Iridium-catalyzed asymmetric isomerization of primary allylic alcohols

Article abstract of DOI:10.1002/anie.200901863

Nothing to sm(Ir)k at: Under appropriate reaction conditions, iridium hydride catalysts promote the isomerization of primary allylic alcohols. The best catalysts, like (R)-1 (P green, O red, N blue, Ir yellow), deliver the desired chiral aldehydes with excellent enantioselectivity and good yields. Mechanistic hypotheses have been developed on the basis of preliminary investigations.

Full text of DOI:10.1002/anie.200901863

Angewandte
Chemie
DOI: 10.1002/anie.200901863
Asymmetric Catalysis
Iridium-Catalyzed Asymmetric Isomerization of
Primary Allylic Alcohols**
Luca Mantilli, David Gꢀrard, Sonya Torche, Cꢀline Besnard, and Clꢀment Mazet*
Undoubtedly, the catalytic asymmetric isomerization of
allylic amines into enamines stands out as one of the most
accomplished and well-studied reaction in asymmetric catal-
ysis as illustrated by its industrial application.[1,2] In contrast,
the related asymmetric isomerization of primary allylic
alcohols to the corresponding aldehydes still constitutes a
significant challenge in organic synthesis.[2] Successful exam-
ples of highly active and selective catalysts for this trans-
formation remain rare and rely almost solely on the use of
chiral rhodium complexes.[3] Furthermore, high catalyst load-
ings, elevated temperatures, long reaction times, poor catalyst
accessibility, and limited substrate scope have prevented
widespread use of this method. We have recently shown that
rates. On one hand, the mild reaction conditions appeared
well-suited for studying the asymmetric version of the
reaction. On the other hand, any variation of the electronic
and steric requirements of 1 induced a complete loss of
catalytic activity, suggesting this might impose severe con-
straints on the design of related chiral complexes. Herein, we
report the identification of a highly active and selective
iridium catalyst for the asymmetric isomerization of primary
allylic alcohols into the corresponding chiral aldehydes.
Initial investigations began with a survey of known chiral
(P,N) iridium complexes which have been shown to be highly
active and selective catalysts in the asymmetric hydrogena-
tion of olefins (2–6a, Figure 1).[5–6] Reactions were carried out
on the model substrate 7 in THF at room temperature for
18 hours using 5 mol% of the precatalyst (Table 1, entries 1–
5). The iridium complexes were activated by slowly bubbling
hydrogen directly through the solution for 5 minutes. To favor
isomerization and to avoid competing hydrogenation, the
substrate was added only after complete extrusion of excess
hydrogen by degassing the solution. Whereas complexes 2–4
did not display any catalytic activity, the pyridyl/phosphinite
catalyst 5 afforded 19% of the aldehyde 8, albeit in a nearly
the hydrogenation catalyst [Ir(PCy3)(pyridine)(cod)]BArF
1
4
(Cy = cyclohexyl, cod = 1,5-cyclooctadiene, BArF4 = tetrakis-
[3,5-bis(trifluoromethyl)phenyl]borate) promoted exclusively
the isomerization of primary allylic alcohols under appropri-
ate experimental conditions (Scheme 1).[4]
Very low loadings of this analogue of Crabtreeꢀs catalyst
were used to quantitatively isomerize a wide range of
substrates at room temperature with appreciable reaction
Scheme 1. Iridium-catalyzed isomerization of primary allylic alcohols.
[*] L. Mantilli, D. Gꢀrard, S. Torche, Dr. C. Mazet
Department of Organic Chemistry, University of Geneva
Quai Ernest Ansermet 30, 1211 Geneva 4 (Switzerland)
Fax: (+41)22-379-3215
E-mail: clement.mazet@unige.ch
Dr. C. Besnard
Laboratoire de Cristallographie, University of Geneva
Quai Ernest Ansermet 24, 1211 Geneva 4 (Switzerland)
[**] This work was supported by the State Secretariat for Education and
Research. Prof. A. Alexakis is warmly thanked for unrestricted access
to his analytical facilities. We thank S. Rosset for GC analyses and
Dr. B. Vitorge and A. Pinto for assistance in NMR measurements.
Johnson-Matthey is also thanked for the generous loan of iridium
precursors. Prof. A. Pfaltz (University of Basel) is acknowledged for
a gift of catalysts 2 and 5.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200901863.
Figure 1. Scope of iridium catalysts investigated. Cy=cyclohexyl,
Ad=adamantyl.
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Table 1: Catalyst survey for the asymmetric isomerization of allylic
alcohol 7.
the best results being obtained at ꢀ108C wherein the rates are
still acceptable (Table 1, entries 11–13 and 16).
A series of other (E)-3,3-disubstituted allylic alcohols
were examined to assess the scope and limitations of our
methodology (Table 2).[8] The catalysts 6 f,g, which displayed
Entry[a]
Catalyst
T [8C]
t [h]
Conv. [%][b]
ee [%][c]
Table 2: Asymmetric isomerization of 3,3-disubstituted allylic alcohols.
1
2
3
4
5
6
7
8
(S)-2
(R)-3
(S)-4
(R)-5
23
23
23
23
23
23
23
23
23
18
18
18
18
18
18
2
2
2
2
4
14
22
4
14
22
12
12
22
<5
<5
<5
19[d]
98
n.d.
n.d.
n.d.
5 (S)
Entry[a] Catalyst R1
R2
t [h] Conv. [%][b] ee [%][c]
(S)-6a
(S)-6b
(S)-6c
(S)-6d
(R)-6e
(R)-6 f
(R)-6 f
(R)-6 f
(R)-6 f
(R)-6g
(R)-6g
(R)-6g
(S)-6h
(S)-6i
(S)-6j
11 (R)
28 (S)
30 (S)
45 (S)
75 (R)
84 (R)
90 (R)
90 (R)
90 (R)
91 (R)
90 (R)[e]
93 (R)
89 (S)
95 (S)
97 (S)
75
1
2
3
4
5
6
7
8
(R)-6 f
(R)-6g
(S)-6j
(R)-6g
(S)-6j
(R)-6g
(S)-6j
(R)-6 f
(R)-6g
(S)-6h
(R)-6 f
(R)-6g
(R)-6 f
(R)-6g
(S)-6j
(R)-6g
(S)-6j
(R)-6 f
(R)-6g
(S)-6j
(R)-6g
(R)-6g
iPr
iPr
iPr
iPr
iPr
iPr
iPr
4-Me-C6H4
4-Me-C6H4
4-Me-C6H4
4-MeO-C6H4
4-MeO-C6H4
4-Cl-C6H4
22
22
22
50
84
71
56 (R)
86 (R)
95 (S)
90 (R)
94 (S)
82 (R)
94 (S)
34 (S)
57 (S)
n.d.
60 (S)
73 (S)
87 (R)
94 (R)
98 (S)
99 (R)
>99 (S)
60 (S)
68 (S)
76 (R)
46 (S)
25 (R)
>99
>99
>99
>99
>99
>99
85
95
20
6
>99
98
9
22 >99
10
11
12
13
14
15
16
17
18
19
23
22
22
22
22
22
91
88
60
30
10
ꢀ10
ꢀ30
ꢀ50
23
4-Cl-C6H4
Me C6H5
Me C6H5
Me C6H5
Et
Et
Cy
Cy
Cy
tBu C6H5
tBu C6H5
Me Cy
Me Cy
Me Cy
9
23
10
11
12
13
14
15
16
17
18
19
20
21
22
22 <5[d]
8
22
6
ꢀ10
C6H5
C6H5
C6H5
C6H5
C6H5
35
30
78
85
88
80
81
70
90
25
26[e]
18
23
23
23
75
6
[a] Reported results are the average of at least two runs. [b] Determined
by GC or 1H NMR methods. [c] Determined by GC or SFC methods using
a chiral stationary phase. Absolute configuration (shown in parentheses)
based on the sign of the optical rotation and by comparison with
literature data. See Ref. [3b]. [d] The remaining 81% is a 2:1 mixture of
E/Z isomers of 7. [e] 2.5 mol% of the catalyst used. n.d.=not
determined.
22
22
22
22
22
18
22
22
Ph
Ph
iPr
Me
[a] Reported results are the average of at least two runs. [b] Determined
by GC or 1H NMR methods. [c] Determined by GC or SFC methods using
a chiral stationary phase. Absolute configuration (shown in parentheses)
based on the sign of the optical rotation and by comparison with
literature data. See Ref. [3b]. [d] A 3:1 mixture of E/Z isomers was
recovered. [e] The remaining 74% is a 3.5:1 mixture of E/Z isomers.
racemic form. In any case the saturated alcohol resulting from
competitive hydrogenation was detected. Despite a very low
but measurable enantiomeric excess, catalyst 6a furnished the
isomerization product almost quantitatively. At this point, we
reasoned that a bulky trialkylphosphine moiety might be
crucial to restore a catalytic activity similar to that of 1,
whereas switching from a pyridine to a less basic oxazoline
ring has little impact on the outcome of the reaction. The high
modularity of the readily available chiral (P,N) ligands 6
prompted us to synthesize a series of 10 different dialkyl-
phosphanylmethyl oxazoline–iridium complexes 6a–j using
literature procedures.[7] The model substrate 7 was used to
evaluate the potential of these catalysts in promoting the
asymmetric isomerization (Table 1). The identity of the
substituent on both the phosphorus atom and the oxazoline
ring has an impact on the rate and the selectivity of the
reaction. For a given substituent, R2, at the stereogenic center
of the oxazoline ring, increasing the steric demand of the
substituents (iPr, tBu, 1-Ad) on the phosphorus atom system-
atically improved the enantiomeric excess of the aldehyde 8.
Catalysts having an aryl-substituted oxazoline moiety (6c–f;
R1 = CH2Ph or Ph) displayed a markedly faster rate than the
alkyl-substituted analogues 6a,b (R1 = iPr) and 6h–j (R1 =
tBu) (compare entries 5, 6 and 17–19 with 7–10 and 14,
respectively, in Table 1). Higher enantioselectivities were
achieved by carrying out the reaction at lower temperatures,
high levels of enantioinduction and impressive reaction rates
in the isomerization of 7, and catalyst 6j, which gave the
highest enantioselectivity, were systematically investigated.
High enantioselectivities were still achieved when either
electron-donating or electron-withdrawing substituents were
introduced in the para position of the aryl group R2 (Table 2,
entries 1–7). Reducing the size of the alkyl substituent affects
both the yield and the enantioselectivity of the isomerization
reaction (Table 2, entries 8–12). In contrast, it is apparent that
a bulky alkyl substituent R1 favors excellent to virtually
perfect enantioselectivities (Table 2, entries 13–17). Interest-
ingly, when both the steric hindrance on the phosphorus atom
of the ligand and on the substrate is reduced, E/Z isomer-
ization of the olefin is exclusively observed (Table 2,
entry 10). Promising levels of enantioselectivity were mea-
sured for a 3,3-dialkyl allylic alcohol (Table 2, entries 18–20).
The isomerization of Z-configured primary allylic alcohols
did not provide satisfactory results (Table 2, entries 21 and
22).
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Crystals of suitable quality for X-ray
crystal analysis were obtained by slow
diffusion of hexanes in a concentrated
dichloromethane solution of the PF6
analogue
of
complex
(R)-6g
(Figure 2).[9] The iridium complex
adopts a distorted square-planar coor-
dination geometry (P-Ir-N = 82.43(17)8)
where the metallacyclopentene chelate
lies in a rigid and almost flat conforma-
tion. Consequently, the two bulky 1-
adamantyl groups on the phosphorus
atom do not display axial/equatorial
orientations but instead occupy nearly
Figure 3. Activation of (R)-6g by H2 at room temperature in [D8]THF. The hydride region of the
1H NMR spectrum (recorded at 223 K) is shown.
from the oxidative addition of molecular hydrogen to the
putative solvate complex [Ir(P,N)(C4D8O)2]BArF .[11] Other
4
hydride intermediates present in minor quantity can also be
seen in the 1H NMR spectrum depicted on Figure 3.[12,13]
2
The small JHP coupling constants (19 and 23 Hz) imply
that both hydrides are cis to the phosphorus donor. Since the
position of the hydride perpendicular to the plane of the
metallacycle could not be ascertained by two-dimensional
NMR spectroscopy, the identity of the major isomer (9 or 10)
remains unclear.
Labeling experiments employing dideuterated test sub-
strate [D2]-7 were conducted using the standard procedure for
the isomerization reaction (Scheme 2).[14] At the end of the
Figure 2. Crystal structure of complex (R)-6g; side view (left) and front
view (right). H atoms and PF6 counter anions have been omitted for
clarity. The methylene fragments of the cod moiety have been removed
in the front view. Selected bond distances [ꢁ] and angles [8]: Ir–C(91)
2.202(8), Ir–C(92) 2.178(8), Ir–C(95) 2.138(7), Ir–C(96) 2.125(8), Ir–N
2.110(6), Ir–P 2.345(5); N-Ir-P 82.43(17).
equivalent positions. The cyclooctadiene ligand exhibit a
strong clockwise twist bringing C(92) and C(96) almost in
plane with the P-Ir-N chelate. The characteristically stronger
trans influence of the phosphorus atom is manifested by
Scheme 2. Labeling experiments using dideuterated model substrate
[D2]-7 and (R)-6g.
ꢀ
longer Ir Ccod bond distances. Two-dimensional NMR anal-
yses recorded in [D8]THF indicate that the solid-state
structure of (R)-6g essentially remains in solution. Interest-
ingly, a NOE contact was observed between the methylene
protons of the “lower” adamantyl cage and the olefinic cod
proton on C(91). This indicates that voluminous substituents
on the phosphorus may exert a steric influence on both the
cis- and trans-coordination sites, sites where the isomerization
reaction is expected to take place.
reaction, along with the dideuterated product [D2]-8, the
monodeuterated aldehyde [D1]-8 was detected by HRMS,
revealing that an intermolecular process is at play. After
1
completion of the reaction, H NMR analysis showed exclu-
sive incorporation of hydrogen at C3. The level of incorpo-
ration of hydrogen was found to be essentially twice propor-
tional to the initial loading in (R)-6g, indicating that all the
exogenous hydrogen had been transferred from the catalyst to
the substrate during the course of the isomerization reaction.
A mechanistic rationale consistent with our experimental
observations is depicted in Scheme 3. Initial chelation of the
substrate on the cis-dihydride intermediate 9 or 10 is
supported by the Stork[15] and Crabtree[16] studies of the
directed hydrogenation of allylic alcohols catalyzed by [Ir-
(PCy3)(pyridine)(cod)]PF6.[17] The productive isomerization
pathway starts with migratory insertion at C2 to produce a
To get a better understanding of the important features of
the active species responsible for high activity and enantio-
selectivity in the asymmetric isomerization of primary allylic
alcohols, the reactivity of (R)-6g with molecular hydrogen
was investigated (Figure 3). Activation of the precatalyst in
[D8]THF was performed at room temperature as for standard
catalytic experiments (see above). After degassing, the NMR
tube was sealed and transferred to a 500 MHz spectrometer,
which was precooled to 223 K to avoid decomposition into
1
catalytically inactive iridium clusters.[10] The H NMR spec-
trum displays a major cis-dihydride species, formally resulting
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of primary allylic alcohols. Deviating hydrogenation catalysts
from their initial goal towards productive isomerization by
adequately tuning the experimental setup allows this most
challenging transformation to take place under mild reaction
conditions. Preliminary investigations have helped our under-
standing and rationalization of crucial features of the reaction
mechanism. Additional work to completely elucidate the
mechanism and the development of new catalysts to circum-
vent the current limitations of the present system is ongoing in
our laboratory.
Received: April 7, 2009
Revised: May 5, 2009
Published online: June 12, 2009
Keywords: allylic compounds · asymmetric catalysis · iridium ·
.
isomerization · P,N ligands
[1] For reviews, see: a) S. Akutagawa in Comprehensive Asymmetric
Catalysis, Vol. 2 (Eds.: E. N. Jacobsen, A. Pfaltz, H. Yamamoto),
Springer, Berlin, 1999, chap. 23; b) S. Akutagawa in Compre-
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Pfaltz, H. Yamamoto), Springer, Berlin, 1999, chap. 41.4. Semi-
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Otsuka, T. Sato, R. Noyori, J. Am. Chem. Soc. 1990, 112, 4897 –
4905.
Scheme 3. Proposed mechanisms for the isomerization of allylic alco-
hols (left) and the competing E/Z isomerization pathway (right).
[2] For recent reviews, see: a) R. C. Van der Drift, E. Bouwman, E.
Drent, J. Organomet. Chem. 2002, 650, 1 – 24; b) R. Uma, C.
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secondary alkyl hydride intermediate (11a!12). Reversible
binding of the alcohol functionality allows subsequent b-
hydride elimination from C1 to generate the enol dihydride
intermediate 13. Rapid tautomerization, presumably outside
the catalytic cycle, leads to the desired aldehyde.[18]
[3] For rhodium-catalyzed asymmetric isomerization of primary
allylic alcohols, see: a) C. Botteghi, G. Giacomelli, Gazz. Chim.
Ital. 1976, 106, 1131 – 1134; b) K. Tanaka, S. Qiao, M. Tobisu,
M. M.-C. Lo, G. C. Fu, J. Am. Chem. Soc. 2000, 122, 9870 – 9871;
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f) M. Ito, S. Kitahara, T. Ikariya, J. Am. Chem. Soc. 2005, 127,
6172 – 6173.
[4] a) L. Mantilli, C. Mazet, Chimia 2009, 63, 35 – 38; b) L. Mantilli,
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relevant contributions on iridium-catalyzed isomerization of
allylic alcohols, see: c) C. S. Chin, J. Park, C. Kim, S. Y. Lee, J. H.
Shin, J. B. Kim, Catal. Lett. 1988, 203 – 205; d) D. Baudry, M.
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1327.
Interestingly, the cis dihydride regenerated at the end of
the first cycle has a different coordination geometry than that
of the initial active catalyst, and therefore offers a different
stereochemical environment to the next molecule of substrate
(i.e. if 9 was initially involved, 10 is generated after one
turnover, and vice versa). A rapid isomerization between 9
and 10 is therefore likely to occur.[19] This hypothesis is in line
with the results of the labeling experiments. In the key
intermediate 11a only one hydride is stereoelectronically
aligned with the s*C to undergo migratory insertion. The
=
C
transfer of all the hydrides from the catalyst to the product
therefore implicates the existence of a fast cis-hydride
exchange mechanism at some stage in the catalytic cycle.[20]
Migratory insertion is likely to be the rate-determining step
regarding the significant effect of olefin substituents and
olefin geometry on the reaction rate. Decreasing the size of
the R1 substituent leads to migratory insertion at C3 and
competitive E/Z isomerization (11b!14!15) (Table 2,
entries 10 and 21). Reversible binding of the hydroxy group
[5] For recent reviews: a) X. Cui, K. Burgess, Chem. Rev. 2005, 105,
3272 – 3296; b) K. Kꢂllstrꢃm, I. Munslow, P. G. Andersson,
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Pfaltz, Acc. Chem. Res. 2007, 40, 1402 – 1411.
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Chem. 1998, 110, 3047 – 3050; Angew. Chem. Int. Ed. 1998, 37,
2897 – 2899; For catalyst 3, see: b) X. Li, L. Kong, Y. Gao, X.
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c) J. W. Faller, S. C. Milheiro, J. Parr, J. Organomet. Chem. 2006,
691, 4945 – 4955; for catalyst 5, see: d) S. Kaiser, S. P. Smidt, A.
Pfaltz, Angew. Chem. 2006, 118, 5318 – 5321; Angew. Chem. Int.
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E. Neumann, A. Pfaltz, Angew. Chem. 2007, 119, 8422 – 8424;
Angew. Chem. Int. Ed. 2007, 46, 8274 – 8276.
ꢀ
in 14 allows free rotation around the C3 C2 bond and b-
hydride elimination of the diastereotopic proton on C2 to
form 15, wherein the olefin has the opposite conformation. In
the absence of hydrogen pressure, the typical reductive
elimination (14!16) in the hydrogenation pathway is not
observed.
In conclusion, we have identified highly active and
selective iridium catalysts for the asymmetric isomerization
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[7] a) J. Sprinz, G. Helmchen, Tetrahedron Lett. 1993, 34, 1769 –
1772; b) H. Danjo, M. Higuchi, M. Yada, T. Imamaoto,
Tetrahedron Lett. 2004, 45, 603 – 606; Catalysts 6a–f and 6h,i
have been previously described, see reference [6e].
[8] Geometrically pure E and Z substrates were synthesized accord-
ing to literature procedures. See the Supporting Information for
details.
[9] CCDC 724940 for complex [(R)-6g]·PF6 contains the supple-
mentary crystallographic data for this paper. These data can be
obtained free of charge from The Cambridge Crystallographic
Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
[10] a) D. F. Chodosh, R. H. Crabtree, H. Felkin, G. E. Morris, J.
Organomet. Chem. 1978, 161, C67 – C70; b) H. Wang, A. L.
Casalnuovo, B. J. Johnson, A. Mueting, L. H. Pignolet, Inorg.
Chem. 1988, 27, 325 – 331; c) S. P. Smidt, A. Pfaltz, E. Martinez-
Viviente, P. S. Pregosin, A. Albinati, Organometallics 2003, 22,
1000 – 1009; d) Y. Xu, M. A. Celik, A. L. Thompson, H. Cai, M.
Yurtsever, B. Odell, J. C. Green, D. M. P. Mingos, J. M. Brown,
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[11] R. H. Crabtree, Acc. Chem. Res. 1979, 12, 331 – 337.
[12] The 31P NMR spectrum shows complete consumption of (R)-6g,
a major signal that corresponds to 9 (or 10), and the presence of a
non-hydride species according to 31P/1H HMQC. Monitoring of
the stoichiometric or catalytic reactions by NMR spectroscopy,
even at low temperature, did not allow detection of other
intermediates. See the Supporting Information.
[13] Up to four different cis-dihydride species may be formed upon
addition of molecular hydrogen. For similar studies using iridium
complexes: a) R. H. Crabtree, H. Felkin, T. Fillbeen-Kahn, G. E.
Morris, J. Organomet. Chem. 1979, 168, 183 – 195; b) R. H.
Crabtree, P. C. Demou, D. Eden, J. M. Mihelic, C. A. Parnell,
J. M. Quirk, G. E. Morris, J. Am. Chem. Soc. 1982, 104, 6994 –
7001; c) R. H. Crabtree, R. Uriarte, Inorg. Chem. 1983, 22, 4152 –
4154; d) R. H. Crabtree, G. G. Hlatky, C. A. Parnell, B. E.
Segmueller, R. J. Uriarte, Inorg. Chem. 1984, 23, 354 – 358;
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Meuwly, A. Pfaltz, J. Am. Chem. Soc. 2004, 126, 14176 – 14181.
[14] The reverse experiment using D2 and 7 would not necessarily
produce the same results since scrambling between iridium
deuterides and cyclooctadiene protons during the activation step
has been observed previously. See: a) J. M. Brown, A. E.
Derome, G. D. Hughes, K. P. Monaghan, Aust. J. Chem. 1992,
45, 143 – 153; b) B. F. M. Kimmich, E. Somsook, C. R. Landis, J.
Am. Chem. Soc. 1998, 120, 10115 – 10125.
[15] G. Stork, D. Kahne, J. Am. Chem. Soc. 1983, 105, 1072 – 1073.
[16] a) R. H. Crabtree, M. W. Davis, Organometallics 1983, 2, 681 –
682; b) R. H. Crabtree, M. W. Davis, J. Org. Chem. 1986, 51,
2655 – 2661.
[17] In agreement with theory, the ligands with stronger trans
influence (Hꢀ, PR3) are coordinated trans to those with
=
weaker trans influence (N, C C, OH) in intermediates 11. See:
a) T. G. Appleton, H. C. Clark, L. E. Manzer, Coord. Chem. Rev.
1973, 10, 335 – 422. For a discussion on substrate directed
reactions, see: b) A. H. Hoveyda, D. A. Evans, G. C. Fu, Chem.
Rev. 1993, 93, 1307 – 1370.
[18] We assume that the tautomerization is taking place outside the
catalytic cycle, since attempts to isomerize 2,3-disubstituted
allylic alcohols gave the corresponding a-substituted aldehydes
in racemic form. For 2,3-disubstituted allylic alcohols, the
tautomerization is the enantiodetermining step. If the metal is
not involved or if background tautomerization is too fast,
racemic product is expected. For a relevant discussion, see: S.
Bergens, B. Bosnich, J. Am. Chem. Soc. 1991, 113, 958 – 967.
[19] Labilization of a weakly bound molecule of solvent in 9 (or 10)
generates a pentacoordinated intermediate. Subsequent Berry
pseudo-rotation may be invoked for the isomerization between 9
and 10.
[20] For related interexchange on Group 9 metal cis-dihydride
complexes, see for example: I. D. Gridnev, N. Higashi, K.
Asakura, T. Imamoto, J. Am. Chem. Soc. 2000, 122, 7183 – 7194.
Angew. Chem. Int. Ed. 2009, 48, 5143 –5147
ꢀ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
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