C bond cleavage

To gain an insight into the likely hydrolytic behavior of sulfated simple sugars and polysaccharides, Brimacombe, Foster, Hancock, Overend, and Stacey carried out a rigorous set of experiments with the cyclic sulfates of cyclohexane cis-and trims-1,2-diol as model compounds. The results were interpreted on the reasonable assumption that, in all cases, the cyclic sulfates initially afford a diol monosulfate. Examples of both S-0 and C-0 bond cleavage were encountered. A qualitative reaction mechanism was proposed for use as a working hypothesis for the hydrolysis of sugar sulfates. 

The complete elimination of functional groups is often an undesirable side reaction in organic synthesis, but on the other hand it is a possibility for the recycling of environmentally harmful compounds, for example phenols and haloarenes such as polychlorinated dibenzodioxins (PCDDs or dioxins ). For example, aryl chlorides can be effectively dechlorinated with Pd(0) NPs in tetra-butylammonium salts with almost quantitative conversions also after 19 runs (entry H, Table 1.4) [96]. On the other hand, a C-0 bond cleavage reaction also seems suitable for the fragmentation of sugar-based biomass such as cellulose or cello-biose in that way, sugar monomers and bioalcohol can be derived from renewable resources (entry F, Table 1.4) [164]. 

The initial C-0 bond cleavage (by attack of a/the bromide ion at the benzylic carbon, followed by recyclization of the intermediate 301 by selective alkylation at sulfur) was suggested to be responsible for a stereospecific rearrangement of 2-substituted-1,3,2 -oxazaphospholidine-2-thiones 299 derived from (-) pseudoephedrine into... 

Therefore, we prefer the formal reduction of CO toward a formyl species. Subsequently, C-0 bond cleavage gives coordinated methylene and an oxide ion that is transformed to OH- and H20. 

Indeed, it was shown that aposcopolamine was not formed by direct dehydration of scopolamine, but via the conjugate scopolamine O-sulfate generated by a sulfotransferase [127]. This explains the species differences observed, and indicates a mechanism of heterolytic C-0 bond cleavage made possible by the electron-withdrawing capacity of the sulfate moiety. The reaction is also facilitated by the acidity of the departing proton carried by the vicinal, stereogenic C-atom. This acidity also accounts for the facile base-catalyzed racemization of scopolamine and hyoscyamine [128]. 

Kinetic studies of the hydrolysis of aryl Af-(methoxycarbonyl)sulfamates (350) are reported for the first time. The compounds are fairly strong acids with g>Ka = 0.5-2.4, and in acid both S-O and C-0 bond cleavages occur (Scheme 43). From an... 

The implications of the above observations may be important, especially if similar trends are observed in pyranose anomers. For example, with respect to the mechanism of acid-catalyzed hydrolysis of pyranosides, endocyclic C-0 bond cleavage (preceeded by 05 protonation) may be assisted in P-anomers in which the Cl-01 bond is equatorial, since the 04-Cl bond may already be extended in these anomers. By a similar argument, exocyclic C-0 scission (preceeded by 01 protonation) may be assisted in the hydrolysis of a-pyranosides in which the Cl-01 is axial and extended, thus resembling the transition state. Post and Karplus have recently suggested that enzyme-catalyzed glycoside hydrolysis of P-pyranosides may indeed take place by ring oxygen protonation, followed by endocyclic C-0 bond scission. 

Alternatives to this mechanism, which differ in some cases only relatively subtly, but in other cases quite dramatically, are not well supported, but include a possible mechanism involving an initial endocyclic C-0 bond cleavage (5), and one involving a full oxocarbonium ion intermediate. The case against such mechanisms is discussed in some detail elsewhere (2). 

But GO is also an archetype for more complex polyols. Catalytic hydrogenolysis of polyols leads to C-C and C-0 bond cleavage as well as activation via dehydrogenation of HCOH to C=0 sites (8, 9). With its three vicinal hydroxyl groups, and its two types of carbon atoms (all bearing hydroxyl functionalities) GO can serve as a simple model in which to study the competition between these processes. We aim to gain insight into the individual steps and apply that information to increase selectivity toward desired products. 

They also found that at pH 9, 130 was hydrolyzed 3.4 times more slowly than 131. It was shown, however, that the rate determining steps are not the same. In 130, it is the C-0 bond cleavage whereas in J 31, it is the hydration of the corresponding oxonium ion. Consequently, a direct comparison between the rates of the spontaneous hydrolysis of 130 and 131 cannot be... 

Thiolysis with H2S as the nucleophile is employed to open the oxazoline ring in 13.24 Attack on the sp2 carbon in the heterocycle leads to tetrahedral intermediate 63 which decomposes preferentially under C-0 bond cleavage. 

Position of Bond Cleavage PNP (EC 2.U.2.1) from human erythrocytes (homogeneous, purified by formycin B affinity chromatography) as well as from E, coli were allowed to equilibrate a mixture of R-l-[1 0lj]-P, pl Oij, hypoxanthine and inosine at pH 7-00 in 10 mm NMR tubes. The chemical shift differences of the 31p nuclei of the two R-l-P s (13 9 Hz for the human erythrocytic and 13.1 Hz for the E. coli enzyme) as well as of the two P3 resonances (13 9 Hz for erythrocytic and 13-7 Hz for E. coli source) clearly indicated C-0 bond cleavage by these enzymes as well. In addition, no evidence was found over the time course of the NMR measurements (l hr) for purine nucleoside phosphorylase catalyzed exchange of pl o + H2O (solvent) J randomized P. Therefore,... 

C-0 bond cleavage in R-l-P and the absence of a phosphoryl enzyme intermediate appear to be general characteristics of PNP s from a wide variety of sources human erythrocytes, calf spleen and E. coli. 

Castellan, A., Vanucci, C., Bous-Laurent, H., "Photochemical Degradation of Lignin Through a C-0 Bond Cleavage of Non-phenolic Benzyl Aryl Ether Units. A Study of the Photochemistry of a(2 ,4 -Trimethyl-Phenoxy)-3,4 Dimethoxy Toluene.", Hohforschmg, 1987, 41(4), 231. 

Epoxides undergo a repertoire of fairly well-characterized reactions, perhaps the most typical being the nucleophilic ring opening process. Since the epoxide in question is usually found in a compound with multiple functionality, many new methodologies seek to optimize chemo- and regioselectivity. Fortunately, the oxirane lone pair provides a convenient handle to effect activation of the heterocycle toward C-0 bond cleavage thus, efficient catalysts are of particular interest. 

The acetal radicals have to be trapped by a scavenger before the C-0 bond cleavage takes place, if the desired product should be the one with the acetal ring (or group) preserved. Terminal olefins have been shown to be efficient scavengers for these radicals. Thus, irradiation of a mixture of dioxolan and terminal olefins in the presence of acetone leads to the desired 1 1 adducts (17) ... 

Similarly, the acetal 146 can undergo a thermal Wolff rearrangement to afford the intermediate ketene. Intramolecular nucleophilic attack of the acetal oxygen onto the ketene forms the zwitterion 147. Subsequent C-0+ bond cleavage and cyclization then furnishes dimethyl l,3-dimethoxy-l//-isochromen-4-yl phosphonate in excellent yield (Scheme 47) <1998T6457>. 

Deoxygenative reduction can be carried out at room temperature with Et3B as an initiator [19,20]. However, this reaction is limited to thiocarbonates derived from sec- and tert-alcohols, since the C-0 bond cleavage of thiocarbonates derived from pr/m-alcohols does not occur easily at room temperature. 

Aside from oxidative dimerizations [175-186] vanadium catalysts are able to induce C-C and C-0 bond cleavages. Momose and coworkers developed a catalytic system to fragment a-hydroxy ketones 101 to diesters or ketoesters 102 (Fig. 31) [190]. Using 1 mol% of EtOVOCl2 as a catalyst and oxygen as the terminal oxidant, 55-87% of 102 was obtained. The reaction mechanism is not known, but the reaction is inhibited by 2,6-di-tert-butylcresol (BHT) pointing to the involvement of a radical process. 

The detethering of one such trans cycloadduct (208), containing a tertiary ether linkage, was carried out. An El process, mediated by a Lewis acid, effected selective C-0 bond cleavage to give the most stable of the alternative regioisomeric carbo-cations, and finally olefin (209) (Scheme 54). 

A different approach for the in situ preparation of 2-zirconated 1,3-dienes 68 was reported by Szymoniak and Bertus et al.,by treatment of 2-silyloxy-l,3-di-enes 81 with dialkylzirconocene 21 (Scheme 28) [61]. The zirconocene induced the C-0 bond cleavage of 81 and led to the dienyl zirconium compound 68,... 

Scheme 15. Regioselective C-0 bond cleavage as a key step in the synthesis of ptaquilosin [57]. Kinetic and thermodynamic aspects...

---