Dimerization, 1,3-cyclohexadiene

Tricyclo 4.4.2.0l 6]dodeca-3,11-dien E17f, 740 (2-C1 — tricycl.-thiolan — 5,5-dioxid/ROK) ejru-Tricyclo 6.2.2.02,7 dodeca-3,9-dien IV/5a, 296/E19c, 90 (1,3-Cyclohexadien-Dimer.) Tricyclo]6.4.0.02 7]dodeca-3,5-dien E17e, 241 [Cyclooctatetraen + Br—(CH2)4 — Br/LiNH2] Tricyclo 6.4.0.0I 7 dodeca-3,ll-dien IV/5a, 296... 

Many Diels-Alder reactions have been shown to be accelerated in the presence of Lewis acid species (Inukai and Kojima, 1971). Several workers have capitalized on the highly organized character of the clay surface, in conjunction with its ability to hold water and bind potentially catalytic Lewis acids, to successfully carry out Diels-Alder syntheses that have been difficult to accomplish in solution. For example, 1,3-cyclohexadiene dimerized at 0°C in dichloromethane containing Fe(III)-doped... 

A recent report of Schuster and Sussman244 indicates that the lowest triplet of eucarvone sensitizes cyclohexadiene dimerization while an upper triplet apparently undergoes cycloaddition to this diene. 

The 1,4-dicyanonaphthalene-sensitized photocyclodimerization of the vinyl ethers (192), yielding (193) has been described. 9,10-Dicyanoanthracene-sensitized dimerization of cyclohexa-1,3-diene affords the two [4+2] adducts (194) and (195) in a total yield of 60% and in a ratio of 4 1. This is to be contrasted with the previous report of cyclohexadiene dimerization where [2 + 2] dimers were also obtained." A detailed study of the photodimerization of some acenaphthylenes (196) has been reported. " The structures and configurations of the products have been elucidated by spectroscopic methods." ... 

Thiophene 1,1-dioxide (61) is too unstable to isolate and dimerizes with loss of S02 to give 3a, 7a-dihydrobenzothiophene 1,1-dioxide (172) in 34%113. However, alkyl-substituted thiophene 1,1-dioxides can serve as dienes in the Diels-Alder reaction, since the aromatic properties of the thiophene nucleus are lost completely and the n-electrons of the sulfur atom are used for forming the bond with oxygen. The sulfones 173-178 are found to react with two moles of maleic anhydride at elevated temperature to give bicyclic anhydrides114. Thus, at high reaction temperature, S02 is split off to give cyclohexadiene... 

The dimerization of 1,3-cyclohexadiene gives 30% adduct after 20 h at 30 °C [32]. In the presence of a catalytic amount of tris(p-bromophenyl) aminium hexachloroantimonate (ArsN SbCle Ar = /iBrC6H4) in CH2CI2 at 0°C, the cyclodimerization occurs in 15 min with 70% yield with a greater diastereos-electivity endojexo = 5 1) than that observed under thermal... 

A derivative of cyclopentyne has been trapped in a matrix. Although cycloheptyne and cyclohexyne have not been isolated at room temperatures, Pt(0) complexes of these compounds have been prepared and are stable." The smallest cyclic allene" so far isolated is l-/err-butyl-l,2-cyclooctadiene 107." The parent 1,2-cyclooctadiene has not been isolated. It has been shown to exist transiently, but rapidly dimerizes." " The presence of the rert-butyl group apparently prevents this. The transient existence of 1,2-cycloheptadiene has also been shown," and both 1,2-cyclooctadiene and 1,2-cycloheptadiene have been isolated in platinum complexes." 1,2-Cyclohexadiene has been trapped at low temperatures, and its structure has been proved by spectral smdies." Cyclic allenes in general are less strained than their acetylenic isomers." The cyclic cumulene 1,2,3-cyclononatriene has also been synthesized and is reasonably stable in solution at room temperature in the absence of air." ... 

The radical dissociation of the Gomberg dimer , [3-(diphenyl-methylidene)-6-(triphenylmethyl)-l,4-cyclohexadiene] [48], is familiar to organic chemists as the original observation of carbon-carbon a bond dissociation in a solution (Gomberg, 1900 Lankamp et al., 1968). The yellow colour of the triphenylmethyl radical in the benzene solution should have been an observation convincing synthetic organic chemists of the stable existence of the triphenylmethyl radical [8-j. 

Irradiation of 1,3-cyclohexadiene in the presence of a sensitizer affords dimers (19) and (20) as well as two other dimers 24-26 ... 

As with cyclopentadiene, the relative yields of products (19)—(21) were relatively insensitive to the sensitizer triplet energy(37) although an effect of temperature on dimer distribution has been noted.0,28 The direct photolysis of cyclohexadiene with wavelengths greater than 330 nm yielded products (19)—(21), although the product distribution in this case was more nearly statistical [(19) (20) (21) = 44% 2470 33%] 28 . 

In the paper published in 1900, he reported that hexaphenylethane (2) existed in an equilibrium mixture with 1. In 1968, the structure of the dimer of 1 was corrected to be l-diphenylmethylene-4-triphenylmethyl-2,5-cyclohexadiene 3, not 2 [38]. Since Gomberg s discovery, a number of stable radicals have been synthesized and characterized, e.g., triarylmethyls, phenoxyls, diphenylpicryl-hydrazyl and its analogs, and nitroxides [39-43]. The radical 1 is stable, if oxygen, iodine, and other materials which react easily with it are absent. Such stable radicals scarcely initiate vinyl polymerization, but they easily combine with reactive (short-lived) propagating radicals to form non-paramagnetic compounds. Thus, these stable radicals have been used as radical scavengers or polymerization inhibitors in radical polymerization. 

Irradiation of mixtures of cyclohexene with 1,3-cyclohexadiene leads to high yield of dimers (G = 6.3)86. Schutte and Freeman87 found that radiolysis of 1,3-cyclohexadiene dissolved in various solvents gives dimers mainly via cationic Diels-Alder addition,... 

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. 

The C3S11XY geometry is also found, but involving a chelate ring as in the set136 of molecules (15), Me2SnX (1,4-cyclohexadiene-COOMe) for X = Cl, Br, I and also Me. When X = F, a further very weak interaction occurs (Sn- -F = 364.1 pm) to form a six-coordinate dimer with an unsymmetric Sn—F- -Sn—F- four-membered ring. Parameters are listed in Table 8. 

The dichlororuthenium arene dimers are conveniently prepared by refluxing ethanolic ruthenium trichloride in the appropriate cyclohexadiene [19]. The di-chloro(pentamethylcyclopentadienyl) rhodium dimer is prepared by refluxing Dewar benzene and rhodium trichloride, whilst the dichloro(pentamethylcyclo-pentadienyl)iridium dimer is prepared by reaction of the cyclopentadiene with iridium trichloride [20]. Alternatively, the complexes can be purchased from most precious-metal suppliers. It should be noted that these ruthenium, rhodium and iridium arenes are all fine, dusty, solids and are potential respiratory sensitizers. Hence, the materials should be handled with great care, especially when weighing or charging operations are being carried out. Appropriate protective clothing and air extraction facilities should be used at all times. 

Beyond dimerization and oligomerization, [2 + 2]- and [4+ 2]-cycloadditions with conjugated dienes and styrenes and the addition of nucleophiles are typical reactions of strained cyclic allenes. These transformations have been studied most thoroughly with 1,2-cyclohexadiene (6) and its derivatives [1, 2]. Concerning the cycloadditions, a theoretical study had the surprising result that even the [4+ 2]-cycloadditions should proceed in two steps via a diradical intermediate [9]. In the case of nucleophiles, the sites of attack at several 1,2-cyclohexadiene derivatives having an... 

The cycloadditions of 1-substituted 1,2-cyclohexadienes and among them their dimerization are of interest because of the position selectivity. Does the reaction occur at the substituted or the unsubstituted ethylene subunit For that question to be answered, 1-methyl- (74), 1-phenyl- (75), 1-cyclopropyl- (76), l-(3-phenylpropyl)-(77) and l-trimethylsilyl-l,2-cyclohexadiene (79) were generated from the corresponding 1-substituted 6,6-dibromobicyclo[3.1.0]hexanes with methyllithium. Several of these dibromides are thermolabile, which particularly applies to the phenyl (93) [76] and the cydopropyl derivative [70], In those cases, it is advisable or necessary to prepare the dibromide in situ, that is, the dibromocarbene is liberated from tetrabro-momethane with methyllithium at -60 °C in the presence of the respective cyclopen-tene. Without workup, from the thus formed 6,6-dibromobicyclo[3.1.0]hexane, the 1,2-cyclohexadiene is then generated by addition of methyllithium at -30°C. 

Scheme 6.74 Generation of 1-oxa-2,3-cyclohexadiene (351) from 6,6-dibromo-2-oxabicyclo[3.1.0]hexane (350), dimerization of351 and its trapping by styrene.

A review of Diels-Alder reactions of fullerenes with acyclic and cyclic dienes has been presented. The addition of substituted pyrimidine o-quinodimethanes (75) to [60]fullerenes yields novel organofullerenes (76) bearing a pyrimidine nucleus covalently attached to the Ceo cage (Scheme 26). The Diels-Alder dimerization of cyclopenta[/]phenanthrene (77) with isobenzindene (78) yields the dimer (79) in 85% yield (Scheme 27). Further evidence has been supplied to support an early reorganization of the r-network in the dimerization of 2-methoxycarbonylbuta-1,3-diene. The Lewis acid-catalysed Diels-Alder reactions of acrylate derivatives of new carbohydrate-based chiral auxiliaries with cyclohexadiene show excellent endo. exo... 

Of conrse, the cyclic cation-radical formed should be less stable than the alkene cation-radical (which contains a double bond that is favorable for the spin-charge scattering). However, the cation-radical product and corresponding nentral species are generated in a concerted process. The process involves simultaneous covalent bond formation and one-electron reduction of the cyclic product (Karki et al. 1997). Similar to other branched-chain processes, the cation-radical dimerization is characterized by an activation enthalpy that is not too high. These magnitudes are below 20 kJ mol for the pair of cyclohexadiene and trani-anethole (p-MeOCgH4CH=CHCHMe, Z-form Lorenz and Bauld 1987). It is clear that the cation-radical variant of cyclodimerization differs in its admirable kinetic relief. For cyclohexadiene and tran -anethole, catalytic factors are 10 and 10, respectively (Bauld et al. 1987). 

The reaction of o-xylene showed the formation of dimethylphenols in about 63% yield together with 21% of the dimer of 6,6-dimethyl-2,4-cyclohexadien-l-one, which involves ipso attack followed by a methyl shift and cycloadditive dimerization of the intermediate 6,6-dimethyl-2,4-cyclohexadien-l-one catalyzed by the acid (Scheme 5). 

Photoaddition reactions can destroy the original sensitizer, form a new sensitizer, or form a substance that quenches the desired reaction completely. Since substrate would also be consumed, its measured rate of disappearance will be anomalously high and that of product appearance too low. The importance of ensuring that the sensitizer acts as a true catalyst in a photo-reaction is demonstrated by a study88 in which 1,2-benzanthracene (13) was used to sensitize the dimerization of cyclohexadiene. The quantum yield of dimer formation was found to decrease at higher sensitizer concentration ... 

This rearranges by proton shift to the 2-hydro-phosphinic acid 85b, two molecules of which associate by hydrogen bridging to the dimer. The long-wave maximum at 275 nm (e = 4075), (Amax2 = 237 nm, e = 2925) confirms the phospha-cyclohexadiene (2,4) system. 

Diets-Alder catalysis.2 This cation radical enhances the reactivity of a neutral or electron-rich eis-1,3-diene in Diels-Alder reactions. Thus 1,3-cyclohexadiene undergoes Diels-Alder dimerization only at temperatures around 200°. The presence of 5-10 mole % of this salt effects dimerization even at —78°, with the usual endo/ exo selectivity (5 1). It also permits facile condensation of 1,3-cyclohexadiene with a hindered dienophile such as 2,5-dimethyl-2,4-hexadiene (equation 1) the dimer of the former diene is a minor product (20% yield). 

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