Photolysis dimers

In weaker acid systems, other reactions involving the triplet state supervene to the exclusion of dimerization. Photolysis of 85 in 3-3% sulfuric acid, 96-5% acetic acid, and 0-2% water gave as products tri-phenylmethane (93), 9-phenylfluorene (94), 6is-9-phenylfluorenyl peroxide (95) and benzophenone (96). When benzene was present, tetra-phenylmethane (97) was also formed in addition to the other products. When the triphenylmethyl cation is irradiated in 3-3% H2SO4, 80 1% HOAc, 16-4% toluene, and 0-2% H2O, the products observed were... 

However, attempts by Kiefer and Carlson59 to prohibit undesired bi-molecular reactions by irradiating 2,3,3-trimethyl-l-penten-4-one adsorbed onto silica gel were unsuccessful (due probably to steric inhibition of adsorption) the product composition was the same as that previously obtained in solution. Werbin and Strom80 attempted to restrain the freedom of movement of the radicals formed from the photolysis of vitamin K3 (2-methyl-1,4-naphthoquinone) by adsorption onto silica gel, but obtained the same mixture of dimers as that obtained from the irradiation in acetone solution, viz., syn and anticyclobutanes, an oxetane dimer, and a binaphthoquinone dimer. Photolysis of the solid substrate, however, produced only the syn isomer of cyclobutane, in this case no migration of radicals is possible, hence only one product. 

Figure Bl.16.2. X-band TREPR spectra obtained at 0.1 ps after 308 mn photolysis of a fliiorinated peroxide dimer in Freon 113 at room temperature. Part A is the A/E RPM spectrum obtained upon direct photolysis part B is the E/A RPM spectrum obtained upon triplet sensitization of this reaction using benzophenone.

Irradiation of ethyleneimine (341,342) with light of short wavelength ia the gas phase has been carried out direcdy and with sensitization (343—349). Photolysis products found were hydrogen, nitrogen, ethylene, ammonium, saturated hydrocarbons (methane, ethane, propane, / -butane), and the dimer of the ethyleneimino radical. The nature and the amount of the reaction products is highly dependent on the conditions used. For example, the photoproducts identified ia a fast flow photoreactor iacluded hydrocyanic acid and acetonitrile (345), ia addition to those found ia a steady state system. The reaction of hydrogen radicals with ethyleneimine results ia the formation of hydrocyanic acid ia addition to methane (350). Important processes ia the photolysis of ethyleneimine are nitrene extmsion and homolysis of the N—H bond, as suggested and simulated by ab initio SCF calculations (351). The occurrence of ethyleneimine as an iatermediate ia the photolytic formation of hydrocyanic acid from acetylene and ammonia ia the atmosphere of the planet Jupiter has been postulated (352), but is disputed (353). 

The photosensitized dimerization of isoprene in the presence of henzil has been investigated. Mixtures of substituted cyclobutanes, cyclohexenes, and cyclooctadienes were formed and identified (53). The reaction is beheved to proceed by formation of a reactive triplet intermediate. The energy for this triplet state presumably is obtained by interaction with the photoexcited henzil species. Under other conditions, photolysis results in the formation of a methylcydobutene (54,55). 

Photolysis of Cp2TiAr2 in benzene solution yields titanocene and a variety of aryl products derived both intra- and intermolecularly (293—297). Dimethyl titan ocene photolyzed in hydrocarbons yields methane, but the hydrogen is derived from the other methyl group and from the cyclopentadienyl rings, as demonstrated by deuteration. Photolysis in the presence of diphenylacetylene yields the dimeric titanocycle (28) and a titanomethylation product [65090-11-1]. 

The fluorinated titanocycle related to (28) is not obtained from C H CMUCgF. Photolysis of CP2T1X2 always gives first scission of a Cp—Tibond. In a chlorinated solvent, the place vacated by Cp is assumed by Cl. In the absence of some donor, the radical dimerizes (298—299). 

The trans isomer is more reactive than the cis isomer ia 1,2-addition reactions (5). The cis and trans isomers also undergo ben2yne, C H, cycloaddition (6). The isomers dimerize to tetrachlorobutene ia the presence of organic peroxides. Photolysis of each isomer produces a different excited state (7,8). Oxidation of 1,2-dichloroethylene ia the presence of a free-radical iaitiator or concentrated sulfuric acid produces the corresponding epoxide [60336-63-2] which then rearranges to form chloroacetyl chloride [79-04-9] (9). 

The photolysis of azirines has been shown to result in dimerization to pyrazines (72JA1395) and although this formally corresponds to a type B synthesis it involves an isolable intermediate (105) and does not proceed by simple dimerization (Scheme 70). 

Photolysis of spiro[fluorene-9,3 -indazole] (384) to the tribenzopentalene (385) has been rationalized in terms of the initial formation of triplet diradical (386) (76JOC2120). The spiroindazole (387) behaves differently and on irradiation in THF is converted into the dimer (388) and the stable iV-ylide (389) (76CB2596). 

The photolysis of 1,2-benzisoxazole in the absence of air in acetonitrile gave salicylonitrile and benzoxazole (67AHC(8)277). When air-saturated acetonitrile was employed, 2,2 -dimeriz-ation to (38) occurred, accompanied by benzoxazole. Photolysis of the 2,2 -dimer (38) and benzoxazole did not alter the ratio, thus indicating that neither one arose from the other. Selective excitation also ruled out dimer formation from benzoxazole under the reaction conditions (Scheme 9). This dimerization is similar to that observed for benzimidazole, except that in that series no 2,2 -dimerization was observed (74TL375). 

Additional work showed that the dimerization of arylazirines to 1,3-diazabicy-clo[3.1.0]hex-3-enes is a general reaction which is independent of the nature of the substituent groups attached to the C atom of the azirine ring. Care is required in the choice of solvent, photolysis time and substituents since the 1,3-diazabicyclohexenes are themselves photochemically labile (72JA7788). 

Preparation of pentalene is followed by immediate dimerization. Low-temperature photolysis produces a new species believed to be pentalene, but the compound reverts to dimer at — 100°C. The matrix-isolated monomer has been characterized spectroscopically. The results are in accord with the predicted lack of stabilization. ... 

Nitriles from 19-nitroso-dimers, 270 Nitrite photolysis, 239 19-Nitroso-5a-pregnane-3/8,6/3,20/3-triol... 

When uradiated, fluonnated isomers of Dewar benzene yield pnsmane derivatives that rearrange thermally to benzene Photolysis of hexakis(mfluororaethyl)benzvalene ozonide gives tetrakis(tnfluoromethyl)cyclobutadiene and its dimer [J47]... 

Subsequent reactions depend on conditions. Ultraviolet photolysis of isolated molecules in an inert matrix yields the radicals CIO and CIOO. At room temperature, photolysis of diy gaseous CIO2 yields CI2, O2, and some CIO3 which either dimerizes or is further photolysed to CI2 and O2 ... 

An intensely colored by-product of the photolysis reaction of methyl-2-azidobenzoate has been identified as the first known derivative of 3,3 -diazaheptafulvalene 70 (94LA1165). Its molecular mass was established by elemental analysis and mass spectroscopy as that of a formal nitrene dimer, whereas and NMR studies demonstrated the twofold symmetry as well as the existence of a cross-conjugated 14 7r-electron system in 70. Involving l-azido-2,3-dimethoxy-5,6-dimethoxycarbonylbenzene in thermal decomposition reactions, the azaheptafulvalene 71 could be isolated and characterized spectroscopically and by means of X-ray diffraction. Tliis unusual fulvalene can be regarded as a vinylogous derivative of azafulvalenes (96JHC1333) (Scheme 28). 

Tile a-dithione 91, generated by photolysis of 92, is transformed into the dithiin 93 (417o) in the absenee of a trapping agent. The eonversion was proposed to proeeed by a [4 + 2] eyeloaddition of 91 with its dithiete tautomer 94 leading to the dithietane 95, whieh was followed by loss of S2 (85JOC1550). Sueh [4 + 2] dimerizations are often eneountered in the ehemistry of 1,2-dithietes as diseussed later. 

Tile following photochemical conversions also involve 1,2-dithietes as intermediates whose chemical trapping was reported in most cases. Tlie formation of the dithiin 249 from 250 may best be explained by the formation of the dithiete dimer 251 and the loss of S2 (73ZC424).Tlie formation of 252 and 253 from 254 (78NJC331) should be compared with the sulfuration of the acetylene 182 with elemental sulfur (93BCJ623).Tlie photolysis of 255 provides a rare example when the ejection of a nitrile was employed for the generation of a 1,2-dithiete (73ZC431). 

Azaquadricyclanes2,formed by photolysis of the Diels-Alder [4 + 2]cycloadductsl of 1-acyl-or 1-sulfonylpyrroles with dimethyl acetylenedicarboxylate at room temperature, undergo isomerization in high yield to l//-azepine-4,5-dicarboxylates 3 (cf. Houben-Weyl, Vol.4/5b, p 1094).122,128 129 However, the azepines are difficult to purify since they dimerize, even at low temperatures (20-40°C). 

On photolysis in benzene solution, the bisamides 22 (n > 14) form internal [2 + 2] photodimers 23, the maximum quantum yield being noted with n = 26. The shorter alkyl chain analogs 22 (n = 2-12), under the same conditions, form intcrmolccular dimers.236... 

The halide groups can be replaced by other substituents like hydride or alkyl [86]. When the naphthyl (np) RuHnp(dmpe)2 is heated, the ligand undergoes an internal metallation to afford a dimer [87] (it was originally believed to be a monomer), though Ru(dmpe)2 has been isolated by photolysis of its dihydride, in matrixes at 12K (Figure 1.29) [88],... 

Photolysis of the rhodium(III) complex of octaethylporphyrin gives a rhodium(II) dimer that readily undergoes addition reactions to afford rhodium(III) species (Figure 2.42). 

Dihydrodithiin sulphoxides, synthesis of 243 Dihydrothiophene dioxides, reactions of 653 /(,/( -Dihydroxyketones 619 Dimerization, photochemical 877, 884 Dimethyl sulphoxide anion of - see Dimsyl anion hydrogen bonding with alcohols and phenols 546-552 oxidation of 981, 988 photolysis of 873, 874, 988 radiolysis of 890-909, 1054, 1055 self-association of 544-546 Dimsyl anion... 

Much has been learned in recent years about the 00 dimer , O2O2, produced in reaction 17. It is actually dichlorine peroxide, OOOCl its geometry is now well established from submillimeter wave spectroscopy (15). Photolysis of OOOO around 310 nm the atmospherically important wavelengths -- yields chlorine atoms and ClOO radicals (16), as given in reaction 18, rather than two OO radicals, even though QO-OQ is the weakest bond (it has a strength of about 17 Kcal/mol (17)). Thermal decomposition of QOOQ (the reverse of reaction 17) occurs very fast at room temperature, but more slowly at polar stratospheric temperatures. Hence, photolysis is the predominant destruction path for CIOOQ in the polar stratosphere and two Q atoms are produced for each ultraviolet photon absorbed. 

In another reductive coupling, substituted alkenes (CH2=CH Y Y = R, COOMe, OAc, CN, etc.) can be dimerized to substituted alkanes (CH3CHYCHYCH3) by photolysis in an H2 atmosphere, using Hg as a photosensitizer. Still another procedure involves palladium-catalyzed addition of vinylic halides to triple bonds to give 1,3-dienes. ... 

The HS2 radical was detected by its infrared absorption spectrum and the S2 molecule by luminescence spectroscopy. In addition, infrared bands assigned to dimers of disulfane molecules were observed at higher H2S2 concentrations. The HS2- radicals may further be split into hydrogen atoms and S2 molecules during the photolysis since the concentration of HS2- first increases and then decreases while that of S2 steadily increases. No evidence for the thiosulfoxide H2S=S was found, and the probably formed HS- radicals are assumed to be unable to leave their cage in the matrix and either recombine to H2S2 or form H2+S2 [69]. 

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