Functional phosphines XII. Heterolytic H2 cleavage and homogeneous 2 bonds on the lefthand side signC=O hydrogenation catalyzed by platinum metal β-aminophosphine complexes

Article abstract of DOI:10.1016/S1387-7003(03)00009-1

Acetophenone was enantioselectively reduced to 1-phenylethanol (21-71% e.e.) by the use of base-modified Ir and Rh catalysts derived from optically active β-aminophosphines, [{(1R,2S)-, (1R,2R)-, (1S,2S)-Ph₂PCH(Ph)CH(Me)N(H)R}M(COD)]BF₄-KOH (R=H, Me, i-Pr, CH₂Ph), in methanol under H₂ (10 bar). The isolation of an equally active amidoiridium catalyst, [(Ph₂PCH₂CMe₂NH)Ir(COD)], its ability to oxidatively add dihydrogen, and the observation of both H₂/D⁺ and H₂/D₂ exchange reactions during catalysis, which crucially depends on the use of protic solvents, provided evidence for a mechanism involving hydride and proton transfer as well as heterolytic H₂ cleavage on dihydrogen-hydrido-amido and dihydrido-amine tautomers, [{'P ∩ NR'}Ir(H₂)H]⁺ and [{'P ∩ N(H)R'}IrH₂]⁺, respectively.

Full text of DOI:10.1016/S1387-7003(03)00009-1

Inorganic Chemistry Communications 6 (2003) 443–446
www.elsevier.com/locate/inoche
Functional phosphines XII. Heterolytic H2 cleavage
and homogeneous ˜C@O hydrogenation catalyzed by
platinum metal b-aminophosphine complexes q
*
€
Lutz Dahlenburg , Rainer Gotz
Institut f€ur Anorganische Chemie der Universit€at Erlangen-N€urnberg, Egerlandstraße 1, Erlangen D-91058, Germany
Received 26 July 2002; accepted 25 November 2002
Abstract
Acetophenone was enantioselectively reduced to 1-phenylethanol (21–71% e.e.) by the use of base-modified Ir and Rh catalysts
derived from optically active b-aminophosphines, [{(1R,2S)-, (1R,2R)-, (1S,2S)-Ph2PCH(Ph)CH(Me)N(H)R}M(COD)]BF4–KOH
(R ¼ H, Me, i-Pr, CH2Ph), in methanol under H2 (10 bar). The isolation of an equally active amidoiridium catalyst,
[(Ph2PCH2CMe2NH)Ir(COD)], its ability to oxidatively add dihydrogen, and the observation of both H2=Dþ and H2=D2 exchange
reactions during catalysis, which crucially depends on the use of protic solvents, provided evidence for a mechanism involving
hydride and proton transfer as well as heterolytic H2 cleavage on dihydrogen–hydrido–amido and dihydrido–amine tautomers,
[{ÔP \ NRÕ}Ir(H2)H]þ and [{ÔP \ NðHÞRÕ}IrH2]þ, respectively.
Ó 2003 Elsevier Science B.V. All rights reserved.
Keywords: Chirality; P,N ligands; Iridium; Rhodium; Catalysis; Hydrogenation
1. Introduction
hydrogen and specifically address the hydrogen-activat-
ing step during catalysis.
Starting from commercially available (pseudo)ephed-
rine diastereomers, we have recently worked out several
useful procedures for the synthesis of optically active
bidentate b-aminophosphines Ph2PCH(Ph)CH(Me)
N(H)R (R ¼ H, alkyl) [1a], P,N-chelated iridium and
rhodium complexes of which, viz. [{Ph2PCH(Ph)
CH(Me)N(H)R}M(COD)]BF4 (COD ¼ g4-1,5-C8H12),
were prepared to serve as catalysts for the enantioselec-
tive hydrogenation of prochiral ketones [2]. In this article
we report on the results of our studies on the application
of such IrI and RhI catalysts to the asymmetric reduction
of the model substrate acetophenone with molecular
2. Experimental
All manipulations were performed under nitrogen
using standard Schlenk techniques. Physical measure-
ments were made as outlined in previous work [1]. The
diverse b-aminophosphines (1R,2S)-, (1R,2R)-, and
(1S,2S)-Ph2PCH(Ph)CH(Me)N(H)R (R ¼ H, Me, i-Pr,
CH2Ph; C-1 linked to P; C-2 bonded to N) as well as the
achiral Ph2PCH2CMe2NH2 ligand were obtained by
ring-opening with Ph2PH of the respective BF3-acti-
vated aziridines using our published procedure [1a] or
slight modifications thereof. The complexes [{Ph2PCH
(Ph)CH(Me)N(H)R}M(COD)]BF4 and [(Ph2PCH2
CMe2NH2)Ir(COD)]BF4 resulted from treatment of
½MðCODÞ2ŠBF4 with equimolar quantities of the re-
quired P,N ligands in THF as detailed earlier [1a] for
[{(1S,2S)-Ph2PCH(Ph)CH(Me)NH2}Ir(COD)]BF4 (1)
and [{(1R,2R)-Ph2PCH(Ph)CH(Me)N(H)Me}M(COD)]
BF4, where M ¼ Ir (2-Ir) or Rh (2-Rh). Combination of
q Presented at the XXXVth International Conference on Coordina-
tion Chemistry (Heidelberg; July 21–26, 2002); Part XI: [1a]; simul-
taneously Part XIV of ‘‘Chiral bisphosphines’’ (Part XIII: [1b]).
* Corresponding author. Tel.: +49-9131-85-27353; fax: +49-9131-85-
27387.
E-mail
address:
dahlbg@anorganik.chemie.uni-erlangen.de
(L. Dahlenburg).
1387-7003/03/$ - see front matter Ó 2003 Elsevier Science B.V. All rights reserved.
doi:10.1016/S1387-7003(03)00009-1

444
L. Dahlenburg, R. G€otz / Inorganic Chemistry Communications 6 (2003) 443–446
½Ir2ðl-ClÞ2ðCODÞ2Š with Ph2PCH2CMe2N(H)Li in 1:2
molar ratio in hydrocarbon solution gave the iridium(I)
amide [(Ph2PCH2CMe2NH)Ir(COD)]. The identity of
the new compounds [{(1S,2S)-Ph2PCH(Ph)CH(Me)
N(H)R}Ir(COD)]BF4 [R ¼ i-Pr (3), CH2Ph (4)], [{(1R,
of hydrogenation from the solvent because Me3COH,
unlike CH3OH, cannot serve as a hydrogen source.
The enantiomeric excesses e.e. for (R)- or (S)-
PhCH(OH)Me ranged from 21% (S) for the iridium
system 6 based on the (1R,2S)-Ph2PCH(Ph)CH(Me)
N(H)Pr-i ligand to 71% (S) for the rhodium catalyst 2-
Rh derived from(1R,2R)-Ph2PCH(Ph)CH(Me)N(H)Me.
The related iridium complex 2-Ir compared to 2-Rh in
activity (ꢀ7 vs. 6 h for quantitative substrate conver-
sion) but afforded a decreased optical yield of only 55%
e.e. In the series of iridium catalysts having primary or
secondary amino functions supported on a ligand
backbone with like, i.e., (S,S) or (R,R), configurations at
the 1- and 2-positions, the e.e. values increased on
substituting –N(H)Me for –NH2 and then decreased
in the order –NðHÞMe > –NðHÞPr-i > –NðHÞCH2Ph.
Hence, a medium-sized nitrogen substituent appears to
be sufficient for attaining the maximal enantioselectivity
that can be achieved with these systems. Furthermore,
catalyst systems based on N-alkylated b-aminophos-
phines with unlike carbon configurations, viz. 5 and 6,
required longer times for complete ˜C@O hydrogena-
tion and showed lower selectivities than those derived
from their diastereomers 2-Ir and 3 with like configu-
rations in the C2 linkages. Finally, the configuration at
C-1 bearing the phenyl group turned out to be decisive
for the preferred formation of the (S) or (R) product in
that the (R) alcohol predominated in the isomeric mix-
tures resulting from hydrogenations that were catalyzed
by systems based on ligands with (S) configuration at
the 1-position, while the (S) enantiomer was formed
preferentially in reactions promoted by catalysts bearing
aminophosphines with (R) configuration at C-1.
2S)-Ph2PCH(Ph)CH(Me)N(H)R}Ir(COD)]BF4
[R ¼
Me (5), i-Pr (6)], [(Ph2PCH2CMe2NH2)Ir(COD)]BF4
(7), and [(Ph2PCH2CMe2NH)Ir(COD)] (8) was estab-
lished, inter alia, by the following analytical data: (3)
Found: C, 51.63%; H, 5.77%; N, 1.73%. C32H40 BF4
IrNP (748.73) Calc.: C, 51.34%; H, 5.39%; N, 1.87%. (4)
Found: C, 54.52%; H, 5.21%; N, 1.62%. C36H40BF4
IrNP (796.77) Calc.: C, 54.27%; H, 5.06%; N, 1.76%. (5)
Found: C, 50.24%; H, 5.32%; N, 2.11%. C30H36BF4
IrNP (760.62) Calc.: C, 50.00%; H, 5.04%; N, 1.94%. (6)
Found: C, 51.65%; H, 5.30%; N, 2.00%. C32H40BF4
IrNP (748.33) Calc.: C, 51.34%; H, 5.39%; N, 1.87%. (7)
Found: C, 44.42%; H, 5.09%; N, 2.09%. C24H32BF4
IrNP (644.57) Calc.: C, 44.73%; H, 5.00%; N, 2.17%. (8)
Found: C, 52.55%; H, 5.97%; N, 2.12%. C24H31IrNP
(556.74) Calc.: C, 51.78%; H, 5.61%; N, 2.52%.
Hydrogenation experiments were carried out in
methanol at 25 °C under 10 bar of H2 at a catalyst-to-
ketone ratio of 1:100, in the presence of 1.1 equiv of
added KOH. Enantiomeric excesses were determined
polarimetrically as previously described [1a] and by
HPLC using a Daicel Chiralcel OD column. H2=Dþ and
H2=D2 exchange reactions were run in NMR tubes
pressurized to 10 bar of H2 and H2=D2 (1:1), respectively.
3. Results and discussion
When coupled with an equimolar amount of an al-
kaline base (KOH) or – less efficiently – of an amine [1a]
in methanol or tert-butanol under H2 (10 bar) at 25 °C,
complexes 1–6 act as catalysts for the homogeneous
hydrogenation of acetophenone to 1-phenylethanol
(Table 1). In the absence of base or in aprotic solvents
such as benzene or THF, virtually no hydrogenation was
observed. The successful use of the tertiary alcohol as a
reaction medium rules out a pathway involving transfer
The mandatory presence of base in the catalytic sys-
tem suggests that in situ-generated species that have lost
Hþ, possibly amido complexes, are likely to be the ac-
tual catalysts. Attempts were therefore made to isolate
such amides from treatment of 1–6 with a number of
bases varying in strength, such as alkoxides, amines, and
organolithium compounds. However, none of these re-
actions did lead to any defined –N(H)R deprotonation
product: While the NH function remained totally un-
Table 1
Enantioselective hydrogenation of acetophenone at 100% substrate conversion: performance of different cationic Ir- and Rh-catalysts based on chiral
b-aminophosphine ligandsa
Complex used
Reaction time (h)
e.e. (%)
[{(1S,2S)-Ph2PCH(Ph)CH(Me)NH2}Ir(COD)]BF4 (1)
[{(1R,2R)-Ph2PCH(Ph)CH(Me)N(H)Me}Ir(COD)]BF4 (2-Ir)
[{(1R,2R)-Ph2PCH(Ph)CH(Me)N(H)Me}Rh(COD)]BF4 (2-Rh)
[{(1S,2S)-Ph2PCH(Ph)CH(Me)N(H)Pr-i}Ir(COD)]BF4 (3)
[{(1S,2S)-Ph2PCH(Ph)CH(Me)N(H)CH2Ph}Ir(COD)]BF4 (4)
[{(1R,2S)-Ph2PCH(Ph)CH(Me)N(H)Me}Ir(COD)]BF4 (5)
[{(1R,2S)-Ph2PCH(Ph)CH(Me)N(H)Pr-i}Ir(COD)]BF4 (6)
7
7
38 (R)
55 (S)
71 (S)
51 (R)
26 (R)
38 (S)
21 (S)
6
12
32
32
50
a 0.014–0.030 mmol of Ir or Rh complex, 1.1 equiv. of KOH, and 1.4–3.0 mmol of acetophenone in 2.5 ml of methanol (substrate-to-catalyst ratio
100:1); T ¼ 25 °C; pðH2Þ ¼ 10 bar. Entries in the tables represent the average values of, at least, duplicate runs. Configurations at the 1- and 2-
positions of the ligand backbones refer to the carbon atoms bonded to P (C-1) and N (C-2), respectively.

L. Dahlenburg, R. G€otz / Inorganic Chemistry Communications 6 (2003) 443–446
445
attacked when triethyl amine, (-)-sparteine, or potas-
sium hydroxide/methanol were used at equimolar con-
centrations, ketonic and/or imine-type compounds were
identified by IR in the mixtures resulting from combi-
nation of 1–6 with excess LiOMe or KOBu-t or with n-
butyl lithium. It therefore appears that the coordin-
atively unsaturated IrI and RhI pre-catalysts tend to
undergo degradation of their P,N ligands in the presence
of an excess of strong base, probably by b-elimination.
Hence, the b-aminophosphine H2NCMe2 CH2PPh2,
which lacks hydrogens adjacent to the amino group, was
used for further studies. As anticipated, the amine
complex [(Ph2PCH2CMe2NH2)Ir(COD)]BF4 (7) as well
accompanied by the appearance of a broad 1H reso-
nance at d ¼ 3:65, assignable to partially re-exchanged
[{(1R,2S)-Ph2PCH(Ph)CH(Me)N(H)Pr}Ir(COD)]BF4.
For all amine complexes containing –NH(R) functions
– but not for [{(1R,2R)-Ph2PCH(Ph)CH(Me)NMe2}
M(COD)]BF4(!) – catalytic H2=D2 scrambling was ob-
served at room temperature by 1H NMR, when CD3CN
solutions of these compounds were exposed to 10 bar of
an H2=D2 equimolar mixture. This isotope exchange
between the two gases corresponds to the intermediate
coordination of H2 or D2 to iridium, although the for-
mation of the adducts could not be monitored by NMR
down to )60 °C. Similar to the H2=Dþ exchange, the
observed H2=D2 scrambling process should involve het-
erolytic splitting of the dihydrogen molecule [4a,7] be-
cause the homolytic alternative would depend on
simultaneous coordination of H2 and D2 to the same
central atom and is therefore considered as highly un-
likely.
as
its
conjugate
amide
[(Ph2PCH2CMe2NH)
Ir(COD)] (8) could be made without difficulties (see
Experimental). Remarkably, amido complex 8 catalyzed
the hydrogenation of acetophenone to 1-phenylethanol
without the addition of a base, quantitatively transform-
ing the ketone to the alcohol under the usual conditions
in less than 2 h. By use of the combined catalytic system
7–KOH (1:1), the reduction was complete after ꢀ3 h.
Additional facts that pertain to the establishment of a
catalytic cycle arise from the following observations:
The ability of base-free amidoiridium(I) complex 8 to
act as a catalyst for ˜C@O hydrogenation likewise de-
pends on the use of alcohols (MeOH, t-BuOH) as sol-
vents, wherein this compound remains unprotonated!
This inertness of 8 toward protonation by alcohols is
attributed to the involvement of the nitrogen lone-pair
in p-bonding to the central metal with consequent
sp3 ! sp2 rehybridization and decreased amide basicity,
similar to the situation that has previously been en-
countered for the related anilide [ð2-Ph2PC6H4
NMeÞIrðCOÞðPPh3Þ2], where the nitrogen atom features
a trigonal-planar environment [3].
In apparent contrast to the heterolytic H–H cleavage
concluded from the foregoing findings, the initial
product obtained after short exposure to H2 (1 bar) of 1
in THF-d8 was identified as resulting from conventional
oxidative H2 addition; viz., cis-[{(1S,2S)-Ph2PCH(Ph)
1
CH(Me)NH2}IrH2(COD)]BF4 (9) [dð HÞ ¼ ꢁ16:31
(cis-2JðP; HÞ ¼ 11:5 Hz; IrH trans NH2), )12.44 (cis-
2JðP; HÞ ¼ 14:8 Hz; IrH trans ˜C@C—)]. Similarly, the
adduct cis-[(Ph2PCH2CMe2NH2)IrH2(COD)]BF4 (10)
1
was characterized by NMR [dð HÞ ¼ ꢁ15:78 (cis-2J
ðP; HÞ ¼ 11:0 Hz; IrH trans NH2), )10.95 (cis-
2JðP; HÞ ¼ 15:9 Hz; IrH trans ˜C@C—)] in CD3CN so-
lutions of amine complex 7 kept under 5 bar of H2 in an
NMR tube. Prolonged exposure of the amine precursors
to 10 bar of H2 in an autoclave led to solvate-stabilized
addition products by hydrogenolytic loss of COD, such
as, e.g., cis,cis-[{(1S,2S)-Ph2PCH(Ph)CH(Me)N(H)Me}
IrH2ðNCMeÞ2]BF4 (11) [dðIrHÞ ¼ ꢁ21:20, )21.14 (cis-
2JðP; HÞ ¼ 17:2, 22.1, cis-2JðH; HÞ ¼ 7:1 Hz)]. In line
with the low basicity displayed by its amide function and
similar to [(2-Ph2PC6H4NMe)Ir(CO)ðPPh3Þ2] [3], amido
complex 8 was also seen to oxidatively add dihydrogen
When ketones, e.g., acetone, aceto-, and benzophe-
none, were reduced by H2 in MeOD using either amide
complex 8 or a (2-Ir)–tert-amine system as catalysts, the
product alcohols were invariably found to contain the
isotopomers R2CHODand R CDODin near 1:1 molar
2
1
1
ratio, HD gas [dð HÞ ¼ 4:70; JðH; DÞ ¼ 42:5 Hz] being
identified simultaneously. This points to the occurrence
of an H2=Dþ scrambling process ‘‘MeOD þ H2 ¼
MeOH þ HD’’, thereby giving evidence that heterolytic
H2 activation [4–6] takes place during ˜C@O hydroge-
nation. Catalytic H2=Dþ exchange was also observed
when MeODsolutions of the amineiridium(I) complexes
1–7 or of amide 8 were pressurized, in an NMR tube, to
5–10 bar of H2 but did not occur on attempted catalysis
by the N,N-dialkylated complex [{(1R,2R)-Ph2PCH
(Ph)CH(Me)NMe2}M(COD)]BF4! This clearly demon-
strates that the H2=Dþ exchange between the gas and
the solvent is supported by indispensable Ir–NH(R)
entities. Consistently, an H2=Dþ exchange experiment
that used pre-formed 6-d1, i.e., [{(1R,2S)-Ph2PCH(Ph)
CH(Me)N(D)Pr}Ir(COD)]BF4, in MeODunder H2, was
1
giving [(Ph2PCH2CMe2NH)IrH2(COD)] (12) [dð HÞ ¼
ꢁ18:01 (cis-2JðP; HÞ ¼ 8:9 Hz; IrH trans NH), )6.96
(cis-2JðP; HÞ ¼ 13:2 Hz; IrH trans ˜C@C—)] rather than
a monohydride, [(Ph2PCH2CMe2NH2)IrH(COD)], that
would result from heterolytic H2 addition.
In sharp contrast to [(Ph2PCH2CMe2NH2)Ir(COD)]
BF4 (7) and its – formally(!) – conjugate amide
[(Ph2PCH2CMe2NH)Ir(COD)] (8), amine- and amido-
iridium(III) dihydrides 10 and 12 were observed to exist
in an acid–base equilibrium in protic solvents: Addition
of small quantities of H2O or MeOH to amide 12, in situ
generated in CD3CN, caused the formation of amine
complex 10, which in turn could be deprotonated to 12
by adding potassium hydroxide at low concentrations.
The basic properties of amido complex 12 can safely be

446
L. Dahlenburg, R. G€otz / Inorganic Chemistry Communications 6 (2003) 443–446
attributed to its coordinative saturation which prevents
the nitrogen lone-pair from being involved in p-bonding
to the central metal.
similar equilibrium has previously been observed di-
rectly for a cationic Ir(H2)H complex having a pendant
NH2 group bonded to a cyclometalated benzoquinoli-
nate ligand [5]. Furthermore, cationic Ir(H2)H species
such as E are notorious for their propensity to support
H2=D2 scrambling in addition to exchange of H2 with
protons [4]. Finally, their acidity is such that they can be
deprotonated easily by the methoxide released during
product formation (or even by the solvent itself) to re-
generate A, thus maintaining the catalysis without the
requirement of added base (step f), as found for
amidoiridium(I) pre-catalyst 8. An acid–base equilib-
rium such as f would also account for the observed ex-
change of H2 gas with solvent protons. As an established
alternative to step f, deprotonation of dihydrido amine
tautomer F by added base (catalysis by amine complexes
1–7) can also serve to close the catalytic cycle.
Taken together, these observations would be consis-
tent with the proposed mechanism depicted in Scheme 1.
Basically, the cycle rests on the formation of a COD-
containing or solvated amido dihydride A (e.g., 12) as
the first key intermediate, accessible from amide 8 and,
by deprotonation of the corresponding acids F (e.g., 9–
11), from the amine side (1–7) as well. Because of their
coordinative saturation, IrIII complexes of type A can
act as bases to generate F at equilibrium concentrations
(vide supra) and, equally important, are inert toward
degradation by b hydrogen transfer, even if they bear
P,N ligands having hydrogen atoms adjacent to the ni-
trogen atom. The formation of ketone and alkoxide
species B and C (steps a and b) follows the generally
accepted Schrock–Osborn pathway [8,9], as does the
protolytic elimination of the product alcohol (step c),
which would explain that use of protic solvents is crucial
for Ir-catalyzed ˜C@O reduction. This pathway is fa-
vored over the metal–ligand bifunctional mechanism
proposed by Noyori [2a] as well as by Morris [6a,6c] for
Ru-catalyzed ˜C@O hydrogenations, because the latter
reactions do not depend on the presence of protic sol-
vents but occur with equal results in benzene [6b]. The
readily occurring H2=D2 and H2=Dþ exchange pro-
cesses, for which the presence of –NH(R) functions is
imperative (see above), are suggestive of an equilibrium
e between dihydrogen–hydrido–amido and dihydrido–
amine tautomers E and F (e.g., 9–11), where the g2-H2
species (formed from H2 addition to hydrido amide D;
step d) remains undetected by NMR even at low tem-
perature, probably because of too low concentration. A
Acknowledgements
Financial support from the Deutsche Forschungs-
gemeinschaft (SFB 583) and the Fonds der Chemischen
Industrie is gratefully acknowledged. We are also in-
debted to Degussa AG for a generous gift of platinum
salts.
References
€
[1] (a) L. Dahlenburg, R. Gotz, J. Organomet. Chem. 619 (2001) 88;
(b) L. Dahlenburg, V. Kurth, Inorg. Chim. Acta 319 (2001) 176.
[2] (a) For leading overviews on stereoselective ˜C@O reduction, see:
R. Noyori, T. Ohkuma, Angew. Chem. 113 (2001) 40;
Angew. Chem. Int. Ed. 40 (2001) 40;
(b) R. Noyori, Angew. Chem. 114 (2002) 2108;
Angew. Chem. Int. Ed. 41 (2002) 2008 (2001 Nobel Lecture in
Chemistry).
H
N
CH3O−
N
P
[3] L. Dahlenburg, K. Herbst, Chem. Ber./Recueil (1997) 1693.
[4] (a) For pertinent review articles, see: P.G. Jessop, R.H. Morris,
Coord. Chem. Rev. 121 (1992) 155;
Ir
Ir
(1−7)
(8)
CH3OH
P
(b) D.M. Heinekey, W.J. Oldham Jr., Chem. Rev. 93 (1993) 913;
(c) R.H. Crabtree, Angew. Chem. 105 (1993) 828;
Angew. Chem. Int. Ed. Engl. 32 (1993) 789;
(d) M.A. Esteruelas, L.A. Oro, Chem. Rev. 98 (1998) 577;
(e) G. Jia, C.-P. Lau, Coord. Chem. Rev. 190–192 (1999) 83.
[5] D.-H. Lee, B.P. Patel, E. Clot, O. Eisenstein, R.H. Crabtree, Chem.
Commun. (1999) 297.
H2
H2
H
H
H
Ir
C
O
CH3O−
CH3OH
N
N
P
Ir
H
P
H
a
e
F (e.g., 9−11)
A (e.g., 12)
H
H
Ir
H
H
N
P
E
N
P
[6] (a) K. Abdur-Rashid, A.J. Lough, R.H. Morris, Organometallics
19 (2000) 2655;
f
Ir
H
(b) K. Abdur-Rashid, A.J. Lough, R.H. Morris, Organometallcis
20 (2001) 1074;
CH3O−
CH3OH
O
B
(c) K. Abdur-Rashid, M. Faatz, A.J. Lough, R.H. Morris, J. Am.
Chem. Soc. 123 (2001) 7473.
CH3O−
H
N
Ir
H
N
Ir
d
c
b
[7] A. Llamazares, H.W. Schmalle, H. Berke, Organometallics 20
(2001) 5277.
P
P
H2
O
[8] R.R. Schrock, J.A. Osborn, J. Chem. Soc. Chem. Commun. (1970)
567.
H
C
H
CH3OH
D
C
OH
[9] Q. Jiang, Y. Jiang, D. Xiao, P. Cao, X. Zhang, Angew. Chem. 110
(1998) 1203;
Angew. Chem. Int. Ed. 37 (1998) 1100.
Scheme 1. Proposed catalytic cycle for ˜C@O hydrogenation; unla-
beled coordination sites occupied by CODor solvent molecules.