Cyclopentadiene, irradiation

Strongly affaited by the presence of cationic substrates. 2,4-Dichloro-l,3-diazaanthracene and 2,4-dimethoxy-l,3-diazaanthracene yield the anti-KY dimers, (140) and (141) respectively, and the corresponding anti-HH dimers on irradiation in benzene. In the solid state only the anti-HT dimers are formed. In the presence of 1,3-cyclopentadiene, irradiation yields single [4+4]cycloadducts (142) and (143) respectively. ... 

Methylene thiirane is obtained by thermolysis of several spirothiirane derivatives which are formally Diels-Alder adducts of methylenethiirane and cyclopentadiene or anthracene <78JA7436). They were prepared via lithio-2-(methylthio)-l,3-oxazolines (c/. Scheme 121). A novel synthesis of the allene episulfide derivatives, 2-isopropylidene-3,3- dimethylthiirane (good yield) or its 5-oxide (poor yield), involves irradiation of 2,2,3,3-tetramethyl-cyclopropanethione or its 5-oxide (81AG293). Substituents on the thiirane ring may be modified to give new thiiranes (Section 5.06.3.9). The synthesis of thiirane 1-oxides and thiirane 1,1-dioxides by oxidation is discussed in Section 5.06.3.3.8 and the synthesis of 5-alkylthiiranium salts by alkylation of thiiranes is discussed in Section 5.06.3.3.4. Thiirene 1-oxides and 1,1-dioxides may be obtained by dehydrohalogenation of 2-halothiirane 1-oxides and 1,1-dioxides (Section 5.06.4.1.2). 

There has been new information on the products of photolysis of derivatives of compound 1. Low temperature irradiation of the ester 254 gives a ketene (93JACS8621) the isolation of an isomeric ketene from a 3-pyridyldiazo ester suggests the involvement of the open chain form 255. Photolysis of the 3-phenyl derivative 256 in the presence of cyclopentadiene gives exo and endo cyclopropanes and a dipyridylstilhene, suggesting the intermediacy of the carhene 257 (99JOC6635). 

While still slightly warm from the drying oven, the photolysis vessel with a water-jacketed quartz immersion well (Note 1) (section A of Figure 2) is charged with 500 ml. of anhydrous tetrahydrofuran (Note 2) and 10 ml. (8.05 g., 0.122 mole) of cyclopentadiene (Note 3). The solution is cooled in an ice bath and purged with dry nitrogen for 2 minutes. Then the vessel is sealed, the lamp inserted, and the solution irradiated at 0° for 30 minutes. During this period, sections B and C... 

A series of studies on CpMn(CO)j and related compounds showed similarly that -Mn(CO)5 compounds are formed in substantial yield. The yield of MnCp2 could not be assessed properly inasmuch as the carrier seems to exchange rapidly with some radioactive species not containing cyclopentadiene 80). The data for various manganese-containing compounds are summarized in Table VII. In addition to those compounds reported in the table, two other, as yet unidentified, radioactive compounds were found in neutron-irradiated CpMn(CO)3 20). 

Another case in which bonds were made where none existed previously is that of irradiation of AsClj in benzene solution (76) which led to formation of PhAsCl2 and PhjAsCl, and even PhjAs. Yet another example 51) is the formation of FeCpj by irradiation of Fe(CO)j in cyclopentadiene (monomer) solution. While these examples may not tell us when the reaction does occur, they do show that initial bonding is not indispensable. 

CuS04-catalyzed decomposition of the (l-sila-cyclopentadienyl)diazomethane 398 did not furnish defined products. The desired rearrangement reactions to a silabenzene and a l-ethylidene-l-sila-2,4-cyclopentadiene, both trapped by /-butanol, were brought about by irradiation of 398, however 388... 

Microwave heating has also been employed for performing retro-Diels-Alder cycloaddition reactions, as exemplified in Scheme 6.94. In the context of preparing optically pure cross-conjugated cydopentadienones as precursors to arachidonic acid derivatives, Evans, Eddolls, and coworkers performed microwave-mediated Lewis acid-catalyzed retro-Diels-Alder reactions of suitable exo-cyclic enone building blocks [193, 194], The microwave-mediated transformations were performed in dichloromethane at 60-100 °C with 0.5 equivalents of methylaluminum dichloride as catalyst and 5 equivalents of maleic anhydride as cyclopentadiene trap. In most cases, the reaction was stopped after 30 min since continued irradiation eroded the product yields. The use of short bursts of microwave irradiation minimized doublebond isomerization. 

Another example of the retention of volatile DA reagents is that of cyclopentadiene in a tandem retro-DA/DA prime reaction [15, 16, 38], This reaction type is the thermal decomposition of a DA adduct (A) and the generation of a diene (generally the initial diene) which is trapped in situ by a dienophile leading to a new adduct (B) [39]. Cyclopentadiene (22) (b.p. 42 °C) is generated by thermolysis of its dimer at approximately 160 °C [40]. An equimolar mixture of commercial crude dicyclopenta-diene (21) and dimethyl maleate was irradiated in accordance with the GS/MW process, in an open reactor, under 60 W incident power, for 4 min (8 x 30 s). The expected adduct 23 was isolated in 40% yield (Scheme 7.1). The isomeric composition of 23 (endo-endoIexo-exo = 65/35) was identical with that obtained under classical conditions from 22 and methyl maleate [41]. The overall yield of this tandem reaction can be increased from pure dimer 21 (61%) and the same tandem reaction has also been reported using ethyl maleate as dienophile [31]. 

Cyclopentadiene behaves differently than the cyclohexadienes in that its radiolysis leads to high molecular weight polymer via a cationic mechanism89, whereas such compounds are not formed in high yield from cyclohexadienes irradiated in the liquid phase. 

One method of producing penta(methoxycarbonyl)cyclopentadiene radical is by irradiation of [SnBu3(OH2)2]+[C5(C02Me)5]" (reaction 36)58. The radical thus produced is remarkably stable, with the ESR signal not decreasing in intensity one hour after cessation of photolysis. 

Reactions of methoxycarbonylformonitrile, furonitrile and substituted benzoni-trile oxides (4-Me, 4-OMe, 3-OMe, 4-C1, 3-C1, 2,4-di-Cl, 4-F as substituents) with dimethyl 7-(diphenylmethylene)bicyclo[2.2. l]hept-2-ene-5,6-dicarboxylate led exclusively to exo cycloadducts 82 (R = C02Me, 2-furyl, substituted phenyl), which, on irradiation with a low-pressure mercury lamp, afforded 3-azabicyclo [4.3.0]nonadiene-7,8-dicarboxylates 83 as the only products. The 1,3-dipolar cycloaddition, followed by a photorearrangement, provides a new method for obtaining tetrahydro-27/ -pyridine derivatives from cyclopentadiene (245). 

Gedanken and colleagues136 investigated the Diels-Alder reactions of trichloromethyl allenyl sulfoxides 203 and cyclopentadiene under ultrasound irradiation. Allenes 203 are generally very sluggish in reactivity. However, when ultrasound was applied, the reactions of allenes 203 with cyclopentadiene were completed within 2 hours (equation 57). Mixtures of endo (204) and exo (205) isomers were obtained in all instances. When the y-position of the allenyl sulfoxides was substituted, additional mixtures of E and Z isomers were obtained. 

TABLE 6. Influence of ultrasonic ))) irradiation on the yield and [endo]/[exo] product ratio of the reaction of cyclopentadiene with methyl vinyl ketone in various organic solvents84... 

Irradiation of an aqueous solution at 296 nm and pH values from 8 to 13 yielded different products. Photolysis at a pH nearly equal to the dissociation constant (undissociated form) yielded pyrocatechol. At an elevated pH, 2-chlorophenol is almost completely ionized photolysis yielded cyclopentadienic acid (Boule et al., 1982). Irradiation of an aqueous solution at 296 nm containing hydrogen peroxide converted 2-chlorophenol to catechol and 2-chlorohydroquinone (Moza et al, 1988). In the dark, nitric oxide (10 vol %) reacted with 2-chlorophenol forming 4-nitro-2-chlorophenol and 6-nitro-2-chlorophenol at yields of 36 and 30%, respectively (Kanno and Nojima, 1979). 

Equation (2) proceeds as written when irradiation is carried out either with incandescent light (from a 300-W bulb for 1 hr) or sunlight.41 Besides the N-methyl compound some cyclopentadiene is also reported. This photolysis is carried out in dilute aqueous sodium hydroxide. 

Exercise 28-17 1,3-Cyclopentadiene gives the following substances on irradiation... 

The Diels-Alder adduct (Formula 190) from cyclopentadiene and benzo-quinone gives a photoisomer formulated as Structure 191 on the basis of the absence of double bonds and the infrared carbonyl absorption of the photoprbduct (74). Similar products are obtained from the hexachlorocyclopentadiene-benzoquirione adduct and the cyclo-pentadiene-chloranil adduct (74). Irradiation of the cyclooctatetraene-benzoquinone adduct (Formula 192) gives a photoproduct formulated as Structure 193 (74). 

Upon UV irradiation in hydrocarbon solution, the hexacarbonyls of chromium, molybdenum, and tungsten react differently with conjugated dienes like 1,3-butadiene (la), ( )-l,3-pentadiene (lb), 2-methyl-1,3-butadiene (lc), ( , )-2,4-hexadiene (Id), ( )-2-methyl-l,3-pentadiene (le), 2-ethyl-1,3-butadiene (If), or 1,3-cyclohexadiene (Ig). Chromium hexacarbonyl (2) yields, with the acyclic dienes la-lf, tetracarbonyl-r/2-dienechromium(0) complexes (3a-3f) in a smooth reaction (8-10). With 1,3-cyclohexadiene, in addition to 3g, dicarbonylbis(>/4-l,3-cyclohexadiene)chromium(0) (4g) is obtained [Eqs. (7) and (8)j. During chromatography on silica gel, the 1,3-cyclohexadiene complex 3g dismutates readily to [Cr(CO)6] and 4g [Eq. (9)]. Under the same conditions with 2 1,3-cyclopentadiene (lh) yields, in a hydrogen-transfer reaction, the stable dicarbonyl- / 5-cyclopentadienyl-r/ 3-cyclopent-enylchromium (5) (11-13) [Eq. (10)]. 

The next three chapters are by Inoue and Mori, Albini, and Rossi, and deal with alkene photoisomerization reactions, the modification of benzylic positions and photochemical aromatic substitution reactions. (E)-2-cyclo-heptenone is produced upon irradiation of the Z-isomer at — 50 °C and can be trapped by cyclopentadiene to afford the adduct 13 [13]. Benzyl-substituted dihydroisoquinolinium derivatives can be used for the photochemical synthesis of tetrahydroisoquinolines. The corresponding... 

Sensitized addition of cyclic dienes with chlorinated alkenes, employing an excess of the latter, yields mixtures of [4+2] and [2+2] adducts (Sch. 10). A substantial proportion of [4+2] adduct 46a is formed when cyclopentadiene 32 is the diene, but cyclohexadiene 36 yields almost entirely the [2+2] adduct 46b. Use of acyclic 1,3-dienes leads only to [2+2] products. The regioselectivity of the cycloadditions is consistent with a biradical intermediate 48 [47]. Sensitized irradiation of cyclopentadiene with 1-acetoxy acrylonitrile 49 also gives a [4+2] and [2+2] mixture, but with a higher proportion of the [4+2] adduct than the reactions using chloro-alkenes 45 [33-35]. 

Cyclic enones with ring sizes of six-to-eight carbons can be photochemically induced to undergo [4+2] cycloadditions via isomerization to a strained trans isomer (Schs. 22-24). Irradiation of 2-cycloheptenone 99 leads to [2+2] dimerization of an intermediate r -2-cycloheptenone 100, but if this irradiation is conducted with an excess of cyclopentadiene 32 at —50 °C, a single [4+2] adduct 101 is isolated in very high yield [65,66]. The somewhat less strained nms-2-cyclo-octenone can be generated and trapped by subsequent addition of a cyclopentadiene [67,68]. Extension of this reaction to intramolecular examples has recently been reported [69]. 

Furans do not undergo [4+4] dimerization, however irradiation of a mixture of 2,5-dimethylfuran with an excess of cyclopentadiene leads to a mixture of two cross products 132 and 133 (Sch. 30) [84]. The alkene position in 133 was not determined, but 133 could have been formed by Cope rearrangement of the cis [4+4] adduct 134 that was not observed. 

Upon direct irradiation at 300 nm (Z)-2-cyclo-octenone 80Z isomerizes to the (ii)-isomer 80E, affording an 80 20 EfZ mixture. The (ii)-isomer 80E is trapped by 80Z or 5,5-dimethoxy-l,2,3,4-tetrachlorocyclopentadiene at room temperature to give the cyclodimers 81 and 82 or the Diels-Alder adduct 83, respectively (Sch. 27) [127]. Similarly, ( )-2-cycloheptenone 84E is produced upon irradiation of 84Z at — 50 °C, and is also trapped by cyclopentadiene to afford the adduct 85 (Sch. 28) [128]. The formation of the (ii)-isomer as a transient intermediate has been proved by IR spectroscopy at — 160°C [129]. Laser flash photolysis of (Z )-2-cyclohep-tenone 84Z revealed that the ( )-isomer has a lifetime of 45 s in cyclohexane... 

The 3,3-dimethyl-5-alkynyl-3H-pyrazoles 28, obtained by addition of 2-diazopropane to diacetylene, were irradiated to give the alkynylvinylcarbenes 29 and 30 the relative reactivity of which depended mainly on the substituent R on the triple bond. These carbenic species were trapped by cyclopentadiene or furan to give the ethynylcyclo-propane derivatives 31 and 32, respectively, Eq. (10) 23). 

Common to these molecules with their cyclopentadiene moieties is the so-called fulvene subunit 27. The first fulvenes, 6,6-dialkylfulvenes, were prepared as early as 1906 by Thiele et al. from sodium cydopentadienide and ketones [16]. The parent hydrocarbon 27 and many other derivatives have been thoroughly studied since the 1960s [17-19]. Diazocyclo-pentadiene (28), which is also easily prepared from cydopentadienide, is a heteroanalogue of fulvene. It has frequently been used as a precursor to other theoretically interesting molecules containing annelated cydopentadiene moieties, because its irradiation readily generates the cyclopentadienylidene 29. This carbene has, for example, been trapped with alkynes to form spiro-annelated cydopentadiene derivatives 30 (Scheme 5) [20]. It has been proved by UV spectroscopy [21] and supported by calculations [22] that these spiro[2.4]heptatrienes (so-called [1.2]spirenes) 30 experience a spedal kind of electronic... 

Perhaps the best qualitative and quantitative scavenger studies in these ionic polymerizations have been made with ammonia and several amines. Busier, Martin, and Williams (8) have shown that, at —78°C. the initial polymer yield from irradiated cyclopentadiene increases linearly with the ratio of [C5H6]/[NH3] over the range 1-10. This would be consistent with a mechanism in which all growing chains are terminated by reaction with ammonia, according to ... 

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