Photoinduced diels-alder

Table 4.13 Photoinduced Diels-Alder reactions of indoles 64 and dienes 61-63 with sensitizers 59 and 65...

Kochi and co-workers studied photoinduced Diels-Alder cycloadditions via direct photoexcitation of anthracene as a diene with maleic anhydride and various maleimides as dienophiles. Here, fluorescence-quenching experiments, time-resolved absorption measurements, and the effect of solvent polarity provide striking evidence for an ion-radical pair to be the decisive intermediate [83],... 

Keywords Mg2+-catalyzed Diels-Alder reactions, anthracenes, p-benzoquin-one, photoinduced electron transfer... 

Another photoinduced 1,5-hydrogen shift/twofold Diels-Alder process to give substituted aromatic compounds has been reported by Nair and coworkers [34]. 

The thermal Diels-Alder reactions of anthracene with electron-poor olefinic acceptors such as tetracyanoethylene, maleic anhydride, maleimides, etc. have been studied extensively. It is noteworthy that these reactions are often accelerated in the presence of light. Since photoinduced [4 + 2] cycloadditions are symmetry-forbidden according to the Woodward-Hoffman rules, an electron-transfer mechanism has been suggested to reconcile experiment and theory.212 For example, photocycloaddition of anthracene to maleic anhydride and various maleimides occurs in high yield (> 90%) under conditions in which the thermal reaction is completely suppressed (equation 75). 

Photoinduced intramolecular electron transfer in the donor-acceptor complex 87 (R = H) generates transient charge-separated open-shell species with the remarkably long lifetime of about 75 ps [89]. Dyads that contain Tt-extended tetrathiafulvalene units also form stable cationic species upon oxidation [90]. The dumbbell shaped triad 91 [91-93] (Scheme 4.13) was obtained by carrying out the reaction with the in situ generated bis-diene at room temperature, in the dark and in o-dichlorobenzene as a solvent in 50% yield. The product is thermally unstable and easily undergoes a retro-Diels-Alder reaction [91]. 

The aziridinofullerenes and azafulleroids obtained by Mattay et al. were subjected to further reactions to yield fullerene adducts of the second generation. As a model compound 4-methyl-l,2,4-triazoline-3,5-dione (NMTAD) 187, which is one of the most reactive dienophiles for Diels-Alder reactions, was selected. Sheridan et al. studied the photoinduced [4 + 2] cycloadditions of NMTAD to arenes [295], With alkenes, however, NMTAD reacts in photoinduced cycloadditions in a [2 + 2] fashion under formation of diazetidines [296], Both the sulfonyl-azafulleroids (185) and aziridinofullerene (186) were irradiated at 420 nm to give both the same [2 + 2] cycloadduct 186 (Scheme 71) [291], This indicates that at a certain stage rearrangements must be involved. 

The Diels-Alder cycloaddition of 1,2-dimcthylcnc 2.n]mclacyclophancs (83) with DMAD followed by aromatization (84) and photoinduced transannular cycliza-tion produced phenanthrene-anellated polycyclic aromatic hydrocarbons (85) (Scheme... 

The Diels-Alder reaction is a well-established synthetic method that allows the creation of two new carbon-carbon bonds and leads to the formation of six-membered rings. Eventually, the photochemical reaction can advantageously compete with the thermal process. For instance, anthracene undergoes thermal and photochemical Diels-Alder reactions with alkenes, but the photoinduced addition of maleic anhydride to the homochiral anthracene, as depicted in Scheme 9.28, is faster than the thermal reaction and occurs with excellent diastereoselectivity (only one diastereoisomer) [42]. 

The photoinduced electron transfer pathway has been unequivocally demonstrated in a photochemical Diels-Alder reactions between C6o and Danishefsky s dienes (Scheme 11) [304, 305]. 

The transient spectra of C6o at Amax = 1080 nm formed in photoinduced electron transfer from Danishefsky s diene to Cso have been detected, accompanied by the decay of Ceo in a laser flash photolysis of the reaction system [305]. The observed rate constant agrees well with the predicted rate constant on the basis of an electron transfer mechanism shown in Scheme 11 [305], In the photochemical Diels-Alder reaction of Cgo, a stereochemicaUy defined (l , 3Z)-l,4-disubstituted Danishefsky s diene is used as a stereochemical probe. In the photochemical Diels-... 

Several reviews have been published within the year which are of general relevance to the photoreactions of aromatic compounds. The subjects of these reviews include photochemistry in ionic liquids and in isotropic and anisotropic media, organic synthesis utilizing photoinduced electron-transfer reactions," heteroatom-directed photoarylation processes, photochromism, and photochemical molecular devices. Reviews more directly pertinent to the sections in the present chapter include those of the photoisomerization of five-membered heteroaromatic azoles, the photocycloaddition of benzene derivatives to alkenes, Diels-Alder additions of anthracenes, advances in the synthesis of polycyclic aromatic compounds, diarylethene-based photochromic switches, the photo-Fries rearrangement, and the application of Diels-Alder trapping of photogenerated o-xylenols to the synthesis of novel compounds. " A number of chapters in the two recently published handbooks of photochemistry and photobiology and in the revised edition of the text on photochromism are also pertinent to the current subject matter. 

From the cycloaddition reaction standpoint, the photo-generated reactive species suitable for both 1,3-dipolar cycloaddition reactions and Diels-Alder reactions have been studied in the literature. In this review, we will focus our discussion on four cycloaddition reactions (1) 1,3-dipolar cycloaddition between aUcenes and photogenerated nitrile imines (2) 1,3-dipolar cycloaddition between aUcenes and photogenerated nitrile ylides (3) photoinduced hetero Diels-Alder reactions and (4) photoinduced strain promoted azide-alkyne cycloaddition (Scheme 3). It is... 

The involvement of a transient photogenerated irans-cyclohe-xene-CuOTf intermediate was also adduced to explain CuOTf catalysis of photoinduced 2jr + 4jr cycloaddition between cis-cyclohexene and 1,3-butadiene (eq 17). In contrast to thermal Diels-Alder reactions, this reaction generates ira/js-A -octalin rather than the cis cycloadduct expected for a 2its +Aits cycloaddition. A mechanism was proposed that involves the 2ns +4jrs cycloaddition of a fraas-cyclohexene with 1,3-butadiene in the coordination sphere of Cu (eq 18). ... 

Attempts have been made to deposit TIPS-pentacene from solution as the functional layer in a pentacene/C60 bilayer photovoltaic device. Careful optimization of deposition conditions, optimal concentration of mobile ion dopants, thermal postfabrication annealing, and the addition of an exciton-blocking layer yielded a device with a moderate white-light PCE of 0.52% [41]. Since TIPS-pentacene derivatives rapidly undergo a Diels-Alder reaction with fiillerene, the assembly of potentially more efficient bulk-heterojunction photovoltaic devices from TIPS-pentacene and fiillerene derivatives were not possible [42]. The energy levels of the TIPS-pentacene-PCBM adduct (PCBM is [6,6]-phenyl C61-butyric acid methyl ester) ineffectively supports the photoinduced charge transfer. 

Reaction of 3,4-di-ferf-butylthiophene-l-oxide in ionic liquid l-butyl-3-methylimidazolium tetrafluoroborate ([Bmim]BF4) with dienophile parasorbic acid afforded the Diels-Alder product 33 in 91% yield. But the resulting oxidation products 34 and 35 showed no significant photoreactivity, rendering it an inappropriate strategy for the synthesis of a two-stage photobase generator (PBG) based on photoinduced aromatization (Scheme 24) [38]. 

The chemical selectivity in the photocycloaddition of ethylene (78) to benzene (77) was discussed. Diels-Alder cycloadducts of 1,2-benzo-quinones (82) with benzynes generated from (83) were converted to naphthalene derivatives (85) by photoinduced double decarbonylation of (84) (Scheme 22). ... 

Enantiopure alkaloids (89) were synthesized via photoinduced intramolecular [4 + 2] cycloaddition of azajq lylenes (87) to pyrroles. Similar [4 + 2] intermolecular photocycloaddition of (91) with (92) gave Diels-Alder cycloadducts (93) (Scheme 23). ... 

Maleimide. Maleimide and maleic anhydride are simple olefins and very useful molecules in organic syntheses for introducing various substituents. Addition reactions, cyclodimerization, Diels-Alder reactions and polymerization reactions are as well-known as their photochemical reactions. They are also known as electron acceptors in photoinduced electron-transfer reactions. The photosensitized reaction of maleimide with xanthone in 2-propanol has been investigated by TR EPR. The emissive CIDEP spectrum observed in 2-propanol is predominantly assigned to two kinds of maleimide alkyl-type radicals. On the other hand, the absorptive spectrum of the maleimide radical-anion was observed in 2-propanol in the presence of hydrochloric acid. The hydrochloric acid addition effect on the CIDEP patterns indicates the existence of two mechanisms for the photosensitization of maleimide by xanthone. One is a T-T energy transfer to maleimide followed by hydrogen abstraction by maleimide... 

Mikami et al. investigated the photochemical Diels-Alder reaction of anthracenes with in the solid state. Irradiation of a mixture of and 9-methylanthracene led to the formation of mono- and bisadducts. No formation of 9-methylanthracene dimers as in solution was observed. The reaction did not work with less bulky anthracenes. The reaction rate depends on the ionization potential of the anthracenes. With decreasing ionization potential, the Diels-Alder reaction of proceeds much more easily. Therefore, the reaction may proceed via photoinduced electron transfer from anthracenes to the triplet excited state of Q . The energetics for the photoinduced electron transfer in the solid state are significantly different from those in solution where solvation plays an important role. Such a difference leads to the different reactivity of the anthracene derivatives in the solid state as compared to that in solution. 

Mikami, K., Matsumoto, S., Tonoi, T, and Okubo, Y, Solid state photochemistry for fullerene functionalization solid state photoinduced electron transfer in the Diels-Alder reaction with anthracenes, Tetrahedron Lett., 39, 3733, 1998. 

---