Frustrated Lewis Pair Mediated 1,2-Hydrocarbation of Alkynes

Article abstract of DOI:10.1002/anie.201705100

Frustrated Lewis pair (FLP) chemistry enables a rare example of alkyne 1,2-hydrocarbation with N-methylacridinium salts as the carbon Lewis acid. This 1,2-hydrocarbation process does not proceed through a concerted mechanism as in alkyne syn-hydroboration, or through an intramolecular 1,3-hydride migration as operates in the only other reported alkyne 1,2-hydrocarbation reaction. Instead, in this study, alkyne 1,2-hydrocarbation proceeds by a novel mechanism involving alkyne dehydrocarbation with a carbon Lewis acid based FLP to form the new C?C bond. Subsequently, intermolecular hydride transfer occurs, with the Lewis acid component of the FLP acting as a hydride shuttle that enables alkyne 1,2-hydrocarbation.

Full text of DOI:10.1002/anie.201705100

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Accepted Article
Title: Frustrated Lewis Pair mediated 1,2-hydrocarbation of alkynes
Authors: Michael Ingleson, Valerio Fasano, James Radcliffe, and Liam
Curless
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Frustrated Lewis Pair Mediated 1,2-Hydrocarbation of Alkynes
V. Fasano,a L. D. Curless,a J. E. Radcliffea and M. J. Inglesona*
Abstract: Frustrated Lewis pair (FLP) chemistry enables a rare
example of alkyne 1,2-hydrocarbation to be achieved using N-Me-
acridinium salts as the carbon Lewis acid. This 1,2-hydrocarbation
process does not proceed via a concerted mechanism as per alkyne
syn-hydroboration, or via an intramolecular 1,3-hydride migration as
operating in the only other reported alkyne 1,2-hydrocarbation
reaction. Instead, in this work alkyne 1,2-hydrocarbation proceeds by
a novel mechanism involving alkyne dehydrocarbation using a
carbon Lewis acid based FLP to form the new C-C bond.
Scheme 2. Stepwise 1,2-hydrocarbation of ynamides by Mayr and co-workers.
Subsequently, intermolecular hydride transfer proceeds, with the
Lewis acid component of the FLP acting as a hydride shuttle that
determined that: (a) alkyne 1,2-hydrocarbation is a stepwise
enables alkyne 1,2-hydrocarbation.
process, (b) that only highly nucleophilic alkynes were amenable,
and (c) that hydrocarbation only occurs when the carbocation
The functionalisation of alkynes using borane Lewis acids is
ubiquitous and best exemplified by the alkyne syn-1,2-
hydroboration reaction (route A, scheme 1).[1] More recent work
has led to the development of other metal free alkyne
hydroboration reactions (e.g. 1,1-hydroboration, route B, and
trans-1,2-hydroboration reactions, route C).[2-5]
formed post 1,3-hydride transfer is significantly stabilized.
Therefore alkyne hydrocarbation reactions are currently limited
and new mechanistic pathways for achieving this conversion are
highly desirable as breakthroughs in this area will help realise a
new way of creating C-C and C-H bonds in one step.
One approach to access novel products from alkynes and
Lewis acids is to react alkynes with Lewis acid / Lewis base
mixtures that form frustrated Lewis pairs (FLPs).[8] This approach
has led to alkyne dehydroelementation (e.g. dehydroboration,
scheme 1 top left) and 1,2-trans-addition of the FLP components
to the alkyne (e.g. scheme 3, top).[9-12] Whilst most FLPs utilise
boron based Lewis acids cationic carbon Lewis acids can be
effective in FLPs.[13] Of specific relevance to this work is the
reaction of alkynes with [Ph3C][BF4] in a FLP with (o-tolyl)3P that
resulted in either intramolecular Friedel-Crafts reactivity or 1,2-
addition to the alkyne (Scheme 3).[14] In these cases, the
absence of a C-H functionality at the electrophilic carbon centre
precludes hydrocarbation reactivity. As part of our studies into
carbon Lewis acid based FLPs[15] we were interested in
determining the reaction outcome(s) from combining the carbon
Lewis acid N-methyl-acridinium ([1]+) with alkynes with and
without additional Lewis bases. This has led to the discovery of
a new FLP mediated route to achieve the 1,2-hydrocarbation of
alkynes. This is distinct to the mechanisms reported by Mayr et
al. and those established in alkyne hydroboration chemistry.
Scheme 1. Metal free alkyne hydroborations. ArF = (2,4,6-(CF3)3-C6H2).
Carbenium ions are isoelectronic to boranes; however, the
metal catalyst free hydrocarbation of alkynes remains extremely
rare in contrast to alkyne hydroboration for reasons discussed in
detail by Mayr and co-workers.[6] 1,2-hydrocarbation involves the
addition of a C-H of a [R2CH]+ electrophile across a triple bond
and thus is distinct to the more common Lewis acid activation of
an alkyne for a subsequent SEAr reaction.[7] To the best of our
knowledge there is only one previous report of alkyne 1,2-
hydrocarbation, specifically the addition of benzhydrylium
cations to ynamides (Scheme 2).[6] This previous study
[*]
V. Fasano, Dr. L. D. Curless, Dr. J. E. Radcliffe and Dr. M. J.
Ingleson*
Scheme 3. 1,2-addition vs. 1,2-hydrocarbation using carbon Lewis acids.
School of Chemistry, University of Manchester,
Oxford Road, Manchester, UK, M13 9PL
E-mail: michael.ingleson@manchester.ac.uk
For alkyne 1,2-hydrocarbation via an analogous stepwise
route to that reported by Mayr and co-workers to be feasible the
cation formed post the 1,3-hydride shift (the hydrocarbation
product, [2]+) has to be more stable than the vinyl cation initially
formed from the interaction of the carbon Lewis acid and the
Supporting information and the ORCID identification number(s)
for the author(s) of this article can be found under
http://dx.doi.org/10.1002/.
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alkyne. Calculations at the M06-2X-6-311G(d,p) level (with PCM
(DCM)) confirmed that this was indeed the case for the
combination of the N-Me-acridinium cation ([1]+) with 4-
ethynylanisole with the 1,2-hydrocarbation product, [2]+, being
energetically favoured over the vinyl cation (Scheme 4).
As no hydrocarbation reaction proceeds in the absence of
2,6-lutidine a concerted mechanism (analogous to syn-1,2-
hydroboration of alkynes with R2BH) and a stepwise mechanism
(analogous to the 1,2-hydrocarbation of ynamides with
benzhydrylium salts) are precluded. The FLP of [1]+ / 2,6-lutidine
conceivably can react with alkynes by a number of pathways
including dehydrocarbation or 1,2-addition. In order to preclude
the 1,2-addition pathway a non-nucleophilic Lewis base was
utilised instead of 2,6-lutidine. On replacing 2,6-lutidine with the
hindered base 2,4,6-tri-tert-butylpyridine (TBP), [2][BArCl] was
again formed in situ as the major product from [1][BArCl] / 4-
OMe
1,2-hydrocarbation
product ([2]+)
vinyl cation
H
H
OMe
H
H
1,3-hydride shift
E = -45.7 kcal mol-1
N
N
o
ethynylanisole (after 72 h at 60 C). The formation of [2][BArCl]
Scheme 4. Energy difference between the vinyl cation (from reaction of [1]+
with 4-ethynyl anisole) and the 1,2-hydrocarbation product [2]+.
using TBP indicates that a deprotonation (dehydrocarbation)
pathway is most likely as alkyne 1,2-addition products are
precluded using this extremely hindered base. The most
probable proton source is the Lewis acid activated alkyne (the
vinyl cation, Scheme 4, left) which on deprotonation would yield
the dehydrocarbation product 4 (Scheme 6) containing a new C-
C bond, along with an equivalent of protonated base. Direct
deprotonation of the alkyne by these two pyridines is not feasible
due to the large difference in pKa values of the pyridiniums and
terminal alkyne (~ 6 and ~25 in water, respectively).
However, the combination of [1][BArCl] ([BArCl] = [B(3,5-
Cl2-C6H3)4]- this anion was chosen as it was an effective
counterion in previous FLP chemistry using [1]+)[15] and 4-
ethynylanisole led to no reaction in dichloromethane (DCM) at
20 oC or on heating at 60 oC (in a sealed tube). This is in
contrast to the reactivity of benzhydrylium salts and ynamides
o
(which proceeds at 20 C) and is presumably due to the lower
electrophilicity of [1]+ (relative to benzhydrylium salts) and the
lower nucleophilicity of 4-ethynyl anisole (relative to ynamides)
resulting in a larger kinetic barrier which prevents hydride
migration in this case despite the favourable thermodynamics.[16]
Furthermore, an attempt to achieve trans-hydrocarbation using
mixtures of [1]+ and N-Me-acridane (as a hydride source) and 4-
ethynylanisole (analogous to pathway C, scheme 1) led to
complex mixtures with minimal hydrocarbation product observed.
In contrast, an equimolar mixture of [1][BArCl], 2,6-lutidine
(which forms an FLP)[15a] and 4-ethynylanisole resulted in a slow
reaction. After 48 hours at 20 oC in DCM only partial
consumption of [1][BArCl] had occurred; nevertheless,
crystallisation (by layering the sample with pentane) and X-ray
diffraction studies revealed formation of the Z-1,2-
hydrocarbation product [3][BArCl] (Scheme 5). Repeating the
reaction at higher temperature (60 oC, 72 hours) accelerated the
Scheme 6. Synthesis of the 1,2-hydrocarbation product [2][BArCl] from 4.
To determine if 4 is indeed a possible intermediate on the
hydrocarbation pathway an equimolar mixture of independently
synthesized 4 and [TBP-H][BArCl] was dissolved in DCM, both
in the presence and absence of 10 mol % [1][BArCl]. The latter
variable was explored as the FLP mediated production of
[2]+/[3]+ is slow therefore any intermediates formed during
alkyne hydrocarbation (such as 4) will exist in solution in the
1
reaction but led to a different product being observed in the H
NMR spectrum consistent with the trans isomer, [2]+ (which has
3
two diagnostic vinylic doublets with a JHH of 16 Hz). The
presence of [1]+ Upon mixing 4, [TBP-H][BArCl] and 10 mol %
.
formation of the E-1,2-hydrocarbation product [2][BArCl] was
confirmed by NMR and mass spectroscopy and X-ray diffraction
studies. It should be noted that: (i) identical outcomes were
observed performing reactions in the dark, and (ii) during these
reactions N-Me-acridane (1-H) plus a new acridinium species
were observed as intermediates.
[1][BArCl] an immediate colour change was noted, with
[1][BArCl] completely converted to 1-H, along with ~ 10 mol %
formation of a new N-methyl-acridinium species, proposed to be
[5]+ (inset, Scheme 6) which was confirmed by subsequent
independent synthesis. Upon heating this reaction at 60 oC,
[2][BArCl] was detected as the major product. Upon repeating
the reaction in the absence of [1][BArCl] conversion to [2][BArCl]
still proceeds although it is notably slower. Therefore, these
results are consistent with the 1,2-hydrocarbation reaction
proceeding via initial dehydrocarbation of the terminal alkyne by
the FLP combination of [1]+ / pyridyl base to form 4. Compound
4 is then converted by reaction with the hydridophilic Lewis acid
[1]+ ultimately to form [2]+. To preclude any hydride transfer
processes mediated by Lewis acidic boranes derived from
decomposition of [BArCl],[17] a different anion was utilised. An
equimolar mixture of 4 and [TBP-H][AlCl4] also led to the
formation of the hydrocarbation product [2][AlCl4] after heating at
60 °C confirming these conversions are not borane mediated.
With 4 being identified as a viable intermediate on the
hydrocarbation pathway we focused our attention on the details
of the hydride transfer / protonation steps that convert 4 to [2]+.
Scheme 5. Synthesis of the alkyne 1,2-hydrocarbation products [2][BArCl] and
[3][BArCl]. Inset the solid-state structures of [2]+ and [3]+.
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The formation of [2]+ could proceed from 4 by concerted
protonation / 1,2-hydride transfer (analogous to pathway B,
scheme 1). However, when the migrating hydrogen in 4 was
replaced with deuterium (Scheme 6), the deuterium was
transferred from position 1 to position 3 precluding a 1,2-hydride
transfer mechanism. Furthermore,
a
related mechanism
involving protonation at position 2 and an intramolecular 1,3-
hydride shift is disfavoured as this would proceed via the original
vinyl cation formed on combination of [1]+ and the alkyne
(Scheme 4, left) and thus would be expected to proceed in the
absence of pyridyl base which is not observed. Compound 4 is
not observed in situ on reacting 4-ethynylanisole with the FLP
2,6-lutidine / [1]+ indicating it is rapidly consumed, presumably by
reaction with [1]+ as discussed above. This would generate an
equivalent of [5]+ and 1-H. The chemical structure of [5]+
Scheme 8. Synthesis of the 1,2-hydrocarbation product [7][BArCl], inset the
solid-state structure of [7]+.
contains an α,β,γ,δ-unsaturated system and is
a Michael
It should be noted that the hydrocarbation product [2]+ is
also a Michael acceptor so could undergo further reduction
initiated by hydride transfer from 1-H. However, this was not
observed in any reaction which is potentially due to the fact that
[1]+ has a higher HIA than [3]+ (HIA = -51 kcal mol-1 in the β and -
52 kcal mol-1 in the δ–position). The lower electrophilicity in the
δ–position of [2]+ relative to that in [5]+ is attributed to the
reduced conjugation between the γ,δ-system and the acridinyl
moiety (in the solid state structure of [2]+ the torsion angle
Cα,Cβ,Cγ,Cδ = 46.0o whereas it will be 0° in [5]+). Combined,
the data indicate that this 1,2-hydrocarbation reaction is the
result of an initial FLP-type process: Lewis acid activation of the
alkyne / deprotonation (dehydrocarbation) to form 4, followed by
stepwise intermolecular hydride transfer / protonation steps to
yield the final products [2]+ or [3]+ with [1]+ acting as an
exogenous Lewis acid to facilitate hydride shuttling (Scheme 9).
The computationally optimized structure of allene 6 reveals that
the least hindered face of the allene would lead, on protonation,
to the Z-isomer [3]+, which is consistent with this being the
kinetic product observed at 20oC. We hypothesize that due to
greater unfavourable steric interactions of the substituents in the
Z-isomer [3]+, heating is sufficient to form predominately the
thermodynamic E-isomer [2]+ (possibly by reversible
protonation/deprotonation of [3]+. Similar reactivity has been
observed for the protonation of allenolates, with the Z-isomer
being the kinetic product and the E-isomer the thermodynamic
one.[19]
acceptor. Therefore it is feasible that 1-H will transfer a hydride
to the δ position of [5]+ which would be consistent with the
deuterium labelling studies and represent an intermolecular
hydride shuttle route to achieve the 1,3-hydride migration. To
gain insight into this hypothesis, the hydride ion affinity (HIA) of
[5]+ (relative to Et3B) was computationally determined (at the
M06-2X/6–311G(d,p) level (DCM solvation (PCM)). The HIA of
[5]+ at the β position (-47 kcal mol-1) is lower than that of [1]+ (-53
kcal mol-1) confirming the intermolecular hydride transfer from 4
to [1]+ is favoured, consistent with the formation of 1-H observed
during the FLP reaction. More notably, the HIA of [5]+ at the δ
position is higher than that at the β position and higher than that
of [1]+. This indicates that the hydride maybe transferred from 4
to the δ position of 5 (yielding allene tautomer 6, inset Scheme
7) in a process mediated by [1]+ (as the direct conversion of 4 to
6 does not proceed). An analogous transformation is reported in
the acid catalyzed rearrangement of tertiary propargyl alcohols
to allenenols (known as Meyer–Schuster rearrangement).[18]
Protonation of 6 would then lead to the observed product, [2]+.
Scheme 7. Hydride ion affinities of [1]+, [2]+ and [5]+, inset compound 6.
In order to assess experimentally the reactivity of [5]+
equimolar amounts of [5][I] and N-methyl-acridane (1-H), both
independently synthesized, were heated in solution at 60 °C.
Interestingly, no formation of the expected product from hydride
transfer, 6, was detected. Instead after 30 hours, the major
product observed was the alkene [7]+ (confirmed
crystallographically) presumably formed by reaction between
allene 6 and the by-product from hydride transfer [1]+ (Scheme
8). Characterisation of [7]+ enabled the minor species observed
in the formation of [2]+/[3]+ from the reaction of [1]+ / 2,6-lutidine /
4-ethynylanisole to be identified as [7]+. This supports the
intermediacy of allene 6 in the FLP reaction to form [2]+/[3]+ and
also indicates that protonation of 6 is favoured over its alkylation.
The latter was confirmed by the fact that adding one equivalent
of [2,6-lutidinium][AlCl4] to [7][I] led to the hydrocarbation product
[2]+ and [1]+. The conversion of [7]+ to [2]+ also suggests
reversibility of the C-C bond formation involving allene 6 and [1]+.
Scheme 9. Proposed mechanism for the formation of the 1,2-hydrocarbation
products [2]+/[3]+/[7]+.
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The rate determining step of the overall reaction appears
to be the dehydrocarbation of the alkyne with the Lewis acid /
Lewis base FLP. This was supported by the fact that on
replacing 4-ethynylanisole with the less nucleophilic alkyne 4-
ethynyltoluene, minimal reactivity was observed with the carbon
Lewis acid based FLP combination after 72 h at 60 oC (Scheme
10, left). However, starting with independently synthesized 8, the
hydrocarbation product [9]+ was observed as the major product
on heating in the presence of the 2,6-lutidinium cation and 10
mol % [1]+ to facilitate hydride transfer. Notably, the conversion
from 8 to [9]+ is significantly quicker when using [1]+ as an
exogenous hydridophilic Lewis acid to mediate the 1,3-hydride
migration than in the absence of [1]+ as observed in the
formation of [2]+ from 4.
data supporting this publication are available as Supporting
Information accompanying this publication.
Conflict of Interest
The authors declare no conflict of interest.
Keywords: 1,2-hydrocarbation • Frustrated Lewis Pairs •
hydride transfer • alkynes • carbocations
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[9]
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Scheme 10. Synthesis of the 1,2-hydrocarbation product [9]+ from 8 and [2,6-
lutidinium]. Inset, crystal structure of [9][AlCl4] (thermal ellipsoids at 50%
probability; [AlCl4]- is omitted for clarity).
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In summary, the first example of alkyne 1,2-hydrocarbation
enabled by FLP chemistry has been presented. It is based on
extending a well-established FLP reaction involving alkyne
activation / deprotonation to carbon Lewis acid based FLPs
which in this case results in dehydrocarbation. This is then
followed by hydride transfer, which is an intermolecular process
enabled by the Lewis acidic component of the FLP acting as a
hydride shuttle. Finally, protonation delivers the hydrocarbation
product. A series of control experiments has disfavoured a
mechanism based on an intramolecular 1,3-hydride migration,
thus this work represent a new alkyne 1,2-hydrocarbation
mechanism that requires a Lewis base and a free Lewis acid.
The ability to circumvent the intramolecular 1,3-hydride
migration step in this hydrocarbation mechanism is particularly
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exothermic) as highlighted by Mayr and co-workers.[6] Therefore,
in previous work slow intramolecular 1,3-hydride migration led to
the carbocation derived from addition of [R2CH]+ to an
alkyne/olefin reacting with an external nucleophile and not
undergoing hydrocarbation.[20] The intermolecular hydride shuttle
mechanism disclosed herein offers the potential to circumvent
the intramolecular 1,3-hydride migration step and make
hydrocarbations more general.
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Acknowledgements
We acknowledge the Royal Society (M.J.I.) for financial support.
We also acknowledge the EPSRC (grant number EP/M023346/1
and EP/K039547/1) for financial support. Additional research
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Entry for the Table of Contents
COMMUNICATION
1,2-hydrocarbation? Yes, please! :
Frustrated Lewis pair (FLP) chemistry
has been employed to achieve a rare
example of alkyne 1,2-hydrocarbation.
The reaction proceeds via a novel
mechanism involving FLP mediated
alkyne dehydrocarbation, followed by
hydride transfer enabled by an
intermolecular hydride shuttle, and
finally protonation.
V. Fasano, L. D. Curless, J. E. Radcliffe,
M. J. Ingleson*
Page No. – Page No.
Frustrated Lewis Pair mediated 1,2-
hydrocarbation of alkynes
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