Heterolytic S-S bond cleavage by a purely carbogenic frustrated lewis pair

Article abstract of DOI:10.1002/anie.201004149

Boranes were getting borin': Owing to the Lewis acidic character of the central carbon atom, a bisfluorenyl-substituted allene 1 could be used instead of B(C₆F₅)₃ for the generation of frustrated Lewis pairs. Mixtures of the allene and the bulky N-heterocyclic carbene shown in red (Dipp=diisopropylphenyl) caused heterolytic cleavage of the S-S bond in disulfides.

Full text of DOI:10.1002/anie.201004149

Angewandte
Chemie
DOI: 10.1002/anie.201004149
Organic Frustrated Lewis Pairs
ꢀ
Heterolytic S S Bond Cleavage by a Purely Carbogenic Frustrated
Lewis Pair**
Blanca Inꢀs, Sigrid Holle, Richard Goddard, and Manuel Alcarazo*
Dedicated to Professor Eloꢁsa Martꢁn Zamora
Since the discovery by Stephan and co-workers of the unique
ability of frustrated Lewis pairs (FLPs) to activate small
molecules,[1] FLPs have been used to cleave many different
[2]
[3]
[4]
[5]
ꢀ
ꢀ
ꢀ
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bonds, such as H H, C O, C H, S S, and B H
bonds.[6] Furthermore, the type of Lewis bases employed in
FLP chemistry has expanded from the original phosphines to
include amines,[7] pyridines,[3a,8] N-heterocyclic carbenes,[9]
and carbodiphosphoranes.[10] In contrast, the acid partner
has remained limited to polyfluorinated boranes or, in some
exceptional cases, polyfluorinated alanes.[4] Attempts to over-
come this limitation by the use of carbon-based Lewis acids,
for example, trityl cations, were not successful owing to the
strong tendency of this system to undergo nucleophilic
aromatic substitution at a position para to the central
carbon atom.[11]
Scheme 1. Resonance extremes of allene 1 and trityl cation 2 empha-
sizing the different charge distribution.
Herein we describe the reactivity of an electron-poor
allene 1[12] towards different Lewis bases and demonstrate
that the combination of 1 with a very bulky N-heterocyclic
carbene, such as 1,3-bis(2’,6’-diisopropylphenyl)imidazol-2-
ylidene, generates an FLP able to heterolytically cleave the
ꢀ
S S bond in some disulfides. In this remarkable FLP, not only
the Lewis base but also the Lewis acid is carbon-based.
Allene 1 exhibits an intrinsic charge separation that comes
from the natural tendency of the fluorene moieties to accept a
pair of electrons to gain aromatization. In this way, the central
allene carbon atom has partial carbocationic character, and
negative charge is delocalized at both termini.[13] Since this
electronic distribution does not result in the delocalization of
positive charge on the aromatic substituents, undesired
nucleophilic attack by the base, as observed in trityl cations
(e.g. 2), is prevented (Scheme 1).
Scheme 2. Reactivity of allene 1 towards N-heterocyclic carbenes: a) 3,
toluene, ꢀ788C!RT (94%); b) 4, toluene, ꢀ788C!RT (98%); c) 7,
toluene, ꢀ788C!RT.
In early attempts, combinations of 1,3,4,5-tetramethylimi-
dazol-2-ylidene (3) and 1,3-di-tert-butylimidazol-2-ylidene
(ItBu, 4) with allene 1 were investigated as possible FLPs
(Scheme 2). However, in both cases, mixing of the carbene
and the allene in toluene at ꢀ788C generated deep-blue
solutions, from which classical Lewis adducts (5 and 6,
respectively) were isolated (see Figure 1 and the Supporting
Information).[14] In contrast, when 1 was mixed with 1,3-
bis(2’,6’-diisopropylphenyl)imidazol-2-ylidene (IDipp, 7),
NMR spectroscopy indicated no interaction between the
partners and thus the formation of a frustrated Lewis pair.[15]
The Lewis acidity of allene 1 was quantified by the Childs
method with crotonaldehyde as the reference base.[16] The
obtained Dd values rank the acidity of 1 as clearly weaker
than that of polyfluorinated boranes but still of the order of
that of B(OPh)3 (see the Supporting Information).[17] Owing
to the limited Lewis acidity of 1, dihydrogen cleavage was
neither expected nor realized experimentally. However, we
recognized that a weaker nonpolar covalent bond, such as the
[*] Dr. B. Inꢀs, S. Holle, Dr. R. Goddard, Dr. M. Alcarazo
Max-Planck-Institut fꢁr Kohlenforschung
45470 Mꢁlheim an der Ruhr (Germany)
Fax: (+49)208-306-2994
E-mail: alcarazo@mpi-muelheim.mpg.de
[**] We thank Prof. A. Fꢁrstner for generous support and constant
encouragement. The analytical departments of our institute are also
acknowledged for excellent support. B.I. thanks the regional
government of the Basque Country (Spain) for support.
ꢀ
S S bond in disulfides, could in principle be activated by this
system.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201004149.
Angew. Chem. Int. Ed. 2010, 49, 8389 –8391
ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
8389

Communications
yellow solids in 93 and 81% yield, respectively, and the
imidazolium hexafluorophosphate salts 12 and 14 were
isolated in 84 and 76% yield, respectively. The formation of
these compounds (11 and 12 were also characterized crystal-
lographically; Figure 2 and Figure 3) proves the proposed
heterolytic cleavage of disulfides by the FLP formed upon the
mixing of 1 and 7.
Figure 1. Molecular structure of adduct 6 in the solid state. Thermal
ellipsoids are drawn at 50% probability. Hydrogen atoms and solvent
molecules have been removed for clarity.[21]
Figure 2. Molecular structure of 11 in the solid state. Thermal ellip-
soids are drawn at 50% probability. Hydrogen atoms have been
removed for clarity.[21]
To check this hypothesis, we added several disulfides to
mixtures of 1 and 7 in toluene at ꢀ788C (Scheme 3). The
initially orange solutions gradually acquired an intense blue
color upon warming to room temperature. This color is
Scheme 3. Activation of disulfides by an organic frustrated Lewis pair:
a) disulfide (1 equiv), toluene, ꢀ788C!RT, (8: 86%; 9: 91%; 10:
78%); b) HPF6, CH2Cl2.
Figure 3. Molecular structure of 12 in the solid state. Thermal ellip-
soids are drawn at 50% probability. Hydrogen atoms and solvent
molecules have been removed for clarity.[21]
indicative of negative-charge delocalization on the fluorene
rings.[18] Removal of the solvent in vacuo and washing with
pentane afforded compounds 8–10, which were isolated as
foams with an intense blue-violet color. Unfortunately, these
compounds did not crystallize despite many attempts.[19] No
reaction was observed when tBuSStBu was added to the
combination of 1 and 7, probably as a result of the bulkiness of
the tBu groups.[20]
In an effort to obtain further structural evidence to
confirm the composition of these salts, we treated solutions of
compounds 8 and 9 with HPF6 (1 equiv). The addition of
HPF6 led to rapid decolorization of the solutions. In each case,
two main products were isolated by chromatography (see the
Supporting Information). Compounds 11 and 13, formed by
protonation of the bisfluorenyl anion, were obtained as light-
In conclusion, these results demonstrate that a pure
hydrocarbon, such as allene 1, can be used as a Lewis acid in
the framework of FLP chemistry. Even though the grade of
“frustration” attained with combinations of 1 and a bulky
NHC is lower than that observed for some well-established
borane/phosphine pairs, it can be envisaged that modifica-
tions, such as the introduction of electron-withdrawing groups
on the fluorene skeleton, might make it possible to increase
the Lewis acidity of 1 to the level necessary for the activation
of a wider range of molecules. Similarly, the decoration of
allene 1 with bulky substituents may enable the use of other
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Received: July 7, 2010
Published online: September 30, 2010
Keywords: allenes · carbon-based Lewis acids · disulfides ·
.
frustrated Lewis pairs · N-heterocyclic carbenes
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