Lateral Nuclear Shifts

The photoreactivity of phenyl benzoates in which the para positions of both rings are substituted has been examined in liquid crystalline media and compared with the results obtained in isotropic solution.The photoreactivity of phenyl esters of cyclohexane carboxylic acids in which the para position of the phenyl ring and the 4-position of the cyclohexane ring are substituted were also studied under the same conditions.The products were not identified but were assumed to arise from photo-Fries rearrangement based upon the development of absorption in the ultra-violet spectrum assignable to ortho-hydroxyphenyl ketones. The relative quantum yields of the rearrangements were correlated with the viscosity and order of the liquid crystalline phase. 

The photo-Fries rearrangement of polymers containing phenyl formate units (e.g. the polymer derived from para-formyloxystyrene) has been examined. 

The effect of complexation by j9-cyclodextrin on the photo-Fries rearrangement of benzene sulphanilide has been investigated. 

In methanol or benzene solution the major photolysis product is aniline while para-aminodiphenylsulphone is obtained as a minor product. However, when the reaction is performed in aqueous solution in the presence of /3-cyclodextrin relatively little aniline is produced and instead the major product is the para-aminodi-phenylsulphone along with substantial quantities of the ortho-isomer. Dramatically different results were obtained when the solid complex of the sulphanilide with p-cyclodextrin was photolysed in the absence of solvent under these circumstances the ortho-amino-diphenylsulphone is the exclusive product. In previous years this chapter has reported several examples of the photo-Fries reaction being conducted in cyclodextrin cavities but this degree of selectivity is much higher than that seen before. Unfortunately, the authors do not provide details of how the solid complexes were prepared. 

The photochemistry of the para-substituted phenyl sulphamates (308)-(312) has been found to be substituent dependent. Photolysis of (308) and (309) in methanol gave photo-Fries products (i.e. the expected aniline sulphonic acids) as well as aniline, while the nitro derivative (311) was photochemically inert, and the halogen substituted sulphamates (310) all gave the photosolvolysis product (312) 

Photochemical rearrangement of 2-arylamino-l-(4-tert-butylphenoxy)-9,10-anthraquinones involves migration of the tert-butylphenoxy group either to the peri carbonyl oxygen atom to produce 2-arylamino-9-(4-tert-butylphe-noxy)-l,10-anthraquinones, or to the nitrogen atom to give 2-aryl(4-tert-butylphenyl)amino-l-hydroxy-9,10-anthraquinones.  

The photo-Fries reaction of 2-benzoyl-4-benzoyloxyphenol (167) affords 2, 3- and 2,5-dibenzoyl-1,4-dihydroxybenzenes (168) and (169) in respective yields of 48 and 19% on 254 nm irradiation/ and similarly both 2- and 

R = Me, CH = CHMe, CH = CH-CH = CHMe, Ph, 2-MeC6H4, 2-CIC6H4, 1-naphthyl orCH = CHPh 

The photochemistry of diphenylether in methanol solution has been reported to be affected by ultrasound and, while the rate of rearrangement to 2-hydroxybiphenyl remains unchanged by sonication, that of the 4-hydroxy isomer is decreased and phenol production is increased. 

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Lateral nuclear shifts have been found to contribute to the photochemistry of spiro[cycloalkyIphenalenes] (334) The cyclopropyl and cyclopentyl derivatives of (334), i.e. n=0 or 2, appear to react by cleavage to biradicals (335) which recombine to produce the isolated products (336) and (337). The cyclohexyl homologue (334), n=3, yields (338) which may be a di-m-methane rearrangement product. 

Silicon-silicon bond homolysis and migration/recombination account for the photochemically induced rearrangement of the bis-(disilanyl)naphthalene (339) to (340). However, analogous products from a photochemical lateral nuclear shift are not seen for the digermane (341) even though germanium-germanium bond homolysis does occur upon excitation. 

The photolysis of triarylsulphonium salts yields diarylsulphides and products of lateral nuclear shift reactions which are ortho, meta, and para aryl substituted diaryldisulphides such as (342). A by-product in these reactions is a proton because of this these reactions have been applied to photoinitiation of cationic polymerisations. A full paper describing a detailed study of the reaction mechanism has been published.In addition, the product distribution obtained by photolysis of triphenylsulphonium salts in films of the polymer of 4-(tert -butoxycarbonyloxy)styrene has been compared with that obtained in solution.The synthesis of some new triarylsulphonium salts and their application for photoinitiation of cationic polymerisation has also been reported.The formation of the products arising from lateral nuclear shifts in sulphonium salts occurs under direct photolysis but not under triplet sensitisation. 

Back in 1938 Rice and Teller [88] formulated the general principle which stated that those elementary reactions are the most favored which exhibit the fewest possible alterations in the positions of atomic nuclei and in electronic configuration. The part referring to the electron configuration was later developed into the Woodward-Hoffmann rules, while that concerning the nuclear shifts became known as the principle of least motion of nuclei or simply the principle of least-motion (PLM) [89,90] ... 

In addition to the chemical shift information, an NMR spectrum may also contain coupling information. The types of couplings frequently present in NMR experiments include scalar (J) couplings between high-abundance nuclei such as protons, dipolar couplings that are important for cross-relaxation processes and the determination of nuclear Over-hauser effect (NOE) (described later in this chapter), and quadrupolar coupling associated with quadrupolar nuclei (/> 1/2). 

Nuclear magnetic resonance spectra may be so simple as to have only a single absorption peak, but they also can be much more complex than the spectrum of Figure 9-23. However, it is important to recognize that no matter how complex an nmr spectrum appears to be, it involves just three parameters chemical shifts, spin-spin splittings, and kinetic (reaction-rate) processes. We shall have more to say about each of these later. First, let us try to establish the relationship of nmr spectroscopy to some of the other forms of spectroscopy we already have discussed in this chapter. 

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