Di- and triarylmethylium ions as probes for the ambident reactivities of carbanions derived from 5-benzylated Meldrum's acid

Xi Chen, Yue Tan, Guillaume Berionni, Armin R. Ofial, Herbert Mayr

Research output: Contribution to journalArticle

Abstract

The kinetics of the reactions of carbocations with carbanions 1 derived from 5-benzyl-substituted Meldrum's acids 1-H (Meldrum's acid=2,2-dimethyl-1,3- dioxane-4,6-dione) were investigated by UV/Vis spectroscopic methods. Benzhydryl cations Ar2CH+ added exclusively to C-5 of the Meldrum's acid moiety. As the second-order rate constants (kC) of these reactions in DMSO followed the linear free-energy relationship lg k=s N(N+E), the nucleophile-specific reactivity parameters N and s N for the carbanions 1 could be determined. In contrast, trityl cations Ar3C+ reacted differently. While tritylium ions of low electrophilicity (E<-2) reacted with 1 through rate-determining β-hydride abstraction, more Lewis acidic tritylium ions initially reacted at the carbonyl oxygen of 1 to form trityl enolates, which subsequently reionized and eventually yielded triarylmethanes and 5-benzylidene Meldrum's acids by hydride transfer.

Original languageEnglish
Pages (from-to)11069-11077
Number of pages9
JournalChemistry - A European Journal
Volume20
Issue number35
DOIs
Publication statusPublished - 25 Aug 2014
Externally publishedYes

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Ions
Acids
Hydrides
Positive ions
Cations
Nucleophiles
Free energy
Rate constants
Dimethyl Sulfoxide
Kinetics
Oxygen
Meldrum's acid

Keywords

  • alkylation
  • ambident reactivity
  • carbocations
  • kinetics
  • linear free energy relationships

Cite this

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abstract = "The kinetics of the reactions of carbocations with carbanions 1 derived from 5-benzyl-substituted Meldrum's acids 1-H (Meldrum's acid=2,2-dimethyl-1,3- dioxane-4,6-dione) were investigated by UV/Vis spectroscopic methods. Benzhydryl cations Ar2CH+ added exclusively to C-5 of the Meldrum's acid moiety. As the second-order rate constants (kC) of these reactions in DMSO followed the linear free-energy relationship lg k=s N(N+E), the nucleophile-specific reactivity parameters N and s N for the carbanions 1 could be determined. In contrast, trityl cations Ar3C+ reacted differently. While tritylium ions of low electrophilicity (E<-2) reacted with 1 through rate-determining β-hydride abstraction, more Lewis acidic tritylium ions initially reacted at the carbonyl oxygen of 1 to form trityl enolates, which subsequently reionized and eventually yielded triarylmethanes and 5-benzylidene Meldrum's acids by hydride transfer.",
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Di- and triarylmethylium ions as probes for the ambident reactivities of carbanions derived from 5-benzylated Meldrum's acid. / Chen, Xi; Tan, Yue; Berionni, Guillaume; Ofial, Armin R.; Mayr, Herbert.

In: Chemistry - A European Journal, Vol. 20, No. 35, 25.08.2014, p. 11069-11077.

Research output: Contribution to journalArticle

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