O cleavage

Hydrolysis of dialkyl sulfites under acidic and alkaline conditions, which is followed by the use of OH2, proceeds by attack at sulfur to give S—O cleavage (72). The rate of hydrolysis is generally faster for cycHc and aryl sulfites than for dialkyl sulfites (73). Activation parameters of hydrolysis are known for some sulfites, and the increased rate for ethylene sulfite results from a reduced entropy of activation which results from a rigid ring stmcture (74). 

Oxirane on thermolysis or photolysis suffers C—O homolysis to give a plethora of products (Scheme 2). Substituted oxiranes behave similarly on thermolysis although some C—C cleavage is observed (Scheme 3). Cyclopentene and cyclohexene oxides undergo only C—O cleavage (Scheme 4). 

The small tendency of oxaziridines to undergo hydrolytic C—O cleavage is underlined by the observation that oxaziridine (52) in a mixture of acid and 0-labeled water yields up to 50% labeled hydroxylamine. NH transfer to water successfully competes with hydrolysis (B-67MI50800). 

Oxaziridines substituted in the 2-position with primary or secondary alkyl groups undergo decomposition at room temperature. In the course of some weeks, slow decomposition of undiluted compounds occurs, the pattern of which is analogous to that of acidic or alkaline N—O cleavage (Sections 5.08.3.1.3 and 4), Radical attack on a C—H bond in (109) effects N—O cleavage, probably synchronously (57JA5739). In the example presented here, methyl isobutyl ketone and ammonia were isolated after two hour s heating at 150 °C. 

Perfluorinated epoxides, which are generally susceptible to C-O cleavage during nucleophihc attack, are cleaved by lithium alummum hydnde Of the available examples, reduction of epoxides from two types of internal olefm to give alcohols IS shown [4S] (equations 37a and 37b)... 

Solution 2-Methyl-3-pentanol, an open-chain alcohol, has M+ = 102 and might be expected to fragment by o- cleavage and by dehydration. These processes would lead to fragment ions of m/z = 84, 73, and 59. Of the three expected fragments, dehydration is not observed (no m/z = 84 peak), but both a clea iges take place (m/z = 73, 59). 

These reactions are all considered at one place because they are formally similar, but though some of them involve R—O cleavage and are thus nucleophilic substitutions... 

A number of sulfonates in which R contains a branching, for example PH2C(CF3)S020R, can be used to ensure that there will be no S—O cleavage Netscher, T. Prinzbach, H. Synthesis, 1987, 683. 

Figure 4.11 Optimized structures of CHj,0 species, as indicated, over aqueous-solvated Pt(lll) as determined by DFT in Cao et al. [2005]. Horizontal and vertical arrows indicate C—and O—cleavage steps, respectively. Reaction energies are included for the aqueous phase [Cao et al., 2005] and the vapor phase (in parentheses) [Desai et al., 2002]. The thermodynamically preferred aqueous phase pathway is indicated by bold arrows (in blue).

Several supported metalhc catalysts were evalrrated for the selective hydrogenolysis of glycerol. Initially, the reactions were performed tmder acidic conditions in order to promote the formation of 1,3-PDO. Rutheniirm-based catalysts were found to be the most active catalysts but significant amount of tmdesired products resulted from C-C cleavages were detected. On the contrary, Rh/C catalysts were found selective to C-O cleavages. As far as the selectivity to 1,3-PDO was concerned, we previously reported that the addition of iron salts in the medium improved the l,3-PDO/l,2-PDO selectivity (11). A systematic study on the influence of additives was therefore carried out in the present investigation. Mineral and organic acids were evaluated for this purpose (Table 35.1). 

Cis 2-butene (48a) thus yields the meso 1,2-diol (47), i.e. the overall hydroxylation is stereoselectively SYN, as would be expected from Os—O cleavage in a necessarily cis cyclic ester (46). The disadvantage of this reaction as a preparative method is the expense and toxicity of 0s04. This may, however, be overcome by using it in catalytic quantities only, but in association with H202 which re-oxidises the osmic acid, (HO)2OsOz, formed to 0s04. 

Diepoxy-p-menthane, 6, rearranges when heated with alumina in toluene 73). What is the product of this reaction An organic chemist would predict that acid treatment of the diepoxide 6 (Figs. 3 and 32) would induce one or other of the oxirane rings to open. But which of the two will be the more reactive, and would overall reaction necessarily involve such an initial step Furthermore, for each oxirane there are two possible C—O cleavages. 

The reaction of oxaziridines 378 with isothiocyanates has been established as a route to l,2,4-oxadiazolidin-5-thiones 379 (Scheme 64). Two mechanistic possibilities exist via which the products can form, each of which relies upon C-O cleavage, shown as paths a and b in Scheme 64. Interestingly, one of these paths involves the generation of nitrone 380, a species which could be identified as a reaction product <1998H(48)1935>. The nitrone could be generated separately and reacted with isothiocyanates to form identical l,2,4-oxadiazolidin-5-thiones 379. 

Fig. 19 Bond length-reactivity plots for phosphate triesters and for monoester dianions. Note the increased slope for the monoesters, which undergo SNl(P)-type P-O cleavage in solution. Reprinted with permission from Jones and Kirby (1984). Copyright 1984 American Chemical Society.

Generally, the pentacoordinate silicon compounds described in this chapter are sensitive to water and very easily undergo hydrolytic Si-O cleavage reactions in solution. This has been used for the synthesis of the octa(silases-quioxane) 46, which was obtained in 90% yield by treatment of compound 35 with water in boiling acetonitrile (Scheme 8).34... 

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