Homobenzylic Ions

This section deals with the participation of the electrons of the adjacent n-bond (the olefin double bond or the aromatic ring) at the stage of the cation centre development, as well as with the formation of intermediate nonclassical homoallylic and homobenzylic ions. 

Stereocbemical and Kinetic Data on n-Participation and Formation of Intermediate Homoallylic and Homobenzylic Ions... 

Subsequently the same authors established that the solvolysis of optically active 6,7-dimethoxy-l,2-dimethyl-2-exo-norbornyl chloride or -p-nitrobenzoate is accompanied by anchimeric acceleration as distinct from the unsul tituted analogues 275 and 276 the resulting products are optically active Thus, anchimeric assistance leads to the formation of an unsymmetrical cation which may be the homobenzylic ion 278 while the partial loss of activity is due to the interconversion of enantiomeric ions. The authors have also shown that the accelerating effect of the homopara-methoxyl group is practically the same for the solvolysis in methanol, 50% acetone... 

The latter process is well modelled by the halogenation of the tetrafluoroben-zonorbomadiene 293. The stereospeciHc endo-side attack of Br and the absence of Wagner-Meerwein rearrangement in reactions with 7-isopropylidentetrafluoro-benzonorbornadiene are well accoimted for by the structure of the homoallylic ion A which is not converted into the homobenzylic ion. The fact that the bromine atoms go in to the most sterically unfavourable cis-vincinal position is difficult to explain if the intermediate cation has the classical structure B. 

The intermediate formation of the homobenzylic ion 303 gives a good account of all the facts of stereospecificity noted above. If the intermediate compound were the classical ion 304 then, while the exo-side attack by a halogen cation occurred, both homoallylic conjugation with formation of tricyclene products and rearrangements would be possible due to migration of the C —C and C —C bonds as well as addition of a nucleophile from a sterically more accessible equatorial side in ion 305. 

The configuration of the intermediate halogenonium ion determines unequivocally the direction of its isomerization into nonclassical carbocations — homo-allylic or homobenzylic ions. These are attacked by the nucleophile strictly stereo-specifically contrary to the steric factors but in agreement with the steroelectronic requirements of nonclassical ions. 

The assumed intermediate formation of homobenzylic ions reflects the fact that with rising nucleophility of the anion it attacks not only C and C , but also the homo-para position of the fluorinated ring... 

The chlorination of the same substrate in the weakly nucleophilic trifluoroacetic acid makes it possible to study the rearrangements of the homobenzylic ion ... 

Ion 555 was one of the first representatives of stable nonclassical ions of the homobenzylic type. Its structure was corroborated by NMR H, F and spectra they allow to reject the alternative assumptions of super-rapid equilibria of classical ions or unsymmetrical homobenzylic ions ... 

In 1975 Cristol, after additional research, admitted his earlier monosta scheme of solvolysis to be incorrect and suggested a two-stage scheme including the an intermediate homobenzylic ion this scheme is quite similar to the one proposed... 

Homoallylic and Homobenzylic Ions under Long-Life Conditions... 

The first stable homobenzylic ions were generated in works ... 

Arnold s demonstration" that oxocarbenium ion intermediates can be formed through homobenzylic ether radical cation fragmentation reactions shows that mild oxidizing conditions can be used to prepare important reactive intermediates. Scheme 3.2 illustrates a critical observation in the development of an explanatory model that allows for the application of radical cation fragmentation reactions in complex molecule synthesis. In Arnold s seminal work, cleavage of the benzylic carbon-carbon bond in substrate 1 is promoted by 1,4-dicyanobenzene (DCB) with photoirradiation by a medium-pressure mercury vapor lamp. With methanol as the solvent, the resulting products were diphenylmethane (2) and formaldehyde dimethyl acetal (3). 

A diastereoselective ewrfo-cyclization into an oxidatively generated oxocarbenium ion was a key step in a formal synthesis of leucascandrolide A. Exposing 56 to CAN provided cw-tetrahydropyran 57 in high yield and with excellent stereocontrol (Scheme 3.20). This transformation provides further evidence that oxidative electrophile formation is tolerant of several functional groups and can be applied to complex molecule synthesis. The synthetic sequence also utilized a Lewis acid mediated ionization reaction to form an oxocarbenium ion in the presence of the homobenzylic ether (58, 59), illustrating that two carbocation precursors that ionize through chemically orthogonal conditions can be incorporated into the same structure. 

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. 

The C —Br bond is ionized without the participation of the C —C bond to form the classical ion 318. Thus, if the arising cation centre at in the bicyclo-[3,2,l]octane system can be stabilized by the classical p,7t-interaction, the non-classical participation does not manifest itself (cf. besides, the homobenzylic participation would result in the antiaromatic (4jt-electrons) destabilized cation 319 (cf. But in the ion 315 the arising secondary cation centre can be effectively stabilized only by the nonclassicai a- or 7t-participation formating ion 316 of the bishomoaromatic type. 

We beUcve the benzylic ion 363 will hardly convert into ion 362 since the interaction of the vacant p-orbital of in the benzylic ion 363 with the homo-located benzene ring would finally lead to the formation of an antiaromatic homobenzylic-benzylic ion 364. This can be supported by Winstein showing that while the 6,7-olefine double bond in a bicyclooctadienylic anion 365 stabilizes the latter... 

This project arose from a desire to find a tmique application for some methodological work that my group had developed on oxidative carbocation formation. This process (Scheme 1) showed that oxocarbenium and acyliminium ions could be formed from oxidative cleavage reactions of homobenzylic ethers and amides, respectively. Cychzation reactions ensued when a nucleophilic group was incorporated into the cationic fragment. 

Alkyl iodides afford mixtures of radical- and ion-derived photoproducts in solution, with the latter usually predominating. Indeed, this is a powerful method for generating carbocations, including many that cannot be readily prepared by other methods. Alkyl bromides display similar photobehavior, but with a lower proportion of ionic products. Analogous behavior has also been observed for phenyl thioethers and selenoethers, as well as some organosilicon iodides. In a process related to the formation of ionic intermediates, irradiation of dihalomethanes in the presence of alkenes results in cyclopropanation, a synthetically useful procedure that complements traditional methods. This chapter, which is concerned with alkyl halides, is a major expansion of an earlier review. The solution-phase photobehavior of aryl, benzylic, and homobenzylic hahdes has been reviewed, along with that of alkyl systems." The photobehavior of alkyl halides in the gas phase has also been reviewed. ... 

---