Photolysis sensitization

Pola18 used laser flash photolysis sensitized by SF6 to study the decomposition of 1-methyl-l-vinylsilacyclobutane which yielded 1-methyl-l-vinylsilene. The reaction followed first-order kinetics. 

Thermal, in dibutylcarbinol. Direct photolysis in hexane solution. c Photolysis sensitized by triphenylene. 

Photolysis, sensitizer, CH3CN, 2h, 76-100% yield. " Irradiation of buffered solutions of p-hydroxyphenacyl esters releases the acid."... 

Iodonium salt photolysis sensitized by dialkoxyanthracenes is thus sensitive to solvent viscosity in respect to bimolecular quenching [100]. 

PROBABLE FATE photolysis, sensitized process may be important, reacts in the vapor phase with photochemically produced hydroxyl radicals at an estimated half-life of 6.2 hr, suggesting that this reaction is the predominate chemical removal process oxidation photooxidation half-life in air 5.2-51.7 hrs hydrolysis not an important process, first-order hydrolytic half-life >197,000 yrs volatilization probably an important process, can volatilize significantly from soil surfaces from which it is sprayed, particularly moist soil surfaces, volatilization half-life from a model pond, river, and lake is 18-26,3.6-5.2, and 14.4-20.6 days respectively sorption probably an important process biological processes bioaccumulation is an important process... 

A sensitizer is a substance that makes a reaction more sensitive to photolysis. Sensitizers absorb light more strongly than the reactant and then transfer the absorbed energy to the reactant. The sensitizer must have an electronic excited state that is sufficiently long-lived to allow for the energy transfer to the reactant, and the energy of this state must be similar to that of the acceptor state of the reactant. 

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.

Transient species, existing for periods of time of the order of a microsecond (lO s) or a nanosecond (10 s), may be produced by photolysis using far-ultraviolet radiation. Electronic spectroscopy is one of the most sensitive methods for detecting such species, whether they are produced in the solid, liquid or gas phase, but a special technique, that of flash photolysis devised by Norrish and Porter in 1949, is necessary. 

Fig. 20. Proposed photochemical mechanisms for the generation of acid from sulfonium salt photolysis. Shown ate examples illustrating photon absorption by the onium salt (direct irradiation) as well as electron transfer sensitization, initiated by irradiation of an aromatic hydrocarbon.

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 ring opening of 2//-azirines to yield vinylnitrenes on thermolysis, or nitrile ylides on photolysis, also leads to pyrrole formation (B-82MI30301). Some examples proceeding via nitrile ylides are shown in Scheme 92. The consequences of attempts to carry out such reactions in an intramolecular fashion depend not only upon the spatial relationship of the double bond and the nitrile ylide, but also upon the substituents of the azirine moiety since these can determine whether the resulting ylide is linear or bent. The HOMO and second LUMO of a bent nitrile ylide bear a strong resemblance to the HOMO and LUMO of a singlet carbene so that 1,1-cycloadditions occur to carbon-carbon double bonds rather than the 1,3-cycloadditions needed for pyrrole formation. The examples in Scheme 93 provide an indication of the sensitivity of these reactions to structural variations. 

Photodecomposition of A -l,2,3-triazolines gives aziridines. In cyclohexane the cis derivative (304) gives the cis product (305), whereas photolysis in benzene in the presence of benzophenone as sensitizer gives the same ratio of cis- and trans-aziridines from both triazolines and is accounted for in terms of a triplet excited state (70AHC(ll)i). A -Tetrazo-lines are photolyzed to diaziridines. 

Photochemical studies on the ring degradation of 3-hydroxy-l,2-benzisoxazole also yielded benzoxazolone, and (40), (41) and (42) (Scheme 14) were believed to be potential intermediates. Low temperature IR measurements indicated the presence of (42) during the photochemical reaction (73JA919, 71DIS(B)4483, 71JOC1088). Sensitization studies indicate that the rearrangement is predominantly a triplet reaction, and the keto tautomer is believed to be the key orientation for the photolysis. 

Benzene-sensitized photolysis of methyl 3-cyclohexene-1-carboxylate in acetic acid leads to addition of acetic acid to the double bond. Only the trans adducts are formed. What factor(s) is (are) responsible for the reaction stereochemistry Which of the two possible addition products, A or B, do you expect to be the major product ... 

Methylfuran, irradiated in the presence of mercury vapor, gave carbon monoxide and a fraction containing 1,3-butadiene and 3-methylcyclopropene (45 55) (67JA1758). Subsequently, it was found that in both sensitized and direct photolysis of 2-methylfuran a more complex mixture of products was obtained, where 3-methylfuran was present (Scheme 5) (68JA2720 70JPC574). 3-Methylfuran was the only product when 2-methylfuran was irradiated in diethyl ether (68JA2720). 

An essentially quantitative yield (98%) of the thermally stable hexafluoro-2-aza-bicyclo[3.2.0]hepta-3,6-diene 18 is obtained from the small-scale photolysis of the hexafluoro-l//-azepine 17.230 Hexafluoro-l//-azepine-l-carbonitrileand-l-carboxamidebehave similarly. On scaling up the process for the l-(ethoxycarbonyl) derivative 17, however, a much lower yield (59%) of bicycle 18 resulted, although this was improved to 81% by carrying out the photolysis in hexane or rm-butyl alcohol in the presence of a triplet sensitizer, e. g. ben-zophenone. 

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