Benzyl ethers photochemical reactions

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. 

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 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. ... 

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. 

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

Heptakis 6-0-t-butyldimethylsilyl-P-cyclodextrin, on reaction with 4-chloro-methyl-A -methyl-2-nitroaniline, affords the mono-3-substituted benzyl ether. P-Cyclodextrin carrying a 2-(naphthylmethyl) group at 0-6 exists with the aromatic rings within the cavity to an extent which is very temperature dependent. Temperature can therefore be used to control the degree of complexing of the aromatic system with a fluorescent naphthalene compound. Heptakis-[2,3-di-0-acetyl-6-deoxy-6-iodo]-P-cyclodextrin treated with 6-methoxycarbonyl-2-naphthol allowed access to the compound having naphthoic acid substituents at all of the primary positions, and this forms a very stable 1 1 complex with a merocyanine laser dye which is a mimic of the antenna function in photosynthesis and shows promise as a photochemical microreactor. Mono-[6-0-(8-qui-nolyl)]-P-cyclodextrin has been reported, and the stabilities of inclusion complexes with amino acid guests have been described. ... 

Photochemical and photophysical properties of a poly(propylene amine) dendri-mer (2) functionalized with -stilbene units have been studied [102]. Z-photoisome-rization and photocyclization of the Z-isomer of the stilbene units were investigated in air-equilibrated acetonitrile solutions. The quantum yields of the E Z photoisomerization reaction and the fluorescence quantum yield of the E were found to be equal to 0.30 and 0.014, respectively. Stilbene dendrimers prepared by coupling 4,4 -dihydroxystilbene with first-, second-, third-, or fourth-generation benzyl ether-type dendrons underwent photoisomerization with the same efficiency as that of 4,4 -dimethoxystilbene [103], The lifetime of the core structure was found to be shorter then 1 ns. According to [104], polyphenylene-based stilbene dendrimers, Gl, G2, and G3, underwent mutual cis-trans isomerization upon direct irradiation with 310 nm light at room temperature. In a solvent glass at 77 K, one-way cis-trans isomerization was observed for G2. 

This wide development is observed in the study of the participation of ARs in various photochemical reactions, and the phototransfer of electrons and electronic energy. For some radicals the basic process is dissociation with the detachment of nitric oxide, whereas other types of ARs mainly abstract hydrogen atoms from solvents. The quantum yield of such process is very high (-0.5) [42]. Di- -alkylaminoxyls are poorly stable under exposure to UV light. The photolysis of radical IV (Rj = OH) by light with A, = 350 nm in toluene completely converts them into equal quantities of hydroxylamine and benzyl ether of hydroxylamine [43]. Thus, the capability of some excited ARs to abstract a hydrogen atom with the subsequent recombination of formed radicals and ARs provides a method of the functionalisation of macromolecules. 

Scheme 10.36 Photochemical CDC reactions of benzylic ethers and amines using CdS.

The nanospace of zeolite may impose a restriction on the rotational and translational motions of substrate molecules and reaction intermediates. This would promote or discourage specific reactions. The photochemical reaction of phenyl phenylacetates 1-4 (Scheme 10.25) within zeolites can be regarded as a good example [110, 111]. Photolysis of 1-4 in a homogeneous solution results in the formation of ortho-hydroxyphenones (40-60%), para-hydroxyphenones (20-25%), phenols (5-15%), diphylethanes (5-15%), and phenyl benzyl ethers (3-8%). However, the photolysis of all four esters in NaY zeolite can produce only ortho-hydroxyphenones. Molecular models suggest that the esters 16-19 can enter into... 

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... 

In several instances, Mannich-type cyclizations can be carried out expeditiously under photochemical conditions. The photochemistry of iminium ions is dominated by pathways in which the excited state im-inium ion serves as a one-electron acceptor. The photophysical and photochemical ramifications of such single-electron transfer (SET) processes as applied to excited state iminium ions have been expertly reviewed. In short, one-electron transfer to excited state iminium ions occurs rapidly from one of several electron donors electron rich alkenes, aromatic hydrocarbons, alcohols and ethers. Alternatively, an excited state donor, usually aromatic, can transfer an electron to a ground state iminium ion to afford the same reactive intermediates. Scheme 46 adumbrates the two pathways that have found most application in intramolecular cyclizations. Simple alkenes and aromatic hydrocarbons will typically suffer addition processes (pathway A). However, alkenic and aromatic systems with allylic or benzylic groups more electrofugal than hydrogen e.g. silicon, tin) commonly undergo elimination reactions (pathway B) to generate the reactive radical pair. 

The initial laboratory-scale syntheses of kresoxim-methyl started from o-bro-motoluene, which was first brominated in the side-chain with NBS. Substitution with o-cresol leads to the benzyl phenyl ether. The carbon skeleton of the final product resulted from a Grignard reaction with methyl 1-imidazolyloxalate. Reaction with methoxyamine gave a mixture of ( )- and (Z)-isomers, which can be isomerised, either with the aid of acids or photochemically. Fortunately, the biologically active isomer is also the thermodynamically more stable one. [57, 58]... 

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