Carbon-oxygen Bond Fission Research Articles

A Mechanistic Study of the Alkaline Hydrolysis of Diaryl Sulfate Diesters

Nearly all of the reported studies of reactions of sulfate diesters are for dialkyl or alkyl aryl diesters, which undergo reaction by carbon-oxygen bond fission. Sulfuryl transfer reactions of sulfate diesters (RO-SO(2)-OR') proceeding by attack at sulfur have been little explored. When both ester groups are aryl groups the hydrolysis reaction (sulfuryl transfer to water) occurs by way of attack at sulfur. The alkaline hydrolysis of diaryl sulfate diesters was shown to obey first-order kinetics with respect to [(-)OH] and proceed through S-O bond fission, in a mechanism that is most likely concerted. Activation parameters for 4-chloro-3-nitrophenyl phenyl sulfate and 4-nitrophenyl phenyl sulfate gave the following respective values: Delta H(++) = 88.0 +/- 0.1 and 84.83 +/- 0.06 kJ mol(-)(1) and Delta S(++) = -37 +/- 1 and -50.2 +/- 0.5 J mol(-)(1) deg(-)(1). The dependence of the second-order rate constant for hydrolysis on leaving group pK(a) was analyzed giving a beta(lg) slope of -0.7 +/- 0.2 and a Leffler alpha parameter value of 0.36. A (15)k kinetic isotope effect (KIE) for the hydroxide attack on 4-nitrophenyl phenyl sulfate of 1.0000 +/-0.0005 and an (18)k(lg) KIE value of 1.003+/-0.002 were obtained.

Radical Ring-Opening Polyaddition of Bis(vinyloxirane) Derivatives and Dithiols

Radical polyadditions of bifunctional vinyloxiranes [1,4-bis(2-vinylepoxyethyl)benzene (3) and 1,4-bis(2-isopropenylepoxyethyl)benzene (4)] and dithiols are described. 3 and 4 were prepared by the phase transfer reaction of terephthaladehyde and the corresponding sulfonium salts. The radical polyaddition of 4 and aromatic dithiols [1,4-benzenedithiol (5a), 4,4'-isopropylidenebis(thiophenol) (5b), and bis(4-mercaptophenyl) sulfide (5c)] was carried out at 60 °C in PhCl in the presence of azobis(isobutyronitrile) (AIBN) as an initiator to obtain the corresponding polymers with high molecular weights in excellent yields. The same polyaddition also proceeded at 20 °C under photoirradiation to give polymers of molecular weights lower than those obtained using AIBN. On the other hand, only a gelled polymer was obtained by the polymerization of 4 and 5a using di-tert-butyl peroxide (DTBP) at a higher temperature (120 °C) in PhCl. When 3 was used instead of 4 in the polyaddition with 5a at 60 °C, a gelled polymer was also obtained, probably due to cross-linking of the formed polymer by the attack of a propagating thiyl radical on the resulting vinylene group in the main chain. The polyadditions of 4 and aliphatic dithiols gave the corresponding polymers with molecular weights lower than those from 4 and aromatic dithiols. The results of the model reactions of vinyloxiranes [2-phenyl-3-vinyloxirane (1) and 2-isopropenyl-3-phenyloxirane (3)] and thiols (benzenethiol and benzyl mercaptan) were consistent with those of the radical polyadditions of dithiols to bifunctional vinyloxiranes. The structure of the obtained polymers was confirmed by comparison with IR and NMR spectra of the model compounds obtained from 2 and benzenethiol in the presence of AIBN. These results indicate that the radical polyaddition proceeded mainly through the carbon-carbon bond cleavage of the oxirane ring of 4, accompanied by a small amount of carbon-oxygen bond fission, and led to the formation of the polymers with a main unit having vinyl ether and sulfide groups and a minor unit having hydroxy groups.

THE SYNTHESIS, SOLVOLYSIS AND REARRANGEMENT OF BENZYL TRIFLUOROMETHANESULFINATES

Abstract The synthesis and reactivity of benzyl trifluoromethanesulfinates have been investigated. These esters are easily and almost quantitatively obtained by selective oxidation of the corresponding sulfenates. A study of their behavior has revealed some unique features. In sharp contrast to benzyl arenesulfinates, which undergo ethanolysis with complete sulfur-oxygen bond fission, the corresponding trifluorornethanesulfinates undergo ethanolysis with exclusive carbon-oxygen bond fission, and with a rate enhancement by a factor of 6 powers of ten. The unusual high reactivity of these esters, comparable to that of the corresponding tosylates, is discussed. A kinetic study of the solvent and substituent effects on the rate of solvolysis has been performed. Also in contrast with benzyl arenesulfinates, these esters undergo facile rearrangement to sulfone on heating in polar nonhydroxylic solvents such as acetonitrile, in high yields. The mechanisms of solvolysis and rearrangement are discussed.

Intramolecular amidoseleniation of N-alkenyl imidates to N-heterocycles

The reactions of N-alkenyl imidates with benzeneselenenyl chloride and bromide in chloroform at ambient temperature afford pyrrolidine and piperidine derivatives having a phenylseleno moiety in good to excellent yields under neutral conditions. The key step of this new reaction consists of an intramolecular carbon–nitrogen bond formation and a simultaneous carbon–oxygen bond fission.

Mechanism of Acetal Cleavage with Methylmagnesium Iodide

The reaction of methylmagnesium iodide with methyl (1R,3S,5R)-1-(furan- 3′-yl)-5-methyl-2,8-dioxabicyclo[3.2.1]octane-3-carboxylate (2) gives products arising from regioselective carbon-oxygen bond fission and intermolecular transfer of the methyl group of the Grignard reagent to the intermediate oxocarbonium ion, in addition to the usual tertiary alcohol.

Reactions of acyl phosphates. Base hydrolysis of [Co(NH3)5OPO3COCH3]+ and the metal hydroxide promoted hydrolysis of acetyl phenyl phosphate

Tracer 18O studies have established that base hydrolysis of coordinated acetyl phosphate in [(NH3)5Co-OPO3COCH3]+ (kOH 0.53 mol-1 dm3 s-1, 25°C, I 1.0 NaClO4) proceeds with exclusive carbon-oxygen bond fission. The hydrolysis of acetyl phenyl phosphate monoanion is significantly catalysed by the exchange-inert hydroxo complex [(NH3)5Co-OH]2+ (kMOH 2.9 × 10-2 mol-1 dm3 s-1, 25°) which operates through a nucleophilic pathway involving attack at carbonyl carbon.

Sulphur-oxygen versus carbon-oxygen bond fission in the solvolysis of benzyl sulphenates

In contrast to p-anisyl trichloromethanesulphenate 1, which readily undergoes ethanolysis at room temperature with carbon-oxygen bond fission, the ethanolysis of the corresponding 2-nitrobenzenesulphenate 2 proceeds at a similar rate only at 100°, and involves sulphur-oxygen bond cleavage. While the solvolysis of 1 showed first-order kinetics, the solvolysis of 2 was second-order (first-order with respect to ester and to added base). The solvolysis rate of 2 decreases on going from 100% to 80% ethanol and by using pyridine instead of acetate as base, consistent with an S N2 type mechanism involving nucleophilic displacement at sulphur by the base or lyate ion. The rate of solvolysis of 1 is greatly enhanced in polar solvents and correlates satisfactorily with the ionization of p-methoxyneophyl tosylate. An ionization mechanism to some ion pair species is suggested for the solvolysis of 1.

Sulfur-oxygen versus carbon-oxygen bond fission in the solvolysis of benzyl sulfenates

Sulfur-oxygen versus carbon-oxygen bond fission in the solvolysis of benzyl sulfenates

Allylic Phosphate Hydrolysis: Kinetics and Mechanism of Degradation of 3β,17α-Dihydroxy-6α-methylpregn-4-en-20-one 17-Acetate, 3-Phosphate

The kinetics and mechanism of degradation of 3β, 17α-dihydroxy-6α-methylpregn-4-en-20-one 17-acetate, 3-phosphate (I) was studied as a function of pH and temperature. At low pH the least stable moiety is the allylic phosphate ester at C3, while the most reactive function at high pH is the acyl ester at position 17. At pH 7 and 37°, the rate of phosphate ester hydrolysis is approximately 104 times faster than hydrolysis of the 17α-acetoxy function. Between pH 4 and 10 at 37° and between pH 2 and 6 at 4°, the rate of phosphate ester hydrolysis is first order with respect to hydrogenion concentration. The major products of allylic phosphate hydrolysis are an epimeric mixture of C3 alcohols, supplying evidence that the reaction occurs principally by carbon—oxygen bond fission. The activation energy for phosphate ester hydrolysis of I is 21.5 ± 0.9 kcal./mole.

The reactions of organic phosphates. Part VII. The mechanisms of hydrolysis of neopentyl dihydrogen phosphate and a comparison with methyl dihydrogen phosphate

Neopentyl dihydrogen phosphate shows an acid-catalysed hydrolysis whose rate increases smoothly with acid concentration. Unlike the acid-catalysed hydrolysis of p-acetylphenyl and p-nitrophenyl dihydrogen phosphates, no rate maximum is observed in strong acid solution, although all three reactions proceed with phosphorus–oxygen bond fission. Because of its greater protonating power, sulphuric acid is a slightly better catalyst than hydrochloric acid. In contrast, the rate of hydrolysis of methyl dihydrogen phosphate in strong acid solution is greatly increased by the presence of halide ions, which act as nucleophiles towards the neutral and conjugate acid species. Both these processes proceed by carbon–oxygen bond fission and involve simple SN2 displacements on the methyl carbon atom.

Alkyl hydrogen sulphates. Part II. The hydrolysis in aqueous acid solution

Oxygen-18 tracer studies were carried out on the hydrolysis of sodium methyl, ethyl, and isopropyl sulphates in aqueous perchloric acid and sodium hydroxide solutions. The reaction proceeds by sulphur–oxygen bond fission in acid solution and carbon–oxygen bond fission in alkaline solution. An A-1 mechanism is proposed for the hydrolysis reaction in aqueous acid solution.

THE SOLVOLYSIS OF ARENESULFINATES, SULFUR–OXYGEN VERSUS CARBON–OXYGEN BOND FISSION

The base-catalyzed solvolysis of p-methoxyneophyl benzenesulfinate and the corresponding 2,6-dimethyl-, 2-methyl-, 4-methyl-, and 2-chloro-benzenesulfinates has been studied in ethanol and aqueous ethanol. The reactions involve sulfur–oxygen bond fission. The rate of sulfur–oxygen bond fission is decreased by the introduction of two methyl groups ortho to the sulfur. The rate decrease may be partly due to steric hindrance to nucleophilic attack on the sulfinate sulfur, but this effect is very small compared to the effect in carboxylate ester hydrolysis. The reactions are catalyzed by ethoxide, acetate, and pyridine. Reaction is very slow in the presence of 2,6-lutidine. With certain arenesulfinate esters one can control the reaction to give sulfur–oxygen or carbon–oxygen bond fission by choice of an appropriate base.