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Low-temperature ozonation of cumene (1a) in acetone, methyl acetate, and tert-butyl methyl ether at -70 degrees C produced the corresponding hydrotrioxide, C(6)H(5)C(CH(3))(2)OOOH (2a), along with hydrogen trioxide, HOOOH. Ozonation of triphenylmethane (1b), however, produced only triphenylmethyl hydrotrioxide, (C(6)H(5))(3)COOOH (2b). These observations, together with the previously reported experimental evidence, seem to support the "radical" mechanism for the first step of the ozonation of the C-H bonds in hydrocarbons, i.e., the formation of the caged radical pair (R(**)OOOH), which allows both (a) collapse of the radical pair to ROOOH and (b) the abstraction of the hydrogen atom from alkyl radical R(*) by HOOO(*) to form HOOOH. The B3LYP/6-311++G(d,p) (ZPE) calculations revealed that HOOO radicals are considerably stabilized by forming intermolecularly hydrogen-bonded complexes with acetone (BE = 8.55 kcal/mol) and dimethyl ether (7.04 kcal/mol). This type of interaction appears to be crucial for the relatively fast reactions (and the formation of the polyoxides in relatively high yields) in these solvents, as compared to the ozonations run in nonbasic solvents. However, HOOO radicals appear to be not stable enough to abstract hydrogen atoms outside the solvent cage, as indicated by the absence of HOOOH among the products in the ozonolysis of triphenylmethane. The decomposition of alkyl hydrotrioxides 2a and 2b involves a homolytic cleavage of the RO-OOH bond with subsequent "in cage" reactions of the corresponding radicals, while the decomposition of HOOOH is most likely predominantly a "pericyclic" process involving one or more molecules of water acting as a bifunctional catalyst to produce water and singlet oxygen (Delta(1)O(2)).
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PMID:Evidence for HOOO radicals in the formation of alkyl hydrotrioxides (ROOOH) and hydrogen trioxide (HOOOH) in the ozonation of C-H bonds in hydrocarbons. 1179 9

Low-temperature ozonation (-78 degrees C) of 2-methyl-1,3-dioxolane (1a) in acetone-d6, methyl acetate, and tert-butyl methyl ether produced both the corresponding acetal hydrotrioxide (3a, ROOOH) and the hemiortho ester (2a, ROH) in molar ratio 1:5. Both intermediates were fully characterized by 1H, 13C, and 17O NMR spectroscopy, and they both decomposed to the corresponding hydroxy ester at higher temperatures. The mechanism involving the HOOO- anion formed by the abstraction of the hydride ion by ozone to form an ion pair, R+ -OOOH, with its subsequent collapse to either the corresponding hemiortho ester (ROH) or the acetal hydrotrioxide (ROOOH) was proposed. This mechanism is supported by the PISA/B3LYP/6-311++G(3df,3pd) calculations.
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PMID:Evidence for the HOOO(-) anion in the ozonation of 1,3-dioxolanes: hemiortho esters as the primary products. 1223 27

A theoretical study specifically addresses the question of whether nucleophilic addition to the carbonyl groups of acid chlorides, esters, and anhydrides involves an addition-elimination pathway or proceeds by a concerted S(N)2-like mechanism in the absence of the generally assumed tetrahedral intermediate. Density functional calculations [B3LYP/6-31+G(d,p)] establish that chloride ion exchange reactions with both formyl and acetyl chloride proceed by a pi attack on the C=O bond. No discernible tetrahedral intermediate typical of an addition-elimination pathway was found in either case. While a tetrahedral intermediate does exist for the addition of fluoride ion to (Cl)(2)C=O, halide exchange of LiCl with both ClFC=O and (Cl)(2)C=O also proceeds by a concerted S(N)2-like pathway. The formation of a tetrahedral intermediate from the addition of methanol to acetyl chloride is slightly exothermic (4.4 kcal/mol). The ion-dipole complex of methanol weakly bonded to the carbonyl carbon of protonated acetyl chloride is stabilized by 13.8 kcal/mol but does not collapse to a tetrahedral intermediate. When four CH(3)OH molecules are H-bonded to protonated acetyl chloride, a tetrahedral intermediate is not completely formed and this solvated complex more closely resembles the precursor to an S(N)1-type ionization of Cl(-). With six H-bonding methanol molecules, a methanol adds to the carbonyl carbon and a proton relay occurs with formation of a tetrahedral-like structure that immediately loses chloride ion in an S(N)1-like solvolysis. These results corroborate earlier suggestions (Bentley et al. J. Org. Chem. 1996, 61, 7927) that the methanolysis of acetyl chloride does not proceed through the generally assumed addition-elimination pathway with a discrete tetrahedral intermediate but is consistent with ionization of Cl(-). The reaction of methoxide ion with methyl acetate proceeds via a multiple-well energy surface and involves the intermediacy of an asymmetrical species with differing C-OMe bond lengths. Models of synthetic applications of acyl transfer reactions involving anhydrides that form N-acyloxazolidinones also proceed by a concerted S(N)2-type pathway even with the carboxylate leaving group. Concerted transition states were observed for the reactions of each enantiomer of a 1,3-diphenylcycloprop-2-ene carboxylic anhydride by S-3-lithio-4-phenyloxazolidinone. Despite close structural similarities between the diastereomeric transition states, the relative energies correlated closely with the experimental results.
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PMID:Computational studies of nucleophilic substitution at carbonyl carbon: the S(N)2 mechanism versus the tetrahedral intermediate in organic synthesis. 1547 86