Contrasting Frustrated Lewis Pair Reactivity with Selenium- and Boron-Based Lewis Acids

Article abstract of DOI:10.1002/anie.201605239

The activation of π-bonds in diynyl esters has been investigated by using soft and hard Lewis acids. In the case of the soft selenium Lewis acid PhSeCl, sequential activation of the alkyne bonds leads initially to an isocoumarin (1 equiv PhSeCl) and then

Full text of DOI:10.1002/anie.201605239

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Communications
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International Edition: DOI: 10.1002/anie.201605239
German Edition: DOI: 10.1002/ange.201605239
Heteroannulation
Contrasting Frustrated Lewis Pair Reactivity with Selenium- and
Boron-Based Lewis Acids
Lewis C. Wilkins, Benjamin A. R. Gꢀnther, Melanie Walther, James R. Lawson, Thomas Wirth,
and Rebecca L. Melen*
Abstract: The activation of p-bonds in diynyl esters has been
investigated by using soft and hard Lewis acids. In the case of
the soft selenium Lewis acid PhSeCl, sequential activation of
the alkyne bonds leads initially to an isocoumarin (1 equiv
PhSeCl) and then to a tetracyclic conjugated structure with the
isocoumarin subunit fused to a benzoselenopyran (3 equiv
PhSeCl). Conversely, the reaction with the hard Lewis acidic
borane B(C6F5)3 initiates a cascade reaction to yield a complex
p-conjugated system containing phthalide and indene subunits.
which can occur in an intra- or intermolecular fashion.[3] Two
different mechanisms for the 1,2-addition of Lewis acid/base
combinations to alkenes and alkynes have been proposed,
which either involve concerted addition or initial activation of
the p-bond followed by 1,2-addition.[4] In the latter case,
p···Lewis acid van der Waals interactions have been
observed.[4] However, unlike transition metals, which are
able to activate p-bonds in a synergic fashion through (metal–
ligand) bonding and back-bonding interactions, boron relies
solely on the vacant pz orbital to activate such unsaturated
frameworks. In these reactions, the nucleophilic center can be
a wide variety of atoms including hydrogen,[5] carbon,[6]
nitrogen,[7] oxygen,[8] phosphorus,[9] or sulfur,[10] while the
Lewis acid component has been limited to the first two rows
of the p-block. Typically, hard Lewis acids such as boranes,
alanes, or borenium and carbenium cations have been
employed.
T
he advent of frustrated Lewis pairs (FLPs) a decade ago has
been a major advance in main-group chemistry, particularly in
metal-free catalysis.[1] FLP chemistry combines sterically
hindered Lewis acids and Lewis bases (Scheme 1, top),
Other reactivities associated with the use of strong boron
Lewis acids involve the synthetically useful carboboration
reactions as well as relevant cascade reactions of unsaturated
À
C C bonds. Recent studies by the Erker and Yamaguchi
research groups showcase the use of B(C6F5)3 in a series of
rearrangements of aryl-functionalized diynes to form com-
plex, extensively fused polyaromatic systems such as diben-
zopentalenes and derivatives thereof.[11] The aforementioned
activation of alkynes is preeminent in this work, highlighting
the fact that, contrary to the traditional concept of HSAB
theory, hard Lewis acids are indeed very effective for the
activation of soft Lewis bases such as alkynes. It is of note that
the synthesis of such polyaromatic systems is seldom
described in the literature, with these reports involving rare,
noble, or heavy toxic metals such as palladium, gold, or tin.[12]
This provides the impetus to find new synthetic methods to
prepare such extended conjugated networks.
Scheme 1. Frustrated Lewis pairs (top) and 1,2-addition of FLPs to
alkynes (bottom).
where the steric demands of the two components suppress
conventional adduct formation and lead to truly unique
reactivity. This ever-expanding field of chemistry now encom-
passes a vast range of Lewis acid/base combinations which
have been shown to effect novel transformations and metal-
free catalytic processes, including small-molecule activation
reactions (e.g. H2, N2O, CO2, CO, SO2, alkenes, and
alkynes).[2]
Indeed, with a few exceptions,[13] the use of heavier p-
block elements with FLP-type reactivity is rarely reported.
These heavier p-block elements are much softer due to their
larger atomic radii and lower charge densities, and are more
susceptible to polarization, thus potentially offering distinct
differences in reactivity compared to their lighter counter-
parts. Many of these elements are also capable of acting
simultaneously as Lewis acids (because of the presence of
low-lying vacant orbitals or s*-orbitals) and Lewis bases
(because of the presence of lone pairs of electrons when in the
nÀ2 or nÀ4 oxidation state). Here we compare the reactivity
of methyl-2-{[2-(phenylethynyl)phenyl]ethynyl} benzoate (1)
with the soft Se-centered Lewis acid PhSeCl and the hard B-
centered Lewis acid B(C6F5)3 in the pursuit of obtaining novel
heterocyclic frameworks such as isocoumarin derivatives.
A particular theme within this area is the cooperative
À
addition of FLPs across C C p-bonds (Scheme 1, bottom),
[*] L. C. Wilkins, B. A. R. Gꢀnther, M. Walther, Dr. J. R. Lawson,
Prof. T. Wirth, Dr. R. L. Melen
School of Chemistry, Cardiff University
Main Building, Park Place, Cardiff, Cymru/Wales, CF10 3AT (UK)
E-mail: MelenR@cardiff.ac.uk
Supporting information for this article can be found under:
http://dx.doi.org/10.1002/anie.201605239.
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Indeed, isocoumarins are well known for their biological
activity and their derivatives have many applications as
antimicrobial,[14] anticancer,[15] and anti-inflammatory[16]
agents etc.[17]
nium center and 2) a neutral selane. The counterion is
a [PhSeCl2]À anion.
The tetracyclic core of 2 consists of a planar isocoumarin
fragment fused to the isoselenochromene as a result of the
annulation step. The selenonium cation adopts the expected
The diyne starting material 1 was prepared by a palla-
dium-catalyzed cross-coupling reaction according to litera-
ture procedures.[18] The subsequent reactivity of 1 in the
presence of excess phenylselenyl chloride (in a 1:3 molar
ratio) in CDCl3 was monitored by multinuclear NMR
spectroscopy. After a few hours at ambient temperature, the
1H NMR spectrum showed the formation of a new compound
with only aromatic protons, thus indicating the loss of the
methyl ester. This observation was confirmed by slow
evaporation of the solvent to yield a crop of orange crystals
(37%), which were characterized by single-crystal X-ray
diffraction and unequivocally confirmed the product to be the
salt 2 (Scheme 2, Figure 1). The cationic fragment contains
two different selenium environments: 1) a cationic seleno-
À
À
tetragonal geometry, with C Se C bond angles between
À
96.57(12)8 and 100.86(12)8 and with Se C bonds having single
bond character (1.912(2)–1.939(3) ꢀ). These observations
indicate that the delocalization of the isocoumarin unit does
not fully extend throughout the isoselenochromene fragment.
Indeed, the lack of planarity across these two moieties further
substantiates this observation. The counteranion [PhSeCl2]À
adopts a T-shaped geometry with the chlorine atoms occupy-
À
ing the axial positions. The Se Cl bond lengths (2.445(1) and
À
À
2.457(1) ꢀ) and Cl Se C bond angles of 90.47(10)8 and
90.72(10)8 are similar to those described for the only
previously reported [PhSeCl2]À anion.[19]
The reaction was then performed with 1:1 and 2:1 molar
ratios of PhSeCl/1. When a 1:1 molar ratio was used, selective
activation of the alkynyl functionality proximal to the ester
occurred through a 6-endo-dig cyclization to afford the
phenylselenyl-substituted isocoumarin 3a (Scheme 2), which
was isolated in 68% yield. The product was fully character-
ized by multinuclear NMR spectroscopy and mass spectrom-
etry. However, despite repeated attempts, we were unable to
grow single crystals of 3a suitable for single-crystal X-ray
diffraction. The 77Se NMR spectrum of compound 3a showed
a singlet whose chemical shift (d = 289.5 ppm) was in agree-
ment with previously reported selanyl compounds.[20]
To probe this reactivity further, multinuclear NMR
spectroscopic studies were undertaken to follow the reaction
of compound 3a with two further equivalents of PhSeCl.
These studies revealed 1H NMR signals corresponding to the
formation of 2, thus suggesting that 3a is an intermediate
en route to 2. A plausible mechanism for the reaction of 1 to 2
via 3a is the electrophilic addition of PhSeCl to the alkyne in
1 with formation of a selenirenium cation. This then under-
goes an intramolecular ring-opening reaction by nucleophilic
attack from the carbonyl oxygen atom of the ester moiety,
thereby resulting in isocoumarin 3a with a concomitant loss of
Scheme 2. Reactions of 1 with phenylselenyl chloride (PhSeCl) to
generate 2 and 3.
1
chloromethane, as observed in the in situ H and 13C NMR
spectra (d = 3.05 and 21.5 ppm, respectively).[21] There is some
literature precedent for such selenolactonization reactions.[22]
The reaction of intermediate 3a to give the zwitterion 2 may
be envisaged as an FLP-type trans-1,2-addition of the Lewis
basic selane in 3a and a second equivalent of the Lewis acidic
PhSeCl across the second alkyne (again via the three-
membered selenirenium cation). The chloride ion that is
eliminated following the 1,2-addition is intercepted by the
third equivalent of PhSeCl to generate the selenium-contain-
ing counterion [PhSeCl2]À.
Attempts to promote the second cyclization of 3a using
the harder Lewis acid B(C6F5)3 failed to lead to the tetracyclic
structure reminiscent of 2. Instead, this afforded the Lewis
acid/base adduct (3b, Scheme 2), thus reflecting the hard
nature of the lone pair of electrons on the O atom relative to
the softer alkyne. Indeed, the structure of the Lewis adduct 3b
was unambiguously determined by X-ray diffraction
(Figure 2). Such adduct formation has previously been
Figure 1. Solid-state structure of 2. The chloroform solvate molecule is
removed for clarity.
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conjugated compound 4 (Scheme 3, Figure 3). In this case, an
FLP domino reaction occurs in which the sterically encum-
bered borane activates the less hindered alkyne, thus
prompting nucleophilic attack from the more protected
alkyne, which is itself attacked by the nucleophilic ester
moiety. Interestingly, the formation of 4 from the cyclization
of 1 with the Lewis acid B(C6F5)3 is in stark contrast to the
formation of 2 from 1 by using PhSeCl. Although bio- and
biomimetic domino reactions are often acid-catalyzed, the use
of boron compounds in Lewis acid mediated annulation
reactions is comparatively rare and remains largely unex-
plored.[24,25]
The structure of 4 consists of an isobenzofuran bicycle
connected by a double bond to an indene moiety. Although
the isobenzofuran heterocycle and the indene ring are
conjugated, they do not lie perfectly in the same plane in
the solid state, with the angle between the two rings being
17.99(11)8. The fact that this system does not lie in the same
plane clearly reflects the steric constraints in the molecule,
which is also revealed in the NMR data. In addition, the
phenyl substituent in 4 also rotates out of the plane of the
indene ring by 67.87(10)8, thus permitting it to adopt a near
coplanar geometry (12.52(9)8) with one of the C6F5 rings of
the B(C6F5)3 moiety with a centroid···centroid distance of just
3.4785(16) ꢀ. This value is somewhat shorter than that
observed for other C6F5···C6H5 contacts (3.6421(10) ꢀ).[3e]
The 11B NMR chemical shift for 4 in solution occurs at d =
À15.8 ppm as a very sharp singlet, which is synonymous with
four-coordinate vinyl borate species.[3] Conversely, the
19F NMR spectrum is more complex and indicates chemical
inequivalence of the three C6F5 rings, thus suggesting some
degree of restricted rotation around the boron–indene bond
brought about by the coordination of B(C6F5)3 to the bulky
organic fragment in 4. Three discrete resonances are observed
in the 19F NMR spectrum for the para- and meta-fluorine
atoms: d = À160.7 ppm (para-F) and d = À165.5 (meta-F). In
addition, the ortho-positions within each C6F5 ring are also
inequivalent, thereby resulting in six distinguishable signals
between d = À123.0 and À136.1 ppm. This steric hindrance is
also observed in the space-filling model (see the Supporting
Information), which clearly shows a very sterically encum-
bered boron center in which the arene ring of the indene unit
interlocks between two C6F5 rings.
Figure 2. Solid-state structure of 3b.
observed using extended-chain alkynyl benzoates, where
adduct formation is also found to be preferred over alkyne
activation.[23]Indeed, no evidence for alkyne activation was
observed in 3b by 11B NMR spectroscopy, even after heating
in toluene at 1008C for several days.
To compare the Lewis acidity of PhSeCl with more
conventional Lewis acids, we investigated the reaction of
1 with B(C6F5)3. The stoichiometric reaction of B(C6F5)3 with
1 (1:1) in toluene resulted in an immediate color change to
a dark-green solution, which afforded block-shaped red
crystals (84%) upon standing for several hours. Single-crystal
X-ray diffraction analysis revealed the product to be the
Scheme 3. Reaction of 1 with B(C6F5)3.
In conclusion, the reactions of soft and hard Lewis acids
with dienyl esters show contrasting reactivity that is unpre-
cedented in FLP chemistry. The divergent reactivity of these
systems allows novel heterocyclic systems to be generated
that incorporate main-group elements. To date, the addition
reactions of FLPs to alkynes have been limited to hard Lewis
acids, with the use of softer Lewis acids in FLP-type chemistry
being comparatively rare. Indeed, we are continuing to
explore the utility of softer Lewis acids in FLP-type reactions.
Acknowledgments
B.A.R.G. and M.W. are grateful to ERASMUS + for funding.
R.L.M. would like to acknowledge the EPSRC (grant number
Figure 3. Solid-state structure of 4. The disordered toluene solvent is
omitted for clarity.
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Keywords: annulation · cyclization · domino reactions ·
frustrated Lewis pairs · main-group elements
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Received: May 29, 2016
Published online: && &&, &&&&
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Communications
Heteroannulation
Making a difference: The activation of p-
bonds in diynyl esters by soft and hard
Lewis acids has been investigated in
a one-pot reaction. The soft selenium
reagent PhSeCl affords a salt containing
three distinct selenium environments,
while the hard Lewis acid borane B(C6F5)3
yields a complex conjugated zwitterionic
system in a domino reaction.
L. C. Wilkins, B. A. R. Gꢁnther,
M. Walther, J. R. Lawson, T. Wirth,
R. L. Melen*
&&&& — &&&&
Contrasting Frustrated Lewis Pair
Reactivity with Selenium- and Boron-
Based Lewis Acids
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