Frustrated lewis pair reactions with bis-acetylenic substrates: Exploring the narrow gap separating very different competing reaction pathways

Article abstract of DOI:10.1021/om200615n

Tris(pentafluorophenyl)borane reacts with 9,9-dipropargylfluorene (5) in a 2:1 ratio under ambient conditions to yield a statistical mixture of the double-1,1-carboboration products (Z,Z)-, (E,Z)-, and (E,E)-6. Photolysis of the product mixture yields a new mixture that is substantially enriched in the (Z,Z)-6 isomer. Treatment of 5 with the P(o-tolyl)₃/B(C ₆F₅)₃ frustrated Lewis pair gave the zwitterionic eight-membered heterocycle 7a, which is the product of a 1,1-carboboration of one alkynyl unit followed by an intramolecular 1,2-addition of the in situ generated new FLP to the remaining alkynyl moiety. The reaction of the P^tBu₃/B(C₆F₅)₃ FLP gave an analogous result, yielding 7b. Treatment of 1,2-diethynylbenzene (9) with the P^tBu₃/B(C₆F₅)₃ FLP resulted in deprotonation, forming the phosphonium/alkynylborate salt 10. The reaction of triarylphosphine/B(C₆F₅)₃ pairs using the phosphines P(o-tolyl)₃ and P{Ph₂[2,5- bis(trifluoromethyl)phenyl]} resulted in clean 1,2-P/B FLP additions to a C≡C triple bond to yield the products 11 and 12, respectively. Eventually, the reaction of 9 with the poorly nucleophilic phosphine P(C₆F ₅)₃ in combination with B(C₆F₅) ₃ gave the zwitterionic product 14, which is thought to result from a two-step reaction involving 1,1-carboboration followed by an exo-dig cyclization by means of a 1,2-P/B FLP reaction of the in situ generated alkenylborane Lewis acid and P(C₆F₅)₃ to the remaining alkynyl unit. The products (Z,Z)-6, 7a,b, 10, 12, and 14 were characterized by X-ray crystal structure analyses.

Full text of DOI:10.1021/om200615n

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issue entitled:
Frustrated Lewis Pairs
Guest Editor: Douglas Stephan
Published in issue 30, 2012 of Dalton Transactions
Articles in this issue include:
Communication
Hydrogen activation by 2-boryl-N,N-dialkylanilines: a revision of Piers’ ansa-aminoborane
Konstantin Chernichenko, Martin Nieger, Markku Leskelä and Timo Repo
Paper
Frustrated Lewis pair addition to conjugated diynes: Formation of zwitterionic 1,2,3-butatriene
derivatives
Philipp Feldhaus, Birgitta Schirmer, Birgit Wibbeling, Constantin G. Daniliuc, Roland Fröhlich,
Stefan Grimme, Gerald Kehr and Gerhard Erker
Paper
Fixation of carbon dioxide and related small molecules by a bifunctional frustrated pyrazolylborane
Lewis pair
Eileen Theuergarten, Janin Schlösser, Danny Schlüns, Matthias Freytag, Constantin G. Daniliuc,
Peter G. Jones and Matthias Tamm
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COMMUNICATION
Novel H2 activation by a tris[3,5-bis(trifluoromethyl)phenyl]borane frustrated
Lewis pair†
Thomas J. Herrington, Alex J. W. Thom, Andrew J. P. White and Andrew E. Ashley*
Received 17th February 2012, Accepted 13th April 2012
DOI: 10.1039/c2dt30384a
Tris[3,5-bis(trifluoromethyl)phenyl]borane (1,
BArF18),
has been synthesised on a practical scale for the first
time. According to the Gutmann–Beckett method it is a
more powerful Lewis acid than B(C6F5)3. It forms a
‘frustrated Lewis pair’ with 2,2,6,6-tetramethylpiperidine
which cleaves H2 to form a salt containing the novel anion
[μ-H(BArF18)2]−.
In recent years the concept and reactivity of “frustrated Lewis
pairs” (FLPs) continues to develop apace.1 Within these systems,
dative bond formation is restricted by steric encumbrance about
the donor and acceptor atoms which leads to ‘unquenched’ reac-
tivity. This enables the activation of small molecules such as
CO2,2 and importantly the heterolytic cleavage of H2, which has
led to application for the metal-free hydrogenation of polar
organic substrates (e.g. nitriles and imines),1e and even the weak
oxidant CO2.3 Typically, FLPs consist of an electrophilic borane
(most commonly B(C6F5)3 or derivatives thereof), whose Lewis
acidity is promoted by electron-withdrawing substituents, in
Fig. 1 Commonly used fluorinated aryl borates and their ‘parent’
Lewis acid boranes.
Scheme 1 Synthesis of tris[3,5-bis(trifluoromethyl)phenyl]borane, (1).
t
combination with a hindered phosphine or amine e.g. Bu3P or
2,2,6,6-tetramethylpiperidine (TMP).
was reported, and no subsequent reactivity studies have been
conducted.
Tetraaryl borate anions [B(C6F5)4]− and tetrakis[3,5-bis-
(trifluoromethyl)phenyl]borate, [BArF24]−, (Fig. 1) have widely
gained use as weakly-coordinating counterions for reactive cat-
ionic metal centres (e.g. in homogeneous olefin polymerisa-
tion).4 Their high stability in acidic and oxidative conditions is
attributed to the electron-withdrawing properties of their F-
substituents (which lower aromatic π-basicity and hence suscep-
tibility towards electrophilic B–C bond cleavage), and the
strength of their C–F bonds.5 Remarkably, in view of the rich
chemistry developed for B(C6F5)3 6 (which can be viewed as the
Lewis acid ‘parent’ of [B(C6F5)4]−) only one report exists for
the synthesis of the analogous tris[(3,5-trifluoromethyl)phenyl]
borane (BArF18), resulting from decomposition of the
[BArF24]− anion by the electrophilic platinum complex
trans-[(Ph3P)2Pt(Me)(OEt2)]+.7 Only X-ray crystallographic data
In continuation of our interest in FLP-H2 activation chemistry,
we herein report a practical synthesis of BArF18 and communi-
cate preliminary findings of its Lewis acidic properties and dif-
fering reactivity with H2 in an FLP system, relative to B(C6F5)3.
Na[BArF24] is synthesised via reaction of excess [3,5-bis-
(trifluoromethyl)phenyl]MgX (X = Cl, Br) with NaBF4 or
BF3·OEt2;5a,8 we reasoned that BArF18 should be an intermedi-
ate en route to the borate anion and decided to employ a rigid
stoichiometry. Accordingly, the Grignard was generated via
metal-halogen exchange of iPrMgCl and 1-bromo-3,5-bis-
(trifluoromethyl)benzene in THF, which was subsequently
reacted in situ with BF3·OEt2 (3 : 1) (Scheme 1).9 Facile
work-up followed by high vacuum sublimation (80 °C, 1 × 10−6
mbar)
afforded
tris[3,5-bis(trifluoromethyl)phenyl]borane
(1, BArF18) in good yield (65–70%, 2–5 gram scale) as a free-
flowing white powder (Scheme 1).‡ The reaction solvent
appeared to be important; Grignard formation can also be con-
ducted in Et2O, yet metathesis with BF3·OEt2 led to formation of
[BArF24]−, as shown by 11B NMR spectroscopy. It is thought
the use of THF may retard the competitive addition of a fourth
Grignard equivalent by coordinating to 1 as it is formed in solu-
tion; indeed the sublimation step is required to remove THF
from the moderately labile adduct 1·THF, which is the actual
Department of Chemistry, Imperial College London, South Kensington,
London SW7 2AZ, UK. E-mail: a.ashley@imperial.ac.uk;
Tel: +44 (0)20 759 45810
†Electronic supplementary information (ESI) available: Experimental
details, X-ray crystallographic details for 2·Et2O, synthetic procedures
and NMR spectral data for 1 and 2, and free volume plots obtained from
X-ray crystal structures of C5H5N·A (A = 1, B(C6F5)3). CCDC 868317.
For ESI and crystallographic data in CIF or other electronic format see
DOI: 10.1039/c2dt30384a
This journal is © The Royal Society of Chemistry 2012
Dalton Trans., 2012, 41, 9019–9022 | 9019

product extracted immediately after the Grignard step, as evinced
by 1H, 19F and 11B NMR spectroscopy.
1 is practically insoluble in aliphatic hydrocarbons, moderately
so in aromatic solvents and displays optimum solubility in
CH2Cl2 or CHCl3; this property contrasts with B(C6F5)3 (soluble
in most common non-donor media). This behaviour may be
attributed to intermolecular H⋯F bond interactions between the
para proton and CF3 groups on neighbouring molecules in the
solid-state for 1; a distance of 2.63 Å is found in the reported
crystal structure [sum of vdW radii, rw(F) + rw(H) = 2.67 Å],
which would obviously be lacking for B(C6F5)3.7,10
Scheme 2 Generation of 2 from heterolytic activation of H2 by 1 and
TMP.
1
1 has been fully characterised by H, 13C, 19F and 11B NMR
spectroscopy; the latter shift (δ 68.1 ppm; CD2Cl2) lends support
for a three-coordinate geometry in the solution-phase and is
noticeably deshielded in comparison with that found for
B(C6F5)3 (δ 61.2 ppm; CD2Cl2). Whilst B(C6F5)3 has been
shown to be inert to pure oxygen at room temperature,11 admis-
sion of dry O2 to a CD2Cl2 solution of 1 led to rapid decompo-
sition (numerous uncharacterisable resonances in the 1H, 19F and
11B NMR spectra). Despite strongly electron-withdrawing CF3
groups in 1 (rationalised to contribute to the observed oxidative
stability of the [BArF24]− anion), it is possible that the ortho-F
substituents in B(C6F5)3 are more important in suppressing reac-
tion with O2; the absence of this structural feature in 1 might
then lead to the heightened reactivity observed for this trigonal
borane in this case. Interestingly, H2O reversibly forms the
dative complex 1·OH2; the donor can be removed under vacuum
or through addition of 3 Å molecular sieves in CH2Cl2 solution,
in contrast with the tightly bound analogue (C6F5)3B·OH2.12
In order to better understand the reactivity of 1, Lewis acidity
assessments were performed by employing the Gutmann–
Beckett (Et3PO probe; 31P NMR) and Childs (trans-crotonalde-
Fig. 2 (a) Diagram of 2·Et2O. H atoms (except attached to B and N)
have been removed for clarity; thermal ellipsoids are shown at 50%
probability. (b) View showing staggered geometry along the B2–H1–B1
axis.
donor to B(C6F5)3 compared with the more ionic (harder) PvO
bond, favoured by B(OC6F5)3.
Since Lewis acidity is a composite of both steric and elec-
tronic factors at the acceptor site, it would be useful to compare
the steric profile of 1 with B(C6F5)3; however, to date no solid-
state structure of the latter has been reported. Fortunately the pyr-
idine adducts, C5H5N·A (A = 1, B(C6F5)3), have been crystallo-
graphically characterised for both boranes in which both have
virtually identical B–N bond lengths (1.63 Å, within e.s.u.), per-
mitting valid comparison.16 Excision of the pyridine ligand
enabled a comparison of the relative free volume from the
B centre, at a given radius, for the remaining pyramidalised
borane fragments.† The results show that 1 is less hindered in
the 2–4 Å region (i.e. that occupied by the pyridine molecule),
as anticipated from the smaller size of the ortho-H in 1 relative
to the ortho-F in B(C6F5)3; in conjunction with the 11B NMR
spectroscopic data (an electronic probe at the B atom)15a this
supports the Gutmann–Beckett assignment that 1 is more Lewis
acidic than B(C6F5)3.
1
hyde; H NMR of H3 resonance) methods; both rely on respect-
ive chemical shift differences (Δδ) upon complexation of the
probe to the Lewis acid, which is proportional to the Lewis acid
strength of the acceptor site.13 The results, compared with data
acquired for B(C6F5)3, are tabulated in Table 1. It can be seen
that 1 displays a Lewis acidity ca. 6% greater than that for
B(C6F5)3 using the former method, which contrasts markedly
with a ca. 38% reduction observed employing the Childs. A
linear correlation is usually documented between methods,14
although an increasing number of boron systems oppose this
observation.15 Notably, Britovsek et al. reported a non-linear
trend for the series B(C6F5)3−x(OC6F5)x (x = 1–3), where prefer-
ence for Et3PO binding over crotonaldehyde is observed as x
increases.15b This was rationalised using Pearson’s HSAB prin-
ciple where the covalent (softer) CvO bond is a preferable
Addition of 1 to 2,2,6,6-tetramethylpiperidine (TMP) in
CD2Cl2 (1 : 1) demonstrated the formation of an FLP, as evi-
1
denced by unchanged resonances in the H, 19F and 11B NMR
Table 1 31P and 1H NMR spectral data derived for Lewis acidity
measurements of 1 and B(C6F5)3
spectra relative to the species in isolation. Subsequent admission
of H2 (1 atm) led to the rapid precipitation of a white solid, and
1H NMR spectroscopy revealed exactly half of the initial TMP
remained in solution, whereas 11B NMR showed complete con-
sumption of 1, indicating complete sequestration of the borane.
Elemental analysis of the solid was consistent with the molecular
formula unit (1)2(TMP)(H2) (2, Scheme 2).‡ Remarkably,
H2 activation occurs even in Et2O, and led to the generation of
large single crystals suitable for study by X-Ray diffraction,
which solved as the novel [TMPH][μ-H(BArF18)2]·Et2O
(2·Et2O; Fig. 2). The anion geometry approximates to D3
Et3PO
trans-Crotonaldeyde
Lewis acid 31P NMR/ppma Δδ/ppmb 1H NMR/ppma Δδ/ppmc
None
B(C6F5)3
1
50.7
77.3
78.9
—
26.6
28.2
6.85
7.93
7.52
—
1.08
0.67
a CD2Cl2 at room temperature. b Δδ = [Et3PO(coordinated) − Et3PO-
(CD2Cl2)]. c Δδ = [H3(coordinated) − H3(CD2Cl2)].
9020 | Dalton Trans., 2012, 41, 9019–9022
This journal is © The Royal Society of Chemistry 2012

symmetry, and the bridging borohydride unit is virtually linear
(BĤB = 176.3°). The B–H bond lengths (1.40 and 1.42 Å) are
similar to those for seen in Li[μ-H(BEt3)2] (1.376(6) Å)17 yet
distinct from [TMPH][H–B(C6F5)3] (1.18(2) Å);18 the longer
bonds reflect the electron-deficient B–H–B interactions relative
to terminal B–H. The aryl rings adopt an almost staggered con-
formation (torsion angles 58.7–61.5°). The [TMPH] cation
shows H-bonding to an Et2O molecule with N⋯O and H⋯O
separations of 2.869(4) and 1.97 Å respectively, the N–H⋯O
angle being ca. 178°. Evidently the ammonium ion binds the
neutral O atom in preference to the charged borohydride anion.
This is the first example of H2 cleavage by an FLP to produce a
bridging borohydride salt.
The only other example of arylborane-mediated H2 activation
in ethereal solvent is by the FLP (Fmes)2BH/DABCO [Fmes =
2,4,6-tris(trifluoromethyl)phenyl, DABCO = diazobicyclo[2.2.2]-
octane];19 here the Lewis acidity remains unquenched because
the steric bulk around the B centre in (Fmes)2BH prevents Et2O
binding. In contrast, both 1 and B(C6F5)3 coordinate Et2O;
1·OEt2 is observable in Et2O solution as shown in the 11B NMR
spectrum (δ 19.0 ppm), a shift indicative of four-coordinate
boron. However, whilst B(C6F5)3 forms a strong adduct that
quenches FLP activity,20 1·OEt2 must dissociate sufficiently at
ambient temperature to allow participation of 1 in H2 heterolysis.
This is analogous to the behaviour exhibited by the 2,6-
dimethylpyridine/B(C6F5)3 FLP, which cleaves H2 only upon
dissociation of the weakly-bound classical adduct.21
Fig. 3 1H NMR spectrum of 2 (pyridine-d5). Solvent peaks denoted
by *, ▲ and ■ denote 1·pyridine-d5 and [1-H]− respectively.
oxygen donors H2O and Et2O more reversibly under comparable
conditions. BArF18 forms an FLP in the presence of TMP which
reacts rapidly with H2 to form the anion [μ-H(BArF18)2]− and
has been crystallographically characterised. This is the first
example of a bridging borohydride resultant from FLP-mediated
H2 heterolysis. Interestingly this reaction also proceeds in Et2O,
which is believed to be an example of a system operating on the
classical/frustrated Lewis pair borderline. Current research is
focusing on the use of different Lewis base partners whilst
exploring catalytic hydrogenation chemistry of these FLP/H2
protocols.
An unambiguous structural assignment of 2 by NMR spec-
troscopy was hampered by the insolubility of the compound in
most non-donor media; only using 1,2-difluorobenzene at 80 °C
(a solvent with a high dielectric constant reported to dissolve
1
poorly soluble ionic salts)22 was a H NMR spectrum obtained
that correctly reproduced the 1 : TMP ratio in 2. Despite this, we
1
were unable to observe any resonances in either the H or 11B
NMR spectra which could be assigned to the B(μ-H)B unit in 2.
This property is reminiscent of the related [(C6F5)3B(μ-H)B-
(C6F5)3] anion; here low temperature 1H and 19F NMR (<183 K)
provided the only means of identification, with 11B NMR unable
to distinguish a B(μ-H)B environment.23 IR spectroscopy of 2
and its deutero analogue, obtained from D2 and TMP : 1 (1 : 2),
also failed to unambiguously reveal a B–H(D) stretch.
Acknowledgements
We thank Imperial College (Junior Research Fellowships; AEA
and AJWT), the EPSRC (EP/P505550/1), and the Royal Society
(Research Grant RG100606) for funding this research. AEA also
thanks Dr George Britovsek for sponsorship in the Imperial
College Junior Research Fellowship scheme, and the Royal
Commission for the Exhibition of 1851 for a Research
Fellowship.
On the other hand, dissolution occurred readily in pyridine-d5
to give readily assignable 1H (Fig. 3), 19F and 11B NMR spectra,
which corresponded to the species [1·pyridine-d5], [TMPH]+,
and the borohydride anion [1-H]− (1 : 1 : 1 ratio). The latter was
cleanly resolved by a diagnostic terminal BH (1 : 1 : 1 : 1 quartet)
1
1
in the H NMR (δ = 4.53 ppm, J11-BH = 84 Hz), accompanied
by an intense doublet in the 11B NMR spectrum (δ = −7.2 ppm,
1J11-BH = 84 Hz). This confirms the composition of 2, and
reveals the behaviour of the anion in donor media as both a
source of terminal borohydride [1-H]−, and a labile equivalent of
the Lewis acid 1.
Notes and references
‡Data for compound 1: 1H NMR (CD2Cl2, 400 MHz): δ 8.24 (s, 3H,
para-H), δ 8.02 (s, 6H, ortho-H). 13C{1H} NMR (CD2Cl2, 101 MHz):
δ 142.8 (s, br, ipso-C), δ 138.3 (s, ortho-C), δ 132.2 (q, 2JCF = 33.5 Hz,
1
meta-C), δ 127.0 (s, para-C), δ 123.7 (q, JCF = 272.9 Hz, CF3).
19F NMR (CD2Cl2, 376 MHz): δ −63.4 (s, CF3). 11B NMR (CD2Cl2,
128 MHz): δ 68.1 (s, br). HRMS (EI, m/z): for BC24F18H9 Calcd:
650.0510. Found: 650.0510. IR (KBr, cm−1): 1615 (m), 1607 (m), 1385
(m), 1283 (s), 1227 (m), 1169 (s), 1127 (s), 909 (m), 844 (w), 720 (m),
708 (w), 683 (m), 657 (m). Anal. Calcd for C24H9BF18: C 44.34;
H 1.40; N 0.00. Found: C 44.22; H 1.29; N 0.00.
Conclusions
The facile synthesis of tris[3,5-bis(trifluoromethyl)phenyl]
borane (1) has been achieved on a multi-gram scale. Gutmann–
Beckett measurements indicate BArF18 to be a stronger Lewis
acid than the ubiquitous B(C6F5)3, yet it appears to bind the
Data for 2: 1H NMR (C6H4F2, 400 MHz, 353 K): δ 7.84 (s, 6H, para-
H), δ 7.73 (s, 12H, ortho-H), δ 4.00 (br, 2H, NH2), 1.57 (m, 2H, CH2),
1.46 (m, 4H, CH2), 1.23 (s, 12H, CH3). IR (KBr, cm−1): 3274 (m), 3234
(m), 3095 (m), 3034 (m), 2983 (m), 1616 (m), 1577 (w), 1459 (w),
This journal is © The Royal Society of Chemistry 2012
Dalton Trans., 2012, 41, 9019–9022 | 9021

8 (a) K. Fujiki, M. Kashiwagi, H. Miyamoto, A. Sonoda, J. Ichikawa,
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1365 (s), 1279 (s), 1165 (s), 1126 (s), 900 (s), 841 (m), 710 (s), 682 (s),
649 (s). Anal. Calcd for C57H39B2F36N: C 47.43; H 2.72; N 0.97.
Found: C 47.34; H 2.63; N 1.02.
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ˉ
M = 1517.63, triclinic, P1 (no. 2), a = 12.0325(5), b = 15.7928(8), c =
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