Ruthenium-catalyzed ionic hydrogenation of iminium cations. Scope and mechanism

Article abstract of DOI:10.1021/ja0506861

Catalysis by CpRu(P-P)H (where P-P Is a chelating diphosphine) of the ionic hydrogenation of an iminium cation inolves (1) the transfer of H^- to form an amine, (2) the coordination of H₂ to the resulting Ru cation, and (3) the transfer of H⁺ from the coordinated dihydrogen to the amine formed in (1). With CpRu(dppe)H the principal Ru species during catalysis remains the hydride complex, and H₂ pressure has no effect on either the ee or the turnover frequency. Step (1), H^- transfer, can be carried out stoichiometrically if the H₂ is replaced by a coordinating solvent. A methyl substituent on the Cp ring decreases the H ^- transfer rate and the turnover frequency slightly. Electron-donating substituents on the phosphine increase the H^- transfer rate and increase the turnover frequency up to a point: eventually the hydride ligand (i.e., the one in Cp*Ru(dmpe)H) becomes sufficiently basic to deprotonate the iminium cation to the corresponding enamine, and this pre-equilibrium competes with H^- transfer. Ionic hydrogenation of enamines is possible when a Ru(H₂) cation (i.e., [CpRu(dppm) (η²-H₂)]⁺) is used as the catalyst and the enamine is more basic than the product amine. Ionic hydrogenation of an α/β-unsaturated iminium cation saturates both the C=C and the C=N bonds. A C=N bond is more reactive toward ionic hydrogenation than a C=C one, but in some cases (i.e., CH=CH₂) the latter may compete with H ₂ for a coordination site and decrease the turnover frequency.

Full text of DOI:10.1021/ja0506861

Published on Web 05/07/2005
Ruthenium-Catalyzed Ionic Hydrogenation of Iminium Cations.
Scope and Mechanism
Hairong Guan, Masanori Iimura, Matthew P. Magee, Jack R. Norton,* and
Guang Zhu
Contribution from the Department of Chemistry, Columbia UniVersity,
New York, New York 10027
Received February 2, 2005; E-mail: jrn11@columbia.edu
Abstract: Catalysis by CpRu(P-P)H (where P-P is a chelating diphosphine) of the ionic hydrogenation
of an iminium cation inolves (1) the transfer of H- to form an amine, (2) the coordination of H2 to the
resulting Ru cation, and (3) the transfer of H+ from the coordinated dihydrogen to the amine formed in (1).
With CpRu(dppe)H the principal Ru species during catalysis remains the hydride complex, and H2 pressure
has no effect on either the ee or the turnover frequency. Step (1), H- transfer, can be carried out
stoichiometrically if the H2 is replaced by a coordinating solvent. A methyl substituent on the Cp ring
decreases the H- transfer rate and the turnover frequency slightly. Electron-donating substituents on the
phosphine increase the H- transfer rate and increase the turnover frequency up to a point: eventually the
hydride ligand (i.e., the one in Cp*Ru(dmpe)H) becomes sufficiently basic to deprotonate the iminium cation
to the corresponding enamine, and this pre-equilibrium competes with H- transfer. Ionic hydrogenation of
enamines is possible when a Ru(H2) cation (i.e., [CpRu(dppm)(η2-H2)]+) is used as the catalyst and the
enamine is more basic than the product amine. Ionic hydrogenation of an R,â-unsaturated iminium cation
saturates both the CdC and the CdN bonds. A CdN bond is more reactive toward ionic hydrogenation
than a CdC one, but in some cases (i.e., CHdCH2) the latter may compete with H2 for a coordination site
and decrease the turnover frequency.
Introduction
Such an ionic hydrogenation mimics biological reductions:
it cleaves H2 heterolytically and adds H- and H+ separately to
the substrates. Coordination of the substrate is not required prior
to hydride transfer; the hydride ligand is transferred intermo-
lecularly to the substrate from the ruthenium. Hydrogenation
by an ionic mechanism potentially offers advantages, such as
(1) compatibility with ionizing solVents such as water6 and
alcohols and (2) selectiVity for polar CdX double bonds oVer
CdC double bonds. Noyori has emphasized the need for
“preferential hydrogenation of a carbonyl function over olefinic
and acetylenic bonds” and has noted that “currently available
hydrogenation catalysts, either homogeneous or heterogeneous,
are mostly selective for CdC functions over CdO bonds”,7
although he has reported a catalyst system (RuCl2L3/en/KOH)
that reverses that selectivity.8
The homogeneous hydrogenation of CdN double bonds
remains a challenge to catalysis, particularly when enantioface
selectivity is needed.1-4 Slight modifications in the substrate
imine can have a substantial effect on the activity of various
catalysts,2c,4 or even destroy them.2c In a preliminary com-
munication, we have reported5 that CpRu(P-P)H can catalyze
the hydrogenation of iminium cations by an ionic mechanism;
we have reported ee’s up to 60% with chiral diphosphines.
(1) Reviews on the asymmetric hydrogenation of CdN: (a) Blaser, H.-U.;
Spindler, F. Hydrogenation of Imine Groups. In ComprehensiVe Asymmetric
Catalysis; Jacobsen, E. N., Pfaltz, A., Yamamoto, H., Eds.; Springer-
Verlag: New York, 1999; Vol. 1, pp 247-265. (b) Ohkuma, T.; Noyori,
R. Hydrogenation of Imino Groups. In ComprehensiVe Asymmetric
Catalysis, Supplement; Jacobsen, E. N., Pfaltz, A., Yamamoto, H., Eds.;
Springer: Berlin, 2004; Vol. 1, pp 43-53. (c) Kobayashi, S.; Ishitani, H.
Chem. ReV. 1999, 99, 1069-1094. (d) Tang, W.; Zhang, X. Chem. ReV.
2003, 103, 3029-3069.
Ionic hydrogenation has been known for some time as a
stoichiometric process with a silane, or a transition-metal
hydride, serving as a source of the H-.9 We have reported the
(2) A chiral titanocene catalyst that gives high ee’s only for the hydrogenation
of cyclic imines: (a) Willoughby, C. A.; Buchwald, S. L. J. Am. Chem.
Soc. 1992, 114, 7562-7564. (b) Willoughby, C. A.; Buchwald, S. L. J.
Org. Chem. 1993, 58, 7627-7629. (c) Willoughby, C. A.; Buchwald, S.
L. J. Am. Chem. Soc. 1994, 116, 8952-8965. (d) Willoughby, C. A.;
Buchwald, S. L. J. Am. Chem. Soc. 1994, 116, 11703-11714.
(3) An Ru catalyst for the transfer of hydrogen from formic acid that also
gives high ee’s only for the hydrogenation of cyclic imines: (a) Uematsu,
N.; Fujii, A.; Hashiguchi, S.; Ikariya, T.; Noyori, R. J. Am. Chem. Soc.
1996, 118, 4916-4917. (b) Noyori, R.; Hashiguchi, S. Acc. Chem. Res.
1997, 30, 97-102.
(6) A water-soluble ruthenium hydride, CpRu(PTA)2H (PTA ) 1,3,5-triaza-
7-phosphaadamantane), has just been isolated and characterized: (a) Frost,
B. J.; Mebi, C. A. Organometallics 2004, 23, 5317-5323. The same hydride
has also been observed in solution: (b) Akbayeva, D. N.; Gonsalvi, L.;
Oberhauser, W.; Peruzzini, M.; Vizza, F.; Bru¨ggeller, P.; Romerosa, A.;
Sava, G.; Bergamo, A. Chem. Commun. 2003, 264-265. Such hydrides
are potential catalysts for the ionic hydrogenation of polar double bonds
in water.
(4) An Ir catalyst that gives high ee’s with acyclic imines that have bulky
substituents on N: (a) Zhu, G.; Zhang, X. Tetrahedron: Asymmetry 1998,
9, 2415-2418. (b) Xiao, D.; Zhang, X. Angew. Chem., Int. Ed. 2001, 40,
3425-3428.
(7) (a) Noyori, R. Acta Chem. Scand. 1996, 50, 380-390. (b) Noyori, R.;
Ohkuma, T. Pure Appl. Chem. 1999, 71, 1493-1501.
(8) Ohkuma, T.; Ooka, H.; Ikariya, T.; Noyori, R. J. Am. Chem. Soc. 1995,
117, 10417-10418.
(5) Magee, M. P.; Norton, J. R. J. Am. Chem. Soc. 2001, 123, 1778-1779.
9
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A R T I C L E S
Guan et al.
example in eq 1 with the Bullock group,10 and the example in
eq 2 with the Tilset group.11
system in eq 4, Ba¨ckvall and co-workers have catalyzed the
transfer hydrogenation (with the H+ and H- coming from
propan-2-ol) of an imine by an ionic mechanism; attempts to
isolate a secondary amine complex were unsuccessful, although
the primary amine complex 4 was isolated and shown to be an
efficient catalyst for the transfer hydrogenation of imines.15
Neither the Casey group nor the Ba¨ckvall group has catalyzed
an asymmetric hydrogenation with the 1/2 system, which does
not lend itself to the incorporation of chiral ligands.
Bullock and co-workers have reported the catalytic (although
not the asymmetric) ionic hydrogenation of ketones with
[CpM(CO)2(PR3)(ketone)]+A-, where M ) Mo or W, R ) CH3,
Ph, or Cy, and A- ) PF6-, BF4-, or BAr′4 [Ar′ ) 3,5-bis-
-
(trifluoromethyl)phenyl].12
Morris has observed H+/H- transfer from RuH2(chiral
diphosphine)(diamine) to a substrate ketone (eq 5), regeneration
of the starting material with H2, and thus a catalytic cycle for
the asymmetric hydrogenation of ketones by an ionic mecha-
nism.16 Because the catalyst in eq 5 is closely related to those
of Noyori,3b,17 it is possible that the latter catalysts also operate
by ionic mechanisms, with the necessary H+ and H- coming
either from H2 or from an alcohol (transfer hydrogenation).18
The Casey group has shown that the system in eq 3, derived
from the one reported by Shvo,13 can catalyze the ionic
hydrogenation of PhCHO; 1a can be regenerated from 2a and
H2. They have reported isotope effects suggesting that the proton
and the hydride are transferred simultaneously to the benzal-
dehyde in eq 3.14a
The Casey group has also reported transfer of hydrogen from
1a to an imine to give 3, but have not reported catalysis (the
amine complex 3 is stable).14 With the closely related 1b/2b
We now report the scope of the ionic hydrogenation of
iminium cations by CpRu(P-P)H and related catalysts. We have
(9) With silanes as the H- donors: (a) Kursanov, D. N.; Parnes, Z. N.; Loim,
N. M. Synthesis 1974, 633-651. With transition-metal hydrides as the H-
donors: (b) Gaus, P. L.; Kao, S. C.; Youngdahl, K.; Darensbourg, M. Y.
J. Am. Chem. Soc. 1985, 107, 2428-2434. (c) Gibson, D. H.; El-Omrani,
Y. S. Organometallics 1985, 4, 1473-1475. (d) Geraty, S. M.; Harkin, P.;
Vos, J. G. Inorg. Chim. Acta 1987, 131, 217-220. (e) Kelly, J. M.; Vos,
J. G. J. Chem. Soc., Dalton Trans. 1986, 1045-1048. (f) Bullock, R. M.;
Rappoli, B. J. J. Chem. Soc., Chem. Commun. 1989, 1447-1448. (g)
Bullock, R. M.; Song, J.-S. J. Am. Chem. Soc. 1994, 116, 8602-8612. (h)
Bakhmutov, V. I.; Vorontsov, E. V.; Antonov, D. Y. Inorg. Chim. Acta
1998, 278, 122-126. (i) Minato, M.; Fujiwara, Y.; Ito, T. Chem. Lett. 1995,
647-648. (j) Minato, M.; Fujiwara, Y.; Koga, M.; Matsumoto, N.;
Kurishima, S.; Natori, M.; Sekizuka, N.; Yoshioka, K.-i.; Ito, T. J.
Organomet. Chem. 1998, 569, 139-145.
(12) (a) Bullock, R. M.; Voges, M. H. J. Am. Chem. Soc. 2000, 122, 12594-
12595. (b) Voges, M. H.; Bullock, R. M. J. Chem. Soc., Dalton Trans.
2002, 759-770. (c) Bullock, R. M. Chem.-Eur. J. 2004, 10, 2366-2374.
(13) Shvo, Y.; Czarkie, D.; Rahamim, Y.; Chodosh, D. F. J. Am. Chem. Soc.
1986, 108, 7400-7402.
(14) (a) Casey, C. P.; Singer, S. W.; Powell, D. R.; Hayashi, R. K.; Kavana, M.
J. Am. Chem. Soc. 2001, 123, 1090-1100. (b) Casey, C. P.; Johnson, J. B
J. Am. Chem. Soc. 2005, 127, 1883-1894.
(15) (a) Samec, J. S. M.; Ba¨ckvall, J.-E. Chem.-Eur. J. 2002, 8, 2955-2961.
(b) Samec, J. S. M.; EÄ ll, A. H.; Ba¨ckvall, J.-E. Chem. Commun. 2004,
2748-2749.
(16) (a) Abdur-Rashid, K.; Lough, A. J.; Morris, R. H. Organometallics 2000,
19, 2655-2657. (b) Abdur-Rashid, K.; Faatz, M.; Lough, A. J.; Morris, R.
H. J. Am. Chem. Soc. 2001, 123, 7473-7474. (c) Abdur-Rashid, K.;
Clapham, S. E.; Hadzovic, A.; Harvey, J. N.; Lough, A. J.; Morris, R. H.
J. Am. Chem. Soc. 2002, 124, 15104-15118. (d) Rautenstrauch, V.; Hoang-
Cong, X.; Churlaud, R.; Abdur-Rashid, K.; Morris, R. H. Chem.-Eur. J.
2003, 9, 4954-4967.
(10) Song, J.-S.; Szalda, D. J.; Bullock, R. M.; Lawrie, C. J. C.; Rodkin, M. A.;
Norton, J. R. Angew. Chem., Int. Ed. Engl. 1992, 31, 1233-1235.
(11) Smith, K.-T.; Norton, J. R.; Tilset, M. Organometallics 1996, 15, 4515-
4520.
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Ru-Catalyzed Ionic Hydrogenation of Iminium Cations
A R T I C L E S
Scheme 1
compared the effectiveness of various ruthenium hydrides in
catalyzing this reaction, and we have explored its ability to
hydrogenate various types of iminium cations as well as its
selectivity for CdN hydrogenation over other reactions. Details
of the enantioface selectivity that can be achieved in such
hydrogenations will be reported separately.
Results and Discussion
“Stoichiometric” Hydride Transfer from LRu(dppe)H (L
) Cp (or C5H5), Cp′ (or C5H4Me), and Cp* (or C5Me5)) to
an Iminium Cation. Both the turnover and the enantioselec-
tivity of the catalytic hydrogenations in eqs 6 and 7 are
unaffected by the hydrogen pressure, implying that H2 uptake
is a relatively fast step in the catalytic cycle (see Scheme 1) at
any pressure. The transfer of an H+ to the product amine must
also be relatively fast,19,20 as 1H NMR observation of a catalytic
reaction shows most (>80 mol %) of the ruthenium to be CpRu-
(dppe)H. Thus, the hydride transfer step determines the enan-
tioselectivity and turnover frequency (TOF) of a catalytic
hydrogenation by an ionic mechanism.5
(5a) to the iminium cation 6 is independent of [CH3CN], but is
first-order in both the ruthenium hydride and the iminium cation
(eq 9).21
We have also reported that the ring size formed by the
chelating diphosphine affects the rate of hydride transfer: the
smaller the chelate ring size, the faster the transfer (CpRu-
(dppm)H > CpRu(dppe)H ≈ CpRu(dpbz)H > CpRu(dppp)H
. CpRu(dppb)H).21 The energy of the LUMO of the coordi-
natively unsaturated, 16-electron, cation CpRu(P-P)+ (which
remains after H- transfer) decreases as its chelate ring becomes
larger, making it a better H- acceptor and decreasing the rate
of H- transfer away from CpRu(P-P)H.
Stoichiometric H- transfer reactions such as the ones in eqs
8 and 9 are convenient for comparing different Ru hydride
complexes (e.g., those with different substituents on the cyclo-
pentadienyl ring). When we treated the cation 6 with Cp′Ru-
(19) Rate constants for the isomerization of dihydrogen complexes to the
corresponding dihydrides have been reported by Chinn and Heinekey for
related [CpRu(P-P)(η2-H2)]BF4.20 From their data at room temperature
with other P-P, the first-order rate constant for [CpRu(dppe)(η2-H2)]BF4
f [CpRu(dppe)(H)2]BF4 can be estimated as 10-2 s-1. At the same
temperature, the first-order rate constant for the disappearance of CpRu-
(dppe)H by H- transfer is 10-3 s-1 under catalytic conditions, suggesting
(because the η2-H2 cation never reaches an observable concentration) that
the rate constant for deprotonation of the η2-H2 cation is at least 10-1 s-1
and that deprotonation of that cation is probably faster than its isomerization
to the Ru(H)2 cation.
(20) Chinn, M. S.; Heinekey, D. M. J. Am. Chem. Soc. 1990, 112, 5166-5175.
(21) Guan, H.; Iimura, M.; Magee, M. P.; Norton, J. R.; Janak, K. E.
Organometallics 2003, 22, 4084-4089. Abbreviations: dppm ) bis-
(diphenylphosphino)methane, dppe ) 1,2-bis(diphenylphosphino)ethane,
dpbz ) 1,2-bis(diphenylphosphino)benzene, dppp ) 1,3-bis(diphenylphos-
phino)propane, dppb ) 1,4-bis(diphenylphosphino)butane.
We can thus screen catalysts by examining the rate of the
hydride transfer step when H2 is not present. To go to
completion, such a “stoichiometric” hydride transfer (eq 8)
requires that a coordinating ligand, such as CH3CN, fill the
vacant coordination site that remains after H- transfer. We have
already reported that the rate of H- transfer from CpRu(dppe)H
(17) (a) Haack, K.-J.; Hashiguchi, S.; Fujii, A.; Ikariya, T.; Noyori, R. Angew.
Chem., Int. Ed. Engl. 1997, 36, 285-288. (b) Noyori, R.; Ohkuma, T.
Angew. Chem., Int. Ed. 2001, 40, 40-73.
(18) (a) Aranyos, A.; Csjernyik, G.; Szabo´, K. J.; Ba¨ckvall, J.-E. Chem. Commun.
1999, 351-352. (b) Laxmi, Y. R. S.; Ba¨ckvall, J.-E. Chem. Commun. 2000,
611-612.
9
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A R T I C L E S
Guan et al.
Figure 1. Plot of kobs versus [6] for hydride transfer from 5b to the iminium
cation 6 at 300 K with CH3CN present.
Figure 2. Molecular structure of Cp′Ru(dppe)H (5b) (20% probability
level). Selected bond lengths (Å) and angle (deg): Ru-P1, 2.2248(6); Ru-
P2, 2.2277(7); Ru-H1, 1.60(3); P1-Ru-P2, 85.40(2).
(dppe)H (5b) in the presence of CH3CN, the free amine 8 and
[Cp′Ru(dppe)(CH3CN)]BF4 (7b) were formed (eq 10). Monitor-
ing the disappearance of 5b by H NMR in the presence of
1
excess 6 (10 equiv) gave a pseudo-first-order rate constant kobs
.
Variation of [6] from 0.12 to 0.25 M (Figure 1) showed a linear
relationship between kobs and [6], confirming that this H-
transfer is also a second-order reaction.
The slope of Figure 1 implies a value of 8.9(1) × 10-3 M-1
s-1 for the rate constant k2. Comparison with the analogous rate
constant k1 for 5a (2.0(1) × 10-2 M-1 s-1 21
)
shows that the
addition of one methyl group to the cyclopentadienyl ring
decreases the rate of H- transfer by a factor of 2; apparently
the steric effect of the methyl group outweighs its electronic
effect.
Figure 3. Molecular structure of Cp*Ru(dppe)H (5c) (20% probability
level). Selected bond lengths (Å) and angle (deg): Ru-P1, 2.2363(6); Ru-
P2, 2.2291(6); Ru-H1, 1.59(2); P1-Ru-P2, 83.73(2).
able in solution to interfere slightly with the approach of
substrate.
Effect of Additional Methyl Substituents on the Rate of
Hydride Transfer. The work of Bullock and co-workers has
shown that, with [Ph3C]BF4 as the hydride acceptor, H- transfer
is about 20 times faster for Cp*W(CO)3H than for CpW(CO)3H,
and about 20 times faster for Cp*Mo(CO)3H than for CpMo-
(CO)3H.24 Apparently the electronic effect of the methyl
substituents on the Cp ring is dominant.
Structural Effects of Methyl Substituents on the Cyclo-
pentadienyl Ring. Structures of Cp′Ru(dppe)H (5b) and
Cp*Ru(dppe)H (5c). The solid-state structures of 5b and 5c
have been determined by X-ray crystallography; thermal el-
lipsoid plots are shown in Figures 2 and 3, respectively. The
structure of 5a has been reported previously.21
However, with the iminum cation 6 as the hydride acceptor,
we have found that complete methyl substitution can suppress
hydride transfer in favor of side reactions. A solution of 6, 5c,
and CH3CN in CD2Cl2 showed negligible H- transfer to 6 (<2
1
mol % by H NMR) after 24 h. However, a new triplet with
JP-H ) 28.8 Hz appeared in the hydride region alongside the
signal of 5c. The chemical shift of this new triplet was consistent
with [Cp*Ru(dppe)(H)2]BF4,25 suggesting the proton-transfer
equilibrium in eq 11.26 However, overlap with the resonances
of 6 precluded identification of the enamine peaks, and further
complications arose from the reaction of 5c with the solvent
The methyl substituents have little effect on the P-Ru-P
bite angle, which is 84.50° in 5a, 85.40° in 5b, and 83.73° in
5c, or on the angle between the Cp ring and the Ru-P-P
plane,6,22,23 which is 69.7° in 5a, 71.0° in 5b, and 71.3° in
5c. The methyl substituent of 5b, which in the crystal
(Figure 2) lies over a phenyl ring of the diphosphine, must be
(24) (a) Cheng, T.-Y.; Bullock, R. M. Organometallics 1995, 14, 4031-4033.
(b) Cheng, T.-Y.; Brunschwig, B. S.; Bullock, R. M. J. Am. Chem. Soc.
1998, 120, 13121-13137.
(22) The utility of this interplanar angle as a way of assessing the geometry of
such hydride complexes has been pointed out by Lemke and Brammer,23
and Frost and Mebi have noted6 that it (the angle between the Cp ring and
the Ru-P-P plane) decreases with increasing P-Ru-P bite angle within
the CpRu(P-P)H series in ref 21.
(25) Jia, G.; Ng, W. S.; Chu, H. S.; Wong, W.-T.; Yu, N.-T.; Williams, I. D.
Organometallics 1999, 18, 3597-3602.
(26) We were not able to determine the equilibrium constant accurately because
of the side reaction, but a 1:1 mixture of 5c and 6 showed about 20 mol %
[Cp*Ru(dppe)(H)2]BF4 after 24 h.
(23) Lemke, F. R.; Brammer, L. Organometallics 1995, 14, 3980-3987.
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Ru-Catalyzed Ionic Hydrogenation of Iminium Cations
A R T I C L E S
Table 1. pKa Determination of Ruthenium Dihydrogen and Dihydride Complexes at Room Temperature
+
a
RuH
BH+
K1
pKa (Ru(
η
2-H2)+)a
K2
pKa (Ru(H)2 )
Cp′Ru(dppe)H (5b)
Cp*Ru(dppe)H (5c)
CpRu(dpbz)H (5e)
Cp*Ru(dmpe)H (5f)
(Ind)Ru(dppm)H (5g)
(Ind)Ru(dppe)H (5h)
[CpRu(dppm)(η2-H2)]BF4
[Cp*Ru(PPh3)2(H)2] BF4
[CpRu(dppm)(η2-H2)]BF4
[Cp*Ru(PPhMe2)2(H)2]BPh4
[CpRu(dppm)(η2-H2)]BF4
[CpRu(dppm)(η2-H2)]BF4
2.49
7.5
6.3
10.8
8.1
11.0
6.0
0.75b
0.15
0.080
4.30b,c
14.9
>0.14
>0.060
>6.2
>5.9
>0.036
>5.7
a Aqueous pKa. b Equilibrium was established in THF-d8. c For the reverse reaction, [Cp*Ru(dmpe)(H)2]BF4 was used, instead of [Cp*Ru(dmpe)(H)2]BPh4.
CD2Cl2, forming Cp*Ru(dppe)Cl.27
well as CpRu(dppm)H (5d); equilibrium was achieved within
1 h (as monitored by H NMR). The same equilibrium was
1
approached from the reverse direction by treating 5d with a
mixture of Cp′Ru(dppe)(η2-H2)]BF4 and Cp′Ru(dppe)(H)2]BF4
(prepared by the direct protonation of 5b with HBF4‚Me2O).
The equilibrium constants and pKa values (given in Table 1)
were then calculated from eqs 13-16 (in this case BH+
)
[CpRu(dppm)(η2-H2)]BF4, and pKa (BH+) ) 7.128). The dif-
ference between the pKa values for Cp′Ru(dppe)(η2-H2)]+ and
Cp′Ru(dppe)(H)2]+ in Table 1 (a ∆pKa of 0.6, implying a K2/
K1 of 4.3) is consistent with the observed equilibrium ratio (3.9)
of the dihydride to the dihydrogen complex.
In an effort to find a less ambiguous example of eq 11, we
treated Cp*Ru(dmpe)H(5f) (dmpe ) 1,2-bis(dimethylphosphi-
no)ethane) (which only reacts very slowly with CD2Cl2) with
the iminium cation 6 in the presence of CH3CN, and indeed
we did observe complete deprotonation of the iminum cation
to the enamine (eq 12). We also found, however, that complete
H- transfer eventually occurred despite the presence of the
unfavorable pre-equilibrium.
[Ru(η2-H2)+][B]
[RuH][BH+]
K1 )
(13)
(14)
+
[Ru(H)2 ][B]
[RuH][BH+]
K2 )
pKa[Ru(η2-H2)+] ) pKa(BH+) + log K1
(15)
(16)
pKa[Ru(H)2 ] ) pKa(BH+) + log K2
+
Similar results (the pKa values of the resulting dihydrogen/
dihydride cations are also given in Table 1) were obtained when
we treated CpRu(dpbz)H (5e) with [CpRu(dppm)(η2-H2)]BF4.
However, [CpRu(dppm)(η2-H2)]BF4 completely protonated 5c
and 5f, and therefore less acidic RuH2 cations (the established28
pKa of [Cp*Ru(PPh3)2(H)2]BF4 is 11.1, and that of [Cp*Ru-
(PPhMe2)2(H)2]BPh4 is 14.3) were used to determine the pKa
values (Table 1) of [Cp*Ru(dppe)(H)2]+ and [Cp*Ru(dmpe)-
(H)2]+.
Equilibrium Constants for Deprotonation of Iminium
Cations by Electron-Rich Ru Hydride Complexes. Jia and
Morris28 have estimated the aqueous pKa values of many
ruthenium dihydrogen and/or dihydride cations containing
Cp*Ru(P-P) or CpRu(P-P) by comparing their relative acidi-
ties in THF or CH2Cl2 with those of “secondary standard” acids
with established aqueous pKa values; as secondary standards
they used protonated tertiary phosphines R3PH+ and other
dihydrogen/dihydride cations. Aqueous pKa values for most of
the ruthenium dihydrogen/dihydride complexes in the present
study are available from their compilation.28
We attempted to determine the pKa values of the related
indenyl complexes (Ind)Ru(dppe)H(5h) (Ind ) indenyl) and
(Ind)Ru(dppm)H (5g). Protonation of 5h by CF3SO3H or HBF4‚
Et2O at -60 °C in THF-d8 has been reported by Lau, Jia, and
co-workers to give a mixture of [(Ind)Ru(dppe)(η2-H2)]+ and
[(Ind)Ru(dppe)(H)2]+,29 and by 1H NMR we obtained a mixture
of the same cations (along with 5d) when 5h was treated with
[CpRu(dppm)(η2-H2)]BF4 in CD2Cl2 at room temperature.
However, we were unable to isolate [(Ind)Ru(dppe)(η2-H2)]BF4
and [(Ind)Ru(dppe)(H)2]BF4 and were thus unable to approach
the equilibrium in eq 17 from the right. Furthermore, even when
When we treated 5b with [CpRu(dppm)(η2-H2)]BF4 in CD2-
Cl2 at room temperature, a mixture of Cp′Ru(dppe)(η2-H2)]BF4
and Cp′Ru(dppe)(H)2]BF4 (ratio 1:3.9, distinguished by a broad
singlet versus a triplet for the hydride signals) was formed, as
(27) The reaction of 5c with CD2Cl2 was much slower than that previously
reported21 for CpRu(dppp)H (which reacts with CD2Cl2 in minutes). In
one case, 46 mol % of 5c ([5c]0 ) 6 × 10-3 M in CD2Cl2) was converted
to Cp*Ru(dppe)Cl after 24 h. As before,21 treatment with 1% (by weight)
TEMPO (TEMPO ) 2,2,6,6-tetramethyl-1-piperidinyloxy) stopped the
reaction.
(28) (a) Jia, G.; Morris, R. H. Inorg. Chem. 1990, 29, 581-582. (b) Jia, G.;
(29) Hung, M. Y.; Ng, S. M.; Zhou, Z.; Lau, C. P.; Jia, G. Organometallics
2000, 19, 3692-3699.
Morris, R. H. J. Am. Chem. Soc. 1991, 113, 875-883.
9
J. AM. CHEM. SOC. VOL. 127, NO. 21, 2005 7809

A R T I C L E S
Guan et al.
Table 2. Relative Catalytic Activity of Various Ruthenium Hydrides for Hydrogenationh
1
initial TOF (h-
)
+
c
catalyst
exptl
calcda
time (h)
conversion (%)
yieldb (%)
pKa of Ru(H)2
CpRu(dppe)H (5a)
8.9
(13.7)
40
50
70
70
40
2
40
95
3
95
95
74
84
<1
98
99
<5
97
63
99
2
81
83
51
70
-
84
83
-
84
40
86
-
7.3 (7.0)d
7.9e
2.4f
4.0
Cp′Ru(dppe)H (5b)
Cp*Ru(dppe)H (5c)e
CpRu(dppm)H (5d)
CpRu(dpbz)H (5e)
Cp*Ru(dmpe)H (5f)e
(Ind)Ru(dppm)H (5g)
(Ind)Ru(dppe)H (5h)
CpRu(dmpe)H (5i)
CpRu(PPh3)2H (5j)
(6.1)
8.1 (7.5)
11.0
<0.025g
297
(437)
(14.4)
(7.1)d
9.1
6.0 (6.3)
14.9
<0.05g
153
(>6.2)
0.75
579
0.04
95
2
50
>5.7 (>5.9)
(9.8)d
8.3d
a The predicted initial TOF was calculated from the rate constants of the H- transfer at 300 K (k2 for 5b, ref 21 for others). b Isolated yield. c Estimated
aqueous pKa of Ru(η2-H2)+ is listed in the parenthesis. d From ref 28. e CH3OH was used as the solvent. f Acetone was used as the solvent. g Other unidentified
products were present. h Reaction conditions: [6] ) 0.2 mol/L, [catalyst] ) 0.002 mol/L, H2 pressure 50 psi, in CH2Cl2, T ) 298 K.
we approached the equilibrium from the left, decomposition
began (after 30 min) before it was established; the extent of
product formation we observed thus gave a lower limit to the
equilibrium constant for eq 17 and implied a pKa > 5.9 for
[(Ind)Ru(dppe)(η2-H2)]BF4 and a pKa > 5.7 for (Ind)Ru(dppe)-
(H)2BF4. Similar experiments gave a pKa > 6.2 for [(Ind)Ru-
(dppm)(η2-H2)]BF4. All three pKa values are shown in Table 1.
CD2Cl2
(Ind)Ru(dppe)H + [CpRu(dppm)(η2-H2)]BF4 y
z
5h
[(Ind)Ru(dppe)(η2-H2)]BF4 + [(Ind)Ru(dppe)(H)2]BF4 +
CpRu(dppm)H (17)
5d
factor of about two, similar to the EIEs observed for Cp2W-
(D)CH3/Cp2W(H)CH2D31 and related exchanges.32
When we generated [(Ind)Ru(dppe)(η2-H2)]BF4 and [(Ind)-
Ru(dppe)(H)2]BF4 by eq 17, we did not reproduce one observa-
tion of Lau, Jia, and co-workers.29 Upon warming to room
temperature, their dihydrogen and dihydride cations (generated
from CF3SO3H or HBF4‚Et2O at -60 °C) isomerized to an η6-
indene complex, [(η6-C9H8)Ru(dppe)H]+. We did not observe
the formation of the η6-indene complex when we carried out
eq 17 at room temperature, suggesting that the isomerization
of [(Ind)Ru(dppe)(η2-H2)]BF4 and/or [(Ind)Ru(dppe)(H)2]BF4
to the η6-indene complex is probably not intramolecular as
proposed.29
Activity of Various Catalysts for Hydrogenation of the
Iminium Cation 6. To compare activities, reaction 19 was
performed under 50 psi H2 pressure with 1 mol % of various
ruthenium catalysts; the solvent used was CH2Cl2 (or CH3OH
if the ruthenium hydride proved unstable in CH2Cl2). The initial
TOF was determined when less than 10% of the 6 had been
hydrogenated. The results are given in Table 2.
H/D Exchange between the Iminium Cation 6 and CpRu-
(dppe)D. When equimolar amounts of iminium cation 6 and
2
CpRu(dppe)D were mixed in CH2Cl2 at room temperature, H
NMR showed a rapid H/D exchange between two reagents, with
the formation of deuterated 6 and a decrease in the amount of
CpRu(dppe)D (Figure S-2 in Supporting Information); 1H NMR
in CD2Cl2 confirmed the formation of CpRu(dppe)H(5a). Over
time, the amine product 8 was formed smoothly by H- transfer.
Presumably 6 transfers H+ to the CpRu(dppe)D,30 giving the
enamine and [CpRu(dppe)(η2-HD)]BF4 (which may not be in
equilibrium with [CpRu(dppe)(H)(D)]BF419); transfer of D+
back to the enamine carbon finishes the exchange shown in
eq 18.
The slow H- transfer made it difficult to measure the
equilibrium isotope effect (EIE) for the H/D exchange in eq 18
with high precision, but it appears to be about 6.1 (see
Supporting Information) at room temperature. This EIE reflects
the operation of a statistical factor of 3 and a zero-point energy
The TOF observed for 6 at the beginning of its catalytic
hydrogenation (the second column in Table 2) agrees well with
(30) The aqueous pKa values of related iminium cations have been compiled:
Acidity and Basicity of Enamines. In The Chemistry of Enamines;
Rappoport, Z., Ed.; Wiley & Sons: New York, 1994; p 709. The
comparable acidities of Cp*Ru(dppe)H (pKa ) 11.0, see Table 2) and
iminium 6 in eq 11 suggest that the pKa of 6 is about 11.
(31) Bullock, R. M.; Headford, C. E. L.; Hennessy, K. M.; Kegley, S. E.; Norton,
J. R. J. Am. Chem. Soc. 1989, 111, 3897-3908.
(32) (a) Churchill, D. G.; Janak, K. E.; Wittenberg, J. S.; Parkin, G. J. Am.
Chem. Soc. 2003, 125, 1403-1420. (b) Janak, K. E.; Churchill, D. G.;
Parkin, G. Chem. Commun. 2003, 22-23. (c) Janak, K. E.; Parkin, G. J.
Am. Chem. Soc. 2003, 125, 6889-6891.
9
7810 J. AM. CHEM. SOC. VOL. 127, NO. 21, 2005

Ru-Catalyzed Ionic Hydrogenation of Iminium Cations
A R T I C L E S
Table 3. Scope of Ionic Hydrogenation of Various Iminium
Cationsd
which TOF data were given in Table 2). We observed no
hydrogenation under standard conditions when the pyrrolidinium
ring in 6 was replaced with two ethyl substituents on the
iminium nitrogen (entry 3); however, stoichiometric H- transfer
from 5d to 13 was facile (eq 21), and catalytic hydrogenation
did occur when we increased the catalyst loading to 10 mol %.
At low catalyst loading the hydride 5d is probably protonated,
to [CpRu(dppm)(η2-H2)]BF4, by traces of acid remaining from
the preparation of the iminium cation 13; we observed the
formation of [CpRu(dppm)(η2-H2)]BF4 when we used an excess
(10 equiv) of the iminium cation 13 in eq 21. The iminium
cation 13 itself (aqueous pKa ≈ 1130) is not a strong enough
acid to protonate 5d.
a All entries other than 3 (1 mol %) and 5 have >95% conversion for
the given reaction time. b Isolated as the ammonium salt instead of the amine
product. c 6% conversion by 1H NMR suggested three turnovers. d Reaction
conditions: [iminium cation] ) 0.1 mol/L, H2 pressure 50 psi, room
temperature.
We would expect an ionic mechanism to hydrogenate CdN
double bonds selectively in preference to CdC double bonds,
and this result was observed stoichiometrically with the substrate
15 (eq 22 and entry 5, Table 3) and catalytically with substrates
15 and 16 (entry 6, Table 3), although the turnover with 16 far
exceeded that with 15. Apparently the terminal CdC double
bond in 15 competes effectively with H2 for the coordination
site remaining after an H- transfer,35,36 whereas the substituents
on the olefin in 16 prevent its coordination and promote catalytic
hydrogenation.
that calculated (in parentheses in the same column) from the
rate of the stoichiometric H- transfer, confirming that (as shown
in Scheme 1) the turnover-limiting step in the catalytic cycle is
the H- transfer step.33
With a Cp ligand, alkyl-substituted phosphines make hydro-
genation more facile; hydride transfer, and thus turnover,
decreases in the order CpRu(dmpe)H (5i) > 5a > CpRu-
(PPh3)2H (5j). However, such hydride complexes remain
sufficiently weak bases that their Ru(η2-H2)/Ru(H)2 cations,
which have aqueous pKa values (Table 1 and ref 28) in the range
of 5-10, are acidic enough to protonate the product amine34
(the “Proton Transfer” step in Scheme 1). Substitution of a Cp*
ligand makes the hydride ligand sufficiently basic that a proton
transfer like the one in eq 12 will decrease [RuH] and retard
hydride transfer and catalytic hydrogenation. The pKa of
[Cp*Ru(dmpe)(H)2]BF4 is 14.9, whereas that of 6 is about 11.30
Cp*Ru(dmpe)H is thus a very poor catalyst (TOF < 0.05 h-1),
even though the stoichiometric hydride transfer in eq 12 is
reasonably fast (complete in 10 h).
Ionic Hydrogenation of Other Iminium Cations. We have
examined the scope of ionic hydrogenation catalyzed by 5a and
5d (eq 20). The results are summarized in Table 3.
The hydrogenation of alkyl-alkyl disubstituted iminium
cations 11 and 12 was straightforward (entries 1 and 2 in Table
3), as was that of aryl-alkyl substituted ones such as 6 (for
When a CdC double bond was conjugated with a CdN
double bond, both were hydrogenated (entry 4 in Table 3).
Presumably a 1,4-hydride transfer (and protonation) is followed
by a 1,2-hydride transfer (and protonation) (Scheme 2).
The protonated imine 17 (entry 7) also underwent ionic
hydrogenation. Unprotonated imines were, as expected, unre-
active.
(35) Performing the same catalytic reaction with 20 mol % catalyst loading gave
upfield vinyl protons at δ 3.47, 3.02, and 1.42, which were similar to those
reported for other olefin complexes of CpRu cations.36
(36) (a) Davies, S. G.; Scott, F. J. Organomet. Chem. 1980, 188, C41-C42.
(b) Consiglio, G.; Fregosin, P.; Morandini, F. J. Organomet. Chem. 1986,
308, 345-351. (c) Consiglio, G.; Morandini, F. J. Organomet. Chem. 1986,
310, C66-C68. (d) Ohkita, K.; Kurosama, H.; Hirao, T.; Ikeda, I. J.
Organomet. Chem. 1994, 470, 189-190. (e) Ohkita, K.; Asano, H.;
Kurosama, H.; Hirao, T.; Miyaji, Y.; Ikeda, I. Can. J. Chem. 1996, 74,
1936-1944.
(33) The catalytic TOF values were measured at a temperature (298 K) two
degrees below that (300 K) at which the rate constants for hydride transfer
were determined, and thus the agreement between the rates of the catalytic
and stoichiometric reactions is even closer than the numbers in Table 2
suggest.
(34) The pKa of trialkylamines lies between 10 and 11: Smith, M. B., March,
J. Acids and Bases. In March’s AdVanced Organic Chemistry; 5th ed.;
Wiley & Sons: New York, 2001; p 330.
9
J. AM. CHEM. SOC. VOL. 127, NO. 21, 2005 7811

A R T I C L E S
Scheme 2
Guan et al.
Scheme 3
Catalytic Hydrogenation of an Enamine. The results in eqs
12 and 18, and the interpretation in Scheme 2 of the result in
entry 4 of Table 3, suggested that ionic hydrogenation of an
enamine to a neutral amine might be feasible with a dihydrogen
complex as the catalyst. The potential catalytic cycle is shown
in Scheme 3.
To carry out Scheme 3, the enamine must compete with the
product amine for a proton from the dihydrogen complex.
Known equilibrium basicities are consistent with this require-
ment: most enamines are more basic than their saturated
counterparts,37 although there are exceptions.38 Indeed, the
enamine 9 proved to deprotonate the ammonium cation 10,
although side reactions occurred as well (which prevented the
successful catalytic hydrogenation of 9).
rate of the slow step in the catalytic cycle, H- transfer, can be
measured directly by observing the rate at which the hydride
complex reacts with the iminium cation in the presence of a
coordinating solvent instead of H2sa “stoichiometric” reaction.
These stoichiometric H- transfers become faster as the chelate
ring in CpRu(P-P)H becomes smaller or as more electron-
donating substituents are introduced onto the phosphine. How-
ever, with sufficient electron density the hydride ligand becomes
more basic as well as more hydridic and is able to deprotonate
the substrate. As the deprotonation of an iminium cation to an
enamine occurs at a pKa of around 11, an Ru(H2) cation with a
pKa between 10 and 11 would be ideal. (CpRu(dmpe)H, with a
pKa of 9.8, is the best we have found to date.) The ionic
hydrogenation of enamines is possible, but complicated by the
competition between the enamine and the amine product for
the deprotonation of the Ru(H2) cation.
Ionic hydrogenation shows the expected preference for the
hydrogenation of CdN over the hydrogenation of CdC,
although a terminal CdC can block the coordination of another
H2 after H- transfer. With an R,â-unsaturated iminium cation
the initial H- transfer is 1,4, giving an enamine that is then
protonated and hydrogenated to an ammonium cation.
Enamine 25, on the other hand, cleanly deprotonated 10 in
eq 24.
Experimental Section
General Procedures. All air-sensitive compounds were prepared
and handled under a N2/Ar atmosphere using standard Schlenk and
inert-atmosphere box techniques. Et2O and CH2Cl2 were deoxygenated
and dried over two successive activated alumina columns under argon.
Benzene and THF-d8 were distilled from Na and benzophenone under
a nitrogen atmosphere. CD2Cl2 was dried over CaH2, degassed by
three freeze-pump-thaw cycles, and then purified by vacuum trans-
fer at room temperature. CpRu(dppe)H (5a),39 CpRu(dppe)D,36a
Cp*Ru(dppe)H (5c),40 CpRu(dppm)H (5d),39 CpRu(dpbz)H (5e),21
(Ind)Ru(dppm)H (5g),41 (Ind)Ru(dppe)H (5h),41 CpRu(dmpe)H (5i),20
CpRu(PPh3)2H (5j),42 [Cp*Ru(dppe)(H)2]BF4,25 Cp′Ru(dppe)Cl,43
Cp*Ru(dmpe)Cl,441-(1-phenylethylidene)pyrrolidinium tetrafluoroborate
(6),45 1-isopropylidenepyrrolidinium tetrafluoroborate (12),45 1-(R-
As we expected from this result, it proved possible to carry
out the ionic hydrogenation of 25, in 71% yield (eq 25), with
10 mol % [CpRu(dppm)(η2-H2)]BF4 as catalyst.
(37) (a) Adams, R.; Mahan, J. E. J. Am. Chem. Soc. 1942, 64, 2588-2593. (b)
Hinman, R. L. Tetrahedron 1968, 24, 185-190.
(38) Stamhuis, E. J.; Maas, W.; Wynberg, H. J. Org. Chem. 1965, 30, 2160-
2163.
(39) Bruce, M. I.; Humphrey, M. G.; Swincer, A. G.; Wallis, R. C. Aust. J.
Chem. 1984, 37, 1747-1755.
(40) Lee, D.-H. J. Korean Chem. Soc. 1992, 36, 248-254.
(41) Bassetti, M.; Casellato, P.; Gamasa, M. P.; Gimeno, J.; Gonza´lez-Bernardo,
C.; Mart´ın-Vaca, B. Organometallics 1997, 16, 5470-5477.
(42) Baird, G. J.; Davies, S. G.; Moon, S. D.; Simpson, S. J.; Jones, R. H. J.
Chem. Soc., Dalton Trans. 1985, 1479-1486.
Summary and Conclusions
The ionic hydrogenation of an iminium cation (Scheme 1)
involves (1) the coordination of H2, followed by (2) the transfer
of H+ to the amine product. Both processes are sufficiently fast
that their rate has little impact on the turnover frequency. The
(43) Alonso, A. G.; Revento´s, L. B. J. Organomet. Chem. 1988, 338, 249-
254.
(44) Luo, L.; Zhu, N.; Zhu, N.-J.; Stevens, E. D.; Nolan, S. P.; Fagan, P. J.
Organometallics 1994, 13, 669-675.
(45) Leonard, N. J.; Paukstelis, J. V. J. Org. Chem. 1963, 28, 3021-3024.
9
7812 J. AM. CHEM. SOC. VOL. 127, NO. 21, 2005

Ru-Catalyzed Ionic Hydrogenation of Iminium Cations
A R T I C L E S
Table 4. Summary of Crystallographic Data
[Cp*Ru(dmpe)(H)2]BF4. To a solution of Cp*Ru(dmpe)H (77.4 mg,
0.20 mmol) in 20 mL of Et2O was added the solution of HBF4‚OMe2
(100 µL, 0.72 mmol) in 20 mL of Et2O. A white precipitate was formed
immediately, and the resulting suspension was stirred for 30 min. The
solid was collected by filtration, washed with Et2O, and dried under
5b
5c
empirical formula
fw
C32H32P2Ru
579.59
C36H40P2Ru
635.69
243(2)
monoclinic
P21/c
15.7131(12)
15.8159(12)
12.8624(9)
90
101.421(2)
90
3133.2(4)
4
temp, K
cryst syst
space group
a, Å
b, Å
c, Å
R, deg
â, deg
γ, deg
V, Å3
243(2)
orthorhombic
Pna21
17.6272(10)
11.2434(6)
13.4570(7)
90
90
90
2667.0(2)
4
1.443
1
vacuum to give a white powder (75 mg, 79% yield). H NMR (400
MHz, CD2Cl2): δ -9.97 (t, Ru(H)2, JP-H ) 32.1 Hz, 2H), 1.57-1.60
(m, PMe, 12H), 1.71-1.74 (m, PCH2CH2P, 4H), 2.04 (s, C5Me5, 15H).
31P {1H} NMR (162 MHz, CD2Cl2): δ 41.84 (s). [Cp*Ru(dmpe)(H)2]-
BF4/[Cp*Ru(dmpe)(η2-H2)]BF4 > 50:1. Anal. Calcd for C16H33P2-
RuBF4: C, 40.44; H, 7.00. Found: C, 40.75; H, 7.09.
[CpRu(dpbz)H2]BF4 was prepared in 73% yield by a procedure
similar to that used for [Cp*Ru(dmpe)(H)2]BF4. [CpRu(dpbz)(H)2]BF4:
1H NMR (400 MHz, CD2Cl2): δ -7.96 (t, Ru(H)2, JP-H ) 28.4 Hz,
2H), 5.44 (s, Cp, 5H), 7.16-7.71 (m, Ar). 31P NMR (162 MHz, CD2-
Cl2): 67.2. [CpRu(dpbz)(η2-H2)]BF4: 1H NMR (400 MHz, CD2Cl2):
Z
d
calcd, g/cm3
1.348
λ (Mo KR), Å
0.71073
0.727
16 969
5174
5174/1/322
1.053
0.71073
0.625
21 053
µ, mm-1
δ -8.90 (bs, Ru(η2-H2), 2H), 4.95 (s, Cp, 5H), 7.16-7.71 (m, Ar). 31
P
No. of data collected
No. of unique data
No. of data/restraints/params
goodness-of-fit on F2
R1, wR2 (I > 2σ(I))
R1, wR2 (all data)
{1H} NMR (162 MHz, CD2Cl2): 78.7. [CpRu(dpbz)(H)2]BF4/[CpRu-
(dpbz)(η2-H2)]BF4 ) 1:1.8. Anal. Calcd for C35H31P2RuBF4: C, 59.93;
H, 4.45. Found: C, 58.78; H, 4.33 (the carbon analysis remained low
in repeated analyses).
7139
7139/0/357
1.036
0.0300, 0.0709
0.0499, 0.0759
0.0217, 0.0555
0.0232, 0.0563
[Cp′Ru(dppe)H2]BF4 was prepared in 74% yield by a procedure
similar to that used for [Cp*Ru(dmpe)(H)2]BF4. [Cp′Ru(dppe)(H)2]BF4:
1H NMR (400 MHz, CD2Cl2): δ -8.60 (t, Ru(H)2, JP-H ) 28.1 Hz,
2H), 1.60 (s, CH3, 3H), 2.45-2,50 (m, PCH2CH2P, 4H), 5.36 (bs, Cp,
2H), 5.40 (bs, Cp, 2H), 7.40-7.55 (m, Ar). 31P {1H} NMR (162 MHz,
CD2Cl2): 71.0. [Cp′Ru(dppe)(η2-H2)]BF4: 1H NMR (400 MHz, CD2-
Cl2): δ -8.84 (bs, Ru(η2-H2), 2H), 1.87 (s, CH3, 3H), 2.34-2.84 (m,
PCH2CH2P, 4H), 4.50 (bs, Cp, 2H), 4.95 (bs, Cp, 2H), 7.28-7.69 (m,
Ar). 31P {1H} NMR (162 MHz, CD2Cl2): 81.0. [Cp′Ru(dppe)(H)2]BF4/
[Cp′Ru(dppe)(η2-H2)]BF4 ) 3.9:1. Anal. Calcd for C32H33P2RuBF4: C,
57.59; H, 4.98. Found: C, 57.45; H, 4.92.
methylcinnamylidene)pyrrolidinium tetrafluoroborate (14),46 1-(1-pyr-
rolidinyl)-1-phenylethene (9),47 R-(diethylamino)styrene (25),47 and
N-(1-phenylethyl)pyrrolidine (8)48 were prepared as described in the
literature. Experimental details on eqs 6 and 7 were reported in the
Supporting Information of ref 5.
Cp′Ru(dppe)H (5b). Under a nitrogen atmosphere, sodium (142
mg, 6.17 mmol) was added to an orange suspension of Cp′Ru(dppe)Cl
(1.12 g, 1.82 mmol) in 40 mL of CH3OH. Boiling the resulting mixture
for 3 h gave a yellow suspension, from which (after cooling to room
temperature) the supernatant liquid was removed by cannula. The solid
was washed with CH3OH (3 × 20 mL) and then dried under vacuum,
leaving 580 mg (55% yield) of a yellow powder. 1H NMR (400 MHz,
C6D6): δ -13.25 (t, RuH, JP-H ) 34.4 Hz, 1H), 1.72 (s, CpMe, 3H),
1.86-1.92 (m, PCH2CH2P, 2H), 2.07-2.12 (m, PCH2CH2P, 2H), 4.65-
4.66 (m, CpMe, 2H), 4.73-4.74 (m, CpMe, 2H), 6.99-7.18 (m, Ar,
12H), 7.48-7.53 (m, Ar, 4H), 7.89-7.93 (m, Ar, 4H). 31P {1H} NMR
(162 MHz, C6D6): δ 94.10 (s). Anal. Calcd for C32H32P2Ru: C, 66.31;
H, 5.56. Found: C, 66.20; H, 5.64.
X-ray Structure Determinations of 5b and 5c. Crystals were grown
by letting a layer of CH3OH slowly diffuse into a saturated benzene
solution of the hydrides. Data collection and refinement parameters
are summarized in Table 4. Data were collected on a Bruker P4
diffractometer equipped with a SMART CCD detector. The structures
were solved using direct methods and standard difference map
techniques and refined by full-matrix least-squares procedures using
SHELXTL. Hydrogen atoms on carbon were included in calculated
positions.
Cp*Ru(dmpe)H (5f). Under a nitrogen atmosphere, sodium (110
mg, 4.78 mmol) was added to an orange suspension of Cp*Ru(dmpe)-
Cl (600 mg, 1.42 mmol) in 40 mL of CH3OH. The mixture was refluxed
for 3 h to give a bright yellow solution. The solvent was removed under
vacuum, and the residue was extracted with benzene (2 × 15 mL) to
give a yellow solution. The solvent was removed again under vacuum
to afford a yellow solid, which gave the pure product (365 mg, 66%
yield) after sublimation (100 °C at 0.01 mmHg). 1H NMR (300 MHz,
C6D6): δ -14.37 (t, RuH, JP-H ) 37.3 Hz, 1H), 1.08-1.15 (m, PMe,
6H), 1.12-1.24 (m, PCH2CH2P, 4H), 1.28-1.32 (m, PMe, 6H), 2.03
(s, C5Me5, 15H). 31P {1H} NMR (121 MHz, C6D6): δ 52.17 (s). Anal.
Calcd for C16H32P2Ru: C, 49.60; H, 8.32. Found: C, 49.77; H, 8.33.
Determination of Equilibrium Constants for Exchange of H+
between a Ruthenium Dihydrogen (and/or Dihydride) Cation and
a Neutral Hydride Complex. The dihydrogen (and/or dihydride) cation
and the neutral hydride complex were dissolved in CD2Cl2 or THF-d8
under N2. The resulting solution was transferred into a J-Young NMR
1
tube, and the approach to equilibrium was monitored by H NMR of
the hydride signals. (A delay of 5 s was inserted between the end of
each data acquisition and the next pulse; the observed ratios did not
change appreciably with a 10-s delay time.) The equilibrium was
established within 1 h for unhindered hydrides, in several hours for
more crowded hydrides.
N-(1-Cyclohexylethylidene)pyrrolidinium Tetrafluoroborate (11).
Pyrrolidinium tetrafluoroborate (3.18 g, 20 mmol), acetylcyclohexane
(5.04 g, 40 mmol), and two or three drops of morpholine were added
to 50 mL of benzene, and the resulting mixture was refluxed under
nitrogen as the water formed was collected in a Dean-Stark trap. When
the volume of the water reached 0.5 mL, the reaction mixture was
cooled to room temperature and the solvent was removed under vacuum.
The addition of 20 mL of Et2O caused the residue to solidify; the solid
was collected by filtration, washed with Et2O, and recystallized in CH2-
Cl2/Et2O to give colorless needles (4.18 g, 78% yield). 1H NMR (400
MHz, CDCl3): δ 1.21-1.43 (m, Cy, 5H), 1.74-1.94 (m, Cy, 5H),
2.17-2.20 (m, NCH2CH2, 4H), 2.35 (s, CH3, 3H), 2.70-2.77 (m, Nd
CCH, 1H), 3.96-3.98 (m, NCH2CH2, 4H). 13C {1H} NMR (100 MHz,
CDCl3): δ 20.09, 24.04, 24.49, 25.18, 25.42, 28.11, 46.79, 53.55, 54.94,
190.31. IR (in fluorolube): 1665 cm-1 (CdN). Anal. Calcd for C12H22-
BF4N: C, 53.96; H, 8.30; N, 5.24. Found: C, 53.87; H, 8.55; N, 5.27.
N-(1-Methylpent-4-enylidene)pyrrolidinium Tetrafluoroborate
1
(15) was prepared by a known procedure46 in 52% yield. H NMR
(400 MHz, CDCl3): 2.10-2.14 (m, NCH2CH2, 4H), 2.37 (s, NdCCH3,
3H), 2.34-2.43 (m, NdCCH2CH2, 2H), 2.70-2.74 (m, NdCCH2CH2,
2H), 3.87-3.92 (m, NCH2CH2, 4H), 5.02-5.10 (m, CdCH2, 2H),
5.72-5.82 (m, CH2dCH, 1H). 13C {1H} NMR (100 MHz, CDCl3): δ
23.03, 24.19, 24.45, 28.60, 37.39, 53.87, 54.52, 116.93, 134.74, 186.87.
(46) Yamaguchi, M.; Shiraishi, T.; Hirama, M. J. Org. Chem. 1996, 61, 3520-
3530.
(47) Lee, N. E.; Buchwald, S. L. J. Am. Chem. Soc. 1994, 116, 5985-5986.
(48) Shapiro, S. L.; Soloway, H.; Freedman, L. J. Am. Chem. Soc. 1958, 80,
6060-6064.
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A R T I C L E S
Guan et al.
IR (in fluorolube): 1676 cm-1 (CdN), 1643 cm-1 (CdC). Anal. Calcd
for C10H18BF4N: C, 50.24; H, 7.59; N, 5.86. Found: C, 50.31; H, 7.52;
N, 5.94.
data was reported. NMR data of 21: 1H NMR (400 MHz, CDCl3): δ
1.18 (d, CH3, JH-H ) 6.4 Hz, 3 H), 1.63-1.76 (m, NCHCH2, 1H),
1.78-1.85 (m, NCH2CH2, 4 H), 1.90-1.99 (m, NCHCH2, 1H), 2.38-
2.45 (m, CHCH3, 1H), 2.51-2.79 (m, PhCH2, 2H), 2.55-2.72 (m,
NCH2CH2, 4H), 7.14-7.30 (m, Ar, 5H). NMR data of 23: 1H NMR
(300 MHz, CDCl3): δ 1.08 (d, NCHCH3, JH-H ) 6.4 Hz, 3 H), 1.31-
1.39 (m, NCHCH2, 1H), 1.53-1.62 (m, NCHCH2, 1H), 1.61 (s, CHd
CCH3, 3H), 1.69 (s, CHdCCH3, 3H), 1.74-1.79 (m, NCH2CH2, 4 H),
1.89-2.11 (m, CdCHCH2, 2H), 2.31-2.38 (m, CHCH3, 1H), 2.53-
2.57 (m, NCH2CH2, 4H), 5.05-5.14 (m, CdCH, 1H).
N-(1,5-Dimethylhex-4-enylidene)pyrrolidinium Tetrafluoroborate
(16) was prepared in 60% yield by a procedure similar to that used for
N-(1-cyclohexylethylidene)pyrrolidinium tetrafluoroborate. 1H NMR
(300 MHz, CDCl3): δ 1.61 (s, CdCCH3, 3H), 1.68 (s, CdCCH3, 3H),
2.15-2.20 (m, NCH2CH2, 4H), 2.33-2.41 (m, NdCCH2CH2, 2H), 2.43
(s, NdCCH3, 3H), 2.68-2.73 (m, NdCCH2CH2, 2H), 3.91-3.97 (m,
NCH2CH2, 4H), 5.04-5.09 (m, CdCH, 1H). 13C {1H} NMR (75 MHz,
CDCl3): δ 17.69, 23.04, 23.49, 24.19, 24.46, 25.58, 38.33, 53.87, 54.56,
120.52, 135.19, 187.81. IR (in fluorolube): 1680 cm-1 (CdN). Anal.
Calcd for C12H22BF4N: C, 53.96; H, 8.30; N, 5.24. Found: C, 53.67;
H, 8.27; N, 5.35.
1-Isopropylpyrrolidinium Tetrafluoroborate (19) from 12. When
the hydrogenation of 1-isopropylidenepyrrolidinium tetrafluoroborate
(12) was complete, the reaction mixture was transferred to a Schlenk
flask and the solvent was removed under vacuum. The residue was
washed with Et2O several times and dried under vacuum to afford a
We were unable to prepare N-(1-phenylethylidene)diethylammo-
nium tetrafluoroborate (13) by the condensation of acetophenone and
diethylammonium tetrafluoroborate, under azeotropic distillation condi-
tions or with TiCl4. We were, however, able to prepare it by protonation
of the corresponding enamine. In a Schlenk flask, HBF4‚OMe2 (2.68
mL, 22 mmol) was added dropwise to a cooled (-10 °C) solution of
R-(diethylamino)styrene (3.82 g, 22 mmol) in 40 mL of Et2O. As the
mixture warmed to room temperature, an oily product settled to the
bottom of the flask. The top (ether) layer was removed by cannula,
while the product was washed several times with ether and dried under
vacuum to give a pale yellow oil (4.79 g, 84% yield). At -30 °C, the
1
white powder (84% yield). H NMR (300 MHz, CDCl3): δ 1.41 (d,
CH3, JH-H ) 6.5 Hz, 6 H), 2.10-2.16 (m, NCH2CH2, 4 H), 3.03-3.09
(m, NCH2CH2, 2H), 3.37-3.48 (m, NCH, 1H), 3.67-3.72 (m, NCH2-
CH2, 2H), 7.40 (bs, NH, 1H). 13C {1H} NMR (75 MHz, CDCl3): δ
18.90, 23.03, 52.50, 58.13. Anal. Calcd for C7H16BF4N: C, 41.83; H,
8.02; N, 6.97. Found: C, 41.94; H, 7.86; N, 6.85.
1-(1-Phenylethyl)pyrrolidinium Tetrafluoroborate (10). A solution
of 1-(1-phenylethyl)pyrrolidine (8) (200 mg, 1.14 mmol) in 30 mL of
Et2O was added dropwise to the diluted HBF4‚OMe2 (173 µL, 1.25
mmol) in 10 mL of Et2O. A white precipitate formed immediately.
The suspension was stirred for 15 min, removed by filtration, washed
with Et2O several times, and dried under vacuum to afford a white
1
oil solidified. H NMR (400 MHz, CDCl3): δ 1.30 (t, CH2CH3, JH-H
) 7.3 Hz, 3 H), 1.51 (t, CH2CH3, JH-H ) 7.4 Hz, 3 H), 2.78 (s, Nd
CCH3, 3H), 3.74 (q, CH2CH3, JH-H ) 7.3 Hz, 2H), 4.09 (q, CH2CH3,
JH-H ) 7.4 Hz, 2H), 7.45-7.48 (m, Ar, 5H). 13C {1H} NMR (100
MHz, CDCl3): δ 12.27, 12.94, 26.11, 49.93, 52.04, 125.38, 129.45,
131.63, 134.34, 187.85. IR (in fluorolube): 1651 cm-1 (CdN). Anal.
Calcd for C12H18BF4N: C, 54.78; H, 6.90; N, 5.32. Found: C, 52.92;
H, 6.93; N, 5.35. While a satisfactory carbon analysis was not obtained
even after repeated purification, 1H NMR showed >99% purity. HRMS-
FAB (m/z): (aggregate [2A + B]+ for an ionic compound A+B-) calcd
for C24H36N2BF4 439.2912 (for 11B); found: 439.2903.
1
powder (278 mg, 93% yield). H NMR (300 MHz, CD2Cl2): δ 1.78
(d, CH3, JH-H ) 6.84 Hz, 3H), 2.00-2.08 (m, NCH2CH2, 2H), 2.12-
2.23 (m, NCH2CH2, 2H), 2.84-2.91 (m, NCH2CH2, 1H), 3.10-3.19
(m, NCH2CH2, 2H), 3.91-3.96 (m, CHCH3, 1H), 4.16-4.21 (m, NCH2-
CH2, 1H), 7.47-7.49 (m, Ar, 5H), 7.83 (bs, NH, 1H). 13C {1H} NMR
(75 MHz, CD2Cl2): δ 19.47, 23.06, 23.40, 54.39, 67.29, 127.88, 129.96,
130.35, 136.10. Anal. Calcd for C12H18BF4N: C, 54.78; H, 6.90; N,
5.32. Found: C, 54.56; H, 6.90; N, 5.32.
Catalytic Hydrogenation of Enamines. [CpRu(dppm)(η2-H2)]BF4
(0.02 or 0.10 mmol) was added to a Fisher-Porter bottle under a
nitrogen atmosphere. The bottle was flushed several times with
hydrogen gas and placed in a 25 °C water bath. A deoxygenated solution
of the enamine (1.0 mmol) in 10 mL of CH2Cl2 was added by syringe
under a flow of hydrogen, and the resulting solution was stirred under
50 psi of hydrogen. When 1H NMR showed that the reaction was
complete (as in the formation of 20 from 25), excess aq HCl (1 M)
was added to acidify the mixture; the product was isolated as that of
hydrogenation of iminium cations.
Catalysis of the Hydrogenation of Iminium Cations by Ruthe-
nium Hydrides. Determination of the Initial Turnover Frequency.
A solid iminium cation (2 mmol) and ruthenium hydride (0.02 mmol)
were added to a Fisher-Porter bottle under a nitrogen atmosphere. The
bottle was flushed several times with hydrogen gas and placed in a 25
°C water bath. Then 10 mL of CH2Cl2 (or CH3OH) was added by
syringe under a flow of hydrogen, and the resulting solution was stirred
under 50 psi of hydrogen. At appropriate intervals an aliquot was
removed by syringe. The solvent was removed from the aliquot, and
1
the residue was dissolved in CDCl3 and its H NMR was recorded.
Acknowledgment. This research was supported by NSF
Grants CHE-0211310 and CHE-0451385. We thank Prof. G.
Parkin for assistance with the X-ray structure determinations.
The conversion was calculated from the relative integrals of the starting
material and the hydrogenation product. The initial TOF was determined
from data collected when less than 10% of the iminium cation had
been hydrogenated.
Supporting Information Available: Details of the crystal-
lographic study (PDF and CIF), kinetic data, plots of these data,
2H NMR spectra for the reaction between iminium cation 6 and
CpRu(dppe)D, a derivation of the equations used in determining
the EIE, and a complete citation for ref 49. This material is
available free of charge via the Internet at http://pubs.acs.org.
Isolation of Amines from the Catalytic Hydrogenation of Imi-
nium Cations (except 12). When the reaction was complete, the solvent
was removed under vacuum. The residue was treated with 20 mL of
water, then washed by ether (2 × 20 mL). A saturated aq KOH solution
(10 mL) was added, and the amine was removed by ether extraction (3
× 20 mL). The combined organic layers were dried over Na2SO4, then
the ether was removed to afford a light yellow oil. The results are
1
summarized in Table 3. The H NMR spectra of the products 18,49
JA0506861
20,50 and 2451 were consistent with those reported in the literature.
Amines 2152 and 2353 were prepared previously; however, no NMR
(51) Salvatore, R. N.; Nagle, A. S.; Jung, K. W. J. Org. Chem. 2002, 67, 674-
683.
(52) Suwa, T.; Sugiyama, E.; Shibata, I.; Baba, A. Synlett 2000, 556-558.
(49) Busacca, C. A. et al. J. Org. Chem. 2004, 69, 5187-5195.
(50) Smith, P. J.; Amin, M. Can. J. Chem. 1989, 67, 1457-1467.
(53) Cocolas, G. H.; Avakian, S.; Martin, G. J. J. Med. Chem. 1965, 8, 875-
877.
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7814 J. AM. CHEM. SOC. VOL. 127, NO. 21, 2005