Phenyl ring proximity

The proximity of the reaction centre to the second phenyl ring makes the aryl cation, formed by heterolytic dediazoniation, a serious competitor to the aryl radical. This is evident in Table 10-6 from various examples where the yield obtained in aqueous mineral acid (varying from 0.1 m to 50% H2S04) is higher than in the presence of an electron-transfer reagent. This competition was studied in three types of product analyses by Cohen s group (Lewin and Cohen, 1967 Cohen et al., 1977), by Huisgen and Zahler (1963 a, 1963 b), and by Bolton et al. (1986). 

These observations are consistent with earlier observations, indicating that those SPC isomers with a greater resistance to further degradation stem from (Ci2-)LAS in which the phenyl ring is attached to central positions of the alkyl chain, or those which are oxidised on the shorter side of the alkyl moiety [10]. In the course of microbial degradation, subsequent shortening of the alkyl chain via (3-oxidation becomes hindered in close proximity to the aromatic ring. 

Fig. 14. A mechanism to explain heme modification in the P. vitcde catalase and possibly E. coli HPII. For simplicity, the phenyl ring of T3rr415 is not shown, and only ring III of the heme and the heme iron are shown. Compound I is an oxyferryl species formed, along with water, in the reaction of one H2O2 with the heme. The iron is in a formal Fe oxidation state, but one oxidation equivalent is delocalized on the heme to create the 0x0-Fe" -heme cation, shown as the starting species, compound I. A water on the proximal side of the heme is added to the heme cation species of compound 1 shown in A to generate a radical ion in B. The electron flow toward the oxo-iron would generate the cation shown in (C), leading to the spirolactone product shown in D. In E, an alternate mechanism for the His-Tyr bond formation in HPII is presented that could occur independently of the heme modification reaction. Reprinted with permission of Cambridge University Press from Bravo et al. (93).

Diphenylmethylene acetals are used to protect 1.2-diols including catechols. They can be cleaved by a variety of methods including add hydrolysis, dissolving metal reduction and hydroge nolysis. The steric bulk of the phenyl rings has been used to control the stereochemistry of reactions at proximate centres.1 181... 

This reaction is considered to proceed via initial coordination of pyridine to one of ruthenium centers, after which the adjacent ruthenium cleaves the ortho C-H bond followed by successive insertion of CO and olefin. When 2-phenylpyridine is employed in a similar system, acylation in the phenyl ring takes place via prior coordination of the N atom followed by cleavage of proximal C-H bond in the phenyl group (Scheme 14.7) [19]. 

The presence of the phenyl ring is not essential for the carbometallation to proceed. For example, treatment of diallylamines, or related systems, with alkyllithium reagents leads to several trianionic intermediates [87] (Scheme 7-74). The interaction of the lithium atom of the amide with the alkyl group of the organolithium reagent together with the simultaneous proximity effect allows the coordination of the double bond to the lithium atom and consequently the addition reaction occurs. 

R = CHjPh), which showed a marked shielding effect of the 8a proton by the phenyl ring. In the unsubstituted compound (81, R = H) the triplet due to the 8a proton is found at S 4.93, whereas in both of the dl isomers of the benzyl compound (81, R = CH2Ph) the 8a proton resonates at S 3.07. The explanation of this remarkable shielding effect is the proximity of the aromatic ring of the pseudoaxial 3-benzyl substituent to the 8a... 

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