen with Alkenes Bearing an Electron‐Withdrawing Group at β‐Position.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Regioselectivity in the ene reaction of singlet oxygen with alkenes bearing an electron withdrawing group at β- position

Electronic repulsions between a perepoxide intermediate and the allylic functionality in the product forming transition state, direct the regioselectivity in the photooxygenation of trisubstituted alkenes bearing an electron withdrawing group at β- position.

ChemInform Abstract: Solvent Effects in the Stereoselectivity of the Ene Reaction of Singlet Oxygen with Allylic Alcohols.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Pressure effect on the ene reaction of singlet oxygen with tetramethylethylene in different solvents

AbstractRate constants kR of the ene reaction of singlet oxygen (1Δg) with tetramethylethylene (TME) have been determined in six solvents with different polarity at elevated pressures up to 1200 bar using time‐resolved phosphorescence technique. Experimental activation volumes, ΔV, resulted in the range between −20.7 cm3 · mol−1 (in benzonitrile) and −35.1 cm3 · mol−1 (in n‐hexane). These results were analysed taking into account contact complex formation as the initial step. The reaction volume ΔVc of the formation of the contact complex was evaluated using the statistical model developed by Yoshimura and Nakahara. The data obtained confirm the assumption of a polar intermediate resembling a perepoxide. The intrinsic activation volume ΔV was found to be −14.5 cm3 · mol−1 and the dipole moment of the transition state was estimated to be μ≠ = 3.4 Debye.

Solvent effects in the stereoselectivity of the ene reaction of singlet oxygen with allylic alcohols

The stereoselectivity of the ene reaction of singlet oxygen with E-2,4-dimethylpent-3-en-2-ol-5,5,5-d 3 depends on solvent; polar solvents favor hydrogen abstraction from the methyl anti to the alcohol; nonpolar solvents favor the opposite.

Photo-sensitized oxygenation of phenethylguanidoxime: a possible chemical model for the biological oxidation of Nω-hydroxy-L-arginine to L-citrulline

Photo-sensitized oxygenation of phenethylguanidoxime led to the effective production of an expected urea derivative along with generation of nitric oxide (NO) or its equivalent. The formation of both products could be reasonably explained by the mechanism based on singlet oxygen ene reaction of olefins. This should give a possible chemical model for the biological oxidation of N ω -hydroxy-L-arginine into L-citrulline and NO.

ChemInform Abstract: Diastereoselective Synthesis of Merucathin: The Singlet Oxygen Ene Reaction (Schenck Reaction) as a Key Step Towards an E‐Configurated . beta.‐Amino Allylic Alcohol.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Diastereoselective Singlet Oxygen Ene Reaction (Schenck Reaction) and Diastereoselective Epoxidations of Heteroatom-Substituted Acyclic Chiral Olefins: A Mechanistic Comparison

The directing propensity of allylic and homoallylic heteroatom substituents on the diastereoselective singlet oxygen ene reaction (Schenck reaction) of acyclic, chiral olefins has been investigated. These extensive stereochemical results offer evidence for our proposed mechanism of the diastereoselective singlet oxygen ene reaction. Thus, provided a substrate possesses sufficient 1,3-allylic strain to populate preferentially the appropriate conformation, the photooxygenation is directed threo-selectively by intramolecular hydrogen bonding in the threo-A exciplex of allylic alcohols, while electron-accepting or bulky substituents result in the erythro-configurated products. A detailed comparison of the diastereoselectivities for the singlet oxygen ene reaction with those of the epoxidation by m-chlorobenzoic acid (mCPBA) and dioxirane of several judiciously chosen substrates showed that both oxygen transfer processes are influenced similarly by allylic and homoallylic substituents. Thus, it is shown that the acyclic, allylic alcohol 1a is epoxidized highly threo-selectively by methyl(trifluoromethyl)dioxirane due to hydrogen bonding in the transition state and that bulky and/or electron-accepting substituents direct all three oxidants erythro-selectively. These pronounced steering effects on the diastereoselectivity of oxygen transfer processes (photooxygenation and epoxidation) should be valuable for the rational design of stereocontrolled oxyfunctionalizations by 1O2, mCPBA, and dioxiranes.

Unusual Regioselectivity in the Self-Sensitized Singlet Oxygen Ene Reaction of Cyclohexenobuckminsterfullerenes

Unusual Regioselectivity in the Self-Sensitized Singlet Oxygen Ene Reaction of Cyclohexenobuckminsterfullerenes

Diastereoselective Synthesis of Merucathin: The Singlet Oxygen Ene Reaction (Schenck Reaction) as a Key Step Towards an E-Configured β-Amino Allylic Alcohol

An enantio- and diastereoselective synthesis of the naturally occuring merucathin is reported. The singlet oxygen ene reaction of the bis-Boc-protected allylic amine 3, which was prepared from L-alanine, was employed as key step for this regio- and diastereoselective synthesis. The ene reaction is highly erythro-selective and the newly formed double bond in the allylic hydroperoxide 5 is exclusively E-configured. Reduction of the allylic hydroperoxide 5 and base-catalyzed deprotection provides a convenient and unprecedented synthesis of the optically active E-configured β-amino allylic alcohol merucathin from the corresponding acylated allylic amine 3.

ChemInform Abstract: Substituent Effects in the Highly Regioselective and Diastereoselective Ene Reaction of Singlet Oxygen with chiral Cyclohexadienes.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Substituent Effects in the Highly Regioselective and Diastereoselective Ene Reaction of Singlet Oxygen with Chiral Cyclohexadienes

Substituent Effects in the Highly Regioselective and Diastereoselective Ene Reaction of Singlet Oxygen with Chiral Cyclohexadienes