Cyclopentenes irradiation

Copper(i) triflate is an excellent catalyst for the photochemical cyclodimerization of cyclopentene. Irradiation of the annelated cyclopentene (336) with olefins and acetylenes provides a useful route to propellanes such as (337). ... 

Dehalogenation of 2-chlorocarbonyl-l,3-dithian in the presence of cyclopentadiene alfords the bicycloheptenone (341), which can be stereospecifically opened to either the CIS (342) or irons (343) disubstituted cyclopentene. Irradiation of the cyclo-... 

The thermal or photolytic fragmentation of furazans to nitriles and nitrile Af-oxides has been reported (73JOC1054, 75JOC2880). The irradiation of dimethylfurazan (419) in the presence of cyclopentene, and benzofurazan (420) in the presence of dimethyl acety-lenedicarboxylate, gave isoxazoline (421) and isoxazole (422), respectively, in good yields. The thermolysis of acenaphtho[l,2-c]furazan (423) in the presence of phenylacetylene gave isoxazole (424) in 55% yield. 

This reachon, of industrial interest, utilizes singlet oxygen generated by irradiation in the presence of Rose Bengal [40]. An endoperoxide is formed as intermediate which is converted to 2-cyclopentene-l,4-diol by reduchon with thiourea. 

Varions possibilities were considered for the nnderlying reaction of the biradical. No radical signals grew when the biradical decayed, so H-abstraction from the matrix did not appear to be occnrring. Analysis of products formed from irradiations of 8 at 5.5 K showed both bicyclopentane 10 and cyclopentene, in a ratio of 30 1. Very similar ratios, ca. 25 1, were observed in solution irradiations at room temperature. It was noted that if the major tnnneling reaction was H-shift to produce cyclopentene, this product should be enhanced as temperatnres were lowered, in contrast to the experimental observations. Hence, it was conclnded that the observed decay of the EPR spectrum of 9 was due to ring closure to give 10. 

With two y,8 double bonds, two a,/3 double bonds, and the possibilities of cis and trans ring fusions with syn and anti configurations, 20 isomeric dimers are possible. Surprisingly, only one product is formed in a head-to-tail fashion. The sole product of the irradiation of the 3,5-diene-7-ketosteroid (76), however, is the head-to-head dimer. The specificity and mode of addition arise presumably through the effect of the specific environment of the chromaphore. The dimerization of (75) is believed to involve the addition of the a,fi double bond of a photoexcited molecule to the less hindered y,8 double bond of a ground state molecule. The photocondensation of (76) with cyclopentene, in which steric hindrance should not be a controlling factor, was found to yield a cyclobutane product involving the a,/ bond of the steroid in contrast to dimerization across the y,8 bond. 

The research groups of Mariano and West developed a photoinduced electrocydi-zation/nucleophilic addition sequence. Thus, irradiation of N-alkylpyridinium perchlorates as 5-19 in an aqueous solution led to the aziridine cations 5-20, which react in a nucleophilic addition with OH to give the isolable azabicyclo[3.1.0]hex-2-enols 5-21. These can be further transformed by a nucleophilic ring-opening of the aziridine moiety under acidic conditions to lead to useful unsymmetrically trans,trans-trisubstituted cyclopentenes 5-22 (Scheme 5.5) [10]. 

Increased ring strain in the C-4 ( = 2) and C-5 (n — 3) products may be a reason for lower yields (and longer irradiation times). The effect of strain on cyclizations is well-documented outside of cycloaromatization chemistry, as well. For example, annealing of a cyclopentene ring leads to a decrease in the cyclization rate and an inversion of the 5-exo/6-endo selectivity for all three patterns shown in Fig. 22. 

An equilibrium mixture of the cyclohexane- 1,3-dione (42) and its enol form (43) was irradiated in the presence of cyclopentene in MeOH to afford the intermediate (44), which was readily transformed to the tricyclic intermediate (45) and subsequently followed an retroaldolization sequence to give the cyclooctanedione (46) in 90 % yield. When refluxed with titanium trichloride and K metal in THF for 5 min., compound (46) gave the diol (47) 21K... 

Sunlamp irradiation of butynyl iodide (6) in the presence of hexabutylditin generates an alkyl radical that reacts with an electron-deficient alkene (7) to form an (iodomethylene)cyclopentene (8) in moderate yield. This product can be reduced by Bu3SnH (AIBN) to the methylenecyclopentane (9).2... 

Data on c clohexene and a pinene aerosols were reported by Schwartz after a preliminary report from the Battelle Institute group. The experimental conditions and analytic techniques were identical with those just described for the toluene aerosol study. Here again, only the methylene chloride-soluble, water-insoluble fractions were studied. They accounted for about 7% and 65% of the total aerosol mass generated from cyclohexene and a pinene, respectively. Grosjean (unpublished data) has investigated the chemical composition of cyclopentene, cyclohexene, and 1,7-octadiene aerosols. Experiments were conducted in an 80-m Teflon smog chamber filled with ambient air, with irradiation by... 

The first example of a tri-Tr-methane reaction was reported by Zimmerman et al. in their smdy of the irradiation of triene 135 in the solid state that affords cyclopentene 136 exclusively (for reviews, see Refs. 75-78). In this instance the cisoid conformation of the biradical, necessary for cyclopentene formation, is... 

Another interesting observation in this study is the boron trifluoride etherate-catalyzed rearrangement of tri-ir-methane systems 141 that afford the corresponding cyclopentenes 142. These reactions can be considered as the first examples of tri-ir-methane rearrangements in the ground state. Interestingly, compounds 141 only undergo conventional di-TT-methane reactions on irradiation. The mechanism shown in Scheme 25 is proposed to account for this novel reaction [79]. 

Little is known about the modes of thermal and photochemical cleavage of -triazoles. 2,4,5-Triphenyltriazole is reported to be photostable, but 2,4,5-trimethyltriazole gives acetonitrile (22%) and several other products when irradiated in ether. In cyclopentene, an adduct to be expected from a nitrilimine intermediate is isolated (Scheme 59). 

Ring contraction also resulted on irradiation (185 nm) of cyclopentenes, to form methylenecy-clobutanes in low yield. Thus, photolysis of cyclopentene gave methylenecyclobutane (36) in 27% and bicyclo[2.1.0]pentane in 28% yield,110 via an intermediate carbene shown by rearrangement of [3,4-2H2]cyclopentene. Labeling in the product was found almost exclusively in endocyclic positions,111 Analogously, 1 -methyl-1 -vinylcyclobutane (37) was obtained from 1,2-dimethylcyclopentene in 20% yield.112... 

Irradiation of 2,5,5-trimethyl-2-prop-l-ynylcyclopentanone in pentane (0.1 M) for 72 hours gave, with 70% total conversion, the cyclobutane 3 as the major product (38%) together with various open-chain products and cyclopentene derivatives.150... 

Several photochemically induced vinylcyclopropane to cyclopentene rearrangements of nor-carene derivatives to form bicyclo[3.2.0]heptenes can be understood as ring contractions of cyclohexenes to cyclobutanes. Upon direct irradiation of norcar-2-ene (bicyclo[4.1.0]hept-2-ene) at 214 nm (pentane solution), however, complex product mixtures were obtained containing only small amounts of bicyclo[3.2.0]hept-2-ene, while toluene sensitized photolysis in 50 millimolar solution in degassed pentane at 254 nm gave mainly the cyclobutane derivative 13 in addition to EjZ-isomeric hepla-l,3,6-trienes.72... 

Cyclopentenes behave differently and often act through radical mechanisms this can lead to photoreduction to cyclopentanes, or photoaddition of the kind exemplified by norborneneand propan-2-ol 12.57). The photoadduct in this process is linked through the carbon atom of the alcohol, and not the oxygen atom. A related addition to acetonitrile 12.58) takes place when norbornene is irradiated in the presence of a silver(i) compound. It is likely thal a metal complex of the alkene is the real irradiation substrate, and the same may be true for copper(i)-promoted additions of haloalkanes to electron-deficient alkenes (2.59). When dichloromelhane is used in such a reaction the product can be reduced electrochemically to a cyclopropane (2.60), which is of value because the related thermal addition of CH.I, to alkenes in the presence of copper does not succeed with electron-poor compounds. 

Irradiation of 2,4,6-trimethylpyrylium perchlorate in water gives a rearranged pyrylium salt (4) which is cleaved to the 5-oxohexenal (3). Replacement of the 4-methyl by A-t-butyl alters the course of the photolysis to give the cyclopentene (5) as the main product, together with small amounts of several others (72CC1240,73JA2406). 

Many [2 + 2] cycloadditions that do not occur by simply heating the possible reactants can be achieved by irradiation with ultraviolet light. The following example, [2 + 2] addition of 2-cyclopentenone to cyclopentene, occurs photochemically but not thermally ... 

The most commonly observed dimerization is that of alkenes to form cyclobutane derivatives. Nonconjugated alkenes such as cyclopentene and norbornene are dimerized in the presence of a sensitizer, whereas conjugated alkenes dimerize directly dimerizations of the second type have been observed in dienes, phenyl-ethylenes, and a,/9-unsaturated carbonyl, cyano, and nitro derivatives. The precise structure and stereochemistry of many of these dimers is uncertain, although it is known to be influenced by the solvent, by the presence and nature of substituents, and by the use of a sensitizer. The structures of dimers formed in solid-state irradiations are often determined by crystal structure. 

Mercury-sensitized irradiation of cyclopentene (Formula 414) gives vinylcyclopropane (Formula 415) (182a). This process shows a reasonably high quantum efficiency (0.24) (182a). The reverse reaction takes place thermally. 

The vinylcyclopropane radical cation, generated at 77 K by X-irradiation of (139) in a Freon-113 matrix, was shown to rearrange at 105-110 K to afford two ring-opened distonic radical cationic species.300 The rearrangement reactions of the radical cations of 1,3- and 1,4-pentadiene and cyclopentene and the formation of spin adducts with 2.4.6-tri-/-butylnitrosobenzene (BNB) are discussed. The pulse radiolysis of 1,1 -binaphthyl-2,2,-diyl hydrogenphosphate (BiNPCUH) (140) in deaerated f-butanol at... 

Meta photocycloaddition was discovered simultaneously and independently by two groups in 1966. Wilzbach and Kaplan [4] found that the adducts from m-but-2-ene, cyclopentene, and 2,3-dimethylbut-2-ene with benzene are substituted tricyclo 3.3.0.02X]oct-3-enes. The adducts were formed by irradiation of solutions (-10%) of the olefins in benzene, at room temperature under nitrogen, with 2537-A light. Bryce-Smith et al. [5] subjected an equimolar mixture of m-cyclooctene at room temperature or in the solid phase at 60°C to ultraviolet radiation of wavelength 235-285 nm. A mixture of 1 1 adducts was obtained from which the main component (-85%) was readily obtained pure by treatment of the mixture with methanolic mercuric acetate. This 1 1 adduct proved to be a meta photocycloadduct (Scheme 2). The minor nonaromatic adduct (10-15%) could, at that time, not yet be obtained completely free from the meta photocycloadduct the structure of a rearranged ortho adduct was provisionally assigned to this isomer. 

Para photocycloaddition of arenes to the benzene ring was first reported in 1971 by Wilzbach and Kaplan [7] as a minor process accompanying ortho and meta photocycloaddition. Since that time, relatively few cases of para photocycloaddition have been described. Para adducts were found as minor products from benzene with cyclobutene [8], ra- 3,4 - dimethyIcycIobu(ene [9], vinylene carbonate [10], 2,3-dihydropyran [11,12], and 1,3-dioxole [13,14] and from a,a,a-tri-fluorotoluene with vinylene carbonate [15], Intermolecular para photocycloadducts were major products from the irradiations of benzene and allene [16,17], benzene and cyclonona-1,2-diene [16,17], and from fluorobenzene and cyclopentene [18], Intramolecular para photocycloadducts were found as major products from the irradiations of phenethyl vinyl ether [19-21] (Scheme 3) and 2,3-dimethyl-6-phenylhex-2-ene [22], No detailed mechanistic investigations have been published. 

In 1966, two groups simultaneously discovered photochemical addition reactions of simple alkenes to excited benzenes. Wilzbach and Kaplan [4] reported that upon irradiation of 10% solutions of r/.v-but-2-ene, cyclopentene, and 2,3-dimethylbut-2-ene in benzene with light of 254 nm, meta photocycloadducts are formed. Bryce-Smith, et al. [5] irradiated an equimolar mixture of benzene and cA-cyclooctene with 254-nm light which yielded a mixture of 1 1 adducts. The main component ( 85%) was a 1 1 adduct similar to that found by Wilzbach and Kaplan, a meta photocycloadduct. The minor (10-15%) 1 1 adduct was unstable and underwent thermal isomerization. It was suggested with some reservations that the minor adduct was tetracydo[6.6.0.02,7.03,6]tetradec-4-ene (I, Fig. 1). In 1968, this structure was rejected [37], and in 1973, the product was shown to be the ortho adduct, tricyclo[6.6.0.02,7]tetradeca-3,5-diene [38] (II, Fig. 1). 

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