Nucleophilic addition to olefins. 7. Kinetic deuterium isotope effects as criterion for an enforced preassociation mechanism in the hydrolysis of substituted benzylidene Meldrum's acids

Journal Article · Wed Sep 22 04:00:00 EDT 1982 · J. Am. Chem. Soc.; (United States)

DOI:https://doi.org/10.1021/ja00383a025· OSTI ID:6234337

Bernasconi, C F; Leonarduzzi, G D

The hydrolysis of the title compounds occurs in four steps: (1) nucleophilic attack by water or hydroxide ion to form the addition complex T/sub OH//sup -/; (2) carbon protonation of T/sub OH//sup -/ to form T/sub OH//sup 0/; (3) oxygen deprotonation of T/sub OH//sup 0/ to form T/sub OH//sup 0 -/; (4) collapse of the tetrahedral intermediate T/sub OH//sup -/ into the respective benzaldehyde and Meldrum's acid anion. There is also a water-catalyzed collapse of T/sub OH//sup 0/ which becomes dominant in strongly acidic solution. In basic solution carbon protonation of T/sub OH//sup -/ (step 2) is rate limiting; in strongly acidic media the water-catalyzed collapse of T/sub OH//sup 0/ is rate limiting for all substrates. In moderatly acidic solution two types of behavior were observed. With the p-nitro derivative step 4 is rate limiting at high, step 3 at low buffer concentrations. The latter situation is equivalent to a diffusion-controlled trapping mechanism in the reverse direction. With the parent and the p-methoxy derivative, collapse of T/sub OH//sup 0 -/ occurs before the protonated base catalyst generated in step 3 can diffuse away; this is equivalent to an enforced preassociation mechanism in the reverse direction and is analogous to the reaction of thiol anions with acetaldehyde studied by Gilbert and Jencks. Our interpretation is strongly supported by (1) ..cap alpha.. secondary kinetic deuterium isotope effects which are large for the preassociation mechanism but essentially nil for the trapping mechanism and (2) by Bronsted ..beta.. values around 0.8 in the case of the preassociation mechanism and 1.0 for the trapping mechanism. The mechanism for the water-catalyzed collapse of T/sub OH//sup 0 -/ is probably concerted, a conclusion which is supported by a large positive deviation from the Bronsted plot for base catalysis and by a large ..cap alpha.. secondary kinetic deuterium isotope effect.

• Research Organization: Univ. of California, Santa Cruz
• OSTI ID: 6234337
• Journal Information: J. Am. Chem. Soc.; (United States), Journal Name: J. Am. Chem. Soc.; (United States) Vol. 104:19; ISSN JACSA
• Country of Publication: United States
• Language: English

Related Subjects

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY
400301 -- Organic Chemistry-- Chemical & Physicochemical Properties-- (-1987)
400302* -- Organic Chemistry-- Isotope Effects-- (-1987)
ALKENES
CHEMICAL REACTION KINETICS
CHEMICAL REACTIONS
DATA
DECOMPOSITION
DEUTERIUM
EXPERIMENTAL DATA
HYDROCARBONS
HYDROGEN ISOTOPES
HYDROLYSIS
INFORMATION
ISOTOPE EFFECTS
ISOTOPES
KINETICS
LIGHT NUCLEI
LYSIS
NUCLEI
NUMERICAL DATA
ODD-ODD NUCLEI
ORGANIC ACIDS
ORGANIC COMPOUNDS
REACTION KINETICS
SOLVOLYSIS
STABLE ISOTOPES