Stability−Reactivity Relation on the Reaction of β,β-Disubstituted Vinyl Cations with Ethanol

Abstract

A family of β,β-disubstituted α-(p-methoxyphenyl)vinyl cations has been generated by the laser flash photolysis of the corresponding vinyl bromides, and the rates of reactions of the cations with ethanol in acetonitrile have been measured at 25 °C. The observed rate constants differ greatly depending on the substituents, ranging from 3.05 × 105 L mol-1 s-1 to 8.18 × 107 L mol-1 s-1. The thermodynamic stabilities of the vinyl cations have been estimated by means of ab initio MO calculations for model compounds, which reveals that their stability is almost unaffected by β,β-dialkyl substituents. In the present system, therefore, the stability−reactivity relation, in which a less stable cation is expected to show higher reactivity, breaks down. Several model transition structures have been considered in the MO calculations, and the results indicate that the β substituents can move away from the incoming nucleophile to avoid steric congestion but that there still exists a large steric repulsion in the transition state. The calculations indicate that the reactivity of the vinyl cations is primarily controlled by this steric effect, which is the reason for the breakdown of the stability−reactivity relationship.

Vinyl Triflimides—A Case of Assisted Vinyl Cation Formation

“…[10] It is well known for its innocent, non-nucleophilic character as ac ounteranion. [17] Hence,e ven if any selectivity occurs,i ti sh eavily substrate-dependent and thus unpredictable. [14] These studies evidence that regioselectivity is at hird challenge.T he addition of the TFSI through one of its sulfonyl Oatoms is clearly preferred [13] and mechanistic rationales and studies of what governs the selectivity are lacking.…”

DABCO and DMAP—Why Are They Different in Organocatalysis?

“…We now report on the use of laser flash techniques [6,7] for the determination of the nucleophilicities of the tertiary amines 1 and 2. We will then compare the nucleophilicities [3c] and the corresponding carbon basicities [8] (derived from equilibrium constants) of 1 and 2 with those of other organocatalysts and discuss the implications.…”

DABCO oder DMAP – worauf beruht ihr Unterschied in der Organokatalyse?

“…Weniger elektrophile Benzhydrylium-Ionen (E < À9) reagierten Wir berichten nun über die Verwendung von Puls-LaserTechnik [6,7] (1) nur halb so groß, was sich durch den elektronenziehenden induktiven Effekt des zweiten Stickstoffs erklären lässt. Werden Reaktionen mit elektrophileren Benzhydrylium-Ionen betrachtet, wobei man sich der Diffusionsgrenze nähert, [10,11] wird DABCO etwas reaktiver als Chinuclidin (2) (Abbildung 2).…”