Methylene chloride, photolysis

Photolysis of the sulphinyl-3H-pyrazole 587 in ether or methylene chloride leads to the formation of a relatively stable carbene 588 that can be identified by physical methods. When the irradiation is performed in ethyl vinyl ether or in furan, the expected cyclopropanes are formed smoothly and stereospecifically683 (equation 374). 

The yield of trans product (18) is decreased by the presence of a radical scavenger such as 1,1-diphenylethylene and increased by dilution of the reactants with methylene chloride or butane, indicating this product to result from the triplet carbene. A heavy-atom effect on the carbene intermediate was observed by photolysis of a-methylmercuridiazoacetonitrile. With c/s-2-butene as the trapping agent either direct photolysis or triplet benzophenone-sensitized decomposition results in formation of cyclopropanes (19) and (20) in a 1 1 ratio ... 

Figure 3. Stern-Volmer plots for quenching of yellowing following photolysis of PVCa solutions in methylene chloride by (a) piperylene and (b) naphthalene. Yellowing is measured as the increase in absorption at 390 nm.

Photolysis of PATE Films. Photolysis of 2-8 micron films of PATE on glass or steel under the full arc of a focused 450 Watt medium pressure mercury lamp for 10 minutes yields totally insoluble films in water and a variety of organic solvents. These include ethyl ether, MEK, MIBK, THF, carbon tetrachloride, cyclohexane, pyridine, methylene chloride, methoxy ethanol, benzene, xylenes and acetone. DMSO alone swelled the film. Upon soaking in warm water for 10 minutes, the films could be removed intact. 

New photochemical cleavage reactions of ortho-substituted C=C double bonds were reported by introducing a 2-nitrophenyl group to the double bond104. Photolysis of 1-(2-nitrophenyl)-l-alkenes 174 in methylene chloride solution without oxygen affords aryl... 

Photolysis of the amide 31 in methylene chloride at room temperature results in the formation of the tricyclic lactam 32 in both syn and anti forms <99TL6001>. Treatment of 32 with BF3 etherate brings about cleavage of the cyclobutane ring in the syn isomer only, with the formation of 33. (Scheme 5). The corresponding esters undergo similar transformations, if somewhat less efficiently. 

Two other observations are noteworthy. First, the yield of isocyanate (9) produced on photolysis of 7 in methylene chloride (an inert solvent) is 40%. Photolysis of 7 in cyclohexene leads to a 45% yield of aziridine adduct 10 and a 41% yield of isocyanate 9. Trapping the nitrene does not depress the yield of isocyanate Hence, isocyanate 9 and adduct 10 cannot be derived from the same reactive intermediate. Instead, the isocyanate must be formed from the excited state of the azide, that is, the excited azide (7 ) must partition between the formation of isocyanate and nitrene. 

The UV spectrum of the 3-oxide in ethanol exhibits maxima at 254 nm (e = 19300) and 320 nm (e = 3000), the latter appearing as a shoulder. On irradiation (A = 250 nm, in methylene chloride), benzo-nitrile and phenyl isothiocyanate are formed in 65% and 3% yields, respectively. A similar result is obtained on photolysis of 5-phenylthiatri-azole (Section III, C). The major part of the products was shown to be formed from the singlet excited state.23... 

The amount of isothiocyanate formed on irradiation of 5-phenylthiatriazole is remarkably independent of the conditions employed.20 In a number of solvents, yields of 6.0-7.696 were found at room temperature. At 193°K the yield was 4.6—4.8% in methylene chloride. 5-(2-Methylphenyl)-l,2,3,4-thiatriazole behaved similarly, giving rise to 7.6% isothiocyanate. This insensitivity of the yield of isothiocyanate under various photolytic conditions is believed virtually to rule out nitrene intermediates. Furthermore, characteristic nitrene products were sought for in the photolysis mixture but not found. ESR spectroscopy also failed to provide evidence for the intermediacy of a thioacyl nitrene (10). 

The photolysis of the meso-ionic 1,4-diphenyl-1,2,4-triazol-3-one (200, R1 = R3 = Ph, R2 = H) was stated97 to yield phenyl isocyanate (13%), iV.N -diphenylurea (23%), and the bicyclic compound 215 (49%). These results were interpreted in terms of the fragmentation of the photo-intermediate 213 yielding the JV-phenyldiazirine (214).97 A later publication by the same group97b reports different results. Photolysis of meso-ionic 1,4-diphenyl-1,2,4-triazol-3-one (200, R1 = R3 = Ph, R2 = H) was stated to yield97 phenylisocyanate and the bicyclic compound 215. Later studies,7b have shown that the bicyclic compound 215 is not produced by the photolysis of 200, R1 = R3 = Ph, R2 = H. Irradiation in methanol-methylene chloride gave methyl phenylcar-bamate (25%), benzimidazole (18%), and azobenzene (7%). 

Polyatomic anions can yield complications. Broomhead and Ileperuma1351 found [Ru(bpy)3](NCSe)2 in methylene chloride yielded [Ru(bpy)2(SeCN)2], while in ethanol the product probably was the cyanate. Wallace and Hoggard1321 had claimed the azido-complex [Ru(bpy)2(N3)2] to be the photolysis product in ethanol. 

Various compounds were shown to sensitize the photochemical decomposition of pyridinium salts. Photolysis of pyridinium salts in the presence of sensitizers such as anthracene, perylene and phenothiazine proceeds by an electron transfer from the excited state sensitizer to the pyridinium salt. Thus, a sensitizer radical cation and pyridinyl radical are formed as shown for the case of anthracene in Scheme 15. The latter rapidly decomposes to give pyridine and an ethoxy radical. Evidence for the proposed mechanism was obtained by observation of the absorption spectra of relevant radical cations upon laser flash photolysis of methylene chloride solutions containing sensitizers and pyridinium salt [64]. Moreover, estimates of the free energy change by the Rehm-Weller equation [65] give highly favorable values for anthracene, perylene, phenothiazine and thioxanthone sensitized systems, whilst benzophenone and acetophenone seemed not to be suitable sensitizers (Table 5). The failure of the polymerization experiments sensitized by benzophenone and acetophenone in the absence of a hydrogen donor is consistent with the proposed electron transfer mechanism. 

The dynamics of reversible onium ion formation has been studied by generating carbenium ions in the presence of nucleophiles using pulse radiolysis or flash photolysis, and following the rate of disappearance of the carbenium ions by UV. As discussed in Chapter 2, the kinetics of reaction of various electrophiles with nucleophiles obey a general reactiv-ity/selectivity relationship. The rates of reaction of various nucleophiles with carbenium ions are summarized in Table 9. These rates often approach diffusion controlled limits (k 10 ° mol-,-L-sec l). The rates are slower for less nucleophilic and less electrophilic compounds, and are particularly slow with sterically hindered amines such as lutidine (2,6-dimethylpyridine) [63]. Solvent effects are minimal when the reactions are diffusion controlled, although tributyl amines react slower with carbenium ions in more nucleophilic dichloroethane than in methylene chloride. 

Photolysis of the arene complexes in the presence of monodentate ligands, e.g. carbon monoxide, leads to new complexes of the type CpFe(L) whereas in pure aprotic solvents, ferrocene and iron salts are formed Investigation of the photo-lytic reaction of an iron arene complex with excess ethylene oxide in methylene chloride solution (Meier and Rhis ) showed that a crystalline crown ether complex (structure shown in Fig. 9) was obtained in high yield. Only traces of dioxane could be detected. 

When released into the environment as a vapor, methylene chloride does not usually undergo direct photolysis but does degrade by reaction with photo-chemically produced hydroxyl radicals. Its half-life in the atmosphere is estimated at 119 days. In soil and water, a major fate pathway involves volatilization to the atmosphere and subsequent degradation. Activated sludge studies have, however, demonstrated biodegradation of methylene chloride. Little bioconcentration in aquatic species can be expected. 

Photolysis of 1.00 g (3.90 mmol) of 9-diazothioxanthene-5,5-dioxidel l in 20 mL of p-methylstyrene, contained in 25 mL-size Vycor glass, using a Hanovia lamp (450 W), was continued until disappearance of the starting diazo-red color. After vacuum distillation of the unreacted styrene, column chromatographic separation of the residue (silica gel, methylene chloride/n-pentane (1 2)), solvent removal and recrystallization from ether afforded 1.10 g (81%) of 2.10b, mp 190 °C. 

Philbin et al.52 studied in detail the photochemistry of the disproportionation reaction producing Mo(CO)2(PPh3)2Cp + Mo(CO)3Cp . It was proposed that the initial ionic product was Mo(CO)3(PPh3)Cp) + Mo(CO)3Cp but that this was capable of back reaction. The back reaction was proposed to occur in benzene, but to be slowed down in more polar solvents such as methylene chloride. Furthermore, wavelength-dependent photolysis on the Mo(CO)3(PPh3)Cp + cation to produce... 

In contrast, photolysis of 1-substituted isoquinoline /V-ethoxycarbon-ylimines in methylene chloride gives the 1H-1,3-benzodiazepines 128 in approximately 20% yields.244 The formation of the diazepines may involve a ring expansion of the diaziridine intermediates 127 (Eq. 43). 

Photolysis of 1,2,4-triazole 4-acetyIimines in methylene chloride gives a different product composition.133 The N-N cleavage is still dominant, but s-triazole-5-carboxamides (113) are formed, in a rearrangement unobserved with other iV-imines and of unknown mechanism. 

Full details have been published of the preparation of 1A, 2,4-benzothiadiazines from N-arylbenzamidines thus treatment of N-phenylbenzamidine (Ph CPhNHg) and phenylsulphenyl chloride (PhSCl) with N-chlorosuccinimide in methylene chloride gives the benzothiadiazine (399). On heating, (399) undergoes a 1,4-sigmatropic shift to give the -benzothiadiazine (4.00) photolysis of (399) yields the benzimidazole (398) (Scheme 88). ... 

Photolysis of deaerated l-phenyl-l,3,5-hexatriyne with unsymmetrical olefins such as AN, MA, and ST in methylene chloride gives three 1 1 photoadducts (40-48) [55]. 

Photolytic or thermal treatment of a-azidoketones offers a route to azahomosteroids. Thus, for example, photolysis of the azide (247) in methanol affords the lactone (248), whilst irradiation in methylene chloride or heat forms the iminolactone (249) <92ac2405>. 

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