Reversible C?H Activation, Facile C?B/B?H Metathesis and Apparent Hydroboration Catalysis by a Dimethylxanthene-Based Frustrated Lewis Pair

Article abstract of DOI:10.1002/chem.201801871

A dimethylxanthene-based phosphine/borane frustrated Lewis pair (FLP) is shown to effect reversible C?H activation, cleaving phenylacetylene, PhCCH, to give an equilibrium mixture of the free FLP and phosphonium acetylide in CD₂Cl₂ solution at room temperature. This system also reacts with B?H bonds although in a different fashion: reactions with HBpin and HBcat proceed via C?B/B?H metathesis, leading to replacement of the -B(C₆F₅)₂ Lewis acid component by -Bpin/-Bcat, and transfer of HB(C₆F₅)₂ to the phosphine Lewis base. This transformation underpins the ability of the FLP to catalyze the hydroboration of alkynes by HBpin: the active species is derived from the HB(C₆F₅)₂ fragment generated in this exchange process.

Full text of DOI:10.1002/chem.201801871

A Journal of
Accepted Article
Title: Reversible C-H activation, facile B-C/B-H metathesis and
apparent hydroboration catalysis by a xanthene-based frustrated
Lewis pair
Authors: Petra Vasko, Ili A Zulkifly, M. Ángeles Fuentes, Zhenbo Mo,
Jamie Hicks, Paul C. J. Kämer, and Simon Aldridge
This manuscript has been accepted after peer review and appears as an
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To be cited as: Chem. Eur. J. 10.1002/chem.201801871
Link to VoR: http://dx.doi.org/10.1002/chem.201801871
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Reversible C-H activation, facile C-B/B-H metathesis and apparent
hydroboration catalysis by a dimethylxanthene-based frustrated
Lewis pair
Petra Vasko,*[a] Ili A. Zulkifly, [a] M. Ángeles Fuentes, [a] Zhenbo Mo,[a] Jamie Hicks,[a] Paul C. J.
Kämer,*[b] and Simon Aldridge*[a]
Abstract: A dimethylxanthene-based phosphine/borane frustrated
Lewis pair (FLP) is shown to effect reversible C-H activation,
cleaving phenylacetylene, PhCCH, to give an equilibrium mixture of
the free FLP and phosphonium acetylide in CD2Cl2 solution at room
temperature. This system also reacts with B-H bonds although in a
different fashion: reactions with HBpin and HBcat proceed via C-B/B-
H metathesis, leading to replacement of the –B(C6F5)2 Lewis acid
component by –Bpin/–Bcat, and transfer of HB(C6F5)2 to the
phosphine Lewis base. This transformation underpins the ability of
the FLP to catalyze the hydroboration of alkynes by HBpin: the
active species is derived from the HB(C6F5)2 fragment generated in
this exchange process.
of ambient temperature and pressure,6 and a related system
(1a) can catalyze dehydrogenation chemistry, including B-N
dehydrocoupling processes.7
Recent work has shown that – in addition to shuttling
dihydrogen – FLPs can also act as catalysts for various hydro-
elementation processes.9 One such process, which has
widespread synthetic potential is the hydroboration of CC
multiple bonds, not least because organoboron derivatives are
very useful building blocks in organic chemistry.10 ‘Metal-free’
approaches to the hydroboration of alkynes in particular have
attracted significant attention.11,12 For example, Stephan and co-
workers recently showed that the hydroboration of alkynes by
pinacolborane (HBpin, HBO2C2Me4) could be effected by a
variety of strong Lewis acids,12 and is sensitive to the choice of
catalyst. In particular, Piers’ borane, HB(C6F5)2,13 was shown to
act as an efficient and versatile pre-catalyst, generating in situ
1,1-diboryl species of the type RCH2CR'(Bpin)(B(C6F5)2) which
were reported to act as the active catalytic species.12
Considering the track record of the dimethylxanthene-
derived FLPs 1a and 1b in the activation of E-H bonds,6,7 and
the use of other systems featuring the -B(C6F5)2 function in
hydroboration chemistry,12,13 we were interested to investigate
their potential to carry out similar transformations. In this account,
we describe the stoichiometric reactions of FLPs 1a/b with both
terminal and internal alkynes, together with their reactivity
towards boranes and their unexpected role as pre-catalysts in
the catalytic hydroboration of alkynes.
Introduction
Frustrated Lewis pairs (FLPs) were first identified conceptually in
1942,1 but it was not until a landmark 2006 report from Stephan
and co-workers on the cleavage of H2 that their use in small
molecule capture/activation was established.2 In the intervening
12 years, numerous examples of both inter- and intramolecular
FLPs have been reported, together with their applications in the
binding and/or activation of both polar and non-polar substrates.3
Although the interaction of FLPs with H2 – and derived
applications in catalytic hydrogenation – have been the focus of
a significant proportion of this research,4 more recent reports
have even seen this extended to the activation and
functionalization of C-H bonds.5
In this FLP context we have been interested in exploiting the
dimethylxanthene framework as a scaffold on which to install the
Lewis acid and base components.6,7 This offers a degree of
preorganization – based on a separation of 4.0-4.5 Å between
the phosphine and borane units – which is optimal for the
cleavage of H2.8 Thus an FLP of this type (1b, Figure 1) has
been shown to reversibly activate H2 in solution under conditions
F
F
B
F
F
F
F
O
F
F
PR2
B(C6F5)2
H
F
1a: R = Ph
1b: R = Mes = 2,4,6-Me3C6H2
F
n
Figure 1. Key compounds relevant to the current study: (left)
dimethylxanthene-based FLPs 1a/1b and (right) Piers’ borane (n = 1, 2).6,7,13
[a]
Dr. P. Vasko, I. A. Zulkifly, Dr. M. Ángeles Fuentes, Dr. Z. Mo, Dr.
J. Hicks and Prof. S. Aldridge
Department of Chemistry, University of Oxford
Inorganic Chemistry Laboratory
South Parks Road, Oxford, OX1 3QR
E-mail: petra.vasko@chem.ox.ac.uk,
simon.aldridge@chem.ox.ac.uk
Results and Discussion
FLP reactivity towards alkynes
[b]
Prof. P. C. J. Kämer
Leibniz Institute for Catalysis (LIKAT Rostock), Albert-Einstein-
Str. 29 a, 18059 Rostock, Germany.
E-mail: Paul.Kamer@catalysis.de
In our initial experiments we scoped out the potential for FLP 1a
to act as a catalyst for the hydroboration of alkynes using
commercially available pinacolborane. Accordingly, we found
that the hydroboration of PhCCMe proceeds to 95% completion
Supporting information for this article is given via a link at the end
of the document.
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in dichloromethane solution at room temperature over a period
of 3 h at 10% catalyst loading. The reaction is shown by 1H NMR
spectroscopy to proceed with the expected syn stereochemistry
and to be ca. 90% selective for the anti-Markovnikov product,
PhC(H)=C(Me)Bpin. Similar chemistry carried out on PhCCPh
proceeds more slowly, with the reaction taking ca. 12 h to
achieve 50% conversion under similar conditions. While such
behaviour is consistent with the more sterically hindered nature
of this alkyne, more difficult to rationalize is the behaviour of the
terminal alkyne PhCCH. In this case, turnover is also apparently
slower than with PhCCMe, with >95% completion achieved only
after ca. 12 h at 10 mol% loading, and 80-85% after a similar
period at 5 mol%. Intrigued by this unusual difference in rate as
a function of alkyne, we set out to probe the mechanism(s) of
the catalysis. With this in mind, we first investigated the
stoichiometric reactions involving 1a and the respective
alkynes/boranes.
as a function of temperature (K = 5.4 mM at 308 K for the loss of
PhCCH; 0.61 mM at 198 K), and a Van’t Hoff analysis (see ESI)
yields thermodynamic data (∆HƟ = +34 kJ mol-1; ∆SƟ = +68 J
mol-1 K-1) for the loss of PhCCH from 2a. These data can be put
into context by the corresponding values of +38 kJ mol-1 and
+102 J mol-1 K-1 for the loss of H2 from the phosphonium borate
derived from the related FLP 1b.6
Over a period of several hours at room temperature,
solutions of 2a in dichloromethane undergo
a
further
transformation, as shown by in situ NMR monitoring. This
instability is intrinsically linked to the reversible nature of the
reaction of 1a with PhCCH, in that the onward reactivity of 2a is
related to the presence of free alkyne in solution (vide infra).
However, a consequence of this lability is that our attempts to
crystallize 2a for X-ray crystallographic analysis were inhibited.
Reasoning that the use of the more strongly P-donating FLP 1b
(in which the phosphine phenyl groups are replaced by mesityl)
might render the C-H activation reaction effectively irreversible,
we examined its chemistry towards phenylacetylene.
Accordingly, 1b was reacted with PhCCH in dichloromethane,
and the corresponding phosphonium acetylide 2b was isolated
in 44% yield. Compound 2b can also be characterized in
solution by multinuclear NMR spectroscopy, although in this
case neither 1H nor 31P NMR data gave any indication of
reversibility. Thus, the 31P NMR spectrum shows a single
resonance at δP = -27.3 ppm (1JP-H = 532 Hz) and the 11B{1H}
NMR spectrum a singlet at δB = -17.9 ppm. Moreover, in this
case, X-ray quality crystals could be grown from a mixture of
dichloromethane and hexane at 25 °C; 2b crystallizes in the
FLP 1a does not react in isolation with either of the internal
alkynes PhCCMe or PhCCPh, but does react with PhCCH at
room temperature in either dichloromethane or toluene. In the
latter solvent, the reaction is essentially complete after 30 min,
with a white solid precipitating out of solution. The product could
be isolated in good yield (81%) and characterized by
multinuclear NMR spectroscopy in CD2Cl2 solution. These data
suggest the formation of the C-H activated product 2a (Scheme
1).
a)
PhCCH
slow
O
monoclinic P21/n space group with
a
molecule of
O
O
Reversible
C6F5
H
Ph2
P
B
PPh2
B(C6F5)2
Ph2P
B(C6F5)2
dichloromethane, and confirms the structure as resulting from
cleavage of the C-H bond in phenylacetylene across FLP 1b
(Figure 2). The xanthene backbone is virtually planar with
tetrahedral boron and phosphorus centres. The phosphorus-
bound hydrogen was found in the electron difference map with a
P(1)-H(1) distance of 1.19(4) Å, and the boron-bound
phenylacetylide moiety shows an almost linear B(1)-C(46)-C(47)
angle of 171.7(4)°.
H
C
C
Ph
Ph
Ph
C6F5
Ph
H
3
1a
2a
b)
PhCCH
O
O
PMes2
B(C6F5)2
Mes2P
B(C6F5)2
H
C
C
Ph
2b
1b
Scheme 1. Stoichiometric reactions (a) between 1a and PhCCH; and (b)
between 1b and PhCCH.
The 31P NMR spectrum of 2a features a doublet at δP = -9.1
1
ppm with a JP-H coupling constant of 544 Hz, and the 11B{1H}
spectrum shows a sharp signal at δB = -18.4 ppm, consistent
with a tetra-coordinate boron atom. These NMR shifts are in
good agreement with data previously published for zwitterionic
phosphonium/alkynylborate compounds formed via the reactions
of FLPs reactions with phenylacetylene. For example, Erker et al.
report 11B and 31P NMR shifts of δB = -17.6 ppm and δP = 1.6
ppm (1JP-H = 490 Hz) for the product of C-H cleavage by an
anthracene-based intramolecular FLP,14 and δB = -16.5 ppm and
δP = 0.0 ppm (1JP-H = 470 Hz) for a cyclopentane-derived
system.15
The 31P and 1H NMR spectra of 2a in CD2Cl2, however, also
feature signals due to the FLP 1a and free phenylacetylene,
which we hypothesized might be due to the presence of an
equilibrium between PhCCH and its C-H activated form at room
temperature. Investigation by variable temperature NMR
spectroscopy revealed that the relative proportions of 1a/2a vary
Figure 2. Molecular structure of 2b as determined by X-ray crystallography.
Most H atoms and disordered solvent molecules are omitted and Mes/C6F5
groups are shown in wireframe format for clarity; thermal ellipsoids are drawn
at the 35% probability level. Key bond lengths (Å) and angles (o): B(1)-C(46)
1.604(5), C(46)-C(47) 1.190(5), P(1)-H(1) 1.19(4), B(1)-C(46)-C(47) 171.7(4).
As noted above, solutions of 2a in dichloromethane undergo
onward reactivity over a period of several hours (while similar
solutions of 2b do not). Monitoring by in situ NMR in CD2Cl2
shows that on standing at room temperature for 2 hours this
process starts to give rise to two new signals at around 20 ppm
in the 31P{1H} NMR spectrum. Specifically, these are a broad
partially collapsed quartet at 19.1 ppm (with a coupling constant
of ca. 20 Hz) and a sharp singlet at δP = -20.1 ppm. The latter
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signal increases in intensity at the expense of the former over
time. In addition, changes in the 1H NMR spectrum over this time
period include the appearance (and then decay) of a sharp
doublet at δH = 9.06 ppm (J = 38.5 Hz). Reasoning that this
further chemistry might involve the reaction of 2a with free
PhCCH present in the equilibrium mixture, we examined the
reaction between 1a and excess PhCCH. After stirring the
reagents in toluene for 14 hours, the solution was concentrated
and layered with pentane. Slow diffusion produced colourless
crystals of the major product (3) suitable for single-crystal X-ray
diffraction studies (Scheme 1 and Figure 3). Crystallographic
analysis revealed 3 to be a multi-insertion product, in which
three molecules of PhCCH have been assimilated by one
molecule of the FLP 1a. Compound 3 features an unusual bora-
cyclohexadiene heterocycle, with one B-bound C6F5 group
having migrated to form a new C-C bond to the terminal carbon
of one of the phenylacetylene moieties. In addition, a further
PhCCH molecule has undergone a 1,2-hydrogen shift and
bridges between the phosphine and the heterocycle. The 31P
NMR spectrum of crystals of 3 in CD2Cl2 confirm that the signal
at δP = 20.1 ppm which appears in the spectrum of 2a on aging,
is due to this multi-insertion product. Moreover, the 3:1 ratio of
alkyne:FLP in 3 is consistent with the idea that it is formed from
2a by assimilation of the ‘free’ PhCCH liberated through the
reversible nature of the C-H activation chemistry. Of additional
interest is the observed migration of one of the B-bound C6F5-
groups: carbo-boration processes of this sort (involving formal
insertion of unsaturated substrates into B-C bonds) are well
known, including examples involving the migration of a fluoroaryl
ring.16
4-MeOC6H4CCH
O
4
O
R2P
B(C6F5)
2
PR2
B(C6F5)2
H
MeO
1a
Scheme 2. (left) Stoichiometric reaction between 1a and 4-MeOC6H4CCH.
(right) Molecular structure of 4 as determined by X-ray crystallography. Most H
atoms and disordered solvent molecules are omitted and Ph/C6F5 groups are
shown in wireframe format for clarity; thermal ellipsoids are drawn at the 35%
probability level. Key bond lengths (Å): B(1)-C(40) 1.6319(18), C(40)-C(41)
1.3480(18), C(41)-P(1) 1.8034(12).
Thus, its 31P{1H} NMR spectrum shows a broad (partially
collapsed) 1:1:1:1 quartet at δP = 18.6 ppm with a coupling
constant of 3JP-B = 19.3 Hz, while the alkyne CH is observed as a
doublet at 9.81 ppm (3JP-H = 38.1 Hz). By means of comparison,
the corresponding data measured for Ph3P(Ph)CC(H)B(C6F5)3
are δP = 25.2 (3JP-B = 15 Hz and JP-H = 36 Hz).17 Confirmation
3
that 4 is indeed the 1,2-addition product of 1a and 4-
MeOC6H4CCH can be obtained from single-crystal X-ray
diffraction studies on crystals grown from a mixture of toluene
and pentane at room temperature (Scheme 2). Both B(1) and
P(1) are tetrahedral with the two heteroatoms being linked via a
two-carbon bridging unit. The associated bond lengths, B(1)-
C(40) = 1.6319(18) Å, C(40)-C(41) = 1.3480(18) Å and C(41)-
P(1) = 1.8034(12) Å, are consistent with the presence of
a
double bond between C(40) and C(41). Thus, these bond
metrics are in good agreement with other 1,2 addition products
of phenylacetylene with P/B FLPs (e.g. d(B-C) = 1.639(2) Å,
d(C=C)
=
1.345(5) Å, d(P-C)
=
1.806(1) Å, for
Ph3P(Ph)CC(H)B(C6F5)3).17
While the reactions of PhCCH with 1a do not, on the face of
it, shed much light on the mode of action of the FLP in catalytic
hydroboration, they do provide potential insight into the widely
differing turnover frequencies obtained in the reactions of HBpin
with (for example) PhCCH and PhCCMe. It is apparent from
stoichiometric studies with the terminal alkyne, that its reactivity
towards 1a in isolation provides a potential avenue through
which much of the FLP might be ‘tied up’ off-cycle via reversible
CH activation, and even undergo irreversible catalyst
decomposition through the formation of multi-insertion
compound 3. PhCCMe and PhCCPh do not appear to react with
1a in isolation over the timeframe of the hydroboration chemistry.
O
C6F5
H
Ph2P
Ph
B
H
Ph
C6F5
Ph
H
Figure 3. Molecular structure (left) and diagrammatic representation (right) of
3 as determined by X-ray crystallography. Most H atoms and disordered
solvent molecules are omitted and selected Ph/C6F5 groups are shown in
wireframe format for clarity; thermal ellipsoids are drawn at the 35% probability
level. Key bond lengths (Å): P(1)-C(39) 1.881(2), C(38)-C(39) 1.548(2), B(1)-
C(34) 1.608(2), C(34)-C(35) 1.339(2), C(35)-C(36) 1.512(2), C(36)-C(37)
1.540(2), C(37)-C(38) 1.339(2), B(1)-C(38) 1.641(2).
FLP reactivity towards boranes
In view of the facts (i) that the solid-state structure of 3
features a bridging unit between the phosphorus and boron
atoms derived from a single molecule of PhCCH; and (ii) the 1,2-
addition of phosphine/borane FLPs across alkynes has literature
precedent,17 we hypothesized that the species giving rise to the
signal at δP = 19.1 ppm which precedes the formation of 3 might
be the corresponding 1,2-alkyne addition product of 1a. While
we were unable to isolate this labile species in the case of
PhCCH, the corresponding (stoichiometric) reaction of 1a with
the more electron rich alkyne, 4-MeOC6H4CCH, did yield more
promising results. The product of this reaction (4, Scheme 2)
can be isolated in 79% yield and gives rise to two diagnostic
NMR signals.
With a view to shedding light on the mechanism by which 1a
effects catalytic hydroboration, we turned our attention to its
reactivity towards pinacolborane and related boranes.18 This
system has previously been shown to react with tetra-coordinate
systems of the type R3N.BH3 via heterolytic B-H bond cleavage,7
but a different mode of reactivity is revealed towards tri-
coordinate boranes. Thus, the stoichiometric reaction between
1a and HBpin can be shown by 11B NMR spectroscopy to give
rise to two signals (at δB = -21.2 and 31.6 ppm) consistent with
the formation of one tetra-coordinate and one tri-coordinate
boron atom. The corresponding 31P{1H} NMR spectrum features
two broad singlets at δP = 14.8 and 18.2 ppm. A preparative
scale reaction carried out using excess HBpin in
dichloromethane over a period of 72 h, afforded a pale-yellow
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solution from which colourless crystals could be grown by
concentration and layering with n-hexane. Single-crystal X-ray
diffraction allows for structural characterization of the product, 5
(Scheme 3 and Figure 4).
of 5 (Scheme 3 and Figure 4). As such, analogous migration of
the B(C6F5)2 group to the phosphine has occurred with
accompanying formation of a new aromatic carbon-boron bond.
Compound 6 features four coordinate P(1) and B(2) centres,
with the associated P-B bond length (1.978(2) Å) being notably
shorter than that found in 5 (2.021(3) Å), despite the presence of
Piers' borane
Excess HBR2
CH2Cl2
.
identical –Ph2PBH(C6F5)2 functions in each case. Closer
inspection shows that the rotational alignment of the P- and B-
bound substituents about the P-B bond in 6 is close to an ideal
staggered arrangement (with torsional angles falling in the range
of 50-70o), while in 5 the same substituents are much closer to
being eclipsed (torsions: 10-20o). This in turn reflects the
positioning of the HB(C6F5)2 group itself: in 6 rotation about the
P(1)-C(1) bond places it away from the ArBcat moiety in space,
while in 5 it is projected towards the ArBpin unit and is
O
O
O
PPh2
B(C6F5)2
PPh2
(C6F5)2B
BR2
PPh2
BR2
H
1a
5, R2 = pinacolato
6, R2 = catecholato
7, R2 = pinacolato
8, R2 = catecholato
Scheme 3. Formation of 5 and 6 by the reaction of 1a with HBpin and HBcat
and via the addition of Piers’ borane to 7 and 8.
presumably subject to
a much more sterically crowded
environment. Consistently, NMR studies carried out on 6 reveal
only one 31P{1H} NMR resonance at room temperature (at δP =
13.4 ppm) in contrast to the two signals observed for 5.
The -Bcat unit offers a less demanding three-dimensional steric
profile than -Bpin, which therefore presumably presents less of a
barrier to rotation about the P-Ar bond.
The ‘migrated’ species 5 and 6 both feature Piers’ borane,13
HB(C6F5)2, coordinated to the phosphine moiety, and we wanted
to know if the two compounds could be synthesized via a more
logical approach – e.g. via the direct reaction of Piers’ borane
itself with the FLP analogues of 1a featuring -Bpin or -Bcat as
the Lewis acid component (7 and 8; Scheme 3, right).
Accordingly, compounds 7 and 8 were synthesized by reacting
Figure 4. Molecular structures of 5 (left) and 6 (right) as determined by X-ray
crystallography. Most
H atoms and solvent molecules are omitted and
Mes/C6F5 groups are shown in wireframe format for clarity; thermal ellipsoids
are drawn at the 35% probability level. Key bond lengths (Å): P(1)-B(1)
2.021(3) (for 5); 1.978(2) (for 6).
4-(diphenylphosphino)(5-lithio)-9,9-dimethylxanthene
with
ClBpin or ClBcat, respectively, in n-hexane, and characterized in
each case by standard spectroscopic, analytical and
crystallographic techniques (Figure 5). Both compounds are
shown to contain unquenched (tri-coordinate) borane and
phosphine components, featuring P-B separations of 4.548(2)
and 4.383(3) Å, respectively. In C6D6 solution, 7 and 8 show
signals at 31.0 and 32.0 ppm (11B{1H}) and -14.0 and -13.0 ppm
(31P{1H}), respectively. Moreover, as expected, the addition of
one equivalent of Piers’ borane to 7/8 gives identical spectra to
samples of 5/6 obtained from 1a; uptake of the HB(C6F5)2
fragment at phosphorus is apparently quantitative, with no
evidence being found for the existence of free borane in solution
at equilibrium.
Compound 5 crystallizes in the monoclinic C2/c space group
with one molecule of 5, and disordered hexane and dichloro-
methane solvent molecules in the asymmetric unit. The
structural analysis shows that FLP 1a has reacted with one
molecule of HBpin via borane exchange, leading to formation of
an ArBpin function with accompanying migration of the B(C6F5)2
group (as HB(C6F5)2) to the phosphine through construction of a
P-B coordinate bond. Both P(1) and B(1) are tetrahedral with a
P(1)-B(1) bond length of 2.021(3) Å, while the geometry around
B(2) is trigonal planar as expected for a tri-coordinate boron
atom.
The 1H, 11B, 31P and 19F NMR spectra measured for isolated
crystalline samples of 5 are found to be identical to those
obtained in situ for the reaction of 1a and HBpin. The solid-state
structure of 5 does not, however, explain the presence of two
sets of signals in these spectra (e.g. in the 31P NMR spectrum at
δP = 14.8 and 18.2 ppm). Looking closely at the X-ray structure
of 5 we wondered whether the origin of the two signals could be
the presence of rotational isomers in solution – relating to
restricted rotation about the Ar-P bond, given the large steric
bulk of the P-bound HB(C6F5)2 unit and the presence of the –
Bpin function (with four pendant CH3 groups) acting, in effect, as
a ‘ratchet’. Two further observations are consistent with this
observation: (i) VT-NMR shows that the two 31P signals coalesce
into a singlet at 15.9 ppm at 373 K (see ESI); and (ii) the use of
the less bulky borane HBcat – while bringing about similar
chemistry – gives a product which is characterized by only one
31P signal at room temperature.
Figure 5. Molecular structures of 7 (left) and 8 (right) as determined by X-ray
crystallography. H atoms are omitted for clarity; thermal ellipsoids are drawn at
the 35% probability level. Key distances (Å): P(1)…B(1) 4.383(3)/4.377(2) (for
7); 4.548(2) (for 8).
The reaction of excess catecholborane (HBcat) with 1a
yields 6, the structure of which is superficially very similar to that
An additional feature of interest in respect of the
phosphine/borane adducts 5 and 6 is the mechanism by which
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they are accessed from 1a and HBpin/HBcat. To probe whether
this rearrangement is unique to 1a (or at least is facilitated in
some way by the presence of the phosphine component), we
probed the simpler reaction between PhB(C6F5)2 and HBpin for
evidence of borane exchange under ambient conditions. Thus, a
mixture of PhB(C6F5)2 and ca. 0.5 equivalents of HBpin in C6D6
was monitored by 1H, 11B and 19F NMR, over a period of 72 h at
room temperature. These studies reveal the formation of three
new boron-containing species in addition to unreacted
PhB(C6F5)2. The 1H NMR spectrum features signals for H2,
PhBpin, and Piers’ borane,13 with the latter two and the third new
HBpin/PhCCH reaction by {HB(C6F5)2}n proceeds via initial
hydroboration of the alkyne by the more reactive borane to give
trans-PhC(H)=C(H)B(C6F5)2 (and then to the proposed active
form of the catalyst, PhCH2CH(Bpin)B(C6F5)2).12 With this in
mind, we could show that 5 reacts with excess PhCCH to form
PhC(H)=C(H)B(C6F5)2 (9) in a timeframe consistent with the
observed catalysis (in effect, faster than a 1H NMR spectrum
could be acquired). In a similar fashion, the reactions with
excess PhCCMe or PhCCPh rapidly yield the corresponding
hydroboration products PhC(H)=C(R)B(C6F5)2 (R = Me, Ph),
respectively.13b
In the case of PhCCMe and PhCCPh, it can readily be
shown that 1a is not only chemically competent to produce the
pre-catalyst PhC(H)=C(R)B(C6F5)2, but kinetically competent too.
Thus, the performance of 1a in the catalytic hydroboration of
both of these alkynes by HBpin (Figure 7) is very similar to the
data reported by Stephan using Piers’ borane as the pre-catalyst
(e.g. for PhCCMe: 95% conversion in 3 h at room temperature
using 10 mol% of 1a; cf. 94% in 5 h at room temperature using 5
mol% {HB(C6F5)2}n). Moreover, in the case of PhCCMe the
regioselectivity of the catalytic hydroboration using 1a (89:11 in
favour of the anti-Markovnikov product) is essentially identical to
that reported using {HB(C6F5)2}n (88:12).12 These data
suggested to us that it is rearrangement of 1a to 5 and the
subsequent availability of the HB(C6F5)2 fragment which is
responsible for the catalytic hydroboration of PhCCMe/PhCCPh
by 1a.
compound, B(C6F5)3, being additionally identified via 11B and 19
F
NMR spectroscopy. Evidently substituent exchange under these
conditions is facile, with the formation of {HB(C6F5)2}n and
PhBpin in this simple system being consistent with the C-B/B-H
exchange observed in the formation of 5/6 from 1a.
In addition to model experimental studies, the reaction
mechanism of substituent exchange between PhB(C6F5)2 and
HBpin was studied computationally by DFT calculations. All
geometry optimizations were carried out using the PBE1PBE
exchange-correlation functional in combination with the def-
TZVP basis set. Ph/H exchange was found to be exergonic
overall by 10 kJ mol-1 with a transition state being located at 111
kJ mol-1 corresponding to concerted transfer of the two
fragments between the Bpin and B(C6F5)2 centres (Figure 6).
The calculated barrier is consistent with the experimental
findings (exchange at room temperature and above) and implies
more generally that carbon-boron bonds in systems of the type
ArB(C6F5)2 are potentially labile in the presence of boranes of
this type.
H
+111
(C6F5)2B
Bpin
H
(C6F5)2B
Bpin
+91
)
1
-
l
o
m
J
k
(
G
Figure 7. Plot of % conversion vs time for the hydroboration of PhCCMe ()
and PhCCPh () by HBpin catalysed by 1a (10 mol%, CD2Cl2 solution, room
temp.).
0
PhBpin
+
HB(C6F5)2
PhB(C6F5)2
-10
+
HBpin
Figure 6. Calculated reaction barrier for exchange of Ph/H substituents
between PhB(C6F5)2 and HBpin.
Mode of FLP action in catalysis
With the stoichiometric reactions between 1a with PhCCH/HBpin
mapped out, the mode of action of the FLP in the catalytic
hydroboration of alkynes was investigated. In particular given (i)
the formation of 5 from 1a in the presence of HBpin; and (ii)
previous reports from Stephan and co-workers of alkyne
hydroboration using {HB(C6F5)2}n as the pre-catalyst,12 we
hypothesized that the apparent catalytic activity of 1a might be
due to its conversion to 5 and subsequent release of Piers’
borane into solution. Although NMR studies show that isolated
samples of 5 do not spontaneously release the HB(C6F5)2
fragment, Stephan has shown that catalysis of the
Figure 8. Plot of % conversion vs time for the hydroboration of PhCCH by
HBpin catalysed by 1a at different catalyst loadings in CD2Cl2 solution. Key: 
20 mol%;  10 mol%;  5 mol%;  2.5 mol%).
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10.1002/chem.201801871
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With this mode of operation in mind, the catalytic
performance of 1a in the hydroboration of PhCCH is apparently
anomalous. The use of {HB(C6F5)2}n as the pre-catalyst in this
reaction is reported to bring about 90% conversion in 5 h using 5
mol% catalyst at room temperature. In the case of reaction
catalysed by 1a, conversion is somewhat slower (Figure 8): the
reaction only reaches completion in <12 h at room temperature
at very high (10-20 mol%) catalyst loadings, and at 5 mol% is
only 80-85% complete after 12 h. Moreover, catalytic
performance at a range of loadings is characterized by a
noticeable induction period which is absent in the kinetic plots
for either of the internal alkynes.
formation of 5 from 1a in the presence of PhCCH. Moreover, the
comparable performance of 1a to {HB(C6F5)2}n in the
hydroboration of PhCCMe and PhCCPh12 suggests that it is the
presence of the terminal alkyne which is inhibiting the
conversion of 1a to 5. This hypothesis is readily confirmed by in
situ NMR monitoring under catalytic conditions (1a plus 10
equivalents of PhCCH/HBpin). 31P{1H} NMR shows that at short
reaction times 1a is converted almost exclusively into the C-H
activation product 2a, together with traces of the multi-insertion
compound 3. While the formation of the former is known to be
reversible, and evidence for the formation of 5 can be seen after
30 min, it is apparent that the reaction of 1a with PhCCH is
much faster than that with HBpin, such that even after 120 min,
much of the FLP is ‘tied up’ as 2a, and therefore not able to
contribute catalytically (Scheme 4).
The use of 5 (or 9) as the pre-catalyst, by contrast, gives
hydroboration activity which is comparable to that reported using
{HB(C6F5)2}n, suggesting that the poorer performance using 1a
(and the induction period seen in Figure 8) is related to the
PhCCH
PhCCH
O
O
O
fast,
reversible
slow
C6F5
H
Ph2P
Ph
B
Ph2P
B(C6F5)2
PPh2
B(C6F5)2
H
C
C
Ph
Ph
C6F5
1a
HBpin
Ph
H
2a
3
As per ref 12
H
PhCCH
O
Ph
PPh2
Bpin
B(C6F5)2
alkyne
coordination
(C6F5)2B
H
Ph
Bpin
5
PhCCH
HBpin
B(C6F5)2
H
B(C6F5)2
+ HBpin
hydro-
boration
Ph
Bpin
Ph
H
9
product
release
O
Bpin
H
PPh2
Bpin
H
H
Bpin
H
7
Ph
Ph
B(C6F5)2
Ph
Bpin
Scheme 4. Proposed catalytic cycle for the hydroboration of PhCCH by FLP 1a.
Conclusions
by -Bpin/–Bcat and transfer of HB(C6F5)2 to the phosphine Lewis
base. Moreover, it also proves to be at the heart of the ability of
1a to act as a catalyst for the hydroboration of alkynes by HBpin
(Scheme 3): the active species is derived from the HB(C6F5)2
fragment generated in this exchange process. In the case of the
internal alkynes PhCCMe and PhCCPh, this leads to catalytic
performance which is comparable to that reported using
{HB(C6F5)2}n itself, while with PhCCH, the apparently faster rate
of C-H activation chemistry means that much of the FLP is
We have shown that the xanthene-based FLP 1a – which has
previously been shown to be capable of the reversible activation
of H2,6 can also effect similar (reversible) cleavage of the C-H
bond in phenylacetylene, PhCCH. FLP 1a also reacts with B-H
bonds, such as those in HBpin and HBcat, although in a different
fashion, proceeding via C-B/B-H exchange. This process leads
to replacement of the –B(C6F5)2 Lewis acid component
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10.1002/chem.201801871
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FULL PAPER
sequestered as the phosphonium acetylide, and catalytic
turnover is much slower.
1
148.6 (d, JC-F = 253 Hz) 153.6, and 155.4 ppm (ArC). 11B NMR (128
MHz, CD2Cl2, 298 K): δ = -18.4 ppm (br). 31P NMR (162 MHz, CD2Cl2,
298 K): δ = -9.1 ppm (d, 1JP-H = 544 Hz). 19F{1H} NMR (376 MHz, CD2Cl2,
3
298 K): δ = -131.4 (br, o-CF, 4F), -161.2 (t, JF-F = 20.7 Hz, 2F, p-CF),
and -166.0 ppm (m, m-CF, 4F). No reliable microanalysis was obtained
due to the instability of the compound.
Experimental Section
General methods: All manipulations were carried out using standard
Schlenk line or dry-box techniques under an atmosphere of argon or
dinitrogen. Solvents were degassed by sparging with argon and dried by
passing through a column of the appropriate drying agent and stored
over potassium or 4 Å sieves. NMR spectra were measured in benzene-
d6 (dried over potassium), dichloromethane-d2 (dried over CaH2), or
toluene-d8 (dried over potassium), with the solvent then being distilled
under reduced pressure and stored under argon in Teflon valve
ampoules. NMR samples were prepared under argon in 5 mm Wilmad
507-PP tubes fitted with J. Young Teflon valves. The 1H, 13C, 11B, 31P,
and 19F NMR spectra were recorded on Bruker Avance III HD nanobay
400 MHz or Bruker Avance III 500 MHz spectrometer at ambient
temperature and referenced internally to residual protio-solvent (1H) or
solvent (13C) resonances and are reported relative to tetramethylsilane (δ
Preparation of Compound 2b: FLP 1b (0.164 g, 0.20 mmol) was
dissolved in dichloromethane (3 mL). Phenylacetylene (25 μL, 0.23
mmol) was added to the solution yielding an instant colour change from
bright yellow to colourless. The solution was stirred at room temperature
for 16 h, filtered, concentrated, and layered with hexane to yield
colourless crystals of 2b (0.081 g, 44%). 1H NMR (400 MHz, CD2Cl2, 298
K): δ = 1.73 (br, 6H, XAN-C(CH3)2), 1.88 (br, 12H, o-CH3 of Mes), 2.32 (s,
3
6H, p-CH3 of Mes), 6.71 (d, JH-H = 6.9 Hz, 1H, ArH), 6.83-6.91 (m, 4H,
3
3
MesH), 6.96 (m, 2H, ArH), 7.11 (td, JH-H = 7.7 Hz, JH-H = 2.7 Hz, 1H,
ArH), 7.16 (m, 3H, ArH), 7.24 (m, 2H, ArH), 7.33 (dd, 3JH-H = 7.9 Hz, 3JH-H
= 1.6 Hz, 1H, ArH), 7.86 (d, 3JH-H = 7.7 Hz, 1H, ArH), and 10.18 ppm (d,
1JP-H = 532 Hz, 1H, P-H). 13C{1H} NMR (126 MHz, C6D6, 298 K): δ = 21.0
(Mes-CH3), 21.4 (XAN-C(CH3)2), 21.8 (Mes-CH3), 21.9 (Mes-CH3), 34.7
(XAN-C(CH3)2), 104.3 (PhCCB), 105.0 (PhCCB), 123.5, 124.2, 124.3,
124.4, 126.6, 127.2, 127.9, 128.4, 128.8, 131.9, 132.1 (br), 132.4, 133.3
= 0 ppm), 31P resonances are referenced externally to H3PO4 (85 %), 19
F
chemical shifts to CF3COOH, and 11B chemical shifts to BF3۰Et2O.
Chemical shifts are quoted in δ (ppm) and coupling constants in Hz.
Elemental analyses were carried out by London Metropolitan University.
1
1
2
(d, JP-C = 9.4 Hz), 133.8, 134.0 (d, JP-C = 6.4 Hz), 134.6 (d, JP-C = 1.5
Hz), 135.8 (br), 137.5 (br), 139.3 (br), 144.2 (br), 146.3 (br), 147.8 (br),
2
149.7 (br), 154.0 and 155.1 ppm (d, JP-C = 2.4 Hz). 11B{1H} NMR (128
4-(Diphenylphosphino)-9,9-dimethylxanthene,20 FLPs 1a and 1b,6,7
MHz, CD2Cl2, 298 K): δ = -17.9 ppm (s). 31P NMR (162 MHz, CD2Cl2, 298
23
ClBpin,21 ClBcat,22 and PhB(C6F5)2
were prepared by literature
1
3
K): δ = -27.3 ppm (dd, JP-H = 532 Hz, JP-H = 14 Hz). 19F{1H} NMR (376
MHz, CD2Cl2, 298 K): δ = -129.7 (br, 2F, o-CF), -131.9 (br, 1F, o-CF), -
135.3 (br, 1F, o-CF), -159.8 (br, 1F, p-CF), -162.9 (br, 1F, p-CF), -166.4
(br, 4F, m-CF). 19F{1H} NMR (376 MHz, CD2Cl2, 240 K): δ = -130.0 (dd,
3JF-F = 26.0 Hz, 4JF-F = 6.0 Hz, 1F, o-CF), -130.4 (dd, 3JF-F = 25.4 Hz, 4JF-F
= 8.2 Hz, 1F, o-CF), -131.9 (d, 3JF-F = 25.9 Hz, 1F, o-CF), -136.2 (d, 3JF-F
= 25.1 Hz, 1F, o-CF), -158.8 (t, 3JF-F = 21.2 Hz, 1F, p-CF), -162.3 (t, 3JF-F
= 21.2 Hz, 1F, p-CF), -165.4 (m, 2F, m-CF), -166.3 (t, 3JF-F = 23.1 Hz, 1F,
methods. All other reagents were used as received.
Details of the X-ray diffraction measurements: Single-crystal X-ray
diffraction data for 2b–8 were collected at 150
K on Oxford
Diffraction/Agilent SuperNova diffractometers with Cu-Kα (λ = 1.54184 Å)
radiation equipped with nitrogen gas Oxford Cryosystems Cryostream
unit.24 Raw frame data were reduced using CrysAlisPro.25 The structures
were solved using SHELXT26 and refined to convergence on F2 and
against all independent reflections by full-matrix least-squares using
SHELXL27 in combination with the SHELXLE28 program. Distances and
angles were calculated using the full covariance matrix. Selected
crystallographic data are summarized in the tables S1–S2 and full details
are given in the supplementary deposited CIF files (CCDC 1827962-
1827968). These data can be obtained free of charge from the
3
m-CF) and -166.6 ppm (t, JF-F = 23.2 Hz, 1F, m-CF). Elemental anal.
calc. for C47H30BF10OP + CH2Cl2: C 64.24% and H 4.19%, found C
64.67% and H 4.66%.
Synthesis of compound 3: Phenylacetylene (0.35 ml, 3.17 mmol) was
added to a stirred solution of 1a (0.150 g, 0.32 mmol) in toluene (10 mL).
The mixture was stirred for 20 h at room temperature after which the
yellow solution was concentrated and layered with pentane. Multi-
insertion product 3 crystallized as colourless crystals suitable for single-
crystal X-ray diffraction studies. The solvent was decanted, and the
crystals washed with pentane (3 x 5 mL) to yield 0.035 g (12%) of 3. 1H
NMR (400 MHz, C6D6, 298K): δ = 1.43 (s, 3H, XAN-C(CH3)2), 1.67 (s, 3H,
XAN-C(CH3)2), 4.79 (s, 1H, PhCCH), 6.40 (s, 1H B-Ar-CH), 6.44 (d, 3JH-H
= 6.5 Hz, 1H, XAN-ArH), 6.45-6.53 (m, 5H, ArH), 6.61-6.66 (m, 5H, ArH)
Cambridge
Crystallographic
Data
Centre
via
http://www.ccdc.cam.ac.uk/data_request/cif.
Computational details: All computational work reported here utilized the
density functional theory (DFT) level with Gaussian09 (Revision D.01)
program package.29 Geometry optimizations were performed with the
PBE1PBE exchange-correlation functional using def-TZVP basis sets.30
The nature of all stationary points found (minimum or saddle point) was
confirmed by full frequency calculations.
3
6.71-6.82 (m, 10H, P-ArH), 6.92 (t, JH-H = 7.08 Hz, 1H, XAN-ArH) 7.03-
3
1
7.12 (m, 5H, ArH), 7.26 (dd, JH-H = 7.4 Hz, JP-H = 13.8 Hz, 1H, XAN-
ArH), 7.36 (t, 3JH-H = 7.4 Hz, 1H, ArH), 7.51 (s, 1H, ArH), 7.58 (d, JH-H
=
3
3
7.5 Hz, 1H, XAN-ArH), and 7.88 ppm (d, JH-H = 7.2 Hz, 1H, XAN-ArH).
13C{1H} NMR (101 MHz, C6D6, 298 K): δ = 26.0 (XAN-C(CH3)2), 33.3
(XAN-C(CH3)2), 58.4 (d,1JP-C =12.2 Hz, Ph2P-C-Ph), 59.6, 106.4, 107.4,
119.7, 120.3, 125.3, 125.5 (d, 1JP-C = 11.1 Hz), 125.6, 125.8, 127.1, 127.5
(d,1JP-C = 9.1 Hz), 128.5, 128.8 (d, JP-C = 4.8 Hz), 129.1 (d, JP-C = 5.6
Hz), 129.3, 129.6, 131.6 (d, JP-C = 12.5 Hz), 132.6 (d, JP-C = 8.6 Hz),
133.0 (d, 1JP-C = 7.3 Hz), 133.0, 133.3 (d, 1JP-C = 4.8 Hz), 133.5 (d, 1JP-C
Synthesis of compound 2a: FLP 1a (0.100 g, 0.14 mmol) was
dissolved in toluene (1.5 mL) and phenylacetylene (16 µL, 0.14 mmol)
was added at room temperature. After 30 min, a white precipitate formed
and the solvent was removed by filtration. The precipitate was washed
with pentane (3 x 5 mL) and dried in vacuo to afford 2a as a white solid
(0.075 g, 81%). The compound is stable in CD2Cl2 for 2 h at room
temperature. Attempts to crystallize compound 2a from various solvents
1
1
1
=
=
1
1
1
4.2 Hz), 134.3, 134.4 (d, JC-F = 235 Hz), 134.5, 135.4, 136.5 (d, JP-C
were unsuccessful as it converted into multi-insertion product 3. H NMR
6.3 Hz), 136.7, 136.8 (d, 1JC-F = 234 Hz), 137.0 (d, 1JC-F = 251 Hz), 138.7
(d, 1JC-F = 245 Hz), 143.0 (d, 1JC-F = 234 Hz), 144.4, 145.6, 144.4, 145.8,
(400 MHz, CD2Cl2, 298 K): δ = 1.72 (br, 6H, XAN-C(CH3)2), 6.68 (d, 3JH-H
3
= 6.4 Hz, 1H, ArH), 6.72-6.78 (m, 1H, ArH), 6.95 (d, JH-H = 7.6 Hz, 2H,
1
146.2 (d, JC-F = 238 Hz), 152.4, and 158.3 ppm (ArC). 11B NMR (128
ArH), 7.01-7.11 (m, 8H, ArH), 7.13-7.17 (m, 2H, ArH), 7.34-7.39 (m, 4H,
ArH), 7.62-7.66 (m, 2H, ArH), 7.88 (d, 3JH-H = 8.0 Hz, 1H, ArH), and 10.50
MHz, C6D6, 298 K): δ = 11.8 ppm (s). 31P NMR (162 MHz, C6D6, 298K): δ
= 20.1 ppm (s). 19F{1H} NMR (376 MHz, C6D6, 298 K): δ = -125.7 (d, 3JF-
1
ppm (d, JP-H = 544 Hz, 1H, P-H). 13C{1H} NMR (101 MHz, CD2Cl2, 258
3
= 42.0 Hz, 1F, o-CF), -130.6 (d, JF-F = 37.6 Hz, 1F, o-CF), -132.2 (d,
F
K): δ = 21.3 (XAN-C(CH3)2), 34.7 (XAN-C(CH3)2), 100.9 (PhCCB), 114.6
3JF-F = 39.9 Hz, 1F, o-CF), -135.1 (d, 3JF-F = 37.1 Hz, 1F, o-CF), -156.7 (t,
3JF-F = 24.3 Hz, 1F, p-CF), -159.2 (m, 1F, m-CF), -162.2 (m, 1F, m-CF), -
1
(PhCCB), 122.7, 123.8, 124.1 (d, JC-P = 19.4 Hz), 125.2, 126.2, 126.9,
127.9, 128.2, 128.5, 129.0, 129.9 (d, 1JC-P = 13.4 Hz), 131.7 (d,2JC-P = 4.5
3
1
1
163.2 (t, JF-F = 25.3 Hz, 1F, p-CF), -166.1 (m, 1F, m-CF), and -167.8
Hz), 132.3, 133.5 (d, JC-P = 11.6 Hz), 134.0 (d, JC-P = 5.9 Hz), 134.9 (d,
1JC-P = 2.7 Hz), 137.0 (dm, JC-F = 235 Hz), 137.8 (dm, JC-F = 238 Hz),
1
1
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10.1002/chem.201801871
Chemistry - A European Journal
FULL PAPER
ppm (m, 1F, m-CF). Elemental anal. calc. for C63H40BF10OP + C7H8 + 0.5
(dm, 1JC-F = 260 Hz), 140.0 (dm, 1JC-F = 251 Hz), 148.1, 148.5, 148.7 (dm,
2
C5H12: C 74.24% and H 4.64%, found C 74.41% and H 4.46%.
1JC-F = 237 Hz), 153.7 (d, JP-C = 2.0 Hz), and 154.6 ppm (ArC). 11B{1H}
NMR (128 MHz, C6D6, 298 K): δ = -21.8 (s, HB(C6F5)2) and 31.8 ppm (s,
B-O). 31P{1H} NMR (162 MHz, C6D6, 298 K): δ = 13.4 ppm (s). 19F{1H}
Synthesis of compound 4: FLP 1a (0.100 g, 0.14 mmol) was dissolved
in toluene (1.5 mL). 4-MeOC6H4CCH (19 µL, 0.15 mmol) was then added
and the mixture was stirred for 14 h at room temperature. The solvent
was removed by filtration, the white precipitate washed with pentane (3 x
5 mL) and dried in vacuo to afford compound 4 as a white solid (0.075 g,
79%). Single crystals suitable for X-ray diffraction studies were grown by
slow diffusion from a toluene/pentane mixture at room temperature. 1H
NMR (500 MHz, C6D6, 298 K): δ = 1.42 (s, 3H, XAN-C(CH3)2), 1.60 (s,
NMR (376 MHz, C6D6, 298 K): δ = -127.4 (br, 4F, o-CF), -157.9 (t, 3JF-F
=
19.6 Hz, 2F, p-CF) and -164.1 ppm (m, 4F, m-CF). Elemental anal. calc.
for C45H27B2F10O3P: C 62.97% and H 3.17%, found C 62.90% and H
3.17%.
Synthesis
of
compound
7:
4-(Diphenylphosphino)-9,9-
dimethylxanthene (2.138 g, 5.39 mmol) was dissolved in Et2O (40 mL)
and cooled to −78 °C. nBuLi (3.6 mL, 1.6 M, 5.76 mmol) was added
dropwise. The mixture was slowly warmed up to room temperature and
stirred for 16 h. The resulting off-white precipitate was allowed to settle,
and solution was decanted. The white powder product was washed with
3
3H, XAN-C(CH3)2), 3.23 (s, 3H, OCH3), 6.21 (d, JH-H = 7.81, 1H, ArH),
3
6.49-6.56 (m, 4H, MeO-ArH), 6.67 (t, JH-H = 7.8, 2.8 Hz, 1H, ArH), 6.79
1
3
(dd, JP-H = 14.2 Hz, JH-H = 7.5 Hz, 1H, ArH), 7.02-7.16 (m, 10H, ArH),
3
3
3
7.39 (d, JH-H = 7.8 Hz, 1H, ArH), 7.56 (dd, JH-H = 12.7, JH-H = 7.7 Hz,
1H, ArH), 7.82 (d,
1
3JH-H = 6.6 Hz, 1H, ArH), and 9.81 ppm (d, JP-H
=
cold Et2O (2
(diphenylphosphino)(5-lithio)-9,9-dimethyl-xanthene (1.160 g, 50%),
which was used directly to prepare compound 7. 4-
x 10 mL) and dried under vacuum to give 4-
38.1 Hz, 1H, P-H). 13C{1H} NMR (101 MHz, C6D6, 298 K): δ = 21.1 (XAN-
C(CH3)), 26.5 (XAN-C(CH3)2), 27.6 (XAN-C(CH3)2), 55.4 (CH3-O-PhCC),
1
120.4, 121.0, 121.2, 121.8, 123.4, 123.6 (d, JP-C = 12.1 Hz), 125.4
(diphenylphosphino)(5-lithio)-9,9-dimethylxanthene (0.500 g, 0.57 mmol)
was dissolved in n-hexane (20 mL) and ClBpin (0.170 g, 1.04 mmol) was
added dropwise as an n-hexane solution (10 mL) at −78 °C. The white
slurry was stirred at −78 °C for 15 min and then at room temperature for
16 h. The precipitate was allowed to settle and the solution filtered and
concentrated to incipient crystallization. Single crystals suitable for X-ray
diffraction studies were grown from n-hexane solution at 6 °C. Yield
(MeO-PhC=CH-B(C6F5)2), 127.4, 129.0 (d, 1JP-C = 11.7 Hz), 128.4, 129.2,
1
1
130.3 (d, JP-C = 12.6 Hz, MeO-Ph-C-PPh2), 131.6 (d, JP-C = 2.9 Hz),
1
1
132.3 (dm, JC-F = 237 Hz), 132.7, 132.8 (d, JP-C = 10.2 Hz), 132.9 (d,
1JP-C = 3.4 Hz), 135.5 (dm, JC-F = 257 Hz), 133.8 (d, JP-C = 8.7 Hz ),
134.8 (d, 1JP-C = 9.9 Hz), 135.4, 138.2 (dm, 1JC-F = 236 Hz), 140.3 (d, 1JP-
C = 3.1 Hz), 156.5 ppm (d, 1JP-C = 3.1 Hz) and 160.2 ppm (ArC). 11B NMR
(160 MHz, C6D6, 298 K): δ = -11.9 ppm (s). 31P NMR (202 MHz, C6D6,
1
1
1
0.210 g (39%). H NMR (400 MHz, C6D6, 298 K): δ = 1.17 (s, 12H, Bpin
3
3
298 K): δ = 18.6 ppm (q, JP-B = 19.3 Hz). 19F{1H} NMR (470 MHz, C6D6,
298 K): δ = -127.6 (br, 4F, o-CF), -162.3 (t, JF-F = 45.7 Hz, 2F, p-CF),
CH3), 1.39 (s, 6H, XAN-C(CH3)2), 6.76 (t, JH-H = 7.6 Hz, 1H, ArH), 6.85-
3
3
6.88 (m, 1H, ArH), 6.98 (t, JH-H = 7.5 Hz, 1H, ArH), 7.05-7.11 (m, 6H,
and -167.3 ppm (br, 4F, m-CF). Elemental anal. calc. for C48H30BF10O2P:
C 66.23% and H 3.47% found C 66.34% and H 3.60%.
ArH), 7.13 (d, 3JH-H = 7.7 Hz, 1H, ArH), 7.25 (d, 3JH-H = 7.5 Hz, 1H, ArH),
3
3
7.50 (t, JH-H = 7.0 Hz, 4H, ArH) and 8.01 ppm (d, JH-H = 7.3 Hz, 1H,
ArH). 13C{1H} NMR (126 MHz, C6D6, 298 K): δ = 25.0 (Bpin CH3), 31.9
(XAN-C(CH3)2), 34.5 (XAN-C(CH3)2), 83.6 (Bpin C(CH3)2), 118.6 (B‒C),
Synthesis of compound 5: FLP 1a (0.136 g, 0.18 mmol) was dissolved
in dichloromethane (4 mL) and HBpin (0.1 mL, 0.69 mmol) was added
dropwise at room temperature. The yellow solution was stirred at room
temperature for 3 d after which the colour had faded to pale yellow. The
reaction mixture was filtered, concentrated, and layered with n-hexane to
yield colourless crystals suitable for single-crystal X-ray diffraction
studies upon standing at −30 °C. Yield 0.051 g (33%). 11B{1H} NMR (128
MHz, C7D8, 298 K): δ = -21.2 (s, HB(C6F5)2) and 31.6 ppm (s, B-O).
31P{1H} NMR (162 MHz, C7D8, 298 K): δ = 14.8 (br s) and 18.2 ppm (br s).
1
123.2, 123.8, 125.9, 126.1, 126.9, 128.0, 128.2, 128.4, 128.6 (d, JP-C
=
1
6.2 Hz), 129.0, 130.2, 130.6, 132.7, 134.4 (d, JP-C = 19.6 Hz), 135.8,
1
1
139.3 (d, JP-C = 15.7 Hz), 153.9 (d, JP-C = 18.2 Hz), and 155.6 ppm
(ArC). 11B{1H} NMR (128 MHz, C6D6, 298 K): δ = 31.0 ppm (s). 31P{1H}
NMR (162 MHz, C6D6, 298 K): δ = −14.0 ppm (s). Elemental anal. calc.
for C33H34BO3P: C 76.16% and H 6.59%, found C 76.03% and H 6.72%.
Synthesis of compound 8: 4-(diphenylphosphino)(5-lithio)-9,9-
dimethylxanthene (0.400 g, 0.46 mmol) was dissolved in n-hexane (20
mL). An n-hexane solution (10 mL) of ClBcat (0.140 g, 0.91 mmol) was
added dropwise at −78 °C. The white slurry was stirred at −78 °C for 15
min and then at room temperature for 16 h. The white precipitate was
allowed to settle and the solution filtered and concentrated to incipient
crystallization. Single crystals suitable for X-ray diffraction studies were
grown from n-hexane solution at −30 °C. Yield 0.120 g (26%). 1H NMR
(400 MHz, C6D6, 298 K): δ = 1.39 (s, 6H, XAN-C(CH3)2), 6.81-6.98 (m,
3
19F{1H} NMR (376 MHz, C7D8, 298 K): δ = -127.2 (d, JF-F = 23.7 Hz, 4F,
3
3
o-CF), -158.1 (t, JF-F = 19.2 Hz, 2F, p-CF), and -164.2 ppm (td, JF-F
=
23.5 Hz, 4JF-F = 8.3 Hz, 4F, m-CF). 1H NMR (400 MHz, C7D8, 373 K): δ =
0.84 (s, 12H, Bpin-CH3), 1.33 (s, 6H, XAN-C(CH3)2), 4.70 (br, 1H, B-H),
3
6.87 (t, 3JH-H = 7.5 Hz, 2H, ArH), 6.95-7.04 (m, 7H, ArH), 7.18 (d, JH-H
=
7.6 Hz, 1H, ArH), 7.33 (m, 1H, ArH), 7.61 (br, 4H, ArH) and 7.75 ppm (d,
3JH-H = 6.5 Hz, 1H, ArH). 11B{1H} NMR (128 MHz, C7D8, 373 K): δ = -20.8
(s, HB(C6F5)2) and 30.2 ppm (s, B-O). 31P{1H} NMR (162 MHz, C7D8, 373
K): δ = 15.9 ppm (br s). We were not able to obtain 13C{1H} NMR
spectrum due to decomposition of the compound at elevated
temperatures. Elemental anal. calc. for C45H35B2F10O3P: C 62.39% and H
4.07%, found C 62.31% and H 3.81%.
3
12H, ArH), 7.20-7.24 (m, 3H, ArH), 7.33 (t, JH-H = 7.5 Hz, 4H, ArH), and
7.77 ppm (dd, 3JH-H = 7.3 Hz, 3JH-H = 1.6 Hz, 1H, ArH). 13C{1H} NMR (126
MHz, C6D6, 298 K): δ = 30.9 (XAN-C(CH3)2), 34.7 (XAN-C(CH3)2), 113.0
(Bcat O‒C), 115.2
(B‒C), 122.7, 123.5, 124.1, 126.3, 126.6, 126.8,
128.0, 128.2, 128.3, 128.5 (d, 1JP-C = 6.7 Hz), 128.6, 129.5, 130.9, 130.9,
132.1, 134.3 (d, 1JP-C = 21.1 Hz), 135.1, 137.9 (d, 1JP-C = 13.6 Hz), 149.3,
153.4 (d, 1JP-C = 16.3 Hz), and 156.1 ppm (ArC). 11B{1H} NMR (128 MHz,
C6D6, 298 K): δ = 32.0 ppm (s), 31P{1H} NMR (162 MHz, C6D6, 298 K): δ
= −13.0 ppm (s). Elemental anal. calc. for C33H26BO3P: C 77.36% and H
5.12%, found C 77.28% and H 5.19%.
Synthesis of compound 6: FLP 1a (0.127 g, 0.17 mmol) was dissolved
in dichloromethane (4 mL) and HBcat (0.05 mL, 0.47 mmol) was added
dropwise at room temperature. The yellow solution was stirred at room
temperature for 3 d to afford a colourless solution. The reaction mixture
was filtered, concentrated, and layered with n-hexane to yield colourless
crystals suitable for single-crystal X-ray diffraction studies upon standing
at −30 °C. Yield 0.075 g (51%). 1H NMR (400 MHz, C6D6, 298 K): δ =
1.26 (s, 6H, XAN-C(CH3)2), 4.60 (br, 1H, B-H) 6.69-6.74 (m, 6H, ArH),
Acknowledgements
3
3
6.80 (t, JH-H = 7.5 Hz, 1H, ArH), 6.84-6.92 (m, 4H, catH), 6.96 (t, JH-H
=
7.8 Hz, 1H, ArH), 7.12 (d, 3JH-H = 7.5 Hz, 1H, ArH), 7.25 (d, 3JH-H = 7.5 Hz,
3
3
We thank the EPSRC Catalysis Hub (EP/K014714/1). P.V.
thanks the Magnus Ehrnrooth Foundation, the Finnish Cultural
Foundation and Emil Aaltonen Foundation for funding. Oxford
Advanced Research Computing are acknowledged for providing
1H ArH), 7.46 (t, JH-H = 9.5 Hz, 4H, ArH), 7.54 (d, JH-H = 6.9 Hz, 1H,
ArH), and 8.26 ppm (br, 1H, ArH). 13C{1H} NMR (126 MHz, C6D6, 298 K):
δ = 30.4 (XAN-C(CH3)2), 34.7 (XAN-C(CH3)2), 112.6, 112.7 (Bcat O‒C),
1
115.0, 116.7
(B‒C), 122.8, 123.0, 124.3, 124.4 (d, JP-C = 13.5 Hz),
1
125.9, 126.4, 128.0, 128.2, 128.3, 129.2, 130.5, 131.3, 131.3 (d, JP-C
2.7 Hz), 132.8 (d, JP-C = 8.5 Hz), 133.3 (d, JP-C = 3.8 Hz), 135.7, 137.5
=
1
1
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10.1002/chem.201801871
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computational resources. Dr E.L. Kolychev is thanked for
assistance with X-ray crystallography.
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Entry for the Table of Contents
Layout 2:
FULL PAPER
Petra Vasko,* Ili A. Zulkifly, M. Ángeles
Fuentes, Zhenbo Mo, Jamie Hicks, Paul
C. J. Kämer,* and Simon Aldridge*
Page No. – Page No.
Reversible C-H activation, facile B-
C/B-H metathesis and apparent
hydroboration catalysis by a
dimethylxanthene-based frustrated
Lewis pair
A dimethylxanthene-based FLP can effect reversible cleavage of the C-H bond in
PhCCH, and can also react with B-H bonds, such as those in HBpin and HBcat via
a B-C/B-H exchange. The latter reactivity gives rise to Piers’ borane, HB(C6F5)2,
which acts as the active catalyst for the hydroboration of alkynes by HBpin.
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