Intramolecular decomposition

Intestinal carcinogenesis, dietary Upid hydroperoxides, 614 Intramolecular decomposition,... 

The bidirectional course of the oxidation may be rationalized by assuming that the peroxide intermediate A formed in the first reaction step undergoes the intramolecular decomposition and/or intermolecular reaction as shown in Scheme III. 

Smith and Hinshelwood - interpreted the maximally inhibited reaction as the intramolecular decomposition of the aldehyde. As already mentioned, such a conclusion must be treated with reserve. The participation of free radicals has been confirmed experimentally by Ingold and Lossing , who identified the phenyl and benzoyl radicals by mass-spectrometry. 

Temperature increase and added NO have only slight effect on C3Hg forma-tion , suggesting that intramolecular decomposition in primary step II is the only significant source of propene. 

In the polymerization of ethylene by (Tr-CjHsljTiClj/AlMejCl [111] and of butadiene by Co(acac)3/AlEt2Cl/H2 0 [87] there is evidence for bimolecular termination. The conclusions on ethylene polymerization have been questioned, however, and it has been proposed that intramolecular decomposition of the catalyst complex occurs via ionic intermediates [91], Smith and Zelmer [275] have examined several catalyst systems for ethylene polymerization and with the assumption that the rate at any time is proportional to the active site concentration ([C ]), second order catalyst decay was deduced, since 1 — [Cf] /[Cf] was linear with time. This evidence, of course, does not distinguish between chemical deactivation and physical occlusion of sites. In conjugated diene polymerization by Group VIII metal catalysts -the unsaturated polymer chain stabilizes the active centre and the copolymerization of a monoolefin which converts the growing chain from a tt to a a bonded structure is followed by a catalyst decomposition, with a reduction in rate and polymer molecular weight [88]. 

Figure 1. A schematic presentation of the intermolecular and intramolecular decomposition of cyclohexenyl hydroperoxide.

The intramolecular decomposition of cyclohexenyl hydroperoxide can be envisaged as proceding via p-hydrogen elimination in an alkenyl-peroxochromium(VI) complex (Figure 3). 

The mechanism of the P-M oxidation consists of three distinct steps 1) activation of the DMSO by a protonated dialkyl carbodiimide 2) activation of the alcohol substrate and the formation of the key alkoxysulfonium ylide intermediate and 3) the intramolecular decomposition of the alkoxysulfonium ylide to afford the product ketone or aldehyde and the dialkyl urea by-product (established by isotopic labeling studies). The alkoxysulfonium ylide is a common intermediate in all other oxidations using activated DMSO. 

Depending on the nature of the counterion, triarylsulfonium salts are more or less stable. The most stable compounds have a non-nucleophilic counterion. On the other hand, the presence of a nucleophilic counterion such as a halide (Cl, Br or I) frequently results in the relatively facile decomposition by thermolysis to afford quantitatively the diarylsulfide and the corresponding aryl halide. The observation of the preferential relief of the steric strain in the case of phenyl-p-tolyl-(2,5-dimethylphenyl)sulfonium salts (23) was explained by the intramolecular decomposition of a first formed tetracoordinate intermediate. 35.36... 

Intramolecular decomposition of (aryloxy)tetraphenylantimony (9) by thermolysis leads to the corresponding diarylethers (15) in modest to Wgh yields. 

Though the almost unique catalytic effects of cuprous salts, or of metallic copper, in the Sandmeyer reaction have been contested by Hodgson (7), the careful kinetic work of Cowdrey and Davies (8) has established that the anion (CuCh)" does play a special role. These workers, however, suggested that an intramolecular decomposition of complex cuprous salts (At.N2, CuCh) produced the aryl halide. If this concept is correct, then the Sandmeyer reaction should not yield detectable free aryl radicals and should differ from many associated reactions of diazonium salts (9,10) for which the homolytic character is in less doubt. 

Another successful approach for blocking the intramolecular decomposition illustrated in Fig. 2 involved inhibition of the dehydration step leading to the anhydrohemiketal by replacement of the C-8 proton of erythromycin with fluorine. Preparation of flurithromycin (8-fluoroerythromycin, see Fig. 4) has been achieved by both chemical and biochemical methods. Addition of 8(S)-8-fluoroerythronolide A to a mutant strain of Streptomyces erythreus blocked in biosynthesis of its endogenous lactone (erythronolide) yielded the desired fluorinated erythromycin [40]. This technique of mutasynthesis has been further employed for the production of other fluorinated derivatives of erythromycin [40, 41]. Fluorination of different 8,9-anhydro-6,9-hemiketal derivatives of erythromycin by chemical means with reagents such as trifluoromethyl hypo-fluorite or perchloryl fluoride (with subsequent reduction of the N-oxide) has also been reported [42, 43]. 

The decomposition, catalysed by copper at elevated temperatures, can be improved by palladium acetate [10] or rhodium acetate at room temperature [11]. Sterically hindered diazoacetic esters give a higher content of trans-isomer in the reaction product [12], as does the employment of diazoacetonitrile [13]. Intramolecular decomposition leads to precursors of cis-chrysanthemic acid [14]. Substituted furfu-ryldiazoacetats yield the final insecticides directly [15]. 

Figure 5.11 Intramolecular decomposition of secondary and tertiary hydrogen peroxides excited state [38]...

In isotactic pol5 ropylene, the intramolecular hydrc en abstraciion via the favoured six-membered transition state results in the formation of adjacent hydroperoxides (sequences of hydroperoxides) during the induction period (19, 20). The tertiary hydroperoxides bound to the polymer chain undergo an intramolecular decomposition to give rise to a fast radical chain process (19. 20). 

---