Cyclic phosphonium bis(fluoroaryl)boranes- Trends in Lewis acidities and application in Diels-Alder catalysis

Article abstract of DOI:10.1002/ejic.201101098

The rigid cyclic Lewis acids [(C₆F₅)B(CH ₂)(C₆F₄)P(tBu)₂]⁺ ([1]⁺), [(C₆F₅)B(CH₂)(C ₆F₃H)P(tBu)₂]

Full text of DOI:10.1002/ejic.201101098

FULL PAPER
DOI: 10.1002/ejic.201101098
Cyclic Phosphonium Bis(fluoroaryl)boranes – Trends in Lewis Acidities and
Application in Diels–Alder Catalysis
Anna Schnurr,[a] Michael Bolte,[a] Hans-Wolfram Lerner,[a] and Matthias Wagner*[a]
Keywords: Boron / Homogeneous catalysis / Lewis acids / Perfluoroarenes / Phosphorus
The rigid cyclic Lewis acids [(C6F5)B(CH2)(C6F4)P(tBu)2]+
([1]+), [(C6F5)B(CH2)(C6F3H)P(tBu)2]+ ([2]+), and [(C6H5)-
B(CH2)(C6F4)P(tBu)2]+ ([3]+), possess Gutmann acceptor
numbers of AN = 87.3, 85.7, and 85.7, respectively, which are
among the highest for organoboranes. Starting from (1)OTf,
adducts [(1)Do][OTf] (Do = OPEt3, pyridine, H2O) have been
prepared and fully characterized. In all three cases, X-ray
crystallography revealed significantly shorter B–O/N bond
lengths than those in the corresponding adducts (C6F5)3B·Do.
Both the free Lewis acid [1][Al(O(tBuF))4] and its triflate ad-
duct (1)OTf have successfully been employed as catalysts for
the [4+2] cycloaddition reaction between 2,5-dimethyl-1,4-
benzoquinone and cyclopentadiene. Moreover, we have
shown that chiral phosphonium boranes are accessible by re-
placement of one tBu group in [1]+ by a Me substituent {cf.
[4]+ = [(C6F5)B(CH2)(C6F4)P(Me)(tBu)]+}.
Introduction
Lewis acidic arylboranes find applications as homogen-
eous (co)catalysts[1–6] and anion sensors.[7–10] The use of
perfluorinated aryl substituents or the incorporation of a
positive charge into the molecular framework are tools to
optimize the Lewis acid strength for any given task.[11–20]
The reinforcing effect of a positive charge on the adduct
bond of an anionic Lewis base is merely of an electrostatic
nature and therefore distance-dependent. Thus, in order to
guarantee a stabilizing effect that is invariable in time, a
rigid molecular framework is required so that the distance
between the boron atom and the cationic center always re-
mains the same.
Figure 1. General structural motif of the cationic phosphonium
boranes investigated in this work.
Results and Discussion
Assessment of the Lewis Acidities of [A]+-Type Compounds
by NMR Spectroscopic Methods
Following this concept, we have recently prepared cyclic
phosphonium boranes [A][WCA] (Figure 1). Because of
their high Lewis acidity, the free boranes are only existent in
combination with the extremely weakly coordinating anion
[WCA]– = [Al(O(tBuF))4]–.[21,22] In this paper, we report on
the quantitative trend in the Lewis acidities of these com-
pounds as a function of the degree of fluorination at the
exocyclic phenyl substituent (i.e., R1 = C6F5, C6H5) or the
phenylene bridge (i.e., R2 = F, H). Moreover, we compare
the performance of the free acid [A][WCA] (R1 = C6F5, R2
= F, R3 = tBu) and its corresponding triflate adduct
(A)OTf[21] as Diels–Alder catalysts, and we explore whether
chiral Lewis acids can be generated by variation of the sub-
stituent R3 at the phosphorus atom.
For Lewis acidity determinations by the Gutmann–
Beckett method[23–25] we used a protocol similar to that de-
scribed by Stephan et al.[15a] (Scheme 1): First, the free
[a] Institut für Anorganische und Analytische Chemie, Goethe-
Universität Frankfurt,
Max-von-Laue-Strasse 7, 60438 Frankfurt (Main), Germany
Fax: +49-69-798-29260
E-mail: Matthias.Wagner@chemie.uni-frankfurt.de
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejic.201101098.
Scheme 1. Preparation of cationic phosphonium boranes [A][WCA]
(A = 1, 2, 3) from their corresponding chloro adducts (A)Cl and
subsequent addition of OPEt3 for Lewis acidity determination by
the Gutmann–Beckett method. (i) CD2Cl2, room temp., 2 min.
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Cyclic Phosphonium Bis(fluoroaryl)boranes
Lewis acids [1]+, [2]+, and [3]+ were liberated from their conclude that the solution equilibrium contains essentially
corresponding chloro adducts (1)Cl,[21] (2)Cl, and (3)Cl[21] no uncoordinated OPEt3.
by addition of [Ag(CH2Cl2)][WCA].[26] A substoichiometric
[(1)OPEt3][OTf] crystallizes with two crystallographically
amount of OPEt3 was added, and the 31P{1H} NMR spec- independent molecules in the asymmetric unit {[(1)OPEt3]-
trum was recorded in CD2Cl2 to determine the Gutmann [OTf], [(1)OPEt3][OTf]A}, of which only [(1)OPEt3][OTf] is
acceptor number (AN; Table 1).
discussed further (Figure 2).
Table 1. 31P{1H} NMR chemical shift values of OPEt3 in the
Lewis acid–base adducts [(1)OPEt3][WCA], [(2)OPEt3][WCA], and
[(3)OPEt3][WCA]. Gutmann acceptor numbers (AN) of the Lewis
acids [1][WCA], [2][WCA], and [3][WCA].
Boron-containing
Lewis acid
δ(31P{1H}) [ppm][a]
OPEt3
AN[b]
None
50.5
77.1[d]
80.4
79.7
79.7
21.0[c]
80.0
87.3
85.7
85.7
(C6F5)3B
[1][WCA]
[2][WCA]
[3][WCA]
[a] CD2Cl2, 27 °C. [b] AN = [δ(31P{1H})sample – 41.0]ϫ[100/
(86.14 – 41.0)].[24] [c] AN(CH2Cl2) according to our own measure-
ment; literature value: AN(CH2Cl2) = 20.4.[23] [d] Our own mea-
surement; literature value: δ(31P{1H}, CD2Cl2) = 78.1.[15b]
All three phosphonium borane derivatives possess a
higher acceptor number (AN = 87.3, 85.7, 85.7, respec-
tively) than the neutral molecule (C6F5)3B (AN = 80.0) and
even the phosphonium borane [(C6F5)2B–(p-C6F4)–
P(H)(tBu)2]+ (AN = 80.2).[15a] The less exhaustively fluorin-
ated species [2]+ and [3]+ are slightly weaker Lewis acids
than [1]+ {Δ(AN) = 1.6}. Somewhat surprisingly, replace-
ment of one F atom on the phenylene bridge (cf. [2]+) ap-
pears to have a similar effect on the acceptor number than
substitution of all five F atoms for H atoms on the exocyclic
phenyl ring in [3]+ (this result has been reproduced twice).
The 31P{1H} NMR chemical shift value of OPEt3 in its
Lewis acid–base adducts is influenced by the position of the
association/dissociation equilibrium. In order to obtain
data of a sample consisting exclusively of an adduct, we
prepared [(1)OPEt3][OTf] from the triflate (1)OTf[21]
(Scheme 2) and measured its 31P{1H} NMR spectrum in
the solid state (see the Supporting Information for a plot of
the spectrum). The chemical shift values of 86.1 ppm
(BCH2P) and 79.9 ppm (OPEt3) deviate by a maximum of
1.1 ppm from those obtained for [(1)OPEt3][OTf]
{85.2 ppm (BCH2P), 80.9 ppm (OPEt3)} and [(1)OPEt3]-
[WCA] {85.0 ppm (BCH2P), 80.4 ppm (OPEt3)} in CD2Cl2
solution. Since the OPEt3 phosphorus atom of [(1)OPEt3]-
[OTf] is less shielded in solution than in the solid state, we
Figure 2. Molecular structure and numbering scheme of the cation
of compound [(1)OPEt3][OTf]. Displacement ellipsoids are drawn
at the 30% probability level; H atoms have been omitted for clarity.
Selected bond lengths [Å], bond angles [°], and torsion angles [°]:
O(1)–P(2) 1.526(3), B(1)–O(1) 1.523(5), B(1)–C(1) 1.656(6), B(1)–
C(11) 1.623(6), B(1)–C(21) 1.643(6), P(1)–C(1) 1.803(4), P(1)–C(12)
1.809(4); B(1)–O(1)–P(2) 152.7(3), B(1)–C(1)–P(1) 107.5(3), C(1)–
B(1)–C(11) 106.2(3), C(1)–P(1)–C(12) 98.6(2); C(1)–B(1)–C(11)–
C(12) 1.2(5), C(1)–P(1)–C(12)–C(11) 5.3(3).
The X-ray crystal structure analysis confirms that the
triflate ion has been replaced by the neutral OPEt3 mole-
cule, thereby maintaining the tetracoordinate state of the
boron atom. This substitution reaction does not lead to any
significant changes in key structural parameters of the bo-
rane fragment. The B(1)–O(1) bond length amounts to
1.523(5) Å, which is shorter by 0.066(5) Å than that of the
starting compound (1)OTf [1.589(3) Å][21] and still slightly
shorter than that of (C6F5)3B·OPEt3 [1.533(3) Å].[25]
A
more significant difference is found in the O–P bond
lengthsof[(1)OPEt3][OTf][1.526(3) Å]comparedto(C6F5)3B·
OPEt3 [1.497(2) Å][25] (cf. also OPCy3: O–P = 1.490(2) Å;
Cy = cyclohexyl).[27] We are aware of the fact that bond
lengths in the solid state do not necessarily correlate with
bond strengths. However, in the present case, the differences
in the Gutmann acceptor numbers of [1]+ vs. (C6F5)3B are
nicely reflected by the trends in the B–O and O–P bond
lengths of their corresponding OPEt3 adducts.
As a second independent measure of Lewis acidities, the
Childs NMR spectroscopic method,[28] with crotonaldehyde
instead of OPEt3, is often employed.[15a] Since crotonalde-
hyde is not stable over time in the presence of strong Lewis
acids, we prepared the pyridine adduct [(1)py][OTf] instead
and took its 13C{1H} NMR chemical shift values as a diag-
nostic tool.
Scheme 2. Substitution of the triflate ligand in (1)OTf by un-
charged donors Do. (i) CH2Cl2, room temp., 12 h.
Eur. J. Inorg. Chem. 2012, 112–120
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A. Schnurr, M. Bolte, H.-W. Lerner, M. Wagner
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The compound readily precipitates from a concentrated
CH2Cl2 solution of (1)OTf upon addition of 1 equiv. of pyr-
idine (Scheme 2); crystals of [(1)py][OTf] suitable for X-ray
diffraction were grown by slow concentration of a dilute
CH2Cl2 solution.
The 11B{1H} NMR spectrum of [(1)py][OTf] is charac-
terized by a resonance at 0.7 ppm. In Table 2, the pyridine
1H and 13C{1H} chemical shift values of [(1)py][OTf] are
compared with those of free pyridine, other selected bo-
rane–pyridine adducts, and pyridinium salts. Pyridine pro-
tonation results in the shielding of the C-2,6 nuclei and the
deshielding of C-4 and C-3,5. Qualitatively similar effects
are observed upon borane–pyridine coordination. A closer
inspection of the data reveals a continuous upfield shift of
the C-2,6 resonances with an overall Δ(δ) of –3.6 ppm along
the sequence py Ǟ Ph3B·py Ǟ (C6F5)3B·py Ǟ [(1)py][OTf],
while the C-4 signal and the C-3,5 resonances are shifted to
lower field by Δ(δ) = 8.0 and 4.0 ppm, respectively. We note
analogous trends also for the chemical shift values of 4-H
and 3,5-H (the 2,6-H resonances are less meaningful, be-
cause they are closer to the borane frameworks and there-
fore influenced by magnetic anisotropy effects). Even
though these measurements do not provide quantitative ac-
ceptor numbers, they nevertheless support our previously
established order of Lewis acidities, that is, [1]+ Ͼ (C6F5)3B
Ͼ Ph3B.
Scheme 3. Top: water adducts of (C6F5)3B known in the literature;
bottom: hydrolysis of (1)OTf.
δ(31P{1H}) = 86.9 ppm}; in the 1H NMR spectrum, the
OH2 protons give rise to an extremely broad signal at ap-
proximately 5 ppm {cf. the resonance of water in CD2Cl2
appears at δ(1H) = 1.52 ppm}.[30]
Single crystals of [(1)OH2][OTf] were grown from
CH2Cl2. X-ray crystal structure analysis revealed a water
molecule coordinated to the boron atom (Figure 3). Both
OH2 atoms were located in the difference Fourier map and
were freely refined. In the solid state, the ion pair [(1)-
OH2]+[OTf]– exists as a centrosymmetric dimer, which is
Table 2. Compilation of 1H and 13C{1H} NMR chemical shift val-
ues of free and coordinated pyridine.
δ(1H)[a]
2,6-H 4-H
δ(13C{1H})[a]
3,5-H C-2,6 C-4 C-3,5
Pyridine
Ph3B·py
(C6F5)3B·py
[(1)py][OTf]
[H·py][OTs][b]
[H·py][Cl]
8.58
8.57
8.61
8.57
8.93
8.90
7.67
8.06
8.21
8.30
8.43
8.51
7.27
7.56
7.71
7.92
7.94
8.02
150.2 136.1
148.3 141.0
147.2 143.2
146.6 144.1
142.3 146.3
141.2 146.1
124.0
125.6
126.2
128.0
127.6
127.5
[a] CD2Cl2, 27 °C. [b] OTs = p-toluenesulfonate.
The most revealing structural parameter of [(1)py][OTf]
(see the Supporting Information for details of its solid-state
structure) is the B–N bond length [1.599(4) Å], which is
shorter by 0.029(4) Å than the B–N bond in (C6F5)3B·py
[1.628(2) Å].[29]
In this context, we also became interested in the molecu-
lar structure of the water adduct [(1)OH2]+, which we
wanted to compare with the structures of the (C6F5)3B·OH2
complexes B, C, and D (Scheme 3). The main emphasis lay
on B–O bond length variations as an approximate measure
of the degree of O–H bond polarization, which should in
turn be influenced by the Lewis acidity of the boron center.
Both the chloroborate (1)Cl[21] and the acetoxyborate
(1)OAc[21] are water-stable, and the solids can be handled
and stored in air for extended periods of time. In contrast,
(1)OTf readily hydrolyzes under similar conditions. Conse-
quently, [(1)OH2][OTf] formed in essentially quantitative
yield upon stirring an Et2O solution of (1)OTf in air for
48 h (Scheme 3). The NMR spectroscopic data of the com-
Figure 3. Dimeric structure of [(1)OH2][OTf] in the solid state. Ex-
cept on the H2O molecule, H atoms have been omitted for clarity.
Selected bond lengths [Å], atom···atom distances [Å], and bond
angles [°]: O(1)–H(1) 0.80(2), O(1)–H(2) 0.98(3), B(1)–O(1)
1.553(2), B(1)–C(1) 1.642(2), P(1)–C(1) 1.794(1), H(1)···O(4)
1.84(2), H(2)···O(3A) 1.67(3), O(1)···O(4) 2.647(2), O(1)···O(3A)
2.640(1); O(1)–H(1)···O(4) 177(2), O(1)–H(2)···O(3A) 171(3), B(1)–
C(1)–P(1) 108.3(1). Symmetry transformation used to generate
pound are unexceptional {δ(11B{1H})
=
1.9 ppm; equivalent atoms: A = –x + 1, –y + 1, –z.
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Cyclic Phosphonium Bis(fluoroaryl)boranes
held together by four HO–H···OTf hydrogen bonds. Bond-
ing of the triflate anion to [1]+ benefits to a greater degree
from Coulomb attraction than water coordination. Never-
theless, the B–O bond of (1)OTf [1.589(3) Å] is elongated
by 0.036(3) Å relative to that of [(1)OH2][OTf] [1.553(2) Å].
The (C6F5)3B·OH2 adducts B, C, and D possess B–O bond
lengths of 1.608(3),[31] 1.583(3),[31] and 1.577(1) Å,[32]
respectively. Thus, an increasing number of hydrogen bonds
about the water molecule, which should in turn increase the
average O–H polarity, results in a gradual decrease of the
B–O bond lengths. Of all species B, C, and D, compound
D, which also establishes two hydrogen bonds per water mo-
lecule, is most closely related to [(1)OH2][OTf]. We observe
a further significant shortening of the B–O bond from
1.577(1) Å in D to 1.553(2) Å in [(1)OH2][OTf]. This result
conforms to the higher Lewis acidity of [1]+ compared to
that of (C6F5)3B.
Scheme 4. Synthesis of (4)OEt and its transformation into the
chloro derivative (4)Cl: (i) toluene, –78 °C Ǟ room temp., 12 h;
(4)OEt is obtained as a mixture of two diastereomers in a 1:3 ratio.
(ii) Et2O, room temp., 18 h.
not only crucial for the entire chemistry of [A]+-type com-
pounds, but it is also clean and quantitative and therefore
qualifies as a test reaction for the stereospecificity of nucle-
ophilic transformations at the boron atom. As expected,
upon addition of 4 equiv. of HCl in Et2O to the dia-
stereomeric mixture of (4)OEt in Et2O (Scheme 4), the ex-
clusive formation of two diastereomers of the chloro adduct
(4)Cl was observed (for an X-ray crystal structure analysis
of the diastereomer in which the Cl atom and the Me sub-
stituent are located on the same side of the five-membered
heterocycle, see the Supporting Information). The experi-
ment was repeated five times; however, the diastereomeric
ratio of (4)Cl turned out to be poorly reproducible and
changed upon standing and workup. We therefore isolated
a pure sample of the major diastereomer of (4)OEt by col-
umn chromatography and protolyzed it with HCl in Et2O
as described above. Again, 1H NMR spectroscopy revealed
a dynamic behavior with no obvious dependence on solvent
polarity or temperature. Since the stereomeric information
at boron is lost upon going from (4)OEt to (4)Cl, we con-
clude that the phosphonium borane [4]+ {presumably in the
form of a weak adduct [4(OEt2)]+} is present in a significant
amount in the reaction mixture.
Stereoselectivity of Nucleophilic Substitution Reactions at
the Boron Centers of [A]+-Type Compounds
So far, we have gathered spectroscopic and crystallo-
graphic evidence for a higher Lewis acidity of [A]+-type cat-
ions relative to the neutral Lewis acid (C6F5)3B. Using a
prototypical substitution reaction at boron, we next studied
whether the free cationic Lewis acid [A]+ is present in the
association/dissociation equilibrium with its donor (Do) ad-
duct (A)Do to a sufficiently high degree to influence the
outcome of the reaction.
The boron atom in (A)Do is a chiral center. Introduction
of a second chiral center into the molecule will therefore
generate (A)Do in a specific diastereomeric ratio. If Do is
quantitatively substituted by another Lewis base DoЈ, two
different scenarios can be envisioned: (1) The dia-
stereomeric ratio is strictly reversed in (A)DoЈ, which would
be indicative of an SN2 reaction mechanism. (2) The dia-
stereomeric ratio changes as a result of a competing SN1
pathway (possibly assisted by intermediate OEt2 coordina-
tion). The extent of this change, in turn, correlates with the
proportion of the free Lewis acid [A]+ {or [A(OEt2)]+} in
the reaction mixture.
Assessment of the Catalytic Activity of [1]+ in a Diels–
Alder Reaction
Replacement of one tBu group in [1]+ by a Me substitu-
During the last decade, the potential of (cationic) bor-
ent offers a convenient way to introduce a second chiral anes for the catalysis of, for example, [4+2] and [3+2] cyclo-
center into the molecule. We therefore treated (C6F5)2B- addition reactions has been recognized, and even enantio-
(OEt)[33] with LiCH2P(Me)(tBu) at –78 °C in toluene and selective transformations have been developed.[35] We there-
obtained (4)OEt in a 1:3 diastereomeric ratio according to fore decided to undertake an exploratory investigation of
NMR spectroscopy [Scheme 4; LiCH2P(Me)(tBu) (cf. the the catalytic activity of [1]+ in the well-studied [4+2] cyclo-
Supporting Information) was prepared in a fashion similar addition reaction between 2,5-dimethyl-1,4-benzoquinone
to LiCH2P(tBu)2[34]]. The 11B{1H} and 31P{1H} NMR and cyclopentadiene[36] (Scheme 5).
chemical shift values of both diastereomers of (4)OEt are
Both the free Lewis acid [1][WCA] and the triflate adduct
virtually the same, but the tBu proton resonances are well (1)OTf were employed. Since [1][WCA] is extremely sensi-
separated and appear as doublets at δ = 0.66 ppm (major tive to any kind of nucleophile, a solution of this compound
diastereomer; C6D6) and δ = 0.53 ppm (minor dia- in CH2Cl2 was freshly prepared as described above. Even
stereomer; C6D6).
though the catalyst solution was separated from the AgCl
(4)OEt is a fairly inert compound and therefore has to precipitate prior to use, the presence of residual [Ag]+ traces
be activated by transformation into the chloro derivative cannot be fully excluded. Thus, we first performed blind
(4)Cl with the help of HCl in Et2O. This latter reaction is tests of the catalytic activity of [Ag(CH2Cl2)][WCA] in
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A. Schnurr, M. Bolte, H.-W. Lerner, M. Wagner
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Interestingly, we observed a slow decrease in the product
concentration (76% after 24 h) with a concomitant increase
in the amount of the 2,5-dimethyl-1,4-benzoquinone start-
ing material. Parallel to that, signs of cyclopentadiene poly-
merization were visible in the NMR spectrum. Thus, ex-
tended reaction times result in lower yields of 5, because
cyclopentadiene is continuously removed from the dynamic
equilibrium, which favors the back reaction. To validate this
interpretation, we repeated the reaction with 5 mol-% cata-
lyst loading, but this time added another 0.2 equiv. of cyclo-
pentadiene 1 h after the beginning of the reaction. In that
way, a further improved spectroscopic yield of 95% (2.5 h)
was achieved, which, again, decreased over time. In a sec-
ond run under the same conditions, the reaction was
quenched after 2.5 h by admission of air and provided 5 in
87% isolated yield after chromatographic workup.
In a next step, (1)OTf was tried as the catalyst, because
now any [Ag]+ contaminants can be excluded and this stor-
able compound is easier to handle than [1][WCA]. (1)OTf
also catalyzes the reaction of 2,5-dimethyl-1,4-benzoqui-
none with cyclopentadiene, albeit at a slower rate: With
5 mol-% of the catalyst, the conversion to 5 was 55% com-
plete after 2.5 h (entry 6). After 24 h, the reaction mixture
contained 81% of the target compound; cyclopentadiene
polymerization was still negligible. An increase in the
amount of added cyclopentadiene improved the spectro-
scopic yield of 5 to 91% (24 h; entry 7). Finally, the use of
10 mol-% of the catalyst together with 1.4 equiv. of cyclo-
pentadiene resulted in virtually quantitative yield (entry 8).
Scheme 5. Application of [1][WCA] and (1)OTf as catalysts for the
reaction of cyclopentadiene (CpH) with 2,5-dimethyl-1,4-benzo-
quinone: (i) 1.2 equiv. CpH, CH2Cl2, room temp., 2.5 h. (ii)
1.4 equiv. CpH, CH2Cl2, room temp., 24 h.
CH2Cl2 solution. Addition of the salt (5 mol-%) to the
diene/dienophile mixture at room temp. caused an immedi-
ate color change from yellow to dark brown. After 2.5 h,
the cycloaddition product 5[36] was already detectable in the
1H NMR spectrum. In addition to that, broad and ill-de-
fined signals appeared in the alkyl region of the spectrum,
and a silver mirror gradually formed on the walls of the
reaction vessel. It is known that cyclopentadiene tends to
polymerize in the presence of Lewis acids,[37] which, in our
case, is most likely a major side reaction.
Table 3 summarizes all catalysis results obtained with
[1][WCA] and (1)OTf (CH2Cl2, room temp.). The transfor-
mations were monitored by 1H NMR spectroscopy (C6D6);
conversion rates were determined by signal integration. In
the uncatalyzed background reaction, the first product res-
onances[36] appeared after approximately 5 h (entry 1).
About 20 h later, the conversion to 5 had reached 27%, the
yellow color of the solution persisted, and no side products
were detectable. Similarly moderate yields were obtained in
the presence of the precatalyst (1)Cl (entry 2). The use of
2 mol-% of [1][WCA] gave 5 in 81% spectroscopic yield af-
ter only 2.5 h (entry 3). Prolonged stirring did not induce
further significant changes. An increase in the catalyst load-
ing to 5 mol-% had only a small effect on the obtained yield
of 5 (87% after 2.5 h; entry 4), thereby indicating that the
equilibrium concentration has been reached at this stage.
Conclusions
According to the Gutmann–Beckett NMR spectroscopic
method, the rigid cyclic phosphonium boranes [(C6F5)-
B(CH2)(C6F4)P(tBu)2]+ ([1]+), [(C6F5)B(CH2)(C6F3H)-
P(tBu)2]+ ([2]+), and [(C6H5)B(CH2)(C6F4)P(tBu)2]+ ([3]+)
possess acceptor numbers of AN = 87.3, 85.7, and 85.7,
respectively, and are therefore stronger Lewis acids than the
popular neutral borane (C6F5)3B (AN = 80.0). This conclu-
sion is supported further by the results of X-ray crystal
structure analyses of adducts [(1)Do]+ and (C6F5)3B·Do
(Do = OPEt3, pyridine, H2O), which consistently reveal sig-
nificantly shorter B–O/N bond lengths in the former case.
Table 3. Investigation of the Diels–Alder reaction between 2,5-di-
methyl-1,4-benzoquinone and cyclopentadiene (CpH) in the pres-
ence of different catalysts.[a]
Entry Catalyst c(Catalyst) Dienophile/
Yield[b] [%] after
[mol-%]
CpH ratio
2.5 h 5 h 10 h 24 h
Starting from diastereomerically pure (4)OEt {[4]+
=
1
2
3
4
5
6
7
8
None
(1)Cl
[1][WCA]
0
5
2
5
5
5
5
10
1
1
1
1
1
1
1
1
1
1
1
1
0
0
3
4
11
27
19
78
76
[(C6F5)B(CH2)(C6F4)P(Me)(tBu)]+}, the protolytic trans-
formation (4)OEt Ǟ (4)Cl proceeds with the complete loss
of stereomeric information at the chiral boron center,
thereby indicating that a significant amount of the free
Lewis acid [4]+ is present in the reaction mixture.
[c]
[c]
[c]
[c]
[c]
81
87
95[e]
55
49
55
83
85
87
58
53 76
63 90
1.2[d]
1
76
(1)OTf
81
1.4[f]
1.4[f]
91[g]
96[g]
In line with that, (1)OTf, which possesses an even better
leaving group than chloride, can be used to catalyze the
[4+2] cycloaddition reaction between 2,5-dimethyl-1,4-
benzoquinone and cyclopentadiene. With respect to the un-
masked Lewis acid [1][Al(O(tBuF))4], (1)OTf leads to a
slower conversion rate, but, nevertheless, to quantitative
yields of the cycloaddition product with no detectable con-
comitant polymerization of cyclopentadiene.
[a]
Reaction
conditions:
2,5-dimethyl-1,4-benzoquinone
(0.15 mmol), CpH (0.15 mmol), CH2Cl2, room temp. [b] Reaction
monitored by H NMR spectroscopy. [c] Yield not determined. [d]
1
1 equiv. of CpH was present at the beginning of the reaction; fur-
ther CpH was added after 1 h (0.2 equiv.). [e] Workup after 2.5 h;
isolated yield: 87%. [f] 1 equiv. of CpH was present at the beginning
of the reaction; further CpH was added after 4 h (0.2 equiv.) and
6 h (0.2 equiv.). [g] Workup after 24 h; isolated yield: 80%.
116
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© 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2012, 112–120

Cyclic Phosphonium Bis(fluoroaryl)boranes
On the basis on the experience gathered with [4]+, we are
currently developing enantiomerically pure chiral deriva-
tives, which offer promising perspectives for enantioselec-
tive Lewis acid catalysis.
1 d. Single crystals suitable for X-ray diffraction were isolated by
decantation. For further characterization, the colorless crystalline
material was washed with hexane (4ϫ0.5 mL), ground, and dried
1
in vacuo over a period of 6 h. Yield: 0.12 g (0.16 mmol, 67%). H
2
3
NMR (300.0 MHz, CD2Cl2): δ = 2.03 (dq, JP,H = 11.8, JH,H
=
=
3
7.7 Hz, 6 H, PCH2CH3), 1.61 (m, 2 H, BCH2P), 1.48 [d, JP,H
16.3 Hz, 9 H, C(CH3)3], 1.24 [d, JP,H = 16.2 Hz, 9 H, C(CH3)3],
1.06 (dt, JP,H = 18.7, JH,H = 7.7 Hz, 9 H, PCH2CH3) ppm.
11B{1H} NMR (96.3 MHz, CD2Cl2): δ = 2.3 (h1/2 = 250 Hz) ppm.
13C{1H} NMR (75.5 MHz, CD2Cl2): δ = n.o. (FC, BCAr, PCAr),
= 32 Hz, C(CH3)3], 35.9 [d, JP,C = 35 Hz,
C(CH3)3], 27.8 [m, C(CH3)3], 27.1 [m, C(CH3)3], 18.1 (d, JP,C
65 Hz, PCH2CH3), 13.9 (m, BCH2P), 5.6 (d, JP,C = 5 Hz,
PCH2CH3) ppm. 19F{1H} NMR (282.3 MHz, CD2Cl2): δ = –79.0
(s, 3 F, OSO2CF3), –120.7 (m, 1 F), –127.4 (m, 1 F), –133.1 (m, 2
3
Experimental Section
3
3
General Considerations: Unless stated otherwise, all manipulations
were carried out under a nitrogen atmosphere by using Schlenk-
tube techniques or in an argon-filled glovebox. Solvents were dried
by distillation from Na (pentane, hexane), Na/benzophenone
(Et2O, benzene, toluene), or CaH2 (CH2Cl2, pyridine). iPrOH was
treated with a small amount of Na, distilled, and degassed through
several freeze-pump-thaw cycles. NMR spectroscopic data were
collected with a Bruker Avance 300 or Avance 400 spectrometer.
Chemical shift values (1H/13C{1H}) are reported in parts per mil-
lion (ppm) relative to Me4Si and were referenced to (residual) sol-
vent signals (C6D6: 7.15/128.0; CD2Cl2: 5.32/53.8; CDCl3: 7.26/
77.2; [D8]THF: 3.58/67.2). Heteronuclear chemical shift values were
referenced to external F3B·OEt2 (11B{1H}), FCCl3 (19F{1H}), and
85% H3PO4 (31P{1H}). Abbreviations: s = singlet, d = doublet, t
= triplet, q = quartet, m = multiplet, br. = broad, n.r. = multiplet
1
1
36.1 [d, JP,C
1
=
2
3
F, C6F5), –144.9 (m, 1 F), –150.9 (m, 1 F), –155.7 (tt, JF,F = 20,
4JF,F = 2 Hz, 1 F, C6F5), –162.2 (m, 2 F, C6F5) ppm. 31P{1H} NMR
(121.5 MHz, CD2Cl2): δ = 85.2 (m, 1 P, BCH2P), 80.9 (s, 1 P,
OPEt3) ppm. MS (ESI+): m/z (%) = 485 (6) [1]+, 619 (100) [(1)-
OPEt3]+. MS (ESI–): m/z (%)
=
149 (100) [OSO2CF3]–.
C28H35BF12O4P2S (768.37): calcd. C 43.77, H 4.59, S 4.17; found
C 43.59, H 4.61, S 4.30.
[(1)py][OTf]: A solution of pyridine in CH2Cl2 (0.52 m; 0.10 mL,
0.052 mmol) was added at room temp. by syringe to a stirred solu-
tion of (1)OTf (0.030 g, 0.047 mmol) in CH2Cl2 (1 mL). The reac-
tion mixture was stirred for 12 h, whereupon a colorless solid pre-
cipitated. The solvent and excess pyridine were removed in vacuo to
obtain analytically pure [(1)py][OTf]. Yield: 0.033 g (0.046 mmol,
98%). Single crystals suitable for X-ray crystallography were grown
by slow concentration of a CH2Cl2 solution of [(1)py][OTf] at room
temp. 1H NMR (300.0 MHz, CD2Cl2): δ = 8.57 (d, 3JH,H = 6.1 Hz,
1
expected in the H NMR spectrum but not resolved, n.o. = signal
not observed. The compounds [Ag(CH2Cl2)][WCA],[26] (C6F5)2B-
(OEt),[38]
(C6F5)2BH·SMe2,[39]
LiCH2P(tBu)2,[34]
(1)Cl,[21]
(1)OTf,[21] (2)OEt,[21] (3)Cl,[21] [1][WCA],[21] and [3][WCA][21] were
synthesized according to literature procedures. A convenient opti-
mized synthesis of the known compound (C6F5)2B(OiPr)[40] and its
transformation into (1)OiPr are described in the Supporting Infor-
mation. Me2P(tBu) has been mentioned in the literature;[41] how-
ever, the isolation of the free phosphane has not been described
yet. We therefore provide details of the synthesis, isolation, and
NMR spectroscopic characterization of Me2P(tBu) and
LiCH2P(Me)(tBu) in the Supporting Information. 2,5-Dimethyl-
1,4-benzoquinone (Aldrich) was sublimed prior to use. OPEt3 was
purchased from Aldrich and used as received.
3
4
2 H, pyH-2,6), 8.30 (tt, JH,H = 7.7, JH,H = 1.3 Hz, 1 H, pyH-4),
3
7.92 (m, 2 H, pyH-3,5), 2.33–2.10 (m, 2 H, BCH2P), 1.42 [d, JP,H
3
= 16.1 Hz, 9 H, C(CH3)3], 1.37 [d, JP,H = 15.9 Hz, 9 H, C(CH3)3]
ppm. 11B{1H} NMR (96.3 MHz, CD2Cl2): δ = 0.7 (h1/2 = 130 Hz)
ppm. 13C{1H} NMR (75.5 MHz, CD2Cl2): δ = n.o. (FC, BCAr
,
PCAr), 146.6 (pyC-2,6), 144.1 (pyC-4), 128.0 (pyC-3,5), 36.9 [d,
1JP,C = 31 Hz, C(CH3)3], 36.4 [d, 1JP,C = 32 Hz, C(CH3)3], 27.7 [m,
C(CH3)3], 27.5 [m, C(CH3)3], 13.7 (m, BCH2P) ppm. 19F{1H}
NMR (282.3 MHz, CD2Cl2): δ = –78.9 (s, 3 F, OSO2CF3), –120.6
(m, 1 F), –126.8 (m, 1 F), –131.7 (m, 2 F, C6F5), –143.5 (m, 1 F),
(2)Cl: HCl in Et2O (0.91 m; 0.88 mL, 0.80 mmol) was added at
room temp. by syringe to a stirred solution of (2)OEt (0.10 g,
0.20 mmol) in Et2O (7 mL), whereupon a colorless precipitate im-
mediately formed. After stirring the resulting suspension for 48 h,
all volatiles were removed in vacuo to obtain (2)Cl in analytically
pure form. Single crystals suitable for X-ray crystallography were
obtained by slow concentration of a CH2Cl2 solution of (2)Cl at
3
4
–149.8 (m, 1 F), –154.6 (tt, JF,F = 20, JF,F = 3 Hz, 1 F, C6F5),
–162.1 (m, 2 F, C6F5) ppm. 31P{1H} NMR (121.5 MHz, CD2Cl2):
δ = 87.5 (m) ppm. MS (ESI+): m/z (%) = 485 (100) [1]+, 564 (9)
[(1)py]+. MS (ESI–): m/z (%)
=
149 (100) [OSO2CF3]–.
room temp. Yield: 0.095 g (0.19 mmol, 95%). 1H NMR
2
C27H25BF12NO3PS (713.32): calcd. C 45.46, H 3.53, N 1.96, S 4.50;
found C 44.52, H 3.66, N 1.78, S 4.61. Note: The deviation from
the calculated value for carbon is due to boron carbide formation
during combustion.
(400.1 MHz, C6D6): δ = 6.58 (m, 1 H, Ar–H), 1.90 (dd, JH,H
=
2
2
2
16.3, JP,H = 11.6 Hz, 1 H, BCH2P), 1.33 (dd, JH,H = 16.3, JP,H
3
=
8.7 Hz, 1 H, BCH2P), 0.73 [d, JP,H = 15.8 Hz, 9 H,
C(CH3)3], 0.45 [d, JP,H = 15.5 Hz, 9 H, C(CH3)3] ppm. 11B{1H}
NMR (96.3 MHz, C6D6): δ = –2.3 (h1/2 = 120 Hz) ppm. 13C{1H}
NMR (75.5 MHz, C6D6): δ = n.o. (FC, BCAr, PCAr), 111.2 (m, Ar-
CH), 34.7 [m, C(CH3)3], 34.3 [m, C(CH3)3], 27.1 [m, C(CH3)3], 26.1
[m, C(CH3)3], 15.2 (m, BCH2P) ppm. 19F{1H} NMR (282.3 MHz,
C6D6): δ = –98.2 (m, 1 F), –130.9 (m, 1 F), –131.7 (br., 2 F, C6F5),
3
[(1)OH2][OTf]: A solution of (1)OTf (0.12 g, 0.19 mmol) in Et2O
(10 mL) was stirred in air at room temp. for 48 h. The solvent was
removed in vacuo yielding [(1)OH2][OTf] in analytically pure form.
Yield: 0.12 g (0.18 mmol, 95%). Slow concentration of a CH2Cl2
solution of [(1)OH2][OTf] at room temp. gave X-ray quality crys-
3
–134.9 (m, 1 F), –159.3 (t, JF,F = 21 Hz, 1 F, C6F5), –164.7 (m, 2
1
tals. H NMR (300.0 MHz, CD2Cl2): δ = 4.90 (br., h1/2 = 300 Hz,
F, C6F5) ppm. 31P{1H} NMR (121.5 MHz, C6D6): δ = 86.0 (m)
ppm. MS (ESI+): m/z (%) = 467 (100) [2]+. C21H21BClF8P (502.61):
calcd. C 50.18, H 4.21; found C 50.37, H 4.43.
2
2
H2O), 1.89 (dd, JH,H = 16.5, JP,H = 13.3 Hz, 1 H, BCH2P), 1.52
2
2
3
(dd, JH,H = 16.5, JP,H = 8.5 Hz, 1 H, BCH2P), 1.47 [d, JP,H
=
3
16.4 Hz, 9 H, C(CH3)3], 1.26 [d, JP,H = 16.0 Hz, 9 H, C(CH3)3]
[(1)OPEt3][OTf]: A solution of (1)OTf (0.15 g, 0.24 mmol) and ppm. 11B{1H} NMR (96.3 MHz, CD2Cl2): δ = 1.9 (h1/2 = 380 Hz)
OPEt3 (0.032 g, 0.24 mmol) in CH2Cl2 (5 mL) was stirred at room ppm. 13C{1H} NMR (75.5 MHz, CD2Cl2): δ = n.o. (FC, BCAr
,
=
1
1
temp. for 12 h, then concentrated under reduced pressure to a vol-
ume of 0.3 mL and layered with pentane (0.2 mL). Crystallization
of [(1)OPEt3][OTf] started after 30 min and continued for another
PCAr, BCH2P), 36.2 [d, JP,C = 31 Hz, C(CH3)3], 35.8 [d, JP,C
33 Hz, C(CH3)3], 27.7 [m, C(CH3)3], 27.1 [m, C(CH3)3] ppm.
19F{1H} NMR (282.3 MHz, CD2Cl2): δ = –79.3 (s, 3 F, OSO2CF3),
Eur. J. Inorg. Chem. 2012, 112–120
© 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
117

A. Schnurr, M. Bolte, H.-W. Lerner, M. Wagner
FULL PAPER
–122.5 (m, 1 F), –127.6 (m, 1 F), –133.7 (m, 2 F, C6F5), –145.5 (m,
1
Major Diastereomer: H NMR (300.0 MHz, CDCl3): δ = 2.07 (d,
3
1 F), –152.0 (m, 1 F), –157.3 (t, JF,F = 20 Hz, 1 F, C6F5), –163.9
2JP,H = 13.1 Hz, 3 H, PCH3), 1.94–1.71 (m, 2 H, BCH2P), 1.26 [d,
(m, 2 F, C6F5) ppm. 31P{1H} NMR (121.5 MHz, CD2Cl2): δ = 86.9 3JP,H = 17.1 Hz, 9 H, C(CH3)3] ppm. 11B{1H} NMR (96.3 MHz,
(m) ppm. MS (ESI+): m/z (%) = 485 (100) [1]+. MS (ESI–): m/z (%)
= 149 (100) [OSO2CF3]–. C22H22BF12O4PS (652.24): calcd. C 40.51,
H 3.40, S 4.92; found C 40.56, H 3.61, S 5.44.
CDCl3): δ = –2.1 (h1/2 = 120 Hz) ppm. 13C{1H} NMR (75.5 MHz,
1
CDCl3): δ = n.o. (FC, BCAr, PCAr), 31.0 [d, JP,C = 45 Hz,
1
C(CH3)3], 24.5 [m, C(CH3)3], 16.8 (m, BCH2P), 7.1 (d, JP,C
=
46 Hz, PCH3) ppm. 19F{1H} NMR (282.3 MHz, CDCl3): δ =
Synthesis of (4)OEt: A suspension of LiCH2P(Me)(tBu) (0.075 g,
0.60 mmol) in toluene (10 mL) was cooled to –78 °C. A solution of
(C6F5)2B(OEt) (0.23 g, 0.60 mmol) in toluene (3 mL) was added
quickly by syringe. The resulting slurry was gradually warmed to
room temp., stirred overnight, and quenched with deionized H2O
(1 mL). The mixture was dried with Na2SO4, filtered, and the insol-
uble material was extracted with toluene (3ϫ3 mL). The combined
organic phases were concentrated to dryness in vacuo, the oily resi-
due was redissolved in benzene (8 mL) and freeze-dried in vacuo
to remove trace amounts of toluene. The colorless solid (4)OEt was
obtained as a 1:3 mixture of two diastereomers (1H NMR spectro-
scopic control). Yield: 0.25 g (0.51 mmol, 85%). The major dia-
stereomer was isolated by column chromatography (silica gel, hex-
ane/EtOAc, 1.5:1). Yield: 0.13 g (0.27 mmol, 45%).
–126.0 (m, 1 F), –129.5 (m, 1 F), –132.4 (m, 2 F, C6F5), –143.9 (m,
3
4
1 F), –154.0 (m, 1 F), –158.3 (tt, JF,F = 20, JF,F = 2 Hz, 1 F,
C6F5), –164.0 (m, 2 F, C6F5) ppm. 31P{1H} NMR (121.5 MHz,
CDCl3): δ = 67.4 (m) ppm.
1
Minor Diastereomer: H NMR (300.0 MHz, CDCl3): δ = 1.92 (d,
2JP,H = 12.9 Hz, 3 H, PCH3), 1.94–1.71 (m, 2 H, BCH2P), 1.42 [d,
3JP,H = 17.3 Hz, 9 H, C(CH3)3] ppm. 11B{1H} NMR (96.3 MHz,
CDCl3): δ = –2.1 (h1/2 = 120 Hz) ppm. 13C{1H} NMR (75.5 MHz,
1
CDCl3): δ = n.o. (FC, BCAr, PCAr), 31.4 [d, JP,C = 44 Hz,
1
C(CH3)3], 25.3 [m, C(CH3)3], 16.8 (m, BCH2P), 7.3 (d, JP,C
=
46 Hz, PCH3) ppm. 19F{1H} NMR (282.3 MHz, CDCl3): δ =
–127.2 (m, 1 F), –129.3 (m, 1 F), –133.0 (m, 2 F, C6F5), –143.7 (m,
3
4
1 F), –154.0 (m, 1 F), –158.7 (tt, JF,F = 20, JF,F = 2 Hz, 1 F,
C6F5), –164.2 (m, 2 F, C6F5) ppm. 31P{1H} NMR (121.5 MHz,
CDCl3): δ = 68.1 (m) ppm. MS (ESI+): m/z (%) = 443 (100) [4]+.
C18H14BClF9P (478.52): calcd. C 45.18, H 2.95; found C 45.15, H
3.01.
1
Major Diastereomer: H NMR (300.0 MHz, C6D6): δ = 3.94 (m, 1
3
H, OCH2CH3), 3.30 (m, 1 H, OCH2CH3), 1.35 (t, JH,H = 6.9 Hz,
2
3 H, OCH2CH3), 1.27 (m, 1 H, BCH2P), 0.70 (d, JP,H = 12.8 Hz,
3
3 H, PCH3), 0.66 [d, JP,H = 16.9 Hz, 9 H, C(CH3)3], 0.54 (m, 1 H,
BCH2P) ppm. 11B{1H} NMR (96.3 MHz, C6D6): δ = 2.1 (h1/2
=
,
Gutmann–Beckett Measurements for Lewis Acidity Determination:
The Lewis acidities of compounds [1][WCA], [2][WCA], and
[3][WCA] were determined by an NMR spectroscopic method as
follows: (1) OPEt3 was placed in an NMR tube. (2) In a different
flask, CD2Cl2 was added to a mixture of [Ag(CH2Cl2)][WCA] and
the chloro adduct of the respective Lewis acid. (3) The resulting
suspension was stirred for 2 min at room temp., filtered, and the
clear yellow filtrate was transferred to the NMR tube. (4) The
NMR tube was flame-sealed, and the 31P{1H} NMR spectrum was
recorded at 27 °C. In a similar fashion, a sample of (C6F5)3B as
the Lewis acid was investigated to ensure maximum comparability
of the data. Note: The same NMR spectrometer was used for all
measurements.
15 Hz) ppm. 13C{1H} NMR (75.5 MHz, C6D6): δ = n.o. (FC, BCAr
PCAr), 60.0 (OCH2CH3), 30.5 [d, 1JP,C = 44 Hz, C(CH3)3], 24.7 [m,
1
C(CH3)3], 18.6 (OCH2CH3), 15.5 (m, BCH2P), 6.5 (d, JP,C
=
46 Hz, PCH3) ppm. 19F{1H} NMR (282.3 MHz, C6D6): δ = –126.8
(m, 1 F), –130.2 (m, 1 F), –136.5 (m, 2 F, C6F5), –145.8 (m, 1 F),
3
–155.5 (m, 1 F), –160.3 (t, JF,F = 21 Hz, 1 F, C6F5), –164.6 (m, 2
F, C6F5) ppm. 31P{1H} NMR (121.5 MHz, C6D6): δ = 64.8 (m)
ppm.
1
Minor Diastereomer: H NMR (300.0 MHz, C6D6): δ = 3.73 (m, 1
3
H, OCH2CH3), 3.20 (m, 1 H, OCH2CH3), 1.28 (t, JH,H = 6.9 Hz,
2
3 H, OCH2CH3), 1.07 (d, JP,H = 13.4 Hz, 3 H, PCH3), 1.01 (m, 1
3
H, BCH2P), 0.58 (m, 1 H, BCH2P), 0.53 [d, JP,H = 16.8 Hz, 9 H,
C(CH3)3] ppm. 11B{1H} NMR (96.3 MHz, C6D6): δ = 1.2 (h1/2
=
,
[(1)OPEt3][WCA]: (1)Cl (0.058 g, 0.111 mmol), [Ag(CH2Cl2)]-
[WCA] (0.129 g, 0.111 mmol), and OPEt3 (0.005 g, 0.037 mmol) in
CD2Cl2 (0.7 mL). 31P{1H} NMR (121.5 MHz, CD2Cl2): δ = 85.0
(m, 1 P, BCH2P), 80.4 (s, 1 P, OPEt3) ppm.
20 Hz) ppm. 13C{1H} NMR (75.5 MHz, C6D6): δ = n.o. (FC, BCAr
PCAr), 59.9 (OCH2CH3), 29.8 [d, 1JP,C = 44 Hz, C(CH3)3], 24.0 [m,
1
C(CH3)3], 18.7 (OCH2CH3), 15.5 (m, BCH2P), 6.2 (d, JP,C
=
46 Hz, PCH3) ppm. 19F{1H} NMR (282.3 MHz, C6D6): δ = –127.6
[(2)OPEt3][WCA]: (2)Cl (0.048 g, 0.096 mmol), [Ag(CH2Cl2)]-
[WCA] (0.111 g, 0.096 mmol), and OPEt3 (0.004 g, 0.032 mmol) in
CD2Cl2 (0.6 mL). 31P{1H} NMR (121.5 MHz, CD2Cl2): δ = 84.1
(m, 1 P, BCH2P), 79.7 (s, 1 P, OPEt3) ppm.
(m, 1 F), –127.8 (m, 1 F), –135.7 (m, 2 F, C6F5), –146.5 (m, 1 F),
3
–156.1 (m, 1 F), –160.4 (t, JF,F = 21 Hz, 1 F, C6F5), –164.7 (m, 2
F, C6F5) ppm. 31P{1H} NMR (121.5 MHz, C6D6): δ = 66.3 (m)
ppm. MS (ESI+): m/z (%) = 443 (100) [4]+. MS (ESI–): m/z (%) =
487 (100) [M – H]–. C20H19BF9OP (488.13): calcd. C 49.21, H 3.92;
found C 49.54, H 3.99.
[(3)OPEt3][WCA]: (3)Cl (0.048 g, 0.111 mmol), [Ag(CH2Cl2)]-
[WCA] (0.129 g, 0.111 mmol), and OPEt3 (0.005 g, 0.037 mmol) in
CD2Cl2 (0.7 mL). 31P{1H} NMR (121.5 MHz, CD2Cl2): δ = 84.3
(m, 1 P, BCH2P), 79.7 (s, 1 P, OPEt3) ppm.
Synthesis of (4)Cl: A solution of HCl in Et2O (0.91 m; 2.4 mL,
2.18 mmol) was added quickly by syringe at room temp. to a stirred
solution of (4)OEt (1:3 mixture of two diastereomers; 0.27 g,
0.55 mmol) in Et2O (8 mL), whereupon a colorless precipitate im-
mediately formed. The resulting suspension was stirred for 18 h to
drive the reaction to completion. All volatiles were driven off under
reduced pressure, and (4)Cl was obtained as a colorless solid in
analytically pure form. Yield: 0.25 g (0.52 mmol, 95%). Note:
NMR spectroscopic measurements (CDCl3) on samples after
workup revealed strong variations in the diastereomeric ratio
(range = 1:3 to 1:12; the major diastereomer was always the same).
Using the 1:12 mixture, X-ray quality crystals of one diastereomer
were grown by slow concentration of a dilute Et2O/CH2Cl2 solution
(3:1) at room temp.
(C6F5)3B·OPEt3: (C6F5)3B (0.057 g, 0.111 mmol) and OPEt3
(0.005 g, 0.037 mmol) in CD2Cl2 (0.7 mL). 31P{1H} NMR
(121.5 MHz, CD2Cl2): δ = 77.1 (s) ppm.
Lewis Acid Catalyzed Diels–Alder Reaction of 2,5-Dimethyl-1,4-
benzoquinone and Cyclopentadiene (CpH)
Catalysis with [1][WCA] (5 mol-%): (1)Cl (0.004 g, 7.5 μmol) and
[Ag(CH2Cl2)][WCA] (0.009 g, 7.5 μmol) were suspended in CH2Cl2
(1 mL) at room temp. The mixture was stirred for 2 min and fil-
tered. 2,5-Dimethyl-1,4-benzoquinone (0.020 g, 0.15 mmol) was
dissolved in a stock solution of freshly distilled CpH in CH2Cl2
(0.073 m; 2.1 mL, 0.15 mmol), and the catalyst solution was added
118
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Eur. J. Inorg. Chem. 2012, 112–120

Cyclic Phosphonium Bis(fluoroaryl)boranes
quickly by syringe. The color of the resulting mixture immediately
changed from bright yellow to red. After 1 h, more of the CpH
stock solution (0.073 m in CH2Cl2; 0.41 mL, 0.03 mmol) was
added, the mixture was stirred for another 1.5 h and then opened
to air. The solvent was removed in vacuo, and the crude product
was purified by column chromatography (hexane/EtOAc, 19:1).
Diels–Alder product 5 was obtained as a pale yellow solid. Yield:
CCDC-841735 {(2)Cl}, -841736 {[(1)OPEt3][OTf]}, -841737 {[(1)py]-
[OTf]}, -846304 {[(1)OH2][OTf]}, -841738 {(1)OiPrϫC6H6}, and
-841739 {(4)Cl} contain the supplementary crystallographic data
for this paper. These data can be obtained free of charge from The
Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/
data_request/cif.
31P{1H} MAS NMR Spectroscopic Measurements: 31P{1H} solid-
state MAS NMR spectroscopic experiments were carried out with
a Bruker Avance WB 600 spectrometer equipped with a 4 mm
MAS DVT double resonance probe at Larmor frequencies of 600.1
and 150.9 MHz for 1H and 31P, respectively. Single-pulse excitation
spectra were recorded by using 10 kHz sample spinning, a 4 μs exci-
tation pulse, and 100 kHz SPINAL64[46] 1H decoupling during the
acquisition time of 50 ms (recycle delay: 5 s). The spectrum was
acquired with 128 transients and was referenced to 85% H3PO4
indirectly by setting the central line of crystalline triethylphosphane
sulfide to 58.4 ppm.
1
0.026 g (0.13 mmol, 87%). The H NMR chemical shift values of
5 were in agreement with the literature values.[36]
Catalysis with (1)OTf (5 mol-%): A stock solution of freshly dis-
tilled CpH in CH2Cl2 (0.073 m; 2.1 mL, 0.15 mmol) was added at
room temp. by syringe to a solid mixture of 2,5-dimethyl-1,4-
benzoquinone (0.020 g, 0.15 mmol) and (1)OTf (0.005 g, 7.5 μmol).
More of the CpH stock solution (0.073 m in CH2Cl2; 0.41 mL,
0.03 mmol) was added after 4 h and a further 0.2 equiv. after 6 h,
whereupon the color of the mixture gradually changed from bright
yellow to dark green. Stirring was continued for 18 h before open-
ing the reaction mixture to air. The workup procedure was the same
as described above. Yield: 0.025 g (0.12 mmol, 80%).
Supporting Information (see footnote on the first page of this arti-
cle): Synthesis and analytical data of (C6F5)2B(OiPr) and (1)OiPr;
synthesis and NMR spectroscopic characterization of Me2P(tBu)
and LiCH2P(Me)(tBu). Selected crystallographic data, plots of the
molecular structures, and selected geometric parameters of [(1)py]-
[OTf], (1)OiPrϫC6H6, (2)Cl, and (4)Cl. 31P{1H} NMR spectrum
of [(1)OPEt3][OTf] in CD2Cl2 solution and 31P{1H} MAS NMR
spectrum in the solid state. NMR spectroscopic data of [2][WCA].
X-ray Crystal Structure Analysis of (2)Cl, [(1)OPEt3][OTf], [(1)py]-
[OTf], [(1)OH2][OTf], (1)OiPr؋C6H6, and (4)Cl
Data were collected with a STOE IPDS II two-circle diffractometer
with graphite-monochromated Mo-Kα radiation. Empirical absorp-
tion corrections were performed for all structures by using the
MULABS[42] option in PLATON.[43] The structures were solved by
direct methods with the program SHELXS[44] and refined against
F2 with full-matrix least-squares techniques with the program
SHELXL-97.[45] The crystal of [(1)OPEt3][OTf] was twinned (emu-
lating monoclinic symmetry) with a contribution of 9.4(1)% for the
minor domain. The twin law was (1 0 0/1 –1 0/0 0 –1). In
[(1)OH2][OTf], the H atoms bonded to O were freely refined (see
also Table 4).
Acknowledgments
The authors gratefully acknowledge financial support by the
Beilstein Institut, Frankfurt/Main, Germany, within the research
collaboration NanoBiC. We wish to thank Dr. Johanna Becker-
Baldus (BMRZ, AK Glaubitz, Institute of Biophysical Chemistry,
Goethe University, Frankfurt) for solid-state 31P{1H} MAS NMR
spectroscopic measurements.
Table 4. Crystal data and structure refinement details for
[(1)OPEt3][OTf] and [(1)OH2][OTf].
[(1)OPEt3][OTf]
[(1)OH2][OTf]
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Formula
FW
Color, shape
Temperature [°C]
Radiation
C28H35BF12O4P2S C22H22BF12O4PS
768.37
colorless, block
–100(2)
Mo-Kα,
652.24
colorless, plate
–100(2)
Mo-Kα,
0.71073 Å
monoclinic
C2/c
20.2478(13)
16.5460(9)
18.0243(12)
90
119.441(5)
90
5258.7(6)
8
0.71073 Å
triclinic
Crystal system
Space group
a [Å]
b [Å]
c [Å]
α [°]
β [°]
¯
P1
11.4760(4)
14.7143(5)
21.5525(8)
86.549(3)
89.043(3)
67.235(3)
3349.7(2)
4
γ [°]
V [Å3]
Z
Dcalcd. [gcm–3]
F(000)
1.524
1576
0.293
1.648
2640
0.299
μ [mm–1]
Crystal size [mm]
Reflections collected
Indep. reflections (Rint
0.35ϫ0.29ϫ0.27 0.33ϫ0.18ϫ0.09
41059
11828 (0.0709)
35094
)
6041 (0.0567)
6041/0/378
1.055
0.0352, 0.0947
0.0406, 0.0976
0.325/–0.454
Data/restraints/parameters 11828/0/866
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2009, 131, 52–53.
GOOF on F2
1.054
R1, wR2 [IϾ2σ(I)]
R1, wR2 (all data)
Largest diff. peak and
hole [eÅ–3]
0.0626, 0.1661
0.0707, 0.1786
1.041/–0.553
Eur. J. Inorg. Chem. 2012, 112–120
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119

A. Schnurr, M. Bolte, H.-W. Lerner, M. Wagner
FULL PAPER
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Received: October 11, 2011
Published Online: November 24, 2011
120
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Eur. J. Inorg. Chem. 2012, 112–120