1.2- Dimethyl-1.4-cyclohexadiene

Addition of benzene in methanol to an excess (25-100%) of sodium in liquid ammonia afforded 84-88% yields of 1,4-cyclohexadiene [392]. o-Xylene under the same conditions gave 70%-92% yield of 1,2-dimethyl-1,4-cyclohexadiene [392, 395]. Lithium in neat ammonia at 60° gave 91% of 1,4-cyclo-hexadiene and 9% of cyclohexene (conversion 58.4%), while calcium under the same conditions yielded 21% of cyclohexadienes and 79% of cyclohexene in conversions of 13-60% [393]. If benzene dissolved in ether was added to the compound Ca(NH3)s, preformed by dissolving calcium in liquid ammonia and evaporating ammonia, a 75% conversion to pure cyclohexene was achieved [394]. 

A. 1,2-Dimethyl-1,4-cyclohexadiene. Caution This step should be conducted in a hood to avoid exposure to ammonia fumes. A 5-1. three-necked flask, cooled in a dry ice-isopropyl alcohol bath, is fitted with an efficient stirrer and a dry ice condenser. I he flask is charged with approximately 2.5 1. of li< uid ammonia, the stirrer is started, and 450 g. of anhydrous diethyl ether, 4(>0 g. (10 moles) of absolute ethanol, and 318.5 g. (3.0 moles) of t>-xylene (Note 1) are added slowly in that order (Note 2). I hen 207 g. (9.0 g. atoms) of sodium is added in small pieces over a 5-hoiir... 

Reaction Conditions. A typical procedure for the reduction of o-xylene to 1,2-dimethyl-1,4-cyclohexadiene is as follows sodium metal (or Li wire) is cut into small pieces and slowly added to a solution of the aromatic substrate in a solvent mixture of liquid NH3, EtjO (or THF), and EtOH (or t-BuOH). ° The alcohol does not react with the metal at —33 °C (bp of liquid ammonia). Relative rates of benzene reduction are Li = 360, Na = 2, and K = 1. 

I lie flask is then equipped with a reflux condenser and 800 ml. of ice water is run In slowly with stirring to dissolve the salts (exothermic reaction). The organic layer is washed well with water, dried, and the 1,2-dimethyl-1,4-cyclohexadiene Is distilled. 

A soln. of 1,2-dimethyl-1,4-cyclohexadiene in 3.6 10 water/acetone added to a yellow slurry of bis(acetonitrile)dichloropalladium(II), KHCO3, and CUCI2 in acetone at 20°, and worked up after 24 h di-p-chlorobis[(l,2,3-r )-5-hydroxy-l,2-dimethyl-2-cyclohexen-l-yl]dipalladium (Y 77%), in methanol containing Na-propionate pressurized in a Fischer-Porter tube with 3.8 atm. CO for 4.5 h methyl trans-S hydroxy-... 

Der Cyolohexadien-(l,3)-RingschluB vollzieht sich photochemisch conrotatorisch. Aus all-trans-Oetatrien-(2,4,6) entsteht iiber die fraJw,cis, rare[Pg.252]

Dimethyl-[cyclohexadien-(2.5)-yl]- 510 Dimethyl-(4-nitro-phcnyl)- 694 (2.4-Dinitro-phenyl)-(5-jod-pentyliden)- 729 Diphenyl- 994.1277,1590 (l.2-Diphenyl-iithyliden)-cyan- 1079 Diphenylmethyl- 1446 Diphenylmethyl-acctyl- 1449 Diphenylmethyl-benzoyl- 1449 Diphenyl methyl-butyl- 1446 Diphcnylmethyl-cyclohexyl- 1446 Diphenylmethyl-diiithyl- 1221 Diphenylmethyl-diphenylmethylen- 1265,1447 Diphenylmethylen- 1127, 1262 [l.l,l,3,3,3-Hexafluor-propyl-(2)]-(a-athoxy-benzyli-den)- 1115... 

Atroposelective cycloaddition reactions of A-2-(r-butylphenyl)- and A-2,5-(di-r-butylphenyl)-maleimide show good to excellent stereoselectivities and the high rotation barriers prevent cycloadducts from interconverting. The stereospeciflc hetero-Diels-Alder reaction of o-quinone methides (80) with o-quinones (79) in MeOH at room temperature produce the 4a,8-di(hydroxymethyl)chromane derivatives (81) and (82) in high yields (Scheme 29). The intramolecular inverse-electron-demand Diels-Alder reaction of o-quinone methides (84) derived from 2-(l-hydroxy-5-alkenyl)phenol derivatives (83) produces l,2,3,3a,4,9b-hexahydrocyclopenta[c][l]benzopyrans (85) under mild acidic conditions (Scheme 30). The Diels-Alder reactions between dimethyl-cyclohexadiene derivatives and di-(-)-menthyl acetylenedicarboxylate exhibit modest diastereoselectivity. ... 

Give the structure of the Diels-Alder adduct of 1 3 cyclohexadiene and dimethyl... 

From West Indian lime oil, a trace low Foiling constituent, 1-methyl-1,3-(or 1,5 /74< 5 -3 7- -cyclohexadiene has been characterized (27). This compound, which possesses an intense and characteristic lime aroma, was later confirmed to be the 1,3-isomer [1489-56-1] (11). This compound can easily be made in a biomimetic way through the reaction of citral [5392-40-5] (3,7-dimethyl-2,6-octadienal) with citric acid (28,29). 

Electrocyclic reactions of 1,3,5-trienes lead to 1,3-cyclohexadienes. These ring closures also exhibit a high degree of stereospecificity. The ring closure is normally the favored reaction in this case, because the cyclic compound, which has six a bonds and two IT bonds, is thermodynamically more stable than the triene, which has five a and three ir bonds. The stereospecificity is illustrated with octatrienes 3 and 4. ,Z, -2,4,6-Octatriene (3) cyclizes only to cw-5,6-dimethyl-l,3-cyclohexadiene, whereas the , Z,Z-2,4,6-octa-triene (4) leads exclusively to the trans cyclohexadiene isomer. A point of particular importance regarding the stereochemistry of this reaction is that the groups at the termini of the triene system rotate in the opposite sense during the cyclization process. This mode... 

Chiral BOX-zinc(II) complexes can also catalyze the cycloaddition reaction of glyoxylates with, e.g., 2,3-dimethyl-l,3-butadiene and 1,3-cyclohexadiene [36]. The reaction gave for the former diene a higher cycloaddition product/ene product ratio compared with the corresponding chiral copper(II) complexes the ee, however, was slightly reduced. For the reaction of 1,3-cyclohexadiene slightly lower yield and ee were also found. 

Chemical Name 6-[D-2-amino-2-(1,4-cyclohexadien-1-vl)acetamido] -3,3-dimethyl-7-oxo-4-thia-1-azabicyclo[3.2.0] heptane-2-carboxylic acid... 

The most striking feature of electrocyclic reactions is their stereochemistry. For example, (2 ,4Z,6 )-2,4,6-octatriene yields only c/s-5,6-dimethyl-l,3-cyclo-hexadiene when heated, and (2 ,4Z,6Z)-2,4,6-octatriene yields only trnns-5,6-dimethyl-l,3-cyclohexadiene. Remarkably, however, the stereochemical results change completely when the reactions are carried out under what are called photochemical, rather than thermal, conditions. Irradiation, or photolysis,... 

Disrotatory cis-S,6-dimethyl- 1,3-cyclohexadiene conrotatory fraw-5,6-d Unethyl -1,3-cvclohexadiene. Disrotatory closure occurs. 

Benzoquinone Fivalic [Propanoic acid, 2,2-dimethyl-] 2-tert-Butyl [2,5-Cyclohexadiene-1,4-dione, 2- (1,1 -di methy lethy 1) - ] 67" 2... 

Engberts [3e, 9] has extensively investigated the Diels Alder reaction in aqueous medium. Recently Engberts and colleagues reported [9c] a kinetic study of a Diels Alder reaction of N-alkyl maleimides with cyclopentadiene, 2,3-dimethyl-1,3-butadiene and 1,3-cyclohexadiene in different solvents. The reaction rates of the cycloadditions with the open-chain diene relative to w-hexane are reported in Table 6.3. The aqueous medium greatly accelerates the Diels Alder reaction and the acceleration increases as the hydrophobic character of the alkyl group of the dienophile increases. These and other kinetic data [3e, 9], along with the observation that the intramolecular Diels-Alder reaction is also accelerated in... 

METHOXY-2,4-CYCLOHEXADlEN-l-YL] -5,5-DIMETHYL-l, 3-CYCLO-HEXANEDIONE]lRON,57, 16 TRICARBONYL [(1,2,3,4,5-n)-2-METH-OXY-2.4-CYCLOHEXADIENE-1-YL ] -IRON(l+)HEXAFLUOROPHOS-PHATE(l-), 57, 107... 

Isoprene or 2,3-dimethyl-l,3-butadiene or 1,3-cyclohexadiene (with Et2NH), 2,3-dimethyl-1,3-butadiene (with n-BuNH2 or piperidine) and 1,3-hexadiene or 2,4-hexa-diene (with PhNH2) similarly give 1 1 telomers in fair to good yields [186]. 

Zimmerman and co-workers were also able to obtain some information regarding the multiplicities of the excited states responsible for the initial /9-cleavage through quenching and sensitization studies. It was found that both trans-to-cis and cis-to-trans isomerizations could be sensitized by chlorobenzene under conditions where the latter absorbed over 95% of the light. The same product ratio was obtained under these conditions as in the direct irradiation of the ketones. With 1,3-cyclohexadiene or 2,5-dimethyl-2,4-hexadiene as quenchers nearly 90% of the reaction of the trans isomer could be quenched. Again the ratio of the quenched reaction products was the same as in the unquenched reaction. The reaction of the cis isomer, on the other hand, could not be quenched by 1,3-cyclohexadiene or 2,5-dimethyl-2,4-... 

Upon treatment with nickeltetracarbonyl, dimethyl 3-vinyl-l,2-dichlorocy-clobutane-l,2-dicarboxylate 169 is rearranged, to dimethyl 1,4-cyclohexadiene-1,2-dicarboxylate 171 with concomitant loss of the chlorine atoms [87], Reduction to dimethyl 3-vinylcyclobutene-l,2-dicarboxylate 170 is involved in the initial step. (Scheme 64)... 

Giguere performed tandem ene-intramolecular Diels-Alder reactions between 1,4-cyclohexadiene (29) and dimethyl acetylenedicarboxylate (26b) in sealed tubes in a commercial microwave oven (Scheme 9.6) [47]. 

For the parent 6-methylene-1,4-cyclohexadiene a planar structure was found149 the d -dimethyl derivative, however, gave a dihedral angle of 8°152, which should diminish slightly the cross-conjugation. 

The products of electrochemical oxidation of conjugated dienes are considerably affected by the reaction conditions such as the material of the electrode, the supporting electrolyte and the solvent. The oxidation of butadiene with a graphite or carbon-cloth anode in 0.5 M methanolic solution of NaClCU mainly yields dimerized products along with small amounts of monomeric and trimeric compounds (equation 5)1. The use of platinum or glassy carbon mainly gives monomeric products. Other dienes such as isoprene, 1,3-cyclohexadiene, 2,4-hexadiene, 1,3-pentadiene and 2,3-dimethyl-l,3-butadiene yield complex mixtures of isomers of monomeric, dimeric and trimeric compounds, in which the dimeric products are the main products. 

Cyclobutyl-cis-4-trans-5-dimethyl- 1,3-dioxolane, by reaction of erythro-3-methane-sulfonyloxy-2-butyl cyclo-butanecarboxylate with sodium borohydride, 51, 12 hydrolysis to cyclobutane-carboxaldehyde, 51, 13 3,5—CYCLOHEXADIENE—1, 2-DI CARBOXYLIC ACID, 50, 50 A1 a-Cyclohexaneacetaldehyde, 53, 104... 

Silylated 1,4-cyclohexadienes, such as 58, are accessible by the Birch reduction of resorcin dimethyl ether and subsequent one-pot silylation-methylation. The reduction of bromo adamantane with 58, occurs readily in the presence of the radical initiator AIBN, with the driving force being the aromatization leading to 59 (Scheme 14). 

Scheme 6.71 Generation of l-oxa-3,4-cyclohexadiene (333) from 6,6-dichloro-3-oxabicyclo[3.1. OJhexane (332) by n-butyllithium and interception of333 by n-butyllithium, styrene, 1,3-butadiene, isopreneand 2,3-dimethyl-l,3-butadiene.

Scheme 6.7S Prod ucts of the cycloadditions of 1 -oxa-2,3-cyclohexadiene (351), liberated from exo-6-bromo-endo-6-fluoro-2-oxabicyclo[3.1.0]-hexane (354), to styrenes, 1,3-butadiene, isoprene, 2,3-dimethyl-l, 3-butadiene,...

Scheme 6.76 Generation of l-oxa-2,3-cyclohexadiene (351) from 5-bromo-3,4-dihydro-2H-pyran (376) and trapping products of351 obtained from furan, 2,3-dimethyl-1,3-butadiene, 1,1-diphenyl-ethylene, ( )-l-phenylpropene, ( )-2-butene, (Z)-2-butene and tert-butyl alcohol , according to Schlosserand co-workers.

---