Solvolytic conditions

Because a neutral molecule is eliminate4 rather than an anion, there is no electrostatic attraction (ion pairing) between the products of the dissociation step. As a result, the carbocations generated by diazonium-ion decomposition frequently exhibit somewhat different behavior from those generated from halides or sulfonates under solvolytic conditions. ... 

The extent to which rearrangement occurs depends on the structure of the cation and foe nature of the reaction medium. Capture of carbocations by nucleophiles is a process with a very low activation energy, so that only very fast rearrangements can occur in the presence of nucleophiles. Neopentyl systems, for example, often react to give r-pentyl products. This is very likely to occur under solvolytic conditions but can be avoided by adjusting reaction conditions to favor direct substitution, for example, by use of an aptotic dipolar solvent to enhance the reactivity of the nucleophile. In contrast, in nonnucleophilic media, in which fhe carbocations have a longer lifetime, several successive rearrangement steps may occur. This accounts for the fact that the most stable possible ion is usually the one observed in superacid systems. 

Cyclization of bromostilbenes 6 to dibenz[/ ,/]oxepins 7 can be achieved by irradiation in excellent yield.102 103 Under solvolytic conditions (60% NaOH, EtOH), the yield is greatly reduced. When a silver salt (AgOAc) is used, however, complete conversion to the tricyclic system is accomplished.102 Tribenz[/>,catalyzed cyclization of biphenyl-2-yl 2 -chlorosulfonylphenyl ether in 44% yield.260... 

It is evident from the foregoing sections that simple alkylvinyl halides do not react via an Sn 1 mechanism, if at all, even under extreme solvolytic conditions (146,149). More reactive leaving groups, such as arylsulfonates, were clearly needed to investigate the possible solvolytic behavior of simple alkylvinyl systems, but the preparation of vinyl sulfonates until recently was unknown. Peterson and Indelicato (154) were the first to report the preparation of vinyl arylsulfonates via reaction of the appropriate disulfonate with potassium t-butoxide in refluxing f-butanol. They prepared and investigated the solvolysis of 1-cyclohexenyl tosylate 169 and c/s-2-buten-2-yl tosylate 170 and the corresponding p-bromobenzenesulfonates (brosylates). Reaction... 

It is well known that allylic substrates are more reactive under solvolytic conditions than their saturated counterparts because of the delocalization of the positive charge in the developing carbonium ion over the tt system and the overlap of the empty p orbital with the double bond in the intermediate ion. 

Consequently, these must be the properties of the norbornyl system, rather than a consequence of cr-participation under solvolytic conditions. 

Reaction under solvolytic conditions such as in ethanol or aqueous tetrahydrofuran caused exclusive C-14—N bond cleavage and introduction of an ethoxyl or hydroxyl group at C-14, giving 30 in excellent yields (34). Addition of a base such as magnesium oxide to the reaction mixture was found to be useful to avoid the recovery of the starting material as the hydrobromide (35). The reaction was used for a synthesis of protopine alkaloids (Section V,E,5). 

These metabolites have been shown to produce arylnitrenium ions 8 under solvolytic conditions and McClelland and Novak have demonstrated, respectively... 

It should be pointed out that, compared to a-bromoboronic esters, p-bromoboronic esters are much less stable. For instance, dibutyl (2-bromoethyl)boronate readily undergoes P-elimination, even under solvolytic conditions [87]. Therefore, the reaction of 18 with NBS also reaffirms the regioselectivity of the hydrozirconation step. The reaction is highly general and works equally well for the preparation of a-chloro- and a-iodoboronic... 

Gas-phase intracomplex substitution in (R)-(- -)-l-arylethanol/CHs OH2 adducts. It is well established that bimolecular Sn2 reactions generally involve predominant inversion of configuration of the reaction center. Unimolecular SnI displacements instead proceed through the intermediacy of free carbocations and, therefore, usually lead to racemates. However, many alleged SnI solvolyses do not give fully racemized products. The enantiomer in excess often, but not always, corresponds to inversion. Furthermore, the stereochemical distribution of products may be highly sensitive to the solvolytic conditions.These observations have led to the concept of competing ° or mixed SNl-SN2 mechanisms. More recently, the existence itself of SnI reactions has been put into question. ... 

On balance, the solvolysis results just described indicate that there is little reason to require the norbomyl ion reached under solvolytic conditions to be non-classical. Before we turn to other aspects of the problem, it is interesting briefly to consider the results of deaimina-tion studies in the norbomyl system. 

Before we turn to other investigations of the stabilized ion it is interesting briefly to compare the rates of hydride shifts found in the strong acid systems with rates obtained under solvolytic conditions. Collins and Lietzke (1967) and Berson et al. (1967) have deduced the rates of 3,2- and 6,2-hydrogen migration relative to the rate of solvent capture for norbornyl ions in acetic acid, formic acid and other common solvents from elaborate C scrambling studies due to Roberts and Lee (1951) and Roberts et al., (1954) and tritium labelling studies of Lee and Lam (1966). From the ratio of these... 

Jencks and Richard, and others, had pioneered the use of the azide elock to quantitatively assess the lifetime of carbenium ions generated under solvolytic conditions. The method relies on the use of product yield data collected at varying [N3 ] to determine the N /solvent selectivity, expressed as the ratio of the second-order rate constant for trapping of the ion by N3 and the pseudo-lirst-order rate constant for trapping of the ion by solvent k Jk. The assumption is made that k z is diffusion limited at ca. 5 x 10 M"" s This assumption allows k to be estimated, and l/kg provides the lifetime of the ion in the solvent in the absence of added nucleophiles. McClelland and Steenken showed by direct measurement of k. , for a series of diarylmethyl and triarylmethyl carbocations that k is approximately constant at... 

Ionic ring-opening reactions of cyclopropanes under solvolytic conditions followed by ring-closure of the primarily produced cations have been employed for the synthesis of lactones, tetrahydrofuranes, and pyranes, respectively, as shown below [176]. 

The cyclopentadienyl cation 1 has been studied extensively both theoretically and experimentally and was recently generated under solvolytic conditions (Allen, A. D. ... 

As a result of investigations directed towards obtaining new, selective, hydroxyl-protecting groups having differing stabilities towards solvolytic conditions and yet favorable stability towards synthetic re-... 

Although the quantitative theory of reactions in moderately concentrated solutions of strong acids is unsatisfactory, we do have a good qualitative idea of the processes involved in the acid-catalyzed hydrolysis and formation of esters. Under conditions where the degree of protonation of the substrate is small it is not possible to separate with confidence the factors which affect the solvolytic process and those which affect the preliminary protonation equilibrium. But there have been a number of recent studies of t ie behaviour of carboxylic acids and esters in very strongly acidic media, in. which they are essentially completely protonated. Under these conditions it is possible to observe the behaviour of the protonated species directly. It is appropriate to summarize the results of this research before discussing the reactions under more normal solvolytic conditions. 

Although under solvolytic conditions carbocations are formed only as short-lived (transient) species, it is of interest to discuss here some recent work involving substituted C4H7+ cations which are relevant to the general discussions in the review. Wiberg and coworkers have carried out extensive studies on solvolysis reactions involving 3-substituted cyclobutyl tosylates 12 (equation 18), and rationalized the observed products by ab initio molecular orbital calculations at the MP2/6-31G level46,47. 

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... 

The reaction of sodium borohydride with benzhydryl halides under solvolytic conditions has been reinvestigated.132 Contrary to the literature, NaBHU was not found to be a convenient trap for benzhydryl carbocations. 

Creary, X. Geiger, C. C. Properties of a-keto cations. Facile generation under solvolytic conditions./. Am. Chem. Soc. 1982, 104, 4151-4162. 

Nitrogen heterocycles are attacked by cyanogen halides at the nitrogen. In the reaction of cyanogen bromide with the saturated, fused oxazole (182), a fused medium-ring heterocycle (183) is formed when the reaction is carried out under solvolytic conditions... 

The activation in the orthoquinonoid structure (669) from the fusion of 1,2,5-thiadiazole makes the 7-chlorine substituent diplaceable under solvolytic conditions in aqueous acetic acid (70RTC5). The high activation is also manifested in the ease of halogen hydrolysis in the fused pyrazine (670) (75JHC451). 

Triple-bond participation has been mainly studied (by Hanack and his school) in reactions of homopropargyl derivatives 31 under solvolytic conditions. In all cases of Table 3, except 1 and 2, compounds 31 yield, besides (and very often instead of) the expected solvolysis products, cyclobutanone derivatives (32) and alkyl cyclopropyl ketones (33). 

Reactions of vinyl derivatives under solvolytic conditions may occur via mechanisms other than a SNl-(El)-type process and yet lead to the same type of products. 

Table 7 shows the relevant examples of substrates investigated, listed according to structural criteria, the solvolytic conditions used and, when available, the first-order rate coefficients and activation parameters. 

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