Low temperatures photoreaction

In the majority of cases, the oxidizing species involved in the photooxidation reactions is not well known because the nature of the species is inferred from indirect experiments. The 02 ion has been invoked as the precursor of the oxidizing species in a number of reactions because it is often the only species observed at room temperature using EPR. However, a variety of other species such as O and OJ have been identified on surfaces when the low-temperature photoreactions are observed by EPR. In addition, O" formed on the surface may have a very short lifetime (as discussed in Ref. 1, p. 93) and can only be detected by its reactivity. With these points in mind, we conclude that O-, and particularly OJ in the presence of excess oxygen, may play a much more important role in photocatalysis than has yet been generally realized. In this connection, the paper of Kubokawa et al. (410) is of particular importance but the use of 170-labeled oxygen is necessary to confirm the nature of the species involved. In order to explore... 

The absorption lines of the low temperature photoreaction products in TS-6 monomer crystals are summarized in the diagram of Fig. 7. The correlation of the A, B, C,. .. photoproduct series to diradical DR intermediates and of the b, c, d,... photoproducts to asymmetric carbene AC intermediates is based on the ESR experiments discussed below. The correlation of the y, 8,6,... series to stable oligomers SO is based on their thermal and optical stability. The correlation of dimer, trimer, tetramer,... molecules follows from the chemical reaction sequences observed in the time resolved optical and ESR measurements as well as from the widths of the one-dimensional potential wells used in the simple electron gas theory , which already has proved successful in its application to dye molecules. Following Exarhos et al. the explicit dependence is given by... 

Photosensitized reaction of 15c with maleimide 19g at low temperature (-10 to -20°C) affords the exo-[4-1-2]-adduct 16 as a major product, together with the [2-1-2]-adduct 17 (Scheme 11). The same reaction was carried out at room temperature to afford the bis-adduct 27, which was formed via decarboxylation of 16cg followed by addition of another molecule of 19g, in addition to the formation of 17cg. The low-temperature photoreactions of 2-pyrones 15a, 15b with 19g or 19g also gives [4-1-2]-adducts 16ag, 16bg, and [2-1-2]-adduct 17bg. Similar photoreactions of 2-pyrones 15b, 15c with maleic anhydride afford exo-[4-1-2]-adducts (major products) and/or [2-1-2]-adducts. Photosensitized reaction of 15a with dimethyl acetylenedicarboxylate affords 1, 2, 4-tris(methoxycarbonyl)benzene 28 in 50% yield byway of decarboxylation of the [4-1-2]-adduct, which was difficult to isolate even from the reaction mixture obtained from the low-temperature irradiation (Scheme 12)." Similar reactions of 15a with... 

In contrast to the a-type crystal, the photoreaction of j8-type diolefin crystals at a very low temperature sometimes does not occur at all, or sometimes proceeds but levels off at a low conversion, suggesting that the photoreaction of /3-type diolefin crystals requires an appropriate thermal motion of the reacting molecules. 

The photochemical dissociation of Me2Ge from 7,7-dimethyl-l,4,5,6-tetraphenyl-2,3-benzo-7-germanorbomadiene (14) has been studied by flash photolysis, low-temperature matrix isolation and CIDNP 3H NMR techniques30. The results suggest that a biradical (15) is formed as an intermediate species in the photoreaction. The biradical is initially formed in the singlet state, which undergoes conversion to the triplet state before irreversible decomposition to form Me2Ge and tetraphenylnaphthalene (TPN) (reaction 19). 

A cycloaddition reaction produces a ring of atoms by forming two new G-bonds, for example the formation of a cyclobutane dimer from two alkene molecules. The direct photoreaction involves the concerted reaction of the singlet Jtpt ) excited state of one alkene with the ground state of the other. Stereospecific reactions in which the dimers preserve the ground-state geometry occur when liquid cis- or trans-but-2-ene are irradiated at low temperature ... 

It is a little difficult to relate these observations of phosphorescence in low-temperature matrices (which in both cases are composed of molecules which at room temperature photoreact with the pyrimidines) to the observed photochemical reactions. The fluorescence of the pyrimidines in frozen aqueous matrices may be weak because the excited molecules are quenching each other reactively—an argument strengthened by the observation110 that addition of ethanol to the solution strengthens the phosphorescence but prevents dimer formation.29 No clear-cut conclusions can yet be provided by these studies. 

In the case of anthracene, the stable monoclinic phase transforms under stress to a triclinic phase in which molecules are favourably oriented for dimerization to occur. Although the triclinic phase has not been isolated as a pure phase, its structure has been established using low-temperature electron microscopy and atom-atom potential calculations (Jones Thomas, 1979). In l,8-dichloro-9-methyl anthracene, isolated dislocations with (201) [010] translation bring the molecules to the required geometry (Fig. 8.17) to facilitate photodimerization. 1, 5-dichloroanthracene is an interesting case. Instead of the expected 100% head-to-head dimers, photoreaction yields 80%... 

The (1,4) substituted naphthalenophanes undergo [4 + 4] photocycloaddition when irradiated at X > 280 nm, in addition to fluorescence. This photoreaction is competitive with fluorescence, and requires a conformational change that can be suppressed at low temperature 93). The few reports of the lifetime or quantum yield of naphthalenophane fluorescence indicate the effects of photocycloaddition. For the anti-[2.2](1,4) isomer, kpu/ku = 0.021 in cyclohexane 93) the lifetime of syn-[3.3](l,4) naphthalenophane fluorescence was given as 15.3 ns107). Both values are low relative to the naphthalene solution excimer (kpu/kjj 0.2 xD 80 ns 71)), and this may be due in part to the photoreaction of the (1,4) naphthalenophanes. 

Although photodimerization of 2-alkoxynaphthalenes [146-151] or 2-cyanonaphthalenes [152-154] was well described, unprecedented photodimerization of 1-substituted naphthalenes was recently reported by Noh et al. [155], Upon irradiation of 1-cyano- or 1-methoxy-naphthalene at -78°C, syn-(2 + 2) cyclodimer (102) was isolated however, no cyclodimer was found in the irradiation of 1-methyl-, 1-methoxy-, or unsubstituted naphthalene (Scheme 30). By the xan-thone-sensitized photoreaction and low-temperature NMR study, 103 was found to be produced through Cope rearrangement of endo-(4 + 4) cyclodimer (101) in the cases of 1-cyano- and 1-methoxy-naphthalene. 

Nishiyama and Mizuno investigated the enantioselectivity by the addition of chiral sources 276-280 into the photoreaction of 255a however, enantiomer excess (ee) was very low [297] (Scheme 80, Table 6). The diastereodifferentiation of compound 281 that connects 1-cyanonaphthalene and an alkene by di-(/)-men-thyl malonate gave 282 in 14% de moreover, the addition of Lewis acid at low temperature increased the de to 17% (Table 7). 

Due to low solubility in hydrocarbon solvents at low temperatures, [(>75-C5H5)Mo(CO)3]2 is not well suited for photoreactions. In order to increase the solubility, the methylcyclopentadienyl derivative, [(t/5-CH3C5H4)-Mo(CO)3]2 (102), has been used. After photoreaction with la-lc and It in n-pentane solution, the reaction mixtures contain two products each, di-nuclear tricarbonylbis(f/5-methylcyclopentadienyl)( /4-diene)molybdenum (103) and dicarbonyl-t -enyl- 5-methylcyclopentadienylmolybdenum complexes (104) (141) [Eq. (56)]. 

The importance of recrystallization to the progress of a reaction was demonstrated again in the solid-state photoreactivity of o-methoxycinnamic acid (24). Cis-o-methoxycinnamic acid (cis-24), when crystallized and irradiated at room temperature, gives rise to the cis and trans acids and to trans/trans dimers (25), as shown in Scheme 18. When the reaction is carried out at low temperature, how-... 

Natural photosynthesis provides the most dramatic demonstration of the potential hidden in this basic photoreaction. In (bacterial) photosynthesis a chlorophyll-dimer (BC)2—the special pair —receives the radiation energy and thereby gains the energy required to enable it to transfer an electron to a pheophytin moiety (BP), an act occurring within 2-3 picoseconds (Martin et al. 1986) even at very low temperatures. Subsequently the electron is transferred to a quinone acceptor (MQ), which once again occurs (Holten et al. 1978) on a very short time scale of about 230 ps. 

A comparison between the photoreactions of 2-chlorophenol and 2-bromo-phenol in a low-temperature argon matrix was carried out by Akai et al. by means of IR spectroscopy [17,18]. The formation of fulvene 6-oxide was evidenced in both systems. Homolytic C - Br cleavage was found as an additional pathway in the case of 2-bromophenol. 

Numerous papers have been published describing the photochemical production of enones from carbonyl compounds and alkynes. The first detection of an oxetene intermediate involved low-temperature (-78 C) photolysis of 2-butyne and benzaldehyde to form the photoproduct (61), which was observed by NMR. The oxetene undergoes further photoreaction with benzaldehyde to form the novel fused oxetane (62). Recently, Friedrich has reported Either studies on the reactivity of oxetenes and developed alternative syntheses of the parent compound and 3-phenyloxete (64). The parent oxetene is found to have a thermal half-life of t proximately 8 h in solution at room temperature. The phenyl-substituted derivative (64) underwent slow ring-opening under acidic conditions to form 2-phenylprq)enal and air oxidation to yield a formate derivative, probably via a radical process. 

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