Titanium salalen catalysts based on cis-1,2-diaminocyclohexane: Enantioselective epoxidation of terminal non-conjugated olefins with H 2O2

Article abstract of DOI:10.1002/anie.201210198

Terminal, non-conjugated olefins, such as 1-octene, are difficult to epoxidize asymmetrically. Ti salalen complexes based on cis-1,2- diaminocyclohexane catalyze this demanding reaction giving high yields and enantioselectivities (up to 95 % ee), with H₂O₂ as the oxidant. The X-ray structures of the μ-oxo and peroxo complexes shed light on the coordination behavior of this novel class of ligands.

Full text of DOI:10.1002/anie.201210198

Angewandte
Chemie
Communications
Asymmetric Catalysis
A. Berkessel,* T. Gꢀnther, Q. Wang,
J.-M. Neudçrfl
&&&& — &&&&
Titanium Salalen Catalysts Based on cis-
1,2-Diaminocyclohexane:
Enantioselective Epoxidation of Terminal
Non-Conjugated Olefins with H2O2
Help for the neglected: Terminal, non-
conjugated olefins, such as 1-octene, are
difficult to epoxidize asymmetrically. Ti
salalen complexes based on cis-1,2-dia-
minocyclohexane catalyze this demand-
ing reaction giving high yields and enan-
tioselectivities (up to 95% ee), with H2O2
as the oxidant. The X-ray structures of the
m-oxo and peroxo complexes shed light
on the coordination behavior of this novel
class of ligands.
Angew. Chem. Int. Ed. 2013, 52, 1 – 6
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DOI: 10.1002/anie.201210198
Asymmetric Catalysis
Titanium Salalen Catalysts Based on cis-1,2-Diaminocyclohexane:
Enantioselective Epoxidation of Terminal Non-Conjugated Olefins
with H2O2**
Albrecht Berkessel,* Thomas Gꢀnther, Qifang Wang, and Jçrg-M. Neudçrfl
In 2005, Katsuki et al. introduced titanium salalen complexes,
such as 1 (Scheme 1), as novel and highly enantioselective
catalysts for the asymmetric epoxidation of a variety of non-
functionalized olefins (both conjugated and non-conjugated)
Our own work in this area aimed at simplifying the
catalyst complexes such as 1, while retaining its remarkable
ability for the asymmetric epoxidation of non-conjugated
olefins. In 2007, we disclosed a two-step approach to salalen
ligands of type 6 (Scheme 2).[4] Our modular synthesis of
salalen ligands begins with the reductive mono-alkylation of
Scheme 1. Titanium salalen catalyst 1, and its salen precursor 2.
Scheme 2. Modular synthesis of the trans-DACH-based ligands 6.
using hydrogen peroxide as the oxidant.[1] Salalen ligands are
mono-reduced salens, carrying one imine and one amine
functionality. For example, the catalyst 1 was reported to
effect the epoxidation of 1,2-dihydronaphthalene with quan-
titative yield and over 99% ee. Even more remarkably, the
epoxidations of 1-octene and vinyl cyclohexane were reported
to proceed with 85% yield/82% ee, and 72% yield/95% ee,
respectively. 1-Octene in particular is a terminal olefin
notoriously difficult to epoxidize asymmetrically—the epox-
ide yields/enantioselectivities achieved by Katsuki et al. in
2005/2007 were the highest ever reported.[1]
Katsukiꢀs seminal discovery was based on the “adventi-
tious” Meerwein-Ponndorf-Verley-type mono-reduction of
the corresponding salen ligand 2 (Scheme 1) when treated
with Ti(OiPr)4, and sparked intensive further studies on
related systems.[2] They furthermore disclosed that fully
reduced and more readily available titanium salan complexes
are similarly active and selective, albeit only for conjugated
olefins.[3]
trans-DACH (trans-3; trans-1,2-diaminocyclohexane) with
a salicylic aldehyde 4. The resulting N-mono-alkylated
trans-DACH 5 is then condensed with a second (or the
same) salicylic aldehyde 4 affording the ligand 6. In situ
complexation of 6 with Ti(OiPr)4 affords the catalytically
active complex.
The readily accessible ligands 6 (e.g. R = 6-Ph) turned out
to be quite efficient for the epoxidation of conjugated olefins,
and afforded for example, indene epoxide in 88% yield and
97% ee.[4] Unfortunately, less-reactive non-conjugated ole-
fins, such as 1-octene, were epoxidized only with low
efficiency, as a result of competing oxidative catalyst deacti-
vation.
Our search for more endurable, yet readily available
catalyst structures led us to exchange trans-DACH (trans-3)
for cis-DACH (cis-3, Scheme 3). We have recently disclosed
a practical one-step preparation of enantiopure mono-Alloc-
(7) or mono-Boc-protected (8) cis-DACH.[5] These com-
pounds were employed for the synthesis of the salalen ligands
12 and ent-12 as outlined in Scheme 3. An initial survey of the
catalytic performance of the titanium complexes derived from
the cis-DACH ligand ent-12, relative to that of its trans-
DACH counterpart 6 (R = 6-Ph), was performed using the
substrate olefins 13–16 and a fixed reaction time of 18 h. The
catalytically active Ti complexes were generated in situ by
combining the respective ligands with one equivalent of
Ti(OiPr)4 in methylene chloride at room temperature (see
Experimental Section and Supporting Information for
details). Olefin and internal standard were added, and the
reaction was started by addition of 1.50 equivalents of
[*] Prof. Dr. A. Berkessel, Dr. T. Gꢀnther,[+] Q. Wang,[+] Dr. J.-M. Neudçrfl
University of Cologne, Department of Chemistry
Greinstrasse 4, 50939 Cologne (Germany)
E-mail: berkessel@uni-koeln.de
Homepage: http://www.berkessel.de
[+] These authors contributed equally.
[**] Financial support from SusChemSys[12] and from the Fonds der
Chemischen Industrie is gratefully acknowledged. We thank Dr. M.-
C. Ong for providing enantiopure monoprotected cis-1,2-diamino-
cyclohexane derivatives.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201210198.
2
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naphthalene (13, entries 1, 2), with higher yield but somewhat
lower ee value for the cis-DACH ligand ent-12, 2) stereospe-
cificity: cis-b-methylstyrene (14, entries 3, 4) as an acyclic
analogue of 13, and trans-b-methylstyrene (15, entries 5, 6)
are epoxidized stereospecifically, pointing to concerted rather
than stepwise oxygen transfer, 3) for the non-conjugated
terminal olefin 16 (1-octene), both epoxide yield and
enantiopurity were higher for the ligand ent-12 (entries 7, 8).
We furthermore observed that alcohols—including 2-
propanol liberated during complex formation from Ti-
(OiPr)4—and 1,2-diols (from hydrolytic oxirane opening)
inhibit the catalytic epoxidation (quantitative data not
shown). We therefore modified the in situ catalyst prepara-
tion so that, after ligand complexation by Ti(OiPr)4 at room
temperature, volatiles were removed in vacuo. The remaining
solid Ti complex was then re-dissolved in methylene chloride,
and the catalytic epoxidation was carried out as described
before. The yield/ee time profile of 1-octene (16) epoxidation
in the presence of the ligands 6 (R = 6-Ph) and 12 is shown in
Figure 1, and reveals the following features: 1) the epoxide
Scheme 3. Preparation of the cis-DACH derived salalen ligands 12 and
ent-12. Reagents and conditions: a) 9, EtOH; NaBH4, MeOH; 93%;
b) same as in (a), 87%; c) dimethylbarbituric acid, Pd(OAc)2, PPh3,
CH2Cl2; 9, EtOH, 38%; d) HCl, MeOH; 9, EtOH, 59%.
aqueous hydrogen peroxide (30%). The results of the
epoxidation experiments are summarized in Table 1.
We were delighted to see that the novel cis-DACH-
derived Ti complexes are active epoxidation catalysts.
Throughout, the epoxide yields achieved with the cis-ligand
ent-12 were higher than those of the trans-ligand 6 (R = 6-Ph).
Note that the sense of induction was the same for ent-12 and 6
(R = 6-Ph), emphasizing the importance of the sense of
chirality at the Ti center (L in both cases). The chirality at the
Ti center, in turn, is defined by the sense of chirality (here R)
at DACHꢀs “amine C-atom” (see Refs. [1,2,4] and X-ray
crystal structures below). Table 1 additionally reflects the
following features of our novel cis-DACH epoxidation
catalyst: 1) both systems work particularly well for the
cyclic cis-1,2-disubstituted conjugated olefin 1,2-dihydro-
Figure 1. Reaction profiles of 1-octene epoxidation, carried out by the
Ti complexes of ligands 6 (R=6-Ph) and 12 (“in situ/vac” procedure).
yield in the presence of ligand 6 (R = 6-Ph) is limited by
catalyst deactivation, thus longer reaction time does not
improve the yield (non-productive peroxide decomposition
was excluded by control experiments); 2) for the more robust
12, the epoxide yield increases continuously with time—after
4 days, 86% yield was achieved; 3) for both ligands, product
ee value is not a function of time, thus excluding concomitant
kinetic resolution.
Table 1: Titanium-catalyzed asymmetric epoxidation: trans- and cis-
DACH-salalen ligands 6 (R=6-Ph) and ent-12.
Table 2 shows the results obtained for the non-conjugated
olefins 16–20, applying the improved “in situ/vac” procedure
described above. With regard to 1-octene (16), we were
delighted to see that the catalyst loading could be reduced to
2 mol% without appreciable loss of epoxide yield or enan-
tioselectivity (Table 2, entries 1–3). Variation of the reaction
temperature (08C, entry 4; 408C, entry 5) did not significantly
affect enantioselectivity. However, at 408C, competing cata-
lyst deactivation severely limited the attainable epoxide yield
(entry 5). Vinyl cyclohexane (17), also a terminal non-
conjugated substrate, behaved analogously (entries 6–8). In
line with the behavior of trans-b-methylstyrene (15, entries 5,
6, Table 1), enantioselectivity was poor for trans-2-octene (18,
entry 9, Table 2), albeit at reasonably high olefin conversion
and epoxide yield (ca. 80%). cis-2-Hexene (19), as a repre-
sentative cis-internal olefin, was converted selectively into the
cis-epoxide, with good conversion and yield, and intermediate
ee value (62–63%, entries 10, 11). As exemplified by 1-
Entry[a] Substrate Ligand Epoxide
Epoxide Epoxide
yield [%][b] ee [%][b] configuration[c]
olefin
1
2
3
4
5
6
7
8
13
13
14
14
15
15
16
16
6
87
98
51
70
17
24
8
95
85
66
59
31
31
62
79
1S,2R
1S,2R
ent-12
6
ent-12
6
ent-12
6
ent-12
cis, 1S,2R
cis, 1S,2R
trans, 1S,2S
trans, 1S,2S
R
20
R
[a] Reactions were performed with a molar ratio of substrate/ligand/
Ti(OiPr)4/aq.H2O2 of 1:0.1:0.1:1.5. [b] Determined by GC or HPLC
analysis. [c] Major epoxide enantiomer, determined by comparison of the
elution order with that of authentic samples in HPLC or GC analysis.
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Table 2: Asymmetric epoxidation of non-conjugated olefins, using the
cis-DACH ligand 12, in situ/vac procedure.
a product yield of 88% (entry 17). In a preparative experi-
ment (1 mol% catalyst; entry 18), even 95% ee resulted. To
our knowledge, those are the best yields/selectivities achieved
to date in the catalytic asymmetric epoxidation of 1-octene
(16). Related 1-hexene (21) afforded the epoxide with
somewhat lower enantioselectivity (89% ee, entry 21). For
vinyl cyclohexane (17), the doubly c-hexyl substituted ligand
25e again afforded the highest enantioselectivity (90% ee) at
89% isolated epoxide yield—and at a catalyst loading as low
as only 2 mol% (entry 20). Most remarkably, switching from
cyclohexyl (ligand 25e) to 1-methylcyclohexyl substitution
(ligand 25d) completely shut down epoxidation activity for
both olefin substrates (entries 15, 16). Furthermore, note that
in the epoxidation of vinyl cyclohexane (17), the loading of
the Ph-substituted catalyst 12 could be reduced to even
1 mol% at invariably high enantioselectivity, although some
loss in epoxide yield occurred upon switching from 10 mol%
(entry 2) to 2 mol% (entry 3) and finally 1 mol% of catalyst
(entry 4). Of the functionalized substrates 22–24, both the
benzoate 23 (entry 23) and the benzyl ether 24 (entry 24) gave
good product yields (91–94%) and 89–90% ee when epoxi-
dized in the presence of ligand 25e. For the alcohol 5-hexen-1-
ol (22), good enantioselectivity (90% ee, entry 22) was
observed, with the expected (see above) partial catalyst
inhibition.
Entry[a]
Substrate
olefin
Olefin
Epoxide
Epoxide
conversion [%][f]
yield [%][f]
ee [%][f]
1
16
16
16
16
16
17
17
17
18
19
19
20
90
86
83
25
68
87
20
56
86
84
83
22
51
87
18
50
89
89
89
88
87
81
80
80
16
63
62
-
2[b]
3[c]
4[d]
5[e]
6
7[d]
8[e]
9
10
11[d]
12
82
78
74
n.d.[g]
n.d.[g]
50
44
trace[h]
[a] In situ/vac procedure for catalyst preparation, reaction at room
temperature with a molar ratio of substrate/ligand/Ti(OiPr)4/30%
aq.H2O2 of 1:0.1:0.1:1.5. [b] 5 mol% of 12/Ti(OiPr)4. [c] 2 mol% of 12/
Ti(OiPr)4. [d] 08C, 50% aqueous H2O2. [e] 408C. [f] Determined by chiral
HPLC or GC. [g] As cis-2-hexene is highly volatile, olefin conversion could
not be determined in a reliable fashion. [h] Epoxide is not stable under
the reaction conditions.
We were able to characterize the Ti complexes 26 and ent-
26, derived from the cis-DACH salalen ligands 12 and ent-12,
by X-ray crystallography.[7] Figure 2 (top) shows the X-ray
crystal structure obtained from ent-26. As in the case of the
Table 3: Asymmetric epoxidation of the olefins 16, 17, 21–24 by the Ti
complexes of ligands 12 and 25a–e.
methylcyclohexene (20, entry 12), trisubstituted olefins are
not efficiently epoxidized by our catalytic system.
Our next optimization step concerned the reaction
solvent. In brief (see Supporting Information for full data
set), halogenated hydrocarbons are the solvents of choice,
with 1,2-dichloroethane being optimal. In this solvent, vinyl
cyclohexane (17) gave an almost quantitative (97%) epoxide
yield, with 82% ee (Table 3, entry 2; compare to entry 6,
Table 2). Finally, a further increase in enantioselectivity was
achieved by variation of the 6-substituents (R’ and R’’) of the
ligand (Table 3). Methoxylation of the phenyl residues
(ligands 25a–c, entries 9–14) was inspired by Katsukiꢀs
observation that this modification was beneficial in the case
of salan ligands.[3] Exchange of Ph for c-hexyl in R’ and R’’
(ligands 25d and 25e, entries 15–24) was hoped to increase
hydrophobic/dispersion interaction between the catalyst and
the olefinic substrates in the presence of aqueous H2O2.[6]
Table 3 reveals the following key features: 1) For the sub-
strate 1-octene (16), methoxylation of the ligand does not
alter the enantioselectivity (90% ee) but both olefin con-
version and epoxide yield are adversely affected (entries 1, 9,
11, and 13). For vinyl cyclohexane (17), ligand methoxylation
again did, at best, result in a slight improvement of
enantioselectivity (84% versus 82% ee, entries 2, 10, 12, and
14). The doubly c-hexyl substituted ligand 25e affords the
highest enantioselectivity for 1-octene epoxide: 94% ee at
Entry[a] L[b] R’
R’’
Olefin Conversion Yield ee
[%][d]
[%][d] [%][d]
1
12 Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
2-MeOPh
2-MeOPh
2-MeOPh
2-MeOPh
Ph
16
17
17
17
21
22
23
24
16
17
16
17
16
17
96
97
95
92
90
97
90
82
2
12 Ph
3
12 Ph
91[e] 82[e]
4
12 Ph
86[f] 82[f]
5
12 Ph
n.d.[g]
51[h]
94[j]
88[j]
72
71
82
6
12 Ph
44[h,i] 86[h]
91[j] 87[j]
70[j] 84[j]
7
12 Ph
8
12 Ph
9
25a Ph
67
48
50
51
71
73
90
84
90
83
90
82
10
11
12
13
14
25a Ph
51
56
54
76
25b 2-MeOPh
25b 2-MeOPh
25c 2-MeOPh
25c 2-MeOPh
Ph
74
4
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Table 3: (Continued)
Entry[a] L[b] R’
DACH derived Ti salalen catalysts provide high yields and
enantioselectivities for non-conjugated terminal olefins—
a notoriously difficult substrate class. The first titanium
salalen peroxo complexes (27, ent-27, and 28) were isolated
and characterized by X-ray crystallography.
R’’
Olefin Conversion Yield ee
[%][d]
[%][d] [%][d]
15
16
17
18
19
20
21
22
23
24
25d Me-c-hexyl[c] Me-c-hexyl[c] 16
25d Me-c-hexyl[c] Me-c-hexyl[c] 17
n.d.
12
91
0
0
88
n.d.
n.d.
94
25e c-hexyl
25e c-hexyl
25e c-hexyl
25e c-hexyl
25e c-hexyl
25e c-hexyl
25e c-hexyl
25e c-hexyl
c-hexyl
c-hexyl
c-hexyl
c-hexyl
c-hexyl
c-hexyl
c-hexyl
c-hexyl
16
16
17
17
21
22
23
24
90
82[f] 95[f]
90
90
90
89[e] 90[e]
70 89
34[h,i] 90[h]
90
n.d.[g]
38[h]
94
94
91
90
89
94
[a] Reactions were performed under the conditions summarized in Table 2,
unless otherwise noted. [b] R’’’=H, except for ligand 25d, where R’’’=Me.
[c] 1-Methylcyclohexyl. [d] Determined by chiral GC or HPLC analysis.
[e] 2 mol% of salalen ligand/Ti(OiPr)4; yield and ee value of isolated product
(1 mmol scale in 1 mL of DCE) after Kugelrohr distillation. [f] 1 mol% of
salalen ligand/Ti(OiPr)4; yield and ee value of isolated product (1 mmol
scale in 0.5 mL of DCE) after Kugelrohr distillation. [g] As 1-hexene is highly
volatile, olefin conversion could not be determined in a reliable fashion.
[h] The reaction was monitored at 18 h. [i] Epoxide is not stable under the
reaction conditions. [j] The reaction was monitored after 72 h.
related complexes derived from the trans-DACH ligands 6
(e.g. R = 6-Ph),[4] a doubly m-oxo-bridged dimer is formed.
The ligands ent-12 are both coordinated to the Ti centers in
a cis-b-mode, that is, with central chirality at the Ti atoms. The
two “halves” of complex ent-26 are homochiral (both
1S,2R,L),[8] and their relative orientation is anti. Note that
the sense of chirality (L) is the same in the Ti complexes
derived from both ligands 6[4] and ent-12.
We were successful in isolating and characterizing (X-ray)
the first three m-oxo-m-peroxo titanium salalen complexes 27,
ent-27, and 28.[10] These complexes were obtained from the
cis-DACH derived ligands 12, ent-12, and 25e, respectively,
and Ti(OiPr)4 in the presence of aqueous H2O2. In both
peroxo complexes ent-27 and 28 (Figure 2, middle and
bottom), the principal structure of the parent m-oxo-bridged
dimer ent-26 (Figure 2, top) is maintained. One of the m-oxo-
bridges is exchanged for a doubly side-on coordinated peroxo
ligand. The O–O distance in the peroxo ligand is 1.444(7) ꢁ
for ent-27, and 1.455 (5) ꢁ for 28, thus in the range typical for
peroxide dianions side-on coordinated to Ti.[11]
The peroxo complex ent-27 alone did not perform
epoxidation, even of easy-to-epoxidize 1,2-dihydronaphtha-
lene (13). Consequently, it does not represent the “loaded”
oxygen-transferring state of the catalytic system. On the other
hand, upon addition of aqueous H2O2, ent-27 promoted the
epoxidation of olefin 13 at a rate and enantioselectivity
indistinguishable from the Ti complex ent-26.
Figure 2. X-Ray crystal structures of Ti salalen complexes derived
from cis-DACH (cis-3). Top: doubly m-oxo-bridged dimer ent-26
(from ligand ent-12). Middle: m-oxo-m-peroxo bridged dimer ent-27
(from ligand ent-12). Bottom: m-oxo-m-peroxo bridged dimer ent-28
[X-ray crystal structure determined for 28 (from ligand 25e),
enantiomer ent-28 shown for ease of comparison].
In summary, we presented the first synthesis of chiral
salalen ligands based on cis-DACH (cis-3), the structural
characterization of their Ti complexes by X-ray crystallog-
raphy, as well as their application in titanium-catalyzed
asymmetric epoxidations with aqueous hydrogen peroxide.
The readily accessible salalen ligand 25e, carrying cyclohexyl
substituents ortho to the phenolic hydroxy groups, proved
particularly effective. As the most significant feature, cis-
Experimental Section
Asymmetric epoxidation of olefins using Ti salalen complexes from
the “in situ/vac” procedure: The salalen ligand (10 mmol, 0.10 equiv)
was added to a solution of Ti(OiPr)4 (2.8 mg, 3.0 mL, 10 mmol,
0.10 equiv) in water-free CH2Cl2 (500 mL). The mixture was stirred for
1 h at room temperature under argon. All volatiles were then
evaporated at room temperature and 10À2 mbar. The olefin
(0.10 mmol, 1.00 equiv), solvent (500 mL), 30% aqueous H2O2
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(15.3 mL, 0.15 mmol, 1.50 equiv), and bromobenzene (22.4 mg, 15 mL,
internal standard) were added. The reaction mixture was stirred at
room temperature. For the epoxidation, no inert atmosphere was
applied. If necessary, additional portions of H2O2 were added
(peroxide test, KI/starch). An aliquot (50 mL) was withdrawn and
diluted with n-hexane or CH2Cl2 (2 mL), and filtered through MgSO4.
The epoxide yield and the ratio of enantiomers were analyzed by
HPLC or GC based on calibrations versus internal standard.
Suzuki, B. Saito, T. Katsuki, Angew. Chem. 2006, 118, 3558 –
3560; Angew. Chem. Int. Ed. 2006, 45, 3478 – 3480.
[4] a) A. Berkessel, M. Brandenburg, E. Leitterstorf, J. Frey, J. Lex,
M. Schꢂfer, Adv. Synth. Catal. 2007, 349, 2385 – 2391; b) A.
Berkessel, M. Brandenburg, M. Schꢂfer, Adv. Synth. Catal. 2008,
350, 1287 – 1294.
[5] A. Berkessel, M.-C. Ong, M. Nachi, J.-M. Neudçrfl, Chem-
CatChem 2010, 2, 1215 – 1218.
[6] C. J. van Oss, D. R. Absolom, A. W. Neumann, Colloid Polym.
Sci. 1980, 258, 424 – 427.
Received: December 21, 2012
Revised: March 22, 2013
Published online: && &&, &&&&
[7] CCDC 915578 (26·DCM), 915579 (27·DCM), 915580 (ent-
26·DCM), 915581 (ent-27·2DCM) and 915582 (28) contain the
supplementary crystallographic data for this paper. These data
can be obtained free of charge from The Cambridge Crystallo-
graphic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
[8] A literature survey revealed that the assignment of the L/D-
descriptor for the chirality of octahedral coordination com-
pounds is not uniform. The assignments made herein are based
on the algorithm proposed by Herrero and Usꢃn (Ref. [9]).
[9] S. Herrero, M. A. Usꢃn, Dalton Trans. 2008, 4993 – 4998.
[10] For a Ti salan peroxo complex by Katsuki et al., see: S. Kondo,
K. Saruhashi, K. Seki, K. Matsubara, K. Miyaji, T. Kubo, K.
Matsumoto, T. Katsuki, Angew. Chem. 2008, 120, 10349 – 10352;
Angew. Chem. Int. Ed. 2008, 47, 10195 – 10198.
Keywords: asymmetric catalysis · cis-DACH · epoxidation ·
.
hydrogen peroxide · titanium
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[3] a) K. Matsumoto, T. Oguma, T. Katsuki, Angew. Chem. 2009,
121, 7568 – 7571; Angew. Chem. Int. Ed. 2009, 48, 7432 – 7435;
b) K. Matsumoto, Y. Sawada, T. Katsuki, Pure Appl. Chem.
2008, 80, 1071 – 1077; c) Y. Shimida, S. Kondo, Y. Ohara, K.
Matsumoto, T. Katsuki, Synlett 2007, 2445 – 2447; d) K. Matsu-
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Sawada, K. Matsumoto, S. Kondo, H. Watanabe, T. Ozawa, K.
[11] A CSD search for Ti peroxo (side-on) complexes gave 40 hits,
À
with O O bond lengths ranging from 1.42–1.49 ꢁ. Among those,
À
the vast majority of O O bond lengths fall in the range 1.44–
1.48 ꢁ.
[12] The project “Sustainable Chemical Synthesis (SusChemSys)” is
co-financed by the European Regional Development Fund
(ERDF) and the state of North Rhine-Westphalia, Germany,
under the Operational Programme “Regional Competitiveness
and Employment” 2007–2013.
6
www.angewandte.org
ꢁ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2013, 52, 1 – 6
These are not the final page numbers!