Isotope effects ring opening

A mechanistic study of acid and metal ion (Ni2+, Cu2+, Zn2+) promoted hydrolysis of [N-(2-carboxyphenyl)iminodiacetate](picolinato)chromate (III) indicated parallel H+- or M2+-dependent and -independent pathways. Solvent isotope effects indicate that the H+-dependent path involves rapid pre-equilibrium protonation followed by rate-limiting ring opening. Similarly, the M2+-dependent path involves rate-determining Cr-0 bond breaking in a rapidly formed binuclear intermediate. The relative catalytic efficiencies of the three metal ions reflect the Irving-Williams stability order (88). 

A study of the stereochemistry and secondary isotope effects for the 2 + 2-cycloaddition of alkyl-substituted buta-1,3-dienes with Ceo indicates the formation of an open biradical intermediate in the rate-determining step leading to the cycloadduct (18) (Scheme 5). The addition of benzyne to C70 produces four isomeric monoadducts. One of these adducts is the first example of an adduct of a 5-6 ring fusion where the ring-fusion bond remains intact. 

Isotope labelling again proved (21) that the decarboxylation involves only carbon 2 of the oxazolidine-2,5-dione, and studies of the kinetic isotope effect (22) demonstrated that this step is not the rate-determining in the overall process. Therefore, the rate of the overall reaction is determined either by (a) — the formation of the complex, or by (b), viz. by opening of the ring between atoms 1 and 5. This conclusion applies also to the "normal amine-propagated NCA polymerisation. 

The parent ion 5 and its naphtho[6] analogue have been generated by treatment of 1 and 11, respectively, with trityl tetrafluoroborate. The cations so formed are captured by water and the final product of reaction is the ring opened aryl aldehyde (equation 24)230 231. Kinetic measurements have shown230 that abstraction of hydride ion from 1 is first order and displays a deuterium isotope effect of 6.5. To the best of the author s knowledge, the isolation and spectroscopic characterization of an unsubstituted cycloproparenyl cation has yet to be achieved. 

Deuterium isotope effect in ring opening of 1,2-diphenylcyclobutanols (440)... 

The mechanism of insertion of 2-alkylphenylnitrenes into a 1,5-related CH bond was studied by three methods 80 determination of isotope effects, stereochemistry, and radical clock. During the formation of indolines, a kn/kD of 12.6-14.7 was observed coupled with complete loss of stereochemical integrity at the CH carbon. When the CH insertion carbon bore a cyclopropane group, ring-opening products were observed. These observations suggest a mainly radical H-atom abstraction mechanism. The sensitivity of the isotope effects to solvent was taken to imply a small concerted nitrene insertion contribution. 

The reaction of 3-unsubstituted isoxazoles with bases, leading to ring opening, has been known for almost 90 years, and has many applications. The mechanism of the reactions with hydroxide ion has been studied by Italian workers.156 157 The reactions showed second-order kinetics (first-order in base and in substrate) and the UV spectra of reaction mixtures showed sharp isosbestic points, indicating the transformation of reactants into products without formation of an intermediate. A primary deuterium isotope effect indicated that fission of the C3—H bond was rate-determining. 

Recently, kinetic isotope effects were studied in the polymerization of DXL. The results reported indicate that the ring opening of the monomer mol ule does not occur in the rate-limiting step of the propagation process and thus support the growth on the cyclic tertiary oxonium ion l... 

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