Crabtree's catalyst revisited; Ligand effects on stability and durability

Article abstract of DOI:10.1039/b711979h

The extent of time-dependent deactivation of monophosphine monoamine iridium hydrogenation catalysts by trimer formation is strongly dependent on ligand structure; attempts to counter this process lead to the observation of an oligomerisation resistant catalyst. The Royal Society of Chemistry.

Full text of DOI:10.1039/b711979h

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Crabtree’s catalyst revisited; Ligand effects on stability and durability
Yingjian Xu, D. Michael P. Mingos and John M. Brown*
Received (in Cambridge, UK) 3rd August 2007, Accepted 24th September 2007
First published as an Advance Article on the web 18th October 2007
DOI: 10.1039/b711979h
The extent of time-dependent deactivation of monophosphine
monoamine iridium hydrogenation catalysts by trimer forma-
tion is strongly dependent on ligand structure; attempts to
counter this process lead to the observation of an oligomerisa-
tion resistant catalyst.
The discovery of active cationic iridium catalysts was one of the
1
milestones of homogeneous hydrogenation. Crabtree’s work with
complex 1a demonstrated a high level of reactivity, particularly for
Our interest in the topic centred on efforts to prepare non-
highly substituted alkenes, which contrasted with the then known
chelate asymmetric catalysts. This needed further information on
rhodium and ruthenium catalysts. Further work demonstrated
the lability of both the phosphorus and nitrogen ligands under
their effectiveness in directed hydrogenation of cyclic alkenes
catalytic turnover conditions, and also the stability of the
carrying coordinating substituents such as OH, CO R or
2
monomeric catalyst. On analysis of the X-ray structure of the all
syn-trimer 2, it was clear that increase in the steric bulk of one of
the two ligands might inhibit its formation. Indeed the X-ray
structure indicates close HH contacts for both P and N ligands.
Correspondingly, we prepared the analogues from quinoline, 4a,
and from 5,6,7,8-tetrahydroquinoline 5a, the latter being char-
acterised by X-ray (Fig. 2a). PM3 calculations were carried out on
the putative trimer from the cation of 5a, using these X-ray
2
. A general drawback of these catalysts has been their
CONR
2
sensitivity to extraneous proton-bearing impurities. In addition
there is a tendency to form an inactive trimeric heptahydride 2
under turnover conditions at low alkene concentration by an
irreversible process, shown in Fig. 1. The trimer formally requires
the addition of the neutral 16-electron species. [IrH (PCx
3
3
)L] to
+
two molecules of [IrH (PCx )L] and the formation of the former
2
3
requires the presence of free base, implying N-ligand dissociation.
structural parameters together with those of the original Ir trimer
8
. These revealed that the minimum energy structure possessed
2
˚
considerable repulsive H–H interactions, several below 2.1 A.
Hydrogenation of complexes 4a and 5a in CD Cl solution was
2
2
1
followed by H NMR, and by ES-MS. For the quinoline complex
2
4
a (PF
6
), the characteristic hydridic peaks at d 23.7 (q, JPH
50 Hz) (bridging) and 219.2, 226.0 ppm (terminal) appeared as
the COD ligand was reduced.{ The analogue 5a has a significantly
bulkier imine ligand, but complex oligomeric hydrides were still
1
observed in the H NMR right from the onset of its hydrogenation
under similar conditions. The ‘‘conventional’’ trimer 6 analogous
Fig. 1 The deactivating trimerisation process in classical Ir homogeneous
hydrogenation; (thQ = 5,6,7,8-tetrahydroquinoline).
Pfaltz’s development of enantiomerically pure PN-chelating
ligands, typified by 3, adds a new dimension, since it extends
3
asymmetric hydrogenation to new classes of reactant. The
importance of this advance has been recognised in subsequent
contributions from several groups, notably Andersson and
4
Burgess. In most applications save the reduction of 1,19-disub-
5
are required. Like their
stituted alkenes, high pressures of H
2
simple achiral analogues, the catalysts are prone to deactivation by
6
trimerisation, and there is a high degree of sensitivity to traces of
water. The use of large non-coordinating counteranions and
especially BARf (tetrakis-(3,5-bis-trifluoromethylphenyl)borate)
provides a practical solution to these difficulties provided that
7
rigorously anhydrous conditions are applied.
Fig. 2 ORTEP diagrams of the X-ray structures of the cationic part of
complexes (a) 5a, NPIr = 93.6u, and (b) 8a, NPIr = 90.3u, with H-atoms
omitted for clarity. The second complex exhibits conformational disorder
Chemistry Research Laboratory, 12 Mansfield Rd, Oxford, OX1 3TA.
E-mail: john.brown@chem.ox.ac.uk; Fax: +44 (0)1865285002
9
in the tetrahydroquinoline entity.
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to 2 was the minor species in this case, accompanied by a novel
incremental improvement rather than an advance. In view of
11
tetrameric iridium species currently under investigation, and which
6
earlier results, we compared the reactivity of PF and BARf
10
will shortly be the subject of an independent publication.
complexes. In our hands, hydrogenation of b-pinene with 4b
(0.5 mol%) was if anything slightly slower than with 4a under
comparable conditions.
Much recent iridium catalysis has been carried out with BARf
as counteranion, and in the simple case of the recently prepared
complex 1b the hydrogenation of 1-octene is somwhat faster than
Variation of the electronic properties of the phosphorus ligand
was considered next. In the iridium asymmetric hydrogenation of
2-substitiuted pyridines to piperidines with arylphosphino analo-
gues of ligand 3, the 4-fluorophenyl complex was the most
11,12
with 1a.
Consequently we monitored the stiochiometric
1
hydrogenation of 4b by H NMR, and noted that trimer
formation as described earlier was accompanied by the formation
of an intermediate, with a characteristic resonance at 216.0 ppm.
In the ES-MS of this same sample a strong monoiridium peak at
m/z 608.2993 is observed (calc. 608.2997), corresponding uniquely
to the cationic hexahydride 7. Diphosphine analogues of this
structure are known and have also been computationally
1
7
effective. Although arylphosphine-derived relatives of complex 1
1
8
have only rarely been applied in catalysis, we decided to
synthesise complex 8 for comparison. The desired iridium complex
8a was readily prepared and characterised by X-ray (Fig. 2b). It
proved to be an effective hydrogenation catalyst that is clearly
1
3
2
1
characterised. DFT calculations support a similar 2g , 2g
2
superior in the rate of H uptake close to completion. The lack of
14
configuration for hydridic entities of the PN-complex. The
tail-off indicates a lower tendency towards deactivation (Fig. 3).
In order to test the mechanism of trimerisation further, a
analogous species is not seen, however, in the parallel experiment
2
when PF
6
2 2
is employed as the anion in CD Cl . Once observed,
crossover experiment was devised. In this a mixture of complexes
1
and the H NMR
the hexahydridic species was seen in other cases. When hydro-
2
1b and 8b was hydrogenated in CD Cl
2
2
t
complex 4a was carried out in Bu OMe, the
genation of the PF
6
monitored over the course of reaction (Fig. 4).
same intermediate monohydridic species was seen by ES-MS.
The previously characterised trimer 2 was observed with only
minor amounts of other hydridic species apparent. The ES-MS
spectrum showed a predominant dication peak at 660.3304 (calc.
6
60.3306) with two other dications at about 10% of its intensity.
2+
The first of these was (2–pyridine) ; as a general observation in
these MS studies, loss of a nitrogen ligand from the trimeric
The relative reluctance of complex 5 to form an inert trimer
under hydrogenation conditions encouraged us to test it in
catalysis. b-Pinene was selected as a moderately hindered alkene.
1
5
Under the hydrogenation reaction conditions in dichloroethane,
interconversion with the more stable trisubstituted alkene a-pinene
16
is rapid. The uptake of H was monitored in a constant volume
2
rig operating at ca. 1.3 bar initial pressure. When complex 1a was
employed, there was a significant tail-off in hydrogenation rate
close to the point of completion, as would be expected as
deactivation sets in through trimerisation (Fig. 3). Over the same
region of the hydrogenation reaction, the loss of activity was less
pronounced when 5a was employed as catalyst under otherwise
identical conditions. Although promising, this represented an
Fig. 3 Hydrogenation of b-pinene with three different Ir complexes in
Fig. 4 The two crossover experiments described in the text. Equimolar
dichloroethane showing the last 15% of hydrogen take-up.
amounts of the two complexes were employed in each case.
2
00 | Chem. Commun., 2008, 199–201
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P. G. Andersson, Org. Lett., 2007, 9, 1659–1661; (c) M. Engman,
J. S. Diesen, A. Paptchikhine and P. G. Andersson, J. Am. Chem. Soc.,
complexes is commonplace whereas phosphine loss is not
observed. The second was a mixed cluster containing two (Cx
3
P
2007, 129, 4536–4537; (d) F. Giacomina, A. Meetsma, L. Panella,
Ir py) units and one (ArFP Ir thq) unit; no other products of
crossed trimerisation could be detected.
L. Lefort, A. H. M. de Vries and J. G. de Vries, Angew. Chem., Int. Ed.,
2007, 46, 1497–1500; (e) J. G. Zhou and K. Burgess, Angew. Chem., Int.
Ed., 2007, 46, 1129–1131; (f) T. T. Co and T. J. Kim, Chem. Commun.,
2006, 3537–3539.
S. P. Smidt, A. Pfaltz, E. Martinez-Viviente, P. S. Pregosin and
A. Albinati, Organometallics, 2003, 22, 1000–1009.
6 A. Pfaltz, J. Blankenstein, R. Hilgraf, E. Hormann, S. McIntyre,
F. Menges, M. Schonleber, S. P. Smidt, B. Wustenberg and
N. Zimmermann, Adv. Synth. Catal., 2003, 345, 33–43.
S. P. Smidt, N. Zimmermann, M. Studer and A. Pfaltz, Chem.–Eur. J.,
2004, 10, 4685.
8 D. A. Fletcher, R. F. McMeeking and D. Parkin, J. Chem. Inform.
The inference is clear; ligand dissociation from the cation of 8 is
an unfavourable process during hydrogenation. By contrast, a
similar experiment in which a mixture of 4b and the electronically
similar complex 9b was hydrogenated showed extensive scram-
5
1
bling. In the H NMR spectrum, a proliferation of peaks in the
215 to 220 ppm region corresponding to different trimers was
7
observed. In the ES-MS spectrum taken immediately following the
completion of hydrogenation, the dicationic trimer from 4b, but
not that from 9b, was observed as its [M-Quinoline] ion. In
addition at least nine crossover products were observed, each
lacking a single imine ligand. This experiment demonstrates that
trimerisation is a consequence of ligand lability, more likely Ir–N
bond dissociation.
Comput. Sci., 1996, 36, 746–749.
Crystal data for 5a: C35H
+
56IrNP , M = 714.38; triclinic, P1; a =
¯
9
˚
1
0.0545(2), b = 11.8176(2), c = 15.5293(3) A, a = 97.6584(8)u, b =
3
103.4087(9)u, c = 99.2889(11)u, V = 1743.13(6) A˚ ; T = 150 K; Z = 2;
m = 3.980 mm ; orange-red plate. Reflections collected = 25277;
21
independent reflections =7896 (Rint = 0.062); R values [I . 3s(I), 6887
reflections]: R1 = 0.0415, wR2 = 0.0489 (all data). Crystal data for 9a:
These experiments provide a rational framework for the
synthesis of novel asymmetric iridium catalysts. Recognising the
likely involvement of ligand dissociation in formation of unreactive
trimers is a key to development of oligomerisation resistant
catalysts.
+
IrNP , M = 750.21; monoclinic, P 2
C
35
H
35
F
3
1
/n; a = 15.2914(2), b =
˚
12.2710(2), c = 17.6370(3) A, a = 90u, b = 91.7169(6)u, c = 90u, V =
3
21
˚
3
307.93(9) A ; T = 150 K; Z = 4; m = 4.211 mm ; orange block.
Reflections collected = 44293; independent reflections = 7880 (Rint
.059); final R values [I . 3s(I), 5480 reflections]: R1= 0.0278, wR2 =
.0293 (all data). For crystallographic data in CIF or other electronic
=
0
0
We thank Merck for an unrestricted grant in support of YX,
and Johnson Matthey for the loan of iridium salts, Dr Andrew
Cowley for the two X-ray structures, Dr Tim Claridge and Dr
Barbara Odell for NMR support. Dr Neil Oldham and Dr James
McCullagh were very helpful in the obtention of FT-ICR MS
spectra.
format, see: CCDC 654176, 654177. For crystallographic data in CIF or
other electronic format see DOI: 10.1039/b711979h.
0 Y. Xu, M. A. Celik, A. Thompson, D. M. P. Mingos and J. M. Brown,
to be published.
11 L. D. Vazquez-Serrano, B. T. Owens and J. M. Buriak, Inorg. Chim.
Acta, 2006, 359, 2786–2797.
1
12 For alternative non-nucleophilic counterions in Ir catalysis, see:
G. L. Moxham, T. M. Douglas, S. K. Brayshaw, G. Kociok-Kohn,
J. P. Lowe and A. S. Weller, Dalton Trans., 2006, 5492–5505.
3 A. C. Cooper, O. Eisenstein and K. G. Caulton, New J. Chem., 1998,
1
Notes and references
22, 307–309; R. H. Crabtree, M. Lavin and L. Bonneviot, J. Am. Chem.
Soc., 1986, 108, 4032–4037.
14 M. A. Celik, unpublished work, Oxford, 2007.
15 A stable dichloroethane Cl, Cl-chelated complex has recently been
{
5
For the parent dication 2, the corresponding values are d 23.4 (q, JPH
0 Hz) (bridging) and 220.1, 225.9 ppm (terminal).
1
2
3
4
(a) R. H. Crabtree, H. Felkin and G. E. Morris, J. Organomet. Chem.,
977, 141, 205–215; (b) R. H. Crabtree, H. Felkin, T. Fillebeen-Khan
and G. E. Morris, J. Organomet. Chem., 1979, 168, 183–195.
characterised for an cationic Ir-diphosphine dihydride cation,
G. L. Moxham, S. K. Brayshaw and A. S. Weller, Dalton Trans.,
2007, 1759–1761.
1
(a) R. H. Crabtree and M. W. Davis, J. Org. Chem., 1986, 51,
16 J. M. Brown, A. E. Derome, G. D. Hughes and P. K. Monaghan, Aust.
J. Chem., 1992, 45, 143–153; J. M. Brown, A. E. Derome and S. A. Hall,
Tetrahedron, 1985, 41, 4647–4656.
17 C. Y. Legault and A. B. Charette, J. Am. Chem. Soc., 2005, 127,
8966–8967.
18 C. S. Chin and B. Lee, J. Chem. Soc., Dalton Trans., 1991, 1323–1327;
I. S. Cho, B. Lee and H. Alper, Tetrahedron Lett., 1995, 36,
6009–6012.
2
655–2661; (b) J. M. Brown, Angew. Chem., Int. Ed. Engl., 1987, 26,
90–203.
A. Lightfoot, A. Schnieder and A. Pfaltz, Angew. Chem., Int. Ed., 1998,
7, 2897–2899; S. Bell, B. Wustenberg, S. Kaiser, F. Menges, T. Netscher
1
3
and A. Pfaltz, Science, 2006, 311, 642, and intervening references.
Most recently: (a) X. Li, L. Kong, Y. Gao and X. Wang, Tetrahedron
Lett., 2007, 48, 3915–3918; (b) P. Cheruku, S. Gohil and
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Chem. Commun., 2008, 199–201 | 201