Cyclopropanes photoreactions

Alkoxycarbene complexes with unsaturation in the alkyl side chain rather than the alkoxy chain underwent similar intramolecular photoreactions (Eqs. 10 and 11) [60]. Cyclopropyl carbene complexes underwent a facile vinyl-cyclopropane rearrangement, presumably from the metal-bound ketene intermediate (Eqs. 12 and 13) [61]. A cycloheptatriene carbene complex underwent a related [6+2] cycloaddition (Eq. 14) [62]. 

The photoelimination of nitrogen from 1-pyrazolines is one of the most thoroughly investigated photoreactions and it has been used extensively in the synthesis of cyclopropane derivatives.334 Both stereospecific and non-stereospedfic processes have been observed and these are believed, at least in simple 1-pyrazolines, to correspond to singlet and triplet excited states, respectively. Two reaction pathways have been proposed in the azoalkane 405335 direct excitation via a thermally activated S, state affords the C6H6 isomers 406 to 409, whereas triplet-sensitized excitation results in a tem-... 

Sarel and co-workers have examined some reactions of alkynylcyclopropanes with iron carbonyl compounds [1]. Treatment of cyclopropylacetylene (5) with iron pentacarbonyl under photolytic conditions gives, after cerium(IV) oxidation, isomeric quinones 6 and 7, derived from two molecules of 5 and two carbonyls with both cyclopropane rings intact [6]. Furthermore, the photoreaction of dicyclopropylacetylene (8) with iron carbonyl gives some ten different products depending on the reagents and the reaction conditions, and some of them have the cyclopentenone skeleton formed by the opening of cyclopropane ring coupled with carbonyl insertion [7] (Scheme 2). 

Exposure of several methyl-substituted derivatives to y-radiolysis at 77 K in cryogenic matrices gave rise to a family of radical cations of the same structure type, some of which had been previously identified on the basis of CIDNP results. We begin with a discussion of the CIDNP investigations, since they preceded the ESR studies of all species but the prototype. The first CIDNP results attributed to a cyclopropane radical cation were observed during the photoreaction between 1,4-dicyanonaphthalene and cis-l,2-diphenylcyclopropane. However, the nature of the cyclopropane radical cation was characterized by CIDNP effects observed during the reaction of chloranil with cis- and /rans-l,2-diphenylcyclo-propane. ... 

Figures, h CIDNP spectra (cyclopropane resonances) observed during the electron transfer photoreaction of chloranil with c/s-1,2-diphenylcyclopropane (fop) and ben-zonorcaradiene (.bottom). The opposite signal directions observed for analogous protons in the two compounds constitute evidence that the two radical cations belong to two different structure types.

Cyclopropane doivatives of various structure types are involved in electron transfer-induced oxygenation reactions. For example, the photoreaction of 9,10-di-cyanoanthracene with 1,2-diarylcyclopropanes generates a mixture of cis- and... 

However, electron transfer-induced photoreactions in the presence of nucleophiles have attracted by far the greatest attention a rich variety of cyclopropane systems have been subjected to these reaction conditions. We will consider several factors that may affect the structure of the radical cations as well as the stereo- and regiochemistry of their nucleophilic capture. Factors to be considCTcd include (1) the spin and charge density distribution in the cyclopropane radical cation (the educt) (2) the spin density distribution in the free-radical product (3) the extent of... 

Numerous studies of the photoreactions of dihydrofurans have been described. On irradiation 2,3-dihydrofurans are known to undergo conversion to acylcyclopropanes by a pathway involving initial carbon-oxygen bond homolysis. The /i-enamino ester (162), for example, affords the cyclopropane (163) on triplet-sensitized irradiation.138 Similar transformations have been observed in 2,2,4-triacyl-2,3-dihydrofurans,139 and a synthesis of a cis-trans mixture of chrysanthemum carboxylic acid has been accomplished in this way.140 The conversion of the 2-thiazolines(164) to the iV-alkenylthio-amides (165) presumably involves an analogous carbon-sulfur bond homolysis, followed by a 1,2-hydrogen shift in the resulting biradical (166).141... 

In an attempt to reduce the size of the photoreactive group, Emi and Khorana (1980) investigated the properties of tetrafluorodialkyl diazirines. Unfortunately these compact molecules (Fig. 3.17) did not yield insertion products on photolysis, but rearranged internally by alkyl migration to give olefins, and underwent intramolecular insertion to yield cyclopropanes. 

To examine the viability of CIM a number of photoreactions (electrocyclic reactions, Zimmerman (di-n) reaction, oxa-di-7i-methane rearrangement, Yang cyclization, geometric isomerization of 1,2-diphenyl-cyclopropane derivatives, and Schenk-ene reaction) which yield racemic products even in presence of chiral inductors in solution have been explored (Sch. 40) [187,189-200]. Highly encouraging enantiomeric excesses (ee) on two photoreactions within NaY have been obtained photocyclization of tropolone ethylphenyl ether (Eq. (1), Sch. 40) and Yang cyclization of phenyl benzonorbornyl ketone (Eq. (3), Sch. 40). The ability of zeolites to drive a photoreaction that gives racemic products in solution to ee >60% provides... 

Cyclopropanation of the C=C bond of an allylic halide as in the cyclopropane 15 greatly suppresses photoreactivity, which is restored by introduction of a vinyl group at the 2-position of the cyclopropyl group, as in 16 (equation 27)134. The shift in UV-absorption maxima of 15 and 16 indicates the presence of electronic interactions of the vinyl and bro-momethyl moieties through the cyclopropane ring in 16. A similar arrangement of chro-mophoric moieties is present in compound 17, which on direct irradiation in acetic acid yields benzo[c]fluorene as sole product (equation 28)135. 

The DCA-sensitized photooxygenation of less electron-rich cyclopropanes in the absence of Mg(C104)2 does not afford 1,2-dioxolanes. However, the photoreaction in the presence of MgfClOJj gives 1,2-dioxolanes in moderate yields [157], In this photoreaction, Mg(C104)2 suppresses not only a back electron transfer from DCA to the radical cations of cyclopropanes, but also the decomposition of the 1,2-dioxolanes produced. 

The photoreaction of electron-rich alkenes and cyclopropanes with cyanoaro-matic compounds and cyanoarylalkenes in the presence of nucleophilic solvents affords the solvent-incorporated adducts (Scheme 62). Three mechanisms have been proposed for these three-component addition reactions. 

Irradiation of the E-isomer of (70) brought about E-Z isomerization as well as cyclization by group migration to the cyclopropane derivatives (71). These derivatives were also photoreactive and ring-opened to the alkenes (72). The kinetics of the photorearrangement of the benzocycloheptene (73) has been studied in detail. Irradiation in methanol affords the six products shown in Scheme 5, and solvent dependence in the reaction was investigated. 

The aminocyclopropane derivative 374 and the propellane 372 were obtained, each in 26 % yield, by photocycloaddition of 370 and ethylene in acetone . Diradicals 371 and 373 were suggested as intermediates in the photoreaction (equation 90). The use of isobutene or 2,3-dimethyl-2-butene instead of ethylene gave no cyclopropane product . 

Previous work by Jeger and his coworkers has studied the photoreactivity of y.5-epoxy enones. More recent work by Ishii et aJ has examined the photochemical reactivity of the related nitriles (145) and (146). The direct irradiation of (145) brings about isomerization by ring opening of the cyclopropane to afford (147) as the major product. This is accompanied by the cyclopropenyl nitrile (148) presumably formed via a carbene intermediate. Triplet sensitized irradiation of (145) only brings about trans-cis isomerization. The epoxide (149) is also photoreactive and direct irradiation affords the products (150) - (156) either by the intermediacy of an ylide (157) or a carbene (156). ... 

Anthraquinonecarboxamide in its Sj state will photoadd two molecules of water. The photoreactions of diphenylhomobenzoquinones in the presence of amine donors have been studied. 1-Bromo substituted diphenylhomobenzoqui-none irradiated in the presence of triethylamine induces opening of the cyclopropane ring with formation of 2-diphenylmethyl-5-methyl-l,4-benzoquinone, and with dimethylaniline, a mixture of an aminated bicyclic dione and bis(p-dimethyl-aminophenyl)methane is produced. Irradiation of boron difluoride complexes derived from 1,3-diketones can lead to a number of different types of process, in particular exciplex formation and slow cycloaddition. Evidence has now been advanced to suggest that excitation of these complexes leads directly to an exciplex without the participation of an encounter complex, and that this solvolyses to free ions. 

Upon UV irradiation of dicyanofpyridinio- or pyridazinio )methanide, dicyanocarbene was generated, which then reacted with benzene to give bicyclo[4.1.0]hepta-2,4-diene-7,7-dicarboni-trile (2) or with (Z)-4-methylpent-2-ene nonstereospecifically to give cis- and trans-2-iso-propyl-3-methylcyclopropane-l,l-dicarbonitrile. However, due to competing photoreactions of the ylide, the carbene-transfer reaction is not efficient and the cyclopropanes were obtained in poor yield only. ... 

Cyclopropyl cyanide is photoreactive on irradiation at 193 nm. This treatment brings about elimination of a cyano radical with the fission of the cyclopropane... 

Homoconjugative interactions between a cyclopropane ring and a double bond were studied for the three bicyclic compounds 7-9, each possessing a locked vinylcyclopropane fragment [84], The polarization patterns in the chloranil-sensitized photoreactions of these substrates were used to probe the spin density distribution in the radical cations. A strong dependence of electronic structure of the radical cations on steric effects was found (see Chart IV). 

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