Vanadium in Modern Organic Synthesis

Highly Efficient Continuous Flow Reactions Using Singlet Oxygen as a “Green” Reagent

Regioselectivity in the Ene Reaction of Singlet Oxygen with Alkenes

Hydroxy Group Directivity in the Epoxidation of Chiral Allylic Alcohols: Control of Diastereoselectivity through Allylic Strain and Hydrogen Bonding

The reaction of peroxotungstates (H2WO4 + H2O2) with H2SeO4 gave the novel selenium-containing dinuclear tungsten species, (TBA)2[SeO4{WO(O2)2}2] (I, TBA = [(n-C4H9)4N]+), which was characterized by elemental analysis, IR, Raman, UV−vis, 77Se NMR, 183W NMR, and CSI-MS. Various kinds of homoallylic and allylic alcohols were efficiently epoxidized to the corresponding epoxy alcohols in high yields with 1 equiv. H2O2 with respect to the substrates. Compound I showed the highest catalytic activity for H2O2-based epoxidation of homoallylic and allylic alcohols among selenium and tungsten complexes. The turnover frequency reached up to 150 h−1 in a 10 mmol-scale epoxidation of cis-3-hexen-1-ol and this value was the highest among those reported for the transition-metal catalyzed epoxidation of homoallylic alcohols with H2O2. The kinetic, mechanistic, computational studies showed that the stabilization of the transition-state by the hydrogen bonding between I and the substrates results in the high reactivity for...

Hydrogen-bond-assisted epoxidation of homoallylic and allylic alcohols with hydrogen peroxide catalyzed by selenium-containing dinuclear peroxotungstate.

Photooxygenation of olefins in the presence of titanium(IV) catalyst. A convienient "one-pot" synthesis of epoxy alcohols.

The reaction of PTAD with 2-trimethylsilyl-, 2-chloro-, 2-cyano-, and 2-methoxycarbonylnorbornadienes 1a–d, respectively, was investigated. In all cases homo Diels-Alder adducts were obtained, for 1a the unexpected regioisomer, the 1-trimethylsilyl derivative 2a, and for other norbornadienes 1b–d the usual cyclopropane-substituted products 2b–d. Except for the trimethylisilyl system 1a, for which also the dicarboximides 4a and 4′a, respectively, (rearrangement urazoles) were obtained, the other norbornadienes 1b–d afforded the insertion products 5b–d. With increasing electron-withdrawal by the 2-substituents the insertion products 5b–d increased at the expense of the homo Diels-Alder adducts 2b–d. These results are mechanistically rationalized in terms of stepwise cycloaddition via 1,5-dipolar intermediates. In the case of 2-chloronorbornadiene (1b), besides the homo Diels-Alder 2b, the rearrangement urazole 4b and the insertion product 5b, also the [2 + 2] cycloadduct 3b was formed.



Die Reaktion von PTAD mit den 2-Trimethylsilyl-, 2-Chlor-, 2-Cyan- oder 2-Methoxycarbonylnorbornadienen 1a–d wurde untersucht. In allen Fallen wurden Homo-Diels-Alder-Addukte erhalten, von 1a das unerwartete Regioisomere 1-Trimethylsilderivat 2a und von 1b–d die gewohnlichen cyclopropansubstituierten Produkte 2b–d. Zusatzlich fuhrte das Trimethylsilylsystem 1a zu den Dicarboximiden 4a bzw. 4′a (Umlagerungsurazole). Die Norbornadiene 1b–d bildeten die Insertionsprodukte 5b–d. Mit steigendem Elektronenzug der 2-Substituenten werden die Einschiebungsprodukte 5b–d auf Kosten der Homo-Diels-Alder-Addukte 2b–d in steigendem Mas gebildet. Diese Ergebnisse erklaren wir unter der Annahme eines mehrstufigen Cycloadditionsmechanismus uber 1,5-dipolare Zwischenstufen. Im Fall von 2-Chlornorbornadien (1b) wurde neben dem Homo-Diels-Alder-Addukt 2b, dem Umlagerungsurazol 4b und dem Einschiebungsprodukt 5b auch das [2 + 2]-Cycloaddukt 3b erhalten.

Cycloaddition Behavior of 2‐Substituted Norbornadienes towards 4‐Phenyl‐4H‐1,2,4‐triazole‐3,5‐dione (PTAD): Homo Diels‐Alder Reactivity versus Insertion, Rearrangement, and [2 + 2] Cycloaddition

A series of 7-alkylidene-2,3-benzonorbornadienes 2a–f was prepared from the corresponding fulvenes 1a–f by reaction with benzyne. The cycloaddition of 2 with PTAD was investigated. While the phenyl derivative 2a was quite unreactive and afforded traces of the [2 + 2] cycloadduct 3a, the phenyl methyl system 2b led to the ene product 3b in fair yield. The diphenyl and bis-(4-chlorophenyl) cases 2d and 2e, respectively, led to a mixture of the rearrangement urazoles 3d and 3e and the double PTAD [4 + 2] cycloadducts 4d and 4e in form of the syn/syn and syn/anti diastereomers. Analogously, the unsymmetrically substituted 7-(diarylmethylene)benzonorbornadienes 2c (X = Ph, Y = p-MeOPh) and 2f (X = Ph, Y = p-NO2Ph) gave similar products, except that the possible diastereomeric rearrangement urazoles were formed (E,Z-3c and E,Z-3f). In the case of 2c the oxidized monocycloadduct 5c was also isolated in low yield. The ratio of rearrangement urazoles 3 to [4 + 2] cycloaddition products 4 increased with increasing electron withdrawal of the substituents on the aryl groups, i.e. it was largest for p-nitrophenyl and smallest for p-methoxyphenyl. NOE studies and X-ray analyses (of 3b, of 4d with NMe instead of NPh, and of 5c) were essential to assign the stereochemistry of these complex products.



Cycloaddition von 4-Phenyl-4H-1,2,4-triazol-3,5-dion (PTAD) an 7-Alkyliden-2,3-benzonorbornadiene



Die Umsetzung einer Reihe von Fulvenen 1a–f mit Dehydrobenzol fuhrte zu den entsprechenden 7-Alkyliden-2,3-benzonorbornadienen 2a–f, deren Cycloadditionen mit PTAD untersucht wurden. Wahrend das Phenyl-Derivat 2a kaum reagierte und nur Spuren des [2 + 2]-Cycloaddukts 3a lieferte, ergab das Phenylmethyl-System 2b das En-Produkt 3b in ansprechender Ausbeute. Im Falle der 6,6-Diphenyl- und 6,6-Bis(4-chlorphenyl)fulvene 2d und 2e wurden die Umlagerungs-Urazole 3d und 3e sowie die syn/syn- und syn/anti-Diastereomere 4d und 4e erhalten, die aus sukzessiven [4 + 2]-Cycloadditionen zweier Aquivalente PTAD resultierten. Die unsymmetrisch substituierten 7-(Diarylmethylen)benzonorbornadiene 2c (X = Ph, Y = p-MeOPh) und 2f (X = Ph, Y = p-NO2Ph) fuhrten zu analogen Produkten, wobei die Umlagerungs-Urazole als Diastereomerenpaare E/Z-3c und E/Z-3f erhalten wurden. Im Falle von 2c konnte auch das oxidierte Monoaddukt 5c in geringer Ausbeute isoliert werden. Das Verhaltnis der aus Umlagerung zu den aus [4 + 2]-Cycloadditionen resultierenden Urazolen steigt mit zunehmendem elektronenziehendem Charakter der Substituenten am Arylrest, d. h. es ist am hochsten fur Y = p-NO2Ph und am niedrigsten fur Y = p-MeOPh. NOE-Untersuchungen und Rontgenstrukturanalysen (von 3b, von 4d mit NMe statt NPh und von 5c) waren zur Bestimmung der Stereochemie dieser komplexen Produkte notig.

Cycloaddition of 4-Phenyl-4H-1,2,4-triazole-3,5-dione (PTAD) to 7-Alkylidene-2,3-benzonorbornadienes

The cycloaddition of the 7-substituted (X; Y) 1,3,5-cyclooctatrienes 3 (N3; H), 4 (OAc; H), 5 (OEt; OEt), 6 (Me; H), and 7 (iPr; H) with singlet oxygen (1O2) and 4-phenyl-4H-1,2,4-triazole-3,5-dione (PTAD) was investigated. Singlet oxygen gave with all cyclooctatrienes the corresponding tricyclic endoperoxides derived from the bicyclic valence isomers (bicyclo[4.2.0]octa-2,4-dienes). The corresponding bicyclic endoperoxides derived from the monocyclic valence isomers (1,3,5-cyclooctatriene) were only obtained for the alkyl-substituted derivatives 6 (Me; H) and 7 (iPr; H). PTAD cycloaddition led to both the bicyclic and tricyclic urazoles, except with the acetal 5 which gave only tricyclic product. The bromo derivative 2 did not react with 1O2 nor PTAD. Control experiments with the parent cyclooctatriene showed that the cycloaddition products were derived from the “static” amounts of the monocyclic and bicyclic valence isomers in the substrate mixture.

Cycloaddition of Singlet Oxygen and 4-Phenyl-4H-1,2,4-triazole-3,5-dione to 7-Substituted 1,3,5-Cyclooctatrienes

α-Silylperoxy aldehydes and ketones 3 have been prepared by ozonolysis of silyl-protected allylic hydroperoxides, followed by subsequent reduction of the latter with dimethyl sulfide. Deprotection of silylperoxy aldehydes 3a-c with fluoride ion led to decomposition with chemiluminescence.

Bernd Will

Papers

Photooxygenation of olefins in the presence of titanium(IV) catalyst. A convienient "one-pot" synthesis of epoxy alcohols.

Cycloaddition Behavior of 2‐Substituted Norbornadienes towards 4‐Phenyl‐4H‐1,2,4‐triazole‐3,5‐dione (PTAD): Homo Diels‐Alder Reactivity versus Insertion, Rearrangement, and [2 + 2] Cycloaddition

Cycloaddition of 4-Phenyl-4H-1,2,4-triazole-3,5-dione (PTAD) to 7-Alkylidene-2,3-benzonorbornadienes

Cycloaddition of Singlet Oxygen and 4-Phenyl-4H-1,2,4-triazole-3,5-dione to 7-Substituted 1,3,5-Cyclooctatrienes

Synthesis of Trialkylsilylated α-Hydroperoxy Aldehydes and Ketones via Ozonolysis of Protected Allylic Hydroperoxides