Amine products and catalyst poisoning in the homogeneous H2 hydrogenation of imines catalyzed by the [Rh(COD)(PPh3)2]PF6 precursor

Article abstract of DOI:10.1021/om020992c

The H₂ hydrogenations of PhN=CHPh and PhCH₂N=CHPh are catalyzed by the precursor [Rh(COD)(PPh₃)₂]PF₆. However, the amine product PhNHCH₂Ph poisons the catalyst by coordination to the Rh through an arene moiety, while the other amine product, (PhCH₂)₂NH, forms a labile N-bonded species that does not poison the catalyst system.

Full text of DOI:10.1021/om020992c

Organometallics 2003, 22, 1177-1179
1177
Am in e P r od u cts a n d Ca ta lyst P oison in g in th e
Hom ogen eou s H2 Hyd r ogen a tion of Im in es Ca ta lyzed by
th e [Rh (COD)(P P h 3)2]P F 6 P r ecu r sor
Paolo Marcazzan, Brian O. Patrick, and Brian R. J ames*
Department of Chemistry, The University of British Columbia, 2036 Main Mall,
Vancouver, British Columbia V6T 1Z1, Canada
Received December 10, 2002
Summary: The H2 hydrogenations of PhNdCHPh
and PhCH2NdCHPh are catalyzed by the precursor
[Rh(COD)(PPh3)2]PF6. However, the amine product
PhNHCH2Ph poisons the catalyst by coordination to the
Rh through an arene moiety, while the other amine
product, (PhCH2)2NH, forms a labile N-bonded species
that does not poison the catalyst system.
Transition-metal-catalyzed hydrogenation of imines
is an area of intense interest, particularly for production
of chiral amines from prochiral ketimine substrates, but
the mechanisms are poorly understood.1,2 One problem
is that the more forcing hydrogenation conditions gener-
ally required, versus those for the corresponding reduc-
F igu r e 1. Plots of percent conversion vs time for the
hydrogenation of PhCH2NdCHPh ([) and PhNdCHPh (9)
catalyzed by 0.53 mM [Rh(H)2(PPh3)2(MeOH)2]PF6 (2) in
MeOH (substrate:catalyst ) 100:1, 293 K, 1 atm of H2).
tions of CdC and CdO functionalities,2 complicate
mechanistic investigations; however, cationic precursors
such as [Rh(COD)(PPh3)2]PF6 (1)3 have long been known
to catalyze homogeneously the H2 hydrogenation of
aldimines (RCHdNR′) under ambient conditions,4 and
we and others5 are studying mechanistic aspects of such
systems. The strong donor character of the NH group
of an amine, with its ability to compete for coordination
at the catalytic site (catalyst poisoning), may be one
factor contributing to the more difficult hydrogenation
of imines.2 We report here on the hydrogenation of two
different aldimines, the findings revealing two limiting
cases for the interaction of the product amines with the
catalytic center.
of reaction were observed for PhNdCHPh; indeed, the
room-temperature reaction of 2 with the product amine
PhNHCH2Ph in MeOH under Ar (amine:Rh ) 2) gave
the red complex [Rh{η4-(C6H5)NHCH2Ph}(PPh3)2]PF6
(3) containing in the solid state the amine ligand bonded
through an η4-π-arene interaction (eq 1).9 X-ray-quality
Treatment of a suspension of 1 in MeOH with 1 atm
of H2 at room temperature (∼20 °C) for 2 h afforded a
pale yellow solution of [Rh(H)2(PPh3)2(MeOH)2]PF6
(2).3,6 For a catalytic run, excess imine (Rh:imine )
1:100) was added to an MeOH solution (10 mL) of 2,
preformed in situ (0.53 mM) under 1 atm of H2, and the
conversion monitored by GC analysis as a function of
time. Of the two substrates tested, benzylidenebenzy-
lamine (PhCH2NdCHPh) and benzylideneaniline (PhNd
CHPh), only the former was converted to the amine
(Figure 1).7,8 Negligible conversion and formation of a
red solution (with trace red solid) during the first 5 min
crystals were obtained from evaporation of a CH2Cl2/
hexanes solution of 3; Figure 2 shows the structure of
the cation, and selected structural parameters are given
in the figure legend.10 The choice of coordination via the
aryl rather than the NH moiety of the amine was not
expected. The solid isolated from the catalytic mixture
* To whom correspondence should be addressed. Tel.: +1-(604)-822-
6645. Fax: +1-(604)-822-2847. E-mail: brj@chem.ubc.ca.
(1) Kobayashi, S.; Ishitani, H. Chem. Rev. 1999, 99, 1069.
(2) J ames, B. R. Catal. Today 1997, 37, 209.
(3) Shapley, J . R.; Schrock, R. R.; Osborn, J . A. J . Am. Chem. Soc.
1969, 91, 2816.
(4) Longley, C. J .; Goodwin, T. J .; Wilkinson, G. Polyhedron 1986,
5, 1625.
(5) Herrera, V.; Munoz, B.; Landeta, V.; Canudas, N. J . Mol. Catal.
2001, 174, 141.
(7) The detailed kinetic analysis and suggested mechanism for the
catalyzed hydrogenation of PhCH2NdCHPh8 will be published else-
where.
(6) Marcazzan, P.; Ezhova, M. B.; Patrick, B. O.; J ames, B. R. C. R.
Chimie 2002, 5, 373 (J . Osborn issue).
(8) Marcazzan, P. Ph.D. Dissertation, University of British Colum-
bia, 2002.
10.1021/om020992c CCC: $25.00 © 2003 American Chemical Society
Publication on Web 02/15/2003

1178 Organometallics, Vol. 22, No. 6, 2003
Communications
significantly, and this has precedents within other RhI-
η4-phenyl structures.14 IR bands are seen for ν(C-N)
and ν(N-H).9
The room-temperature 31P{1H} NMR spectrum of 3
in CD2Cl2 (δ 46.61 d, J RhP ) 211 Hz) shows that the
complex is stable in noncoordinating or weakly coordi-
nating media, the J RhP value being typical for cis
phosphines within RhI-π-bound arene complexes.13,15,16
The three upfield-shifted resonances for the π-arene
protons in a 1:2:2 ratio in the room-temperature 1H
9
NMR spectrum in CD2Cl2 indicate equivalence of the
two meta and the two ortho protons in an η6 coordina-
tion mode; for η4-hapticity, as in the solid state, five
different upfield-shifted resonances would be expected.
The mutually coupled CH2 and NH resonances of the
coordinated amine are observed at δ 3.83 and 4.08,
respectively, upfield of those of the free amine (δ 4.36
and 7.30, respectively). The four upfield-shifted reso-
nances for the C atoms of the coordinating ring in an
approximate 1:2:2:1 ratio (para, ortho, meta, and ipso
C, respectively) in the room-temperature 13C{1H} spec-
F igu r e 2. ORTEP diagram of the cation of 3, [Rh{η4-
(C6H5)NHCH2Ph}(PPh3)2]+, with 50% probability thermal
ellipsoids. Selected bond distances (Å) and angles (deg):
Rh(1)-P(1) ) 2.2636(8), Rh(1)-P(2) ) 2.2421(7), Rh(1)-
C(37) ) 2.524(3), Rh(1)-C(38) ) 2.442(3), Rh(1)-C(39) )
2.288(3), Rh(1)-C(40) ) 2.287(4), Rh(1)-C(41) ) 2.310-
(4), Rh(1)-C(42) ) 2.291(3), N(1)-C(37) ) 1.354(4), N(1)-
C(43) ) 1.468(4), C(37)-C(38) ) 1.405(4), C(38)-C(39) )
1.388(4), C(39)-C(40) ) 1.416(5), C(40)-C(41) ) 1.392(5),
C(41)-C(42) ) 1.416(5), C(42)-C(37) ) 1.430(5); P(1)-Rh-
(1)-P(2) ) 94.98(3), P(1)-Rh(1)-C(37) ) 105.30(7), P(1)-
Rh(1)-C(38) ) 129.33(7), P(1)-Rh(1)-C(39) ) 163.24(9),
P(1)-Rh(1)-C(40) ) 152.61(9), P(1)-Rh(1)-C(41) ) 118.75-
(9), P(1)-Rh(1)-C(42) ) 98.37(8), P(2)-Rh(1)-C(37) )
144.30(7), P(2)-Rh(1)-C(38) ) 112.61(7), P(2)-Rh(1)-
C(39) ) 94.09(7), P(2)-Rh(1)-C(40) ) 102.24(10), P(2)-
Rh(1)-C(41) ) 131.75(9), P(2)-Rh(1)-C(42) ) 165.90(8),
C(37)-N(1)-C(43) ) 124.0(3).
trum in CD2Cl2 are also consistent with the η6 coordi-
9
nation mode.
In more strongly coordinating media, 3 partially
dissociated the amine; at room temperature in acetone-
d6, about half the complex dissociates to form cis-[Rh-
(PPh3)2(acetone)2]+ (4; δP 54.19, d, J RhP ) 202 Hz)6 and
free amine. The corresponding room-temperature 1H
NMR data support the dissociation reaction, the reso-
nances for 3 being seen at values 0.15-0.30 ppm
downfield-shifted from those recorded in CD2Cl2.
Complex 3 dissociates similarly in CD3OD, with the
exception that 3 now exists as two different isomers in
about a 2:1 ratio. The room-temperature 31P{1H} spec-
trum shows the resonance of cis-[Rh(PPh3)2(alcohol)2]+
(δ 57.02 d, J RhP ) 207 Hz)6 and a doublet for each isomer
of 3 (δ 46.71 d, J RhP ) 211 Hz; δ 47.45 d, J RhP ) 212
Hz; ∼2:1); the corresponding 1H NMR spectrum reveals
two sets of upfield-shifted resonances for the η-arene
moieties, as well as two upfield-shifted singlets for the
amine CH2 protons (the NH proton of the coordinated
amine is not detected because of exchange with the
deuterated solvent). For the major isomer, resonances
described above was also 3, formed following hydroge-
nation of 1-2 equiv of the imine; the Rh center is
“sequestered” and the catalytic activity poisoned by the
coordinated amine. While RhI-π-arene complexes are
common, those adopting η4 hapticities are rare.11-14 The
shorter Rh(1)-C(n) (n ) 39-42) distances show that the
arene is coordinated through the C(39)-C(40) and
C(41)-C(42) bonds, while the longer Rh(1)-C(37) and
Rh(1)-C(38) distances are consistent with the C(37)-
C(38) bond being noncoordinating; the C-C bond dis-
tances within the η4-phenyl do not, however, differ
1
3
are seen at δ 3.81 (s, 2H, CH2), 5.21 (t, H, J HH ) 7
Hz, p-(η6-Ph)), 5.51 (d, 2H, 3J HH ) 7 Hz, o-(η6-Ph)), 5.92
3
(pseudo t, 2H, J HH ) 7 Hz, m-(η6-Ph)); for the minor
isomer “corresponding” resonances are seen at δ 3.33,
5.23, 5.55, and 6.02 with the same splitting patterns
and J values as for the major isomer. The nature of the
second isomer is unclear. The 0.48 ppm difference in
the δ(CH2) resonances suggests that one isomer may
coordinate the amine through the benzylic arene; reso-
nance structures of the type shown in eq 2 could be
(9) In a solution of [Rh(H)2(PPh3)2(MeOH)2]PF6 (2; 0.074 g, 0.084
mmol) in MeOH (5 mL), a solution of the amine (0.031 g, 0.168 mmol)
in MeOH (1 mL) was cannulated under Ar and the resulting deep red
solution stirred for 2 h at room temperature. Volume reduction to ∼1
mL afforded a red solid that was collected, washed with Et2O (3 × 2
mL), and dried in vacuo. Yield: 0.060 g (75%). 31P{1H} NMR (CD2-
Cl2): δ 46.61 (d, J RhP ) 211 Hz). 1H NMR (CD2Cl2): δ 3.83 (d, 2H,
3
3
CH2, J HH ) 5 Hz), 4.08 (t, 1H, NH, J HH ) 5 Hz), 5.09 (t, 1H, p-(η6-
3
3
Ph), J HH ) 6 Hz), 5.42 (d, 2H, o-(η6-Ph), J HH ) 7 Hz), 5.94 (pseudo t,
2H, m-(6η-Ph), J HH ≈ 6 Hz), 7.15-7.70 (m, 35H, aromatics). 13C{1H}
3
NMR (CD2Cl2): δ 48.60 (CH2), 90.35 (o-η6-Ph), 91.47 (p-η6-Ph), 104.66
(m-η6-Ph), 115.12 (ipso-η6-Ph). IR (KBr pellet): ν 1567 (C-N, m), 3388
cm-1 (N-H, s). Anal. Calcd for C49H43NP3F6Rh: C, 61.58; H, 4.54; N,
1.47. Found: C, 61.40; H, 4.54; N, 1.65. X-ray-quality crystals were
obtained from evaporation of a CH2Cl2/hexanes solution of 3.
(10) Crystal data for 3: space group P21/c, a ) 13.5394(8) Å, b )
18.4378(7) Å, c ) 18.0210(7) Å, â ) 104.352(1)°, Z ) 4, Fc ) 1.521 g/cm3,
R ) 0.040, Rw ) 0.086.
involved with the relative contributions of the forms
perhaps being dependent on H-bonding interactions
(11) Muetterties, E. L.; Bleeke, J . R.; Wucherer, E. J .; Albright, T.
A. Chem. Rev. 1982, 82, 499.
(12) Hughes, R. P. In Comprehensive Organometallic Chemistry;
Wilkinson, G., Stone, F. G. A., Abel, E. W., Eds.; Pergamon Press:
Oxford, U.K., 1982; Vol. 5, p 277, and references therein.
(13) Maitlis, P. M. Chem. Soc. Rev. 1981, 10, 1.
(15) Singewald, E. T.; Slone, C. S.; Stern, C. L.; Mirkin, C. A.; Yap,
G. P. A.; Liable-Sands, L. M.; Rheingold, A. L. J . Am. Chem. Soc. 1997,
119, 3048.
(16) Werner, H.; Canepa, G.; Ilg, K.; Wolf, J . Organometallics 2000,
19, 4756.
(14) Dunbar, K. R.; Quillevere, A. Organometallics 1993, 12, 618.

Communications
Organometallics, Vol. 22, No. 6, 2003 1179
between the NH moiety and MeOH. In the presence of
excess amine (2:1), no dissociation of amine from 3 is
observed, and 3 is seen as a single isomer (δ 46.71 d,
J RhP ) 211 Hz). Exposure of solutions of 3 to 1 atm H2
at room temperature gave no hydrogenation of the
coordinated arene: 3 in CD2Cl2 remains unaltered (cf.
eq 1), while in MeOH or acetone, free amine and the
respective [Rh(H)2(PPh3)2(solvent)2]+ (2) are the only
species detected. The coordinated amine is labile in
coordinating media under 1 atm of H2 but, in the
presence of excess amine, 3 is formed under catalysis
conditions, and this clearly suppresses catalytic activity
(cf. Figure 1).
tectable. The data at 280 K correspond to a K value of
∼50 M-1, with 5 having a lifetime of ∼3.0 × 10-4 s. The
corresponding 1H NMR spectra reveal a set of two
2
doublets (δ 3.58, 4.01, 2d, J HH ) 12 Hz), indicating
inequivalence of the benzylic protons in 5, and the
resonance of the same protons in the free amine (δ 3.79
s, CH2).
In terms of the catalysis illustrated in Figure 1, the
(PhCH2)2NH amine generated does not inhibit the
hydrogenation; even on complete hydrogenation of
PhCH2NdCHPh, the concentration of accumulated
amine (0.053 M) gives a 5/[Rh(PPh3)2(MeOH)2]+ value
of ∼2, and both of these labile species have appropriate
solvent sites where the imine substrate could be acti-
vated for subsequent hydrogenation. The difference
between the amines is the CH2 “spacer”, and this leads
to binding through the arene (no spacer) or through the
N-atom (spacer). Electronic (versus steric) factors are
likely to be more important, with conjugated resonance
contributions perhaps favoring arene binding (see eq 2),
and also PhN(H)CH2Ph is a weaker base than (PhCH2)2-
NH as an N-donor. That the corresponding imines both
bind to cis-[Rh(PPh3)2(solvent)2]+ to form ortho-meta-
lated species of the type [Rh(H)(PPh3)2(RNdCH(o-
C6H4))(solvent)]+ with η1(N) and η1(C) bonding (R ) Ph,
CH2Ph)8 tends to rule out steric factors as important in
the difference observed in the bonding of the corre-
sponding amines. Inhibition of catalytic hydrogenation
of an imine by catalyst poisoning through the amine
product was predictable, but binding of the amine via
an arene moiety was not.
A quite different interaction with 2 is seen at room
temperature under Ar with dibenzylamine, the hydro-
genation product of PhCH2NdCHPh (amine:Rh ) 2).
The NMR data are consistent with the labile equilibri-
um shown in eq 3, set up after loss of H2 from 2. At
<243 K, the 31P{1H} NMR spectrum is resolved into an
AMX eight-line pattern (δ 43.38 dd, J RhP ) 169 Hz, 2J PP
2
) 55 Hz; δ 58.15 dd, J RhP ) 217 Hz, J PP ) 55 Hz)
consistent with the presence of 5; comparison with
literature data17 leads us to assign the more downfield
resonance with the larger J RhP value to the phosphine
trans to MeOH and the upfield resonance to that trans
to the amine. With an increase of temperature, the
equilibrium shifts to the left-hand side and cis-[Rh-
(PPh3)2(CD3OD)2]+ (δ 57.02 d, J RhP ) 207 Hz)6 is fully
formed at ∼330 K; at intermediate temperatures,
broader resonances are seen, and at ∼280 K the
resonances broaden into the baseline and are nonde-
Ack n ow led gm en t. We thank the Natural Sciences
and Engineering Research Council of Canada for finan-
cial support.
Su p p or tin g In for m a tion Ava ila ble: Tables giving crys-
tallographic data for 3, including crystal data and structure
refinement details, atomic coordinates, bond distances and
angles, and torsion angles. This material is available free of
charge via the Internet at http://pubs.acs.org.
(17) Becalski, A. G.; Cullen, W. R. C.; Fryzuk, M. D.; J ames, B. R.;
Kang, G.-J .; Rettig, S. J . Inorg. Chem. 1991, 30, 5002.
OM020992C