Aromatization of 1,4-cyclohexadienes with tetracyanoethylene: a case of varying mechanisms

Abstract

The aromatization of 1,4-cyclohexadiene and four alkyl-substituted 1,4-cyclohexadienes with tetracyanoethylene was examined and found in four of five cases to involve two competing mechanisms. Most of each reaction proceeded by concerted ene addition followed by breakdown of the ene adduct, probably heterolytically. Rate constants for diene reaction were determined in acetonitrile-d, and p-dioxane-d8. Adducts were isolated in three cases and rate constants for adduct breakdown determined for the isolated compounds. Where the adduct could be observed but not isolated, a constant was calculated through computer simulation of the rate data. The minor mechanism competing with the ene addition displayed no detectable intermediates and seemed most consistent with electron-proton-electron-proton or electron-proton-hydrogen-atom transfer. Total reaction rate varied by a factor of over 4 X lo5, yet with one exception, the ratio of the two pathways varied very little. One possible explanation for this, the presence of a common rate-determining step preceding any hydrogen transfer (such as SET) was ruled out by the finding of a large primary isotope effect for hexadeuterio-1,4-cyclohexadiene disappearance (kH/kD = 5.2). With one diene, 3,3-dimethyl-l,4-~yclohexadiene, the otherwise minor mechanism became the sole one, as the adduct formed was clearly not a concerted ene adduct. However, in this case aromatization also required a 1,2 methyl shift, and the fact that quantitative collapse to an adduct, without rearrangement, occurred instead ruled out a simple cation intermediate from hydride transfer. A reversible electron transfer therefore seems the likeliest first step for the minor mechanism.Tetracyanoethylene (TCNE) reacts with 1,4-cyclohexadienes (1,4-CHD's) to give tetracyanoethane and the corresponding arene.'I2 Although this reaction has not been applied with anything approaching the frequency of similar aromatization reactions using quinones (e.g. ochloranil or 2,3-dichloro-5,6-dicyanobenz~quinone~) yet, it is often very mild, has synthetic utility,* and perhaps might see more use if it were better understood.Nishiguchi et al. proposed a stepwise mechanism involving hydride abstraction or electron-proton-electron transfer to give a cyclohexadienyl cation for the TCNE r e a~t i o n .~ A little earlier, Haselbach and Rossi, during an investigation directed largely a t another matter, had isolated a reactive intermediate, 1, from the reaction of c?

Studies toward the Total Syntheses of Cucurbitacins B and D

“…Eventual conversion of the acetate to the ketone (via hydrolysis and oxidation) and reductive opening of the cyclopropyl ketone would afford the desired C14 methyl group. The diene 4 could be derived in four steps from the known 2,2-dimethylcyclohexane-1,5-dione, 6 . The more complex dienophile 5 could be generated via an intramolecular aldol condensation of the dicarbonyl compound 7 .…”

“…The diene 4 could be derived in four steps from the known 2,2-dimethylcyclohexane-1,5dione, 6. 9 The more complex dienophile 5 could be generated via an intramolecular aldol condensation of the dicarbonyl compound 7. Given that the ethyl ketone and the secondary alcohol are on the same face of the cyclopentene 7, it was envisioned that the two functional groups could be obtained through a methoxide-mediated ring-opening of the lactone 8, which could in turn be synthesized via a Baeyer-Villiger reaction on the corresponding ketone prepared from the norbornenone ketal 9.…”

Tandem Inverse Electron-Demand Diels−Alder Reactions of 4,5-Dicyanopyridazine with Nonconjugated Dienes: An Excellent Direct Entry into Carbo- and Hetero-Cage Systems

“…A quite different reaction course was observed when cyclohexa-1,4-diene ( 10 ) was used in place of diene 2 , and we isolated 4-cyanopyridazine ( 12 ) as the main product (40%) together with a small amount (6%) of the dihydronaphthalene 14 (Scheme ). A hydrogen transfer from 10 to 4 , followed by elimination of HCN from 11 , now prevails over the expected conversion into 13 that, on the other hand, preferentially gives rise to 14 by a partial aromatization process.

3

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Efficient Ruthenium-Catalyzed Transfer Hydrogenation/Hydrogenation of 1,3-Cycloalkanediones to 1,3-Cycloalkanediols Using Microwave Heating

“…3 2,2-Dimethyl-1,3-cyclohexanediol (2c). 24 Following the general procedure, the reaction mixture was heated for 60 min in a microwave oven. Chromatography (CH 2 Cl 2 to remove the yellow band, then n-pentane/EtOAc 2:1 to 1:1) afforded 2c (120 mg, 83%) as a white solid in a cis/trans ratio of 56:44. cis-2c 1 H NMR (400 MHz, CDCl 3 ): δ 3.40 (dd, J ) 3.5, 7.9 Hz, 2H), 1.79 (m, 3H), 1.55 (m, 2H), 1.34 (m, 1H), 1.03 (s, 3H), 1.00 (s, 3H) 13 C NMR (100 MHz, CDCl 3 ): δ 76.2, 39.7, 29.3, 24.6, 17.1; trans-2c 1 H NMR (400 MHz, CDCl 3 ): δ 3.67 (dd, J ) 3.3, 7.1 Hz, 2H), 1.72 (m, 2H), 1.64 (m,2H), 1.50 (m, 2H), 0.99 (s, 6H) 13 C NMR (100 MHz, CDCl 3 ): δ 74.9, 39.8, 29.4, 21.1, 19.1 5-Methyl-1,3-cyclohexanediol (2d).…”