Heterolytic dihydrogen activation with the 1,8-bis(diphenylphosphino) naphthalene/B(C6F5)3 pair and its application for metal-free catalytic hydrogenation of silyl enol ethers

Article abstract of DOI:10.1039/b813286k

The "frustrated Lewis pair" 1,8-bis(diphenylphosphino) naphthalene/B(C₆F₅)₃ reversibly activates dihydrogen; it serves as an active catalyst for the hydrogenation of silyl enol ethers under mild reaction conditions. The Ro

Full text of DOI:10.1039/b813286k

View Online / Journal Homepage / Table of Contents for this issue
COMMUNICATION
www.rsc.org/chemcomm | ChemComm
Heterolytic dihydrogen activation with the 1,8-bis(diphenylphosphino)-
naphthalene/B(C6F5)3 pair and its application for metal-free catalytic
hydrogenation of silyl enol etherswz
¨
Huadong Wang, Roland Frohlich,y Gerald Kehr and Gerhard Erker*
Received (in Cambridge, UK) 31st July 2008, Accepted 1st September 2008
First published as an Advance Article on the web 9th October 2008
DOI: 10.1039/b813286k
The ‘‘frustrated Lewis pair’’ 1,8-bis(diphenylphosphino)-
naphthalene/B(C6F5)3 reversibly activates dihydrogen; it serves
as an active catalyst for the hydrogenation of silyl enol ethers
under mild reaction conditions.
conformative features and studied its ability for heterolytic
H2-activation and its catalytic hydrogenation properties.
When 1,8-bis(diphenylphosphino)naphthalene (5)10 and
B(C6F5)3 were stoichiometrically mixed in d8-toluene, the
31P, 19F and 11B NMR spectra remained practically un-
changed, suggesting no Lewis adduct was formed. This is in
sharp contrast to the parent triphenylphosphine, which forms
a classical Lewis adduct when mixed with B(C6F5)3.11 Conse-
quently, the 1 : 1 mixture of 5 and B(C6F5)3 reacted smoothly
with dihydrogen (2 bar) at room temperature in toluene. We
isolated the mono-phosphonium/hydridoborate salt (6) as a
white solid in 80% yield (see Scheme 2). The formation of the
[HB(C6F5)3ꢀ] anion was confirmed by its typical 19F NMR
spectrum featuring signals at d ꢀ132.0 (o-), d –163.7 (p-) and
d ꢀ166.4 (m-C6F5) in a 6 : 3 : 6 intensity ratio and a 11B NMR
doublet at d ꢀ22.8 (1JBH = 84 Hz; corresponding BH
1H NMR resonance at d 4.49 in d6-benzene). The 31P and
1H NMR spectra of the (previously described)12 5-H+ mono-
cation are dynamic. The broad doublet observed in the 31P
NMR spectrum at 25 1C in d2-dichloromethane decoalesces
upon lowering the temperature to feature a doublet of
doublets at d 10.0 (1JPH E 557 Hz, JPP = 110 Hz) for the
–Ph2PH+ unit (1H NMR PH signal at d 10.1) and a doublet at
d ꢀ18.8 of the adjacent neutral –PPh2 substituent at the
naphthalene peri-positions. The analogous reaction of the
5/B(C6F5)3 pair with D2 gave the corresponding salt 5-D+/
Wittig, Tochtermann and others1 observed that some bulky
borane/phosphane pairs (and related systems) are hindered
toward the usual Lewis acid/Lewis base neutralization reac-
tion. Tochtermann called such systems ‘‘antagonistische
Paare’’. They were used e.g. in the addition reaction to
benzyne to yield ortho-phenylene-bridged zwitterionic
phosphonium/borate compounds. Much later Stephan et al.
observed that ‘‘frustrated’’ borane/phosphane Lewis pairs,
whose direct interaction was inhibited by steric bulk, added
ethene (and other simple alkenes) between them to yield
similarly structured alkylene-bridged zwitterions.2 More im-
portantly, ‘‘frustrated Lewis pairs’’,3 such as e.g. (mesityl)3P/
B(C6F5)3 (1) were shown by Stephan et al. to rapidly activate
dihydrogen at ambient conditions to yield the respective
phosphonium/hydridoborate salts, e.g. (mesityl)3PH+/
ꢀ
HB(C6F5)3 (2).4–6 These systems were shown to act as imine
hydrogenation catalysts at elevated temperature.7 We had
recently described the H2-activation reaction by the weakly
interacting intramolecular Lewis pair 3 to yield the zwitterion
4 (see Scheme 1).8 The 3/4 system was shown to actively
catalyze the hydrogenation reaction of a variety of enamines
and of some bulky imines under very mild conditions at room
temperature.9 We have now found a unique ‘‘frustrated’’ P/B
pair, that evaded a deactivating interaction by its special
2
[D-B(C6F5)3ꢀ] (6-D2). In the H NMR spectrum at 25 1C, it
shows a broad [B]–D resonance at d 4.34 and it features a
2
broad H NMR triplet at d 9.73 with an exchange averaged
coupling constant of JPD E 40 Hz.
The salt 6 was characterized by an X-ray crystal structure
analysis (single crystals were obtained from pentane–
ꢀ
dichloromethane by the diffusion method). The [HB(C6F5)3
]
anion exhibits an averaged set of C–B–C angles of 112.8(3)1
(B–C(C6F5) bond lengths between 1.626(5) A and 1.630(5) A).
The corresponding cation shows a neutral –PPh2 substituent
at the naphthalene framework that features typical angles of a
Scheme 1
Organisch-Chemisches Institut, Universitat Munster, Corrensstr. 40,
¨
¨
D-48149 Munster, Germany. E-mail: erker@uni-muenster.de;
¨
Fax: +49 251-83 36503
w Electronic supplementary information (ESI) available: Experimental
section and details of the quantum chemical calculations. CCDC
697210. For ESI and crystallographic data in CIF or other electronic
format see DOI: 10.1039/b813286k
z Dedicated to Professor Manfred T. Reetz on the occasion of his 65th
birthday.
y X-Ray crystal structure analysis.
Scheme 2
ꢁc
This journal is The Royal Society of Chemistry 2008
5966 | Chem. Commun., 2008, 5966–5968

View Online
Table 1 Hydrogenation of silyl enol ethers (7) catalyzed by
a
5/B(C6F5)3
Substrate
Product
Conversionb (%)
499 (93)
499 (89)
499 (86)
499 (85)
Fig. 1 A view of the molecular structure of the phosphonium/
hydridoborate salt 6 featuring a weak P–Hꢂ ꢂ ꢂH–B contact in the
crystal.14
16/499c
three-coordinate phosphane [C8–P2–C31 105.00(12)1, C8–P2–C41
102.14(13)1, C31–P2–C41 102.51(12)1; corresponding bond
lengths P2–C41 1.824(3) A, P2–C8 1.826(3) A, P2–C31
1.826(3) A). The adjacent cationic –Ph2PH+ substituent shows
markedly larger angles [C1–P1–C11 108.61(12)1, C1–P1–C21
113.60(14)1, C11–P1–C21 107.70(13)1, corresponding bond
lengths P1–C21 1.788(3) A, P1–C11 1.800(3) A, P1–C1
1.805(3) A)].
a
20 mol% of 5/B(C6F5)3 in C6D6 under 2 bar H2 at room temperature
b
for 20 h. Conversion was determined via 1H NMR analysis and
c
isolated yields are given in parentheses. 20 mol% of 5/B(C6F5)3 in
C6D6 under 60 bar H2 at room temperature for 3 h.
An exceptional feature of the structure of salt 6 is the
observed weak P–Hꢂ ꢂ ꢂH–B interaction13 between the phos-
phonium cation and the hydridoborate anion (see Fig. 1) with
a Hꢂ ꢂ ꢂH distance of 2.08 A.
(see Scheme 3)—to the best of our knowledge this was the first
time that this class of compounds has been employed in such a
metal-free catalytic hydrogenation. For the systems 7a–7d (see
Table 1) quantitative hydrogenation was achieved with
20 mol% of the 5 + B(C6F5)3 catalyst system under mild
conditions (2 bar H2, room temperature). Under these condi-
tions the hydrogenation of the least hindered substrate (7e)
was only close to stoichiometric but a complete conversion to
8e was achieved under more forcing conditions at 60 bar H2
pressure.
The H2-activation process by 5/B(C6F5)3 is reversible.
Heating a solution of the salt 6 in d6-benzene at 60 1C resulted
in a practically quantitative formation of 5 and B(C6F5)3,
indicating stoichiometric loss of H2. We note that the
5/B(C6F5)3/H2 $ 6 system is one of the rare examples of an
observed reversible uptake/release of dihydrogen at
‘‘frustrated Lewis pair’’.4
a
The facile manner of H2 release from the salt 6 prompted us
to explore the possibility of applying the 5 + B(C6F5)3/6
system as a hydrogenation catalyst. We chose a series of silyl
enol ethers (7) as substrates for the hydrogenation reaction
We have shown that the 1,8-bis(diphenylphosphino)-
naphthalene/B(C6F5)3 pair reversibly activates dihydrogen
under metal-free conditions with an unprecedentedly low
temperature gap between absorption and release of H2.
Although this bis-phosphine at the naphthalene peri-positions
does not act as a proton sponge,15 it seems to favour a
conformational orientation of the pair of –PPh2 groups that
prevents a direct P/B adduct formation. This system serves as
an active catalyst for alkene hydrogenation in silyl enol ether
substrates. Our approach to use a conformationally restricted
Scheme 3
ꢁc
This journal is The Royal Society of Chemistry 2008
Chem. Commun., 2008, 5966–5968 | 5967

View Online
bifunctional bis-phosphine and B(C6F5)3 to heterolytically
activate dihydrogen extends the concept of ‘‘antagonistic/
frustrated Lewis pairs’’ and will be useful for the development
of novel catalyst systems16 and potential hydrogen storage
materials.
(b) G. C. Welch, T. Holtrichter-Roesssmann and D. W. Stephan,
Inorg. Chem., 2008, 47, 1904–1906.
12 A. Karacar, V. Klaukien, M. Freytag, H. Thonnessen, J.
¸
¨
Omelanczuk, P. G. Jones, R. Bartish and R. Schmutzler,
Z. Anorg. Allg. Chem., 2001, 627, 2589–2603. The NMR
spectra of the cation 5-H+ are dynamic (see ESIw); they
show the presence of an as yet unidentified minor compound
(ca. 5%).
13 R. Custelcean and J. E. Jackson, Chem. Rev., 2001, 101,
1963–1980.
14 Crystal data for 6: C52H28BF15P2, M = 1010.49, triclinic, space
Notes and references
1 (a) G. Wittig and E. Bunz, Chem. Ber., 1959, 92, 1999–2013; (b) W.
Tochtermann, Angew. Chem., 1966, 78, 355–375 (Angew. Chem.,
Int. Ed. Engl., 1966, 5, 351–371) and references cited therein.
2 J. S. McCahill, G. C. Welch and D. W. Stephan, Angew. Chem.,
2007, 119, 5056–5069 (Angew. Chem., Int. Ed., 2007, 46,
4968–4971).
ꢀ
group P1 (No. 2), a = 9.7493(5), b = 13.3554(6), c = 19.3935(9)
A, a = 100.853(2), b = 99.692(2), g = 94.678(4)1, V = 2427.8(2)
A3, Dc = 1.382 g cmꢀ3, m = 1.645 mmꢀ1, Z = 2, l = 1.54178 A, T
= 223(2) K, 29 126 reflections collected (ꢃh, ꢃk, ꢃl), [(sin y)/l] =
0.60 Aꢀ1, 8515 independent (Rint = 0.062) and 6735 observed
3 D. W. Stephan, Org. Biomol. Chem., 2008, 6, 1535–1539.
4 G. C. Welch, R. R. San Juan, J. D. Masuda and D. W. Stephan,
Science, 2006, 314, 1124–1126.
reflections [I Z 2s(I)], 639 refined parameters, R = 0.060,
wR2 = 0.172, CCDC 697210.
15 Review see: H. Staab and T. Saupe, Angew. Chem., 1988, 100,
895–909 (Angew. Chem., Int. Ed., 1998, 27, 865–879).
5 (a) P. A. Chase, G. C. Welch, T. Jurca and D. W. Stephan, Angew.
Chem., 2007, 119, 8186–8199 (Angew. Chem., Int. Ed., 2007, 46,
8050–8053); P. A. Chase, G. C. Welch, T. Jurca and D. W.
Stephan, Angew. Chem., 2007, 119, 9296 (Angew. Chem., Int.
Ed., 2007, 46, 9136); (b) G. C. Welch and D. W. Stephan, J. Am.
Chem. Soc., 2007, 129, 1880–1881; (c) G. C. Welch, L. Cabrera, P.
A. Chase, E. Hollink, J. D. Masuda, P. Wei and D. W. Stephan,
Dalton Trans., 2007, 3407–3414; (d) Review: A. L. Kenward and
W. E. Piers, Angew. Chem., 2008, 120, 38–42 (Angew. Chem., Int.
Ed., 2008, 47, 38–41).
16 For other examples of metal-free hydrogenation reactions see: (a)
C. Walling and L. Bollyky, J. Am. Chem. Soc., 1961, 83,
2968–2969; (b) E. J. DeWitt, F. L. Ramp and L. E. Trapasso, J.
Am. Chem. Soc., 1961, 83, 4672–4672; (c) F. L. Ramp, E. J. DeWitt
and L. E. Trapasso, J. Org. Chem., 1962, 27, 4368–4372; (d) C.
Walling and L. Bollyky, J. Am. Chem. Soc., 1964, 86, 3750–3752;
(e) M. Siskin, J. Am. Chem. Soc., 1974, 96, 3641–3641; (f) J.
Wristers, J. Am. Chem. Soc., 1975, 97, 4312–4316; (g) E. Osthaus
and M. W. Haenel, in Proceedings of the 1987 International
Conference on Coal Science, Coal Science and Technology, ed. J.
A. Moulijin, K. A. Nater and H. A. G. Chermin, Elsevier,
Amsterdam, 1987, vol. 11, pp. 765–768; (h) M. Yalpani, T. Lunow
6 Theoretical calculations: (a) T. A. Rokob, A. Hamza, A. Stirling,
T. Soo
Chem., Int. Ed., 2008, 47, 2319–2513); (b) A. Stirling, A. Hamza, T.
A. Rocob and I. Papai, Chem. Commun., 2008, 3148–3150; (c) Y.
s and I. Papai, Angew. Chem., 2008, 120, 2469–2472 (Angew.
´ ´
´
and R. Koster, Chem. Ber., 1989, 122, 687–693; (i) M. Yalpani and
¨
R. Koster, Chem. Ber., 1990, 123, 719–724; (j) M. Yalpani, R.
¨
Guo and S. Li, Inorg. Chem., 2008, 47, 6212–6219.
7 See also: (a) P. A. Chase, T. Jurca and D. W. Stephan, Chem.
Commun., 2008, 1701–1703; (b) D. Chen and J. Klankermayer,
Chem. Commun., 2008, 2130–2131; (c) V. Sumerin, F. Schulz, M.
Nieger, M. Leskela, T. Repo and B. Rieger, Angew. Chem., 2008,
¨
120, 6090–6092 (Angew. Chem., Int. Ed., 2008, 47, 6001–6003).
8 P. Spies, G. Erker, G. Kehr, K. Bergander, R. Frohlich, S. Grimme
¨
and D. W. Stephan, Chem. Commun., 2007, 5072–5074.
9 P. Spies, S. Schwendemann, S. Lange, G. Kehr, R. Frohlich and G.
¨
Erker, Angew. Chem., 2008, 120, 7654–7546 (Angew. Chem., Int.
Ed., 2008, 47, 7543–7546).
Koster and M. W. Haenel, Erdoel Kohle, Erdgas, Petrochem., 1990,
¨
43, 344–347; (k) M. W. Haenel, J. Narangerel, U.-B. Richter and
A. Rufinska, Angew. Chem., 2006, 118, 1077–1082 (Angew. Chem.,
Int. Ed., 2006, 45, 1061–1066); (l) M. W. Haenel, J. Narangerel,
U.-B. Richter and A. Rufinska, Prepr. Pap.-Am. Chem. Soc., Div.
Fuel Chem., 2006, 51, 741–742; (m) M. Rueping, A. P. Antonchick
and T. Theissmann, Angew. Chem., 2006, 118, 3765–3768 (Angew.
Chem., Int. Ed., 2006, 45, 3683–3686); (n) M. Rueping, A. P.
Antonchick and T. Theissmann, Angew. Chem., 2006, 118,
6903–6907 (Angew. Chem., Int. Ed., 2006, 45, 6751–6755); (o) M.
Rueping and A. Antonchick, Angew. Chem., 2007, 119, 4646–4649
(Angew. Chem., Int. Ed., 2007, 46, 4562–4265); (p) D. Frey, V.
Lavallo, B. Donnadieu, W. W. Schoeller and G. Bertrand, Science,
2007, 316, 439–441.
10 R. D. Jackson, S. James, A. G. Orpen and P. G. Pringle, J.
Organomet. Chem., 1993, 458, C3–C4.
11 (a) H. Jacobsen, H. Berke, S. Doring, G. Kehr, G. Erker, R.
¨
¨
Frohlich and O. Meyer, Organometallics, 1999, 18, 1724–1735;
ꢁc
This journal is The Royal Society of Chemistry 2008
5968 | Chem. Commun., 2008, 5966–5968