Photoaddition reactions Cycloadditions

Schmidt reaction of ketones, 7, 530 from thienylnitrenes, 4, 820 tautomers, 7, 492 thermal reactions, 7, 503 transition metal complexes reactivity, 7, 28 tungsten complexes, 7, 523 UV spectra, 7, 501 X-ray analysis, 7, 494 1 H-Azepines conformation, 7, 492 cycloaddition reactions, 7, 520, 522 dimerization, 7, 508 H NMR, 7, 495 isomerization, 7, 519 metal complexes, 7, 512 photoaddition reactions with oxygen, 7, 523 protonation, 7, 509 ring contractions, 7, 506 sigmatropic rearrangements, 7, 506 stability, 7, 492 N-substituted mass spectra, 7, 501 rearrangements, 7, 504 synthesis, 7, 536-537... 

The photoaddition reaction of 2-phenylbenzimidazole with Michael acceptors was investigated <96JHC1031> as was the preparation and cycloaddition-reactivity of benzimidazole-2-carbonitrile oxide <96AJC199>. The nitration of 1-methylbenzimidazole was found to give only the 5- and 6-nitrated products as a mixture of isomers in 87% yield... 

The most widely used method for synthesizing bicyclic azetidines is by photoaddition reactions, primarily of the [2 + 2] type. Scheme 3 lists several examples of this method (37) (81JA3136), (38) (75JA7288), (39) (77TL431), (40) (74JHC453). Cycloadditions to carbon-nitrogen double bonds have been reviewed (78MI51200). 

Open-chain Systems - Chow and his coworkers have carried out further studies on the cycloaddition reactions of dibenzoyl(methanato)boron difluoride (1) to alkenes and dienes. This present work has examined the mechanistic details of the process. Previously they have reported the addition of the same reactant to enol ethers. The influence of the chiral centre in the alkene (2) on the cycloaddition of a,P-unsaturated nitriles such as acrylonitrile has been evaluated. The products obtained from these (2+2)-photoaddition reactions are the azabicyclooctanes (3)." ... 

In contrast to cyclopentenones and cyclohexenones, medium-ring vinylogous esters are not suitable for photocycloaddition reaction unless the olfin coupling partner is substituted. For example, when ( -3-(but-3-enyloxy)cyclooct-2-enone 78a is irradiated under a variety of conditions, no intramolecular cycloadduction occurs. However, upon substitution of a vinyl or phenyl on the olefin, the cycloaddition proceeds efficiently to give diastereomeric mixtures 79b/c, and 80b/c, respectively. The dramatically enhanced yields and rates of the photoaddition reactions upon olefin substitution result from the stabilization of the 1,4-biradical by a vinyl or phenyl. A mixture of diastereomers is formed presumable because the rotational relaxation of the intermediate 1,4-diradical is faster than [2 + 2] ring closure. 

Benzene and other aromatic hydrocarbons undergo photoaddition reactions with furan, dienes, and allenes to give para-like adducts as major products by allowed (jt" s + Ti s) cycloaddition process. For examples, benzene reacts with furan in molar ratio 1 1 gives para-like adduct as major product [19] and with allene gives para-and meta-adducts in a ratio of 2 1 [20]. 

A novel 1,8-photoaddition of dimethyl 1,4-naphthalenedicarboxylate and 1,4-dicyanonaphthalene to alkenes, a formal [3-1-2] cycloaddition, was reported by Kubo and co-workers. Although yields of the desired products were low, Cu(OAc)2 showed a profound impact on the reaction by increasing the yields considerably [85]. 

A number of examples of photoaddition to alkynes has been described. Dimethyl acetylenedicarboxylate has been found to add to methyl-substituted 3-benzoylthiophens301 and to thiophen and 2,5-dimethylthiophen302 on irradiation. Benzo[f>]thiophen also undergoes cycloaddition reactions with alkynes,303 and in the case of dimethylacetylene dicarboxylate, product formation has been shown to be wavelength dependent.304 Intramolecular [ 2 -(- 2] cycloaddition has been observed on both direct and triplet-sensitized irradiation of the alkyne (353) and gives the cyclobutene (354)305 the isomer (355) is formed on prolonged irradiation. 

A systematic study of the photochemistry of thiones has been undertaken.356 Not surprisingly, cycloaddition to alkenes to give thietans is one pathway that commonly occurs. The reactions appear to be wavelength dependent, and stereospecific addition takes place via the S2 excited state and nonstereospecific addition via the T, state. The photoaddition of... 

In addition to [2+2] photoadditions, thermal [4+2] cycloadditions (Diels-Alder reactions) are also remarkably accelerated in the cavity of cage 2. The reaction of... 

The first such reaction published in 1908 by Ciamician and Silber was the light induced carvone —> carvonecamphor isomerization, corresponding to type b [1]. Between 1930 and 1960 some examples of photodimerizations (type c) of steroidal cyclohexenones and 3-alkylcyclohexenones were reported [2-5]. In 1964, Eaton and Cole accomplished the synthesis of cubane, wherein the key step is again a type b) photocycloisomerization [6]. The first examples of type a) reactions were the cyclopent-2-enone + cyclopentene photocycloaddition (Eaton, 1962) and then the photoaddition of cyclohex-2-enone to a variety of alkenes (Corey, 1964) [7,8]. Very soon thereafter the first reviews on photocycloaddition of a,(3-unsaturated ketones to alkenes appeared [9,10]. Finally, one early example of a type d) isomerization was communicated in 1981 [11]. This chapter will focus mainly on intermolecular enone + alkene cycloadditions, i.e., type a), reactions and also comprise some recent developments in the intramolecular, i.e., type b) cycloisomerizations. 

Using the temperature effect, it was proven that non-emitting exciplex intermediates were also involved in the cycloaddition. For example, in the reaction of naphthalene with diphenylacetylene, with increasing temperature both the quantum yields of photoaddition and the quenching of the naphthalene fluorescence by acetylene decrease by the same magnitude [65],... 

As has been mentioned earlier, it is often very difficult to distinguish between and identify the roles of exciplexes (and excimers) and biradicals in cycloaddition reactions. Caldwell and Creed (1978b) have studied the cycloaddition of dimethyl fumarate to phenanthrene and found that the quantum yield of the cyclobutane photoaddition product is increased in the presence of oxygen. It was suggested that oxygen enhances intersystem crossing in the triplet biradical formed between the two reactants. Nitroxide radicals have also been found to increase intersystem crossing (Sj -> Tj) in carbocyanines when nonpolar solvents are used (Kuzmin et al., 1978). When polar solvents are employed full electron transfer takes place. 

This review article deals with addition and cycloaddition reactions of organic compounds via photoinduced electron transfer. Various reactive species such as exdplex, triplex, radical ion pair and free radical ions are generated via photoinduced electron transfer reactions. These reactive species have their characteristic reactivities and discrimination among these species provides selective photoreactions. The solvent and salt effects and also the effects of electron transfer sensitizers on photoinduced electron transfer reactions can be applied to the selective generation of the reactive species. Examples and mechanistic features of photoaddition and photocycloaddition reactions that proceed via the following steps are given reactions of radical cations with nucleophiles reactions of radical anions with electrophiles reactions of radical cations and radical anions with neutral radicals radical-radical coupling reactions addition and cycloaddition reactions via triplexes three-component addition reactions. 

In the photoaddition of acetone and other ketones to 1-, 2- and 1,2-di-methylimidazoles the products sire a-hydroxyalkylimidazoles (153) which are derived from the selective attack of excited carbonyl oxygen at C-5. In the case of 2-methylimidazole the products are the 4-mono- (8%) and 4,5-di- (14.5%) substituted compounds, but imidazole itself does not react. The suggestion that it is not a sufficiently electron-rich substrate is not particularly convincing. The reaction mechanism (Scheme 72) may reflect the greater radicd reactivity at C-5, and the comparative stabilities of the radical intermediates derived from carbonyl attack at this position. Hiickel calculations of radical reactivity indices show that, indeed, C-5 is more reactive, and the radical intermediate at C-5 is more stable than that at C-4, but a concerted cycloaddition could also give rise to the oxetane (152). Such an oxetane can be isolated in the photochemical addition of benzophenone to 1-acetylimidazole. 

Chiral Auxiliary for Asymmetric Induction. Numerous derivatives of (—)-8-phenylmenthol have been utilized for asymmetric induction studies. These include inter- and intramolecular Diels-Alder reactions, dihydroxylations, and intramolecular ene reactions of a,p-unsaturated 8-phenylmenthol esters. These reactions usually proceed in moderate to good yield with high diastereofacial selectivity. a-Keto esters of 8-phenylmenthol (see 8-Phenylmenthyl Pyruvate) have been used for asymmetric addition to the keto group, as well as for asymmetric [2 -F 2] photoadditions and nucleophilic alkylation. Ene reactions of a-imino esters of 8-phenylmenthol with alkenes provide a direct route to a-amino acids of high optical purity. Vinyl and butadienyl ethers of 8-phenylmenthol have been prepared and the diastereofacial selectivity of nitrone and Diels-Alder cycloadditions, respectively, have been evaluated. a-Anions of 8-phenylmenthol esters also show significant diastereofacial selectivity in aldol condensations and enantiose-lective alkene formation by reaction of achiral ketones with 8-phenylmenthyl phosphonoacetate gives de up to 90%. ... 

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