Photochemical reactions alkene 4- oxygen

Hashimoto, S. and Akimoto, H. (1987). Absorption spectra of contact-charge-transfer bands and photochemical reactions of simple alkenes in the cryogenic oxygen matrix. J. Phys. Chem. 91, 1347-1354... 

Yates and coworkers have examined the mechanism for photohydration of o-OH-8. The addition of strong acid causes an increase in the rate of quenching of the photochemically excited state of o-OH-8, and in the rate of hydration of o-OH-8 to form l-(o-hydroxyphenyl)ethanol. This provides evidence that quenching by acid is due to protonation of the singlet excited state o-OH-8 to form the quinone methide 9, which then undergoes rapid addition of water.22 Fig. 1 shows that the quantum yields for the photochemical hydration of p-hydroxystyrene (closed circles) and o-hydroxystyrene (open circles) are similar for reactions in acidic solution, but the quantum yield for hydration of o-hydroxystyrene levels off to a pH-independent value at around pH 3, where the yield for hydration of p-hydroxystyrene continues to decrease.25 The quantum yield for the photochemical reaction of o-hydroxystyrene remains pH-independent until pH pAa of 10 for the phenol oxygen, and the photochemical efficiency of the reaction then decreases, as the concentration of the phenol decreases at pH > pAa = 10.25 These data provide strong evidence that the o-hydroxyl substituent of substrate participates directly in the protonation of the alkene double bond of o-OH-8 (kiso, Scheme 7), in a process that has been named excited state intramolecular proton transfer (ESIPT).26... 

To have a rough idea of the potential of the method, please have a look to the syntheses below that are among those considered typical in a recent book (Scheme 1). These are a cyclization to hydroxycyclobutane via H-abstraction, a 2 + 2 alkene cycloaddition, a "meta" benzene-olefin cycloaddition, and a dye sensitized addition of singlet oxygen." Please consider whether there are facile thermal alternatives to these straightforward photochemical reactions. 

These photochemical reactions with olefins can be considered a cationic analogue of the Meerwein arylation that occurs with nucleophilic rather than with electrophilic alkenes. The rapid cleavage of excited aryl halides and esters in polar solvent and the efficient trapping of the formed aryl cation render these arylations normally less-sensitive towards dissolved oxygen, in contrast to many other photochemical reactions. These characteristics, along with the mild reaction conditions and the simple experimental set-up, make the photochemical method a complementary and valuable alternative to metal-mediated or -catalyzed reactions. 

Among the photochemical reactions of aromatic compounds, the photocycloadditions are most frequently applied to the synthesis of complex polycyclic compounds [6, 9]. The [2+3] or meta photocycloaddition of aromatic compounds and alkenes is the most prominent example [10]. This transformation also demonstrates complementarities between photochemical and ground state reactions since such reactions are almost impossible using conventional activation. A [2+2] ot ortho photocycloaddition between carbocyclic aromatic compounds and alkenes is observed as well. It is often competitive with other cycloaddition modes, in particular the [2+3] mode [11]. Many of these reactions are reversible, and photostationary equilibria are involved. This reaction was much less applied to organic synthesis. Recently, it was found that an acidic reaction medium may have an influence on the outeome of the reaction. The intramolecular photocycloaddition of resorcinol derivatives such as 1 is difficult due to its reversibility (Scheme 29.1). However, in an acidic reaction medium, the cycloadducts 2a,b are protonated at the oxygen atom of the tetrahydrofuran moiety... 

The visible light excitation of diene - NO reactant pairs has been also found to cause an oxygen-atom transfer from NO to diene to form the oxirane biradical [77], which is similar to the photochemical reactions observed in the alkene - NO - Ar systems. Two isomers are considered for the nitrite radical of butadiene ... 

Abstract Photochemical reactions are generally easily carried out, at least in laboratory scale, and require no expensive apparatus. Some general reactions, e.g. the cycloaddition of enones to alkenes and various oxygenations have been extensively investigated and represent an excellent choice for preparative applications. Many other possibilities are known—and a few are presented below. This suggests that photochemical steps should be considered more often in synthetic planning. 

The important hydrocarbon classes are alkanes, alkenes, aromatics, and oxygenates. The first three classes are generally released to the atmosphere, whereas the fourth class, the oxygenates, is generally formed in the atmosphere. Propene will be used to illustrate the types of reactions that take place with alkenes. Propene reactions are initiated by a chemical reaction of OH or O3 with the carbon-carbon double bond. The chemical steps that follow result in the formation of free radicals of several different types which can undergo reaction with O2, NO, SO2, and NO2 to promote the formation of photochemical smog products. 

The attacking radical need not always be at carbon. Amidyl radical are known and give cyclization reactions. Aminyl radical cyclizations have been reported. Oxygen radical can be generated under photochemical conditions, and they add to alkenes in a normal manner. ... 

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... 

In qualitative terms, the rearrangement reaction is considerably more efficient for the oxime acetate 107b than for the oxime ether 107a. As a result, the photochemical reactivity of the oxime acetates 109 and 110 was probed. Irradiation of 109 for 3 hr, under the same conditions used for 107, affords the cyclopropane 111 (25%) as a 1 2 mixture of Z.E isomers. Likewise, DCA-sensitized irradiation of 110 for 1 hr yields the cyclopropane derivative 112 (16%) and the dihydroisoxazole 113 (18%). It is unclear at this point how 113 arises in the SET-sensitized reaction of 110. However, this cyclization process is similar to that observed in our studies of the DCA-sensitized reaction of the 7,8-unsaturated oximes 114, which affords the 5,6-dihydro-4//-l,2-oxazines 115 [68]. A possible mechanism to justify the formation of 113 could involve intramolecular electrophilic addition to the alkene unit in 116 of the oxygen from the oxime localized radical-cation, followed by transfer of an acyl cation to any of the radical-anions present in the reaction medium. 

Regardless of whether singlet oxygen is formed chemically or photochem-ically, it gives similar products in reactions with alkenes. 

Irradiation of an alkene in the presence of molecular oxygen and an a-diketone furnishes the core-sponding oxirane in high yields. The reaction proceeds in the complete absence of nucleophiles, and thus can avoid formation of by-products arising from the reaction of nuclec hiles with sensitive oxiranes. The photoepoxidation proceeds via addition of an acylperoxy radical to the alkene. Photochemical epoxidation of cholesteryl acetate (176) has been carried out (equation 64a) the major epoxidation product is the sp,6 -epoxide (177a). In MCPBA epoxidation of (176) the major product is (177b). 

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