Reaction mechanism bond cleavage

The Mn + complexed ions appear to be the most selective and efficient initiator system for grafting to polysaccharides so far described. The mechanism of the initiation reaction has been studied in our laboratories by model experiments and ESR spectroscopy. There are two possible reactions indicated. Bond cleavage of a vicinal diol according to reaction (22) is one possibility. Another and faster reaction with Mn3+ giving radicals appears to be oxidation of aldehyde groups (24) to alkoxy radicals ... 

Whitney et al. used DBU or tetramethylguanidine as hindered and nonnucleophilic base for the removal of protected peptides from a 2-[4-(hydroxymethyl)phenylacetoxy]propionyl resin (84T4237). The proposed reaction mechanism involved cleavage of the ester bond between the peptide and resin via a base-catalyzed elimination. This cleavage reaction was mild and rapid, and proceeded in good yield with a very simple work-up procedure. 

The strength of their carbon-halogen bonds causes aryl halides to react very slowly in reactions in which carbon-halogen bond cleavage is rate determining as m nude ophilic substitution for example Later m this chapter we will see examples of such reactions that do take place at reasonable rates but proceed by mechanisms distinctly dif ferent from the classical S l and 8 2 pathways... 

Ozonation ofAlkenes. The most common ozone reaction involves the cleavage of olefinic carbon—carbon double bonds. Electrophilic attack by ozone on carbon—carbon double bonds is concerted and stereospecific (54). The modified three-step Criegee mechanism involves a 1,3-dipolar cycloaddition of ozone to an olefinic double bond via a transitory TT-complex (3) to form an initial unstable ozonide, a 1,2,3-trioxolane or molozonide (4), where R is hydrogen or alkyl. The molozonide rearranges via a 1,3-cycloreversion to a carbonyl fragment (5) and a peroxidic dipolar ion or zwitterion (6). 

Studies of reaction mechanisms ia O-enriched water show the foUowiag cleavage of dialkyl sulfates is primarily at the C—O bond under alkaline and acid conditions, and monoalkyl sulfates cleave at the C—O bond under alkaline conditions and at the S—O bond under acid conditions (45,54). An optically active half ester (j -butyl sulfate [3004-76-0]) hydroly2es at 100°C with iaversion under alkaline conditions and with retention plus some racemization under acid conditions (55). Effects of solvent and substituted stmcture have been studied, with moist dioxane giving marked rate enhancement (44,56,57). Hydrolysis of monophenyl sulfate [4074-56-0] has been similarly examined (58). 

The protonated azirine system has also been utilized for the synthesis of heterocyclic compounds (67JA44S6). Thus, treatment of (199) with anhydrous perchloric acid and acetone or acetonitrile gave the oxazolinium perchlorate (207) and the imidazolinium perchlorate (209), respectively. The mechanism of these reactions involves 1,3-bond cleavage of the protonated azirine and reaction with the carbonyl group (or nitrile) to produce a resonance-stabilized carbonium-oxonium ion (or carbonium-nitrilium ion), followed by attack of the nitrogen unshared pair jf electrons to complete the cyclization. 

A number of studies of the acid-catalyzed mechanism of enolization have been done. The case of cyclohexanone is illustrative. The reaction is catalyzed by various carboxylic acids and substituted ammonium ions. The effectiveness of these proton donors as catalysts correlates with their pK values. When plotted according to the Bronsted catalysis law (Section 4.8), the value of the slope a is 0.74. When deuterium or tritium is introduced in the a position, there is a marked decrease in the rate of acid-catalyzed enolization h/ d 5. This kinetic isotope effect indicates that the C—H bond cleavage is part of the rate-determining step. The generally accepted mechanism for acid-catalyzed enolization pictures the rate-determining step as deprotonation of the protonated ketone ... 

Curvature in a Br nsted-type plot is sometimes attributed to a change in transition state structure. This is not a change in mechanism rather it is interpreted as a shift in extent of bond cleavage and bond formation within the same mechanistic pattern. Thus, Ba-Saif et al. ° found curvature in the Br nsted-type plot for the identity reactions in acetyl transfer between substituted phenolates this reaction was shown earlier. They concluded that a change in transition state structure occurs in the series. Jencks et al." caution against this type of conclusion solely on the evidence of curvature, because of the other possible causes. 

This is the reverse of the first step in the SnI mechanism. As written here, this reaction is called cation-anion recombination, or an electrophile-nucleophile reaction. This type of reaction lacks the symmetry of a group transfer reaction, and we should therefore not expect Marcus theory to be applicable, as Ritchie et al. have emphasized. Nevertheless, the electrophile-nucleophile reaction possesses the simplifying feature that bond formation occurs in the absence of bond cleavage. 

DNA is not susceptible to alkaline hydrolysis. On the other hand, RNA is alkali labile and is readily hydrolyzed by dilute sodium hydroxide. Cleavage is random in RNA, and the ultimate products are a mixture of nucleoside 2 - and 3 -monophosphates. These products provide a clue to the reaction mechanism (Figure 11.29). Abstraction of the 2 -OH hydrogen by hydroxyl anion leaves a 2 -0 that carries out a nucleophilic attack on the phosphorus atom of the phosphate moiety, resulting in cleavage of the 5 -phosphodiester bond and formation of a cyclic 2, 3 -phosphate. This cyclic 2, 3 -phosphodiester is unstable and decomposes randomly to either a 2 - or 3 -phosphate ester. DNA has no 2 -OH therefore DNA is alkali stable. 

Contrary to the dioxetanone pathway, DeLuca and Dempsey (1970) proposed a mechanism of the bioluminescence reaction that involves a multiple linear bond cleavage of luciferin peroxide... 

However, the linear bond cleavage hypothesis of the firefly bioluminescence was made invalid in 1977. It was clearly shown by Shimomura et al. (1977) that one O atom of the CO2 produced is derived from molecular oxygen, not from the solvent water, using the same 180-labeling technique as used by DeLuca and Dempsey. The result was verified by Wannlund et al. (1978). Thus it was confirmed that the firefly bioluminescence reaction involves the dioxetanone pathway. Incidentally, there is currently no known bioluminescence system that involves a splitting of CO2 by the linear bond cleavage mechanism. 

The mechanism of the reaction has been interpreted in terms of a 4- or 6-centre transition state (LV) or (LVI) in which nucleophilic assistance of C-Si bond cleavage occurs this would not be possible for nitration by nitronium tetrafluoroborate. 

There is no clear reason to prefer either of these mechanisms, since stereochemical and kinetic data are lacking. Solvent effects also give no suggestion about the problem. It is possible that the carbon-carbon bond is weakened by an increasing number of phenyl substituents, resulting in more carbon-carbon bond cleavage products, as is indeed found experimentally. All these reductive reactions of thiirane dioxides with metal hydrides are accompanied by the formation of the corresponding alkenes via the usual elimination of sulfur dioxide. 

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. 

Fig. 11. Proposed reaction mechanism for the SbaR bond cleavage reaction...

Scheme 65 The calculated mechanism of H-H bond cleavage reaction of the model complex for [Fe] -hydrogenases...

Figure 9.8. Global reaction mechanism for the hydrodesulfurization of thiophene, in which the first step involves hydrogenation of the unsaturated ring, followed by cleavage ofthe C-S bond in two steps. Butadiene is assumed to be the first sulfur-free product,...

Shyadehi AZ, DC Lamb, SL Kelly, DE Kelly, W-H Schunck, JN Wright, D Corina, M Akhtar (1996) The mechanism of the acyl-carbon bond cleavage reaction catalyzed by recombinant sterol 14a-demethyl-ase of Candida albicans (other names are lanosterol 14a-demethylase, P-450]4p, and CYP51). J Biol Chem 271 12445-12450. 

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