[2.2] Paracyclophane derived bisphosphines for the activation of hydrogen by FLPs: Application in domino hydrosilylation/hydrogenation of enones

Article abstract of DOI:10.1039/c2dt30374d

The heterolytic splitting of hydrogen by two types of [2.2]paracyclophane derived bisphosphines (1, 2a and 2b) in combination with tris(pentafluorophenyl) borane (3) at room temperature is described. The corresponding frustrated Lewis pairs (FLPs) exhibit

Full text of DOI:10.1039/c2dt30374d

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Frustrated Lewis Pairs
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Published in issue 30, 2012 of Dalton Transactions
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Communication
Hydrogen activation by 2-boryl-N,N-dialkylanilines: a revision of Piers’ ansa-aminoborane
Konstantin Chernichenko, Martin Nieger, Markku Leskelä and Timo Repo
Paper
Frustrated Lewis pair addition to conjugated diynes: Formation of zwitterionic 1,2,3-butatriene
derivatives
Philipp Feldhaus, Birgitta Schirmer, Birgit Wibbeling, Constantin G. Daniliuc, Roland Fröhlich,
Stefan Grimme, Gerald Kehr and Gerhard Erker
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Fixation of carbon dioxide and related small molecules by a bifunctional frustrated pyrazolylborane
Lewis pair
Eileen Theuergarten, Janin Schlösser, Danny Schlüns, Matthias Freytag, Constantin G. Daniliuc,
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PAPER
[2.2]Paracyclophane derived bisphosphines for the activation of hydrogen by
FLPs: application in domino hydrosilylation/hydrogenation of enones†
Lutz Greb,a Pascual Oña-Burgos,b Adam Kubas,c Florian C. Falk,a Frank Breher,b Karin Finkc and
Jan Paradies*a
Received 16th February 2012, Accepted 22nd March 2012
DOI: 10.1039/c2dt30374d
The heterolytic splitting of hydrogen by two types of [2.2]paracyclophane derived bisphosphines (1, 2a
and 2b) in combination with tris(pentafluorophenyl)borane (3) at room temperature is described. The
corresponding frustrated Lewis pairs (FLPs) exhibit different behavior in the activation of hydrogen. This
results from diverse steric and electronic properties of the bisphosphines. The reactivity of the frustrated
Lewis pairs was exploited in the first diastereoselective domino hydrosilylation/hydrogenation reaction
catalyzed by FLPs.
highly rigid planar-chiral framework allows arrangement of the
Introduction
two phosphino groups in distances between 4.9 Å in 112 or
4.0 Å in 213 (Fig. 1) providing a unique setup to study their
activity depending to the functional group distance. The FLPs
The metal-free hydrogenation of unsaturated compounds has
emerged as powerful tool in synthetic organic chemistry.1–4
Despite these great achievements, the direct utilization of hydro-
gen in metal free processes has only recently been described uti-
lizing frustrated Lewis pairs5 (FLPs).6–8 The majority of these
reactions utilize bulky monophosphines or monoamines9 with
B(C6F5)3 (3) precluding the formation of a Lewis acid/Lewis
base adduct. In contrast, the application of bisphosphines in FLP
chemistry has found less attention. Clearly, this is attributed to
the fact that apart from H2-activation, multiple quenching or side
reactions are observed as reported for flexible ferrocene,10 zirco-
nocene6a and dppe (diphenylphosphinoethane) derivatives.11
One of the rare examples10,11 of a rigid bisphosphine derived
FLP capable for hydrogen activation was described by Erker
et al.7b The FLP consisting of bis(diphenylphosphino)naphtha-
lene (4)/3 was capable of reversible uptake and release of hydro-
gen7c–e,g–j and was applied in the hydrogenation of silyl enol
ethers. Hence, rigidity and steric bulk are key factors in the acti-
vation of hydrogen by bisphosphine-derived FLPs. Conse-
quently, we focused on the activity of [2.2]paracylophane
derived bisphosphines in the metal-free hydrogen activation. The
were active in a novel domino hydrosilylation/hydrogenation
reaction of enones, providing the silyl-protected alcohols in high
yields and excellent diastereoselectivity in
(Scheme 1).
a single step
Results and discussion
Structure of frustrated Lewis pairs derived from bisphosphines
1 and 2 after hydrogen activation
We initiated our investigations by the reaction of commercially
available PhanePhos (1) with B(C6F5)3 (3). Indeed, 1/3 formed a
aKarlsruhe Institute of Technology, Institute of Organic Chemistry, Fritz-
Haber Weg 6, 76131 Karlsruhe, Germany. E-mail: jan.paradies@
kit.edu; Fax: +49 721 608 45637; Tel: +49 721 608 45344
bKarlsruhe Institute of Technology, Institute of Inorganic Chemistry,
Engesserstrasse 15, 76131 Karlsruhe, Germany. E-mail: frank.breher@
kit.edu; Fax: +49 721 608 47021; Tel: +49 721 608 44855
cKarlsruhe Institute of Technology, Institute of Nanotechnology,
Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen,
Germany. E-mail: Karin.fink@kit.edu; Tel: +49 721 608 26411
†Electronic supplementary information (ESI) available: synthetic
details, NMR spectra and characterization. CCDC 867393 for
[2a·H]+[BF4]− and 867456 for [2b·H]+[BF4]−. For ESI and crystallo-
graphic data in CIF or other electronic format see DOI: 10.1039/
c2dt30374d
Fig. 1 Bisphosphines including relative P–P distances.
Scheme 1 Domino hydrosilylation/hydrogenation of enones.
9056 | Dalton Trans., 2012, 41, 9056–9060
This journal is © The Royal Society of Chemistry 2012

Scheme 3 Hydrogen activation by GemPhos derivatives (2).
Scheme 2 Activation of dihydrogen with PhanePhos (1).
FLP since the corresponding NMR spectra of a 1 : 1 mixture did
not differ from the individual spectra. After the solution was
reacted with hydrogen (2 bar) at room temperature the NMR
spectra changed significantly. The 31P NMR spectrum displayed
two new resonances at 4.3 and −1.2 ppm (31P NMR resonance
of free 1 δ = −0.7 ppm). The resonance at lower frequencies was
split into a doublet (JP–H = 510 Hz) accounting for a protonated
phosphine species. However, the spectrum also displayed the
signal of free bisphosphine 1 in a 1 : 1 ratio to the phosphonium
compound. The 11B NMR and the 19F NMR spectrum indicated
a borohydride anion with its characteristic signals (11B NMR
δ = −24.3 ppm, 19F NMR δ = −132 (o-), −163 (p-) and −166
Fig. 2 Crystal structure of (a) [2a·H]+[BF4]− and (b) [2b·H]+[BF4]−
(selected bond lengths for [2a·H]+[BF4]−: P1–P2 3.681 Å,
P2–H1 3.321 Å; for [2b·H]+[BF4]−: P1–P2 4.040 Å, P1–H2 3.391 Å;
peripheral hydrogen atoms and BF4 anion were omitted for clarity).14
1
(m-C6F5); H NMR resonance at δ = 4.40 ppm ((C6F5)3B–H))
together with the formation of a P–B Lewis adduct (11B NMR
δ = 0.4 ppm, 19F NMR δ = −132 (o-), −158 (p-) and −164
(m-C6F5)). The subsequent addition of a second equivalent of 3
and reaction with hydrogen resulted in conversion to the phos-
at −24.6 ppm with a coupling constant of JB–H = 92 Hz, which
accounts for a borohydride anion. On the contrary, the phos-
phonium motive derived from 2b is structurally different from
2a. When the unsymmetrical bisphosphine 2b was subjected to
hydrogen activation the PCy2H+ species was provided
(Scheme 3) and the resonance for a free PPh2-unit was observed
by 31P NMR (H-PCy2: −14.3 ppm, JP–H = 473 Hz, JP–P = 47 Hz;
PPh2: 2.79 ppm, JP–P = 47 Hz; regioselective protonation was
proved by 31P1H COSY). All attempts to obtain suitable crystals
for single crystal analysis were unsuccessful. However, the struc-
tures of the protonated phosphonium species [2a·H]+ and
[2b·H]+ as BF4−-salts were determined (Fig. 2a and b). The solid
state structure of 2a·HBF4 reveals the monoprotonation of one
phosphino-unit. Additionally, the proton is hydrogen bonded to
the second phosphino-group (P2–H1 3.321 Å). The P1–P2
distance is reduced by 0.387 Å to 3.681 Å compared to unproto-
nated 2a (4.068 Å).13 The crystal structure of [2b·H]+[BF4]− dis-
plays the selective protonation of the PCy2-unit confirming the
NMR spectroscopic results. The two phosphorous moieties are
separated by 4.040 Å, which is significantly larger than in the
[2a·H]+ cation. Clearly, the pseudo-geminal bisphosphines 2a
and 2b offer more efficient activation of dihydrogen compared
1 since only one equivalent of 3 is needed. However, the require-
ment of two equivalents of 3 for exhaustive H2-activation by 1
prompted us to investigate this system in more detail, since it is a
potential deactivation pathway for bisphosphines. Therefore the
phonium cation together with
a
borohydride species
([1·H·3]+[H·3]−, Scheme 2).
Consequently, the activation of hydrogen by PhanePhos (1)
required two equivalents of Lewis acid 3, while only one boro-
hydride anion was formed. This might be the result of a geo-
metrical reorganisation in [1·H]+ making the second phosphine
unit more accessible (3) (vide infra for theoretical considerations)
thus providing a binding/reaction site for the Lewis acid 3. The
product [1·H·3]+[H·3]− was prone for concurrent attack of the
phosphine unit to the para-position in 3 (see ESI† for details).10
Surprisingly, the pseudo-geminal bisphosphine GemPhos deriva-
tives 2 displayed different stoichiometry for H2-activation com-
pared to 1. The NMR spectra of a 1 : 1 mixture of 2a or 2b with
3 displayed no changes compared to the individual spectra,
hence both GemPhos phosphines 2 in combination with 3 form
FLPs. Exposure of these mixtures to hydrogen formed the corre-
sponding phosphonium–borohydride salts as clearly determined
by NMR spectroscopy (Scheme 3). The symmetrical bispho-
sphine 2a provided a binding motive for the proton, which is
similar to 4.7b Accordingly, upon lowering the temperature to
−60 °C the 31P NMR resonance separated into a resonance at
high (−0.42 ppm, JP–H = 515 Hz, JP–P = 86 Hz) and at low fre-
quencies (−4.75 ppm, JP–P = 86 Hz), indicating slow exchange
of the proton between the two phosphorous atoms on the NMR
time scale. The 11B NMR spectrum revealed a doublet centered
This journal is © The Royal Society of Chemistry 2012
Dalton Trans., 2012, 41, 9056–9060 | 9057

Scheme 4 Hydrogenation of silyl enol ether 6 by bisphosphine
derived frustrated Lewis-pairs (yields were determined by 1H NMR).
Fig. 3 Calculated phosphonium-borohydride salts of (a) 1 and (b) 2a
with one additional B(C6F5)3 molecule; (selected interatomic distances:
(a) [1·H·3]+[H·3]−: B1–P1 3.766 Å, B2–P2 2.283 Å; (b)
[2a·H·3]+[H·3]−: B1–P1 4.347 Å, B2–P2 5.673 Å; selected hydrogen
atoms were omitted for clarity).
structures of the phosphonium–borohydride salts derived from
1 and 2a with one additional B(C6F5)3 (3) molecule were studied
by theoretical methods. The corresponding lowest energy struc-
tures optimized at BP86/def2-SVP15 level of theory including
Grimme’s dispersion corrections16 as implemented in the Turbo-
mole17 package are depicted in Fig. 3 (see ESI† for more
details).
The two calculated structures show distinctly different binding
modes: (1) the arrangement in [1·H·3]+[H·3]− exhibits a short
B2–P2 distance (Fig. 3a, 2.283 Å), which confirms our exper-
imental observation that one Lewis acid molecule is consumed
by adduct formation (see Scheme 1); (2) the structure of
[2a·H·3]+[H·3]− shows an intramolecular hydrogen bond of
H1 to P2 (Fig. 3b). This hydrogen bond prevents the intermole-
cular adduct formation with 3 and is in agreement with the
NMR data and solid-state structure of 2a·HBF4. Additionally
this study rationalizes why reaction product [1·H·3]+[H·3]− is
prone for rearrangement.
Scheme 5 Domino hydrosilylation/hydrogenation of α,β-unsaturated
ketones (numbers in parentheses are NMR yields, areaction was per-
formed at 100 °C).
ethers. Accordingly we subjected the three bisphosphines in
combination with 3 to the hydrogenation of silyl enol ether 6 at
room temperature (10 mol% cat. loading, Scheme 4).
Catalytic application
The saturated product 7 was obtained in variable yields depen-
dent on the nature of the bisphosphine. When PhanePhos (1)
was reacted with one equivalent 3 (1 : 1 mixture, 10 mol%) in
the presence of silyl enol ether 6 and hydrogen, the previously
observed adduct formation was prevented due to the lower con-
centration of free borane. This was evidenced by the 31P NMR
spectrum of the reaction displaying only resonances for the cor-
responding FLP. The hydrogenation of 6 with 10 mol%
1/3 (1 : 1) provided the product in highest yield (>95%). The
pseudo-geminal isomer 2a proved less efficient (60% yield) and
the unsymmetrical bisphosphine 2b provided 7 in 40% yield
after 40 h at room temperature. The notable difference in reactiv-
ity might be explained by the increasing basicity of the phos-
Since the three bisphosphines 1, 2a and 2b display different
reactivity when exposed to 3 and H2, we were curious how these
structural features would affect their catalytic potential. We envi-
sioned the application of the new catalyst system in a novel
domino reaction18 consisting of two processes: the B(C6F5)3-cat-
alyzed 1,4-hydrosilylation of enones providing silyl enol ethers
and their subsequent hydrogenation by a FLP consisting of a
bisphosphine and B(C6F5)3 (Scheme 1).
Although both reactions have been reported separately by
Piers et al.19 and Erker et al.,7b the application of such trans-
formations in a sequence employing only one single catalyst
system has not been reported so far.
First we established that the B(C6F5)3-catalyzed 1,4-hydrosily-
lation reaction is not affected by the presence of phosphine. To
our delight, when 4,4-dimethyl cyclohexenone was reacted with
one equivalent of diphenylmethylsilane (5) in the presence of
20 mol% 3/1 the silyl enol ether was provided instantaneously.
Next, we turned our attention to the hydrogenation of silyl enol
phine moieties in the order PPh2-H+ < PPh2-H+-PPh2
<
PCy2-H+. Encouraged by the fact that each step of the domino
reaction was affected by the catalyst system consisting of the
planar-chiral bisphosphines (1 and 2a) and 3, we directed our
interest to the catalytic domino 1,4-hydrosilylation/hydrogen-
ation of α,β-unsaturated ketones (Scheme 5). The domino
9058 | Dalton Trans., 2012, 41, 9056–9060
This journal is © The Royal Society of Chemistry 2012

4,4-Dimethylcyclohexyloxy(methyl)diphenylsilane (9b)
reaction of cyclic and acyclic enones with one equivalent of
silane 5 and hydrogen was efficiently catalyzed by 1/3 employ-
ing 20 mol% catalyst loading at 50 °C. Generally, the corre-
sponding silyl-protected secondary alcohols were obtained in
good yields (50–90% for isolated products after column chrom-
atography; 75–95% determined by NMR from crude material).
Notably, the saturated products derived from tetra-substituted
silyl enol ethers (9d, 9e) were obtained with highest yield and
exclusively as cis diastereomers, which can be rationalized
according to the well established mechanistic model for
B(C6F5)3-catalyzed hydrosilylation of carbonyl compounds.20
Such products are commonly not easily accessed, which under-
scores the potential of this transition metal free hydrogenation
process.
24 h, 48 mg, yield 74%, Rf = 0.15. 1H NMR (300 MHz,
CDCl3): δ = 7.66–7.58 (m, 4H, HAr), 7.43–7.32 (m, 6H, HAr),
3.73 (sept, J = 4.7 Hz, 1H, CH), 1.73–1.50 (m, 4H, CH2),
1.48–1.36 (m, 2H, CH2), 1.19–1.05 (m, 2H, CH2), 0.93 (s, 3H,
CH3), 0.87 (s, 3H, CH3), 0.67 (s, 3H, CH3) ppm. 13C-NMR
(75 MHz, CDCl3): δ = 137.2 (2 × Cquart Ph), 134.5 (4 × CHPh),
129.8 (2 × CHPh), 127.9 (4 × CHPh), 71.9 (CH), 36.9, 31.6,
30.8, 29.7, 26.2, −2.1 ppm.
1,3-Diphenylpropoxy(methyl)diphenylsilane (9c)
1
48 h, 38 mg, yield 78%, Rf = 0.1. H NMR (300 MHz, CDCl3):
δ = 7.51–6.92 (m, 20H, HAr), 4.68 (dd, J = 6.7 Hz, 5.6 Hz, 1H,
CH), 2.68–2.39 (m, 2H, CH2), 2.15–1.83 (m, 2H, CH2), 0.36
(s, 3H, CH3) ppm. 13C-NMR (75 MHz, CDCl3): δ = 144.6,
142.2, 134.7, 134.5, 129.9, 128.8, 128.5, 128.4, 128.3, 127.9,
127.9, 127.3, 126.4, 125.8, 75.3, 42.0, 31.8, −2.3 ppm.
Conclusions
We have shown that planar-chiral bisphosphines are viable Lewis
bases for the activation of hydrogen by frustrated Lewis pairs
and exhibit distinct reactivity to more flexible ferrocene derived
bisphosphines. The two isomeric [2.2]paracylophane-derived
bisphosphines 1 and 2a allowed to study steric aspects without
altering electronic properties. Especially PhanePhos (1) exhibited
significant features for the use of bisphosphines in hydrogen acti-
vation: Although 1 and 3 forms a FLP, one phosphine moiety is
quenched by 3 upon exposure to hydrogen. However, in the
presence of an unsaturated substrate the adduct formation was
not observed and a 1 : 1 ratio of 1/3 can be applied.
cis-2-Methylcyclopentyloxy(methyl)diphenysilane (9d)
24 h, 69 mg, yield 90%, Rf = 0.10. 1H NMR (300 MHz,
CDCl3): δ = 7.66–7.56 (m, 4H, HAr), 7.45–7.32 (m, 6H, HAr),
4.16 (q, J = 4.6 Hz, 1H, CH), 1.88–1.61 (m, 5H), 1.59–1.38
(m, 2H), 1.00 (d, J = 7.0 Hz, 3H, CH3), 0.66 (s, 3H, CH3) ppm.
13C-NMR (75 MHz, CDCl3): δ = 137.3 (2 × Cquart Ph), 134.5
(4 × CHPh), 129.7 (2 × CHPh), 127.8 (4 × CHPh), 40.0, 34.8,
31.0, 21.9, 14.6, −2.3 ppm [CH(O) was not observed]. The
NMR data agrees with the literature.21
A novel domino hydrosilylation/hydrogenation reaction was
developed providing silyl protected alcohols in one step.
The hydrogenation proceeded in high yields and high
diastereoselectivity.
(1S,2R,5S)-2-Methyl-5-(prop-1-en-2-yl)cyclohexyloxy(methyl)-
diphenylsilane and (1R,2S,5S)-2-methyl-5-(prop-1-en-2-yl)-
cyclohexyloxy(methyl)diphenylsilane (9e)
Experimental section
100 h, 29 mg, yield 50%, Rf = 0.6; d : r = 2 : 1; (1S,2R,5S)-9e:
1H NMR (300 MHz, CDCl3): δ = 7.66–7.55 (m, 4H, HAr),
7.44–7.32 (m, 6H, HAr), 5.09 (s, 1H), 4.89 (s, 1H), 3.94
(mc, 1H), 2.75–2.62 (m, 1H), 1.83–1.68 (m, 2H), 1.57 (s, 3H,
CH3), 1.33–1.11 (m, 5H), 0.91 (d, J = 6.6 Hz, 3H, CH3), 0.66
General procedure for the reduction of enones 8
In a glovebox, B(C6F5)3 (3) (10–20 mol%), Ph2MeSiH (5, 1.0 eq.)
and the corresponding enone 9 (25 mg, 1.0 eq.) were dissolved
in toluene (2 ml) followed by immediate addition of PhanePhos
(1) (10–20 mol%) (in case of 9e GemPhos 2a was used). The
solution was transferred to a sealable flask equipped with a
Teflon tap and magnetic stirbar. The solution was freeze–pump
thawed for 2 cycles, charged with H2 at 77 K and stirred for
24–48 h at 50 °C. The reaction mixture was directly subjected to
column chromatography (cyclohexane–EtOAc 99 : 1) yielding
the corresponding silylether as a colorless oil.
1
(s, 3H, CH3) ppm; (1R,2S,5S)-9e: H NMR (300 MHz, CDCl3):
δ = 7.68–7.54 (m, 4H, HAr), 7.45–7.30 (m, 6H, HAr), 4.62 (br s,
2H), 3.95 (br s, 1H), 2.49–2.36 (m, 1H), 1.80–1.66 (m, 2H),
1.62 (s, 3H, CH3), 1.31–1.13 (m, 5H), 0.89 (d, J = 6.4 Hz, 3H,
CH3), 0.65 (s, 3H, CH3) ppm.
Acknowledgements
The authors acknowledge the DFG-funded transregional colla-
borative research center SFB/TRR 88 Cooperative effects in
homo- and hetero-metallic complexes (3MET) for funding. We
like to acknowledge the Landesgraduiertenförderung of the State
of Baden-Württemberg for a Ph.D. grant to L. G. and the Alex-
ander von Humboldt Stiftung for a postdoc grant to P. O.-B. The
Fonds der Chemischen Industrie is acknowledged for a Liebig-
Grant to J. P. The authors gratefully acknowledge S. Schneider,
P. Smie and C. Kiefer for the collection of crystal structural data.
We thank Prof. Dr S. Bräse for his kind support.
Cyclopentyloxy(methyl)diphenylsilane (9a)
1
24 h, 50 mg, yield 58% (10% silane impurity), Rf = 0.20. H
NMR (300 MHz, CDCl3): δ = 7.63–7.56 (m, 4H, HAr),
7.42–7.32 (m, 6H, HAr), 4.40–4.30 (m, 1H, CH), 1.86–1.46
(m, 8H, CH2), 0.93 (s, 3H, CH3) ppm. 13C-NMR (75 MHz,
CDCl3): δ = 137.1 (2 × Cquart Ph), 134.5 (4 × CHPh), 129.7 (2 ×
CHPh), 127.9 (4 × CHPh), 75.3 (CH), 35.7, 23.3, −2.2 ppm.
This journal is © The Royal Society of Chemistry 2012
Dalton Trans., 2012, 41, 9056–9060 | 9059

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