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Table 3: [FeII(L3)(OTf)2], [FeII(L6)(OTf)2], and [FeII(L7)(OTf)2]-catalyzed
AD of styrene and its p-Br, m-Cl, or o-F derivatives with H2O2.[a]
on a 1 mol scale affording the cis-diol, after recrystallization,
in 99% ee and 72% yield.[17]
[FeII(L3)(OTf)2]-catalyzed AD of (Z)-alkenes, (Z)-b-
methyl styrene, indene, dihydronaphthalene, coumarin, and
2-cyclohexene-1-one, gave the corresponding cis-diols in
22.6–83.1% ee (entries 1–5, Table 2). For 1,2-dihydro-
% ee (conversion [%],[b] yield [%][c])
R
H
p-Br
m-Cl
o-F
[FeII(L3)(OTf)2]
17.5 (83, 90)
32.3 (83, 98)
36.9 (81, 95)
15.0 (77, 84)
[FeII(L6)(OTf)2][d]
59.8 (33, 94)
58.5 (54, 65)
49.7 (34, 79)
40.3 (21, 86)
[FeII(L7)(OTf)2]
19.8 (60, 65)
25.7 (60, 63)
32.0 (54, 69)
15.6 (38, 63)
Table 2: [FeII(L3)(OTf)2]-catalyzed AD of (Z)-alkenes and terminal
alkenes with H2O2 as an oxidant.
Entry[a]
Alkene
Conv.
[%][b]
cis-Diol
ee
[%]
[%][c]
[a] Reaction conditions: alkenes (0.2 mmol), catalyst (0.006 mmol,
3 mol%), H2O2 (0.6 mmol, 3.0 equiv), MeCN/MeOH (2:1, 3 mL), 278C,
1.5 h. [b] Determined by 1H NMR or GC. [c] Determined by 1H NMR
based on conversions. [d] Catalyst (5 mol%) with MeOH (2 mL) as
solvent. Change of catalyst loading from 3 to 5 mol% increased the
conversion and cis-diol yield but did not affect the enantioselectivity.
1[d,e]
2[d,e]
62
87
50
54
22.6
40.6
3[d]
89
53
83.1
4[f]
31
52
81
75.1
46.4
21[g]
A number of mechanistic studies have been performed for
the [FeII(L3)(OTf)2]-catalyzed AD reactions with H2O2.
1) Time-course experiments revealed that the AD of methyl
(E)-cinnamate at 278C is > 90% complete within 0.5 h
(Supporting Information, Figure S3). Competitive AD
reactions (Scheme 2) revealed that electron-withdrawing
5
6
7
8
99
31
40
99
94
95
92.3
81.6
84.9
9
24
96
83.8
[a] Reaction conditions: alkenes (0.2 mmol), [FeII(L3)(OTf)2]
(0.006 mmol, 3 mol%), H2O2 (0.6 mmol, 3.0 equiv), MeOH (2 mL),
278C, 1.5 h. [b] Determined by 1H NMR or GC. [c] Yields determined by
1H NMR and based on conversions. [d] MeCN/MeOH (2:1, 3 mL).
[e] [FeII(L3)(OTf)2] (0.01 mmol, 5 mol%) was used. [f] MeCN/MeOH
(2:1, 3 mL). [g] Isolated yield based on conversion.
Scheme 2. Competitive AD of methyl (E)-cinnamates with H2O2 cata-
lyzed by [FeII(L3)(OTf)2].
naphthalene, the cis-diol was obtained in 83.1% ee with
53% yield. For terminal alkenes, tert-butyl acrylate gave the
cis-diol in nearly quantitative yield with 92.3% ee (entry 6,
Table 2); 1-octene, homoallyl benzene, and cyclohexyl ethyl-
ene afforded the cis-diols in 81.6–84.9% ee and with high
selectivity (94–96%, entries 7–9, Table 2).
The AD of styrene with H2O2 (3 equiv) in MeOH
catalyzed by [FeII(L1)(OTf)2], [FeII(L3)(OTf)2], or
[FeII(L7)(OTf)2] gave the cis-diol in 7, 4, and 12% ee,
respectively (Supporting Information, Table S3). With
MeCN/MeOH (2:1, v/v) as solvent, the AD of styrene and
p-Br, m-Cl, and o-F substituted styrenes afforded the cis-diols
in 15.0–36.9% ee for [FeII(L3)(OTf)2], and in 15.6–32.0% ee
for [FeII(L7)(OTf)2]; [FeII(L6)(OTf)2] catalyzed the AD of
these styrenes to give the cis-diols in significantly higher ee
values of 40.3–59.8% (Table 3).
The L6 ligand, with a binaphthyl backbone, could be
a promising chiral ligand scaffold. The (Z)-alkenes, coumarin,
indene, and (Z)-b-methyl styrene, also underwent AD by the
“[FeII(L6)(OTf)2] + H2O2” method, giving the cis-diols in
higher ee values (83, 67, and 56% ee, respectively; Supporting
Information, Scheme S1), than that obtained with
[FeII(L3)(OTf)2]. The increased enantioselectivity achieved
using [FeII(L6)(OTf)2] indicates that the AD of styrenes and
(Z)-alkenes may be feasibly improved by modification of the
N4 ligand system.
para-substituents (Cl, CF3, Br, and NO2) of methyl (E)-
cinnamate significantly accelerate the reaction by up to 6.1-
fold, while electron-donating groups (p-MeO and p-Me)
showed little effect on the reaction, and for para-substituted
styrenes the p-Cl group accelerated the reaction more than
a p-Me group (Supporting Information, Scheme S2). This
coincides with a coordinated “nucleophilic” hydroperoxide
oxidant but disfavors a highly oxidizing FeV-oxo reaction
intermediate for the reaction (DFT calculated E = 1.58 V vs.
SCE for [FeV(L3)(O)(OH)]2+/[FeIII(L3)(OH)(OH2)]2+ at
pH 1; Supporting Information). 2) The ee, cis-diol yield, and
substrate conversion for the AD of methyl (E)-cinnamate
were almost unaffected by the presence of air (compare
entries 1 and 2, Table 1), disfavoring a radical autoxidation
pathway. 3) Treatment of styrene epoxide with H2O2 or H2O
in the presence of catalyst [FeII(L3)(OTf)2] mainly gave
2-methoxy-2-phenylethanol (45% or 97%) with the diol
obtained in a minor amount (9% or 2%; Supporting
Information, Scheme S3). Thus, a pathway featuring alkene
epoxidation and ring-opening of epoxides could be ignored.
4) GC-MS measurements revealed very low 18O-incorpora-
tion into the cis-diol product (16O16O/16O18O/18O18O ratio =
98.6:1.2:0.2) for the AD of styrene in the presence of H218O
(H216O2/H218O = 1:10; Supporting Information, Scheme S4),
Angew. Chem. Int. Ed. 2016, 55, 1 – 6
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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