Metal-free catalytic hydrogenation of enamines, imines, and conjugated phosphinoalkenylboranes

Article abstract of DOI:10.1002/anie.200801432

(Chemical Equation Presented) The metal-free hydrogen activator 1 catalyzes the unique P/B hydrogenation of the frustrated Lewis pair 3, which itself is inactive toward H₂ under the applied conditions, to yield the hydrogenation product 4. System 1/2 (5 mol%) also catalyzes the hydrogenation of a bulky ketimine and of enamines under mild conditions (2.5 bar H₂, RT) to yield the respective amines.

Full text of DOI:10.1002/anie.200801432

Angewandte
Chemie
DOI: 10.1002/anie.200801432
H2 Activation
Metal-Free Catalytic Hydrogenation of Enamines, Imines, and
Conjugated Phosphinoalkenylboranes**
Patrick Spies, Sina Schwendemann, Stefanie Lange, Gerald Kehr, Roland Fröhlich, and
Gerhard Erker*
Dedicated to Professor Reinhard W.Hoffmann on the occasion of his 75th birthday
Catalytic heterolytic splitting of dihydrogen had until recently
been a domain of metal-containing systems, be it for example
in the Fe or Fe/Ni hydrogenase enzymes[1] or the Noyori-type
chelate amine ruthenium catalysts and many conceptually
related systems.[2] Stephan et al. recently reported the reac-
tion of dihydrogen with frustrated phosphane/borane Lewis
pairs, some of which are quite active catalysts for the
hydrogenation of bulky imines at elevated temperatures.[3–9]
We reported the formation of the ethylene-bridged P/B
system 1 (Mes = 2,4,6-trimethylphenyl),[6] a weakly intramo-
Scheme 1. Preparation of the zwitterionic compounds 6.
lecularly interacting phosphane/borane pair that rapidly
activates dihydrogen heterolytically at ambient temperature
to form the linked phosphonium/hydridoborate zwitterion 2
[Eq. (1)]. Herein we show that the system 1/2 is able to
transfer the H+/HÀ pair under mild conditions and can serve
as a catalyst for the hydrogenation of bulky imines and of
enamines.
The hydroboration of compounds 3b (R = CH3) and 3c
(R = Ph, Scheme 1) at room temperature in pentane gave the
bright-red products 5b (82%) and 5c (93%), respectively.
The NMR spectra are consistent with three-coordinate boron
(11B NMR, [D6]benzene: d = 64 (5b), 59 ppm (5c);[12]
19F NMR: Dd(m/p-C6F5) = 11.7 (5b), 11.4 ppm (5c);
31P NMR: d = À27.1 (5b), À22.0 ppm (5c)).
In contrast to 5a, neither compound 5b nor 5c reacts with
dihydrogen under the reaction conditions used (room temper-
ature, pentane or toluene solution, 2.5 bar H2), nor with
dihydrogen at a pressure of 60 bar. Addition of bulky tri-tert-
butyl phosphane (15 mol%) to a solution of 5b led to slow
heterolytic dihydrogen splitting at ambient conditions
(2.5 bar H2, 3 d at RT) to eventually yield 6b. Treatment of
5b with dihydrogen at 60 bar in the presence of P(tBu)3
(15 mol%) in toluene at room temperature led to a complete
conversion into 6b within 3 h.
[10]
ꢀ
Hydroboration of tBu2PC CCH3 (3a) with HB(C6F5)2
(4)[11] at 808C in benzene gave the bifunctional phosphane/
borane system 5a (Scheme 1). The orange-colored oil reacts
slowly with dihydrogen at elevated pressure (60 bar H2,
toluene, 3 h at room temperature) to yield the zwitterionic
product 6a (63%). Compound 6a was characterized by
elemental analysis and NMR spectroscopy (see the Exper-
imental Section).
=
Although the –CH C(CH3)-linked P/B system (5b) itself
proved to be unreactive toward dihydrogen under the
reaction conditions used, it accepted the H+/HÀ pair from
the H2 activator system 1/2. Compound 5b was mixed in
[D6]benzene with an equimolar amount of the zwitterion 2,
and the rapid formation of an equilibrium mixture of 1, 2, 5b,
and 6b was observed (Figure 1).
The heterolytic splitting of dihydrogen and its transfer to
5b was efficiently catalyzed by the 1/2 pair. Treatment of a
mixture of the unreactive compound 5b with H2 (2.5 bar) at
RT in toluene solution in the presence of 10 mol% of 2 (either
added as the substance, or generated in situ from 1 during the
hydrogenation reaction) resulted in a complete conversion
into the hydrogen addition product 6b within 4 h (Scheme 1).
Compound 6b (admixed with some residual 2) was isolated
from the reaction mixture in over 80% yield as a colorless
solid (m.p. 1538C). It features a characteristic 1H NMR signal
[*] P. Spies, S. Schwendemann, S. Lange, Dr. G. Kehr, Dr. R. Fröhlich,[+]
Prof. Dr. G. Erker
Organisch-Chemisches Institut
Westfälische Wilhelms-Universität Münster
Corrensstrasse 40, 48149 Münster (Germany)
Fax: (+49)251-833-6503
E-mail: erker@uni-muenster.de
[+] X-ray crystal structure analyses.
[**] Financial support from the Deutsche Forschungsgemeinschaft and
the Fonds der Chemischen Industrie is gratefully acknowledged. We
thank the BASF for a gift of solvents and Professor C. J. Elsevier for
assistance with some of the NMR experiments.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200801432.
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Figure 1. 31P NMR spectra (upper: {1H} coupled, lower: {1H} decou-
pled) of the equilibrium mixture of 1, 2, 5b, and 6b after reaction of 2
and 5b (1:1) after ca. 30 min in [D6]benzene.
at d = 7.58 ppm (see Experimental Section) for P–H and a
corresponding 31P NMR resonance at d = À36.2ppm.
The analogous reaction of 5b with D2, catalyzed by 1/
Figure 2. Molecular structure of the zwitterionic product 6b. Selected
bond lengths [Š] and angles [8]: P1–C1 1.775(2), C1–C2 1.340(2), C2–
C3 1.506(3), C2–B1 1.616(3), C11-P1-C21 117.7(1), C11-P1-C1
113.5(1), C21-P1-C1 110.0(1), P1-C1-C2 124.4(1), C1-C2-C3 120.7(2),
C1-C2-B1 122.6(2), C3-C2-B1 116.3(2), C31-B1-C41 108.3(1), C31-B1-
C2 115.3(2), C41-B1-C2 113.7(2).
2
[D2]2 (10 mol%) gave [D2]6b. The compound has H NMR
signals at d = 7.55 (1JPD ꢁ 70 Hz, P–D) and 4.1 ppm (br s, B–
D). The 31P NMR spectrum of [D2]6b has a corresponding
1:1:1 triplet for the P–D unit at d = À36.8 ppm (1JPD ꢁ 70 Hz).
The weakly interacting P/B pair 1 also catalyzes the P/B
hydrogenation of the analogous phenyl derivative 5c at
ambient temperature (6c isolated in 80% yield).
The products 6b and 6c were characterized by X-ray
diffraction. Both the phosphorus and the boron atom have
pseudotetrahedral coordination geometries; each carries a
hydrogen atom which was located in the structure. The
structure of 6b (single crystals from THF) has bulky
[Mes2PH]+ and [(C6F5)2BH]À units oriented trans to each
other at the central bridging -CH = CMe-unit (Figure 2).
Compound 6c has a very similar structure (for details, see the
Supporting Information).
The system 1/2 catalyzes the hydrogenation of bulky
imines under mild conditions. Treatment of N-tert-butylbenz-
aldimine (7a) with dihydrogen (2.5 bar) in the presence of
20 mol% of the dihydrogen activation/transfer reagent
system 1/2 leads to the rapid formation of tert-butylbenzyl-
amine (8a) at room temperature (87% yield of isolated
product; Scheme 2). In the case of the ketimine substrate 7b,
using 5 mol% catalyst was sufficient to achieve complete
imine hydrogenation within 180 min (70% amine 8b iso-
lated).
Scheme 2. 1) a: 20 mol% 2, b: 5 mol% 2; 2) 10 mol% 2;
3) a: 5 mol% 2, b: 3 mol% 2.
The 1/2 pair is a good metal-free catalyst for the hydro-
genation of enamines. We achieved quantitative hydrogena-
tion of the enamine 9 to the tertiary amine 10 at ambient
conditions with 10 mol% of the 1/2 catalyst system
(Scheme 2). Similarly, we were able to hydrogenate the
enamines piperidinocyclohexene (11a) and morpholinocy-
clohexene (11b) to the respective tertiary amines N-cyclo-
hexylpiperidine (12a, 88% yield of isolated product) and N-
cyclohexylmorpholine (12b, 78%) at room temperature and
H2 at a pressure of 2.5 bar employing 5 mol% or 3 mol% of
the metal-free 1/2 hydrogenation catalyst system, respec-
tively.
to emerge.[3–9] It seems that a subtle balance of steric and
electronic factors is controlling the potential dihydrogen
activation ability of the P/B frustrated Lewis pairs. It is
remarkable that some of the rather open systems 5 by
themselves do not show activity toward dihydrogen. How-
ever, these systems accept the H+/HÀ pair readily from the
active hydrogen splitting catalyst 1/2 to undergo a unique
catalytic P/B hydrogenation reaction, as do imines and
enamines. The unique pair 1/2 seems to be one of the most
active metal-free catalytic hydrogenation catalysts described
to date. Our observations indicate that the development of
new frustrated Lewis pairs is useful for the development of
novel catalyst systems for reactions with dihydrogen.
Our study has revealed some distinctive features of the
new hydrogen activation systems that are currently beginning
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Angewandte
Chemie
K. Wieghardt), Wiley, New York, 2001, pp. 738 – 751; b) M. Frey,
Experimental Section
J. C. Fontecilla-Camps, A. Volbeda, Handbook of Metallopro-
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Borg, X. Liu, S. K. Ibrahim, C. J. Pickett, S. P. Best, J.Am.Chem.
Soc. 2007, 129, 11085 – 11092, and references therein.
5a: Reaction of 3a (32mg, 0.17 mmol) with HB(C 6F5)2 (4); 59 mg,
0.17 mmol) in benzene at 808C (10 min) gave 5a (90 mg, 100%).
1H NMR (400 MHz): d = 7.31 ppm (1H, dq, 2JPH = 4.5, 4JHH = 1.3 Hz,
P
13
1
B
=
=
CH ); C{ H} NMR (101 MHz): d = 161.7 (br s, C ), 158.0 ppm (d,
1JPC = 33.7 Hz, C ); 19F NMR (282 MHz): d = À129.8 (o), À147.1
(p), À160.6 ppm (m); 31P{1H} (122 MHz): d = 3.2ppm ( n1/2 = 4 Hz);
11B{1H} NMR (96 MHz): d = 62ppm ( n1/2 = 930 Hz). UV/Vis (pen-
tane): lmax(e) = 397 nm (1500).
P
=
[2] a) R. Noyori, S. Hashiguchi, Acc.Chem.Res. 1997, 30, 97 – 102;
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5b: Compound 3b (344 mg, 1.1 mmol) was reacted with HB-
(C6F5)2 (4; 386 mg, 1.1 mmol) in pentane (15 mL) at room temper-
ature to give 5b (591 mg, 82%). 1H NMR: d = 8.17 ppm (1H, d,
P
2JPH = 8.0 Hz, CH ); 13C{1H} NMR: d = 171.4 (d, 2JPC = 22.8 Hz,
=
CH ), 151.8 ppm (br s, C ); 31P{1H} NMR: d = À27.1 ppm (n1/2
=
P
B
=
=
7 Hz); 19F NMR: d = À130.8 (o), À149.2( p), À160.9 ppm (m). UV/
Vis (pentane): lmax(e) = 432nm (10500). Elemental analysis (%)
calcd for C33H26BF10P: C 60.57, H 4.01; found: C 60.13, H 4.04.
6a: Compound 3a (111 mg, 0.60 mmol) was reacted with HB-
(C6F5)2 (4; 208 mg, 0.60 mmol) in toluene (10 mL) at 808C for 10 min.
Subsequent hydrogenation (3 h, 60 bar H2) in a steel autoclave gave
3
6a (200 mg, 63%). 1H NMR: d = 5.11 (1H, dd, 2JPH = 31.5, JHH
=
12.9 Hz, C ), 4.57 (1H, dd, 1JPH = 435, 3JHH = 12.9 Hz, P H),
P
=
À
4.04 ppm (1H, br s. 1:1:1:1 q (partially relaxed),[13] 1JBH = 93 Hz, B
À
H); C{ H} NMR: d = 202.0 (br s, C ), 90.2ppm (d, 1JPC = 65 Hz,
13
1
B
=
Tochtermann, Angew.Chem.
1966, 78, 355 – 375; Angew.
C ); 31P NMR: d = 22.0 ppm (d, 1JPH = 435 Hz); 19F NMR: d =
P
=
Chem.Int.Ed.Engl. 1966, 5, 351 – 371, and references therein.
[5] See also: a) C. Walling, L. Bollyky, J.Am.Chem.Soc. 1961, 83,
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Schoeller, G. Bertrand, Science 2007, 316, 439 – 441; Review:
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Angew.Chem.Int.Ed. 2007, 47, 38 – 41.
[6] P. Spies, G. Erker, G. Kehr, K. Bergander, R. Fröhlich, S.
Grimme, D. W. Stephan, Chem.Commun. 2007, 5072– 5074.
[7] a) G. C. Welch, L. Cabrera, P. A. Chase, E. Hollink, J. D.
Masuda, P. Wei, D. W. Stephan, Dalton Trans. 2007, 3407 –
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À131.4 (o), À161.6 (p), À165.3 ppm (m); 11B NMR: dÀ18.0 ppm (d,
1JBH = 91 Hz). IR (KBr): n˜ = 2345 cmÀ1 (m, nPH/nBH). Elemental
analysis (%) calcd for C23H24BF10P: C 51.91, H 4.55; found: C 52.05,
H 4.89.
6b: Compound 5b was generated in situ from 3b (200 mg,
0.65 mmol) and HB(C6F5)2 (4; 224 mg, 0.65 mmol) in the presence of
the catalyst 2 (42mg, 0.07 mmol). Subsequent hydrogenation (2.5 bar,
toluene, RT) gave 6b (298 mg, 70%). 1H NMR: d = 7.58 (1H, dd,
3
1JPH = 469, 3JHH = 7.8 Hz, P H), 5.47 (1H, dd, 2JPH = 38.4, JHH
=
À
P
=
7.8 Hz, CH ), 4.11 ppm (1H, br s, 1:1:1:1 q (partially relaxed),
1JBH = 95 Hz, B H); C{ H} NMR: d = 201.2 (br s, C ), 96.8 ppm (d,
13
1
B
À
=
1JPC = 68.3 Hz, CH ); 31P NMR: d = À36.2ppm (dd, 1JPH = 469,
P
=
2JPH = 38 Hz); 19F NMR: d = À131.3 (o), À161.6 (p), À165.3 ppm
(m); 11B NMR: d = À18.0 ppm (d, 1JBH = 95 Hz). IR (KBr): n˜ =
2329 cmÀ1 (s, nPH/nBH). Elemental analysis (%) calcd for
C33H28BF10P: C 60.39, H 4.30; found: C 59.87, H 4.18.
X-ray crystal structure analysis for 6b: C33H28BF10P, Mr = 656.33,
colorless crystal 0.35 ” 0.20 ” 0.20 mm3, a = 13.0699(4), b = 16.1472(5),
c = 15.1163(5) Š,
b = 100.271(1)8,
V= 3139.06(17) Š3,
1calc =
1.389 gcmÀ3 m = 1.515 mmÀ1
,
,
empirical absorption correction
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Received: March 26, 2008
Revised: May 10, 2008
Published online: August 25, 2008
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Keywords: boron · catalytic hydrogenation · hydrogen ·
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