Internal adduct formation of active intramolecular C4-bridged frustrated phosphane/borane Lewis pairs

Article abstract of DOI:10.1021/ja413060u

The tetramethylene-bridged PMes₂/B(C₆F ₅)₂ frustrated Lewis pair (FLP) 8 was prepared by hydroboration of Mes₂P-(CH₂)₂CH=CH₂ with HB(C₆F₅)₂. It is an active FLP that splits dihydrogen under mild conditions and, consequently, serves as a metal-free hydrogenation catalyst for a variety of substrates. It also reacts typically with terminal acetylenes. The C₄-bridged FLP 23 was prepared by HB(C₆F₅)₂ hydroboration of 1- dimesitylphosphino-2-vinylferrocene. It represents a rare example of a FLP where the equilibrium between the open form and the closed internal P/B adduct form is experimentally observable. It also shows a variety of typical FLP reactions, including dihydrogen splitting. The FLPs 8 and 23 (open form) and many precursors and products were characterized by X-ray diffraction.

Full text of DOI:10.1021/ja413060u

Article
pubs.acs.org/JACS
Internal Adduct Formation of Active Intramolecular C4‑bridged
Frustrated Phosphane/Borane Lewis Pairs
Xiaowu Wang, Gerald Kehr, Constantin G. Daniliuc, and Gerhard Erker*
Organisch-Chemisches Institut der Universitat Munster, Correnstrasse 40, 48149 Munster, Germany.
̈
̈
̈
S
* Supporting Information
ABSTRACT: The tetramethylene-bridged PMes2/B(C6F5)2
frustrated Lewis pair (FLP) 8 was prepared by hydroboration
of Mes2P−(CH2)2CHCH2 with HB(C6F5)2. It is an active
FLP that splits dihydrogen under mild conditions and,
consequently, serves as a metal-free hydrogenation catalyst
for a variety of substrates. It also reacts typically with terminal
acetylenes. The C4-bridged FLP 23 was prepared by
HB(C6F5)2 hydroboration of 1-dimesitylphosphino-2-vinyl-
ferrocene. It represents a rare example of a FLP where the
equilibrium between the open form and the closed internal P/B adduct form is experimentally observable. It also shows a variety
of typical FLP reactions, including dihydrogen splitting. The FLPs 8 and 23 (open form) and many precursors and products were
characterized by X-ray diffraction.
3, probably via reversal of the LA/LB adduct formation
followed by hydroboration under rapid equilibrium conditions
(see Scheme 1).
INTRODUCTION
■
Frustrated Lewis pairs (FLPs) have pairs of active Lewis acids
and bases in solution that provide the opportunity to observe
their joint reaction with added substrates.1,2 Such cooperative
or synergistic FLP reactions have been observed upon exposure
of, e.g. respective phosphane/3,4 or amine/5borane pairs to
dihydrogen resulting in heterolytic cleavage of the H2 molecule,
often providing a basis for developing metal-free catalytic
hydrogenation reactions.6−8 FLPs also have been observed to
add to alkenes9 or alkynes,10,11 to carbonyl compounds12
including carbon dioxide,13 to SO2,14 and even to nitrogen
oxides.15 Recently, the reduction of carbon monoxide to the
formyl stage has been achieved at a frustrated Lewis pair
template.16
Scheme 1
Steric hindrance (and sometimes special electronic fea-
tures17) have been used to hinder deactivating strong Lewis
acid/Lewis base adduct formation between the frustrated Lewis
pair components. However, that does not imply that FLPs must
be completely devoid of any Lewis acid (LA)/Lewis base (LB)
interaction or even adduct formation. Actually, weak LA/LB
van der Waals contacts seem to be essential in the reactions of
intermolecular FLPs with small molecules to avoid the
“termolecularity trap”.18 Also intramolecular FLPs often feature
pronounced Lewis acid/base interactions,5b,19 although recently
we have described a few geminal and vicinal FLP examples that
seem to have no measurable LA/LB contacts.16a,20 Lewis acid/
Lewis base interaction can go to extremes in active FLPs.
Examples are the systems 1a,b where the addition of the
strongly Lewis acidic HB(C6F5)2 reagent has resulted in the
quantitative formation of the strong B-Lewis acid/enamine C-
Lewis base adducts 2, that were isolated and characterized by X-
ray diffraction.21 Nevertheless, these systems split dihydrogen
under mild reaction conditions to give the zwitterionic products
Most vicinal P/B and N/B FLPs that we had contributed to
the literature show weak internal adduct formation. From
dynamic 19F NMR spectroscopy the activation barriers for ring-
opening were determined at ΔG⧧ ≈ 12 to 14 kcal·mol−1 for
chiral variants of such systems.5b,19 It was thought that the
typical FLP reactions of systems such as 4 took place via their
nonobserved open isomers 5 (see Scheme 2).22
We had also prepared the corresponding trimethylene-
bridged P/B system 7 but have so far not found any indication
of typical FLP activity.23 We have now prepared the next
Received: December 23, 2013
Published: January 27, 2014
© 2014 American Chemical Society
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range between 1.851(2) Å (P1A−C21A) and 1.857(2) Å
(P1A−C11A). The P1A−C1A bond length amounts to
1.856(2) Å. The P-bonded butenyl group was found in an
extended conformation (C1A−C2A 1.533(3) Å, C2A−C3A
1.494(3) Å, C3A−C4A 1.292(4) Å, dihedral angles P1A−
C1A−C2A−C3A 172.7(1)°, C1A−C2A−C3A−C4A
126.3(3)°). In solution, compound 9 shows the typical NMR
features of the terminal vinyl group and a 31P NMR resonance
at δ −22.5.
Scheme 2
The X-ray crystal structure analysis of compound 8 shows a
tetramethylene chain that has a PMes2 group and a B(C6F5)2
substituent attached at its ends (P1−C1 1.855(2) Å, B1−C4
1.636(3) Å). There is a marked bonding contact between the
boron Lewis acid and the phosphorus Lewis base (B1−P1
2.113(2) Å) which is, however, located at the long end of
related B−P systems.25 In the crystal, compound 8 exhibits a
chairlike conformation of its framework with a close to gauche
C1−P1−B1−C4 dihedral angle of 44.9(2)° (see Figure 2).
member in this series, the tetramethylene-bridged P/B FLP 8
and found it to be an active frustrated Lewis pair. We here
report its characterization and some typical reactions, and we
have prepared a closely related C4-bridged P/B FLP where we
were able to experimentally observe both the open and the
closed form of an active frustrated Lewis pair.
RESULTS AND DISCUSSION
■
The Tetramethylene-Bridged P/B FLP 8. The synthesis
of the tetramethylene-bridged phosphane/borane FLP 8 was
rather straightforward. The reaction of the butenyl magnesium
bromide Grignard reagent with dimesitylchlorophosphane gave
butenyldimesitylphosphane (9) in reasonable yield. This was
then reacted with Piers’ borane [HB(C6F5)2].24 A clean
hydroboration reaction ensued with the usual anti-Markovnikov
regiochemistry to yield the P/B system 8 (see Scheme 3).
Compound 8 was isolated as a white solid in 90% yield.
Scheme 3
Figure 2. View of the molecular structure of compound 8 (thermal
ellipsoids are shown with 30% probability).
Both the compounds 9 and 8 were characterized by X-ray
diffraction. Compound 9 features a tricoordinate phosphorus
center with a typical trigonal pyramidal geometry with a sum of
C−P−C bond angles of 316.3° (values given for one of the
independent molecules of 9 in the crystal, see Figure 1). The
individual P−C bond lengths were found in a very narrow
In solution, compound 8 shows heteronuclear magnetic
resonance signals at δ −5.8 (11B) and δ 3.3 (31P), respectively.
It exhibits a set of four 13C NMR signals of the tetramethylene
linker at δ 23.1, 23.1, 21.2, and 18.6 (br). Compound 8 features
dynamic NMR spectra probably due to conformational
inversion of the heterocyclohexane chair conformation of the
six-membered core-ring structure concomitant with “freezing”
of the P−C(mesityl) and B−C6F5 rotation at low temperature
on the respective NMR time scales. Consequently, we have
1
observed a total of six mesityl methyl H NMR signals at low
temperature and four m-arene proton resonances (the spectra
are depicted in the SI). Similarly, we have monitored a
decoalescence of the three broad o,p,m-C6F5 19F NMR signals
observed at 299 K to eventually give four well-separated o-F
resonances, two p-F signals and four m-F signals of the pair of
C6F5 substituents at boron. From the p-F coalescence, we have
obtained a Gibbs-activation energy value of ΔG⧧(268 K) = 11.5
0.3 kcal·mol−1 for this conformational equilibration process
of compound 8.
Despite the fact that compound 8 shows an internal adduct
structure in both the crystal and in solution, it reacted with
dihydrogen. A solution of 8 in dichloromethane was exposed to
a H2 atmosphere (1.5 bar) at ambient temperature for 30 min,
and the mixture was kept under H2 at −30 °C for 2 d to give a
Figure 1. View of the molecular structure of butenyldimesitylphos-
phane 9 (thermal ellipsoids are shown with 30% probability).
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crystalline material (50% isolated) which was identified as the
product 10 of heterolytic splitting of dihydrogen by compound
8 (see Scheme 4).
using between 5 and 10 mol % of the FLP/H2 catalyst system
10/H2 (for details see the SI).
We have also carried out a few other FLP reactions with the
tetramethylene-bridged P/B system 8. Active FLPs usually react
with terminal acetylenes by (sometimes) competing addition or
deprotonation reactions. The latter was observed when
compound 8 was treated with p-tolylacetylene in dichloro-
methane. The reaction went to completion during 1 h, and we
isolated the product 11 as a white solid in 78% yield (Scheme
5). It shows the typical 1H NMR [Mes2PH+] doublet at δ 7.38
Scheme 4
The zwitterionic complex 10 was characterized by X-ray
diffraction, by spectroscopy, and by C,H-elemental analysis. In
Scheme 5
solution it shows the 31P NMR doublet of the [PHMes2 ]
+
moiety at δ −11.9 within the typical large coupling constant of
1JPH ≈ 486 Hz (corresponding 1H NMR feature at δ 7.82). The
11B NMR signal of 10 occurs at δ −20.9 (1JBH ≈ 80 Hz).
Compound 10 shows three 19F NMR resonances of the pair of
symmetry equivalent C6F5 substituents at δ −134.0 (o), −164.6
(p), and −166.8 (m) with a small Δδ19F(m,p) shift difference of
2.2 that is typical of a borate anion structure. The hydrogen
splitting reaction by 10 was confirmed by its reaction with
dideuterium to yield compound 10-D2. It shows the 31P/11B
NMR signals of the [PDMes2 ] unit at δ −11.7 (1JPD = 73 Hz)
+
−
and of the [BD(C6F5)2 ] moiety at δ −21.0 (ν1/2 ≈ 85 Hz) (for
details see the SI).
The X-ray crystal structure analysis of compound 10 (see
(1JPH = 475.5 Hz, corresponding 31P NMR resonance at δ
−10.7), and a borate type 11B NMR feature at δ −18.2. There is
a single set of 19F NMR signals of the pair of C6F5 substituents
at boron with a small Δδ19F(m,p) value of 2.4. We have
monitored one of the 13C NMR signals of the [B]−CC−Ar
unit at δ 94.1, and we have located the four 13C NMR signals of
the bridging tetramethylene moiety at δ 29.0, 27.0, 24.8, and
22.3 (for details see the SI).
Figure 3) shows an extended acyclic structure. The [Mes2PH+]
We had shown that a variety of vicinal P/B FLPs (4, see
Scheme 2) undergo P/B addition to the nitrogen atom of nitric
oxide (NO) under mild conditions to yield the respective
persistent FLP-NO aminoxyl radicals.15b,c Alternatively, phos-
phanes are known to react with NO to give the corresponding
phosphanoxide plus N2O.26 Compound 8 shows a typical free
phosphane reactivity toward NO. Treatment of 8 with nitric
oxide (−78 °C to −30 °C) overnight gave a blue solution from
which workup produced a white solid (89%) that was shown to
be the phosphanoxide derivative 12 by X-ray diffraction
(Scheme 5). In the crystal compound 12 shows a heterocyclic
seven membered ring structure. It contains a phosphanoxide
(P1−O1 1.536(3) Å). Its oxygen atom is internally coordinated
to the B(C6F5)2 group27 (B1−O1 1.570(5) Å, angle P1−O1−
B1 129.9(2)°) (see Figure 4).
Figure 3. Molecular structure of the zwitterionic compound 10
(thermal ellipsoids are shown with 30% probability).
phosphonium substituent is bonded to carbon at one end of the
C4-chain (C1−P1 1.807(4) Å, sum of heavy atom bond angles
at phosphorus: ∑Pccc 339.9°) and the [(C6F5)2BH−]
hydridoborate unit to the other C-terminus (B1−C4
1.632(6) Å, ∑Bccc 334.9°). The central heavy atom chain is
found in a close all-anti-periplanar conformation (dihedral
angles P1−C1−C2−C3 −179.6(3)°, C1−C2−C3−C4
−179.5(4)°, C2−C3−C4−B1 −179.2(4)°).
Many zwitterionic hydridoborate/phosphonium pairs de-
rived from the heterolytic splitting of dihydrogen by internal P/
B frustrated Lewis pairs were shown to be able to transfer the
H+/H− pair to a variety of suitable substrates, and some of
them have served as active catalysts in hydrogenation reactions.
In a small series of qualitative experiments, we were able to
show that the [PH+]/[BH−] zwitterion 10 can also transfer
H+/H− and serve as a metal-free hydrogenation catalyst. We
carried out catalytic hydrogenations of bulky imines, enamines,
and also of alkenylferrocene derivatives by in situ generated 10
In solution, compound 12 shows a typical 31P NMR signal of
a phosphanoxide (δ 64.7). It shows the typical set of 19F NMR
o-, p-, m-signals of the pair of symmetry-equivalent C6F5 groups
at boron with a small Δδ19F(m,p) = 4.5 ppm shift difference.
The 13C NMR resonance of the bridging tetramethylene unit of
12 were located at δ 34.3 (1JPC = 55.5 Hz), δ 23.4 (2JPC = 5.1
Hz), δ 29.0, and δ 24.4 (br).
A C4-bridged P/B FLP at the Ferrocene Framework.
We prepared a couple of related C4-bridged P/B-FLPs derived
from a respective ferrocene framework. Our synthesis followed
a pathway that had been developed previously by Kagan28 and
̆ ̌
by Stepnick
a.29 It started with the enantiomerically pure acetal
(S,S)-14 that was derived from ferrocene carbaldehyde by the
“Kagan method” (see Scheme 6). Directed o-metalation
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Figure 4. Molecular structure of compound 12 (thermal ellipsoids are
shown with 30% probability).
Scheme 6
Figure 5. View of the molecular structure of compound (pS)-19
(thermal ellipsoids are shown with 30% probability).
The NMR spectra of compound (pS)-19 indicate that the
P−B bond is retained in solution. The complex features a 31P
NMR resonance at δ +3.4 and a sharp 11B NMR signal at δ
−10.7. The latter is typical of tetra-coordinated borane in this
environment. Consequently, we have observed a pair of sharp
19F NMR signals of the p-F atoms of the pair of inequivalent
C6F5 groups at boron (δ −157.9 and δ −158.7) and a
corresponding pair of m-C6F5 signals of 2-fold relative intensity
at δ −163.8 and −164.5. Compound (pS)-19 features the
1H/13C NMR signals of a pair of inequivalent phenyl groups at
1
phosphorus and a set of H NMR signals of the CH2−CH2−
[B] moiety at δ 2.58 (m, 2H) and δ 1.98, 1.85 (each m, each
1H).
We prepared the related PMes2 containing ferrocene-based
FLP by an analogous route. It started with the in situ generated
lithio-ferrocene derivative (S,S,pR)-15 (see Scheme 6) that was
quenched with ClPMes2 to give (S,S,pS)-20 (see Scheme 7).
Compound (S,S,pS)-20 was characterized by X-ray diffraction,
and its stereochemical features were confirmed (see Figure 6).
followed by treatment with diphenylchlorophosphane gave the
(S,S,pS)-16 acetal in high diastereomeric excess as described by
Kagan.28 Hydrolysis followed by Wittig olefination of the
resulting aldehyde (pS)-17 eventually yielded the o-
diphenylphosphanyl(vinyl)ferrocene (pS)-18 highly enantio-
Scheme 7
̆ ̌
merically enriched as described by Stepnick
a.29
Compound (pS)-18 was then reacted with Piers’ borane
[HB(C6F5)2]. A clean hydroboration of the vinyl-substituent
was observed with anti-Markovnikov regioselectivity to give the
P/B system (pS)-19 which was isolated as an orange solid in
close to quantitative yield. Compound (pS)-19 was charac-
terized by X-ray diffraction (single crystals were obtained from
benzene/heptane at r.t.). The X-ray crystal structure analysis
confirmed the 1,2-substitution pattern at the “upper” ferrocene
Cp-ring and the regioselective hydroboration of the pendant
vinyl substituent. The most prominent structural feature of
compound (pS)-19 is the presence of a strong P−B bond
between the PPh2 Lewis base and the B(C6F5)2 Lewis acid
(P1−B1 2.046(2) Å). The resulting six-membered heterocyclic
ring system exihibits a distorted cyclohexene-type half-chair
conformation (see Figure 5) with pairs of chemically
inequivalent phenyl substituents at phosphorus and C6F5
groups at boron (θ B1−C12−C11−C2 75.6 (2)°).
Acid-catalyzed hydrolysis then gave (pS)-21. It shows a 31P
NMR signal at δ −41.9 and the typical 1H/13C NMR
resonances of the carbaldehyde function at δ 10.63/192.3
[(IR (CO stretch): v = 1671 cm−1)]. Complex (pS)-21 was
̃
also characterized by X-ray diffraction. It shows the pair of o-
PMes2 and CHO functional groups attached at the “upper”
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Figure 6. Molecular structure of compound (S,S,pS)-20 (thermal
ellipsoids are shown with 30% probability).
Figure 8. Molecular structure of complex (pS)-22 (thermal ellipsoids
are shown with 30% probability).
ferrocene-Cp ring (see Figure 7). The plane of the aldehyde is
rotated slightly out of the Cp-ring plane (C11−O1 1.194(4) Å,
anti-Markovnikov orientation to give the ferrocene-derived
frustrated Lewis pair (pS)-23 in close to quantitative yield. The
X-ray crystal structure analysis revealed an open structure.
Contrary to its PPh2 analogue (pS)-19 (see above), complex
(pS)-23 does not show any appreciable interaction between the
bulky PMes2 Lewis base (P1−C1 1.811(7) Å) and the
B(C6F5)2 Lewis acid (C12−B1 1.559(1) Å). The boron center
is planar tricoordinate (∑BCCC = 359.8°). The Cp−CH2−
CH2−B(C6F5)2 unit features a gauche like conformation (θ
C2−C11−C12−B1 73.7(9)°) that has the C11−C12 vector
pointing away from the phosphorus atom. Actually, the P and B
atoms in compound (pS)-23 are separated by 5.837 Å (see
Figure 9).
Figure 7. View of the molecular structure of compound (pS)-21
(thermal ellipsoids are shown with 30% probability).
θ C3−C2−C11−O1 167.5(3)°) and the carbonyl oxygen atom
is pointing toward the adjacent phosphorus (C1−P1 1.824(2)
Å).
Wittig olefination of the aldehyde function in (pS)-21 gave
the vinylferrocene derivative (pS)-22 in good yield (97%)
[[α]20D = +324; (pS)-18: [α]20D = +34329]. It shows the typical
1H NMR AMX pattern of the vinyl group [δ 6.94, 5.45, 5.09;
Figure 9. A view of the molecular structure of the FLP (pS)-23 in the
crystal (thermal ellipsoids are shown with 30% probability).
3
13C: δ 134.1 (HC=, JPC = 14.4 Hz), δ 111.9 (4JPC = 2.4 Hz,
Compound (pS)-23 shows temperature dependent NMR
spectra. These indicate an equilibrium of the open FLP
structure (pS)-23(open) with the internally P···B coordinated
adduct structure (pS)-23(closed). This system shows dynamic
NMR spectra demonstrating a strongly temperature dependent
23(open) ⇄ 23(closed) equilibrium situation. At high
temperature the open form is favored as expected (probably
mostly entropically controlled), whereas with decreasing
=CH2)] and a 31P NMR resonance at δ −40.9. Complex (pS)-
22 was also characterized by an X-ray crystal structure analysis
(see Figure 8). It features the vinyl group in a conformation
showing away from the phosphorus (C11−C12 1.317(5) Å, θ
C1−C2−C11−C12 −152.2(3)°).
Addition of Piers’ borane [HB(C6F5)2] to (pS)-22 resulted in
a clean hydroboration of the vinyl CHCH2 double bond with
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temperature an increasing equilibrium amount of the closed
isomer is being observed. This is best illustrated with the
temperature dependent 19F NMR spectra. At ambient
conditions, we have monitored three slightly broadened 19F
NMR B(C6F5)2 resonances [298 K, CD2Cl2 solution, 564
MHz: δ −129.8 (o), δ −148.6 (p), δ −161.8 (m)]. On lowering
the temperature these signals broaden and show a slight
shifting. Below 223 K, we start to observe the signals of a
second species that rapidly increases in intensity with
decreasing temperature. Figure 10 shows the 19F NMR
Reactions of the Ferrocene-Derived FLP (pS)-23. We
first reacted the FLP (pS)-23 with typical N- and C-donor
ligands. Treatment of the ferrocene-derived FLP with pyridine
in a 1:1 molar ratio in pentane at r.t. gave a yellow precipitate of
the [B]−pyridine adduct (pS)-24 that was isolated in 88%
yield. Compound (pS)-24 features the 1H/13C NMR signals of
the pyridine moiety and of a pair of diastereotopic mesityl
substituents at phosphorus. Actually the rotation around the
1
P−Mes vectors are “frozen” on the 600 MHz H NMR time
scale at 233 K so that we have observed a total of six methyl
group signals and four m-CH signals of the PMes2 unit in
compound (pS)-24. Similarly, we have observed the 19F NMR
resonances of a pair of diastereotopic C6F5 groups at boron.
The remaining heteronuclear magnetic resonance signals of
compound (pS)-24 occur at δ −44.0 (31P) and δ −1.8 (11B),
respectively.
Compound (pS)-24 was characterized by X-ray diffraction.
In the crystal, compound (pS)-24 features a tetracoordinated
boron atom that has the pyridine unit bonded to it (B1−N13
1.632(5) Å). The B1−C12−C11−C2 unit shows a close to
antiperiplanar conformational arrangement (θ −177.9(3)°)
(see Figure 11 and Scheme 9).
Figure 10. 19F NMR (564 MHz, CD2Cl2) spectra of the (pS)-
23(open), (closed) mixture of isomers at 193 K (2) and 298 K (1).
spectrum at 193 K. It features the three broad signals of the
(pS)-23(open) isomer [δ −127.7 (o), δ −150.1 (p), δ −162.2
(m)] with a large Δδ19F(m,p) separation of 12.1 that is typical
for a Lewis acidic tricoordinate R−B(C6F5)2 situation (see
Scheme 8). The second species is likely the (pS)-23(closed)
Scheme 8
Figure 11. Molecular structure of compound (pS)-24 (thermal
ellipsoids are shown with 30% probability).
Scheme 9
isomer. In contrast to the open isomer, it features the 19F NMR
resonances of a pair of nonequivalent C6F5 groups at boron and
“frozen” rotation about the B−C6F5 vectors of both on the 19F
NMR time scale. Consequently, we observe a total of four o-,
two p- and four m-C6F5 19F NMR signals of (pS)-23(closed)
(see Figure 10) (for further details see the SI). The 19F NMR
Δδ19F(m,p) = 4.7, 4.9 of the (pS)-23(closed) isomer is small, as
it is expected for a tetra-coordinated borane situation.30 We
have observed corresponding temperature dependent spectral
1
changes in the H, 31P and 11B NMR spectra of compound
Treatment of (pS)-23 with tert-butylisocyanide gave a similar
adduct. We isolated compound (pS)-25 as an yellow solid in
>80% yield. It was also characterized by X-ray diffraction (see
Figure 12). In the crystal, the newly formed [B]−CN−tBu
moiety is linear [B1−C13 1.621(1) Å, C13−N14 1.141(9) Å,
angles B1−C13−N14 173.6(9)°, C13−N14−C15 171.4(9)°].
The 13C NMR isonitrile signal of (pS)-25 was located at δ
116.8 as a broad resonance. Complex (pS)-25 shows an
(pS)-23. At 193 K the mixture of the (pS)-23 (open)/(closed)
isomers shows a pair of 31P NMR signals (63: 37) at δ −33.4
and +11.5 whereas at 298 K one resonance at δ −42.8 was
observed. The (pS)-23(closed)/(pS)-23(open) equilibrium
was characterized by a van’t Hoff plot (depicted in the SI).
This equilibrium is characterized by the thermodynamic
parameters of ΔH° = 4.7 0.2 kcal·mol−1 and ΔS° = +25.9
0.2 cal·mol−1·K−1.
isonitrile IR band at v = 2289 cm−1.
̃
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Figure 12. Molecular structure of compound (pS)-25 [thermal
ellipsoids are shown with 30% probability; only one molecule
(molecule A) from two found in the asymmetric unit is shown].
Figure 13. Molecular structure of compound (pS)-26a (thermal
ellipsoids are shown with 30% probability).
Compound (pS)-27 exhibits a 31P NMR signal at δ −18.8 (d,
1JPH = 496.1 Hz). The 19F NMR spectrum shows typical signals
of a tetracoordinated borate signal. The 11B NMR spectrum
Many intramolecular P/B FLPs react with terminal
acetylenes by deprotonation and formation of the respective
phosphonium cation/alkynyl borate anion zwitterions.31
Compound (pS)-23 reacted analogously. In three different
experiments, we treated complex (pS)-23 with p-tolylacetylene,
with 2-methylbutenyne and with 1-pentyne. In each case we
have observed the formation of the corresponding zwitterionic
phosphonium/alkynylborate systems and isolated the respec-
tive complexes (pS)-26 a-c (see Scheme 10).
1
indicates the presence of a [BH−] unit (δ −20.5, d, JBH ≈ 85
Hz). We have also reacted (pS)-23 with D2 and obtained the
corresponding [PD+]/[BD−] product (pS)-27-D2. It shows the
2
expected 1:1:1 intensity triplet 31P NMR signal and the D
NMR [PD+] doublet and a broad [BD−] signal (for details see
the SI) (see Scheme 11).
Scheme 11
Scheme 10
The FLP (pS)-23 serves as a hydrogenation catalyst via its
dihydrogen splitting product (pS)-27. We have briefly tested it
in the successful catalytic hydrogenation of selected imines and
enamine examples. In most cases, the substrates were rapidly
hydrogenated using 5−20 mol % of the FLP catalyst.
Hydrogenation of N-tert-butylacetophenone imine gave the
corresponding amine in up to 26% ee (for details see the SI).
Although this represents only a relatively small asymmetric
induction exerted by the planary chiral FLP catalyst, it
nevertheless proves that the intact FLP (pS)-23, respectively
its dihydrogen activation product (pS)-27, is responsible for
this hydrogenation process. We assume a mechanistic scheme
of iminium ion formation by initial protonation followed by
hydride transfer. Therefore, the (pS)-23 system not unexpect-
edly was able to catalytically hydrogenate vinylferrocene to give
ethylferrocene.
Eventually we reacted the ferrocene-derived system (pS)-23
with nitrogen monoxide (see Scheme 12). Similar to the
reaction behavior of the tetramethylene-bridged PMes2/
B(C6F5)2 system 8 (see above) this also marked the limits of
(pS)-23 in cooperative FLP chemistry. Instead of synergistic P/
B addition as it had been observed for the ethylene-bridged
PMes2/B(C6F5)2 system 4, the phosphane and borane
functionality in (pS)-23 react additively, i.e. each is acting
independently. Consequently, the phosphane of (pS)-23 was
As a typical example compound (pS)-26a shows a 11B NMR
signal at δ −18.9 and a 31P NMR doublet at δ −18.9 (1JPH
≈
1
500 Hz). The corresponding H NMR PH signal occurs at δ
8.55. The acetylene 13C NMR resonances were located at δ
110.3 (BC) and δ 94.2 (C-[p-tolyl]) respectively.
Compounds (pS)-26b and (pS)-26c show similar NMR signals
(see the SI).
The p-tolylacetylene-derived compound (pS)-26a was
characterized by X-ray diffraction (see Figure 13). It shows
the acetylene moiety bonded to the boron atom [B1−C41
1.603(7) Å, C41−C42 1.196(7) Å]. The boron center is
pseudotetrahedrally coordinated.
The intramolecular frustrated Lewis pair (pS)-23 reacts
rapidly with dihydrogen at ambient conditions (r.t., 1.5 bar H2).
From a pentane solution, a yellow precipitate was formed. We
isolated the [PH+][BH−] containing compound (pS)-27 in
>70% yield. Compound (pS)-27 turned out to be rather
sensitive in solution.32 Some of the NMR features of the
product of heterolytic splitting of dihydrogen by the FLP (pS)-
23 were obtained from in situ experiments without workup.
3299
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Journal of the American Chemical Society
Article
MesA), 140.3 (d, 4JPC = 2.2 Hz, p-MesA), 140.2 (br, o′-MesB), 139.5 (d,
Scheme 12
3
4JPC = 1.8 Hz, p-MesB), 131.0 (d, JPC = 6.4 Hz, m′-MesB), 130.8 (d,
3
3JPC = 8.0 Hz, m-MesA), 129.9 (d, JPC = 9.4 Hz, m′-MesA), 129.5 (d,
3
1JPC = 45.3 Hz, i-MesB), 128.9 (d, JPC = 8.9 Hz, m-MesB), 123.0 (dd,
1JPC = 45.8 Hz, J = 2.1 Hz, i-MesA), 25.1 (d, 3JPC = 2.2 Hz, o-CH3MesA),
24.4 (m, o-CH3MesB), 23.12 (d, JPC = 3.3 Hz)t, 23.07 (d, JPC = 37.1
3
Hz)t, 21.2 (br)(CH2), 23.0 (d, JPC = 6.4 Hz, o′-CH3MesA), 21.8 (d,
3JPC = 3.2 Hz, o′-CH3MesB), 20.07 (d, 5JPC = 0.9 Hz, p-CH3MesA), 20.13
(d, JPC = 0.5 Hz, p-CH3MesB), 18.6 (br, BCH2), [C6F5 not listed.
5
t
tentatively assigned]. 11B{1H} NMR (192 MHz, 193 K, [D2]-
dichloromethane): δ = −5.8 (ν1/2 ≈ 1400 Hz). 19F NMR (564
MHz, [D2]-dichloromethane, 193 K): δ = −126.0 (m, o), −127.5 (m,
oxidized upon exposure to NO to give the respective
phosphanoxide which was then κO-bonded to the internal
boron Lewis acid to give the product (pS)-28, which was
isolated as a pale-yellow solid in 79% yield.
3
o″), −158.0 (br t, JFF = 19.5 Hz, p), −165.0 (m, m), −165.1 (m,
B
m′)(each 1F, C6F5 ) [Δδ19Fmp = 5.0, 5.1], −124.5 (m, o), −130.2 (m,
Complex (pS)-28 shows the 19F NMR signals of a pair of
diasterotopic C6F5 groups at boron. The rotation around the P-
3
o″), −159.8 (t, JFF = 21.3 Hz, p), −164.0 (m, m′), −164.6 (m,
A
m)(each 1F, C6F5 ) [Δδ19Fmp = 4.2, 4.8]. 31P{1H} NMR (243 MHz,
1
mesityl vectors is slow on the H NMR time scale, so we have
193 K, [D2]-dichloromethane): δ = 3.3 (ν1/2 ≈ 30 Hz).
observed a total of six methyl group signals and four arene
methine protons of the pair of mesityl groups at the P(O)Mes2
function of (pS)-28. The 11B NMR resonance of (pS)-28 is at δ
3.6 and the 31P NMR signal is at δ 58.6.
Preparation of Compound 10. The P/B FLP 8 (0.12 mmol, 80.0
mg, 1 equiv) was dissolved in dichloromethane (2 mL), and the
solution was degassed by freeze−pump−thaw cycles (two times).
Then H2 gas (1.5 bar) was pressed to the solution for 30 min at
ambient temperature. The reaction mixture was stored in the
refrigerator at −30 °C for 2 d. Batches of crystals were formed at
the bottom of the vessel. Solvent was removed in vacuum, and the
obtained solid was dried at low temperature (∼0 °C) in vacuo to give a
white powder (0.06 mmol, 40.0 mg, 50%). Crystals suitable for the X-
ray crystal structure analysis were obtained from a toluene solution of
10 at −30 °C in an H2 atmosphere (1.5 bar). Mp: 111 °C. Anal. Calcd
CONCLUSIONS
■
We had reported about the highly reactive ethylene-bridged
FLP 4 that has undergone a great variety of typical FLP
reactions, including selective small-molecule activation and
binding.22,33 Since the next homologue, the trimethylene-linked
PMes2/B(C6F5)2 system 7, turned out to be an unreactive FLP
(for reasons that are presently not clear), one might not have
expected quite reactive FLP behavior of the next homologue,
the tetramethylene-bridged PMes2, B(C6F5)2 system 8.
However, 8 is a very reactive FLP. It cleaves dihydrogen and
activates it for metal-free catalytic hydrogenation, deprotonates
alkynes with formation of the respective phosphonium/
alkynylborate products. The ferrocene-derived FLP (pS)-23
behaves similarly. In this very exceptional case of a C4-bridged
PMes2/B(C6F5)2 FLP we can experimentally observe the
reversible P−B adduct formation/dissociation reaction by its
temperature-dependent NMR spectra. This constitutes a very
rare case where the open (active) and closed (dormant)
structures of an intramolecular frustrated Lewis pair have
become experimentally observable. These two C4-bridged
intramolecular P/B FLP systems serve to markedly extend
the structural and conceptional scope of active intramolecular
FLP systems and frameworks. They will probably help us
design a variety of active FLP systems for future applications of
frustrated Lewis chemistry.
1
for C34H32BF10P: C, 60.73; H, 4.80. Found: C, 61.05; H, 4.76. H
NMR (600 MMR, 243 K, [D2]-dichloromethane): δ = 7.82 (br dt,
3
3
1JPH = 482.6 Hz, JHH = 6.3 Hz, 1H, PH), 6.98 (d, JPH = 3.9 Hz, 4H,
m-Mes), 2.76 (br m, 2H, PCH2), 2.33 (s, 12H, o-CH3Mes), 2.29 (s, 6H,
p-CH3Mes), 1.65, 1.24, 0.81 (each br m, each 2H, CH2), n.o. (BH).
13C{1H} NMR (151 MHz, 243 K, [D2]-dichloromethane): δ = 145.9
(p-Mes), 143.0 (d, 2JPC = 9.9 Hz, o-Mes), 131.7 (d, 2JPC = 11.7 Hz, m-
Mes), 111.2 (d, 1JPC = 81.3 Hz, i-Mes), 32.1, 28.1, n.o. (each br, CH2),
24.5 (br d, JPC = 40.1 Hz, PCH2), 22.0 (d, JPC = 7.2 Hz, o-CH3Mes),
21.1 (p-CH3Mes), [C6F5 not listed]. 11B NMR (192 MHz, 299 K, [D2]-
dichloromethane): δ = −20.9 (d, 1JBH ≈ 78 Hz). 19F NMR (564 MHz,
243 K, [D2]-dichloromethane): δ = −134.0 (br m, 2F, o-C6F5), −164.6
1
3
3
(br t, JFF = 20.3 Hz, 1F, p-C6F5), −166.8 (br m, 2F, m-C6F5)
[Δδ19Fmp = 2.2]. 31P NMR (243 MHz, 243 K, [D2]-dichloromethane):
1
δ = −11.9 (br d, JPH ≈ 486 Hz, ν1/2 ≈ 40 Hz).
Preparation of Compound 10-D2. A solution of compound 8
(0.35 mmol, 240 mg, 1 equiv) in dichloromethane (5 mL) was
degassed by freeze−pump−thaw cycles (two times). Then D2 gas (2
bar) was pressed to the solution for 30 min. The reaction mixture was
stirred overnight. Solvent was quickly evaporated in vacuum, and the
obtained solid was dried at low temperature in vacuo to give
1
compound 10-D2 (0.16 mmol, 113 mg, 47%) as a white powder. H
4
NMR (600 MHz, 299 K, [D2]-dichloromethane): δ = 7.06 (d, JPH
=
EXPERIMENTAL SECTION
4.2 Hz, 4H, m-Mes), 2.88, 1.78, 1.43, 0.88 (each m, each 2H, CH2),
■
2.41 (s, 12H, o-CH3Mes), 2.35 (s, 6H, p-CH3Mes). 11B{1H} NMR (192
Preparation of Compound 8. Compound 9 (8.6 mmol, 2.80 g, 1
equiv) and HB(C6F5)2 (8.6 mmol, 2.99 g, 1 equiv) were suspended in
pentane (20 mL) and stirred for 1 h at ambient temperature. The
resulting clouded suspension was filtered, and the clear filtrate was
dried in vacuo to yield compound 8 (7.7 mmol, 5.20 g, 90%) as a pale-
yellow solid which was crystallized in toluene. Crystals suitable for the
X-ray crystal structure analysis were obtained from a solution of 8 in
toluene at −30 °C. Mp: 151 °C. Anal. Calcd for C34H30BF10P: C,
60.92; H, 4.51. Found: C, 61.05; H, 4.76. 1H NMR (600 MMR, 193 K,
[D2]-dichloromethane): δ = 6.96 (m, 1H, m-MesA), 6.86 (m, 1H, m-
MesB), 6.73 (m, 1H, m′-MesB), 6.40 (m, 1H, m′-MesA), 3.14, 2.36,
2.23, 1.89, 1.57, 0.84 (each m, each 1H, CH2), 2.81 (s, 3H, o-
MHz, 299 K, [D2]-dichloromethane)): δ = −21.0 (ν1/2 ≈ 80 Hz). 19
F
NMR (564 MHz, 299 K, [D2]-dichloromethane): δ = −133.7 (m, 2F,
3
o-C6F5), −165.5 (t, JFF = 19.9 Hz, 1F, p-C6F5), −167.5 (m, 2F, m-
C6F5) [Δδ19Fmp = 2.0]. 31P{1H} NMR (243 MHz, 299 K, [D2]-
1
dichloromethane)): δ = −11.7 (1:1:1 t, JPD = 73.0 Hz, ν1/2 ≈ 5 Hz).
Preparation of Compound (pS)-23. NMR Characterization.
Compound (pS)-22 (0.05 mmol, 24.0 mg, 1 equiv) was reacted with
HB(C6F5)2 (0.05 mmol, 17.3 mg, 1 equiv) in deuterated dichloro-
methane (0.5 mL) at room temperature for 30 min to give compound
(pS)-23 (100% conversion). [Comment: two compounds were
observed in CD2Cl2 solution at 193 K, which were tentatively assigned
1
CH3MesA), 2.45 (br d, JPH = 4.2 Hz, 3H, o-CH3MesB), 2.17 (s, 3H, p-
4
as the “open” and “closed” FLP structure]. H NMR (600 MHz, 193
4
CH3MesA), 2.15 (s, 3H, p-CH3MesB), 2.06 (s, 3H, o′-CH3MesB), 1.44,
1.00 (each m, each 1H, BCH2), 1.14 (s, 3H, o′-CH3MesA). 13C{1H}
K, [D2]-dichloromethane): δ[major] = 7.00 (d, JPH = 4.9 Hz), 6.81
(br), 6.54 (br), 6.51 (br, each 1H, m-Mes), 4.30, 4.22, 4.04 (each br m,
each 1H, C5H3), 3.93 (s, 5H, C5H5), 2.83, 2.34 (each br m, each 1H,
CH2), 2.80, 2.23, 2.07, 1.87, 1.80, 1.56 (each s, each 3H, CH3Mes), 2.35,
NMR (151 MHz, 193 K, [D2]-dichloromethane): δ = 142.0 (d, 2JPC
=
14.3 Hz, o-MesB), 141.5 (d, JPC = 11.9 Hz, o′-MesA), 140.9 (br, o-
2
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dx.doi.org/10.1021/ja413060u | J. Am. Chem. Soc. 2014, 136, 3293−3303

Journal of the American Chemical Society
Article
2.02 (each m, each 1H, BCH2). δ[minor] = 7.03, 6.82, 6.81, 6.49(each
br, each 1H, m-Mes), 4.04 (2H), 3.83 (1H, each br m, C5H3), 4.12 (s,
gas atmosphere (1.5 bar). Then all volatiles were removed in vacuo and
the resulting pale-yellow solid was washed twice with pentane (2 × 1
mL) to give compound (pS)-27 (0.036 mmol, 30.0 mg, 73%) as a
pale-yellow solid. Dec: 150 °C. Anal. Calcd for C42H36BF10FeP: C,
60.90; H, 4.38. Found: C, 60.10; H, 4.08.
3
5H, C5H5), 3.71 (s), 2.46 (s), 2.26 (d, JPH = 6.3 Hz), 2.22 (s), 2.19
(s), 0.99 (s) (each 3H, CH3Mes), 3.03, 1.90 (each br m, each 1H, CH2),
3
1.90 (br m), 1.27 (br dm, JPH = 37.7 Hz)(each 1H, BCH2). 11B{1H}
NMR (192 MHz, 298 K, [D2]-dichloromethane): δ = 72.6 (ν1/2
≈
Preparation of Compound (pS)-27-D2. NMR Characterization.
In a Schlenk vessel (10 mL) compound (pS)-22 (0.1 mmol, 48.0 mg, 1
equiv) was reacted with HB(C6F5)2 (0.1 mmol, 34.6 mg, 1 equiv) in
CD2Cl2 (1 mL) at room temperature for 30 min to give an orange
solution. Afterward, 1.5 bar D2 gas was exposed to the reaction
solution. A bright-yellow solution was formed immediately. The
solution was stirred for another 20 min in the presence of a D2 gas
atmosphere (1.5 bar). The solution was immediately transferred to a
NMR tube and sealed. The sample was promptly characterized by
NMR experiments. 1H NMR (600 MHz, 193 K, [D2]-dichloro-
methane): δ = 7.21 (d, 4JPH= 5.1 Hz), 7.00 (d, 4JPH = 4.2 Hz), 6.93 (d,
1200 Hz). 19F NMR (564 MHz, 193 K, [D2]-dichloromethane):
δ[major, ∼70% mol] = −127.7 (br, 2F, o-C6F5), −150.1 (br, 1F, p-
C6F5), −162.2 (br, 2F, m-C6F5)[Δδ19Fp,m = 12.1]. δ[minor, ∼30%
mol] = −121.0, −131.0 (each m, o), −160.3 (br t, 3JFF = 21.0 Hz, p),
−165.0, −165.2 (each m, m) (each 1F, C6F5)[Δδ19Fp,m = 6.4, 6.6],
3
−125.7, −127.4 (each m, o), −157.7 (br t, JFF = 19.8 Hz, p), −164.1
−164.3 (each m, m) (each 1F, C6F5)[Δδ19Fp,m = 4.7, 4.9]. 19F NMR
(564 MHz, 298 K, [D2]-dichloromethane): δ = −129.8 (br m, 2F, o-
C6F5), −148.6 (br, 1F, p-C6F5), −161.8 (br m, 2F, m-C6F5) [Δδ19Fp,m
= 13.2]. 31P{1H} NMR (243 MHz, 193 K, [D2]-dichloromethane): δ =
−33.4 (ν1/2 ≈ 750 Hz, ∼63% mol, “open” structure), 11.5 (ν1/2 ≈ 40
Hz, ∼37% mol, “closed” structure). 31P{1H} NMR (243 MHz, 298 K,
[D2]-dichloromethane): δ = −42.8 (ν1/2 ≈ 55 Hz).
4
4JPH = 4.3 Hz), 6.76 (d, JPH = 4.3 Hz)(each 1H, m-Mes), 4.86, 4.62,
4.16 (each br, each 1H, C5H3), 4.13 (s, 5H, C5H5), 2.86 (3H), 2.33
(3H), 2.20 (6H), 2.11 (3H), 1.74 (3H) (each s, CH3Mes), 2.18 (m, 2H,
2
Isolation of Compound (pS)-23. Compound (pS)-22 (0.1 mmol,
48.0 mg, 1 equiv) was reacted with HB(C6F5)2 (0.1 mmol, 34.6 mg, 1
equiv) in pentane (7.0 mL) at room temperature for 30 min to give a
red-orange solution. The red-orange solution was kept in the
refrigerator at −30 °C. Crystals suitable for the X-ray crystal structure
analysis were obtained from a solution of (pS)-23 in pentane at −30
°C. Quick removal of the pentane solution by pipet and drying of the
resultant red-orange solid in vacuum gave compound (pS)-23 (0.07
mmol, 61.0 mg, 74%). Mp: 176 °C. Anal. Calcd for C42H34BF10FeP: C,
61.05; H, 4.15. Found: C, 59.96; H, 3.94.
CH2), 0.93, 0.82 (each m, each 1H, BCH2). H NMR (92 MHz, 193
1
K, [D2]-dichloromethane): 8.46 (br d, JPD = 73.6 Hz, PD), 2.50 (br,
BD). 11B NMR (192 MHz, 193 K, [D2]-dichloromethane): δ = −21.1
(ν1/2 ≈ 350 Hz). 19F NMR (564 MHz, 193 K, [D2]-dichloromethane):
δ = −134.2 (m, 4F, o-C6F5); −163.9, −164.0 (each br m, each 1F, p-
C6F5); −166.0, −166.1 (each m, each 2F, m-C6F5) [Δδmp = 2.1].
31P{1H} NMR (243 MHz, 193 K, [D2]-dichloromethane): δ = −19.2
1
(1:1:1 t, JPD = 73.4 Hz).
ASSOCIATED CONTENT
* Supporting Information
■
Preparation of Compound (pS)-27. NMR Characterization. In a
Schlenk vessel (10 mL), compound (pS)-22 (0.05 mmol, 24.0 mg, 1
equiv) was reacted with HB(C6F5)2 (0.05 mmol, 17.3 mg, 1 equiv) in
CD2Cl2 (0.5 mL) at room temperature for 30 min to give an orange
solution. Afterward the solution was exposed to an H2 gas atmosphere
(1.5 bar). Within 1 min, a bright yellow solution was formed. The
solution was stirred for another 20 min in the presence of an H2 gas
atmosphere (1.5 bar). Then the solution was immediately transferred
to a NMR tube and sealed. The sample was immediately characterized
S
Experimental details and physical characterization of the new
compounds, hydrogenation reactions, crystallographic data, and
CIF files. This material is available free of charge via the
Internet at http://pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Authors
erker@uni-muenster.de
kehrald@uni-muenster.de
1
by NMR experiments. H NMR (600 MHz, 193 K, [D2]-dichloro-
1
4
methane): δ = 8.45 (d, JPH = 495.4 Hz, 1H. PH), 7.21 (d, JPH = 3.8
Hz, 1H, m-Mesa), 7.01 (br, 1H, m′-Mesa), 6.93 (d, 4JPH = 3.5 Hz, 1H,
m-Mesb), 6.76 (d, 4JPH = 3.3 Hz, 1H, m′-Mesb), 4.86, 4.62, 4.16 (each
br, each 1H, C5H3), 4.13 (s, 5H, C5H5), 2.86 (s, 3H, o-CH3Mesa), 2.33
(s, 3H, p-CH3Mesa), 2.203 (s, 3H, p-CH3Mesb), 2.198 (s, 3H, o-
CH3Mesb), 2.18 (m, 2H, CH2), 2.11 (s, 3H, o′-CH3Mesa), 1.74 (s, 3H,
o′-CH3Mesb), 0.94, 0.85 (each br, each 1H, BCH2), n.o. (BH). 13C{1H}
NMR (151 MHz, 193 K, [D2]-dichloromethane): δ = 145.3 (p-Mesa),
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
Financial support from the European Research Council is
gratefully acknowdged.
■
2
2
144.7 (p-Mesb), 143.8 (d, JPC = 10.1 Hz, o′-Mesb), 142.3 (d, JPC
10.3 Hz, o′-Mesa), 141.4 (d, 2JPC = 19.2 Hz, o′-Mesa), 141.3 (d, 2JPC
21.4 Hz, o-Mesb), 131.9 (d, 3JPC = 12.1 Hz, m′-Mesb), 131.6 (d, 3JPC
11.7 Hz, m′-Mesa), 130.7 (d, 3JPC = 10.5 Hz, m-Mesa), 130.5 (d, 3JPC
=
=
=
=
=
REFERENCES
■
(1) (a) Stephan, D. W.; Erker, G. Angew. Chem., Int. Ed. 2010, 49, 46.
(b) Stephan, D. W.; Erker, G. Top. Curr. Chem. 2013, 332, 85.
(2) (a) Stephan, D. W.; Greenberg, S.; Graham, T. W.; Chase, P.;
Hastie, J. J.; Geier, S. J.; Farrell, J. M.; Brown, C. C.; Heiden, Z. M.;
Welch, G. C.; Ullrich, M. Inorg. Chem. 2011, 50, 12338. (b) Stephan,
D. W. Org. Biomol. Chem. 2008, 6, 1535. (c) Stephan, D. W. Dalton
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10.5 Hz, m-Mesb), 111.7 (d, JPC = 89.8 Hz, i-Mesa), 110.8 (d, JPC
1
1
82.1 Hz, i-Mesb), 99.7 (d, JPC = 12.1 Hz), 75.6 (d, JPC = 10.0 Hz), 72.6
(d, JPC = 11.0 Hz), 71.9 (d, JPC = 14.1 Hz), 56.9 (d, JPC = 96.3
Hz)(C5H3), 70.5 (C5H5), 28.8 (br, CH2), 23.4 (br, BCH2), 21.9 (d,
3JPC = 10.5 Hz, o-CH3Mesa), 21.8 (d, JPC = 3.3 Hz, o′-CH3Mesb), 21.5
3
(d, JPC = 6.8 Hz, o′-CH3Mesa), 21.0 (p-CH3Mesa), 20.50 (p-CH3Mesb),
3
20.49 (d, 3JPC = 8.6 Hz, o-CH3Mesb), [C6F5 not listed]. 11B NMR (192
1
MHz, 273 K, [D2]-dichloromethane): δ = −20.5 (d, JBH ≈ 85
Hz)(diluted sample ∼0.0125 mmol/mL). 19F NMR (564 MHz, 193 K,
[D2]-dichloromethane): δ = −134.2 (m, 4F, o-C6F5); −163.9, −164.0
(each br m, each 1F, p-C6F5); −166.0, −166.2 (each m, each 2F, m-
C6F5). 31P NMR (243 MHz, 193 K, [D2]-dichloromethane): δ =
(3) (a) Welch, G. C.; Juan, R. R. S.; Masuda, J. D.; Stephan, D. W.
Science 2006, 314, 1124. (b) Welch, G. C.; Stephan, D. W. J. Am.
Chem. Soc. 2007, 129, 1880.
1
−18.8 (d, JPH = 496.1 Hz).
Isolation of Compound (pS)-27. Compound (pS)-22 (0.05 mmol,
24.0 mg, 1 equiv) was reacted with HB(C6F5)2 (0.05 mmol, 17.3 mg, 1
equiv) in pentane (10 mL) at room temperature for 30 min to give an
orange solution. Afterward the solution was exposed to an H2 gas
atmosphere (1.5 bar). Within 1 min, a bright-yellow solid was formed.
The solution was stirred for another 5 min in the presence of an H2
(4) Spies, P.; Schwendemann, S.; Lange, S.; Kehr, G.; Frohlich, R.;
̈
Erker, G. Angew. Chem., Int. Ed. 2008, 47, 7543.
(5) (a) Sumerin, V.; Schulz, F.; Nieger, M.; Leskela, M.; Repo, T.;
Rieger, B. Angew. Chem., Int. Ed. 2008, 47, 6001. (b) Sumerin, V.;
Schulz, F.; Atsumi, M.; Wang, C.; Nieger, M.; Leskela, M.; Repo, T.;
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