Ethers benzyl, photochemical

Other methods that have been less regularly used are the dehydration of alcohols with dimethyl sulfoxide to form symmetrical ethers, the photochemical transformation of benzylic chlorides with fert-butyl alcohol, or radical reactions of hcxafluoroacetone with alkanes, Mercury acetate promoted couplings of alcohols with vinyl acetate or vinyl ethyl ether to form vinyl... 

The benzyl ethers (223) photochemically cleave to form alcohols ROH (R = alkyl, cycloalkyl, or 2-pyrrolidinoethyl) in acetonitrile/water. The reaction involves sinsls electron... 

In the present chapter, emphasis is placed on the authors recent studies concerning the molecular design and optical/photochemical properties of Frechet-type poly(benzyl ether) dendrimers. 

Poly(benzyl ether) dendrimers have several unique optical and photochemical properties. In particular, it is quite interesting that some photochemical events are considerably affected by the molecular size and morphology of the dendrimer molecules. These examples, together with those by other researchers [2], will provide a new strategy toward next-generation, nanoscopic photofunctional materials. 

The proposed mechanism for the photochemical cleavage of nBn ethers (see Scheme 3) involves a n tt transition of the nitrogroup and an intramolecular, benzylic hydrogen abstraction by the excited nitro group. Rearrangement leads to a hemiacetal that decomposes to a free alcohol and to 2-nitrosobenzaldehyde that undergoes further thermal and photochemical reactions. 

Scheme 4.—Photochemical Cleavage of Benzyl Ethers in the Presence of Bromine.

Castellan, A., Vanucci, C., Bous-Laurent, H., "Photochemical Degradation of Lignin Through a C-0 Bond Cleavage of Non-phenolic Benzyl Aryl Ether Units. A Study of the Photochemistry of a(2 ,4 -Trimethyl-Phenoxy)-3,4 Dimethoxy Toluene.", Hohforschmg, 1987, 41(4), 231. 

A photochemical approach to the synthesis of complex spiropyrans has been developed. A variety of spiropyrans can be synthesized by replacement of the benzylic ether moiety of previously described photocyclization substrates with allylic ethers composed of cycloalkenes of diverse size (Scheme 46) <1996TL8913>. 

Other recent oxidative methods have yet to make a major impact. Photochemical bromination can be used for the oxidative cleavage of a benzyl ether on a substantial scale.290-291 An intermediate a-bromo ether hydrolyses to a hemiace-tal intermediate that collapses to give benzaldehyde and the liberated alcohol [Scheme 4 156] ... 

Enol benzyl ether, photochemical rearrangement, 431 Enol esters, electrophilic reagents with, 183... 

Photochemically aided hydrogenolysis of allylic C-heteioatom bonds has anracted little interest. A few methods for debenzylation of ethers, amines and thiols, catalyzed by irradiation, have been utilized (see previous section also). An unusual desulfurization of benzyl thiol with triethyl phosphite (equation 94) in high yield, catalyzed by UV irradiation, is a useful alternative to other methods described in other parts of this chapter. 

Oxidative cyclization of benzyl and other 2-hydroxyethyl ethers to afford dioxolanes (Equation 46) can be achieved using A -iodosuccinimide in nitromethane for R = Ph <1998AGE3177> or photochemical reaction with iodine and polymer-supported iodobenzene diacetate in acetonitrile <2005SL923>. In a related process, RuCl2(Ph3P)2 catalyzes the isomerization of allyl 2-hydroxyethyl ether to form 2-ethyl-l,3-dioxolane <2004SL1203>. 

The 4,5-corane (84) is obtained in SOX yield on photo-decarbonylation of the pentacyclic ketone (85). Photochemical decomposition of the carbonate (86), by the loss of carbon dioxide, affords a mixture of products containing oxirane. styrene oxide, bibenzyl and phenylacetaldehyde. Triplet sensitized irradiation yields products solely from benzyl radicals. - An earlier study of the irradiation (at 254 nm) of the carbonate (87) reported that benzaldehyde, phenyl carbene, and carbon dioxide were produced. A reinvestigation of the irradiation of this compound (at 254 nm in acetonitrile) has provided evidence that the cis- and trans-stilbene oxides (88) and (89) are formed as well as deoxybenzoin and smaller amounts of diphenylacetaldehyde and bibenzyl. When methanol is used as the solvent the same products are produced accompanied by benzylmethyl ether, 1,2-diphenylethanol, and 2,2-diphenylethanol. These authors suggest that the oxiranes (88) and (89) are formed by way of... 

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. ... 

The 2-nitrobenzylidene acetal has been used to protect carbohydrates. It can be cleaved by photolysis (45 min, MeOH CF3CO3H, CH2CI2, 0°C, 95% yield) to form primarily axial 2-nitrobenzoates fromdiols containing at least one axial alcohol. As with other benzylidene acetals the ring can be opened to give a benzyl ether and an alcohol. The resulting benzyl ethers can be removed photochemically. 

In polar solvents the excited state of sufficiently electron deficient arenes will accept an electron from donors. The fates of the radical ion pairs produced include formation of products of addition to the arene ring. A new example of this mode of reactivity is the photochemical reaction of 1,4-dicyanonaphthalene with benzyl methyl ether in acetonitrile. This yields stereoisomers of the addition product (120). The reaction most likely involves electron transfer from the ether to the naphthalene excited state and subsequent ionisation of a proton from the benzyl ether radical cation. This produces a benzyl ether radical which adds to the naphthalene derivative. An analogous sequence is proposed to explain the photochemical formation of (121)-(124) from ultra-violet light irradiated solutions of naphthalene-1,2-dicarboxylic acid anhydride in methanolic benzene or acetonitrile containing isobutene, 2-butene or 2-methyl-2-butene. Here it is suggested that the alkene radical cation, formed by electron transfer to the excited state of the naphthalene, is attacked by methanol deprotonation... 

Photoexcitation increases nitroxyl reactivity appreciably. The photochemical reaction of the radical (6) with toluene yields quantitatively the corresponding hydroxylamine and its benzyl ether (41). This reaction simulates to a certain extent the process of polymer stabilization by nitroxyl radicals. 

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