Photochemical version of the

Ti02, with a band gap of 3.2 eV, was successfully used for the photooxidation of acetate ion in acetic acid, a photochemical version of the Kolbe reaction (Kraeutler et al., 1978). The main products formed were methane and carbon dioxide, in addition to small amounts of ethane. The latter is the major product... 

A rational approach to land use planning for air quality maintenance may be built on the basis of diffusion modeling. Although the photochemical versions of the models that relate air quality to emission are... 

The Patemo-Buchl reaction of furan and various aldehydes was shown to be a highly stereoselective photochemical version of the aldol reaction by S.L. Schreiber and co-workers in which the furan serves as an enolate equivalent. This strategy was applied to the total synthesis of the antifungal metabolite (+)-avenaciolide. The photocycloaddition of nonanal with excess furan proceeded in nearly quantitative yield, and the two out of the three required stereocenters were created in a single step. The photocycloadduct was first hydrogenated then hydrolyzed under acidic conditions. 

A photochemical version of the system has also been reported. In this case, irradiation of PtClg " and hexane in AcOH gives a Pt(II) complex of hexene. The alkene is almost certainly formed by P elimination of the intermediate alkyl, a reaction to be discussed in more detail in the next section. With Mc2CO as substrate a stable alkyl, [(H3N)Cl4Pt(CH2COMe)] was isolated by addition of NH3. 

Photochemical versions of the BKR have been exploited and studied from both a mechanistic point of view (2012PPS1383) and for the preparation of the isoxazoline systems 34 (Scheme 9 2009ARK156). 

As we saw briefly in Chapter 53, the photochemical version of the Graebe-Ullmann carbazole-carboUne synthesis can be an excellent route to these heterocycles. Similar to this benzotriazole reaction is the photolysis of aryl azides to give carbazoles and related fused indoles. 

Photolysis of a-diazoketones produces acyl carbenes, which efficiently rearrange to ke-tenes that are subsequently trapped (Eq. 16.73). This photochemical version of the Wolff re-... 

A novel photochemical version of the Bamford-Stevens reaction has been used in the synthesis of A -dehydro-A/D-seco-corrin complexes (183). Irradiation of the seco-corrinoid amidrazone (181) in methanol gave the methoxy-derivative (182) which underwent elimination of methanol in base to give (183) (Scheme 51). 

Studies of di-ir-methane photochemical rearrangements have been one of the main areas of research in organic photochemistry for many years (for reviews, see Refs. 1-4). The first example of a reaction of this type was reported by Zimmerman in 1967 in the sensitized irradiation of barrelene 1 that yields semibullvalene 2 [5] (Scheme 1). The reaction has been extended to a large number of acyclic and cyclic 1,4-dienes that yield the corresponding vinylcyclo-propanes on irradiation, in the di-ir-methane (DPM) version of the rearrangement. This reaction also takes place when a vinyl unit is replaced by an aryl group. A few representative examples of DPM rearrangements are shown in Scheme 1 [6-9]. 

With the same catalysts, carbene transfer from diazoacetates 205 [silyl groups SiMe3, SiEt3, Si(Pr-i)3] to 1-hexene and styrene was studied4,117. The yields of cyclopropanes were usually higher in the metal-catalyzed than in the photochemical version. Interestingly, the metal-catalyzed procedure gave preferentially -210, whereas... 

Cycloaddition of the carbene derived from 205 to bis(trimethylsilyl)acetylene yields the expected cyclopropene in low yield both photochemically (20%) and under catalysis by copper triflate at 80 °C (10-13%)119. The latter version of the reaction is accompanied by [3 + 2] cycloaddition of the diazo compound to the alkyne, and the photochemical route yields a by-product which obviously comes from carbenic C,H insertion at a SiMe3 group of the alkyne. 

A silicon version of the well-studied electrocyclic interconversion of C4H6 (cyclobutene, bicyclobutane, butadiene, etc.) has been investigated both experimentally23,101 and theoretically.102,103 Irradiation of 51 in 3-methylpentane (X >420nm) gives a 1 9 mixture of 51 and bicyclo[1.1.0]tetrasilane 88 at the photostationary state as determined by UV-vis spectroscopy.23 When the mixture is left for 12 h in the dark at rt, 51 is produced quantitatively [Eq. (58)]. The photochemical conversion of 51 to 88 and the thermal reversion can be repeated more than 10 times without any appreciable side reactions. 

According to the Woodward-Hoffmann rules, the concerted [2S + 2J cycloaddition with two alkenes is photochemically symmetry-allowed, but is symmetry-forbidden at the ground state [9]. Photochemical [2 + 2] cycloaddition, in which one of two alkene partners is electronically excited, has been applied to the synthesis of cage hydrocarbons [10]. In such transformations, the intramolecular version of the reaction is particular efficient. The transformation of compound 1, in which two... 

Next, let s consider the photochemical version of this reaction. A photochemical reaction between two molecules involves the excited state of one component and the ground state of the other. (Because most excited states have extremely short lifetimes, the chances of two excited molecules colliding are exceedingly small, so reactions where both components are excited are highly improbable.) The HOMO of the excited ethene is n, and the LUMO of the ground-state ethene is also tt. The overlaps are as follows ... 

The Pauson-Khand reaction typically involves the thermal reaction of a Co2(CO)6(/r- -alkyne) with alkene to form a substituted cyclopentenone. A photochemical version of this reaction has been reported.Matrix studies of Co2(CO)6(/r-jj -alkyne) have established that CO-loss occurs from a single position, but that photochemical rearrangement of the Co2(CO)5(/r- j -alkyne) photointermediate readily takes... 

Cyclohexadienone derivatives that have two alkyl groups in the 4 position undergo, on acid treatment, 1,2 migration of one of these groups from 64 to give the phenol. Note that a photochemical version of this reaction has been... 

The photoaddition of a furan and an aldehyde can serve as a photochemical version of a stereoselective aldol reaction, since the photoadduct can be viewed as a protected aldol, as indicated in Scheme 23 (Schreiber et al., 1983). 

The fj-nitrobenzyl and p-nitrobenzyl ethers can be prepared and cleaved by many of the methods described for benzyl ethers. In addition, the o-nitrobenzyl ether can be cleaved by irradiation (320 nm, 10 min, quant, yield of carbohydrate 280 nm, 95% yield of nucleotide ). This is one of the most important methods for cleavage of this ether. These ethers can also be cleaved oxidatively (DDQ or electrolysis) after reduction to the aniline derivative." Clean reduction to the aniline is accomplished with Zn(Cu) (acetylacetone, rt, >93% yield).Hydrogenolysis is also an effective means for cleavage. A polymeric version of the o-nitrobenzyl ether has been prepared for oligosaccharide synthesis that is also conveniently cleaved by photolysis. An unusual selective deprotection of a bis-o-nitrobenzyl ether has been observed. The photochemical reaction of o-nitrobenzyl derivatives has been reviewed. ... 

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