Diels–Alder reactions of 1,1,3-triarylallyl cations: determination of the free energy of concert

Abstract

Triarylallyl cations react with 1,3-dienes to give Diels-Alder products in low to moderate yields. The observed second-order rate constants agree with those calculated for the stepwise processes by the correlation log k(20 ЊC) = s(N ϩ E ) [eqn. (1)] indicating the absence of a significant free energy of concert. Eqn. (1) can, therefore, also be employed for the prediction of absolute rate constants of Diels-Alder reactions of allyl cations. Scheme 2 Stepwise and concerted pathways of the Diels-Alder reactions of allyl cations 1 with dienes 2.

Electrophilic Reactivities of α,β‐Unsaturated Iminium Ions

“…It is apparent that the iminium ion 3 f , which is derived from MacMillan's catalyst 2 f , is the most active compound of this series, in line with the observation that 2 f is particularly suitable for iminium ion catalyzed reactions employing weak nucleophiles, e.g., for Diels–Alder reactions3a, 11 of unsaturated aldehydes with cyclopentadiene or 2‐methylbuta‐1,3‐diene (isoprene), or Friedel–Crafts reactions with heterocyclic arenes (indoles, pyrroles, furans) 12…”

Nucleophilicity Parameters of Pyridinium Ylides and Their Use in Mechanistic Analyses

“…In line with quantum chemical calculations by Matsumura, which indicated a barrier of only 2.9 kJ/mol for the cyclization of a betaine generated from an isoquinolinium ylide and 1,1-dicyanoethylene, one can conclude that the betaines formed from 5a and 1h , i undergo fast subsequent ring closures with formation of the [3 + 2]-adducts 14h , i (Scheme ), with the consequence that in these cases k exp corresponds to k 2 . The observation k exp ≈ k calcd also excludes concerted 1,3-dipolar cycloadditions 5a + 1h , i with a high energy of concert, because in this case k exp should be considerably larger than k calcd …”

“…In case of concerted cycloadditions k exp / k calcd should be >1, depending on the degree of concertedness . Because of the large error limits of eq , the kinetic data do not allow us to differentiate stepwise cycloadditions from concerted processes with highly unsymmetrical transition states and a small energy of concert.…”

Biomimetic Total Synthesis of (±)-Griffipavixanthone via a Cationic Cycloaddition–Cyclization Cascade

“…The formation of griffi­pavi­xanthone precursor 15 from either 14 or a 16a / b mixture (cf. Scheme ) supports an ionic Diels–Alder reaction cascade in which six-membered ring formation is reversible. , Figure outlines a plausible mechanism which is supported by DFT computations. ,, After acid-catalyzed isomerization of 14 to 10a , the intermediate cation 17 may combine with 10a to afford allylic cations, represented as 18a – c . Cation 18a was the lowest energy structure located; this should interconvert with an energetically favorable xanthonium ion 19 which arises from capture of the benzyl cation by the nucleophilic carbonyl of the pendant xanthone.…”