Cleavage processing

The cleavage process occurs by p-quinonemethide formation after acetate hydrol-ysis. 

TLC analysis of the crude reaction product reveals no additional products. Similarly quantitative results are obtained using benzene, /-butanol and ether as the solvents in the irradiation of (39). The reaction is less sensitive to oxygen than most ketone photolyses resulting in products from a-cleavage processes. 

As a consequence, it appears to be valid to apply Marcus theory (Section 5.3) to Sn2 reactions. Note that we may expect structure-reactivity relationships in Sn2 reactions to be functions of both the bond formation and bond cleavage processes, just as in acyl transfers. 

The following examples illustrate unusual and unexpected cleavage processes because of participation by nearby functionality. 

K2CO3 or CS2CO3, DMF or CH3CN, PhSH, 88-96% yield. This process is not always selective for p-nosylate cleavage. Some amines, especially cyclic ones, tend to form 4-phenyl thioethers by nitro displacement as by-products of the cleavage process. This seems to be true only for the p-nosylate. ... 

In most cases, the behaviour of sulphones at the cathodic interface (obviously rendered basic when insufficiently buffered due to the accumulation of electrogenerated bases) may be strongly modified by the presence of vicinal CH groups in the a position to the S02 group. These acidic groups may transfer protons and are often responsible for low yields of the cleavage processes or for undesirable isomerization reactions. 

In contrast, sulphoxides appear to possess a more classical behaviour in electrochemistry, due to their intermediate oxidation state which allows, in most of the cases, their reduction to sulphides but also their oxidation to sulphones with no cleavage process. Moreover, the increase of the sulphur atom basicity may also produce catalytic hydrogen evolution in acidic solution. 

Here, the relative stability of the anion radical confers to the cleavage process a special character. Thus, at a mercury cathode and in organic solvents in the presence of tetraalkylammonium salts, the mechanism is expected16 to be an ECE one in protic media or in the presence of an efficient proton donor, but of EEC type in aprotic solvents. In such a case, simple electron-transfer reactions 9 and 10 have to be associated chemical reactions and other electron transfers (at the level of the first step). Those reactions are shown below in detail ... 

The same overall cleavage process may occur via one of two main pathways (see Scheme 3) according to proton availability in the electrolysis solution. 

With functionally substituted, alkyltin compounds, the functional substituent may become involved in the cleavage process, resulting in an intramolecular reaction, e.g.. 

There is clear evidence that the addition of a sulfonyl radical to an olefin is a reversible reaction. This has been demonstrated for the case of but-2-ene, where cis-trans isomerization of the olefin accompanies addition . Furthermore, 18a and 18b radicals generated by the reaction of BusSn" with erythro- and t/ireo-2-bromo-3-(phenylsulfonyl)butane, respectively, eliminate benzenesulfonyl radicals to some extent, even at — 67°C, to form 2-butenes in a non-stereospecific manner, providing also indication that the rotation about the central C—C bond is faster than the / -cleavage process . 

Although not formally a type I cleavage process, /1-cleavage of cyclopropyl ketones to form propenyl ketones and/or epimerized cyclopropyl groups bears such a close similarity to a-cleavage that it should be discussed in combination with the latter. The first report of a reaction thought to occur by 0-cleavage was made by Pitts in 1954 for the formation of methyl propenyl ketone from methyl cyclopropyl ketone,<72)... 

There is another mechanistic variation to circumvent the direct cleavage process 11- 12. The isomerization 11- 13 does not necessarily have to involve a skeletal reorganization. For example, it can be envisaged that a specific hydrogen transfer occurs onto a suitable acceptor function Y, 24- 25 (5). In this way a reactive radical site is created which induces the dissociation step (elimination of X ) by... 

It is the purpose of this review11 to show that a fairly substantial number of quite different unimolecular dissociations of gaseous cation radicals can be described conveniently by this concept. Among the reactions discussed are some unusual cleavage processes, the occurrence of which were difficult to explain without the involvement of hidden hydrogen migrations. 

However, there are several experimental observations which are against an interpretation of the reaction as being a one-step C—C cleavage process which would give rise to the formation of the relatively unstable a-oxocarbenium ion 28. 

The above mentioned conclusion is confirmed nicely by energetic measurements63 (Fig. 2). The experimentally derived transition state energy for OH loss from 168 is approximately 175 kcal mol-1. This is much too low for a direct a-cleavage process which would give rise to the formation of 170 and OH , the sum of the heats of formation of which is around 190 kcal mol-1. However, the transition... 

Rice has shown that the cracking of hydrocarbons at high temperatures gives free radicals.48 Only the methyl and ethyl radicals survive long enough to react with mercury to give dialkylmercury. Presumably the others decomposes by a beta cleavage process. 

For the substrates [22] reacting with /3-CD through 1 1 complexes the features are broadly similar (Table A5.10). However, for the C6, C7, C8, 2-ethylhexanoate (2EtC6), and 4-methylpentanoate (4MeC5) esters, a cleavage process involving two molecules of CD is evident at high [/3-CD] (Tee and Du, 1988, 1992). This process, which may be ascribed to attack of a second CD molecule on the 1 1 ester-CD complex (19), can be characterized by a rate constant, /cc2. 

---