Characterization of the Zwitterionic Intermediate in 1,1-Carboboration of Alkynes

Article abstract of DOI:10.1002/anie.202003468

The reaction of a Lewis acidic borane with an alkyne is a key step in a diverse range of main group transformations. Alkyne 1,1-carboboration, the Wrackmeyer reaction, is an archetypal transformation of this kind. 1,1-Carboboration has been proposed to proceed through a zwitterionic intermediate. We report the isolation and spectroscopic, structural and computational characterization of the zwitterionic intermediates generated by reaction of B(C₆F₅)₃ with alkynes. The stepwise reactivity of the zwitterion provides new mechanistic insight for 1,1-carboboration and wider B(C₆F₅)₃ catalysis. Making use of intramolecular stabilization by a ferrocene substituent, we have characterized the zwitterionic intermediate in the solid state and diverted reactivity towards alkyne cyclotrimerization.

Full text of DOI:10.1002/anie.202003468

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Accepted Article
Title: Characterization of the zwitterionic intermediate in 1,1-
carboboration of alkynes
Authors: Alessandro Bismuto, Fernanda Duarte, Gary S. Nichol,
Michael J. Cowley, and Stephen Thomas
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10.1002/anie.202003468
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Characterization of the zwitterionic intermediate in 1,1-
carboboration of alkynes
Alessandro Bismuto,[a] Gary S. Nichol,[a] Fernanda Duarte,[b]* Michael J. Cowley, [a]* Stephen P.
Thomas[a]*
[a]
Dr A, Bismuto, Dr G. S. Nichol, Dr M.J. Cowley, Dr S. P. Thomas
aEaStCHEM, School of Chemistry, University of Edinburgh, Joseph Black Building, David Brewster Road, Edinburgh, EH9 3FJ.
E-mail: Michael.Cowley@ed.ac.uk, Stephen.Thomas@ed.ac.uk
[b]
Dr F. Duarte
bChemistry Research Laboratory, University of Oxford, 12 Mansfield Road, Oxford, OX1 3TA, UK
E-mail: Fernanda.DuarteGonzalez@chem.ox.ac.uk
Experimental procedure and DFT coordinates provided via the link in the Supporting information.
Abstract: The reaction of a Lewis acidic borane with an alkyne is a
key step in a diverse range of main group transformations. Alkyne 1,1-
carboboration, the Wrackmeyer reaction, is an archetypal
transformation of this kind. 1,1-Carboboration has been proposed to
proceed through a zwitterionic intermediate. We report the isolation
and spectroscopic, structural and computational characterization of
the zwitterionic intermediates generated by reaction of B(C6F5)3 with
alkynes. The stepwise reactivity of the zwitterion provides new
mechanistic insight for 1,1-carboboration and wider B(C6F5)3 catalysis.
Making use of intramolecular stabilization by a ferrocene substituent,
we have characterized the zwitterionic intermediate in the solid state
and diverted reactivity towards alkyne cyclotrimerization.
The use of main group species has garnered significant interest
as sustainable catalysis alternatives.[1] Lewis acidic boranes,
particularly tris(pentafluorophenyl)borane (B(C6F5)3), have been
extensively investigated and the reaction of B(C6F5)3, and related
boranes, with an alkyne is key to a number of reactivity pathways
including frustrated Lewis pair chemistry[2] and the Wrackmeyer
Scheme 1. A) Wrackmeyer reaction and FLP chemistry; B) 1,1-Carboboration
of alkynes. C) This work.
reaction[3,4] (Scheme 1, A).
1,1-Carboboration was first reported by Wrackmeyer using
silyl- or stannyl-substituted alkynes.[3,4] The substrate scope was
We began by examining the stoichiometric reaction of
significantly expanded using strongly Lewis acidic boranes
phenylacetylene and B(C6F5)3. Rapid combination of
phenylacetylene and one equivalent of B(C6F5)3 in toluene-d8 at
195 K produced an intensely red solution.[13] On warming to 233
K, a new signal in the 1H NMR spectrum at δ 5.41 was observed,
which was attributed to the alkenyl proton of the zwitterion 2a.
13C{1H} NMR revealed a distinctive downfield signal at δ 199.3,
typical of alkenyl-cations.[14] A 1H-13C HMBC experiment showed
a correlation between these two signals (Figure S1). The
expected high-field chemical shift for the four-coordinate boron
center was observed at δ –12.1 in the 11B NMR spectrum, which
[5,6]
(Scheme 1, B).
Computational studies suggest that 1,1-
carboboration using B(C6F5)3 proceeds by alkyne coordination to
B(C6F5)3 to form a zwitterion I, a σ-adduct of the alkyne and
B(C6F5)3.[7-9] 1,2-Hydride shift (Scheme 1, B; R2 = H) and aryl
migration from boron to carbon gives the alkenyl borane. Direct
experimental evidence for this mechanism is scant; only tentative
evidence from NMR spectroscopy for the zwitterionic intermediate
I has been reported.[10]
B(C6F5)3 is by far the most widely used acid component in
FLPs, enabling the synergistic activation of small molecules and
unsaturated bonds.[2] Stephan[11] and Erker,[7,12] reported the
reactivity of alkynes towards B(C6F5)3/phosphine FLPs, which
result in the apparent trapping of the σ-adduct I by the Lewis
base.[12b]
1
correlated with the alkenyl proton by H-11B HMBC. Using 13C-
labelled (PhC≡13CH), we observed a quartet at δ 51.2 by 13C NMR
spectroscopy, assigned as the boron-coordinated C1 of the
zwitterion 2a (1JB-C = 44 Hz). A corresponding doublet (δ –12.1,
1JB-C = 44 Hz) was observed in the 11B NMR spectrum (Figure 1).
A DEPT 135 13C NMR spectrum confirmed the signal at δ 199.3
was C2 (Figures S2 and S3).[13] Even at 233 K, significant
quantities of unreacted phenylacetylene and B(C6F5)3 remain,
indicating that the zwitterion 2a may exist in equilibrium with the
Here, we report the isolation and full spectroscopic and
structural characterization of the zwitterionic intermediate in
B(C6F5)3-mediated 1,1-carboboration.
1
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starting materials. Upon warming above 243 K, rearrangement to
the 1,1-carboboration product 3a was observed,[8] alongside
unidentified side products.
PBED3BJ/6-31G*) were performed. Both species were found to
be stable minima and slightly higher in energy than the starting
materials (8.5 and 4.0 kcal mol-1, respectively), consistent with
previous work (Figure 1C).[7-9] A transition state connecting 2a and
2b to the experimentally observed 1,1-carboboration products
was also obtained (Figure S5). NBO analysis revealed the
expected decreased C–C Wiberg bond orders in 2a and 2b when
compared to the free alkynes (2a: 2.22; 2b: 2.15). The observed
B–C bond order increased with the addition of the para-OMe
group of 2b (2a: 0.65; 2b: 0.71). Correspondingly, the B–C bond
distance in 2a was longer than that in 2b (1.75 vs 1.71 Å). NPA
analysis reveals significant build-up of positive charge at the two-
coordinate carbon center C2, confirming the cationic character
(Figure 1C, NPA charges, 2a: 0.35; 2b: 0.29).
With solution-state evidence for intermediate 2a, we sought
a more stable analogue. We rationalized that an electron-
donating substituent would improve the zwitterion stability.
Reaction of p-methoxy phenylacetylene with B(C6F5)3
immediately gave the characteristic resonances of a zwitterionic
intermediate 2b, as observed by 1H, 13C, and 11B NMR
spectroscopy (Figure 1B and S4). Intermediate 2b was more
stable than the unsubstituted analogue with no 1,1-carboboration
observed below room temperature, and the zwitterion 2b
persisted for up to 2 days at room temperature. The 13C chemical
shifts of the two-coordinate C2 centers of 2a and 2b (2a δ 199.3;
2b δ 193.5) are typical for alkenyl-cations,[14a-c] and the lower
chemical shift of the methoxy-substituted zwitterion 2b is
consistent with the presence of the more strongly electron-
donating substituent.
Phosphine, amine and B(C6F5)3 FLPs have been reported to
activate alkynes to 1,2-addition of the Lewis acid and base to form
zwitterionic borates.[2a,11] Stephan speculated that the mechanism
of this reaction involved an initial π-interaction of the alkyne with
B(C6F5)3 which enabled nucleophilic attack of the
amine/phosphine. Consistent with previous findings, generation
of the zwitterion in the presence of Lewis base 1,4-
diazabicyclo[2.2.2]octane (DABCO) gave a mixture of the trapped
adducts 4a and 4b (50% combined yield after 24 h). Solution
phase characterization of the DABCO adducts 4a and 4b showed
they exist in equilibrium.[7] Consistent with this, isolated acid-base
pair 4b slowly converted to a mixture of adduct 4a and acid-base
pair 4b in CD2Cl2 (Figure S7).
Scheme 2. Trapping DABCO adduct 4.
Figure 2. Crystal Structure of the zwitterion (4a). Ellipsoids are set to 50%
probability; hydrogen atoms are omitted for clarity. Selected bond lengths (Å)
and angles [º]: B(1)–C(1) 1.630(4), C(1)–C(2) 1.324(4), N(1)–C2) 1.534(3);
B(1)–C(1) –C(2) 128.4(2), C(1)–C(2)–N(1) 121.1(2).
Figure 1. A) Reaction conditions and NMR spectroscopy for the generation of
2a. B) Conditions for the generation of 2b. C) Optimized structures, NPA
charges (red/blue text), and Wiberg bond orders for 2a and 2b. Geometry
optimizations and NBO calculations were carried out at the SMD[toluene]-
PBED3BJ/6-31G*
PBED3BJ/6-31G* levels of theory, respectively).
and
SMD[toluene]-M06-2X/6-311+G**//SMD[toluene]-
Although the NMR spectroscopic evidence for the formation
of zwitterions 2a and 2b was convincing, we were unable to
isolate single crystals of these species. Ferrocenyl substituents
have been used to stabilize carbocations[15-17] and silyl cations.[18-
The formation of 2a has previously been predicted on the
basis of density functional theory (DFT) calculations.[7-9] To gain
additional insights into the structure of 2a and 2b, DFT
calculations (SMD[toluene]-M06-2X/6-311+G**//SMD[toluene]-
20]
Furthermore, short-lived alkenyl cations have been
characterized by NMR spectroscopy.[21] We therefore postulated
that a zwitterion generated from ethynyl ferrocence and B(C6F5)3
may be stable.
2
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Ethynylferrocene was reacted with B(C6F5)3 at 243 K, and the
reaction monitored by NMR spectroscopy. Consumption of the
starting materials and formation of the zwitterionic alkenyl cation
1.286(5) Å, significantly longer than that observed for
ethynylferrocene (1.17(3) Å),[22] comparable to that of the DABCO
adduct 4a (1.324(4) Å), and consistent with a reduction in bond
1
1
2c was observed as the major initial product (see later) by H
order and the observed reduction in JC-C coupling. The three
NMR spectrum (Scheme 3). The alkenyl proton was assigned to
a resonance at δ 6.92, which correlated with signals in the 13C
NMR spectrum at δ 107.8 and 186.7 in HSQC and HMBC
experiments. The signal at δ 107.8 was assigned to the three-
coordinate alkenyl carbon bound to boron. The signal at δ 186.7
was assigned to the cationic carbon (Figure S8). Although at
much higher field than the signals observed for the
phenylacetylene or methoxyphenylacetylene adducts 2a and 2b,
the chemical shift is consistent with previously reported ferrocenyl
coordinate C1 adopts the expected trigonal planar geometry (sum
of angles at C1 = 360.0(3)º) and the B1-C1 bond distance is
1.654(5) Å, also comparable to that in 4a (1.630(4) Å).
The relatively short C(3)–C(2) bond length (1.379(5) Å) and
the Fe(1)–C(2) distance (2.379(4) (Å)) indicate some charge
delocalization to the ferrocene motif. Consistent with this, the tilt
angle of the Cp rings in the ferrocene unit is 11.13(4)º. The dip
angle of the alkenyl substituent from its Cp ring, α*, is 36.1(4)º,
which is significantly greater than that reported for the
diphenylferrocenylcarbenium ion, and begins to approach those
reported for ferrocenylsilylium ions.[16,18] The large dip angle
results in the short Fe1–C2 bond distance (2.379(4) Å), again
substantially less than in ferrocenylcarbenium ion (2.715(6) Å).
The short Fe1–C2 distance and large dip angle of 2c are close to
those calculated for the parent ferrocenylcarbenium ion;[23]
consistent with the reduced steric and increased electronic
alkenyl cations[15-17] and potentially indicates
a stabilizing
1
interaction with the iron center. Unusually, no H-11B or 11B-13C
coupling was observed, even when 2-13C-ethynylferrocene was
used. However, the 13C-labelled ethynylferrocene enabled the
measurement of the 13C–13C coupling constant in the alkenyl unit
of the zwitterion 2c (1JC-C = 99 Hz), which was substantially
decreased compared to the alkyne starting material (1JC = 177
-C
Hz). This decrease is consistent with the lower C–C bond order
demand
at
the
cationic
carbon
compared
to
1
and a change in hybridization from sp to sp2. Simialrly the JC-H
diphenylferrocenylcarbenium. The structural features of the
ferrocene-substituted zwitterion 2c strongly suggest stabilization
of the carbocationic center through an interaction with iron.
coupling constant was obserebd to fall from 250 Hz in the alkyne
1c to 165 Hz in zwitterion 2c (Figure S9). Single crystals of
zwitterion 2c suitable for X-ray diffraction were grown from the
reaction mixture at low temperature (Figure 3) and confirmed the
proposed zwitterionic structure.
DFT calcualtions were used to better understand teh unusual
bonding of 2c. Calculations with the popular M06-2X functional
led to a geometry substantially different to the one obtained
experimentally. Geometry optimization was repeated with a range
of DFT functionals and compared to the X-ray model (Figure 4A,
figure S10). Among them, the computationally less expensive
PBE-D3BJ best reproduced the solid-state structure of 2c (root
mean square deviation (rmsd) = 0.23 vs 0.48 Å for M06-2X).
There was significant bonding character in the iron-C2 interaction,
as apparent from the calculated Wiberg bond order of 0.16. The
C1–C2 bond order was reduced from 2.84 in the alkyne precursor
to 2.09, whilst The C2–C3 (Cp ipso) bond has a degree of multiple
bond character with a bond order of 1.28 (Figure 4B, S11). These
bond orders, together with the observed bond-length alternation
in the substituted Cp ring indicate a degree of allenic character to
the C1–C2–C3 unit. The donor-acceptor character of the C1–B1
bond is apparent from the accumulation of negative charge at C1
(NPA charge –0.43) and the reduced C1–B1 bond order of 0.73,
as well as the heavy polarisation towards C2 (67% C, 33% B).
Calculated NMR chemical shifts of 2a-2c, followed the
experimental trends (Figure S12).
Scheme 3. Reaction of B(C6F5)3 and ethynylphenylacetylene
Figure 3. Crystal Structure of the alkenyl ferrocene zwitterion (2c). Ellipsoids
are set to 50% probability; hydrogen atoms are omitted for clarity with the
exception of the alkenyl C–H bond. Selected bond lengths (Å) and angles [º]:
B(1)–C(1) 1.654(5), C(1)–C(2) 1.286(5), C(2)–C(3) 1.379(5), Fe(1)–C(2)
2.379(4), Fe(1)–C(2) 1.990(4); B(1)–C(1)–C(2) 122.8(3), Fe(1)–C(2)–C(1)–
140.47(2), C(3)–C(2)–C(1) 162.6(4).
Figure 4. A) comparison of solid-state structure and optimized structure of 2c
at the SMD[toluene]- PBE-D3BJ/6-31G*/SDD (Fe) level of theory. B) NBO
analysis of 2c at the SMD[toluene]-M06-2X/6-311+G**//SMD[toluene]-
PBED3BJ/6-31G* level of theory.
A significant distortion of the geometry around the alkenyl unit was
observed in the solid-state indicating significant stabilization from
the iron center. The C–C bond distance in the alkenyl unit was
3
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Although solutions of the simple aryl substituted zwitterions
2a and 2b were observed to progress to 1,1-carboboration
products (Figure 1 and Scheme 2), the fate of ferrocenyl
substituted zwitterion 2c was more complex. Reactions of
ethynylferrocene and B(C6F5)3 at 243 K showed complete
consumption of the starting materials and formation of zwitterion
2c. Upon warming to room temperature complex mixtures of
products were obtained without any 1,1-carboboration products.
In one case we were able to obtain a small quantity of product
sufficient for a single-crystal X-ray diffraction analysis. with the he
compound obtained proving to be the result of a complex
trimerization/multiple carboboration reaction to give bicycle 5. We
tentatively propose that this unique product arises as a result of
the longer lifetime of the stabilized ferrocenyl zwitterion 2c
introducing a Lewis acid of similar potency to B(C6F5)3, and thus
sequestering additional alkyne units. Inter- and, subsequent,
intramolecular reaction of ethynylferrocene with the zwitterion 2c,
and concurrent 1,1- and 1,4-carboboration, lead to the
cyclopropylcyclohexene 5 from a single B(C6F5)3 and three alkyne
units.[24]
Juraj Bella for NMR technical advice, and Dr Martin Stanford for
fruitful discussions. F.D. thanks the EPSRC Tier‐2 National HPC
Facility Service (http://www.cirrus.ac.uk), and the EPSRC Centre
for Doctoral Training for Theory and Modelling in Chemical
Sciences (EP/L015722/1) for providing access to the Dirac cluster
at Oxford.
Keywords: 1,1-carboboration • vinyl cation • mechanistic
investigation • FLP
[1]
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[2]
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In summary, we have observed, characterized and trapped
the proposed zwitterionic intermediates in 1,1-carboboration
reactions. We have shown that these intermediates, like
carbenium ions, can be stabilized by interactions with a ferrocene
substituent, resulting in increased lifetimes and changes to
reactivity patterns. We are currently investigating the role of in situ
generated alkenyl zwitterions in catalysis and investigating the
mechanism of the formation of 5.
[3]
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Figure 5. Crystal Structure of the cyclotrimerization product (5). Ellipsoids are
set to 50% probability; hydrogen atoms are omitted for clarity. Selected bond
lengths (Å): B(1)–C(1) 1.609(5), B(1)–C(5) 1.507(5), C(1)–C(2) 1.527(4), C(2)–
C(3) 1.523(4), C(3)–C(4) 1.483(4); C(4)–C(5) 1.372(4); C(2)–C(6) 1.564(4);
C(3)–C(6) 1.538(4). Fc = Ferrocenyl, ○ = alkyne units.
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MJC and SPT thank the University of Edinburgh for funding. SPT
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5
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Entry for the Table of Contents
We describe the isolation and the characterization of the zwitterionic intermediate in 1,1-carboboration reactions. The highly reactive
zwitterionic intermediates are generated from tris(pentafluorophenyl)borane and alkynes under reaction conditions of 1,1-
carboboration.
Twitter usernames: @AlexBismuto @cowley_group @SPThomasGroup
6
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