Direct synthesis of a geminal zwitterionic phosphonium/hydridoborate system - developing an alternative tool for generating frustrated Lewis pair hydrogen activation systems

Article abstract of DOI:10.1039/c5ob00634a

A convenient way to a new class of geminal Mes₂PH⁺/B(C₆F₅)₂H^- pairs is presented. It utilizes triflic acid addition to trans-Mes₂PCHCHB(C₆F₅)₂ fo

Full text of DOI:10.1039/c5ob00634a

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Journal Name
RSCPublishing
ARTICLE
Direct Synthesis of a Geminal Zwitterionic
Phosphonium / Hydridoborate System -
Developing an Alternative Tool for Generating
Frustrated Lewis Pair Hydrogen Activation
Systems
Cite this: DOI: 10.1039/x0xx00000x
Received 00th January 2012,
Accepted 00th January 2012
Jiangang Yu,a Gerald Kehr,a Constantin G. Daniliuc,a Christoph
DOI: 10.1039/x0xx00000x
Bannwarth,b Stefan Grimme,b and Gerhard Erker*a
www.rsc.org/
A convenient way to the new class of geminal Mes2PH+/B(C6F5)2H- pairs is presented. It utilizes
triflic acid addition to trans-Mes2PCH=CHB(C6F5)2 followed by triflate/hydride exchange.
Thermally induced ring-closure gave a phosphonium/boratacyclopropane zwitterion
8 which
formed the Mes2PH(CHMe)B(C6F5)2H P/B FLP-H2 product 10 by subsequent treatment with triflic
acid and a silane, or alternatively with dihydrogen at 90 °C. The product 10 is an active catalyst
for the hydrogenation of a variety of unsaturated organic substrates, including a quinoline
derivative. Treatment of
8 with HB(C6F5)2 gave a bifunctional borane 14 which selectively
reduced carbon monoxide to the formyl stage.
unsaturated substrates and we have developed first examples of
Introduction
an interesting followꢀup chemistry derived from this new
synthetic FLP development. In this article we will briefly
outline this unusual but very convenient new way to a
catalytically active geminal P/B FLP H2 system.
Frustrated Lewis pairs (FLPs) have shown some remarkable
potential for small molecule binding and activation.1,2
Heterolytic cleavage of the dihydrogen molecule has been the
most prominent feature in this chemistry.3 This activation of H2
by cooperative action of a pair of complementary main group
element functional groups has led to the development of a
catalytic metalꢀfree hydrogenation protocol for quite a variety
of unsaturated substrate types.4ꢀ6 Usually, the interꢀ or intraꢀ
molecular FLPs were prepared and then converted to the
respective zwitterionic H2ꢀactivation products by treatment with
dihydrogen. The formation of the vicinal intramolecular P/B
Chart 1. Intramolecular FLP examples
FLP
1 2 is a typical
and its [P]H+/[B]Hꢀ H2ꢀactivation product
Results and discussion
example (see Chart 1).7ꢀ8 However, there are unfortunate cases
where this straightforward approach has so far not been
achieved. The family of the geminal Mes2P/B(C6F5)2 FLPs
represents an example of such a situation. So far, we had been
Formation of the new geminal PH+/BH- system
Our synthesis started with dimesitylethynylphosphane (4)
which was regioselectively hydroborated with Piers’ borane
able to obtain related systems
3 by simple hydroboration only
[HB(C6F5)2]11 to yield compound
5. It contains a phosphane
with the P(C6F5)2 moiety (see Chart 1) i.e. a Lewis base
functionality that is not basic enough for the dihydrogen
activation process.9ꢀ10 For such situations, we have now
developed an attractive principal synthetic alternative that has
made a respective [P]H+/[B]Hꢀ product directly available
without involving the isolated free P/B FLP. By this new way
we have obtained an active geminal phosphonium/hydridoꢀ
borate system that could then be used directly for the actual
metalꢀfree catalytic hydrogenation reaction of a variety of
Lewis base (31P NMR: δ ꢀ7.7) and a borane Lewis acid (11B
NMR: δ 53.2) bridged by the transꢀCH=CHꢀunit (1H NMR: δ
3
8.30, 6.66, JHH = 18.1 Hz). The structure was confirmed by Xꢀ
ray diffraction (see Figure 1). The Xꢀray crystal structure
analysis shows that the Piers’ borane reagent [HB(C6F5)2] has
regioselectively added to the carbonꢀcarbon triple bond of the
dimesitylphosphanyl substituted acetylene. The resulting
central carbonꢀcarbon double bond [C1ꢀC2 1.350(3) Å] in the
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product
5
is transꢀconfigured. The bulky substituents show a In solution, compound
6
shows a typical borate 11B NMR
dihedral angle of θ P1ꢀC1ꢀC2ꢀB1 = 170.3(2)°. The borane is resonance at δ 0.7 [31P: δ ꢀ16.8 (1JPH ≈ 475 Hz)]. The 19F NMR
planarꢀtricoordinate with a sum of bond angles of ΣB1CCC spectrum features the typical ratio of
o,p,mꢀresonances of the
360.0°. It is found in a conjugated orientation with the adjacent pair of C6F5 substituents at boron and a separate 19F NMR
C=C double bond [θ C1ꢀC2ꢀB1ꢀC31 ꢀ161.6(2)°, C1ꢀC2ꢀB1ꢀC41 signal of the ꢀOSO2CF3 group at δ ꢀ78.6. The connecting C=C
17.5(3)°]. The phosphorus atom in compound
flattened trigonalꢀpyramidal geometry with ΣP1CCC 324.7°.12
5
features a unit in compound
3JHH = 19.0 Hz), 13C: δ 174.1 ([B]CH), 107.9 ([P]CH)]. The
OTf substituent at the borate anion terminus of compound
was then exchanged to hydride by treatment with
chlorodimethylsilane (CH2Cl2, 1h at r.t.) to give the trans
6
is transꢀconfigured [1H NMR: δ 7.67, 6.43
(
6
ꢀ
CH=CHꢀbridged phosphonium/hydridoborate system
7 (see
Scheme 1).
1
Both the PH+ proton (δ 7.82, JPH = 469.3 Hz, with
corresponding 31P NMR signal at δ = ꢀ18.3) and the BHꢀ
hydride signal (broad partially relaxed 1:1:1:1 quartet at δ 3.24,
with corresponding 11B NMR signal at δ = ꢀ21.6, JBH = 89.0
1
Hz) were located. The 1H/13C NMR spectra feature of the trans
ꢀ
CH=CHꢀbridge occur at δ 8.02, 5.74 (3JHH = 18.3 Hz) and δ
189.1/101.5, respectively.
TfO
H
B(C6F5)2
H
H
B(C6F5)2
H
Mes2P
H
4
+
Mes2P
Mes2P
HOTf
HB(C6F5)2
Figure 1. Molecular structure of compound
30% probability).
5
(thermal ellipsoids are shown with
H
5 (76%)
6 (66%)
[Si]H
[B]
Triflic acid was then added11c,13 to compound
phosphonium compound
[PH+: δ 7.90 (1JPH = 475.3 Hz) (1H);
5
to give the
[B]
H
CH3
H
H
TfOH
H
∆
H
6
Mes2P
[B]
OTf
δ ꢀ16.8 (31P)]. The triflate anion was found covalently attached
at the boron atom. This was confirmed by Xꢀray diffraction (see
Figure 2). The Xꢀray crystal structure analysis shows the ꢀ
OSO2CF3 group covalently bonded to the boron atom [B1ꢀO1
1.573(3) Å] which now has altered into a pseudoꢀtetrahedral
coordination geometry (ΣB1CCC 333.4°). The phosphorus base
at the opposite end of the transꢀB1ꢀC2=C1ꢀP1 framework with
[B1ꢀC2 1.594(4) Å, C2ꢀC1 1.322(4) Å, C1ꢀP1 1.777(3) Å,
angles B1ꢀC2ꢀC1 128.0(2)°, C2ꢀC1ꢀP1 122.5(2)°] has become
protonated (ΣP1CCC 342.2°).
H
Mes2P
Mes2P
H
H
H
H
9 (75%)
7 (78%)
8 (75%)
(65%)
CH3
H2
90°C
H
[B]: B(C6F5)2
[Si]: Me2ClSi
[Si]H
Mes2P
H
B(C6F5)2
(88%)
H
10
Scheme 1. Synthesis of compound 10
Thermolysis of compound
7 in toluene solution (30 min,
110 °C) resulted in a clean and complete rearrangement to the
zwitterionic phosphonium/boratacyclopropane isomer 8 14
The
.
Xꢀray crystal structure analysis (see Figure 3) shows the newly
formed anionic heterocyclic threeꢀmembered ring [B1ꢀC1
1.660(3) Å, B1ꢀC2 1.563(3) Å, C1ꢀC2 1.535(3) Å]. The boron
atom is fourꢀcoordinate; it has the pair of C6F5 substituents
bonded to it [B1ꢀC31 1.614(3) Å, B1ꢀC41 1.608(3) Å, angle
C31ꢀB1ꢀC41 112.2(2)°, C1ꢀB1ꢀC2 56.8(1)°]. The Mes2P(H)ꢀ
substituent is found attached at carbon atom C1 of the ring [C1ꢀ
P1 1.749(2) Å]. The phosphorus atom also shows a distorted
pseudotetrahedral coordination geometry (ΣP1CCC 342.5°).
The NMR spectra of compound
8 show typical cyclopropane
features [1H: δ 1.17 (CH), δ 1.00, 0.85 (CH2); 13C: δ 12.4 (CH2),
δ 4.2 (CH, 1JPC = 62.3 Hz)]. The 11B NMR signal was observed
at δ ꢀ23.8 and the signals of the PH unit were located at δ ꢀ1.1
Figure 2. A view of the molecular structure of compound
are shown with 30% probability).
6 (thermal ellipsoids
1
(31P) and 6.94 (1H, JPH = 464.4 Hz), respectively. Due to the
newly formed chiral carbon center (C1) compound
8 shows the
2 | J. Name., 2012, 00, 1-3
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19F NMR resonances of a pair of diastereotopic C6F5 groups at [B1ꢀO1 1.571(5) Å] and the phosphorus atom is protonated
1
boron and the H/13C NMR signals of a pair of diastereotopic (ΣP1CCC 344.0°). In the crystal, product
9 exhibits a
mesityl groups at phosphorus (for details see the Supporting conformation that shows the C1ꢀC2 vector in a gaucheꢀlike
Information).
orientation with the B1ꢀO1 triflate group [θ C2ꢀC1ꢀB1ꢀO1 ꢀ
61.1(4)°].
Subsequent treatment of compound
9 with Me2Si(H)Cl resulted
in clean removal of the OTf group from boron with formation
of the borohydride product 10, which was isolated in 88% yield.
Alternatively, ring opening of
8 was achieved directly by
treatment with dihydrogen at elevated temperature (90 °C,
overnight in toluene, 2.5 bar H2) to give compound 10 (see
Scheme 1). We isolated it after workup involving
crystallization from dichloromethane/pentane in 65% yield.
The geminal phosphonium/hydridoborate zwitterion 10 was
characterized by Xꢀray diffraction (see Figure 5). It features
bonds between carbon atom C1 and phosphorus [P1ꢀC1 1.805(3)
Å] and boron [B1ꢀC1 1.669(5) Å, angle B1ꢀC1ꢀP1 107.2(2)°].
Carbon atom C1 also bears the newly formed methyl group that
was derived from opening of the boratacyclopropane ring of the
Figure 3. Molecular structure of compound
30% probability).
8 (thermal ellipsoids are shown with
product
8 in the course of the hydrogenolysis reaction [C1ꢀC2
1.566(6) Å]. Both the phosphorus and the boron atoms in
compound 10 feature pseudotetrahedral coordination
geometries (ΣP1CCC 343.8°, ΣB1CCC 336.4°). In solution, we
have located both the hydride 1H NMR signal of the
hydridoborate subunit [δ 2.89, 11B: δ ꢀ20.0 (1JBH = 90.5 Hz)]
and the phosphonium proton [δ 7.06 (1JPH = 463.9 Hz), 31P: δ
1
6.3]. The H NMR spectrum feature of the ethylidene bridge
occur at δ 3.26 (CH) and δ 1.13 (CH3) [13C: δ 17.3 (CH), δ 15.8
(CH3)].
Figure 4. A view of the molecular structure of the HOTf addition product
(thermal ellipsoids are shown with 30% probability).
9
Treatment of the zwitterion
8 with triflic acid resulted in a
selective protonolytic ring opening of the heterocyclic threeꢀ
membered ring to give compound
9 in 75% yield. In solution, it
shows heteronuclear magnetic resonance signals at δ 3.6 (11B),
δ ꢀ1.2 (31P) and δ ꢀ76.8 (19F NMR of the CF3 group). At 233 K
Figure 5. A view of the molecular structure of the geminal phosphonium/
hydridoborate system 10 (thermal ellipsoids are shown with 30% probability).
compound
of diastereotopic C6F5 substituents at boron, indicating hindered
rotation around the BꢀC vectors. Compound shows a typical
pattern of H NMR signals of the ꢀCHCH3 unit [δ 3.72 (CH), δ
9
showed a total of ten 19F NMR signals of the pair
9
Reactivity studies
1
We tried to find out more about the unusual hydrogenolytic ring
3
3
1.67 (dd, JPH = 24.2 Hz, JHH = 6.9 Hz (CH3); 13C: δ 19.5,
14.9)]. Compound was characterized by Xꢀray diffraction.
The view of the structure (Figure 4) confirms that the
boratacyclopropane ring of the product has been opened
opening reaction of
few additional reactions with the phosphonium/borataꢀ
cyclopropane zwitterion . Treatment with potassium hydride
8 to give 10. Therefore, we performed a
9
8
8
resulted in a clean deprotonation of the phosphonium unit to
give the Mes2Pꢀsubstituted boratacyclopropane salt 11 [NMR: δ
ꢀ4.4 (31P), δ ꢀ26.8 (11B), δ 1.62, 0.61 (3JPH = 25.4 Hz), 0.18
protonolytically. This resulted in the formation of a methyl
group (C2) attached at the methine carbon atom C1 that is
found bridging between boron and phosphorus [C1ꢀC2 1.552(5)
Å, B1ꢀC1 1.662(5) Å, P1ꢀC1 1.816(3) Å, angle B1ꢀC1ꢀP1
116.0(2)°]. The triflate anion has become attached to boron
(
3JPH = 10.1 Hz) (1H, ꢀCHꢀCH2ꢀ)]. Its treatment with
benzylbromide gave the zwitterionic benzylphosphonium/
boratacyclopropane product 12 isolated as colorless crystals in
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82% yield (see Scheme 2) [NMR: δ 33.7 (31P), δ ꢀ23.1 (11B)]. 1.579(4) Å]. The boratacyclopropane unit of its precursor 12
1
Compound 12 showed an AB H NMR type pattern of the had been opened by addition of the proton to give a methyl
diastereotopic benzylic hydrogens adjacent to phosphorus [δ group (C2) attached at the bridging sp3ꢀcarbon atom C1 [C1ꢀC2
4.45, 4.22, (2JPH
≈
2JHH = 13.4 Hz); 13C: δ 39.3 (1JPC = 49.5 Hz)] 1.547(4) Å, B1ꢀC1 1.652(5) Å, P1ꢀC1 1.844(3) Å, angle B1ꢀ
and the 13C NMR signals of the boratacyclopropane unit at δ C1ꢀP1 118.5(2)°]. Overall, the structural parameters of the
12.1 (CH2) and 10.4 (CH), respectively.
zwitterionic Pꢀbenzyl/BꢀOTf product 13 are similar to those of
the related PH/BꢀOTf compound (see above).
9
[B]
[B]
H
[B]
H
H
PhCH2Br
KH
-H2
H
H
H
H
Mes2P
H
Mes2P
H
Mes2P
K
Ph
H
8
11
12
HOTf
H
H
CH3
(C6F5)2B
H
B(C6F5)2
HB(C6F5)2
H
H
Mes2P
B(C6F5)2
OTf
Mes2P
H
[B]: B(C6F5)2
Ph
14
13
Scheme 2. Reaction of compound
8
Figure 7. A projection of the molecular structure of the ethylidene-bridged P-
benzyl/B-OTf zwitterion 13 (thermal ellipsoids are shown with 30% probability).
In solution, compound 13 is characterized by NMR resonances
at δ 5.1 (11B), δ 38.6 (31P) and ꢀ75.9 (19F of OSO2CF3). The
compound shows the 19F NMR signals of a pair of
diastereotopic C6F5 groups at boron (with hindered rotation on
the 19F NMR time scale at low temperature) and similarly the
1H/13C NMR resonances of the diastereotopic mesityl group at
phosphorus. The benzylic methylene protons are also
diastereotopic as expected, and give rise to a typical pattern at δ
1
4.94/4.40. The H/13C NMR spectra feature of the bridging
ethylidene unit occur at δ 3.95 / 25.1 (CH) and δ 1.84 / 17.2
(CH3), respectively.
Figure 6. Molecular structure of compound 12 (thermal ellipsoids are shown with
30% probability).
Compound 12 was characterized by Xꢀray diffraction (see
Figure 6). It shows structural features of the
boratacyclopropane subunit [B1ꢀC1 1.675(3) Å, B1ꢀC2 1.574(3)
Å, C1ꢀC2 1.523(3) Å, angle C1ꢀB1ꢀC2 55.8(1)°] that are very
similar to those of the related phosphonium/boratacyclopropane
compound
8 (see above). In 12 the P1ꢀC3 linkage of the newly
introduced benzyl substituent amounts to 1.845(2) Å.
Compound 12 was also exposed to a dihydrogen atmosphere at
elevated temperature. In contrast to the PH+ containing system
8
, the Pꢀbenzyl+ analogue 12 did not undergo ringꢀopening
under comparable conditions, but it was cleanly opened by
triflic acid to give 13 (see Scheme 2). The ring opened product
13 was characterized by C,H elemental analysis, by NMR
spectroscopy and by Xꢀray diffraction. The Xꢀray crystal
structure analysis of compound 13 (see Figure 7) showed the
presence of the Mes2Pꢀbenzyl moiety [P1ꢀC3 1.840(3) Å] and
the newly introduced triflate substituent at boron [B1ꢀO1
Figure 8. A view of the molecular structure of compound 14 (thermal ellipsoids
are shown with 30% probability). Selected bond lengths (Å) and angles (deg): C1-
4 | J. Name., 2012, 00, 1-3
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P1 1.816(2), C1-B2 1.664(3), C1-C2 1.558(3), C2-B1 1.602(4), ΣP1CCC 347.2, ΣB1CCC
347.0, ΣB2CCC 349.3, B1-C1-C2-B2 22.9(2).
product of the nonꢀobserved C1ꢀbridged P/B FLP 15’, has
served as a catalyst in the metalꢀfree hydrogenation of a variety
of unsaturated organic substrates (see Table 1). Under the
applied standard test condition (90 °C, 50 bar H2, D2ꢀ
dichloromethane solution) complete conversion was achieved
upon hydrogenation of a bulky imine to the secondary amine,4b
of a tetraꢀsubstituted enamine to the tertꢀamine,4a,17 of a
silylenolether to the silylated alcohol,18 and even of a
substituted quinoline to the respective tetrahydroquinoline
product.19 The respective products were in each case isolated in
reasonable yields (see Table 1 and the Supporting Information).
Compound
8 also underwent rapid opening of the boratacycloꢀ
propane ring by treatment with the Lewis acid [HB(C6F5)2].
Workup gave the zwitterionic product 14 in 65% yield [NMR:
1
δ ꢀ2.2 (31P), H: δ 2.09 (BH), δ 7.01 (1JPH = 460.8 Hz, PH), δ
4.09, 2.09, 1.87 (CHCH2)]. The product 14 was characterized
by Xꢀray diffraction (see Figure 8). The structure shows a
central fiveꢀmembered [B]ꢀHꢀ[B] containing heterocycle that
was formed by opening of the boratacyclopropane ring system.
It has the Mes2PH+ꢀsubstituent attached.
From these experimental observations it had become evident Table 1. Metalꢀfree catalyzed hydrogenation of a small series of substrates
with the [P]H+/[B]Hꢀ system 10a
that the ring opening of the boratacyclopropane ring was
readily achieved by protonolysis. It was, therefore, likely that
substrate
product
substrate
mg (mmol) mol% time (h)
10
reaction
conv.
(iso.)
the phosphonium [P]H+ served as the necessary Brønsted acid
also in the hydrogenolytic ringꢀopening reaction. Quantum
chemical calculations15,16 rapidly ruled out any direct reaction
of H2 with a B1ꢀC2 σꢀbond or a preceeding intramolecular
conversion to the geminal FLP Mes2PꢀCH(CH3)ꢀB(C6F5)2 (15’),
209
(1.0)
>99%
(86%)
1
2
10
10
36
36
215
(1.0)
>99%
(60%)
although the
thermoneutral. According to our density functional theory (DFT)
analysis the isomerisation of to the (not experimentally
observed) geminal FLP 15’ is substantially kinetically hindered
G‡ ~ 55 kcal molꢀ1), even though the subsequent reaction
8 to 15’ isomerization was calculated to be almost
192
(1.0)
>99%
(62%)
3
4
10
10
36
36
8
143
(1.0)
>99%
(70%)
(
∆
calc
of the FLP 15’ with dihydrogen would be feasible (∆
G‡calc ~ 17
a 90 °C, 50 bar H2, in D2ꢀdichloromethane solution.
kcal molꢀ1) (for details see the Supporting Information).
However, we found some indication that the reaction might
Carbon monoxide reduction
proceed intermolecularly via
protonation reaction between two species of
a
mutual protonation/deꢀ
. This would
8
While carbon monoxide reacts readily with trialkyl boranes to
give carbonꢀcarbon coupled products,20 CO is not reduced by
[B]ꢀH boranes. Chart 2 shows two striking examples: diborane
reacts with carbon monoxide (at elevated pressure and
temperature) only to form borane carbonyl [H3BꢀCO (16)]. This
had been described by Schlesinger et al as early as 1937.21,22
Borane carbonyl (16) is a lowꢀboiling liquid (b.p. ꢀ64 °C), it
rapidly looses CO under specific conditions. We recently
reported the related formation of the carbonyl of Piers’ borane
[(C6F5)2BH·CO (17)], which was even characterized by an Xꢀ
ray crystal structure analysis.23
generate the not directly observed salt 15 which would be an
alternative candidate for the heterolytic splitting of the
dihydrogen to eventually form the observed phosphonium/
hydridoborate zwitterion 10 (see Scheme 3). Alternatively a
further proton transfer might then generate the geminal FLP 15’
which, according to the DFT analysis, would react with
dihydrogen to give compound 10
.
H
H
H
H
H
H
(C6F5)2B
Mes2P
(C6F5)2B
(C6F5)2B
H
PMes2
H
Mes2P
H
H2
8
H3C
(C6F5)2B
CH3
H
H
H
(C6F5)2B
PMes2
H
Mes2P
B(C6F5)2
H
Mes2P
H
H
Chart 2. Formation of borane carbonyls
H
15
8
10
H+
- 8
This situation was dramatically changed by applying FLP
chemistry. The thermodynamic restriction imposed on the
borane CO reduction is lifted at a variety of vicinal P/B FLP
templates. As a typical example, compound 18 reacts with CO
and HB(C6F5)2 to give the product 19, which is a formylborane
stabilized at the P/B template (see Chart 3).24 This and a variety
of related examples have been used to initiate a genuine
formylborane derived chemistry.23 We have assumed that the
borane CO reduction may take place by means of the 1,2ꢀP/B
H2
H3C
H
(C6F5)2B
PMes2
15'
Scheme 3. A possible pathway of the formation of compound 10
Catalytic hydrogenation
The new directly prepared geminal phosphonium/hydridoborate
zwitterion, which formally represents the dihydrogen activation
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ARTICLE
DOI: 10.1039J/oCu5OrnB0a0l6N3a4Ame
FLP addition to the borane carbonyl to generate the reactive B(C6F5)3 substituent attached at carbon atom C3 and the
intermediate 20, followed by internal borohydride reduction of remaining P(H)Mes2 group at C1 (see Figure 9).
the activated CO unit.24
[B]
B(C6F5)2
CO/HB(C6F5)2
O
B(C6F5)2
[P]
PMes2
H
18
19
[B]
O
H
B(C6F5)2
H
[P]
20
(C6F5)2B
[B]: B(C 6F5)2
[P]: PMes2
CO
Chart 3. CO reaction at a FLP template
In the course of this study we have now found a closely related
example of a CO reduction to the formylborane stage. It utilizes
the reducing property of the HB(C6F5)2 addition product 14 to
the zwitterionic boratacyclopropane 8. This was first observed
Figure 9. Molecular structure of compound 22 (thermal ellipsoids are shown with
30% probability). Selected bond lengths (Å) and angles (deg): C1-P1 1.804(4), C1-
C2 1.554(5), C1-C3 1.556(5), C2-B1 1.581(6), B1-O1 1.343(5), C3-O1 1.473(4), B2-
C3 1.671(6), B1-O1-C3 112.8(3), P1-C1-C2-B1 -108.6(3), P1-C1-C3-B2 -138.7(3).
when we treated the Hꢀbridged BBP system 14 (see Scheme 4)
with benzaldehyde. This gave the reduction product 21 of benzꢀ
aldehyde and reformed the phosphonium/boratacycloꢀpropane
starting material
Information). This indicated that the compound 14 might serve
as a source of Piers’ borane under specific conditions.
8 (see Scheme 4, for details see the Supporting
1
In solution, compound 22 shows the H/13C resonances of the
incorporated formyl moiety at δ 6.09 (3JPH = 30.3 Hz) / δ 84.2.
The 1H NMR signals of the remaining fiveꢀmembered
heterocyclic framework occur at δ 3.90 (CH), and δ 1.47 / 1.21
(CH2). Compound 22 exhibits two 11B NMR signals, one of the
endocyclic slightly Lewis acidic borane (δ 42.7) and one of the
pending B(C6F5)3 borate unit (δ ꢀ12.9) and compound 22 shows
the [P]H resonances at δ ꢀ5.6 (31P) and δ 6.97 (1H) (1JPH = 457
Hz), respectively.
H
(C6F5)2BO
(C6F5)2
B
(C6F5)2B
Mes2P
B(C6F5)2
+
PhCHO
+
Mes2P
H
H
14
21
8
Scheme 4. Benzaldehyde reaction by compound 14
(C6F5)2
This encouraged us to treat compound 14 with carbon
monoxide. The reaction was carried out in CH2Cl2 at 2.5 bar
CO pressure overnight at 80 °C. Workup of the reaction
mixture involving column chromatography gave the product 22
in 60% yield (see Scheme 5). Compound 22 was characterized
by C,H elemental analysis, by spectroscopy and by Xꢀray
diffraction.
H
B
(a)
(b)
(C6F5)2B
Mes2P
B(C6F5)2
+
HB(C6F5)2
CO
Mes2P
80°C
H
8
14
H
H
O
O
(C6F5)2B
B(C6F5)2
C
+
8
H
(C6F5)2B
Mes2P
(D)
H
H
H
(D)
H
23
H
17
O
(C6F5)2B
Mes2P
B(C6F5)2
(C6F5)3B
B
C6F5
CO
H
H
80°C
O
O
Mes2P
(C6F5)2B
B(C6F5)2
(C6F5)3B
H
B
C6F5
H
14
H
22
H
Scheme 5. Reaction of carbon monoxide by compound 14
Mes2P
Mes2P
H
H
24
22
The Xꢀray crystal structure analysis has revealed that carbon
monoxide was reduced to the formylborate stage.25 The newly
formed formyl moiety is found Cꢀattached at carbon atom C1 of
the framework and Oꢀattached at the ring boron atom B1. In
this situation the “formyl” C3ꢀO1 bond is long at 1.473(4) Å.
Boron atom B1 has only a single C6F5 substituent attached, but
the adjacent O1ꢀB1 bond is rather short [1.343(5) Å] indicating
some oxygenꢀboron π interaction. The framework has the
Scheme 6. A possible pathway of the formation of the CO reduction product 22
The borohydride reduction of CO was additionally confirmed
by an experiment using the deuterium labelled DB(C6F5)2
reagent. Its reaction with the phosphonium/boratacyclopropane
system
8 gave the product 14-D (see Scheme 5). It was
2
characterized by H NMR spectroscopy and showed the broad
[B]2H resonance at δ 2.1 (the corresponding 1H/2H NMR
spectra are depicted in the Supporting Information section).
6 | J. Name., 2012, 00, 1-3
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DOI: 10.1039/C5OB00634A
Treatment of 14-D with CO under the usual condition then gave overnight to give some deep yellow crystalline material. The
the formylborane product 22-D (isolated in 53% yield after solid was collected and dried in vacuo to give compound (340
suitable
selective deuterium incorporation at the formyl group (i.e. ꢀCDꢀ for the Xꢀray crystal structure analysis were obtained by storing
O[B]) shown by the broad 2H NMR at δ 6.3.
a saturated toluene solution of compound at r.t. for several
5
1
column chromatography). The H/2H NMR spectra revealed mg, 0.53 mmol, 76%). Single crystals of compound
5
5
We assume a reaction scheme of the formation of the CO days. Anal. Calcd. for C32H24BF10P: C, 60.03; H, 3.78. Found:
reduction product that is related to the formation of the C, 59.96; H, 3.70.
FLP/formylborane product 19 (see Chart 3 and Scheme 6). It
Synthesis of compound 6.
may be that reversibility of the formation of 14 from
8 and
HB(C6F5)2 under the harsh reaction conditions allows for the in Triflic acid (75 mg, 0.50 mmol) was added to a toluene solution
situ generation of the borane carbonyl (C6F5)2B(H)·CO (17), a (10 mL) of compound (320 mg, 0.50 mmol). The reaction
compound that we had previously isolated and characterized by mixture was stirred at r.t. for 30 min to give a light yellow
Xꢀray diffraction.23 This could then add to
and have the H[B]ꢀ solution. Subsequently all volatiles were removed in vacuo and
C≡O unit reduced via possible intermediates 23 24 in a similar the residue was dissolved in CH2Cl2 (2 mL). Then pentane (10
fashion as obtained for the FLPꢀformylborane formation shown mL) was added to form a suspension. Compound (260 mg,
0.33 mmol, 66%) was obtained as white powder after filtration
of the suspension and drying of the residue in vacuo overnight.
Single crystals of compound suitable for the Xꢀray crystal
structure analysis were obtained by slow diffusion of pentane to
a solution of compound in CH2Cl2 at ꢀ30 °C. Anal. Calcd. for
5
8
/
6
in Chart 3.
6
Conclusions
Our new reaction has made a novel type of active geminal FLP
derivatives readily available that cannot be made by the
conventional hydroboration route because of its wrong
regioselectivity. Our study utilizes the formation of the
phosphonium substituted boratacyclopropane moiety by means
of a ringꢀclosing rearrangement reaction of the respective
alkenylhydridoborate isomer in conjunction with its subsequent
hydrogenolytic ring opening reaction. There is evidence that the
latter reaction is Brønsted acid induced. Our reaction sequence
6
C33H25BF13O3PS: C, 50.15; H, 3.19. Found: C, 49.92; H, 3.00.
Synthesis of compound 7.
Excess Me2Si(H)Cl (0.2 mL, 1.8 mmol) was slowly added to a
CH2Cl2 (4 mL) solution of compound
Then the reaction mixture was stirred at r.t. for 1 h.
Subsequently all volatiles were removed in vacuo and the
residue was dissolved in CH2Cl2 (2 mL). Then pentane (10 mL)
was added to give a suspension. The solution was removed by
6 (320 mg, 0.40 mmol).
has
consequently
arrived
directly
at
the
FLPꢀ
phosphonium/hydridoborate state 10, which is the formal
hydrogen activation product of the respective geminal P/B FLP
15’. It might actually be that this sensitive P/B Lewis
base/Lewis acid system is involved in the protonolytic ringꢀ
opening reaction under dihydrogen. First tests have shown that
the system 10 is even stable toward small amounts of water
added to its solution in e.g. CH2Cl2 (for details see the
Supporting Information). The new geminal P/B FLPꢀH2 system
made available by means of our new synthesis is an active
catalyst for the metalꢀfree hydrogenation of a variety of
substrates, among them the heterocyclic quinoline system (see
Table 1), although it is less active than e.g. the related vicinal
cannula to give compound
white powder which was dried in vacuo overnight. Anal. Calcd.
for C32H26BF10P: C, 59.84; H, 4.08. Found: C, 59.69; H, 3.96.
7 (200 mg, 0.31 mmol, 78%) as
Synthesis of compound 8.
Heating a solution of compound
7 (200 mg, 0.31 mmol) in
toluene (10 mL) at 110 °C for 30 min gave a deep yellow
solution. Then all volatiles were removed in vacuo and the
residue was dissolved in pentane (5 mL). Subsequently the
solution was stored at ꢀ30 °C overnight to give yellow
crystalline material. The solid was collected and dried in vacuo
overnight to give compound
Single crystals of compound
8
8
(150 mg, 0.23 mmol, 75%).
suitable for the Xꢀray crystal
P/B FLP system
1 or 18 (see above). However, the system 8
provides a template for the borane reduction of carbon
monoxide to the formyl stage. We expect such geminal P/B
FLP related system to further enhance the synthetic and
catalytic potential of intramolecular frustrated Lewis pair
chemistry.
structure analysis were obtained by storing solution of
compound in pentane at ꢀ30 °C for several days. Anal. Calcd.
for C32H26BF10P: C, 59.84; H, 4.08. Found: C, 59.51; H, 4.44.
8
Synthesis of compound 9.
Experimental section
Triflic acid (30 mg, 0.20 mmol) was added to the CH2Cl2 (5 mL)
solution of compound
8 (128 mg, 0.20 mmol). The color of the
Synthesis of compound 5.
yellow solution disappeared immediately. After stirring the
reaction mixture at r.t. for another 10 min, all volatiles were
removed in vacuo and the residue was dissolved in CH2Cl2 (1
mL) and then pentane (5 mL) was added. After storing this
solution at ꢀ30 °C overnight, some colorless solid was obtained.
The solid was collected and dried in vacuo overnight to give
The combination of
dimesitylethynylphosphane (
toluene (2.0 mL) solution of bis(pentafluorophenyl)borane (242
mg, 0.70 mmol) give instantaneously a yellow solution. After
stirring the reaction mixture at r.t. for 30 min, it was stored at r.t.
a
toluene (2.0 mL) solution of
) (206 mg, 0.70 mmol) with a
4
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ARTICLE
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DOI: 10.1039/C5OB00634A
compound
9
(120 mg, 0.15 mmol, 75%). Single crystals of
9
Synthesis of compound 14.
Mixing of compound
suitable for the Xꢀray crystal structure analysis were obtained
by slow diffusion of pentane to a solution of compound in
8
(200 mg, 0.31 mmol) and HB(C6F5)2
9
(107 mg, 0.31 mmol) in CH2Cl2 (10 mL) gave a suspension.
The reaction mixture was stirred at r.t. for another 1h. Then the
solid was collected and dried in vacuo overnight to give
compound 14 (200 mg, 0.20 mmol, 65%). Single crystals of
compound 14 suitable for the Xꢀray single crystal structure
analysis were obtained by storing a saturated CH2Cl2 solution
of compound 14 at r.t. for several days. Anal. Calcd. for
C44H27B2F20P: C, 53.48; H, 2.75. Found: C, 53.83; H, 2.38.
CH2Cl2 at ꢀ30 °C. Anal. Calcd. for C33H27BF13O3PS: C, 50.02;
H, 3.43. Found: C, 49.91; H, 3.43.
Synthesis of compound 10.
Excess Me2Si(H)Cl (0.1 mL, 0.90 mmol) was slowly added to a
CH2Cl2 (2 mL) solution of compound
9 (125 mg, 0.16 mmol).
Then the reaction mixture was stirred at 90 °C for 0.5 h.
Subsequently all volatiles were removed in vacuo and the
residue was dissolved in CH2Cl2 (1 mL). Then pentane (5 mL) Synthesis of compound 22.
was added. The obtained solution was stored at ꢀ30 °C
Compound 13 (300 mg, 0.30 mmol) was dissolved in CH2Cl2
overnight to give crystalline material. The solid was collected
and dried in vacuo overnight to give compound 10 (91 mg, 0.14
mmol, 88%).
(10 mL). Then the solution was degassed and purged with CO
gas (2.5 bar). After heating the reaction mixture at 80 °C
overnight, all volatiles were removed in vacuo at rt and the
remaining residue was purified by column chromatography
[silica gel, CH2Cl2] to give compound 22 (183 mg, 0.18 mmol,
60%). Single crystals of compound 22 suitable for the Xꢀray
single crystal structure analysis were obtained by storing a
saturated CH2Cl2 solution of compound 22 at r.t. for several
days. Anal. Calcd. for C45H27B2F20OP (1016.26 g/mol): C,
53.18; H, 2.68. Found: C, 53.06; H, 2.77.
Synthesis of compound 11.
KH (16 mg, 0.40 mmol) was added to the THF solution (4 mL)
of compound
8 (200 mg, 0.31 mmol). The reaction mixture was
stirred at r.t. for 0.5 h (some gas bubbles were observed during
the reaction). Then all volatiles were removed in vacuo and the
residue was dissolved in CH2Cl2/pentane (1 mL/5 mL).
Subsequently the solution was stored at ꢀ30 °C overnight to
give a colorless crystalline material. The solid was collected Compound 10 catalyzed hydrogenation reaction.
and dried in vacuo for 1 h to give compound 11 with
General procedure: compound 10 (0.1 mmol) and the
coordinating ¼ THF (140 mg, ≈ 0.2 mmol, ≈ 50 %).
respective substrate were mixed in CD2Cl2 (2 mL) using a
special ampule4a (10 mL) with magnetic stirrer. Subsequently
the ampule was put into an autoclave and H2 gas (50 bar) was
applied. Then the reaction mixture was stirred at 90 °C for 36 h.
Synthesis of compound 12.
Benzyl bromide (50 mg, 0.29 mmol) was added to a THF
solution (5 mL) of compound 11 (200 mg, ≈ 0.28 mmol) to give
a suspension immediately. The suspension was stirred at r.t. for
10 min. Then the solid was filtrated off. All volatiles of the
solution were removed in vacuo and the resulting residue was
dissolved in CH2Cl2/pentane (1 mL/5 mL). Subsequently the
solution was stored at ꢀ30 °C overnight to give a colorless
crystalline material. The solid were collected and dried in vacuo
overnight to give compound 12 (160 mg, 0.23 mmol, 82%).
Single crystals of compound 12 suitable for the Xꢀray crystal
structure analysis were obtained by slow diffusion of pentane to
a CH2Cl2 solution of compound 12 at ꢀ30 °C. Anal. Calcd. for
C39H32BF10P: C, 63.95; H, 4.40. Found: C, 63.45; H, 4.50.
1
The obtained reaction mixture was directly characterized by H
NMR spectroscopy. The conversion was estimated by
integration of suitable 1H NMR signals.
Acknowledgements
Financial support from the European Research Council (ERC)
is gratefully acknowledged.
Notes and references
a OrganischꢀChemisches Institut der Universität Münster Correnstraβe 40,
48149 Münster, Germany. Email:erker@uniꢀmuenster.de
b
Mulliken Center for Theoretical Chemistry, Institut für Physikalische
Synthesis of compound 13.
und Theoretische Chemie, Universität Bonn, Beringstraβe 4, 53115 Bonn,
Germany.
Triflic acid (30 mg, 0.20 mmol) was added to the CH2Cl2 (4 mL)
solution of compound 12 (150 mg, 0.20 mmol) to give a
colorless solution. The solution was stirred at r.t. for 10 min.
Then all volatiles were removed in vacuo and pentane (5 mL)
was added to the residue to give a suspension. Subsequently the
formed solid was collected and dried overnight in vacuo to give
compound 13 (130 mg, 0.15 mmol, 75%). Crystals of
compound 13 suitable for the Xꢀray single crystal structure
analysis were obtained by slow diffusion of pentane to a
CH2Cl2 solution of compound 13 at ꢀ30 °C. Anal. Calcd. for
C40H33BF13PS: C, 54.44; H, 3.77. Found: C, 54.69; H, 3.56.
†
Electronic Supplementary Information (ESI) available: Details of the
experiments, characterization of all compounds and crystal structure data
as CIF files (CCDC numbers are 1034881 to 1034888 and 1045775). See
DOI: 10.1039/b000000x/
1
(a) D. W. Stephan and G. Erker, Angew. Chem. Int. Ed., 2010, 49, 46; (b)
Frustrated Lewis Pairs I, Uncovering and Understanding in Topics in
Current Chemistry, ed. G. Erker and D. W. Stephan, Springer, Heidelberg,
New York, Dordrecht, London, 2013, vol. 332; (c) Frustrated Lewis Pairs
II, Expanding the Scope in Topics in Current Chemistry, ed. G. Erker and
D. W. Stephan, Springer, Heidelberg, New York, Dordrecht, London,
2013, vol. 334.
8 | J. Name., 2012, 00, 1-3
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DOI: 10.1039/C5OB00634A
2
3
Further reviews: (a) D. W. Stephan and G. Erker, Chem. Sci., 2014, 5,
2625; (b) G. Erker, Pure Appl. Chem., 2012, 84, 2203.
16
(a) J. Tao, J. P. Perdew, V. N. Staroverov and G. E. Scuseria, Phys. Rev.
Lett., 2003, 91, 146401; (b) S. Grimme, J. Antony, S. Ehrlich and H. Krieg,
J. Chem. Phys., 2010, 132, 154104; (c) S. Grimme, S. Ehrlich and L.
Goerigk, J. Comput. Chem., 2011, 32, 1456; (d) F. Weigend and R.
Ahlrichs, Phys. Chem. Chem. Phys., 2005, 7, 3297; (e) S. Grimme, J.
Chem. Phys., 2006, 124, 034108; (f) F. Eckert and A. Klamt, AIChE J.,
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and S. Grimme, J. Comput. Chem., 2013, 34, 1672; (i) S. Grimme, Chem.
Eur. J., 2012, 18, 9955.
(a) G. C. Welch, R. R. S. Juan, J. D. Masuda and D. W. Stephan,
Science, 2006, 314, 1124; (b) G. C. Welch and D. W. Stephan, J. Am.
Chem. Soc., 2007, 129, 1880.
4
(a) P. Spies, S. Schwendemann, S. Lange, G. Kehr, R. Fröhlich and G.
Erker, Angew. Chem. Int. Ed., 2008, 47, 7543; (b) J. S. Reddy, B.-H. Xu,
T. Mahdi, R. Fröhlich, G. Kehr, D. W. Stephan and G. Erker,
Organometallics, 2012, 31, 5638; (c) T. Mahdi, Z. M. Heiden, S. Grimme
and D. W. Stephan, J. Am. Chem. Soc., 2012, 134, 4088-4091.
(a) D. W. Stephan and G. Erker, Top. Curr. Chem., 2013, 332, 85; (b) V.
Sumerin, K. Chernichenko, F. Schulz, M. Leskelä, B. Rieger and T. Repo,
Top. Curr. Chem., 2013, 332, 111.
17
18
19
S. Schwendemann, T. A. Tumay, K. V. Axenov, I. Peuser, G. Kehr, R.
Fröhlich and G. Erker, Organometallics, 2010, 29, 1067.
H. Wang, R. Fröhlich, G. Kehr and G. Erker, Chem. Commun., 2008,
5966.
5
6
(a) A. E. Ashley, A. L. Thompson and D. O’Hare, Angew. Chem., Int. Ed.,
2009, 48, 9839; (b) T. Mahdi and D. W. Stephan, J. Am. Chem. Soc.,
2014, 136, 15809; (c) D. J. Scott, M. J. Fuchter and A. E. Ashley, J. Am.
Chem. Soc., 2014, 136, 15813.
See for a comparison: S. J. Geier, P. A. Chase and D. W. Stephan,
Chem. Commun., 2010, 46, 4884.
20
21
22
H. C. Brown, Acc. Chem. Res., 1969, 2, 65.
A. B. Burg and H. I. Schlesinger, J. Am. Chem. Soc., 1937, 59, 780.
a) S. H. Bauer, J. Am. Chem. Soc., 1937, 59, 1804; (b) W. Gordy, H.
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Chem. Phys., 1950, 18, 1101; (d) G. W. Bethke and M. K. Wilson, J.
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7
8
P. Spies, G. Erker, G. Kehr, K. Bergander, R. Fröhlich, S. Grimme and D.
W. Stephan, Chem. Commun., 2007, 5072.
See also: (a) M. Sajid, G. Kehr, T. Wiegand, H. Eckert, C. Schwickert, R.
Pöttgen, A. J. P. Cardenas, T. H. Warren, R. Fröhlich, C. G. Daniliuc and
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Mömming, G. Kehr, R. Fröhlich and G. Erker, Chem. Eur. J., 2010, 16,
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Sci., 2011, 2, 1842; (d) X. Wang, G. Kehr, C. G. Daniliuc and G. Erker, J.
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23
24
25
M. Sajid, G. Kehr, C. G. Daniliuc and G. Erker, Angew. Chem. Int. Ed.,
2014, 53, 1118.
M. Sajid, L.-M. Elmer, C. Rosorius, C. G. Daniliuc, S. Grimme, G. Kehr
and G. Erker, Angew. Chem. Int. Ed., 2013, 52, 2243.
(a) A. Berkefeld, W. E. Piers, M. Parvez, L. Castro, L. Maron and O.
Eisenstein, J. Am. Chem. Soc., 2012, 134, 10843; (b) R. Dobrovetsky, D.
W. Stephan, J. Am. Chem. Soc., 2013, 135, 4974.
9
(a) C. Rosorius, G. Kehr, R. Fröhlich, S. Grimme and G. Erker,
Organometallics, 2011, 30, 4211; (b) A. Stute, G. Kehr, R. Fröhlich and G.
Erker, Chem. Commun., 2011, 47, 4288.
10
See for a comparison: (a) F. Bertini, V. Lyaskovskyy, B. J. J. Timmer, F. J.
J. de Kanter, M. Lutz, A. W. Ehlers, J. C. Slootweg and K. Lammertsma,
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C. Slootweg, K. Lammertsma and W. Uhl, Dalton Trans., 2012, 41, 9033;
(e) F. Bertini, F. Hoffmann, C. Appelt, W. Uhl, A. W. Ehlers, J. C.
Slootweg and K. Lammertsma, Organometallics, 2013, 32, 6764.
(a) D. J. Parks, R. E. von H. Spence and W. E. Piers, Angew. Chem. Int.
Ed. Engl., 1995, 34, 809; (b) W. E. Piers and T. Chivers, Chem. Soc.
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TOC
11
12
13
B. J. Dunne and A. G. Orpen, Acta Cryst., 1991, C47, 345.
See for a comparison: S. Frömel, R. Fröhlich, C. G. Daniliuc, G. Kehr and
G. Erker, Eur. J. Inorg. Chem., 2012, 2012, 3774.
14
15
See for a comparison: (a) X. Zhao, A. J. Lough and D. W. Stephan,
Chem. Eur. J., 2011, 17, 6731; See also: (b) J. Ugolotti, G. Kehr, R.
Fröhlich, S. Grimme and G. Erker, Chem. Commun., 2009, 6572.
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