DOI: 10.1002/chem.201403161
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Ambident Reactivity
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,* and Herbert Mayr[a]
Dedicated to Professor Hans-Ulrich Reißig on the occasion of his 65th birthday
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 lgk=sN(N+E), the nucleophile-specific
reactivity parameters N and sN for the carbanions 1 could be
determined. In contrast, trityl cations Ar3C+ reacted differ-
ently. While tritylium ions of low electrophilicity (E<ꢀ2) re-
acted with 1 through rate-determining b-hydride abstrac-
tion, more Lewis acidic tritylium ions initially reacted at the
carbonyl oxygen of 1 to form trityl enolates, which subse-
quently reionized and eventually yielded triarylmethanes
and 5-benzylidene Meldrum’s acids by hydride transfer.
Introduction
of the alkylation reactions were mostly interpreted on the
basis of the principle of hard and soft acids and bases (HSAB
principle),[21] which predicts O-alkylation with hard and C-alkyl-
ation with soft electrophiles. Because the HSAB principle nei-
ther differentiates between kinetic and thermodynamic control
nor considers the diffusion limit, it often fails to predict the
correct regioselectivity.[22] An alternative treatment of ambident
reactivity based on Marcus theory has, therefore, been pro-
posed and applied to reactions of enolate ions.[23]
Carbonyl-substituted carbanions (enolates) or related organo-
metallic species are usually generated with the aim to make
use of their reactivity at carbon, for example, for CꢀC bond
forming reactions.[1] In order to quantify the nucleophilic
carbon reactivity of enolate anions, we have previously investi-
gated the kinetics of their reactions with benzhydrylium ions
(Ar2CH+) and structurally related quinone methides. Analysis of
the second-order rate constants (k) of these reactions by the
correlation Equation (1),[2–6] in which E describes the reactivity
of the reference electrophiles, provided the nucleophile-specif-
ic, solvent-dependent nucleophilicity parameters N (and sensi-
tivities sN) for enolates and other types of acceptor-stabilized
carbanions in DMSO,[7–17] water,[9,10] and methanolic solu-
tions.[11,18]
A third reaction channel exists for enolate ions that carry
hydrogen atoms in b-position to the carbonyl group. As de-
scribed for several examples, such enolates may react with car-
bocations or Michael acceptors by b-hydride transfer to give
olefins.[24]
We now report that carbanions derived from 5-benzylated
Meldrum’s acid (1)[25,26] react with carbocations through all
three mentioned paths (Scheme 1), which makes kinetic stud-
ies of their reactivity rather challenging. By using benzhydryli-
um ions and tritylium ions as electrophilic probes it has been
possible to investigate independently the rates of CꢀC bond
formation and hydride transfer of Meldrum’s acid derived car-
banions 1 and to assign reactivity parameters N and sN to the
different positions.
A set of benzhydrylium ions, Ar2CH+, served as reference
electrophiles[27] for the determination of the carbon reactivity
of 1. Tritylium ions (Ar3C+) are sterically more demanding and
cannot react with C-5 of the Meldrum’s acid moiety of 1. Yet,
they have recently been shown to act as reliable reference
electrophiles for the determination of hydride donating abili-
ties of several types of hydride donors,[28] which made them
promising candidates to study the hydride transfer rates of car-
banions 1. Both types of carbocations may alternatively attack
lg k20 o ¼ sNðN þ EÞ
ð1Þ
C
As ambident nucleophiles, enolates can alternatively under-
go O-alkylation with formation of enol ethers, a reaction
course that is frequently used for the protection of carbonyl
groups.[19] The site of attack depends on the structure of the
enolate as well as on the nature of the electrophile, the sol-
vent, and the counterion.[20] In the past, the regioselectivities
[a] Dr. X. Chen,+ Dr. Y. Tan,+ Dr. G. Berionni, Dr. A. R. Ofial, Prof. Dr. H. Mayr
Department Chemie, Ludwig-Maximilians-Universitꢀt Mꢁnchen
Butenandtstr. 5–13, 81377 Mꢁnchen (Germany)
[+] These authors contributed equally to this work.
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201403161.
Chem. Eur. J. 2014, 20, 1 – 10
1
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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