Stilbenes photolysis

The photolysis of phenyldiazirines [53a]-[53c] in an argon matrix was studied in a similar way. Intense bands of the corresponding carbenes [54a]-[54c] (Table 2) have been observed in all cases. The bands disappeared either when the matrix temperature was raised from 12 to 40-45 K (the corresponding stilbene bands appeared at the same time), or when a trapping agent (HCl, CI2) was doped into the matrix (Mal tsev et al., 1985b, 1987a). 

Herkstroeter and Hammond found support for this postulate from a flash photolysis study. They were able to measure directly the rate of sensitizer quenching (energy transfer) by cis- and fra/w-stilbene. When a sensitizer triplet had insufficient excitation energy to promote fims-stilbene to its triplet state, the energy deficiency could be supplied as an activation energy. The decrease in transfer rate as a function of excitation energy of the sensitizer is given by... 

These reactions, while not very important for high-energy donors in the absence of azulene, do help define what the potential energy diagrams should look like, that is, the fram-stilbene triplet is a discrete intermediate while the cij-stilbene triplet is a high vibrational form of the phantom triplet. Herkstroeter and McClure(13> have been able to observe the fro/w-stilbene triplet via flash photolysis using low-temperature glasses however, no similar triplet was observed for cfr-stilbene. Only when they studied cfr-stilbene-like... 

The previous sections have shown that cis-trans isomerization of stilbene can take place via the lowest triplet state of stilbene. The question to be considered now is whether the isomerization upon direct photolysis takes place via the singlet state, the triplet state, or a vibrationally excited ground state.a 7 81 50)... 

A recently popular mechanism involves the intersystem crossing of the cis- or trons-stilbene singlet state, produced upon direct photolysis, to its corresponding triplet states, which would then undergo the type of reactions given in Eqs. (9.8M9.10) and (9.17M9.19) ... 

Photoaddition of fra/w-stilbene to tetramethylethylene yields cyclobutane (42) in high yield. Cw-stilbene also results from the photolysis ... 

The quantum yield for the formation of the cycloaddition product has been found to be temperature dependent, increasing by a factor of approximately three as the temperature is lowered from 65 ( = 0.24) to 5°C ( = 0.69). Photolysis of mixtures of the olefin and f/my-stilbene in the presence of sensitizers yielded no cycloaddition product (42) but rather only m-stilbene. This suggests that the cycloadduct is produced via a singlet reaction. This conclusion is supported by the fact that tetramethylethylene quenches fluorescence from the /rans-stilbene singlet. A plot of l/ (42) vs. 1/[TME] (TME = tetramethylethylene) is linear. The slope of this plot yields rate constants for cycloadduct formation which show a negative temperature dependence. To account for this fact, a reversibly formed exciplex leading to (42) was proposed in the following mechanism<82) ... 

This dependence on energy has been nicely demonstrated by Herkstroeter and Hammond,<57>68) who studied the energy transfer for a series of 18 triplet donors to cis- and tra/w-stilbene and cis- and trans-1,2-diphenylpro-pene by flash photolysis. The results for cis- and Pmr-stilbene are summarized graphically in Figure 6.10. 

The direct photolysis of cis- or ftmr-stilbene in solution gives rise to photo-isomerization. In addition, dihydrophenanthrene is formed from the cis isomer ... 

Evidence that eliminates the triplet mechanism as the mode for the cis-trans isomerization of stilbene upon direct photolysis has been provided by azulene quenching studies.(48) Using the experimentally determined decay ratio a/(l — a) and the triplet mechanism, it is possible to calculate what the effect of azulene is upon the pss. The predicted and observed azulene effects on the direct photoisomerization are shown in Figure 9.6. The failure of the triplet mechanism in predicting the very small changes observed in the pss provides a crucial test that is the basis for rejecting the triplet mechanism. 

Cis- and fro/w-stilbenes have been reported to add to 2,3-dihydropyran upon photolysis to yield adducts (44) and (45) as well as two stilbene dimers(88> ... 

The isomeric stilbene dicarbenes [20] were generated in 2-MTHF matrices in an epr cavity at 16 K (Murata et ai, 1987 Iwamura, 1988). The spectra obtained by the photolysis of the diazo-compounds [20a], precursors to m,p -[20] and w,m -[20], at 16 K exhibited conspicuous signals at ca. 250 mT, characteristic of quintet species. The signals of /w,w -[20] showed a dramatic temperature dependence (Fig. 16). First, their intensity increased as the temperature was raised, reaching a maximum at SO K, and then decreased somewhat and eventually irreversibly at above 65 K. In contrast, the intensity of the strong signal at ca. 250 mT and some weak signals due to m,p -[20] decreased linearly with the reciprocal of the temperature as dictated... 

Laser flash photolysis (30 ns, 50 mj, 347 nm) of 4-nitrostilbene, 4,4 -dinitro-stilbene, 4-nitro-4 -methoxystilbene and 4-dimethyl-amino-4 -nitrostilbene permits the observation of transients 133) por the latter two compounds, the lifetime and the absorption spectra of the transients vary strongly with the polarity of the solvent used. First order decay rate constants are given in Table 4. 

Phosphorane, (dichloromethylene) TRIPHENYL-1, 46, 33 Phosphorus pentachloride, for conversion of D,L-10-camphorsulfonic acid to acid chloride, 46,14 reaction with styrene, 46, 99 Photolysis of substituted stilbenes to phenanthrenes, 46, 91... 

The photocyclization has been found not to occur with stilbenes substituted with acetyl, dimethylamino, or nitro groups, lodo substituents are replaced by hydrogen by photolysis in cyclohexane solution. w-Substituted stilbenes give mixtures of 2- and 4-substituted phenanthrenes which generally are difficult to separate. 

The photolysis of donor-acceptor systems provides unique synthetic opportunities. Direct irradiation of the donor-acceptor systems, such as systems containing arene and amine components, leads to intramolecular electron transfer, that is, to amine cation-radical and arene anion-radical moieties. After generation, these moieties undergo cyclization reactions providing efficient synthetic routes to fV-heterocycles with a variety of ring sizes. Thus, direct irradiation of secondary amino-ethyl and aminopropyl stilbenes leads to benzazepines in improved yields (Hintz et al. 1996). As known, benzazepines are used in medicine as antidepressants. Scheme 7.44 illustrates ion-radical cyclization with the formation of benzazepine derivative (65% yield). 

Similarly, photolysis of 2-carbomethoxy-4,5-methylenedioxy-2 -iodo-a-ni-tro-dj-stilbene (117), which was prepared by condensation of 116 with 101, gave... 

This yellow coloration is undoubtedly due to DHP (i), whose oxidation product, phenanthrene, was observed by Smakula as an oxidation product in the photolysis of cis-stilbene ... 

The valence tautomerism 251 257 has been proposed to account for the formation of iV-methylthiobenzamide by irradiation (2537 A) in methyl cyanide solution. The valence tautomer 257, R = Me, = Ph, has been detected spectroscopically (p , 2060 cm —N=C=S) either during photolysis at 25° or by heating at 160°. Photochemical oxidative cyclization of 4,5-diaryl derivatives (251, R = R = Ar) analogous to the formation of phenanthrene from stilbene has been reported. Thus, irradiation of the 4,5-diphenyl derivative 251, R = R = Ph, yields the tetracyclic meso-ionic compound (258). ... 

In the vapor phase phenanthrene and hydrogen were products of the photolysis of c/.v-stilbene in addition to the expected mww-stilbene.65 The photolysis of 1,4-diphenyl-1,3-butadiene to give a-phenylnaphtha-lene is an analogous process.65 It was proposed that the intermediate dihydrophenanthrene was a cis form.66... 

Photochemical ring cleavage also occurs readily in systems where stable fragments can be extruded. Photolysis of the 1,3,4-oxadiazinone (127) gives an E/Z mixture of stilbenes by initial extrusion of CO2 followed by rearrangement and loss of N2 as shown in Scheme 7 (68JA1061). 

Photoinduced ring cleavage also occurs readily in heterocyclic systems containing both oxygen and nitrogen. A series of dihydro-oxadiazinones, for example, undergo decomposition, and the results obtained parallel those observed on thermal decomposition cis- and Jraras-stilbene are obtained from the diphenyl derivative (71), whereas cis-cyclooctene is the major product of photolysis of the fused cyclooctane (72).62... 

Increasingly, this process has been adapted to the synthesis of heterocyclic systems. One or both of the phenyl substituents in stilbene can be replaced by a heteroaromatic system, and some of the more important of these cyclizations are recorded in Table I. The formation of benz[A]isoquinoline by photolysis of 4-styrylpyridine in cyclohexane is accompanied by the formation of 1- and 3-cyclo-hexylbenz[A]isoquinoline.346... 

Norrish Type II processes from the triplet state involve a biradical even when concerted reaction would be exothermic. Photolysis of 60 gives the products in Equation 13.65. Concerted reaction with the formation of triplet stilbene (a process corresponding to Equation 13.29) would be exothermic, but triplet stilbene is not formed The 7 state of stilbene decays to a 60 40 cis-trans mixture, but photolysis of 60 gives 98.6 percent tra/u-stilbene.94... 

Compound 1 (150 mg, 0.41 mmol from photolysis of stilbene with 2-methylmer-capto-benzothiazole) was sealed under vacuum in a 10-mL Pyrex tube that was previously heated to red glow in order to remove adhering water. The whole tube was heated to 150 °C for 20 h in the dark. Separation by preparative HPLC (RP18, 80% methanol) gave 62 mg (82%) 4 and 25 mg (34%) 5 as well as 18 mg (14%) 2 and 2 mg (2%) 3. 2 and 3 equilibrated in the melt at 150 °C to give a 8.0 1.0 ratio (6.1 1.0 at 200 °C when similarly prepared from 6). 

Compound 1 (500 mg, 1.39 mmol from photolysis of stilbene and 2-methylmer-capto-benzothiazole) was sealed under vacuum in a thick-walled Pyrex tube (the volume was about 100 mL) and heated to 200 °C for 20 h in the dark. Methan-ethiol (from the [2,l,3]-elimination) and dimethyldisulfide were condensed to a cold trap (-196 °C) at a vacuum line (65 mg, 97%). The residue was separated by preparative TLC on 200 g SiC>2 with benzene to give 340 mg (78%) of 2 (mp 108 °C, methanol) and 96 mg (22%) of 3. 

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