Ion-assisted solvolysis

The silver ion assisted carbon-halogen bond cleavage and the unraveling of the cyclopropane ring by the cyclopropyl-allyl rearrangement was first noted in the formation of 2-bromocyclohexen-l-ol from dibromobicyclo[3.l.0]hexane under solvolytic conditions (equation 86).220 The silver ion assisted solvolysis of the dihalocyclopropane adduct (43), derived from a Birch reduction product, smoothly rearranges to the tropone (equation 87).221 A number of other synthetic applications222-226 have beien reported... 

In certain cases, the silver ion assisted solvolysis fails to initiate electrocyclic reaction to give ring-expanded compounds, leading instead to a cleavage product (equation 90).227 The dichlorocarbene adduct (48) on FeCb-promoted cleavage in ether affords exclusively a-trichloromethylcyclohexanone, whereas in DMF a small amount of chlorocycloheptenone also was obtained (equation 91).228... 

In a silver ion assisted solvolysis of the dibromo[3.3.1]propellane 55 in acetone, a mixture is obtained with a surprising product (56) albeit only in 1.2 % yield, accompanying... 

An even more remarkable rearrangement occurs when 16-chloro-l,2-dehydrotabersonine (194) is briefly heated, or subjected to silver-ion assisted solvolysis.The product is the tetrahydro-/3-carboline derivative (195), although the thermal rearrangement also leads to some (196). In Scheme 24 two mechanistic... 

Apart from its use as a synthetic method,the silver-ion-assisted solvolysis of gfcm-dihalogenocyclopropanes has received further detailed scrutiny. The solvolyses of 9,9-dibromobicyclo[6,l,0]-nonane and -non-4-ene both show both first- and second-order dependence on silver(i) ion. That part of the reaction first-order in... 

The mechanistic details of the silver-ion-assisted solvolysis of em-dihalogeno-propellanes has attracted further attention and the route suggested by Warner and Lu in 1975 has been substantiated for compound (179) (Scheme 21). It appears reasonable to assume the operation of a similar pathway in related substrates that are no more strained. However, in more severely strained cases, the initial formation of a transoid bridgehead olefin is not favoured and, for compound (180) at least,... 

The hitherto unknown tropylium ion isomer (317) has been generated from both (315) and (316). ° In carbon tetrachloride at 80°C, (315) affords (31 probably by disrotatory ring cleavage and loss of the endo-bromine atom. Although (317) is stable, silver-ion-assisted solvolysis of (316) causes partial rearrangement of (317) into the norbornene manifold (Scheme 40). 

The Ritter reaction with unsaturated carbenium ions under either silver-assisted solvolysis or photolytic conditions leads to excellent yields of isoquiaolines (173). The ease of preparation of the requited vinyl bromides makes an attractive route to highly substituted isoquiaolines. 

Use of other methods has contributed further to the emerging picture of solvolysis of most secondary systems as being solvent-assisted. For example, the solvolysis rate acceleration on substituting a-hydrogen by CH3 in 2-adamantyl bromide is 107 5, much larger than that found for other secondary—tertiary pairs such as isopropyl-/-butyl. In molecules less hindered than 2-adamantyl, the secondary substrate is accelerated by nucleophilic attack of solvent.100 Rate accelerations and product distributions found on adding azide ion to solvolysis mixtures (Problem 4) also provide confirmatory evidence for these conclu-... 

A set of rapidly equilibrating carbenium ions might account for the rearrangements and the label scrambling but this cannot be the correct explanation, for cyclopropylcarbinyl, cyclobutyl, and allylcarbinyl systems all solvolyze much more rapidly than would be expected from model compounds. Thus, for example, the rate of solvolysis of cyclopropylcarbinyl tosylate is 10 times that of the solvent-assisted solvolysis of isobutyl tosylate.77 Cyclobutyl tosylate solvolyzes 11 times... 

From the large amount of 7r-electron participation it is evident that considerable charge delocalization is developed in the transition state of the assisted solvolysis. This may be represented as 89. The multiplicity of products ranging from linear to three- and four-membered cyclic derivatives may be accommodated by the idea of a non-identical bicyclobutonium cationic intermediate 90 (Jacobs and Macomber, 1969 Santelli and Bertrand, 1969c). Interconversion of various bicyclobutonium ions and possibly conversion of these to various other cations,... 

The reactivity of halides is increased by coordination with Lewis acids. For example, silver ion accelerates solvolysis of methyl and ethyl bromide in 80 20 ethanol water by more than 10 In Section 4.4.1, we will see that the powerful Lewis acids SbFj and SbCl5 also assist in the ionization of halides. 

Howe and Winstein have shown if the skeleton of the endo-norbomyl brosylate molecule is so much deformed that theo-bonds C —and C —OBs become nearly transcollinear, then the a-participation of the C —C bond in the solvolysis of endo-ester becomes possible. Thus, the acetolysis of endo-brosylate 31 yielded 22.5 % of endo-acetate 33 this is explained by the formation of an intermediate nondassical ion 32). Solvolysis without anchimeric assistance would have yielded exo-acetate as the main product since the exo side in this ion is far less sterically hindered than the endo side it is, however, only a side product of 6.5%. 

The hydrolysis of 2,2 -bischloroethyl sulfide (mustard gas) was one of the first neighboring-group-accelerated reactions to get widespread attention. Initially the hydrolysis of this compound is purely first order in the sulfide, and the rate is unaffected by added alkali or other nucleophiles the increasing amount of chloride ion, however, slows the rate with time. Since formation of the primary carbocation is not expected, these data suggest an anchimerically assisted solvolysis [Eq. (1)], leading to the formation of the intermediate sulfonium ion (1), which may react with water, chloride ion, or any other nucleophile present. A second anchimerically assisted process that leads to displacement of the second chloro group can occur. 

The details of trans-medium-ring cycloalkene synthesis by silver-ion-assisted opening of igiem-dihalogenobicyclo[n,l,0]alkanes have been presented. Solvolysis of chlorofluorocyclopropanes (352) is also assisted by silver ion, and it provides a useful route to fluoroallyl alcohols (353). ... 

Raycroft MAR, Maxwell Cl, Oldham RAA, Saiiburi Andrea A, Neverov AA, Brown RS. Trifunctional metal ion-catalyzed solvolysis Cu(II)-promoted methanolysis of A/,A/-bis(2-picolyl) benzamides involves unusual Lewis acid activation of substrate, delivery of coordinated nucleophile and powerful assistance of the leaving group departure. Inorg Chem. 2012 51 10325-10333. 

Belzde M-N, Neverov AA, Brown RS. Cu(II) ion catalyzed solvolysis of N,N-bis(2-picolyl)ureas in alcohol solvents evidence for a cleavage involving nucleophilic addition and strong assistance of bis(2-picolyl)amine leaving group departure. Inorg Chem. 2014 53 7916-7925. 

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