Ethyl biradical

The deoxygenation of tetrahydrofuran (THE, 90), which yields ethylene and carbon monoxide, is an interesting case. While this and other deoxygenations might be expected to proceed through an yild intermediate and a biradical as shown in Eq. 50, calculations (MP2/6-31G ) indicate that neither ylid 91 nor biradical 92 is an intermediate in this reaction. These calculations reveal a concerted removal of oxygen that proceeds to carbon monoxide and two molecules of ethylene without barrier. Experimental evidence that 91 is not an intermediate is provided by the fact that reaction of carbon with a mixture of 90 and 90-d generates ethylene and ethyl-ene-t/g in a 2.7 1 ratio.This secondary isotope effect of 1.13 (per D) would not be expected if 91 (or 92) were an intermediate. 

Mono- and di-alkylated furans were synthesized in a one-pot preparation from 2-propynyl-2-tetrahydropyranyl ether (106), butyllithium and formaldehyde. The intermediate allenyl ether (107) presumably cyclizes via a 2-(2-tetrapyranyloxy)-2,5-dihydrofuran (108) to afford the heterocycle (109) (79AG(E)875). In a similar manner, singly and doubly branched tetrahydropyranyloxybutynolates afforded the substituted furans (110) (Scheme 20). The thermocatalytic isomerization of ethyl l-methyl-2-phenylcyclopropene-3-carboxylate yielded the furan, possibly by a 1,3-sigmatropic displacement step or by a non-concerted biradical intermediate (75T2495). 

A number of cycloadditions have been shown to arise by initial bond homolysis in the heterocycle followed by the addition of the resulting biradical to an alkene. The photoaddition of 2-phenylbenzothiazole (367) to ethyl vinyl ether to give the 1,5-benzothiazepine (368) can be interpreted in this way.313 Similar pathways have been proposed to account for the... 

Furthermore, it was shown that alpha- as well as ortho-substituents in such ketones retard the photocyclization due to other competing reactions. However, photocyclization showed interesting stereochemical trends, which were also strongly affected by the solvent polarity and the phase (Scheme 8.7). For example, the photocyclization of a-(o-ethyl phenyl acetophenone (27) either in benzene or as a solid favored the isomer with the methyl and phenyl group trans to R, due to the conformational preferences in the 1,5-biradical intermediate [9]. 

Horaguchi and coworkers [19, 20] have studied extensively the effect of solvents and the groups attached to the 8-carbon, and have concluded that polar solvents lower both photocyclization yield and diastereoselectivity. Studies with different substrates have revealed that the development of diastereoselectivity is the result of conformational preferences of the biradical rather than of the steric interactions between the two radicals. The same research group has also studied the photocyclization of ethyl oc-(o-benzoylphenoxy)carboxylates (59), and reported that the product yields and Z/E ratios (60a 60b) in benzene and acetonitrile are, interestingly, much the same as for other ketones in which R is not an electron-withdrawing group [20] (Table 8.2). 

Griesbeck and Mattay described photocycloaddition of methyl and ethyl trimethyl pyruvates (25) with di-isopropyl-1,3-dioxol. In contrast to the reaction with ethyl pyruvate, the bicyclic oxetane 26 was formed with very high (>98%) diastereoisomeric excess (Sch. 7) [29]. An X-ray analysis revealed the unusual endo-tert-butyl configuration. Semiempirical calculation indicated that this clearly is the kinetic product formed by a biradical... 

An attempt to affect the ratio of products from [47] by inclusion of ethyl iodide led to negative results. This experiment was run with the hope that intersystem crossing of some intermediate (e.g., S, biradical, or oxacarbene) might lead to a change in the product distribution and the reaction stereospecificity. 

The photochemical reactions of ethyl phenylglyoxalate (217) in benzene have been re-examined. The three new products (218), (219) and (220) have been isolated from the reaction mixture. The quantum yields for the formation of the products are dependent on concentration. Irradiation of the phenylglyoxalate derivatives (221) results in conversion into the lactones (222) from (221, R = H) and (223) and (224) from (221, R = Me). The reactions are proposed to involve an intramolecular electron transfer process forming a zwitterionic biradical. This leads to activation of the methylenes adjacent to the sulfur atom. A similar effect is observed with the nitrogen analogue (225) which affords (226) as the... 

Photocyclisation of 8-alkoxy-l,2,3,4-tetrahydro-l-naphthalenones and 4-alkoxy-6,7,8,9-tetrahydro-5H-benzocyclohepten-5-ones gives naphtho[l, 8-bc]furans and cyclohepta[cd]benzofurans respectively, and conformational and substituent effects of 1,5-biradicals in the cyclisation process are discussed." " The same authors also describe substituent effects on the photocyclisation of ethyl 2-(8-oxo-5,6,7,8-tetrahydro-l-naphthyloxy)acetates and ethyl 2-(5-oxo-6,7,8,9-tet-rahydro-5H-benzocyclohepten-4-yloxy)acetates to give naphtho[l,8-bc]furans and cyclohepta[c,d]benzofurans respectively." Also reported are cyclisations involving photogenerated radical cations of unsaturated silyl enol ethers, fragmentation cyclisations of unsaturated ot-cyclopropyl ketones which occur by photoelectron transfer and give polycyclics, and kinetic and theoretical studies of [2+3] cycloadditions of nitrile ylids. These reactions have been studied mechanistically and their synthetic potential investigated. 

Others have reported that there is a definite effect of aryl substituents and that the derivative 39 undergoes cyclization in the presence of dissolved oxygen to yield the hydroxyflavone 40. The cyclization involves the formation of a biradical 41 that cyclizes in the presence of oxygen to yield the hydroxyflavone 42. The cyclization of such chalcones has been known for many years and studied in some detail . Research showed that the derivatives 43 undergo efficient cyclization to 44 (Scheme 4) on irradiation at wavelengths >365 nm in dioxan or ethyl acetate solution . The reaction is solvent-dependent and poorer yields are obtained in benzene or chloroform solution . Further studies demonstrated, for the conversions shown in Scheme 5, that the cyclizations probably arose from a 7171 transition . ... 

Pincock and his co-workers have studied the photochemical fragmentation reactions of the esters (31). This system has an in-built electron accepting sensitiser. When (31a-c) are irradiated in methanol the principal reaction is fission to yield the styrene (32) and p-cyanobenzoic acid. The other products formed from the reactions are the styrene addition products (33)-(35). The authors propose that the Norrish T) e II process in this instance involves a proton transfer and this occurs within the zwitterionic biradical formed as the primary intermediate on electron transfer. Further proof of the authenticity of this mechanism was obtained by irradiation of the deuteriated derivatives (31 d, e). The results of a study of the photochemical decomposition of benzyl phenylacetate, as a suspension in water over Ti02, have been reported. Bond fission is the result of irradiation of (36) in cyclohexane/ethyl acetate. A Norrish Type II hydrogen abstraction occurs with the elimination of the enone moiety. This affords a path to the CD ring system (37) of vitamin D. 

Mixed cycloadditions, for example, between cyclopentenone and ethyl vinyl ether, have also been observed frequently (Schuster, 1989). As indicated in Scheme 25, attack of the n,K ) excited state of the enone on the olefin will give a triplet 1,4-biradical that ultimately yields cyclization and disproportionation products (Corey et al., 1964 De Mayo, 1971). The regio-... 

Rate data have been obtained from a detailed analysis of the previously reported 1420 maximum and fall off in the photodimerization of acenaphthylene with increasing concentrations of dissolved ethyl iodide.1420 The heavy-atom solvent dibromomethane facilitates the cross cycloaddition of acenaphthylene to trans- and cw-penta-1,3-diene by inducing intersystem crossing to the triplet state of the aromatic hydrocarbon. The intermediacy of a biradical is proposed and the stereospecificity can be understood in terms of the maintenance of the stereochemical integrity of the allylic radical units.143... 

Perfluoro-4-ethyl-2,3,4,5-tetramethyl-4,5-dihydrofuran 78 was prepared from tetrafluoroethene pentamer. The possible reaction mechanism may involve the formation of biradical 79 by one-electron transfer from enolat-ion 80 to the CF2 carbon that undergoes loss of fluoride ion followed by intramolecular cyclization of the brradical 79. " ... 

The regio- and stereochemistry of the Paterno-Buchi reaction depend on the structures of the reactants, on the electronic energy of the excited state carbonyl compound, and on the multiplicity of the excited state. With unsymmetrical alkenes, the products suggest preferential formation of the more substituted radical center in the biradical intermediate, but steric and electronic factors are also important. Stereoselectivities depend on the multiplicity of the excited state. For example, the reaction of excited singlet and triplet states of propanal (84) with 2,3-dihydrofuran (85) gave the diastereomers of 7-ethyl-2,6-dioxabicyclo[3.2.0]heptane (86) in different ratios (equation 12.63). ... 

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