Replacing phosphorus with sulfur for the efficient hydrogenation of esters

Article abstract of DOI:10.1002/anie.201209218

Catalyst tune-up: A readily available, air-stable amino-sulfide catalyst, [RuCl₂(PPh₃){HN(C₂H₄SEt) ₂}], has been developed. This complex displays outstanding efficiency for the hydrogenation of a broad range of substrates with C-X bonds (esters, ketones, imines), as well as for the acceptorless dehydrogenative coupling of ethanol to ethyl acetate (see scheme). Copyright

Full text of DOI:10.1002/anie.201209218

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Angewandte
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DOI: 10.1002/anie.201209218
Catalytic Hydrogenation
Replacing Phosphorus with Sulfur for the Efficient Hydrogenation of
Esters**
Denis Spasyuk, Samantha Smith, and Dmitry G. Gusev*
The reduction of carboxylic esters is a common organic
reaction that is often accomplished with the help of alumi-
unprecedented activity in the hydrogenation of esters and
imines at [Ru] loadings as low as 50 ppm at 408C.
[1]
nium hydrides. Another known approach is Bouveault–
It can be seen that all of the ester hydrogenation catalysts
in Scheme 1 possess amino–phosphine ligands. More gener-
ally, many Noyori-type catalysts incorporate a combination of
[2]
Blanc reduction of esters with alkali metals in ethanol. Both
classical reduction methods present problems: With alumi-
nium hydrides, the reactions are hazardous and have chal-
lenging workups owing to the highly exothermic hydrolysis
step that yields voluminous precipitates. In the Bouveault–
Blanc method, the drawbacks include excessive foaming and
the risk of fires. Both methods produce large amounts of
waste.
[
8]
phosphorus and nitrogen donors. Despite the widespread
application and tremendous success of phosphines in catal-
ysis, they have well-known disadvantages. Their preparations
are often far from trivial and require handling under an inert
atmosphere. As a result, the amino–phosphines are costly
chemicals that can be challenging to make on a large scale.
Not surprisingly, catalysts I–III (available from Strem Chem-
icals) are very expensive, especially I, which costs $680 per
gram. Considering that ruthenium contributes less than 1% to
this cost, it is apparent that the development of practical ester
hydrogenation calls for using practical ligands, preferably
ones containing no phosphorus.
An attractive “green” alternative to the classical methods
is the catalytic hydrogenation shown in Scheme 1, a method
which has attracted much recent interest for the reduction of
[
3–7]
esters under H2.
2
The disclosure of Milsteinꢀs catalysts in
[
3]
006 (such as complex I; Scheme 1) was quickly followed by
Intrigued by the recent observation of the superior
catalytic activity of [CrCl {HN(C H SEt) }] over [CrCl {HN-
3
2
4
2
3
[9a]
(
C H PEt ) }] for the trimerization of ethylene to 1-hexene,
2 4 2 2
we became interested in the preparation of ruthenium
complexes with the HN(C H SEt) (SNS) ligand and the
2
4
2
evaluation of their catalytic activity for ester hydrogenation.
The SNS ligand is obtained nearly quantitatively by adding
bis-(2-chloroethyl)amine hydrochloride to a solution of
Scheme 1. Ester hydrogenation catalysts.
[
9b]
ethanethiol and NaOH in ethanol. This synthesis has the
practical advantages of being straightforward and scalable; it
can be conveniently performed in air, and it provides the SNS
ligand at a small fraction of the cost of the amino–phosphines
used in catalysts I–IV. Herein, we report the preparation of
a readily available, air-stable ruthenium–SNS complex that is
the most efficient catalyst for ester hydrogenation to date,
outperforming the known catalytic systems I–III by a large
margin. The significance of this finding goes beyond ester
hydrogenation. It is now apparent that a new class of catalysts
for the Noyori-type hydrogenation of compounds with C=X
[
4]
the development of the Firmenich catalysts in 2007, among
which [RuCl (H NC H PPh ) ] (II) is effective at 1008C at
a 0.05 mol% catalyst loading. In 2011, a new catalyst, Ru-
2
2
2
4
2 2
[
4a]
[5]
MACHO (III), was patented by Takasago chemists. Ru-
MACHO is useful at a 0.05 mol% loading for the hydro-
genation of methyl lactate and methyl menthoxyacetate,
giving high yields of (R)-1,2-propanediol and 2-(l-menthox-
y)ethanol, respectively. The most recent additions to this list
of efficient catalysts are osmium and ruthenium complexes
[
6]
from our group, particularly the air-stable complex [RuCl2-
PPh ){PyCH NHC H PPh }] (IV), which demonstrated
bonds can be made based on amino–sulfides that have the
potential to replace the ubiquitous phosphorus-based ligands
used in this area.
(
3
2
2
4
2
The ruthenium complexes of Figure 1 were obtained by
the conventional ligand substitution reactions of HN-
[
*] Dr. D. Spasyuk, S. Smith, Prof. D. G. Gusev
Department of Chemistry, Wilfrid Laurier University
(
C H SEt) with [RuCl (PPh ) ], [RuHCl(PPh ) ], [RuCl -
2 4 2 2 3 3 3 3 2
7
5 University Ave. W., Waterloo, ON N2L3C5 (Canada)
E-mail: dgoussev@wlu.ca
dgoussev@hotmail.com
Homepage: http://wlu.ca/homepage.php?grp_id=366
[**] We are grateful to NSERC Canada, the Ontario Government and
Wilfrid Laurier University for financial support
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201209218.
Figure 1. New SNS catalysts.
2
538
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Figure 2. Ortep plots of 1 and 4 with most hydrogens omitted for
clarity. Thermal ellipsoids set at 50%.
(
AsPh ) ], and [RuHCl(CO)(PPh ) ], as documented in the
3 3 3 3
Supporting Information. Complexes 1 and 4 have been
crystallographically characterized and their molecular geo-
[10]
metries are presented in Figure 2.
The catalytic results of this paper are organized in Tables 1
and 2. First, we compare the effectiveness of catalysts I–IV
and our newly developed complexes 1–4. Two typical
substrates were selected for the comparative study: methyl
benzoate and methyl hexanoate. The hydrogenations were
performed at 408C, under H (50 bar), using a catalyst loading
2
of 0.05 mol% in all cases. The reaction mixtures were
1
analyzed by H NMR spectroscopy after 3 h of hydrogena-
tion. In all cases, they were found to contain the product
alcohols together with varied amounts of byproduct from
transesterification, as well as methanol and unreacted starting
material. The results in Table 1 demonstrate that the Milstein
and Takasago catalysts I and III are the least effective in the
group and produce little product under the test conditions.
The Firmenich catalyst II shows a moderate performance,
whereas complex IV is the most effective of the known
systems. The new catalysts 1–4 are all active for ester
hydrogenation; among these, the dichloride and hydrido–
chloride complexes 1 and 2 give the best conversions to
products, accompanied by formation of the smallest amounts
of the symmetrical ester byproducts. To gain a broader
understanding of the catalytic performance of 1, we tested this
complex in the hydrogenation of the diverse group of
substrates shown in Figure 3, accompanied by variation of
the reaction temperature, time, and substrate-to-catalyst
Figure 3. Substrates tested in this work.
of 9800 is achieved in 2 h at 1008C with 1, whereas Firmenich
catalyst II is known to be 6.5 times slower at this temperature
for a similar substrate, methyl octanoate, giving a TON of
[4a]
1880 after 2.5 h. The reduction of neat ethyl acetate (E5) is
particularly impressive with 1, affording a TON of 58400 after
21 h under very mild reaction conditions (T= 408C). A
relatively difficult substrate, methyl phthalate (E3) is also
rapidly reduced with 1 (0.1 mol%) at 1008C.
Catalyst 1 has also been successfully tested for the
hydrogenation of typical imines and ketones shown in
Figure 3, where TONs up to 40000 have been observed
when running the reactions at 23–408C. Catalyst 1 also has
some activity for olefin hydrogenation. Styrene was reduced
relatively rapidly at 408C with 1 (0.05 mol%). However, the
reduction of 1-pentene was considerably slower, and
2-pentene was largely unchanged even after 48 h at 408C.
The latter observation is promising for the selective hydro-
genation of esters, ketones, and imines containing internal
C=C bonds. For example, the hydrogenation of methyl
(
S/C) ratios.
The data in Table 2 support the assessment of complex
1
as an outstanding hydrogenation catalyst possessing unsur-
passed efficiency and distinguished by excellent thermal
stability and longevity in catalytic solutions. Large turnover
numbers were observed for methyl and ethyl benzoates E1
and E2, respectively. For methyl hexanoate (E6), a high TON
3-nonenoate at 408C afforded trans-3-nonen-1-ol in a 73%
yield. Such selectivity is rare among catalysts that are active
[
a]
Table 1: Comparative hydrogenation with catalysts I–IV and 1–4.
Substrate
Catalyst
I
II
III
IV
1
2
3
4
[
b]
[
[b]
[b]
[b]
[b]
[c]
[b]
[c]
[b]
[b]
PhCO Me
4 (3)
63 (6)
4 (3)
75 (6)
86 (2)
98 (1)
84 (3)
96 (1)
57 (6)
45 (7)
2
c]
[c]
[c]
[c]
[c]
[c]
C H CO Me
18 (7)
55 (16)
23 (8)
89 (10)
75 (13)
66 (17)
5
11
2
[
a] Conditions: ester (0.1 mol), catalyst (0.05 mol%), KOMe (5 mol%) in THF (15 mL) under H (50 bar) for 3 h at 408C. [b] Concentration (mol%) of
2
benzyl alcohol in the product mixture; data in parentheses is the concentration (mol%) of benzyl benzoate. The balance of material present was
unreacted starting material. [c] Concentration (mol%) of 1-hexanol in the product mixture; data in parentheses is the concentration (mol%) of hexyl
hexanoate. The balance of material present was unreacted starting material.
Angew. Chem. Int. Ed. 2013, 52, 2538 –2542
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2539

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Table 2: Hydrogenation of the substrates shown in Scheme 3 catalyzed
[
a]
by 1.
[
b]
Entry Substrate S/C
Base
Solvent t [h] T [8C] Conv. [%]
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
E1
E2
E3
E4
E4
E5
E5
E6
E6
E7
E8
E9
E10
E11
E12
I1
4000 tBuOK THF
20000 EtONa THF
6
40
95
85
96
[
c]
16 40
1.2 100
16 40
1000 MeOK
2000 MeOK
1000 MeOK
THF
THF
THF
93
1
100
100
95
73
81
98
[
[
[
[
[
d]
40000 EtONa neat
80000 EtONa neat
14 40
21 40
24 40
d]
Figure 4. Ortep plots of 5·EtOH (left) and 6 (right). Thermal ellipsoids
set at 50%.
c,e]
c]
20000 MeOK
10000 MeOK
10000 MeOK
2000 MeOK
4000 tBuOK
THF
THF
THF
THF
neat
2
100
c]
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
16 60
21 23
1
8
1
2
1.5 23
1
6
100
97
100
40
100
100
100
100
93
[
f]
2000 tBuOK THF
[
g]
2000 MeOK
toluene
toluene
10000 MeOK
99
100
63
20000 tBuOK THF
I2
I3
50000 MeOK
THF
40
40
2000 tBuOK
toluene
100
100
100
100
100
94
[
[
[
[
c,h]
c,h]
c,h,i]
c,h]
K1
K2
K3
K4
A1
O1
O2
O3
40000 tBuOK THF
20000 tBuOK THF
20000 tBuOK THF
20000 tBuOK THF
24 40
1
2
1
23
23
40
Scheme 2. Formation of dihydride 6 from 5·EtOH.
10000 MeOK
toluene 1.5 100
2000 tBuOK THF
20 40
48 40
48 40
100
75
13
[12]
mild heating in toluene, as shown in Scheme 2.
The
2000 MeOK
2000 MeOK
THF
THF
molecular structure of 6 is presented in Figure 4; unlike the
related mer-SNS compounds 1–5, the dihydride 6 adopts a fac-
SNS geometry.
Scheme 2 has mechanistic implications, as the forward
reaction is part of the ADC process, whereas the reverse
reaction is ester hydrogenation. Complex 5·EtOH rapidly
hydrogenates ethyl acetate at room temperature (238C),
giving 77% conversion into ethanol within 1 h, under H2
[
(
a] Unless otherwise noted, the reaction was carried out on substrate
0.02 mol) with base additive (1 mol%) in solvent (6 mL) in a 75 mL Parr
high-pressure vessel. [b] Substrate to catalyst ratio. [c] Substrate
0.1 mol) was hydrogenated in a 0.3 L vessel. [d] Substrate (0.2 mol) was
(
hydrogenated in a 0.3 L vessel. [e] The product also contained hexyl
hexanoate (9%). [f] Trans-3-nonen-1-ol/1-nonanol=73:27. [g] 5 mol% of
base was used. [h] In 15 mL of THF. [i] Cis/trans product ratio=87:13.
(
50 bar), with [Ru] (0.02 mol%) and NaOEt (1 mol%) and
À1
with an efficiency corresponding to TOF = 3850 h . We
believe that dihydride 6 is the hydrogenation catalyst involved
in this process, as well as in the reactions of Table 2, where 6 is
for ester hydrogenation. So far, only one ruthenium catalyst
[4]
[6a]
from Firmenich and an osmium catalyst from our group
have shown good selectivity for the reduction of esters with
presumably produced under H from 1 and base. We further
2
internal C=C bonds.
note that the NH group is crucial for catalysis. We synthesized
[
6b]
As we commented recently, high catalytic efficiency in
ester hydrogenation is expected to correlate with activity in
the reverse reaction of acceptorless dehydrogenative coupling
two analogues of catalyst 1: [RuCl (PPh ){O(C H SEt) }] (7)
2
3
2
4
2
and [RuCl (PPh ){MeN(C H SEt) }] (8), and these complexes
2
3
2
4
2
proved to be inactive in the hydrogenation of methyl
benzoate at 40–1008C, under the reaction conditions given
in Table 1.
(
ADC) of alcohols, affording symmetrical esters. Indeed,
when tested in the ADC reaction of ethanol under reflux, with
S/C = 2000 and 10000, complex 1 gave 97% and 89%
conversion to ethyl acetate in 16 and 24 h, respectively. This
performance is similar to that of catalyst IV, and complexes
The mechanism of ester hydrogenation is poorly under-
[
7i]
[3a]
[4a]
stood. According to Milstein et al and Saudan et al, the
concerted transfer of a metal hydride and a ligand proton to
the C=O group of the substrate first takes place, affording
1
and IV are currently among the most efficient ADC
[
11]
catalysts.
a hemiacetal intermediate. Dissociation of the hemiacetal
gives rise to an aldehyde, which is hydrogenated again by the
catalyst, thus completing the reduction process. In our hands,
When studying the reactions of 1 upon heating in basic
ethanol, we observed a quantitative conversion of the
dichloride into species 5, which was isolated and character-
ized as [RuH(OEt)(PPh ){HN(C H SEt) }] and crystallized
1
examination of the reaction mixtures by H NMR spectros-
copy gave no evidence of the presumed hemiacetal or
aldehyde intermediates of ester hydrogenation, ethanol
dehydrogenation reactions, or the reaction shown in
Scheme 2. It is likely that no free organic intermediate,
hemiacetal or aldehyde, is released into the reaction solution
during the reactions catalyzed by the SNS complexes
presented herein.
3
2
4
2
with one equivalent of hydrogen-bonded ethanol, 5·EtOH
Figure 4). The preparation of 5 containing no ethanol was
(
also possible by treating hydrido–chloride species 2 with
EtONa in toluene. Interestingly, whereas 5 is thermally stable
in solution, 5·EtOH is readily and selectively converted into
[
RuH (PPh ){HN(C H SEt) }] (6) and ethyl acetate upon
2 3 2 4 2
2
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The readily available, air-stable complex [RuCl (PPh ){HN-
2
3
(
C H SEt) }] (1) shows outstanding efficiency for the hydro-
2 4 2
genation of a broad range of substrates with C=X bonds
(esters, ketones, imines) as well as for the acceptorless
dehydrogenative coupling of ethanol to ethyl acetate. This
study has demonstrated that the phosphorus groups of
Noyori-type catalysts can be successfully replaced by sulfide
groups, thus overcoming the many drawbacks of working with
phosphines and phosphine-based catalysts, such as high
synthetic costs and the need for handling under inert
atmosphere. Complex 1 is the first practical and highly
active green hydrogenation catalyst for substrates with C=X
(X = O, N) bonds, and has a high potential to replace the use
of main-group hydrides for the reduction of esters in the
chemical industry and academic laboratories.
Received: November 16, 2012
Revised: January 2, 2013
Published online: January 28, 2013
Scheme 3. Base-free reduction of ethyl acetate catalyzed by 6.
Keywords: dehydrogenative coupling · esters · hydrogenation ·
.
imines · thioethers
A tentative mechanism for the base-free hydrogenation of
ethyl acetate catalyzed by 6 is presented in Scheme 3. Free
energies for all of the intermediates shown in Scheme 3 were
calculated in ethyl acetate using the M06-L functional.
Among the proposed key steps is the insertion of ethyl
acetate into a RuÀH bond of 6, to afford Int 1, which is
[1] a) Comprehensive Organic Synthesis, Vol. 8 (Eds.: B. M. Trost, I.
Fleming), Pergamon, New York, 1991; b) J. Seyden-Penne,
Reductions by the Alumino- and Borohydride in Organic Syn-
nd
thesis, 2 ed., Wiley-VCH, New York, 1997.
[
[
2] P. Vogt, B. Bodnar, Spec. Chem. Mag. 2009, 29/7, 22 – 24.
3] a) J. Zhang, G. Leitus, Y. Ben-David, D. Milstein, Angew. Chem.
analogous to the hemiacetaloxide formation in the reaction of
trans-[RuH {(R)-BINAP}{(R,R)-dpen}] with g-butyrolactone,
2
2006, 118, 1131 – 1133; Angew. Chem. Int. Ed. 2006, 45, 1113 –
[
7g]
as documented by Bergens et al. Inspection of the DFT-
optimized structure of Int 1 reveals an interesting feature:
Int 1 has a six-membered cycle formed by the H-N-Ru-O-
CH(Me)-OEt groups and closed by an NH···O hydrogen bond
1115; b) E. Balaraman, C. Gunanathan, J. Zhang, L. J. W.
Shimon, D. Milstein, Nat. Chem. 2011, 3, 609 – 614; c) E.
Fogler, E. Balaraman, Y. Ben-David, G. Leitus, L. J. W.
Shimon, D. Milstein, Organometallics 2011, 30, 3826 – 3833;
d) C. Gunanathan, D. Milstein, Acc. Chem. Res. 2011, 44, 588 –
(
1
dO···H = 1.92 ꢁ). The single CÀO bond of Int 1 is elongated to
602; e) D. Milstein, E. Balaraman, C. Gunanathan, B. Gnanap-
.493 ꢁ from the corresponding 1.345 ꢁ distance in ethyl
rakasam, J. Zhang, WO 2012/052996A2, 2012.
acetate. It is conceivable that intramolecular nucleophilic
substitution in step b results in the formation of bis(ethoxide)
[
4] a) L. A. Saudan, C. M. Saudan, C. Debieux, P. Wyss, Angew.
Chem. 2007, 119, 7617 – 7620; Angew. Chem. Int. Ed. 2007, 46,
7473 – 7476; b) L. Saudan, P. Dupau, J.-J. Riedhauser, P. Wyss
(Firmenich SA), WO 2006106483, 2006; c) L. Saudan, P. Dupau,
J.-J. Riedhauser, P. Wyss (Firmenich SA), US 2010280273, 2010.
5] a) W. Kuriyama, Y. Ino, O. Ogata, N. Sayo, T. Saitoa, Adv. Synth.
Catal. 2010, 352, 92 – 96; b) Y. Ino, W. Kuriyama, O. Ogata, T.
Matsumoto, Top. Catal. 2010, 53, 1019 – 1024; c) W. Kuriyama, T.
Matsumoto, Y. Ino, O. Ogata, N. Saeki (Takasago Int. Co.), WO
2011048727, 2011.
Int 2, which rearranges in step c to afford the dihydrogen
2
complex Int 3. Heterolytic splitting of the h -H ligand in
2
step d gives Int 4. Ethanol elimination accompanied by H2
coordination and heterolysis in steps e and f regenerate
dihydride 6. The isolated complex 5·EtOH (a mer-SNS isomer
of Int 4) is apparently a resting state of the catalyst. Formation
[
of 5·EtOH from 6, ethyl acetate, and H is favorable by DG =
2
À1
À1
À6.2 kcalmol , which is 6.7 kcalmol more stable than Int 4.
Base has a tremendous accelerating effect on the hydro-
genation rate. Without base, 5·EtOH (0.02 mol%) gave only
[6] a) D. Spasyuk, S. Smith, D. G. Gusev, Angew. Chem. 2012, 124,
826 – 2829; Angew. Chem. Int. Ed. 2012, 51, 2772 – 2775; b) D.
2
Spasyuk, D. G. Gusev, Organometallics 2012, 31, 5239 – 5242.
7] a) Y. Sun, C. Koehler, R. Tan, V. T. Annibale, D. Song, Chem.
Commun. 2011, 47, 8349 – 8351; b) F. Stempfle, D. Quinzler, I.
Heckler, S. Mecking, Macromolecules 2011, 44, 4159 – 4166;
c) M. J. Hanton, S. Tin, B. J. Boardman, P. Miller, J. Mol. Catal. A
2011, 346, 70 – 78; d) W. W. N. O, A. J. Lough, R. H. Morris,
Chem. Commun. 2010, 46, 8240 – 8242; e) T. Touge, T. Haka-
mata, H. Nara, T. Kobayashi, N. Sayo, T. Saito, Y. Kayaki, T.
Ikariya, J. Am. Chem. Soc. 2011, 133, 14960 – 14963; f) M. Ito, T.
Ootsuka, R. Watari, A. Shiibashi, A. Himizu, T. Ikariya, J. Am.
Chem. Soc. 2011, 133, 4240 – 4242; g) S. Takebayashi, S. H.
Bergens, Organometallics 2009, 28, 2349 – 2351; h) I. Carpenter,
S. C. Eckelmann, M. T. Kuntz, J. A. Fuentes, M. B. France, M. L.
Clarke, Dalton Trans. 2012, 41, 10136 – 10140; i) M. L. Clarke,
[
4
% conversion into ethyl acetate in 2 h at 408C under H
2
À1
(
50 bar), which corresponds to TOF = 100
vs. TOF =
À1
4
100 h with NaOEt (1 mol%) in 1 h under otherwise
[
13]
identical conditions. According to Bergens et al,
base
helps by labilizing the alkoxide intermediates (such as Int 2
À
and Int 4) for EtO substitution, through deprotonation of
the NH group. Elucidating all of the complex mechanistic
features of catalytic ester hydrogenation should be the subject
of a dedicated computational study.
In conclusion, herein we report the development of
a novel catalyst type based on the HN(C H SEt) ligand.
2
4
2
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.
Angewandte
Communications
Catal. Sci. Technol. 2012, 2, 2418 – 2423; j) W. W. N. O, R. H.
Morris, ACS Catal. 2013, 3, 32 – 40.
[11] a) J. Zhang, G. Leitus, Y. Ben-David, D. Milstein, J. Am. Chem.
Soc. 2005, 127, 10840 – 10841; b) J. Zhang, M. Gandelman,
L. J. W. Shimon, D. Milstein, Dalton Trans. 2007, 107 – 113; c) J.
Zhang, E. Balaraman, G. Leitus, D. Milstein, Organometallics
2011, 30, 5716 – 5724; d) C. Gunanathan, L. J. W. Shimon, D.
Milstein, J. Am. Chem. Soc. 2009, 131, 3146 – 3147; e) C.
del Pozo, M. Iglesias, F. Sꢂnchez, Organometallics 2011, 30,
2180 – 2188; f) S. Musa, I. Shaposhnikov, S. Cohen, D. Gelman,
Angew. Chem. 2011, 123, 3595 – 3599; Angew. Chem. Int. Ed.
2011, 50, 3533 – 3537; g) M. Nielsen, A. Kammer, D. Cozzula, H.
Junge, S. Gladiali, M. Beller, Angew. Chem. 2011, 123, 9767 –
9771; Angew. Chem. Int. Ed. 2011, 50, 9593 – 9597; h) M. Nielsen,
H. Junge, A. Kammer, M. Beller, Angew. Chem. 2012, 124,
5809 – 5811; Angew. Chem. Int. Ed. 2012, 51, 5711 – 5713.
[12] A related transformation of a ruthenium isopropoxide into
a hydride species, facilitated by isopropanol, has been studied;
see: W. Baratta, M. Ballico, G. Esposito, P. Rigo, Chem. Eur. J.
2008, 14, 5588 – 5595.
[
8] For recent reviews, see: a) C. Wang, X. F. Wu, J. L. Xiao, Chem.
Asian J. 2008, 3, 1750 – 1770; b) S. Gladiali, E. Alberico, Chem.
Soc. Rev. 2006, 35, 226 – 236; c) J. S. M. Samec, J. E. Backvall,
P. G. Andersson, P. Brandt, Chem. Soc. Rev. 2006, 35, 237 – 248;
d) T. Ikariya, K. Murata, R. Noyori, Org. Biomol. Chem. 2006, 4,
393 – 406; e) S. E. Clapham, A. Hadzovic, R. H. Morris, Coord.
Chem. Rev. 2004, 248, 2201 – 2237; f) R. Noyori, Angew. Chem.
2002, 114, 2108 – 2123; Angew. Chem. Int. Ed. 2002, 41, 2008 –
2022.
[
9] a) D. S. McGuinness, P. Wasserscheid, D. H. Morgan, J. T. Dixon,
Organometallics 2005, 24, 552 – 556; b) M. Konrad, F. Meyer, K.
Heinze, L. Zsolnai, J. Chem. Soc. Dalton Trans. 1998, 199 – 205.
10] The mer-SNS complexes form isomers in solution. This is due to
the different arrangements of the SEt groups relative to the SNS
ligand plane. Two of the isomers have eclipsed SEt groups
[
(
arranged on one side of the SNS plane), and the third isomer
has staggered SEt groups (occupying the opposite sides of the
SNS plane).
[13] R. J. Hamilton, S. H. Bergens, J. Am. Chem. Soc. 2006, 128,
13700 – 13701; see also Ref. [7g].
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