Aryl participation

Table 5.22. Extent of Solvolysis with Aryl Participation as a Function of Substituent and Solvent for l-Aryl-2-propyl Tosylates...

Perhaps it is not unexpected and surprising that there is no /3-aryl participation in the solvolysis of triarylvinyl substrates, as the intermediate vinyl cation is especially stable because of charge delocalization into the a-aryl ring and has no need for extra stabilization. 

Measurement of reaction rate and activation parameters showed that in solvolytic reactions the [2.2]paracyclophanyl system is a much more active neighboring group than is the phenyl nucleus in its open-chain counterparts 101 a and 101b Aryl participation (charge delocalization) is greater in paracyclophanyl bromide (100a) than in 101 a. 

B. The unsubstituted aromatic nucleus of the [2.2]paracyclophane system could also participate directly in ionization. This would give an intermediate product like 108a. However, the absence of 109 and of a- and -deuterated species in the final product makes this type of aryl participation unlikely. 

The photochemical decomposition of acyl azides, on the other hand, proceeds by an intermediate nitrene whicn, in many cases, leads to typical nitrene reaction products. - 1- 111 -14 This is not unreasonable since a photochemical reaction provides enough energy to break the N-N, bond without alkyl or aryl participation. Homer, for example, has proved the existence of benzoylnitrene as the primary cleavage product of the photolysis of benzoyl azide in the presence of trapping reagents (Scheme II).1 7... 

They analyzed the product data by assuming that all the threo-acetate formed arises from a phenonium ion and all the erythro-acetate from backside assistance of the solvent to ionization. Then the percentage of aryl participation is synonymous with the percentage of threo-acetate in the product. The product analyses for acetolysis at 75°C are in the sixth column of Table 6.1. 

TT-Aryl participation in bromination of a norbornene derivative has been reported (194 — 280). Acid opening of the corresponding exo-epoxide proceeds in a similar way402. 

Anderson, J.C. and Reese, C.B. (1962) Photo-induced rearrangement involving aryl participation. Tetrahedron Letters, 3, 1-4. 

Aryl participation is more common than simple alkene participation... 

A aim liar aryl participation In saturated compounds to give a phenanrum ion intermediate appears in Chapter 37, p.974. 

Wilt and coworkers have made further studies of aryl participation in solvolysis of the rrans-3-bicyclo[3.1.0]hexyl systems 121 and 122. Neither underwent significant long-range aryl migration. 

The thermal migratory aptitudes varied from Ar/Ph = 6 7 for X = / -NMc2 to 0-20 for X = p-N02- It has been pointed out, however, that these rate differences are small compared with the corresponding migratory aptitudes observed in assisted migrations involving carbonium ion intermediates, and that if tlie effect of substituents on A/fJ only is considered, the inductive order is indeed followed . A study of the effects of mcte-substituents would be of value since these would not be expected to facilitate or retard aryl participation by direct conjugation . 

A similar aryl participation in saturated compounds to give a phenonium ion intermediate appears in Chapters , p. 000. 

Ando and coworkers conducted isotope effect studies (entry 11) on the direct displacement reaction of benzyl arenesulfonates with dimethylaniline (DMA)38. They found that an electron-withdrawing substituent in the substrate (Y = 3-Br) caused the TS to shift to a later position along the reaction coordinate, which is consistent with that predicted by the Thornton rule (or anti-Hammond effect). The anilinolysis of phenylethyl arenesulfonates (entries 12 and 13) proceeds also by an SN2 mechanism. The reaction was found to proceed by a dissociative SN2 mechanism with a relatively small degree of aryl participation. The fraction of the phenonium ion intermediate captured by the aniline nucleophile in the aryl-assisted pathway has been shown to increase with a stronger nucleophile, and a four-center TS in an intermolecular SNi mechanism is suggested for the aryl-assisted pathway39,40. Under the same reaction conditions, benzylamine nucleophiles react at a rate ca two times faster than that of anilines. 

In the reactions of anilines with l-phenyl-2-propyl arenesulfonates61 (YCgH4CH2 CHMe0S02C6H4Z), aryl participation (kA) was found with Y = 4-OMe (Scheme 4). The overall rate constant was 3.55 x 10 5 M s 1 in MeOH at 65.0 °C and the degree of aryl participation as expressed by l00[FkA/(kA + ks + n)L where kx is the rate of... 

Defunctionalization. Aryl participation in the ionization of a homobenzylic tosyloxy group is not unusual. Such a phenonium ion intermediate is readily converted to a lactone when a strategically located ester is present in the same molecule. Removal of A/-Boc groups by silica gel with assistance of microwaves is an expedient method. 

A bridged intermediate appears unlikely in the acetolysis of (338) since the solvolysis of the tertiary system (339) goes with neither aryl participation nor pheno-nium intermediates (cf. Section 7.3.3.). More direct evidence comes from the solvolysis of the chiral 2-methyl-2-phenyl-1-butyl system. Both the mesylate (340) and the amine (341) afford the product of phenyl migration, (342), with substantial racemization285. Similarly, the alkaline deamination of (343) yields largely racemic acetals (344)2S6 These stereochemical results clearly demonstrate the limits of phenonium ion intervention. 

Aryl participation is enhanced by less nucleophilic solvents (cf. formolysis of (351), Y=H, Table 13) and by increasing the donor capacity of the aryl group. Threo-(351), Y=OCH3, X=OTs, gave threo acetate to the virtual exclusion of any other products no erythro acetate was detectable by gas chromatography295. On the other hand, a nitro group accentuates the ks pathway. (351), Y=N02, X=OTs was found... 

The factors which control aryl participation in the decomposition of secondary diazonium ions are illustrated by the nitrous acid deamination of ethyl 2-amino-3-aiylpropionates (356)29 Both the retention of configuration in non-rearranged product (357) and the relative yield of rearranged product (358) increase in the order 02N < H < OCH3 for phenyl substitution, and in the order H20 < CH3C02H < CFjCOjH for the solvent. 

Aryl participation is greatly enhanced in exo-2-benzonorbornenyl tosylate (366) which differs from (364) by an additional C—C bond. The phenyl ring is now fused into a rigid system to provide close to maximal overlap between the aryl 7r and carbenium p orbitals. The aryl group cannot provide anchimeric assistance to the ionization of the endo epimer (565). The exo/endo rate ratio for the acetolysis of the parent brosylates, X=H, is 15000303, 304 (62000 if polarimetric rates are used instead of kt305 ). A 6-methoxy group in (366) increases the rate by a factor of 150, and the exo/endo rate ratio by a factor of 60303, 306. A 7-methoxy group has no... 

Aryl participation in the [2.2] paracyclophane system (378) requires distortion of this already highly strained molecule. Yet solvolysis proceeds with complete re-... 

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