Bifunctional Behavior of Unsaturated Intramolecular Phosphane-Borane Frustrated Lewis Pairs Derived from Uncatalyzed 1,4-Hydrophosphination of a Dienylborane

Article abstract of DOI:10.1002/chem.201501402

Three unsaturated C₄-bridged phospane/borane frustrated Lewis pairs (P/B FLPs) are prepared by uncatalyzed hydrophosphination of a dienylborane. The systems are bifunctional. Consequently, two examples undergo clean hydroboration reactions with

Full text of DOI:10.1002/chem.201501402

DOI: 10.1002/chem.201501402
Full Paper
&
Frustrated Lewis Pairs
Bifunctional Behavior of Unsaturated Intramolecular Phosphane–
Borane Frustrated Lewis Pairs Derived from Uncatalyzed 1,4-
Hydrophosphination of a Dienylborane
Guo-Qiang Chen, Gerald Kehr, Constantin G. Daniliuc, Birgit Wibbeling, and Gerhard Erker*[a]
Dedicated to Professor Uwe Rosenthal on the occasion of his 65th birthday.
Abstract: Three unsaturated C4-bridged phospane/borane
frustrated Lewis pairs (P/B FLPs) are prepared by uncatalyzed
hydrophosphination of a dienylborane. The systems are bi-
functional. Consequently, two examples undergo clean hy-
droboration reactions with HB(C6F5)2 to yield B/B/P systems.
The 1,4-P/B system (C6F5)2BÀCH2CH=CMeCH2PMes2 reacts
with benzaldehyde initially by allylborane addition, followed
by internal P/B FLP addition to the pendant C=C double
bond, to yield a bicyclic product. The corresponding reaction
of (C6F5)2BÀCH2CH=CMeCH2PtBu2 stops at the allylborane/
benzaldehyde addition product. The related system
(C6F5)2BÀCH2CH=CMeCH2PPh2 shows a similar bifunctional
reaction pattern, whereby allylborane addition to benzalde-
hyde is combined with P/B addition to a second aldehyde
equivalent to form the eight-membered heterocyclic 1:2 ad-
dition product.
Introduction
The strongly electron-withdrawing (C6F5)2B substituent renders
a-CH units attached to it quite CÀH acidic. Recent DFT studies
have shown that, for example, (C6F5)2BÀCH3 has a pKa value of
approximately 18, that is, the CÀH acidity of methyl bis(penta-
fluorophenyl)borane is similarly to that of cyclopentadiene.[1]
Scheme 1. Real and hypothetical borata-ethene systems.
À
The thermodynamic stabilization of the (C6F5)2BÀCH2 anion is
approximately 55–60 kcalmolÀ1 relative to the cyclopentyl
anion. This pronounced stabilization results in distinct (C6F5)2B-
substituted carbanion chemical properties. There is good evi-
dence that delocalization of the negative charge of a-boryl car-
banions with the adjacent Lewis acidic borane moiety is gener-
ally quite pronounced. a-Boryl carbanions, therefore, exhibit
a marked borata-alkene character.[2–5] This has been shown for
a variety of typical examples (Scheme 1), for example, by X-ray
diffraction, and this effect is also to be assumed for the
B(C6F5)2-substituted analogues.
vented this general problem. We generated the geminal phos-
phane/borane frustrated Lewis pair (P/B FLP)[6] 2 by in situ hy-
droboration of the diphenylphosphino-substituted conjugated
enyne 1. However, we never observed this product directly,
since it underwent a very rapid subsequent intramolecular ad-
dition of the free phosphane Lewis base to the d-position of
its adjacent dienylborane moiety, giving rise to the formation
of the stable zwitterionic cyclic borata-diene 3 (Scheme 2).[7]
a-Deprotonation of alkylboranes meets with the difficulty of
the (often strongly) competing addition reaction of the (nucle-
ophilic) base to the electrophilic boron Lewis acid. Therefore,
the few cases of successful borata-alkene formation by the
borane deprotonation route required bulky substituents and
bulky base.[2–5] We recently developed a different synthetic
route to stable borata-alkene systems that successfully circum-
Scheme 2. FLP pathway to stable zwitterionic borata-ethenes.
[a] Dr. G.-Q. Chen, Dr. G. Kehr, Dr. C. G. Daniliuc,+ B. Wibbeling,+ Prof. G. Erker
Organisch-Chemisches Institut, Universität Münster
Corrensstrasse 40, 48149 Münster (Germany).
E-mail: erker@uni-muenster.de
[+] X-ray crystal structure analysis.
This posed the question whether a related intermolecular
phosphane attack at a (C6F5)2B-dienyl borane would open an
entry into the respective borata-diene chemistry in general.
This turned out to be the case. Treatment of the boryldiene 4
with a small series of secondary phosphanes HPR2 (R=Ph, me-
sityl, tBu) resulted in an efficient non-catalyzed 1,4-hydrophos-
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201501402.
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phination of the diene unit to yield the products 6a–c. This is
a remarkable result as hydrophosphination reactions, much
like hydroamination reactions, are often catalyzed by metal
complexes[8,9] or sometimes induced by radicals[10] (although
not exclusively[11]). We assumed that in our case nucleophilic
phosphane addition to the dienylborane substrate generated
the borata-diene intermediates 5a–c, taking advantage of the
relatively high thermodynamic stabilization of such systems.
Subsequent intramolecular proton transfer would then
lead to the observed 1,4-hydrophosphination products
6a–c (Scheme 3). The compounds 6 were characterized by X-
to the aldehyde generating the undetected reactive intermedi-
ate 7b (the analogous derivative 7c was detected, however;
for details, see below and the Supporting Information), which
then apparently underwent a typical frustrated Lewis pair (FLP)
reaction, namely the internal 1,2-borane Lewis acid/phosphane
Lewis base addition to the pendant olefinic functionality[13] in
7b to directly yield the final bicyclic product 8b.
Compound 8b was characterized by X-ray diffraction
(Figure 1). Suitable single crystals were grown at À358C from
dichloromethane/pentane by the diffusion method. The X-ray
crystal structure analysis of 8b showed the presence of the
Scheme 3. Boryldiene hydrophosphination reactions.
ray diffraction. They all featured an internal P/B interaction. In
this study, we tested these readily available dormant P/B FLP
systems in a variety of reactions and have found that they
show unique bifunctional reaction behaviors; as allylboranes
and as P/B frustrated Lewis pairs. This will be illustrated herein
by some typical examples.
Figure 1. A view of the molecular structure of compound 8b (thermal ellip-
soids are set at 30% probability). Selected bond lengths (Š) and angles (8):
C1ÀC5 1.576(4), B4ÀO3 1.468(4), B4ÀC5 1.656(4), P7ÀC8 1.812(3), P7ÀC6
1.817(3); O3-B4-C5 102.2(2), C6-P7-C8 90.5(1), C2-C1-C5-C6 126.6(2), B4-C5-
C1-C8 À110.8(2).
Results and Discussion
zwitterionic heterobicyclo[3.3.0]octane framework that has in-
corporated both the borate and phosphonium moieties at
their specific positions. The ring fusion is cis, as is commonly
observed for such frameworks derived from annulated five-
membered rings. The phenyl substituent is endo-oriented at
the ring carbon atom C2.
Reactions of the P/B systems with benzaldehyde
We treated the six-membered P/B system 6b (R=mesityl) in
dichloromethane with benzaldehyde (1 molar equiv). The reac-
tion was slow; it took 10 days at room temperature to go to
completion. Workup then gave the product 8b as a colorless
crystalline solid in 81% yield (Scheme 4). We assume that this
specific product was formed by a two-step reaction sequence
that was initiated by a typical allyl borane addition reaction[12]
In solution (CD2Cl2), compound 8b gives rise to typical
borate 11B NMR (d=1.5 ppm) and phosphonium 31P NMR (d=
1
42.9 ppm) features. It also exhibits the H NMR signals of the
phenyl substituent and the 19F NMR signals of the pair of dia-
1
stereotopic C6F5 groups at boron. The H NMR spectrum of 8b
also shows the signals of a pair of diastereotopic mesityl sub-
stituents at phosphorus.
The proposed reaction pathway (Scheme 4) was supported
by the experimentally observed formation of the open allylbo-
ration product 7c from the reaction of the tert-butyl substitut-
ed P/B system 6c with benzaldehyde. The compound was iso-
lated in 82% yield and characterized by its typical spectroscop-
ic features (11B NMR: d=40.7 ppm; 31P NMR: d=15.6 ppm). Ac-
cording to the 11B NMR chemical shift, there is a marked
oxygen-to-boron p-interaction. Consequently, compound 7c
Scheme 4. Reaction of the FLP 6 with benzaldehyde.
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shows the 19F NMR signals of a pair of different C6F5 groups at
boron. These show a coalescence of the pair of o, m, and p flu-
orine signals (DG°(o)=13.1Æ0.3 kcalmolÀ1).[14] The 1H NMR
spectrum of compound 7c shows the signals of a pair of dia-
stereotopic tBu substituents at phosphorus and the typical
ABX pattern of the pendant vinyl group.
dergone allylborane addition. In which order these two reac-
tions had occurred remained unknown, since we detected nei-
ther of the possible intermediates 7a or 9 (Scheme 5). The X-
ray crystal structure showed that product 10 has a zwitterionic
eight-membered heterocyclic framework (Figure 2). The pend-
ant vinyl group is situated at the ring carbon atom C2, which
also bears the single CH3 substituent from the starting material
6a. The boron atom has a pair of alkoxy groups bonded to it.
The phosphorus atom is bonded to two ring carbon atoms.
The diastereoisomer 10 has the pair of phenyl substituents de-
rived from benzaldehyde oriented trans to each other at the
eight-membered heterocyclic framework, whereas the methyl
group at C2 and the phenyl group at C3 are in a cis orienta-
tion.
The P/B compound 6a (R=Ph) also reacts rapidly with ben-
zaldehyde. Regardless of the starting ratio of these two re-
agents we have always observed the formation of the 1:2 addi-
tion product 10 (Scheme 5). The reaction mixture containing
one molar equivalent of 6a and two molar equivalents of ben-
The 19F NMR spectra of compound 10 indicates the presence
of a pair of diastereotopic C6F5 substituents at boron (11B NMR:
d=3.9 ppm; 31P NMR: d=15.0 ppm). The CH2 protons adjacent
to phosphorus are diastereotopic and we observed the
1H NMR signal for CH3 at d=1.30 ppm. The pendant vinyl sub-
1
stituent gave rise to an H NMR ABX pattern of signals at d=
4.91 ppm (=CH) and 4.70/4.57 (=CH2) with H,H-coupling con-
3
3
stants of Jcis =10.9 Hz and Jtrans =17.4 Hz.
We previously reported that some saturated vicinal P/B FLPs
react cleanly with nitric oxide (NO) cooperatively to form per-
sistent FLP NO nitroxide radicals.[16] The formation of 12 from
11 is a typical example (Scheme 6). In contrast, many tert-phos-
phanes themselves react differently with NO, namely by oxida-
tion to give the respective phosphanoxides and N2O.[17] We
Scheme 5. Formation of the 2:1 benzaldehyde addition product 10.
zaldehyde was stirred for 7 days at room temperature in di-
chloromethane. Workup of the reaction mixture eventually
gave the product 10 as a colorless solid in 86% yield.
The X-ray crystal structure analysis (Figure 2) revealed that
the borane Lewis acid and the phosphane Lewis base of the
FLP 6a underwent 1,2-addition to the carbonyl function of
benzaldehyde[13b,15] and that a second benzaldehyde had un-
Scheme 6. Reaction of FLP or a tert-phosphane with NO.
briefly investigated the reactivity of one example of the new
FLPs 6 with NO and treated the PtBu2-containing example 6c
with NO in pentane solution. In this case, we found no indica-
tion of the unique cooperative P/B addition but isolated the
oxidation product 13c instead in 91% yield (Scheme 7).[18]
Compound 13c gave rise to a single set of 19F NMR signals
of the B(C6F5)2 group with an 11B NMR signal at d=0.8 ppm,
a
31P NMR signal is at d=75.2 ppm and olefinic CH signals at
d=5.87 ppm (1H NMR) and d=135.9 ppm [13C NMR, 3J(P,C)=
6.0 Hz], respectively. Compound 13c was further characterized
by X-ray diffraction (Figure 3). The X-ray structure features
a heterocyclic seven-membered ring with an endocyclic B-O-P
Figure 2. A view of the molecular structure of compound 10 (thermal ellip-
soids are set at 30% probability). Selected bond lengths (Š) and angles (8):
B1ÀO1 1.457(3), B1ÀO2 1.503(3), P1ÀC1 1.820(3), P1ÀC7 1.876(3); O1-B1-O2
110.3(2), C1-P1-C7 111.4(1), B1-O1-C3 115.4(2), B1-O2-C7 113.7(2).
Scheme 7. P-oxidation of the FLP 6c by treatment with NO.
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three signals (o, p, m) of the pendant B(C6F5)2 unit (Dd19F(p,m)
=
11.6 ppm) and a set of two diastereotopic C6F5 substituents of
the tetracoordinate boron inside the ring system.
The hydroboration product 14a was also characterized by X-
ray diffraction (Figure 4). It shows a six-membered heterocyclic
framework with
a
reasonably short BÀP bond [B1ÀP1
2.005(3) Š]. The framework shows a distorted chair-like ring
conformation with the CH3 substituent at C2 and the B(C6F5)2
substituent at C3 oriented trans to each other. The boron atom
B2 is planar-tricoordinate (SB2CCC =359.68).
Figure 3. Molecular structure of compound 13c (thermal ellipsoids are set at
30% probability). Selected bond lengths (Š) and angles (8): B1ÀO1 1.554(2),
B1ÀC4 1.624(2), P1ÀO1 1.537(1), P1ÀC1 1.819(2), C2ÀC3 1.324(3); B1-O1-P1
138.0(1).
unit [B1ÀO1 1.554(2) Š, P1ÀO1 1.537(1) Š; angle B1-O1-P1
138.0(1)8]. The ring system contains a CH2 unit bonded to
boron and another CH2 moiety bonded to phosphorus and the
adjacent C3=C2 carbon–carbon double bond.
Hydroboration of the unsaturated six-membered P/B FLPs 6
The FLPs 6 contain a residual olefinic C=C double bond func-
tionality. Although the tricoordinate C=C bond is quite sterical-
ly shielded, we investigated its potential to serve as a substrate
for hydroboration with the strongly electrophilic HB(C6F5)2 re-
agent (Piers’ borane).[19] We first treated the PPh2 containing
unsaturated FLP 6a with the hydroboration reagent. A suspen-
sion of 6a was stirred overnight in pentane with HB(C6F5)2 at
room temperature. Workup gave the hydroboration product
14a as a colorless solid in 88% yield (Scheme 8). The NMR
spectra showed that a single regioisomer had been formed
that corresponded to the usual anti-Markovnikov orientation
Figure 4. A projection of the molecular structure of the hydroboration prod-
uct 14a (thermal ellipsoids are set at 30% probability). Selected bond
lengths (Š) and angles (8): B1ÀP1 2.005(3), B1ÀC4 1.654(4), B2ÀC3 1.577(6),
P1ÀC1 1.822(3); B1-P1-C1 108.9(2), P1-B1-C4 97.8(2).
We then treated the more bulky PMes2-containing FLP 6b
with Piers’ borane. Again we observed the typical anti-Markov-
nikov [B]ÀH addition reaction (1H NMR of CH3: d=1.05, dd, 3H,
4
3J(H,H)=6.7 Hz, J(H,H)=1.3 Hz). However, the isolated product
15b (76% yield) turned out to result from a secondary rear-
rangement process. The product contains a pair of strongly
Lewis acidic B(C6F5)2 functional groups. Under the conditions
of apparently thermodynamic control, the original BÀP linkage
of the six-membered FLP heterocycle had opened and the
weakly nucleophilic PMes2 group had become associated with
the newly introduced B(C6F5)2 Lewis acid, forming a five-mem-
Scheme 8. Hydroboration of the 1,4-P/B FLP 6a.
bered framework with
a
pendant CH2ÀB(C6F5)2 group
1
of the 1,2 [B]ÀH addition reaction.[19,20] The H NMR signal cor-
(Scheme 9). The alternative isomer 14b was not detected.
In solution, compound 15b gives rise to a pair of 11B NMR
signals at d=73.8 ppm and d=À0.9 ppm. The exocyclic CH2À
[B] unit incorporates diastereotopic hydrogen atoms. The CH2À
B(C6F5)2 group gives rise to a single set of 19F NMR signals (o, p,
m). The endocyclic B(C6F5)2 moiety gives rise to signals of
a pair of diastereotopic C6F5 groups that show hindered rota-
tion around the BÀC vector, which adds up to a total of ten
responding to the methyl substituent at the central framework
was found at d=1.09 ppm (3J(H,H)=6.2 Hz, 4J(P,H)=3.1 Hz).
The product also gave rise to a 31P NMR resonance at d=
0.0 ppm and a pair of 11B NMR signals, one in the typical range
of tetracoordinate boron (d=À13.1) and the other in the
planar-tricoordinate RB(C6F5)2 range (d=64.1 ppm). The system
also showed two sets of 19F NMR signals, a simple one with
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containing hydrophosphination products 6. Some of the exam-
ples of 6 underwent clean anti-Markovnikov hydroboration re-
actions with Piers’s borane HB(C6F5)2. However, neither the FLP
6b (with PMes2 phosphanyl functionality), nor the hydrobora-
tion product 15b that was derived from it were active hydro-
gen-splitting reagents under the normal mild conditions. Nev-
ertheless, compound 6b served as a slow metal-free enamine
hydrogenation catalyst. Given that we had not observed the
formation of a specific hydrogen splitting product upon expo-
sure of 6b to dihydrogen, it is questionable whether 6b·H2
was actually responsible for the observed hydrogenation activi-
ty or the open isomer of the system 6b only served as
a borane Lewis acid in conjunction with the enamine Lewis
base to generate an intermolecular FLP reactivity.[22]
Scheme 9. Formation of the rearranged hydroboration product 15b.
19F NMR signals for this unit. The 31P NMR resonance of com-
pound 15b was located at d=20.0 ppm.
Compound 15b was also characterized by X-ray diffraction.
The structure (Figure 5) features a central five-membered het-
erocyclic ring system that contains a marked BÀP interaction
[B1ÀP1 2.085(4) Š] between an endocyclic tetracoordinate
phosphorus and a tetracoordinate boron atom. A CH2ÀB(C6F5)2
moiety is attached at C3 and a trans-CH3 substituent at C2.
The reaction of the systems 6 with benzaldehyde furnished
an interesting result. Compounds 6 are bifunctional in that
they can be regarded as P/B frustrated Lewis pairs and they
contain an allyl borane moiety. We observed the typical reac-
tivities of both functionalities. System 6c reacted with benzal-
dehyde simply as an allyl borane. The related system 6b also
seemed to undergo an initial allyl borane addition to the reac-
tive aldehyde, but this was then followed by a very rapid typi-
cal FLP reaction, namely the 1,2-borane/phosphane Lewis pair
addition to the internal C=C double bond to form the ob-
served bicyclic reaction product 8. The PPh2-containing system
6a showed similar reaction behavior, only that in this case the
allyl borane/aldehyde addition reaction was coupled with
a 1,2-FLP addition to an additional equivalent of the reactive
aldehyde and not to an internal carbon–carbon double bond.
Our study has shown that typical FLP chemistry can success-
fully be combined with other functional group chemistry, in
this case, that of the allyl borane moiety that is part of the
new FLP systems. This indicates to us that there might be in-
teresting possibilities to combine conventional functional
group chemistry in integrated systems with FLP reactivity. We
will see if this will lead us to novel utilization of specifically de-
signed FLP systems.
Figure 5. Molecular structure of compound 15b (thermal ellipsoids are set
at 15% probability). Selected bond lengths (Š) and angles (8): B1ÀP1
2.085(4), B1ÀC3 1.656(5), B2ÀC5 1.548(5), P1ÀC1 1837(3); B1-P1-C1 94.7(1),
P1-B1-C3 93.1(2).
Experimental Section
General information, detailed reaction procedures, analytical de-
tails, and structural data of the new compounds are given in the
Supporting Information.
We briefly tested the reaction of the FLPs 6b, 14a, 15b with
dihydrogen, but did not observe splitting of H2 under the
usual conditions.[21] However, compound 6b catalyzed the
slow hydrogenation of the enamine pyrrolidinocyclohexene to
the corresponding tert-amine product. With 10 mol% of the
FLP catalyst 6b, we attained approximately 70% conversion at
1.5 bar H2 at 508C during 36 h.
Syntheses
7c: Compound 6c (111.6 mg, 0.2 mmol) and benzaldehyde
(21.2 mg, 0.2 mmol) were dissolved in CH2Cl2 (1 mL). The reaction
mixture was stirred at room temperature for 30 min. Subsequently
all volatiles were removed under reduced pressure to give com-
pound 7c (108.9 mg, 82%) as a viscous liquid.
Conclusion
8b: Compound 6b (102.3 mg, 0.15 mmol) and benzaldehyde
(15.9 mg, 0.15 mmol) were dissolved in CH2Cl2 (1 mL). The reaction
mixture was stirred at room temperature for 10 d. All volatiles
were removed under reduced pressure and the residue was
washed with cold pentane (1 mL). After drying under vacuum,
compound 8b (95.2 mg, 81%) was obtained as a colorless solid.
Crystals suitable for X-ray crystal structure analysis were obtained
We previously developed an uncatalyzed metal-free hydro-
phosphination reaction of a conjugated alkenyl borane to give
a small series of 3-methyl substituted 1-boryl-4-phosphanyl-2-
butene products 6.[1] The reaction took advantage of the sub-
stantial stabilization of the a-boryl carbanion species 5. We
have now investigated some chemical reactivity of the P/B
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(31.8 mg, 0.30 mmol) were dissolved in CH2Cl2 (1 mL). The reaction
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13c: Compound 6c (111.6 mg, 0.2 mmol) was suspended in pen-
tane (4 mL) and the resulting suspension was degassed. Then NO
gas (1 bar) was introduced for 10 min and the reaction mixture
was stirred overnight at room temperature. All volatiles were re-
moved under reduced pressure and the residue was washed with
pentane (3”1 mL). After drying under vacuum, compound 13c
(104.3 mg, 91%) was obtained as a colorless solid. Crystals suitable
for X-ray crystal structure analysis were obtained by slow diffusion
of pentane into a solution of compound 13c in dichloromethane
at À358C. M.p.: 1898C; elemental analysis calcd for C25H26BF10OP: C
52.29, H 4.56; found: C 52.33, H 4.28.
14a: Compound 6a (89.7 mg, 0.15 mmol) and HB(C6F5)2 (51.9 mg,
0.15 mmol) were suspended in pentane (3 mL) and the reaction
mixture was stirred overnight at room temperature. All volatiles
were removed under reduced pressure and the residue was
washed with pentane (3”1 mL). After drying under vacuum, com-
pound 14a (124.1 mg, 88%) was obtained as a colorless solid.
Crystals suitable for X-ray crystal structure analysis were obtained
by slow diffusion of pentane into a solution of compound 14a in
dichloromethane at À358C. M.p.: 2038C; elemental analysis calcd
for C41H19B2F20P: C 52.16, H 2.03; found: C 52.20, H 1.91.
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15b: Compound 6b (102.3 mg, 0.15 mmol) and HB(C6F5)2 (51.9 mg,
0.15 mmol) were suspended in pentane (3 mL) and the reaction
mixture was stirred overnight at room temperature. The resulting
viscous solution was concentrated to a volume of 1 mL and then
stored at À358C. The colorless precipitated solid and the upper
clear solution were separated carefully by pipette. The colorless
solid was collected and dried under vacuum to give compound
15b (117.3 mg, 76%). Crystals suitable for X-ray crystal structure
analysis were obtained by slow diffusion of pentane into a solution
of compound 15b in dichloromethane at À358C. M.p.: 1398C; ele-
mental analysis calcd for C47H31B2F20P: C 54.90, H 3.04; found: C
54.75, H 3.15.
Acknowledgements
Financial support from the European Research Council is grate-
fully acknowledged.
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Received: April 10, 2015
Published online on July 14, 2015
Chem. Eur. J. 2015, 21, 12449 – 12455
www.chemeurj.org
12455
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim