Cyclohexadiene reaction

James and Troughton ( ) obtained ethylene and 1,3,5-hexatriene as the primary products in their study on the reaction of diallyl with the ethyl radical at 134- 175 C. Furthermore, they obtained 1,3-cyclohexadiene as a successive product. Recently Orchard and Thrush (19) reported the thermal isomerization of 1,3,5-hexatriene to 1,3-cyclohexadiene at ca. 400 C and the consecutive formation of benzene at ca. 550 C. In the present work, 1,3-cyclohexadiene (reaction 17) and benzene (reaction 18) were obtained as the secondary products. The hydrogen atom produced in reactions 12,... 

Scheme 5 summarizes the types of displacement reactions that have been reported, and it is evident that these provide facile routes to a whole range of adducts, often in high-yield reactions. This is in contrast to the reactions of Os3(CO)12, when some of the intermediate products cannot be isolated. Thus in the reaction of H2S and Os3(CO)12, the only product identified is the sulfur-capped species, H2Os3(CO)9S the potential intermediate [HOs3(CO)i0HS] is readily formed in the cyclohexadiene reaction and smoothly converts to the capped species on heating. 

Draw the orbitals for the hexatriene-cyclohexadiene reaction 4.47 —>4.48 and its reverse, in the style 4.40, 4.41 and 4,42 used for the corresponding cyclobutene-butadiene reaction, identify the developing overlap, and hence show that the reaction is disrotatory in both directions. 

It is frequent but not invariable that where a longer conjugated system has a geometrically accessible and symmetry-allowed transition structure like that in 5.90, the longer system is used. Thus, the [8+2] and [6+4] cycloadditions on pp. 15 16, and the [14+2] cycloaddition on p. 44 take place rather than perfectly reasonable Diels Alder reactions, and the 8-electron electrocyclic reactions of 4.51 and 4.54 takes place rather than disrotatory hexatriene-to-cyclohexadiene reactions. This kind of selectivity is called periselectivity. 

More interesting were the results obtained in reactions of 99 with acyclic dienes catalyzed by Eu(fod)3 (-20°C) and TiCl4 (-78°C). The resulting adducts 100 are unstable and underwent spontaneous sulfinyl elimination at room temperature, affording cyclohexadienes. Reactions with dienes lacking substituents at C-1 (butadiene, 2-methyl butadiene and 2,3-dimethyl butadiene) yielded optically pure compounds 101 (Scheme 49). These results indicate that the regio-selectivity and the 7r-facial selectivity of the cycloadditions are complete (only one adduct is formed) under both catalytic conditions. Desulfinylation of 100 is also completely regioselective. 

X. Su, K. Y. Rung, J. Lahtinen, Y. R. Shen, G. A. Somorjai, 1-3 and 1-4 cyclohexadiene reaction intermediates in cyclohexene hydrogenation and dehydrogenation on Pt(lll) crystal surface a combined reaction kinetics and surface vibrational spectroscopy study using sum frequency generations, J. Mol. Catal. A 1999, 141, 9-19. 

The first two rows of Table 20.1 summarize the results so far. They also point out that the cyclobutene-1,3-butadiene interconversion is a four-electron process, whereas the hexatriene-l,3-cyclohexadiene reaction involves six electrons. In the cyclobutene-butadiene reaction, the four electrons in the O and it orbitals of the cyclobutene come to occupy the Oi and 2 orbitals of 1,3-butadiene. Similarly, the six electrons in the o, i, and O2 of 1,3-cyclohexadiene become the six electrons in Oj, 2, and 3 of the 1,3,5-hexatriene molecule. 

Compounds 3.934a,b react with the mercury(II) acetate (CCU, 12 hours, 20°C) to form dithiol-2-ones 3.935a,b in 60% yield. These compounds 3.935a,b are stable in solution and do not cyclize when heated to 180°C for 6 hours in the presence of 1,4-cyclohexadiene. Reaction of dithiol 3.935a with two equivalents of potassium ethoxide (MeOH, 15... 

Cationic diarylethenes were synthesized by appending an imidazolium directly connected to the central ethane unit as an aryl group in order to participate in the photochromic hexatriene-cyclohexadiene reaction. Scheme 4.9. [47] At least one of these compounds (1-open form) is described in the experimental part as being an oil, and probably is an intrinsically photochromic ionic liquid, see below. [48] The solvatochromic behaviour of 1 was studied in two ionic liquids, [EMIM][NTf2] and [EMIM] [EtS04], as well as in some common solvents. The closed ring form shows different absorption spectra depending on the anion of the IL. The red form is stabilized in [EMIM] [EtSOJ while the yellow form in... 

We illustrate the method for the relatively complex photochemistry of 1,4-cyclohexadiene (CHDN), a molecule that has been extensively studied [60-64]. There are four it electrons in this system. They may be paired in three different ways, leading to the anchors shown in Figure 17. The loop is phase inverting (type i ), as every reaction is phase inverting), and therefore contains a conical intersection Since the products are highly strained, the energy of this conical intersection is expected to be high. Indeed, neither of the two expected products was observed experimentally so far. 

Alkenes in (alkene)dicarbonyl(T -cyclopentadienyl)iron(l+) cations react with carbon nucleophiles to form new C —C bonds (M. Rosenblum, 1974 A.J. Pearson, 1987). Tricarbon-yi(ri -cycIohexadienyI)iron(l-h) cations, prepared from the T] -l,3-cyclohexadiene complexes by hydride abstraction with tritylium cations, react similarly to give 5-substituted 1,3-cyclo-hexadienes, and neutral tricarbonyl(n -l,3-cyciohexadiene)iron complexes can be coupled with olefins by hydrogen transfer at > 140°C. These reactions proceed regio- and stereospecifically in the successive cyanide addition and spirocyclization at an optically pure N-allyl-N-phenyl-1,3-cyclohexadiene-l-carboxamide iron complex (A.J. Pearson, 1989). 

Barrelene was obtained via a double Diels-Alder reaction from a-pyrone with methyl acrylate (H.E. Zimmerman, I969A). The primarily forming bicyclic lactone decarboxylates in the heat, and the resulting cyclohexadiene rapidly undergoes another Diels-Alder cyclization. Standard reactions have then been used to eliminate the methoxycarbonyl groups and to introduce C—C double bonds. Irradiation of barrelene produces semibullvalene and cyclooctatetraene (H.E. Zimmerman. 1969B). 

The diacetoxylation of E,E)- and ( ,Z)-2.4-hexadiene (351 and 353) is stereospecific, and 2,5-dimethylfurans (352 and 354) of different stereochemistry have been prepared from the isomers. Two different carboxylates are introduced with high cis selectivity by the reaction of 1,3-cyclohexadiene and... 

The wM-diacetate 363 can be transformed into either enantiomer of the 4-substituted 2-cyclohexen-l-ol 364 via the enzymatic hydrolysis. By changing the relative reactivity of the allylic leaving groups (acetate and the more reactive carbonate), either enantiomer of 4-substituted cyclohexenyl acetate is accessible by choice. Then the enantioselective synthesis of (7 )- and (S)-5-substituted 1,3-cyclohexadienes 365 and 367 can be achieved. The Pd(II)-cat-alyzed acetoxylactonization of the diene acids affords the lactones 366 and 368 of different stereochemistry[310]. The tropane alkaloid skeletons 370 and 371 have been constructed based on this chemoselective Pd-catalyzed reactions of 6-benzyloxy-l,3-cycloheptadiene (369)[311]. 

An intramolecular version offers useful synthetic methods for heterocycles. The total syntheses of a- and 7-lycoranes (373 and 374) have been carried out by applying the intramolecular aminochlorination of the carbamate of 5-(2-aminoethyl)-l,3-cyclohexadiene (372) as a key reaction[312,313]. Interestingly, the 4,6- and 5,7-diene amides 375 and 377 undergo the intramolecular amina-tion twice via x-allylpalladium to form alkaloid skeletons ofpyrrolizidine (376) and indolizidine (378), showing that amide group is reactive[314]. 

The TT-allylpalladiLim complexes formed as intermediates in the reaction of 1,3-dienes are trapped by soft carbon nucleophiles such as malonate, cyanoacctate, and malononitrile[ 177-179). The reaction of (o-iodophenyl-methyl) malonate (261) with 1,4-cyclohexadiene is terminated by the capture of malonate via Pd migration to form 262. The intramolecular reaction of 263 generates Tr-allylpalladium, which is trapped by malononitrile to give 264. o-[odophenylmalonate (265) adds to 1,4-cyciohexadiene to form a Tr-allylpalladium intermediate via elimination of H—Pd—X and its readdition, which is trapped intramolecularly with malonate to form 266)176]. 

The cyclization of the enediynes 110 in AcOH gives the cyclohexadiene derivative 114. The reaction starts by the insertion of the triple bond into Pd—H to give 111, followed by tandem insertion of the triple bond and two double bonds to yield the triene system 113, which is cyclized to give the cyclohexadiene system 114. Another possibility is the direct formation of 114 from 112 by endo-rype. insertion of an exo-methylene double bond[53]. The appropriately structured triyne 115 undergoes Pd-catalyzed cyclization to form an aromatic ring 116 in boiling MeCN, by repeating the intramolecular insertion three times. In this cyclization too, addition of AcOH (5 mol%) is essential to start the reaction[54]. 

The cyclohexadiene derivative 130 was obtained by the co-cyclization of DMAD with strained alkenes such as norbornene catalyzed by 75[63], However, the linear 2 1 adduct 131 of an alkene and DMAD was obtained selectively using bis(maleic anhydride)(norbornene)palladium (124)[64] as a cat-alyst[65], A similar reaction of allyl alcohol with DMAD is catalyzed by the catalyst 123 to give the linear adducts 132 and 133[66], Reaction of a vinyl ether with DMAD gives the cyclopentene derivatives 134 and 135 as 2 I adducts, and a cyclooctadiene derivative, although the selectivity is not high[67]. 

Reduction of arenes by catalytic hydrogenation was described m Section 114 A dif ferent method using Group I metals as reducing agents which gives 1 4 cyclohexadiene derivatives will be presented m Section 1111 Electrophilic aromatic substitution is the most important reaction type exhibited by benzene and its derivatives and constitutes the entire subject matter of Chapter 12... 

If the Lewis base ( Y ) had acted as a nucleophile and bonded to carbon the prod uct would have been a nonaromatic cyclohexadiene derivative Addition and substitution products arise by alternative reaction paths of a cyclohexadienyl cation Substitution occurs preferentially because there is a substantial driving force favoring rearomatization Figure 12 1 is a potential energy diagram describing the general mechanism of electrophilic aromatic substitution For electrophilic aromatic substitution reactions to... 

Vinylboranes are interesting dienophiles in the Diels-Alder reaction. Alkenylboronic esters show moderate reactivity and give mixtures of exo and endo adducts with cyclopentadiene and 1,3-cyclohexadiene (441). Dichloroalkenylboranes are more reactive and dialkylalkenylboranes react even at room temperature (442—444). Dialkylalkenylboranes are omniphilic dienophiles insensitive to diene substitution (444). In situ formation of vinyl-boranes by transmetaHation of bromodialkylboranes with vinyl tri alkyl tin compounds makes possible a one-pot reaction, avoiding isolation of the intermediate vinylboranes (443). Other cycloadditions of alkenyl- and alkynylboranes are known (445). 

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). 

The synthesis of natural products containing the quinonoid stmcture has led to intensive and extensive study of the classic diene synthesis (77). The Diels-Alder cycloaddition of quinonoid dienophiles has been reported for a wide range of dienes (78—80). Reaction of (2) with cyclopentadiene yields (79) [1200-89-1] and (80) [5439-22-5]. The analogous 1,3-cyclohexadiene adducts have been the subject of C-nmr and x-ray studies, which indicate the endo—anti—endo stereostmcture (81). 

BenZotrichloride Method. The central carbon atom of the dye is supphed by the trichloromethyl group from iJ-chlorobenzotrichloride. Both symmetrical and unsymmetrical triphenyhnethane dyes suitable for acryhc fibers are prepared by this method. 4-Chlorobenzotrichloride is condensed with excess chlorobenzene in the presence of a Lewis acid such as aluminium chloride to produce the intermediate aluminium chloride complex of 4,4, 4"-trichlorotriphenylmethyl chloride (18). Stepwise nucleophihc substitution of the chlorine atoms of this intermediate is achieved by successive reactions with different arylamines to give both symmetrical (51) and unsymmetrical dyes (52), eg, N-(2-chlorophenyl)-4-[(4-chlorophenyl) [4-[(3-methylphenyl)imino]-2,5-cyclohexadien-l-yhdene]methyl]benzenaminemonohydrochloride [85356-86-1J (19) from. w-toluidine and o-chloroaniline. 

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