Formation and mechanism for reactions of ring‐substituted phenonium ions in aqueous solution

Abstract

The results of studies on the structure and reactivity of spiro[5.2]oct-5,7-diene-4yl carbocation [phenonium ion] have had a significant impact on the course of discussion about the distinction between classical and nonclassical carbocations. This minireview will present a brief overview of the structure, bonding and reactivity of ring substituted phenonium ions (X-4+), with an emphasis on work completed since 2004. The discussion will focus on the development of new experimental protocol for determination of the selectivity for addition of nucleophilic anions to X-4+ in aqueous solution. The existing relationships between carbocation lifetime and nucleophilic selectivity, and the known lifetime of ca 140 sec for spiro[2.5]oct-4,7-diene-6-one provide rough estimates of the lifetimes for 4-Me-X-4+ and 4-MeO-X-4+ in aqueous solution. Evidence is presented that nucleophile addition to X-4+ proceeds through an “exploded” transition state, with relatively weak bonding the nucleophile and leaving group, and the development of significant positive charge at the reacting primary cyclopropyl carbon.

A Hexavalent Basket for Bottom‐Up Construction of Functional Soft Materials and Polyvalent Drugs through a “Click” Reaction

“…In this regard, dibenzobicyclo[2.2.2]octadienes of type 5a/6/6 a are, in aliphatic substitutions, prone to Wagner-Meerwein rearrangements [38] as mediated by phenylium and secondary carbocationic intermediates ( Figure 2D). [43] In light of such reports, [38,42,44] we hypothesize that the principalf ormation of 7 could result from the electron-withdrawing bromides within 6a (or other polybrominated intermediates, Figure 2D)r educing the extent of p-to-s*h omohyperconjugation [38] to retard the departure rate of the leaving group and concurrently lower the stability of carbocationic intermediates.I nt his way,t he reactionp athway (i)i nF igure 2D dominates over (ii-v) to give 7 as the kinetic product. In another scenario, ag reater thermodynamic stabilityo f[ 2.2.2] over [3.2.1] bicyclicf rameworks [42,44] could lead to the formation of 7 as the thermodynamic product if all of the reactions are reversible on the time scale of the experiment.…”

“…Purportedly, the substitution of hydroxyl group in 5 with nucleophilic bromide anion was initiated with HBr, generated in the EAS reaction as the side product. In this regard, dibenzobicyclo[2.2.2]octadienes of type 5 a / 6/6 a are, in aliphatic substitutions, prone to Wagner–Meerwein rearrangements as mediated by phenylium and secondary carbocationic intermediates (Figure D) . In light of such reports, we hypothesize that the principal formation of 7 could result from the electron‐withdrawing bromides within 6 a (or other polybrominated intermediates, Figure D) reducing the extent of π‐to‐σ* homohyperconjugation to retard the departure rate of the leaving group and concurrently lower the stability of carbocationic intermediates.…”

Computational Investigation of Dichloromethane Ligand Substitution in the Enantiopure Cation [(η5-C5H5)Re(NO)(PPh3)(ClCH2Cl)]+, a Functional Equivalent of a Chiral Lewis Acid

“…66 The role of the PCCH linkage in the NGP mechanism can be compared to that played by the phenyl or aryl group in phenonium ion chemistry. 67 As shown in Figure 14, systems such as ArCHRCHRX (14) can undergo X/X′ exchange with overall retention via a double inversion pathway involving participation of C ipso of the aryl ring (see 15 + ). Interestingly, in certain substrates, the onset of this mechanism is also favored by weak nucleophiles.…”

Unlocking the Friedel-Crafts arylation of primary aliphatic alcohols and epoxides driven by hexafluoroisopropanol

“…Indeed, the association of Lewis or Brønsted acid catalysts with HFIP is known to trigger transformations with rather unreactive alcohols, alkenes, and cyclopropanes through the formation of highly reactive hydrogen-bond networks. [55][56][57][58][59][60] Under these reaction conditions, the ring-opening Friedel-Crafts reaction of most epoxides is found to be stereospecific, and the resulting alcohols can react in a second Friedel-Crafts reaction through the intermediacy of a well-established [61][62][63][64][65][66][67][68][69] but underexploited [70][71][72][73][74][75][76][77][78] phenonium ion without pre-activation of the alcohol. Simple aliphatic alcohols can also undergo a Friedel-Crafts reaction through a rare intermolecular S N 2-type mechanism, as supported by density functional theory (DFT) calculations.…”