Ruthenium-catalyzed ionic hydrogenation of aziridinium cations

Article abstract of DOI:10.1021/om050456v

Hydride transfer from CpRu(dppm)H (dppm = bis(diphenylphosphino)methane) to asymmetrically substituted aziridinium cations occurs at the less substituted carbon when the substituents are alkyl, but at the more substituted carbon when the substituent is ph

Full text of DOI:10.1021/om050456v

6358
Organometallics 2005, 24, 6358-6364
Ruthenium-Catalyzed Ionic Hydrogenation of
Aziridinium Cations
Hairong Guan, Sahar A. Saddoughi, Anthony P. Shaw, and Jack R. Norton*
Department of Chemistry, Columbia University, New York, New York 10027
Received June 6, 2005
Hydride transfer from CpRu(dppm)H (dppm ) bis(diphenylphosphino)methane) to
asymmetrically substituted aziridinium cations occurs at the less substituted carbon when
the substituents are alkyl, but at the more substituted carbon when the substituent is
phenyl. Some of the phenyl-substituted aziridinium cations react with the amine resulting
from H- transfer; the reaction of the same phenyl-substituted cation with Cp*Ru(dppf)H
(dppf ) 1,1′-bis(diphenylphosphino)ferrocene) shows evidence of electron transfer. CpRu-
(dppm)H can catalyze the hydrogenation of the same aziridinium cations to ammonium
cations; this reaction occurs with the same regioselectivity as the stoichiometric H- transfer
reactions.
Scheme 1
Introduction
There are relatively few reports of the homogeneous
hydrogenation of epoxides, particularly terminal epox-
ides,1-3 and there is no general method for preparing
primary alcohols from terminal epoxides and hydrogen
(although 2-phenylethanol can be prepared from styrene
oxide and hydrogen with several catalysts).2
The Fujitsu group obtained only a 44% yield of
3-buten-1-ol when the hydrogenation of 3,4-epoxy-1-
butene was catalyzed by various cationic rhodium
complexes.3a The transfer hydrogenation (with isopro-
pyl alcohol as the hydrogen source) of 1,2-epoxyalkanes
gave branched (secondary) alcohols with several cata-
lysts derived from Rh2(OAc)4.3b Ikariya and co-workers
have recently reported secondary alcohols as the domi-
nant products with the ternary catalyst system Cp*Ru-
(1,5-cyclooctadiene)Cl/2-(diphenylphosphino)ethylamine/
KOH.3c
However, the instability of protonated epoxides at
ambient temperature (although they can be spectro-
scopically observed at low temperatures4) makes it
inconvenient to examine their reactions with transition-
metal hydrides M-H. We have therefore examined
instead the regiochemistry of H- transfer to aziridinium
cations, which are isoeletronic with prontonated ep-
oxides.
Aziridinium cations are themselves important in the
synthesis of biologically active compounds,5 and their
ring-opening reactions with nucleophiles, such as
amines,6 hydrazine hydrate,7 alcohols,6f,n naphthols,8
halide ions,6m,9 CN-,9d SCN-,10 AcO-,11 RS-,6o,11 and
several organometallic anions,12 have been studied.
However, the reaction of aziridinium cations with
transition-metal hydrides has not been studied, and, to
the best of our knowledge, the catalytic hydrogenation
of aziridinium cations has not been reported.
One approach to obtaining terminal alcohols from the
catalytic hydrogenation of epoxides would be an ionic
mechanism, with H- transferred to the internal carbon
of the protonated epoxide (Scheme 1).
(1) Hydrogenation of internal epoxides: (a) Shimizu, I.; Oshima, M.;
Nisar, M.; Tsuji, J. Chem. Lett. 1986, 1775-1776. (b) Oshima, M.;
Yamazaki, H.; Shimizu, I.; Nisar, M.; Tsuji, J. J. Am. Chem. Soc. 1989,
111, 6280-6287. (c) Shimizu, I.; Hayashi, K.; Ide, N.; Oshima, M.
Tetrahedron 1991, 47, 2991-2998. (d) Shimizu, I.; Omura, T. Chem.
Lett. 1993, 1759-1760. (e) Torii, S.; Okumoto, H.; Nakayasu, S.;
Kotani, T. Chem. Lett. 1989, 1975-1978. (f) Chan, A. S. C.; Coleman,
J. P. J. Chem. Soc., Chem. Commun. 1991, 535-536. (g) Bakos, J.;
Orosz, AÄ .; Csere´pi, S.; To´th, I.; Sinou, D. J. Mol. Catal. A 1997, 116,
85-97. (h) Noguchi, Y.; Yamada, T.; Uchiro, H.; Kobayashi, S.
Tetrahedron Lett. 2000, 41, 7493-7497. (i) Kakei, H.; Nemoto, T.;
Ohshima, T.; Shibasaki, M. Angew. Chem., Int. Ed. 2004, 43, 317-
320.
In this paper we report H- transfer from ruthenium
hydrides to aziridinium cations and the observation of
one-electron transfer with one ruthenium hydride,
Cp*Ru(dppf)H (dppf ) 1,1′-bis(diphenylphosphino)fer-
rocene).13 We also demonstrate that the hydrogenation
of aziridinium cations can be catalyzed by other ruthe-
nium hydrides.
(2) Hydrogenation of styrene oxide: (a) Kwiatek, J.; Mador, I. L.;
Seyler, J. K. J. Am. Chem. Soc. 1962, 84, 304-305. (b) Love, C. J.;
McQuillin, F. J. J. Chem. Soc., Perkin Trans. 1 1973, 2509-2512. (c)
Fujitsu, H.; Shirahama, S.; Matsumura, E.; Takeshita, K.; Mochida,
I. J. Org. Chem. 1981, 46, 2287-2290.
Results and Discussion
Synthesis of Aziridinium Cations. Although some
aziridinium cations are unstable in regard to dimeriza-
(3) Hydrogenation of other terminal epoxides: (a) Fujitsu, H.;
Matsumura, E.; Shirahama, S.; Takeshita, K.; Mochida, I. J. Chem.
Soc., Perkin Trans. 1 1982, 855-859. (b) Ricci, M.; Slama, A. J. Mol.
Catal. 1994, 89, L1-L4. (c) Ito, M.; Hirakawa, M.; Osaku, A.; Ikariya,
T. Organometallics 2003, 22, 4190-4192.
(4) Olah, G. A.; Szilagyi, P. J. J. Org. Chem. 1971, 36, 1121-1126.
(5) (a) Dahanukar, V. H.; Zavialov, L. A. Curr. Opin. Drug Discovery
Dev. 2002, 5, 918-927. (b) Cossy, J.; Pardo, D. G. Targets Heterocycl.
Syst. 2002, 6, 1-26.
10.1021/om050456v CCC: $30.25 © 2005 American Chemical Society
Publication on Web 11/12/2005

Ruthenium-Catalyzed Ionic Hydrogenation
Organometallics, Vol. 24, No. 26, 2005 6359
tion to piperazinium cations,14 most can be isolated after
their preparation by (1) the reaction of diazomethane
with iminium cations, (2) the treatment of â-haloethyl-
amines with silver salts, or (3) the quaternization of
aziridines.15 We prepared the cations 1a-c according
to eq 1, and 1d according to eq 2.
We therefore examined the reaction of the aziridinium
cation 1a with CpRu(dppm)H (2) under conditions
similar to those in eq 4, i.e., in CD2Cl2 in the presence
of the coordinating ligand CH3CN. The amine products
3a17 and 4a18 were formed smoothly in a ratio of 70:1
(eq 5). The regiochemical preference (attack on the less
substituted carbon) is the same as that reported for
attack on alkyl-substituted aziridinium cations by other
nucleophiles, i.e., amines,6k,o-q PhS-,6o and CN-.9d,19
Stoichiometric Hydride Transfer from Ruthe-
nium Hydrides to Aziridinium Cations. While in-
vestigating the ionic hydrogenation of iminium cations
according to eq 3, catalyzed by CpRu(P-P)H (where
P-P is a chelating diphosphine),16 we learned that the
efficiency and enantioselectivity of the catalyst could be
inferred from the rate constant and the enantioselec-
tivity of the stoichiometric hydride transfer reaction in
eq 4.
(6) (a) Dieter, R. K.; Deo, N.; Lagu, B.; Dieter, J. W. J. Org. Chem.
1992, 57, 1663-1671. (b) Miao, G.; Rossiter, B. E. J. Org. Chem. 1995,
60, 8424-8427. (c) O’Brien, P.; Poumellec, P. Tetrahedron Lett. 1996,
37, 5619-5622. (d) Saravanan, P.; Singh, V. K. Tetrahedron Lett. 1998,
39, 167-170. (e) de Souse, S. E.; O’Brien, P.; Poumellec, P. J. Chem.
Soc., Perkin Trans. 1 1998, 1483-1492. (f) Anderson, S. R.; Ayers, J.
T.; DeVries, K. M.; Ito, F.; Mendenhall, D.; Vanderplas, B. C.
Tetrahedron: Asymmetry 1999, 10, 2655-2663. (g) Colman, B.; de
Sousa, S. E.; O’Brien, P.; Towers, T. D.; Watson, W. Tetrahedron:
Asymmetry 1999, 10, 4175-4182. (h) Chuang, T.-H.; Sharpless, K. B.
Org. Lett. 1999, 1, 1435-1437. (i) Chuang, T.-H.; Sharpless, K. B. Org.
Lett. 2000, 2, 3555-3557. (j) Curthbertson, E.; O’Brien, P.; Towers, T.
D. Synthesis 2001, 693-695. (k) O’Brien, P.; Towers, T. D. J. Org.
Chem. 2002, 67, 304-307. (l) Couturier, M.; Tucker, J. L.; Andresen,
B. M.; DeVries, K. M.; Vanderplas, B. C.; Ito, F. Tetrahedron:
Asymmetry 2003, 14, 3517-3523. (m) Piotrowska, D. G.; Wro´blewski,
A. E. Tetrahedron 2003, 59, 8405-8410. (n) Couty, F.; Evano, G.; Prim,
D. Tetrahedron Lett. 2005, 46, 2253-2257. (o) Richardson, P. F.;
Nelson, L. T. J.; Sharpless, K. B. Tetrahedron Lett. 1995, 36, 9241-
9244. (p) Liu, Q.; Marchington, A. P.; Boden, N.; Rayner, C. M. J. Chem.
Soc., Perkin Trans. 1 1997, 511-525. (q) McKay, C.; Wilson, R. J.;
Rayner, C. M. Chem. Commun. 2004, 1080-1081. (r) Carter, C.;
Fletcher, S.; Nelson, A. Tetrahedron: Asymmetry 2003, 14, 1995-2004.
(s) Katagiri, T.; Takahashi, M.; Fujiwara, Y.; Ihara, H.; Uneyama, K.
J. Org. Chem. 1999, 64, 7323-7329.
We attempted to prepare CpRu(dppm)D, in an effort
to confirm the site of hydride transfer in eq 5. However,
treatment of CpRu(dppm)Cl with NaOCD3/CD3OD re-
sulted in deuteration of the methylene of the diphos-
phine as well as the hydride ligand (eq 6), yielding 2-d3.
Presumably the methylene is easily deprotonated be-
cause the resulting carbanion is stabilized by two
phosphorus atoms.20
Reaction of 1a with 2-d3 (eq 7) led to deuterium
incorporation into the tert-butyl group of the amine 3a,
confirming that the less substituted carbon of the
aziridinium ring had received the transferred D-/H-.
There was also incorporation into the R position of the
pyrrolidinium ring, reflecting the acidity of that C-H
bond20 and the basicity of the D- ligand.
(7) Chuang, T.-H.; Sharpless, K. B. Helv. Chim. Acta 2000, 83,
1734-1743.
(8) Periasamy, M.; Seenivasaperumal, M.; Rao, V. D. Tetrahedron:
Asymmetry 2004, 15, 3847-3852.
(9) (a) Bottini, A. T.; Dowden, B. F.; Sousa, L. J. Am. Chem. Soc.
1965, 87, 3249-3250. (b) Somekh, L.; Shanzer, A. J. Am. Chem. Soc.
1982, 104, 5836-5837. (c) Gmeiner, P.; Junge, D.; Ka¨rtner, A. J. Org.
Chem. 1994, 59, 6766-6776. (d) Weber, K.; Kuklinski, S.; Gmeiner,
P. Org. Lett. 2000, 2, 647-649. (e) Nagle, A. S.; Salvatore, R. N.; Chong,
B.-D.; Jung, K. W. Tetrahedron Lett. 2000, 41, 3011-3014. (f) Sekar,
G.; Nishiyama, H. Chem. Commun. 2001, 1314-1315. (g) Back, T. G.;
Parvez, M.; Zhai, H. J. Org. Chem. 2003, 68, 9389-9393.
(10) Abe, H.; Aoyagi, S.; Kibayashi, C. J. Am. Chem. Soc. 2005, 127,
1473-1480.
(11) Verhelst, S. H. L.; Martinez, B. P.; Timmer, M. S. M.; Lodder,
G.; van der Marel, G. A.; Overkleeft, H. S.; van Boom, J. H. J. Org.
Chem. 2003, 68, 9598-9603.
(12) (a) Overman, L. E.; Sharp, M. J. J. Org. Chem. 1992, 57, 1035-
1038. (b) Graham, M. A.; Wadsworth, A. H.; Thornton-Pett, M.;
Carrozzini, B.; Cascarano, G. L.; Rayner, C. M. Tetrahedron Lett. 2001,
42, 2865-2868. (c) Andrews, D. R.; Dahanukar, V. H.; Eckert, J. M.;
Gala, D.; Lucas, B. S.; Schumacher, D. P.; Zavialov, I. A. Tetrahedron
Lett. 2002, 43, 6121-6125.
(13) Hembre, R. T.; McQueen, J. S.; Day, V. W. J. Am. Chem. Soc.
1996, 118, 798-803.
The reaction of the N,N-dimethyl aziridinium cation
1b with the hydride 2 (eq 8) was much faster than that
of 1a. The same preference for attack on the less-
substituted carbon was observed with 1b, although the
(14) Leonard, N. J.; Klainer, J. A. J. Heterocycl. Chem. 1971, 8, 215-
219.
(15) Crist, D. R.; Leonard, N. J. Angew. Chem., Int. Ed. Engl. 1969,
8, 962-974.

6360 Organometallics, Vol. 24, No. 26, 2005
Guan et al.
selectivity was less than in eq 5: the amines 3b21 and
4b18 were formed in a 30:1 ratio in eq 8.
with 2-d3 led to deuterium incorporation into the R as
well as the â carbon of the phenethyl substituent in the
amine 4c (eq 12)!
With the monomethyl aziridinium cation 1d hydride
transfer from 2 was even faster (eq 9). Attack on the
less-substituted carbon still predominated, although the
selectivity decreased further: the amines 3d22 and 4d18
were formed in a ratio of 24:1.
Treatment of 1d with 2-d3 (eq 10) led to deuterium
incorporation into the isopropyl group of the amine 3d,
confirming that the less substituted carbon of the
aziridinium ring had received the transferred D-/H-.
There was also, by analogy with the pyrrolidinium
result in eq 7, some incorporation into the methyl
substituents of 3d.
The ammonium salt 5 must arise from the opening
of the aziridinium cation 1c by the product amine 4c
(eq 13). An authentic sample of 5 can be prepared from
equimolar amounts of 1c and 4c. In similar experiments
there is no reaction within 1 h between 1c and the more
hindered product amine 3c. In eq 11 the amines 3c23
and 4c24 and the ammonium salt 5 were formed in a
ratio of 2:88:10.
When the reaction of 2 with 1c was examined at low
temperature (-64 °C) with concentrations of 1c high
enough to keep [1c] constant despite the operation of
eq 13, the disappearance of 2 was straightforwardly
second order (eq 14). (Details are given in the Support-
ing Information.) The value of k at -64 °C proved to be
Reaction of the aryl-substituted aziridinium cation 1c
with the hydride 2 was also fast, but the principal
product, 4c, arose from transfer to the more substituted
carbon (eq 11): the same regiochemistry generally
observed for attack by nucleophiles on aryl-substituted
aziridinium cations.6a-n,7,12c However, treatment of 1c
1.30(2) × 10-3 M-1 s-1
.
d[2]
dt
(16) (a) Magee, M. P.; Norton, J. R. J. Am. Chem. Soc. 2001, 123,
1778-1779. (b) Guan, H.; Iimura, M.; Magee, M. P.; Norton, J. R.;
Janak, K. E. Organometallics 2003, 22, 4084-4089. (c) Guan, H.;
Iimura, M.; Magee, M. P.; Norton, J. R.; Zhu, G. J. Am. Chem. Soc.
2005, 127, 7805-7814.
-
) k[2][1c]
(14)
Finally, the product ratios arising from reaction
with 1c were examined over the series of ruthenium
hydrides in Table 1. The product distributions agree
with the relative rates already observed for hydride
transfer to iminium cations:25 when transfer to 1c is
slower (as in entry 4), the amine 4c can compete
(17) Bottini, A. T.; Roberts, J. D. J. Am. Chem. Soc. 1958, 80, 5203-
5208.
(18) Taniguchi, Y.; Horie, S.; Takaki, K.; Fujiwara, Y. J. Organomet.
Chem. 1995, 504, 137-141.
(19) Unimolecular ring opening of 1a (in the fashion shown for 1c
in Scheme 2) would give predominantly the tertiary carbocation and
lead (after hydride transfer) to the formation of 4a.
(20) Stabilization of carbanions by adjoining N or P atoms is well
known. For examples see: Reutov, O. A.; Beletskaya, I. P.; Butin, K.
P. CH-Acids; Pergamon: New York, 1978; pp 69-71; translation editor,
T. R. Crompton.
(23) Lee, N. E.; Buchwald, S. L. J. Am. Chem. Soc. 1994, 116, 5985-
5986.
(24) Katritzky, A. R.; Hughes, C. V.; Rachwal, S. J. Heterocycl. Chem.
1989, 26, 1579-1588.
(21) Bushweller, C. H.; O’Neil, J. W.; Bilofsky, H. S. Tetrahedron
1971, 27, 5761-5766.
(22) Brown, J. H.; Bushweller, C. H. J. Am. Chem. Soc. 1992, 114,
8153-8158.
(25) For the ionic hydrogenation of iminium cations catalyzed by
series of ruthenium hydrides, the relative activities are in the order 2
> 6 > 7 > 8.16c

Ruthenium-Catalyzed Ionic Hydrogenation
Organometallics, Vol. 24, No. 26, 2005 6361
Table 1. Product Ratios for Stoichiometric
Hydride Transfers from Various LRu(P-P)H to
the Aziridinium Cation 1c
entry
LRu(P-P)H
3c:4c:5a
1
2
3
4
CpRu(dppm)H (2)
CpRu(dppe)H (6)
(CpMe)Ru(dppe)H (7)
Cp*Ru(dppe)H (8)
2:88:10
3:75:22
2:73:25
3:13:84
a Determined by the integration of 1H NMR.
Scheme 2
efficiently with Cp*Ru(dppe)H (8) for nucleophilic attack
on 1c and ring opening, and the dominant product
becomes 5.
Electron Transfer Reaction between Cp*Ru-
(dppf)H and Aziridinium Cation 1c. The preference
for attack at the more substituted carbon in eq 11
requires discussion. The regioselectivity can be ex-
plained as the result of an SN1 process: unimolec-
ular ring opening by the aziridinium 1c forms the
more stable carbocation, and subsequent H- transfer
gives 4c (Scheme 2). However, as the rate law in eq 14
shows, eq 11 is second order and occurs too rapidly
(t1/2 < 98 min at -64 °C) for k1 to be rate determining.
From experiments in which the carbocation from 1e is
trapped with excess benzaldehyde (eq 15)26 we know
that k1 for 1e is <10-4 s-1 at room temperature,27 and
we observe little reaction (<10% at room temperature
after 2 days) when 1c is treated with excess benzalde-
hyde.
Figure 1. 1H NMR hydride resonance from eq 16 as a
function of temperature.
1c at room temperature with Cp*Ru(dppf)H (9) (known
to undergo facile one-electron oxidation13). Most of the
hydride remained unchanged, but there was clear
evidence for the electron transfer in eq 16.30 The hy-
1
dride H NMR resonance of 9 broadened a great deal,
It is possible that the H- transfer in eq 11 may occur
through a single-electron-transfer (SET) mechanism: a
two-step H- transfer, with electron transfer followed by
H• transfer.28 Electrochemistry studies have shown that
aryl aziridinium salts are more easily reduced than
alkyl aziridinium salts by 0.6-0.9 V.29
while its Cp* and ferrocenyl resonances broadened
significantly. The line width, as illustrated for the hy-
dride resonance in Figure 1, decreased at lower tem-
peratures.
In an attempt to observe electron transfer and H•
transfer separately, we treated the aziridinium cation
(26) Keenan, T. R.; Leonard, N. J. J. Am. Chem. Soc. 1971, 93,
6567-6574.
This differential broadening implies rapid e- self-
exchange (eq 17) between 9 and its radical cation
Cp*Ru(dppf)H•+ (10) (which has been prepared by
Hembre and characterized by near-infrared spectros-
copy13). The temperature dependence of the broadening
confirms the exchange and implies that this system is
(27) A value of 3.8 × 10-4 s-1 has been measured at 50 °C for the k1
in eq 15,26 which would suggest a t1/2 > 1.9 h at room temperature for
eq 11 if ring opening were the rate-determining step.
(28) Some evidence for an SET mechanism is offered by the
observation of a small hump at δ -6.9 as we monitored the kinetics of
reaction 11 at -64 °C. This signal is probably due to some [CpRu-
(dppm)(H2)]+ (Jia, G.; Morris, R. H. Inorg. Chem. 1990, 29, 581-582),
the formation of which (from CpRu(dppm)H) would require the
presence of some of the strongly acidic cation radical CpRu(dppm)-
(30) Several years ago we reported similar 1H NMR line broadening
from rapid self-exchange between an azazirconacyclobutene and its
radical cation: Harlan, C. J.; Hascall, T.; Fujita, E.; Norton, J. R. J.
Am. Chem. Soc. 1999, 121, 7274-7275.
H+•
.
(29) Crist, D. R.; Borsetti, A. P.; Kass, M. B. J. Heterocycl. Chem.
1981, 18, 991-995.

6362 Organometallics, Vol. 24, No. 26, 2005
Guan et al.
in the “large hyperfine” or “slow exchange” limit of the
de Boer-MacLean equation for NMR line broadening
due to e- exchange.30,31 A simplified form (eq 18) of that
equation thus applies; the line width (T2ex-1) now
depends only on the rate constant for e- exchange (kR)
and the concentration of the radical cation 10.
∆(T2ex-1) ) kR[10]
(18)
The persistence of the line broadening at room tem-
perature implies that 10 is unable to transfer H•,
presumably as the result of the steric congestion around
its hydride ligand. Hydride radical cations such as 10
have been implicated in many reactions13,33 but seldom
-
isolated, although Lapinte has characterized the PF6
salt of an iron hydride radical cation, [Cp*Fe(dppe)H]-
PF6, by X-ray crystallography.32 These radical cations
are known for their kinetic acidity33 and frequently
protonate the neutral hydride from which they are
formed. In the reaction of 1c and 9 we observed (in the
1H NMR) a small amount of the [Cp*Ru(dppf)(H)2]BF4
(11),13 surely the result of the proton transfer reaction
in eq 19. The formation of 11 offers indirect but
persuasive evidence that the radical cation 10 has been
formed.
Cp*Ru(dppe)H•+ + Ph3C‚ + MeCN f
[Cp*Ru(dppe)(MeCN)]+ + Ph3CH (26)
It is possible that all the 1c/RuH reactions in Table 1
occur through an SET mechanism, even though the
subsequent H• transfer step is too fast to prevent the
identification of the radical intermediates.
Catalytic Ionic Hydrogenation of Aziridinium
Cations. Given the established relationship between
turnover in catalytic ionic hydrogenations and the rate
of stoichiometric H- transfers,16 the success of the
stoichiometric H- transfers in eqs 5, 8, 9, and 11 and
Table 1 suggested that the catalytic hydrogenation of
aziridinium cations should be possible. We therefore
treated various aziridinium cations with 50 psi H2 in
the presence of 2 mol % of ruthenium catalyst 2. The
results (eq 27) are given in Table 2.
The addition of cobaltocene to the 9/1c reaction
mixture provided further evidence of the electron trans-
fer in eq 17. The Cp2Co reduced the radical cation 10
(eq 20) and, by suppressing the self-exchange in eq 18,
restored the 1H NMR spectrum of 9 to its original
appearance.
A related SET mechanism explains the formation of
trityl radical as a byproduct of the reaction between
trityl cation and the hydride 8 (eq 21). The dihydride
cation 1234 is presumably formed by H+ transfer to 8
(eq 23) from the radical cation Cp*Ru(dppe)H•+ formed
by the initial electron transfer (eq 22).35 Oxidation of
the resulting Cp*Ru(dppe)• by Cp*Ru(dppe)H•+ gives
[Cp*Ru(dppe)(MeCN)]+ (eq 24).36 Gomberg’s dimer 1337
is in equilibrium (eq 25) with the Ph3C• produced by
the e- transfer; subsequent H• transfer (eq 26) gives
Ph3CH and [Cp*Ru(dppe)(MeCN)]+.
As expected from the stoichiometric hydride transfer
reactions (eqs 5, 8, 9, 11), alkyl-substituted aziridinium
cations gave the more branched ammonium cations
(entries 1-3), while the aryl-substituted aziridinium
cation 1c gave the less branched ammonium cation
(entry 4). To our knowledge this is the first report of the
catalytic hydrogenation of aziridinium cations. We
believe, in light of our studies on the mechanism of the
ionic hydrogenation of iminium cations,16 that the
aziridinium cations undergo hydrogenation by the mech-
anism in Scheme 3 (in which 1c is shown as a repre-
sentative substrate). H- transfer from the hydride
complex to the aziridinium cation (1c in this case)
generates the amine (4c in this case) and a 16-electron
ruthenium complex; coordination of H2 (dihydrogen
complexes are known to be kinetically acidic38) leads to
protonation of the amine (4c here), giving the am-
monium salt (15c here), and regeneration of the ruthe-
nium hydride catalyst.
(31) de Boer, E.; MacLean, C. J. Chem. Phys. 1966, 44, 1334-
1342.
(32) Hamon, P.; Toupet, L.; Hamon, J.-R.; Lapinte, C. Organome-
tallics 1992, 11, 1429-1431.
(33) Ryan, O. B.; Tilset, M.; Parker, V. D. J. Am. Chem. Soc. 1990,
112, 2618-2626.
(34) Jia, G.; Ng, W. S.; Chu, H. S.; Wong, W.-T.; Yu, N.-T.; Williams,
I. D. Organometallics 1999, 18, 3597-3602.
(35) The equivalent of eq 22 has been reported for Cp*Ru(dppf)H
by: Hembre, R. T.; McQueen, J. S. Angew. Chem., Int. Ed, Engl. 1997,
36, 65-67.
(36) A case similar to eq 24 has been reported. See Scheme 1 in:
Smith, K.; Rømming, C.; Tilset, M. J. Am. Chem. Soc. 1993, 115, 8681-
8689.
(38) (a) Jessop, P. G.; Morris, R. H. Coord. Chem. Rev. 1992, 121,
155-284. (b) Heinekey, D. M.; Oldham, W. J. Chem. Rev. 1993, 93,
913-926.
(37) McBride, J. M. Tetrahedron 1974, 30, 2009-2022.

Ruthenium-Catalyzed Ionic Hydrogenation
Organometallics, Vol. 24, No. 26, 2005 6363
Table 2. Ionic Hydrogenation of Aziridinium
Cations Catalyzed by CpRu(dppm)H (2)a
tion was added dropwise to a cold (0 °C) solution of 1-isopro-
pylidenepyrrolidinium tetrafluoroborate (1.59 g, 8.0 mmol) in
80 mL of CH3CN. When a yellow color persisted in the solution,
the addition was stopped. The reaction mixture was stirred
at 0 °C for another 15 min before a few drops of acetic acid
were added to remove the excess CH2N2. The volume of the
solution was reduced to 30 mL, then 170 mL of Et2O was
carefully poured down the sides of the flask. The mixture was
kept at -30 °C for 2 days. The resulting colorless crystals were
filtered, washed with Et2O, and dried under vacuum to give a
1
white solid (1.41 g, 83% yield). H NMR (400 MHz, CD3CN):
δ 1.58 (s, CH3, 6H), 2.05-2.20 (m, NCH2CH2, 4H), 2.92 (s,
NCH2C(CH3)2, 2H), 3.07-3.15 (m, NCH2 CH2, 2H), 3.48-3.57
(m, NCH2CH2, 2H). 13C{1H} NMR (100 MHz, CD3CN): δ 20.81,
24.85, 54.03, 54.27, 57.32. Anal. Calcd for C8H16NBF4: C,
45.11; H, 7.57; N, 6.58. Found: C, 45.21; H, 7.63; N, 6.53.
1,1,2,2-Tetramethylaziridinium Tetrafluoroborate (1b).
The cation has been previously reported with perchlorate as
the counterion.44 Its tetrafluoroborate salt was prepared from
N-methyl-N-(1-methylethylidene)methanaminium tetrafluo-
roborate in 46% yield by a procedure similar to that used for
1a. 1H NMR (300 MHz, CD2Cl2): δ 1.72 (s, NC(CH3)2, 6H),
3.01 (s, NCH2, 2H), 3.11 (s, N(CH3)2, 6H). 13C{1H} NMR (75
MHz, CD2Cl2): δ 20.04, 46.00, 52.87, 56.02. Anal. Calcd for
C6H14NBF4: C, 38.54; H, 7.55; N, 7.49. Found: C, 38.56; H,
7.57; N, 7.50.
a Reaction conditions: [1] ) 0.1 mol/L, [2] ) 0.002 mol/L, H2
pressure 50 psi, in CH2Cl2, room temperature. b Determined by
1H NMR integration of the crude product. c Isolated yield.
Scheme 3
1-Phenyl-3-azoniaspiro[2.4]heptane Tetrafluorobo-
rate (1c). This compound has been previously prepared;45
however, no NMR data were reported. 1H NMR (400 MHz,
CD2Cl2): δ 1.89-2.01 (m, NCH2CH2, 1H), 2.13-2.25 (m,
NCH2CH2, 2H), 2.27-2.36 (m, NCH2CH2, 1H), 2.71-2.78 (m,
NCH2, 1H), 3.09-3.18 (m, NCH2, 1H), 3.67-2.81 (m, NCH2,
4H), 4.56 (t, NCHPh, JH-H ) 8.1 Hz, 1H), 7.45-7.61 (m, Ar,
5H). 13C{1H} NMR (75 MHz, CDCl3): δ 23.89, 24.29, 41.74,
53.56, 55.13, 61.74, 125.99, 129.69, 130.29, 131.43. Anal. Calcd
for C12H16NBF4: C, 55.21; H, 6.18; N, 5.37. Found: C, 54.89;
H, 6.27; N, 5.29.
In the catalytic hydrogenation of 1c (entry 4, Table
2) we did not observe the formation of 5 (or its proto-
nated dication). Presumably in the catalytic cycle the
protonation of 4c is much faster than the reaction of 4c
with the aziridinium cation 1c (as in eq 13).
1,1,2-Trimethylaziridinium Tetrafluoroborate (1d).
The cation has been previously reported with perchlorate as
the counterion.46 Its tetrafluoroborate salt was prepared as
follows: to a cold (5-10 °C) solution of 1-(dimethylamino)-2-
chloropropane (2.43 g, 20 mmol) in 60 mL of benzene was
slowly added a suspension of AgBF4 (3.9 g, 20 mmol) in 5 mL
of benzene. The mixture was warmed to room temperature and
stirred overnight. The brown solid was filtered and washed
with acetone several times. The combined acetone filtrate was
pumped to dryness, and the residue was recrystallized from
acetone/Et2O to afford a white solid (2.47 g, 71% yield). 1H
NMR (400 MHz, CD3COCD3): δ 1.67 (d, NCHCH3, JH-H ) 6.2
Hz, 3H), 2.99-3.02 (m, NCH2, 1H), 3.14 (s, NCH3, 3H), 3.21
(s, NCH3, 3H), 3.20-3.24 (m, NCH2, 1H), 3.39-3.45 (m,
NCHCH3, 1H). 13C{1H} NMR (100 MHz, CD3COCD3): δ 11.65,
42.81, 46.91, 49.68, 50.84. Anal. Calcd for C5H12NBF4: C,
34.72; H, 6.99; N, 8.10. Found: C, 34.99; H, 7.03; N, 8.01.
Stoichiometric Hydride Transfer from Ruthenium
Hydrides to Aziridinium Cations. In a typical experiment,
a ruthenium hydride (20 µmol), CH3CN (2.1 µL, 40 µmol), and
CD2Cl2 (ca. 0.8 mL) were mixed in a vial under an inert
atmosphere. The aziridinium cation 1 (20 µmol) was added to
the mixture in one portion. The resulting solution was
transferred to a J. Young NMR tube. The reaction was
Experimental Section
General Procedures. All air-sensitive compounds were
prepared and handled under a N2/Ar atmosphere using stand-
ard Schlenk and inert-atmosphere box techniques. CH2Cl2 and
Et2O were deoxygenated and dried over two successive acti-
vated alumina columns under argon. Benzene was 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 transfer at room
temperature. An ether solution of CH2N2 was prepared from
Diazald using an Aldrich Diazald kit with Clear-Seal joints.
Caution: CH2N2 is explosive when heated. It should be
handled with care in a well-ventilated fume hood. CpRu-
(dppm)H (2),39 CpRu(dppe)H (6),39 (CpMe)Ru(dppe)H (7),16c
Cp*Ru(dppe)H (8),40 Cp*Ru(dppf)H (9),13 1-isopropylidenepyr-
rolidinium tetrafluoroborate,41 N-methyl-N-(1-methylethylidene)-
methanaminium tetrafluoroborate,42 and 1-(2-phenylethyl)-
pyrrolidine (4c)43 were prepared as described in the literature.
1,1-Dimethyl-3-azoniaspiro[2.4]heptane Tetrafluoro-
borate (1a). Under a nitrogen atmosphere CH2N2 ether solu-
1
monitored by H NMR, and the product ratio was measured
1
by H NMR integration.
1-(2-Phenylethyl)-1-(1-phenyl-2-pyrrolidinylethyl)pyr-
rolidinium Tetrafluoroborate (5). 1-(2-Phenylethyl)pyrro-
(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) Leonard, N. J.; Paukstelis, J. V. J. Org. Chem. 1963, 28, 3021-
3024.
(44) Leonard, N. J.; Paukstelis, J. V.; Brady, L. E. J. Org. Chem.
1964, 29, 3383-3386.
(42) Bao, J.; Park, S.-H. K.; Geib, S. J.; Cooper, N. J. Organometallics
2003, 22, 3309-3312.
(43) Shapiro, S. L.; Soloway, H.; Freedman, L. J. Am. Chem. Soc.
1958, 80, 6060-6064.
(45) Kass, M. B.; Borsetti, A. P.; Crist, D. R. J. Am. Chem. Soc. 1973,
95, 959-961.
(46) Tyssee, D. A.; Baizer, M. M. J. Electrochem. Soc. 1971, 118,
1420-1425.

6364 Organometallics, Vol. 24, No. 26, 2005
Guan et al.
lidine (39 µL, 0.21 mmol) was added dropwise to the aziridin-
ium cation 1c (52 mg, 0.20 mmol) in 10 mL of CH2Cl2, and
the resulting solution stirred at room temperature for 1 h. Et2O
was added until a white precipitate formed. After filtration
the solid was washed with Et2O (3 × 20 mL) and dried under
vacuum to afford the ammonium product 5 (73 mg, 84% yield).
1H NMR (400 MHz, CDCl3): δ 1.72-1.78 (m, NCH2CH2, 4H),
1.95-2.18 (m, N+CH2CH2, 4H), 2.53-2.57 (m, NCH2CH2, 2H),
2.63-2.70 (m, NCH2CH2, 2H), 2.99-3.04 (m, NCH2CHPh, 1H),
3.08-3.13 (m, NCH2CHPh, 1H), 3.14-3.21 (m, N+CH2CH2Ph,
1H), 3.27-3.34 (m, N+CH2CH2Ph, 1H), 3.52-3.71 (m,
N+CH2CH2, 4H), 3.73-3.81 (m, N+CH2CH2Ph, 1H), 3.98-4.05
(m, N+CH2CH2Ph, 1H), 4.82-4.85 (m, N+CHPh, 1H), 7.19-
7.53 (m, Ar, 10H). 13C{1H} NMR (100 MHz, CD2Cl2): δ 22.32,
22.98, 24.02, 30.11, 54.62, 57.51, 60.65, 61.02, 61.36, 73.34,
127.65, 128.84, 129.17, 129.71, 130.56, 130.89, 132.23, 135.16.
Anal. Calcd for C24H33N2BF4: C, 66.06; H, 7.62; N, 6.42.
Found: C, 65.72; H, 7.55; N, 6.43.
NMR tube. The reaction was complete within 10 min at room
temperature, and the product ratios were measured by 1H
NMR integration.
Hydrogenation of Aziridinium Cations by Ruthenium
Hydrides. A solid aziridinium cation (1.0 mmol) and the
ruthenium hydride catalyst (0.02 mmol) were added to a
Fischer-Porter bottle under a nitrogen atmosphere. The bottle
was flushed several times with hydrogen gas. Then 10 mL of
CH2Cl2 was added by syringe under a flow of hydrogen, and
the resulting solution stirred under 50 psi of hydrogen. When
the hydrogenation of aziridinium cation was complete (moni-
1
tored by H NMR), the reaction mixture was transferred to a
Schlenk flask and the solvent removed under vacuum. The
product ratio was measured by 1H NMR integration of the
crude product. The residue was washed with Et2O several
times and dried under vacuum.
1-(tert-Butyl)pyrrolidinium Tetrafluoroborate (14a):
1
white solid (183 mg, 85% yield). H NMR (300 MHz, CDCl3):
δ 1.42 (s, CH3, 9H), 2.05-2.11 (m, NCH2CH2, 2H), 2.12-2.22
(m, NCH2CH2, 2H), 3.05-3.17 (m, NCH2CH2, 2H), 3.54-3.62
(m, NCH2CH2, 2H), 7.66 (br, NH, 1H). 13C{1H} NMR (75
MHz, CDCl3): δ 23.59, 24.84, 49.03, 61.96. Anal. Calcd for
C8H18NBF4: C, 44.68; H, 8.44; N, 6.51. Found: C, 44.92; H,
8.38; N, 6.33.
Deuterium Transfer and Exchange Experiments (eqs
7, 10, 12). Typically 2-d3 (11.1 mg, 20 µmol), CH3CN (2.1 µL,
40 µmol), and CH2Cl2 or CD2Cl2 (ca. 0.8 mL) were mixed in a
vial under an inert atmosphere. The aziridinium salt (20 µmol)
was added in one portion, and the resulting solution was
transferred to a J. Young tube. The reaction was monitored
1
2
N,N-Dimethyl-tert-butylammonium Tetrafluoroborate
by either H or D NMR.
(14b): white solid (143 mg, 76% yield). 1H NMR (400 MHz,
Kinetics of Hydride Transfer from CpRu(dppm)H (2)
to the Aziridinium Cation 1c. Under an inert atmosphere,
the aziridinium salt 1c (24.2 mg, 92.7 µmol) was dissolved in
800 µL of CD2Cl2 and transferred to a screw-cap NMR tube
with a Teflon-coated septum insert; 1.5 µL of a hexamethyl-
cyclotrisiloxane standard (0.0557 M in benzene) was added and
the NMR tube frozen in liquid nitrogen while its contents were
under an atmosphere of nitrogen connected to a bubbler.
Separately a stock solution was prepared from CpRu(dppm)H
(22.1 mg, 40.0 µmol) and CH3CN (6 µL, 115 µmol) in 1000 µL
of CD2Cl2; 200 µL of this solution was added to the NMR tube
with a microliter syringe. The tube was then thawed at -70
°C (with its contents still under N2 connected to the bubbler)
and its contents were mixed; then it was inserted into an NMR
probe precooled to -64 °C. The disappearance of the hydride
signal was monitored via 1H NMR (by comparison to the height
of the hexamethylcyclotrisiloxane standard) for at least three
half-lives. The NMR probe temperature was calibrated with
a Wilmad chemical shift thermometer (99.97% methanol +
0.03% HCl).47 This experiment was repeated with varying
amounts of 1c (129.5, 154.0, and 191.5 µmol).
CDCl3): δ 1.46 (s, NC(CH3)3, 9H), 2.87 (d, N(CH3)2, JH-H
)
5.2 Hz, 6H), 7.32 (br, NH, 1H). 13C{1H} NMR (100 MHz,
CDCl3): δ 24.57, 38.99, 63.83. Anal. Calcd for C6H16NBF4: C,
38.13; H, 8.53; N, 7.41. Found: C, 38.28; H, 8.50; N, 7.25.
N,N-Dimethylisopropylammonium Tetrafluoroborate
(14d): white solid (135 mg, 77% yield). 1H NMR (300 MHz,
CDCl3): δ 1.38 (d, NCH(CH3)2, JH-H ) 6.7 Hz, 6H), 2.87 (d,
N(CH3)2, JH-H ) 5.2 Hz, 6H), 3.60 (dsept, NCH(CH3)2, JH-H
)
3.0 Hz, 6.7 Hz, 1H), 7.38 (br, NH, 1H). 13C{1H} NMR
(100 MHz, CDCl3): δ 16.82, 39.99, 59.18. Anal. Calcd for
C5H14NBF4: C, 34.32; H, 8.06; N, 8.00. Found: C, 34.37; H,
8.29; N, 7.86.
1-(2-Phenylethyl)pyrrolidinium Tetrafluoroborate
1
(15c): white solid (194 mg, 73% yeild). H NMR (400 MHz,
CDCl3): δ 2.10-2.22 (m, NCH2CH2, 4H), 2.95-3.05 (m,
NCH2CH2, 2H), 3.06-3.13 (m, PhCH2, 2H), 3.36-3.42 (m,
NCH2CH2Ph, 2H), 3.81-3.88 (m, NCH2CH2, 2H), 7.21-7.31
(m, Ar, 5H), 7.82 (br, NH, 1H). 13C{1H} NMR (75 MHz,
CDCl3): δ 22.93, 32.33, 55.33, 57.34, 127.46, 128.73, 129.04,
135.42. Anal. Calcd for C12H18NBF4: C, 54.78; H, 6.90; N, 5.32.
Found: C, 54.92; H, 6.93; N, 5.34.
Low-Temperature NMR of the Reaction of Cp*Ru-
(dppf)H (9) with the Aziridinium Salt 1c. Under an inert
atmosphere, Cp*Ru(dppf)H (15.8 mg, 20.0 µmol) and aziri-
dinium salt 1c (5.2 mg, 20.0 µmol) were dissolved in 800 µL
Acknowledgment. This research was supported by
NSF grants CHE-0211310 and CHE-0451385. We thank
Dr. M. P. Magee for awakening our interest in aziri-
dinium cations.
1
of CD2Cl2 and transferred to a J. Young NMR tube. The H
NMR spectrum was recorded at room temperature and at
lower temperatures (-26.1, -33.7, -38.0, and -42.8 °C). The
NMR probe temperature was calibrated with a Wilmad chemi-
cal shift thermometer (99.97% methanol + 0.03% HCl).47
Reaction of Cp*Ru(dppe)H (8) with [Ph3C]BF4. Under
a nitrogen atmosphere, the hydride 8 (12.7 mg, 20 µmol) and
[Ph3C]BF4 (6.6 mg, 20 µmol) were placed in a vial, followed by
the addition of CD2Cl2 (ca. 0.8 mL) containing CH3CN (4.2 µL,
80 µmol). The resulting solution was transferred to a J. Young
Note Added after ASAP Publication. In the ver-
sion of this paper published on the Web on Nov. 12,
2005, eq 26 was incorrect. The version of this equation
that now appears is the correct one.
Supporting Information Available: Rate constants and
plot for pseudo-first-order kinetics of 2 with 1c at -64 °C. This
material is available free of charge via the Internet at
http://pubs.acs.org.
(47) (a) van Geet, A. L. Anal. Chem. 1970, 42, 679-680. (b) Raidford,
D. S.; Fisk, C. L.; Becker, E. D. Anal. Chem. 1979, 51, 2050-2051.
OM050456V