FULL PAPER
DOI: 10.1002/chem.201202839
Towards a Comprehensive Hydride Donor Ability Scale
Markus Horn, Ludwig H. Schappele, Gabriele Lang-Wittkowski, Herbert Mayr, and
Armin R. Ofial*[a]
Dedicated to Professor Klaus Hafner on the occasion of his 85th birthday
Abstract: Rates of hydride transfer
from several hydride donors to benzhy-
drylium ions have been measured at
208C and used for the determination of
empirical nucleophilicity parameters N
and sN according to the linear free
energy relationship logk208C =sNACHTNUTRGNEUG(N N+E).
Comparison of the rate constants of
hydride abstraction by tritylium ions
with those calculated from the reactivi-
ty parameters sN, N, and E showed fair
agreement. Therefore, it was possible
to convert the large number of litera-
ture data on hydride abstraction by tri-
tylium ions into N and sN parameters
for the corresponding hydride donors,
and construct a reactivity scale for hy-
dride donors covering more than 20
orders of magnitude.
Keywords: carbocations · hydride
transfer · kinetics · linear free ener-
gy relationships · reactivity parame-
ters
Introduction
ing abilities based on kinetic data has not been report-
ed.[13,14] Because of the availability of a large number of po-
tential hydride donors, an ordering system concerning their
abilities to deliver hydride anions would be useful for de-
signing synthetic strategies, and would furthermore help to
understand how structure determines reactivity.
The linear free energy relationship [Eq. (1)], in which E is
an electrophilicity, N is a nucleophilicity, and sN is a nucleo-
phile-dependent sensitivity parameter, was introduced in
1994,[15] and has since been shown to be useful in predicting
bimolecular rate constants for many electrophile–nucleo-
phile combination reactions.[16,17]
Hydride transfers play an important role in organic chemis-
try.[1] They are key steps in the Cannizzaro, Meerwein–Ponn-
dorf–Verley, Oppenauer, Tishchenko, Leuckart–Wallach,
and Sommelet reactions, and the conversion of ketones and
aldehydes into alcohols by use of metal hydrides such as
LiAlH4 or NaBH4 is one of the most important methods in
the chemistꢀs toolbox.[2] Hydride-transfer reactions are also
involved in biochemical redox processes, for example, in the
NAD(P)H/NAD(P)+ couple,[3] and are found in initiation,
chain transfer, and termination reactions of cationic poly-
merizations.[4,5]
ꢀC
log k20 ¼ sNðN þ EÞ
ð1Þ
A variety of organic molecules can be oxidized by carbo-
cations, typically tritylium (Ph3C+) or tropylium ions
+
Nucleophiles as different as alkenes, arenes, allyl metal
and diazo compounds, amines, carbanions, phosphanes,
halide anions, alcohols, and ylides have been characterized
by the nucleophile-specific parameters N and sN, which have
been derived from the kinetics of their reactions with benz-
hydrylium ions and quinone methides (defined as reference
electrophiles). The nucleophilicity scale thus established
presently covers a reactivity range of 35 orders of magni-
tude, from Nꢁꢂ5 for non-activated arenes to Nꢁ30 for
cyano-substituted benzyl anions.[18]
Nucleophilicity parameters of various hydride donors
(e.g., dihydropyridines, hydrocarbons, silanes, germanes,
stannanes, and borohydrides) have already been derived
from the rates of their reactions with benzhydrylium
ions.[16,19–21] Several of these hydride-transfer reactions have
been shown to follow a polar mechanism,[20,22] though hy-
dride transfer by a sequence of electron-proton-electron
transfers cannot be excluded in all cases. The previously re-
ported rate constants for reactions of hydride donors with
benzhydrylium ions are collected in Table 3 (see later) and
(C7H7 ), that can act as one-electron oxidants or hydride ac-
ceptors in these reactions.[6] Synthetic applications of such
hydride-transfer reactions to carbocations are, for example,
oxidation of polycyclic hydroaromatic compounds,[7] carbo-
cation-mediated oxidative a-cyanation of tertiary amines,[8,9]
or the use of carbocations as hydride acceptors for the gen-
eration of cationic metal-complexed p systems[10] or silylium
ions.[11]
Thermodynamic studies of hydride affinities have been
pioneered by Parker and greatly extended by Cheng and
Zhu,[12] but a comprehensive comparison of hydride-donat-
[a] Dr. M. Horn, Dr. L. H. Schappele, Dr. G. Lang-Wittkowski,
Prof. Dr. H. Mayr, Dr. A. R. Ofial
Department Chemie, Ludwig-Maximilians-Universitꢁt Mꢂnchen
Butenandtstrasse 5-13 (Haus F), 81377 Mꢂnchen (Germany)
Fax : (+49)89-2180-77717
Supporting information for this article is available on the WWW
Chem. Eur. J. 2013, 19, 249 – 263
ꢃ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
249