SET Induced Reactions

2 SET Induced Reactions - Further studies on ring opening reactions of 1,2-diarylcyclopropanes have focused on compounds (83) bearing an acetyl functional group.All of the cyclopropane derivatives studied show efficient cis.trans isomerism with reasonable quantum efficiency. The isomerism reaction involves an acetophenone-like triplet state with lifetimes shorter than 1 ns. The formation of ring opened products also takes place but inefficiently to give a mixture of the two alkenes (84) and (85). A study of the electron-transfer-induced photochemical reactivity of the cyclopropane (86) has been reported in the co-sensitisation system (biphenyl/phenanthrene/DCA).  

The photochemical reactivity of vinylcyclopropane under electron-transfer conditions has been studied The reactions were carried out using DCB/ phenanthrene as the sensitiser system and in a solvent mixture of acetonitrile/ methanol (3 1). The three products (89)-(91) result from ring opening and trapping by solvent. The radical cation of (92) can be formed photochemically using DCB as the electron-accepting sensitiser. When the reaction is carried out in acetonitrile/methanol as the solvent mixture the two products (93) and 

Miyashi et alP have previously demonstrated that the isomerism and ring opening of the methylene cyclopropanes (95) is initiated by SET processes. The present study has examined in detail the steps within the system. SET photochemistry of (95) can be brought about by the use of DCA, TCNB or NMQ BF4 as sensitisers and the formation of the radical cation (96) was demonstrated as was the formation of the biradical (97) which is formed from (96) by a back electron-transfer step. 

2 Other Reactions Involving Cyclopropane Rings. 2.2.1 SET Induced Reactions The radical cation of phenylcyclopropane can be obtained by irradiation in the presence of chloranil as the electron accepting sensitizer.  

The details of the electron-transfer processes involved on irradiation of mixtures of tetrachloro-p-benzoquinone with the cyclopropane derivatives (58) and (59) have been reported. The intermolecular electron-transfer processes lead to the formation of the radical cation of the cyclopropanes. Ring-opening of the cyclopropanes results from these species.  

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Dienes closely related to the norbornadiene system have also proved of interest in SET-induced reactions. Thus, the diene 86 can be transformed into the corresponding radical... 

A study of the photochemical Cope reaction of the hexadienes 40 has been carried out under photoinduced electron-transfer conditions. Evidence was gathered for the formation of a chair cyclohexane-1,4-radical cation 41 °. In snch systems, where the radical cation is formed using DCA as the sensitizer, a degenerate Cope process is operative. Thus when the tetradeuterio derivative 42 is used, rearrangement affords a (52 48) mixture of the two dienes 42 and 43. Related to this general problem, DCA-sensitized reactions of the isomeric dienes 44 and , -45 and the cyclization prodnct, the bicyclohexane 46, have been studied in considerable detail. At low conversions, the irradiation of 46 affords a mixture of the dienes 44 and , -45 in ratios that are independent of temperature. The influence of the position of the aiyl groups on the diene skeleton has also been studied. This does not appear to affect the conversion to a cyclic radical cation. Thus the SET-induced reaction of the diene 47 has shown that the open chain radical cation of the diene 48 cyclizes preferentially to the radical cation 49. ... 

CF2Br2 and HCF2I have also been utilized in reductively-induced reactions to provide 1 1 adducts [229, 256, 258-260] A mechanistic study by Wu et al. provided strong evidence for the SET nature of the CF2Br2/Zn process for alkene addition [261]. 

SET-induced chain reactions are usually much faster than Sn2 reactions, and if both pathways are accessible for a given pair of starting materials, products resulting from SrnI will usually predominate. This is, for example, observed for the first reaction sketched in Scheme4.4 [11, 16]. Electrochemical or zinc-mediated [17] reduction of the starting pyridinium salt yields a carbanion which, on reaction with Me2tBuS+, can either be methylated via Sn2 or tert-butylated via SrnI. Because the nucleophile... 

CHS, and nitric oxide (Chap. 15). Radicals have a typical set of reactions in the gaseous state, many will remove thin films of metals from hot glass, and in solution they often can induce polymerization of compounds having carbon-carbon double bonds. Thus, a solution of ferrous sulfate and hydrogen peroxide (such a mixture is called Fenton s reagent) qatalyzes the conversion of acrylonitrile, CH2=CH—CN, to its polymer neither H202 alone nor Fe2+ alone is effective as a catalyst. 

An approach based on samarium diiodide-promoted reactions of the iminoketone 279 with aldehydes provides access to a series of substituted or fused pyrroles bearing at least two phenyl groups, for instance, the system 280 (Equation 86) <2001T4881>. In an alternative lanthanide-catalyzed route, a series of pyrroles were constructed from imines and nitroalkenes in the presence of Sm(fPrO)3 <1999T13957>. A set of 1-dimethylaminopyrroles have also been obtained by TiCU-induced reactions between 2-acetoxypropanal hydrazones with silyl enol ethers <1995TL8007>. 

While identification of the relevant electrophores in both examples above has enabled specification of the site at which the reaction will occur, it will be difficult for the non-expert reader to predict the primary reaction of the radical ion. Hence, we have written this review with the intention of providing guidelines for the identification of primary reaction channels of radical ions and information about their kinetic reactivity, which should enable the reader to set up defined ET-induced reactions even in polyfunctional molecules. 

A single electron transfer- (SET) induced ring enlargement has been found in the conversion of substituted 2-furylmethanols (10) into 6-hydroxytetrahydropyran-3-ones (12) by the action of the binary reagent PhI(0Ac)2-Mg(C104)2 it has been suggested that the reaction occurs via a furan radical cation (11) as illustrated in Scheme 10 [44],... 

Zimmerman and Hofacker have studied the photochemically induced SET reactivity of the 1,4-dienes (74). The sensitizers used were dicyanoanthracene and dicyanonaphthalene. The radical cations of the 1,4-dienes undergo regioselective cyclization to the cyclic radical cations (75) which ultimately afford the final products (76). The SET-induced photochemistry of other non-conjugated dienes such as geraniol (77) has been studied. The results demonstrate that with DCA as the sensitizer in methylene chloride a contact radical-ion pair is involved and this yields the cyclopentane derivatives (78) and (79) in the yields shown. The cyclization is the result of a five-centre cyclization. With the more powerful oxidant dicyanobenzene as the sensitizer and in acetonitrile as solvent, separated radical-ion pairs are involved and this leads to the formation of the bicyclic ethers (80) and (81). DCA-sensitized reactions of the dienes (82) and E,E-(S3) and the bicyclohexane (84) have been studied. At low conversion the irradiation of (84) under these conditions affords a mixture of the dienes (82) and , -(83) in ratios that are independent of temperature. 

Miscellaneous Rearrangements and Bond Fission Processes - Chan and Arnold have carried out calculations to confirm that the two electrons in the biscation (145) are delocalized three-dimensionally. A review has given details of specific photoreactions of acyclic and cyclic saturated hydrocarbons that can provide paths for their functionalization. A study of the SET-induced photochemical bond fission processes in the series of 2-alkoxyphenylethers (146), (147) and (148) has been studied. The reaction involves bond cleavage in the inter-... 

Other Addition Reactions - The photochemical addition in the gas phase of ammonia to a,P-unsaturated nitriles has been studied. The reactions are carried under conditions where all, or most, of the light is absorbed by the ammonia and this results in N-H bond fission and the production of NH2 radicals. These radicals add to the a-position of the substrate to yield 2-amino-propionitrile from acrylonitrile and analogous products are formed from crotononitrile, methacrylonitrile and 1-cyclohexenecarbonitrile. Additions also occur to but-2-yne nitrile which yields the Z, -isomers of 3-amino-crotononitrile with the E -isomer being predominant. The SET-induced amina-tion (using 1,4-dicyanobenzene as the sensitiser) of alkenylnaphthalene derivatives affords products of addition to both the alkene and the naphthalene skeleton. Suau et al. have examined the irradiation of phthalimide in the presence of low concentrations of hydroxide ion and alkenes. ° The result of this treatment is addition of the phthalimide moiety to the alkene. A SET... 

The results from a detailed study of the solvent effect on the SET induced photochemical reactions of phthalimides and related maleimides have been reported.For example, irradiation of the phthalimide (218, n = 4) in acetonitrile gives a mixture of the three products (219, n=l), (220) and (221, n = 4). When a water/acetonitrile (35 65) mixture is used as the solvent the reaction is very selective and affords the benzoindolizidine (221, n = 4) in 94% yield. The same selectivity is observed with the phthalimide (218, n = 5) although in this example the change in efficiency is not as dramatic. The results from a study of the formation of the ylide (222) by irradiation of the phthalimide derivatives (223) have been reported. ... 

SET induced cyclisations have been carried out on a series of non-conjugated dienes one component of which is a silyl ether. Irradiation in acetonitrile of dienes such as (330, R = H) using DCA as the electron accepting sensitiser results in the formation of the radical-cation of the diene which on cyclisation affords the cyclic ketone (331) in 25% yield. The reaction is solvent sensitive and in a mixture of acetonitrile/propan-2-ol yields three products which are identified as (331, 30%) and two minor products (332, 11%) and (333, 9%). The reaction is suggested to have some considerable synthetic potential and the effect of chain length and substituents on the reaction has been evaluated. Thus (334) is converted into (335, 11%) while (330, R = Me) affords an isomeric mixture of products (336). Other studies with silyl ether derivatives have examined the electron transfer-induced ring opening processes encountered with the cyclopropane derivatives (337). ... 

Recovery and the restoration to dark state in the rod and cone cells require three sets of reactions, a, b, and c. The light-induced lowering of cytosolic Ca + levels (see Figure 38-9) initiates the process of adaptation to dark (see text for details). Pj, Inorganic phosphate , stimulation I, inhibitory subunit of phosphodiesterase. 

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