Asymmetric epoxidation of unfunctionalized alkenes with ammonium and phosphonium monopersulfates catalyzed by chiral Mn(III)-salen complexes

Article abstract of DOI:10.1016/S0040-4020(99)01008-X

Simple cis-disubstituted and trisubstituted alkenes were enantioselectively epoxidized in mild conditions using various Mn(III)-salen complexes as catalysts and quaternary ammonium and phosphonium monopersulfates (Bu₄NHSO₅, Ph_4<

Full text of DOI:10.1016/S0040-4020(99)01008-X

TETRAHEDRON
Pergamon
Tetrahedron 56 (2000) 417–424
Asymmetric Epoxidation of Unfunctionalized Alkenes with
Ammonium and Phosphonium Monopersulfates Catalyzed by
Chiral Mn(III)–Salen Complexes
ء
¨
Pekka Pietikainen
Department of Chemistry, Laboratory of Organic Chemistry, P.O. Box 55, FIN-00014 University of Helsinki, Finland
Received 16 June 1999; revised 20 October 1999; accepted 11 November 1999
Abstract—Simple cis-disubstituted and trisubstituted alkenes were enantioselectively epoxidized in mild conditions using various Mn(III)–
salen complexes as catalysts and quaternary ammonium and phosphonium monopersulfates (Bu4NHSO5, Ph4PHSO5) as oxidants together
with amine N-oxides as additives. The effect of the catalyst structure on the stereochemical outcome of the epoxidation reactions was studied.
Generally, the 1,2-diphenylethylenediamine-derived complexes were found to give higher asymmetric induction compared to their
1,2-diaminocyclohexane-derived counterparts. Particularly high yields of epoxides (up to 98%) and good enantiomeric excesses (ee up to
93%) were obtained in the epoxidation of 2,2-dialkylchromenes and trisubstituted alkenes. ᭧ 2000 Elsevier Science Ltd. All rights reserved.
Optically active epoxides are valuable intermediates in
organic chemistry because they can undergo stereospecific
ring-opening reactions giving rise to a wide variety of bio-
logically and pharmaceutically important compounds.
During the last decade, chiral (salen)Mn(III) complexes
have emerged as efficient and practical catalysts for the
asymmetric epoxidation of various unfunctionalized cis-
disubstituted, tri- and tetra-substituted alkenes.1–3 Several
different stoichiometric oxidants have been discovered
to be effective oxygen atom donors in these reactions,
most of the epoxidations being conducted using iodo-
sylbenzene2 or NaOCl3. Other common oxidants that
have been explored with (salen)Mn(III) complexes
such as 1–8 (Scheme 1) include MCPBA,4 molecular
oxygen,5 dimethyldioxirane,6 H2O2,7 periodates,8 and
recently also potassium monopersulfate (Oxone᭨,
2KHSO5–KHSO4–K2SO4).4,9
epoxidation of trans-alkenes using chiral ketones as
catalysts.11
Tetrabutylammonium
monopersulfate
(Bu4NHSO5) is a solid easily prepared from Oxone.12,13
Unlike Oxone, it is readily soluble in various organic
solvents and usually used in CH2Cl2 for the mild oxidation
of sensitive compounds.12,14 It has also been used in
oxidations catalyzed by transition metal complexes with
varying results.15
Very recently, simple alkenes were epoxidized in this
laboratory with Bu4NHSO5 catalyzed by Mn(III)–salen
complexes 1 and 5 together with N-methylmorpholine
N-oxide acting as proximal ligand.16 Generally, the results
were good, with electron-rich alkenes the yields and ee’s of
the corresponding epoxides exceeded 90%. Surprisingly,
asymmetric epoxidations conducted using commercially
available “Jacobsen’s” catalyst 5, in many cases the catalyst
of choice, gave considerably lower yields and ee’s
Oxone is a strong, cheap and versatile oxidising agent that
has previously been studied in metalloporphyrin-catalyzed
oxidations.10 It is an efficient single oxygen atom donor
since it contains a non-symmetrical O–O bond which is
heterolytically cleaved during the oxidation cycle catalyzed
by transition metal complexes (porphyrins, salen
compounds). It has some disadvantages: it is insoluble in
organic solvents, buffering is needed due to its acidity, and it
sometimes bleaches the metal catalysts and donor ligands
during oxidation reactions. On the other hand, it has
recently been successfully applied to the asymmetric
Scheme 1. (S,S)-1: R1ˆPh, R2ˆH, R3ˆt-Bu, R4ˆMe. (R,R)-1: R1ˆH,
R2ˆPh, R3ˆt-Bu, R4ˆMe. (S,S)-2: R1ˆPh, R2ˆH, R3ˆt-Bu, R4ˆt-Bu.
(S,S)-3: R1ˆPh, R2ˆH, R3ˆt-Bu, R4ˆOSi(i-Pr)3. (S,S)-4: R1ˆPh, R2ˆH,
R3ˆC(Me)2Ph, R4ˆMe. (S,S)-5: R1,R1ˆ–(CH2)4–, R2ˆH, R3ˆt-Bu,
R4ˆt-Bu.(R,R)-6: R1ˆH, R2,R2ˆ–(CH2)4–, R3ˆt-Bu, R4ˆMe. (R,R)-7:
R1ˆH, R2,R2ˆ–(CH2)4–, R3ˆt-Bu, R4ˆC(Ph)3. (R,R)-8: R1ˆH, R2,R2ˆ
–(CH2)4–, R3ˆC(Me)2Ph, R4ˆt-Bu.
Keywords: asymmetric reactions; epoxidations; catalysts.
ء
Present address: Orion Corporation Fermion, P.O. Box 28, FIN-02101
Espoo, Finland.
0040–4020/00/$ - see front matter ᭧ 2000 Elsevier Science Ltd. All rights reserved.
PII: S0040-4020(99)01008-X

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P. Pietikainen / Tetrahedron 56 (2000) 417–424
418
compared with the values obtained using catalyst 1. These
results prompted the further study of the Bu4NHSO5/NMO
oxidation system. Here we report the results of the
asymmetric epoxidation of simple alkenes using struc-
turally varied catalysts 1–8 together with Bu4NHSO5
and amine N-oxides. Also a quaternary phosphonium mono-
persulfate, Ph4PHSO5, which has earlier been used as a
mild oxidant in kinetic and mechanistic studies concerning
certain Mn(III)-porphyrins,17 was studied as a potential
oxidant in asymmetric salen-catalyzed epoxidations.
reaction conditions using the catalyst 1 (results in Table 1)
together with Bu4NHSO5. The reaction proceeded smoothly
at 2ЊC giving the epoxide of 89% ee in 59% yield (entry 1).
Lowering the temperature from 2 to Ϫ18ЊC increased the
yield to 72% but had practically no effect on the ee of the
epoxide (entry 2). Further lowering of the temperature
resulted in retardation of the reaction rate (entry 3) and a
slight increase in ee. After 5 h reaction time at Ϫ46ЊC the 1H
NMR analysis of the reaction mixture showed the presence
of the epoxide and alkene in a ratio of 85:15, the isolated
yield of the epoxide being 57% (at Ϫ74ЊC the epoxide/
olefin ratio was only 34:66 after 5 h). Epoxidations were
also conducted in CH2Cl2, which resulted in longer reaction
times (e.g. at Ϫ18ЊC reaction time extended from 1.5 to
2.5 h, entry 2), but the yield and ee of the epoxide were
practically identical in both solvents.
Results and Discussion
Asymmetric epoxidation of 6,7-dihydro-5H-benzocyclo-
heptene with monopersulfates under different reaction
conditions
Using an aromatic amine N-oxide, picoline N-oxide, as an
additive in the place of NMO gave equally good yields and
ee’s (entry 5). On the other hand, imidazole was not an
effective donor ligand. The rate of the epoxidation reaction
was similar to that using the amine N-oxides but the yield of
the epoxide and enantioselectivity were lower (entry 6). The
reason may be the tendency of imidazole to degrade in the
presence of many oxidants, e.g. KHSO5,10 although in some
cases it has been used successfully in asymmetric
epoxidations of chromene derivatives.9 Also, when the
epoxidation was performed using substoichiometric amount
of the N-oxide additives the ee of the epoxide was slightly
reduced (entry 4).
Tetrabutylammonium monopersulfate is easily prepared by
two similar methods from Oxone and Bu4NHSO4.12,15a The
original procedure by Trost et al. gave a product with
Bu4NHSO5 content of 37% (the rest consisting of
Bu4NHSO4 and [Bu4N]2SO4) and the modification
of Campestrini et al. afforded Bu4NHSO5 with a purity of
88%. Here, the epoxidations were performed using the purer
oxidant, which produced the epoxides with somewhat
higher asymmetric induction.16 Tetraphenylphosphonium
monopersulfate was obtained using a similar procedure
from Oxone and Ph4PCl with a purity of 89%.17a The epoxi-
dations were conducted in CH3CN containing the substrate,
oxidant, nitrogen heterocycle additive, and salen catalyst
1–8 in a molar ratio of 0.40:0.56–0.65:0.10–0.40:0.0012–
0.0028. The results are summarized in Tables 1 and 2.
Electronic and steric effects in asymmetric epoxidations
with monopersulfates
First, the epoxidation of a simple model substrate, 6,7-dihy-
dro-5H-benzocycloheptene, was studied under different
It was earlier reported, that when the epoxidation of
Table 1. Asymmetric epoxidation of 6,7-dihydro-5H-benzocycloheptene with Bu4NHSO5 and catalysts 1–8 (reactions were performed in CH3CN (2.2 ml).
Molar ratio of alkene:oxidant:additive:catalystˆ0.40:0.65:0.10–0.40:0.012–0.028)
Entry
Catalyst
Additive (equiv.)a
Temperature (ЊC)
Time (h)
Yieldb (%)
eec (%)
Epoxide configurationd
1
2
3
4
5
6
7
8
(S,S)-1
(S,S)-1
(S,S)-1
(S,S)-1
(R,R)-1
(R,R)-1
(S,S)-2
(S,S)-3
(S,S)-4
(S,S)-5
(R,R)-6
(R,R)-7
(R,R)-8
NMO (1.0)
NMO (1.0)
NMO (1.0)
NMO (0.25)
PicNO (1.0)
Imid. (1.0)
NMO (1.0)
NMO (1.0)
NMO (1.0)
NMO (1.0)
NMO (1.0)
NMO (1.0)
NMO (1.0)
2
Ϫ18
Ϫ46
Ϫ18
2
2
Ϫ18
2
1
59
72
89
90
92
84
89
80
88
80
80
72
74
60
83
5S,6R-(ϩ)
5S,6R-(ϩ)
5S,6R-(ϩ)
5S,6R-(ϩ)
5R,6S-(Ϫ)
5R,6S-(Ϫ)
5S,6R-(ϩ)
5S,6R-(ϩ)
5S,6R-(ϩ)
5S,6R-(ϩ)
5R,6S-(Ϫ)
5R,6S-(Ϫ)
5R,6S-(Ϫ)
1.5 (2.5)e
5
1.75
1
57 (80)f
74
72
67
78
62
70
52
60
49
1
1.25
3.5
3
2.5
3.5
3
9
2
2
2
2
10
11
12
13
2
1.5
66
a Relative to olefin. NMOˆN-methylmorpholine N-oxide, PicNOˆpicoline N-oxide, Imid.ˆimidazole.
b Yield of the isolated epoxide.
c Determined by 1H NMR analysis in the presence of Eu(hfc)3.
d Determined by comparison of the sign of [a]D to the literature values.
e Reaction was conducted in CH2Cl2.
f Yield in parentheses is calculated from the reacted olefin.

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P. Pietikainen / Tetrahedron 56 (2000) 417–424
419
Table 2. Asymmetric epoxidation of various alkenes with monopersulfates using catalyst 1 (for general conditions see Table 1. Reactions with Ph4PHSO5 were
performed in 3.2 ml of CH3CN)
Entry
1
Alkene
Oxidant (MHSO5)
Bu4NHSO5
Temperature (ЊC)
Ϫ18
Time (h)
1.5
Yield (%)
72
ee (%)
90
00
2
3
Ph4PHSO5
Bu4NHSO5
Ϫ18
Ϫ18
1.25
1
78
75
91
72
00
00
4
5
6
Ph4PHSO5
Ph4PHSO5
Bu4NHSO5
2
Ϫ18
Ϫ18
1
1
1
79
71
78
75
72a
87b
64c
66d
86
88b
89b
91
00
00
7
8
9
Ph4PHSO5
Ph4PHSO5
Bu4NHSO5
2
Ϫ18
Ϫ18
1
1.25
1.25
00
10
11
Ph4PHSO5
Bu4NHSO5
Ϫ18
Ϫ18
1.5
1
86
97
89
93
12e
13
Bu4NHSO5
Ph4PHSO5
Ϫ18
Ϫ18
2.5
1
86
98
85
91
00
00
a A mixture of cis- and trans-epoxides (8.3:1).
b ee of the cis-epoxide.
c A mixture of cis- and trans-epoxides (7.8:1).
d A mixture of cis- and trans-epoxides (9.4:1).
e Reaction was performed using complex 5 (7 mol%) as the catalyst.
benzocycloheptene with Bu4NHSO5 was catalyzed by
commercially available “Jacobsen’s” catalyst 5 the reaction
proceeded more slowly and the ee of the epoxide dropped
considerably (from 90 to ca. 70%) compared to the reaction
catalyzed by 1.16 The reason for this difference is not clear,
probably the catalyst 5 is partially deactivated during the
catalytic cycle by some unknown mechanism (see Ref. 15
for similar comments). In fact, when the epoxidation was
conducted in a two-phase system using aqueous Oxone as
the oxidant, catalyst 5 was completely bleached during the
reaction (as indicated by TLC and disappearance of the
color of the catalyst). With some other oxidants (e.g.
H2O2) both of these catalysts 1 and 5 give comparable yields
and ee’s in epoxidations of various simple alkenes.7b There-
fore, the electronic and steric effects of substituents on the
different 1,2-diphenylethylenediamine- and 1,2-diamino-
cyclohexane-derived catalysts were studied (Table 1, entries
6–13). In almost all cases the 1,2-diphenylethylenediamine-
derived catalysts gave better ee’s than the corresponding
1,2-diaminocyclohexane-derived complexes.
Altering the electronic and steric environment around the
metal center of the salen catalyst strongly affects the stereo-
chemical outcome of the epoxidations. Jacobsen et al. have
reported that Mn(III)–salen complexes bearing electron-
donating groups exhibited higher asymmetric induction
than those bearing electron-withdrawing groups.18 Here,
introduction of electron-donating groups [e.g. OSi(i-Pr)3
in catalyst 3] at the 5 and 50 positions of the salicylide ligand
(see Scheme 1 for the numbering in salen complexes) atten-
uated the reactivity of the catalyst as expected, but at the
same time both the enantioselectivity and yield of the
epoxide were lowered considerably compared to reactions
catalyzed by, e.g. 1 (entry 2 vs. 8). This somewhat differs
from the results obtained using H2O2 as the oxidant in the
asymmetric epoxidation of benzocycloheptene, where the

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P. Pietikainen / Tetrahedron 56 (2000) 417–424
420
difference in selectivity between catalysts 1 and 3 was not so
deactivation when used with metalloporphyrins.15a,b It was
assumed that Bu4NHSO4 present as an impurity inhibits
epoxidation of olefins by Bu4NHSO5.17a The mechanism
of this action is not known yet. Ph4PHSO5 was then
presumed to react more selectively with metalloporphyrins
than Bu4NHSO5. Therefore, the epoxidation of various di-
and tri-substituted aromatic alkenes was performed using
both monopersulfates as oxidants together with NMO and
catalyst (S,S)-1 (Table 2).
pronounced.7b
Increasing the steric bulk of the substituents on the para
(5,50) positions of the 1,2-diaminocyclohexane-derived
catalysts from methyl (6) and tert-butyl (5) to triphenyl-
methyl (7) resulted in considerably decreased enantioselec-
tivity (entries 10–12). Presumably the large triphenylmethyl
group blocks all sides of the catalyst making the side-on
approach of the olefin more difficult.1a Increasing the size
of the substituents at the ortho (3,30) positions had variable
effects. Introduction of bulky C(Me)2Ph groups on the ortho
positions of 1,2-diphenylethylenediamine-derived salen
ligand (catalyst 4) resulted in decreased asymmetric induc-
tion compared to catalyst 1 as expected on the basis of
earlier studies.19 On the other hand, 1,2-diaminocyclo-
hexane-derived catalyst 8 bearing C(Me)2Ph groups on the
ortho positions was found to be more reactive and selective
(difference in ee 11%) than catalyst 5 bearing t-Bu sub-
stituents (entry 10 vs. 13). Our earlier observations show
that both 1,2-diphenylethylenediamine- (4) and 1,2-diamino-
cyclohexane-derived catalysts (8) bearing C(Me)2Ph groups
at the ortho positions show decreased asymmetric induction
during the asymmetric epoxidation of simple alkenes with
oxidants like NaOCl and H2O2 compared to catalysts 1 and 5
having t-Bu groups in ortho positions (difference in ee ca.
10%).19 The reason for this reversed difference in reactivity
and enantioselectivity between catalyst 5 and 8 is not fully
clear. X-ray crystal structure analysis of 820 gives no straight
answer, since the analysis shows that the salen ligand adopts
the near planar conformation typical for most Mn(III)–salen
complexes.21 In fact, the structure of the complex 8 closely
resembles the known X-ray structures of the catalysts 521a
and 6.21b One possible explanation is that complex 5 is less
stable than the more hindered catalyst 8 towards the
oxidative degradation induced by the monopersulfate
oxidant during the catalytic cycle.
All the reactions proceeded smoothly with high yields and
enantioselectivity. Also both of the oxidants gave almost
identical yields and ee’s with most of the substrates. Only
the epoxidation of indene gave a some what moderate ee
(entries 3–5). Here, the possibility for partial epoxide ring
opening with subsequent kinetic resolution cannot be ruled
out. With this alkene Ph4PHSO5 produced slightly better
results compared to Bu4NHSO5. The epoxidation of (Z)-1-
phenyl-1-propene produced the corresponding epoxide with
a stereoselectivity (cis/transˆ7–9, entries 6–8) comparable
with the results obtained earlier with the MCPBA/NMO-
system.4 Here, the use of Ph4PHSO5 resulted in slightly
lower yield but higher stereoselectivity than Bu4NHSO5.
Particularly useful values (yield, ee) were obtained for
electron-rich substrates spiro[chromen-2,10-cyclohexane]
(entries 9 and 10) and 1,1-diphenyl-1-propene (entries
11–13). We also saw the difference in reactivity between
1,2-diphenylethylenediamine- and 1,2-diaminocyclohexane-
derived catalysts (catalyst 1 vs. 5), the first-mentioned
giving again both higher yields and ee’s. For example, the
epoxidation of 1,1-diphenyl-1-propene with Bu4NHSO5
proceeded with ee’s of 93 and 84% using catalysts 1 and
5, respectively.
In conclusion, ammonium and phosphonium monoper-
sulfates were found to be useful oxidants in Mn(III)–salen
catalyzed asymmetric epoxidations. These readily synthe-
sized oxidants might find more general use since they are
readily soluble in various organic solvents unlike, e.g.
Oxone and PhIO. The reaction system presented here offers
mild reaction conditions, as illustrated by comparing the
Mn(III)–salen catalyzed oxidation of alkenes by
Bu4NHSO5/NMO with that of Oxone (see also Ref. 9). A
number of salen complexes were synthesized and evaluated
in the epoxidation with monopersulfates including two new
catalysts 4 and 8. The 1,2-diphenylethylenediamine-derived
complex 1 was found to be the catalyst of choice.
One problem in comparing the selectivities of different
types of salen complexes is that the structures of the actual
active catalysts [Mn(V)–oxo complexes] are not fully
known and may differ from the Mn(III) complexes.1c In
fact, some research groups have proposed a bent or twisted
structure22 for the active catalyst while others hold to the
planar model.21b Also, it is not fully clear which is the actual
direction of the alkene approach to the metal center in Mn-
oxo complex. Here again, different kinds of approaches
have been proposed at various times by several research
groups.1,22c,23 For example, Jacobsen et al. have proposed
alternative approach models for 1,2-diaminocyclohexane-
derived and 1,2-diphenylethylene-derived catalysts.2a,3a
The common feature in these models is that they all can
reasonably well explain the stereochemical outcome in the
asymmetric epoxidation.1c While the exact mechanism of
the asymmetric Mn–salen catalyzed epoxidation reactions
remains to be elucidated, it is apparent from the results
shown here that the reaction conditions play a fundamental
role.24
Experimental
General
NMR spectra were recorded at 200 MHz on a Varian
Gemini 2000 spectrometer in CDCl3 with Me4Si as internal
standard. IR spectra were acquired by use of a Nicolet
Protege 460 FTIR spectrometer. Optical rotation was
measured with a Jasco DIP-1000 polarimeter at ambient
temperature. EI-MS was acquired by use of a JEOL JMS-
SX102 mass spectrometer. FAB-MS was recorded on a
Finnigan Mat 8200 BE instrument by bombardment of the
samples (in 3-nitrobenzyl alcohol matrix) with Xe.
Elemental analyses were performed by the Analytische
Asymmetric epoxidation of various alkenes with mono-
persulfates
Bu4NHSO5 has previously been reported to cause catalyst

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P. Pietikainen / Tetrahedron 56 (2000) 417–424
421
Laboratorien Prof. Dr. Malissa und G. Reuter GmbH in
Lindlar, Germany. TLC was conducted on Merck aluminum
plates coated with silica gel 60 F254. TLC plates were
visualized with UV and molybdatophosphoric acid–
Ce(SO4)2–H2SO4 with subsequent heating at 120ЊC. Flash
chromatography and dry column flash chromatography25
were performed using Merck silica gel 60 (230–400 mesh
ASTM). (Z)-Phenyl-1-propene was obtained from Tokyo
Chemical Industry Co. 6,7-Dihydro-5H-benzocycloheptene,
spiro-[chromen-2,10-cyclohexane], and 1,1-diphenyl-1-
propene were prepared as indicated in the literature.7b
Oxone was purchased from Sigma–Aldrich Chemie.
Bu4NHSO4 and Ph4PCl were obtained from Fluka Chemie.
Synthesis of the catalysts 1 and 3 is described in a previous
paper.7b Catalyst 5 was from Fluka Chemie.
5-tert-Butyl-2-hydroxy-3-(1-methyl-1-phenylethyl)benz-
aldehyde (salicylaldehyde precursor of 8). Prepared as
above from 4-tert-butyl-2-(1-methyl-1-phenylethyl)phe-
nol.26 The oily product was purified by dry column flash
chromatography (eluent hexane–THF) to afford a pale
solid, yield 68%, mp 76–77.5ЊC. IR (KBr): 3082, 3060,
3027, 2966, 2872, 2835, 1641, 1603, 1460, 1440, 1332,
1
1267, 1212, 1156, 882, 758, 695 cmϪ1. H NMR: d 1.38
(9H, s, t-Bu), 1.75 (6H, s, CMe2), 7.12–7.32 (5H, m, Ph),
7.41 (1H, d, Jˆ2.4 Hz, Ar–H), 7.76 (1H, d, Jˆ2.4 Hz,
Ar–H), 9.83 (1H, s, CHO), 11.22 (1H, s, OH). 13C NMR:
d 29.3, 31.4, 34.3, 42.1, 120.1, 125.4, 125.6, 127.9, 128.2,
132.1, 137.1, 141.6, 149.8, 158.3, 197.1. HRMS (EI) m/z:
calcd for C20H24O2 296.1777, found 296.1782.
(S,S)-N,N0-Bis(3,5-di-tert-butylsalicylidene)-1,2-diphenyl-
ethylenediamine (ligand of catalyst 2). (S,S)-1,2-
Diphenylethylenediamine (0.25 g, 1.18 mmol) and 3,5-di-
tert-butyl-2-hydroxybenzaldehyde27 (0.55 g, 2.35 mmol)
were refluxed in absolute ethanol (10 ml) for 3 h. A small
amount of water was added to the reaction mixture, then it
was allowed cool to 2ЊC and kept at that temperature for 2 h.
The product was collected by suction filtration to afford a
yellow powder, yield 0.53 g (70%), mp 197–198ЊC (lit.29
mp 199–200ЊC). IR (KBr): 3087, 3062, 3030, 2958, 2909,
2869, 1626, 1598, 1469, 1454, 1442, 1362, 1250, 1174, 876,
776, 700 cmϪ1. 1H NMR: d 1.22 (18H, s, t-Bu), 1.42 (18H,
s, t-Bu), 4.72 (2H, s, CH–N), 6.98 (2H, d, Jˆ2.4 Hz, Ar–H),
7.18 (10H, s, Ph), 7.31 (2H, d, Jˆ2.4 Hz, Ar–H), 8.40 (2H,
s, CHyN), 13.59 (2H, s, OH). 13C NMR: d 29.4, 31.4, 34.0,
34.9, 80.0, 117.7, 126.2, 127.0, 127.3, 127.9, 128.1, 136.2,
139.7, 139.9, 157.8, 167.1. HRMS (EI) m/z: calcd for
C44H56N2O2 644.4342, found 644.4329.
2-Hydroxy-5-methyl-3-(1-methyl-1-phenylethyl)benzal-
dehyde (salicylaldehyde precursor of 4). Prepared from
4-methyl-2-(1-methyl-1-phenylethyl)phenol26 using the
procedure of Deng and Jacobsen.27 Yellowish crystals
(from ethanol at Ϫ18ЊC), yield 68%, mp 70–71ЊC. IR
(KBr): 3085, 3052, 3023, 2968, 2938, 2847, 1644, 1599,
1492, 1443, 1324, 1264, 1218, 973, 863, 764, 749,
1
695 cmϪ1. H NMR: d 1.73 (6H, s, CMe2), 2.40 (3H, s,
Ar–CH3), 7.12–7.26 (6H, m, PhϩAr–H), 7.53 (1H, d,
Jˆ2.1 Hz, Ar–H), 9.79 (1H, s, CHO), 11.17 (1H, s, OH).
13C NMR: d 20.7, 29.3, 41.8, 120.5, 125.4, 125.5,
127.9, 128.1, 131.8, 135.5, 137.4, 149.8, 158.3, 196.7.
HRMS (EI) m/z: calcd for C17H18O2 254.1307, found
254.1299.
3-tert-Butyl-2-hydroxy-5-triphenylmethylbenzaldehyde
(salicylaldehyde precursor of 7). 2-tert-Butyl-4-triphenyl-
methylphenol28 (6.58 g, 16.8 mmol) was suspended in dry
benzene (10 ml) under Ar atmosphere together with 2,6-
lutidine (0.8 ml, 6.87 mmol). SnCl4 (0.22 ml, 1.88 mmol)
was added dropwise and the yellowish mixture was stirred
under Ar for 80 min. Solid paraformaldehyde (1.7 g,
56.6 mmol) was then added and the mixture refluxed for
3 h. The mixture was cooled and stirring was continued
while water (30 ml) and ethyl acetate (30 ml) were added.
The mixture was transferred to a separating funnel, and the
reaction flask was rinsed with additional water (30 ml) and
ethyl acetate (30 ml). The mixture was acidified with 2 M
HCl. The resulting emulsion was filtered through a short pad
of Celite to facilitate phase separation, and the Celite was
washed several times with ethyl acetate. The aqueous layer
was extracted with more ethyl acetate (2×25 ml) and the
combined organic extracts were washed with 2 M HCl
(25 ml), water (40 ml) and saturated NaCl solution
(40 ml). Drying with anhydrous Na2SO4, and subsequent
evaporation in vacuo yielded a yellowish solid, 6.72 g
(95%), mp 190–193ЊC (lit.21b mp 189–190ЊC). Recrystalli-
zation from chloroform–methanol afforded a white solid,
mp 203ЊC. IR (KBr): 3086, 3059, 3031, 2991, 2957, 2950,
2909, 2866, 1646, 1615, 1492, 1442, 1415, 1330, 1277,
1202, 1161, 1031, 873, 776, 763, 751, 703, 647,
(S,S)-N,N0-Bis[5-methyl-3-(1-methyl-1-phenylethyl)sali-
cylidene]-1,2-diphenylethylenediamine (ligand of catalyst
4). Prepared as above from (S,S)-1,2-diphenylethylenedia-
mine (220 mg, 1.04 mmol) and 2-hydroxy-5-methyl-3-(1-
methyl-1-phenylethyl)benzaldehyde (528 mg, 2.08 mmol).
Yellow powder, yield 635 mg (91%), mp 103–105ЊC. IR
(KBr): 3082, 3056, 3028, 2964, 2915, 2869, 1627, 1599,
1492, 1453, 1442, 1264, 1029, 860, 776, 764, 697 cmϪ1
.
1H NMR: d 1.69 (6H, s, CH3), 1.72 (6H, s, CH3), 2.29
(6H, s, Ar-CH3), 4.47 (2H, s, CH–N), 6.76 (2H, d,
Jˆ1.9 Hz, Ar–H), 7.0–7.25 (22H, m, PhϩAr–H), 8.11
(2H, s, CHyN), 12.88 (2H, s, OH). 13C NMR: d 20.7,
29.3, 30.0, 42.0, 80.3, 118.4, 124.9, 125.6, 126.3, 127.3,
127.8, 128.0, 128.1, 130.5, 131.2, 135.9, 139.2, 150.6,
157.3, 166.5. HRMS (EI) m/z: calcd for C48H48N2O2
684.3716, found 684.3713.
(R,R)-N,N0-Bis(3-tert-butyl-5-methylsalicylidene)-1,2-
cyclohexanediamine (ligand of catalyst 6). (R,R)-1,2-
Cyclohexanediamine (1.22 g, 10.7 mmol) and 3-tert-butyl-
2-hydroxy-5-methylbenzaldehyde (4.12 g, 21.4 mmol) were
refluxed in absolute ethanol (50 ml) for 1 h and stirred over-
night at 70ЊC. Water was added and the mixture was cooled
at 2ЊC for 2 h to give yellow powder, yield 3.35 g (68%), mp
134–135ЊC (lit.21b mp 134–135ЊC). IR (KBr): 2992, 2956,
2942, 2860, 1628, 1595, 1466, 1441, 1357, 1317, 1265,
634 cmϪ1 1H NMR: d 1.26 (9H, s, t-Bu), 7.16–7.32
.
(16H, m, PhϩAr–H), 7.35 (1H, d, Jˆ2.3 Hz, Ar-H), 9.69
(1H, s, CHO), 11.81 (1H, s, OH). 13C NMR: d 29.2, 34.9,
64.3, 119.4, 126.1, 127.5, 130.9, 132.9, 136.9, 137.5, 138.2,
146.3, 159.5, 197.2. HRMS (EI) m/z: calcd for C30H28O2
420.2089, found 420.2090.
1
1233, 1211, 1165, 866, 772 cmϪ1. H NMR: d 1.40 (18H,
s, t-Bu), 1.4–2.0 (8H, m, CH2), 2.20 (6H, s, CH3), 3.30 (2H,
m, CH–N), 6.78 (2H, d, Jˆ1.8 Hz, Ar–H), 7.04 (2H, d,

¨
P. Pietikainen / Tetrahedron 56 (2000) 417–424
422
Jˆ1.8 Hz, Ar–H), 8.23 (2H, s, CHyN), 13.6 (2H, brs, OH).
13C NMR: d 20.5, 24.3, 29.3, 33.0, 34.6, 72.0, 118.2, 126.3,
129.6, 130.1, 136.6, 157.9, 165.4. HRMS (EI) m/z: calcd for
C30H42N2O2 462.3246, found 462.3242.
1429, 1317, 1252, 1174, 857, 700, 579 cmϪ1. MS (FAB) m/z
697.6 (MϪCl)ϩ. Anal. Calcd for C44H54ClMnN2O2·1/2H2O:
C, 71.19; H, 7.47; N, 3.77. Found: C, 71.36; H, 7.47; N,
3.66.
(R,R)-N,N0-Bis(3-tert-butyl-5-triphenylmethylsali-
cylidene)-1,2-cyclohexanediamine (ligand of catalyst 7).21b
(R,R)-1,2-Cyclohexanediamine (0.141 g, 1.24 mmol), 3-
tert-butyl-2-hydroxy-5-triphenylmethylbenzaldehyde (1.04 g,
2.47 mmol), and anhydrous Na2SO4 (1.5 g) were refluxed
gently in chloroform (10 ml) for 22 h. The cooled reaction
mixture was filtered and the filtrate evaporated. The crude
product was purified by flash chromatography (eluent
n-hexane–THF) to afford yellow foam, yield 1.01 g
(89%). IR (KBr): 3085, 3056, 3029, 2999, 2956-2934,
2861, 1628, 1595, 1492, 1467, 1442, 1279, 1203, 1185,
[(S,S)-N,N0-Bis[5-methyl-3-(1-methyl-1-phenylethyl)-
salicylidene]-1,2-diphenylethylenediamine]chloroman-
ganese(III) (catalyst 4). Prepared as catalyst 2 from (S,S)-
N,N0-bis[5-methyl-3-(1-methyl-1-phenylethyl)-salicylidene]-
1,2-diphenylethylenediamine (602 mg, 0.88 mmol), total
reaction time 4 h. Brown powder, yield 650 mg (96%),
mp 244ЊC. IR (KBr): 3085, 3055, 3028, 2963, 2920, 2869,
1611, 1539, 1494, 1429, 1342, 1299, 1223, 819, 774, 699,
554 cmϪ1. MS (EI) m/z 772 (M)ϩ, 737 (MϪCl)ϩ. Anal.
Calcd for C48H46ClMnN2O2·H2O: C, 72.86; H, 6.18; N,
3.54. Found: C, 73.03; H, 6.18; N, 3.36.
1
1160, 1038, 873, 749, 703, 648, 635 cmϪ1. H NMR: d
1.23 (18H, s, t-Bu), 1.4–1.8 (8H, m, CH2), 3.29 (2H, m,
CH–N), 6.90 (2H, d, Jˆ2.4 Hz, Ar–H), 7.13 (2H, d,
Jˆ2.4 Hz, Ar-H), 7.15–7.26 (30H, m, Ph), 8.17 (2H, s,
CHyN), 13.91 (2H, s, OH). 13C NMR: d 24.2, 29.3, 33.3,
34.8, 64.4, 72.2, 117.4, 125.8, 127.5, 131.1, 131.3, 133.5,
135.7, 135.9, 146.9, 158.6, 165.6. MS (EI) m/z 918 (M)ϩ.
Anal. Calcd for C66H66N2O2: C, 86.24; H, 7.24; N, 3.05.
Found: C, 85.71; H, 7.32; N, 2.88.
[(R,R)-N,N0-Bis(3-tert-butyl-5-methylsalicylidene)-1,2-
cyclohexanediamine]chloromanganese(III) (catalyst 6).
Prepared as catalyst 2 from (R,R)-N,N0-bis(3-tert-butyl-5-
methylsalicylidene)-1,2-cyclohexanediamine (406 mg, 0.88
mmol), total reaction time 16 h. Brown powder, yield
468 mg (97%), mp Ͼ300ЊC (lit.21b mp 311–312ЊC). IR
(KBr): 3032, 3006, 2940, 2910, 2865, 1616, 1543, 1431,
1387, 1338, 1305, 1238, 1208, 1172, 825, 781, 570 cmϪ1
.
MS (EI) m/z 550 (M)ϩ, 515 (MϪCl)ϩ. Anal. Calcd for
C30H40ClMnN2O2·H2O: C, 63.32; H, 7.44; N, 4.92. Found:
C, 63.72; H, 7.85; N, 4.55.
(R,R)-N,N0-Bis[5-tert-butyl-3-(1-methyl-1-phenylethyl)-
salicylidene]-1,2-cyclohexanediamine (ligand of catalyst
8). (R,R)-1,2-Cyclohexanediamine (72 mg, 0.631 mmol)
and
5-tert-butyl-2-hydroxy-3-(1-methyl-1-phenylethyl)-
[(R,R)-N,N0-Bis(3-tert-butyl-5-triphenylmethylsalicyli-
dene)-1,2-cyclohexanediamine]chloromanganese(III)
(catalyst 7). Mn(OAc)2·4H2O (0.53 g, 2.16 mmol) dissolved
in absolute ethanol (5 ml) was added to a suspension of
(R,R)-N,N0-bis(3-tert-butyl-5-triphenylmethylsalicylidene)-
1,2-cyclohexanediamine (0.80 g, 0.87 mmol) in absolute
ethanol (10 ml), and the brown mixture was refluxed for
2 h under air. Solid LiCl (0.12 g, 2.83 mmol) was then
added and the mixture was further refluxed for 1 h. The
reaction mixture was treated as above to afford a brown
powder, yield 0.79 g (90%), mp Ͼ300ЊC (lit.21b mp 325–
326ЊC). IR (KBr): 3083, 3056, 3028, 2999, 2938, 2864,
1621, 1607, 1534, 1491, 1433, 1341, 1308, 1184, 1036,
872, 712, 703, 651, 575 cmϪ1. MS (FAB) m/z 971.4
(MϪCl)ϩ. Anal. Calcd for C66H64ClMnN2O2·H2O: C,
77.29; H, 6.49; N, 2.73. Found: C, 77.34; H, 6.48; N, 2.55.
benzaldehyde (374 mg, 1.262 mmol) were refluxed in
absolute ethanol (6 ml) for 3 h. Water was added to the
reaction mixture and it was cooled to 2ЊC and kept at that
temperature for 2 h. The product was collected by suction
filtration to afford a yellow powder, yield 379 mg (90%), mp
97–99ЊC. IR (KBr): 3083, 3057, 3022, 2963, 2933, 2862,
1628, 1598, 1465, 1442, 1361, 1272, 1161, 771, 762,
698 cmϪ1. H NMR: d 1.28 (18H, s, t-Bu), 1.54 (4H, m,
1
CH2), 1.65–1.80 (4H, m, CH2), 1.69 (6H, s, CH3), 1.73 (6H,
s, CH3), 3.12 (2H, m, CH–N), 6.96 (2H, d, Jˆ2.4 Hz,
Ar–H), 7.10–7.30 (10H, m, Ph), 7.42 (2H, d, Jˆ2.4 Hz,
Ar–H), 8.09 (2H, s, CHyN), 13.19 (2H, s, OH). 13C
NMR: d 24.3, 28.9, 30.2, 31.5, 33.1, 34.1, 42.3, 72.3,
117.9, 125.0, 125.6, 126.3, 127.3, 127.9, 135.6, 139.7,
150.7, 157.4, 165.4. HRMS (EI) m/z: calcd for
C46H58N2O2 670.4498, found 670.4495.
[(R,R)-N,N0-Bis[5-tert-butyl-3-(1-methyl-1-phenylethyl)-
salicylidene]-1,2-cyclohexanediamine]chloromanganese-
(III) (catalyst 8). Prepared as catalyst 2 from (R,R)-N,N0-
bis[5-tert-butyl-3-(1-methyl-1-phenylethyl)salicylidene]-1,2-
cyclohexanediamine (359 mg, 0.535 mmol), total reaction
time 4 h. Brown powder, yield 394 mg (97%), mp 263–
265ЊC. IR (KBr): 3087, 3054, 3021, 2960, 2865, 1613,
[(S,S)-N,N0-Bis(3,5-di-tert-butylsalicylidene)-1,2-diphenyl-
ethylenediamine]chloromanganese(III) (catalyst 2).
Solid Mn(OAc)2·4H2O (0.40 g, 1.63 mmol) was added to a
solution of (S,S)-N,N0-bis(3,5-di-tert-butyl-salicylidene)-
1,2-diphenylethylenediamine (0.51 g, 0.79 mmol) in abso-
lute ethanol (10 ml), and the dark brown mixture was
refluxed for 2 h under air. Solid LiCl (0.11 g, 2.60 mmol)
was then added and the mixture was refluxed for an
additional 2 h and then stirred at 70ЊC overnight. The reac-
tion mixture was cooled, and then water was added resulting
in the precipitation of a brown powder which was collected
by suction filtration. The powder was redissolved in CH2Cl2
and extracted with water and brine. The organic phase was
dried over anhydrous Na2SO4, and solvent evaporated to
afford a brown powder, yield 0.54 g (93%), mp Ͼ300ЊC.
IR (KBr): 3063, 3027, 2956, 2904, 2867, 1610, 1534, 1455,
1538, 1434, 1339, 1311, 1264, 1249, 836, 700, 554 cmϪ1
.
MS (FAB) m/z 723.6 (MϪCl)ϩ. Anal. Calcd for
C46H56ClMnN2O2: C, 72.76; H, 7.43; N, 3.69. Found: C,
72.84; H, 7.55; N, 3.31.
Preparation of the oxidants
Bu4NHSO5.12 To a solution of Oxone (3.4 g, 11.2 mmol of
KHSO5) in distilled water (35 ml) was added Bu4NHSO4
(3.4 g, 10 mmol). The solution was stirred for 20 min and

¨
P. Pietikainen / Tetrahedron 56 (2000) 417–424
423
then extracted with CH2Cl2 (70 ml). The organic phase was
dried over MgSO4, filtered, and evaporated. The crude product
was washed with n-hexane (20 ml) and dried in vacuo to
produce Bu4NHSO5 as a white powder (2.5–2.6 g, 70–
73%). The purity was determined iodometrically to be 88%.
Development Centre of Finland (TEKES). I thank Mr
Jorma Matikainen (PhD) for running the mass spectra.
References
Ph4PHSO5.17a To a solution of Oxone (6.0 g, 19.7 mmol of
KHSO5) in distilled water (60 ml) was added a solution of
Ph4PCl (3.0 g, 8.0 mmol) in CH2Cl2 (60 ml). The mixture
was stirred vigorously for 10 min after which the layers
were separated. The organic phase was dried over MgSO4,
filtered, and evaporated. The crude product was washed
with cold water (20 ml), dried in vacuo and crystallized
once from CH2Cl2–n-hexane to produce Ph4PHSO5 as a
white powder (2.5 g, 69%). The purity was determined
iodometrically to be 89%.
1. Recent reviews: (a) Jacobsen, E. N. In Comprehensive Organo-
metallic Chemistry II; Abel, E. W., Stone, F. G. A., Wilkinson, G.,
Eds.; Pergamon: New York, 1995; Vol. 12, Chapter 11.1. (b)
Katsuki, T. J. Mol. Catal. A: Chem. 1996, 113, 87. (c) Dalton,
C. T.; Ryan, K. M.; Wall, V. M.; Bousquet, C.; Gilheany, D. G.
Top. Catal. 1998, 5, 75. (d) Canali, L.; Sherrington, D. C. Chem.
Soc. Rev. 1999, 28, 85.
2. (a) Zhang, W.; Loebach, J. L.; Wilson, S. R.; Jacobsen, E. N.
J. Am. Chem. Soc. 1990, 112, 2801. (b) Irie, R.; Noda, K.; Ito, Y.;
Matsumoto, N.; Katsuki, T. Tetrahedron: Asymmetry 1991, 2, 481
and references therein. (c) Fukuda, T.; Irie, R.; Katsuki, T. Synlett
1995, 197.
3. (a) Jacobsen, E. N.; Zhang, W.; Muci, A. R.; Ecker, J. R.; Deng,
L. J. Am. Chem. Soc. 1991, 113, 7063. (b) Zhang, W.; Jacobsen, E.
N. J. Org. Chem. 1991, 56, 2296. (c) Brandes, B.; Jacobsen, E. N.
J. Org. Chem. 1994, 59, 4378. (d) Brandes, B.; Jacobsen, E. N.
Tetrahedron Lett. 1995, 36, 5123.
General procedure for the asymmetric epoxidation of
unfunctionalized alkenes with monopersulfates cata-
lyzed by 1–8. To a cooled solution of the alkene
(0.4 mmol), additive (0.1–0.4 mmol), and catalyst
(0.0012–0.0028 mmol) in acetonitrile (2.2–3.2 ml) was
added the monopersulfate oxidant (0.56–0.65 mmol) in
two portions during 20–30 min. The mixture was stirred
at the indicated temperature. After all the olefin had reacted
(monitored by TLC) the reaction was quenched by adding
Me2S (ca. 1.0 mmol). Excess solid K2CO3 was added, the
mixture was allowed to reach room temperature and then
filtered. The filtrate was concentrated and the residue was
purified by flash chromatography (eluent n-hexane–ethyl
4. Palucki, M.; McCormick, G. J.; Jacobsen, E. N. Tetrahedron
Lett. 1995, 36, 5457.
5. Yamada, T.; Imagawa, K.; Nagata, T.; Mukaiyama, T. Bull.
Chem. Soc. Jpn. 1994, 67, 2248.
6. Adam, W.; Jeko, J.; Levai, A.; Nemes, C.; Patonay, T.; Sebok,
P. Tetrahedron Lett. 1995, 36, 3669.
¨
7. (a) Pietikainen, P. Tetrahedron Lett. 1994, 35, 941. (b)
1
acetate). The ee of the epoxide was determined by H
¨
Pietikainen, P. Tetrahedron 1998, 54, 4319. (c) Irie, R.; Hosoya,
NMR analysis in the presence of the chiral shift reagent
N.; Katsuki, T. Synlett 1994, 255.
tris[3-(heptafluoropropylhydroxymethylene)-(ϩ)-campho-
¨
8. Pietikainen, P. Tetrahedron Lett. 1995, 36, 319.
1
rato]europium, Eu(hfc)3. H NMR and chiroptical data of
9. Gurjar, M. K.; Sarma, B. V. N. B. S.; Rama Rao, A. Indian
J. Chem., Sect. B 1997, 36B, 213. See also Ref. 4.
the obtained epoxides are presented in a previous paper.7b
10. Review: Meunier, B. New. J. Chem. 1992, 16, 203 and
references therein.
Typical asymmetric epoxidation of (Z)-1-phenyl-1-
propene with monopersulfates catalyzed by 1 (Table 2,
entry 8). To a cooled (Ϫ18ЊC) solution of (Z)-1-phenyl-1-
propene (48 mg, 0.406 mmol), NMO (48 mg, 0.41 mmol),
and catalyst 1 (18.5 mg, 0.00285 mmol) in acetonitrile
(3.2 ml) was added Ph4PHSO5 (89%, 300 mg, 0.59 mmol)
in two portions during 30 min. After 75 min the reaction
was quenched by adding Me2S (80 ml, 1.1 mmol). Excess
solid K2CO3 was added, the mixture was allowed to reach
room temperature and then filtered through a short pad of
11. See, e.g. Wang, Z.-X.; Tu, Y.; Frohn, M.; Zhang, J.-R.; Shi, Y.
J. Am. Chem. Soc. 1997, 119, 11 224 and references therein.
12. Trost, B. M.; Braslau, R. J. Org. Chem. 1988, 53, 532.
13. Bu4NHSO5 is also commercially available from Fluka Chemie
by the name Oxone tetrabutylammonium salt.
14. Cosstick, R.; Vyle, J. S. Tetrahedron Lett. 1989, 30, 4693.
15. (a) Campestrini, S.; Meunier, B. Inorg. Chem. 1992, 31, 1999.
(b) Hoffmann, P.; Robert, A.; Meunier, B. Bull. Soc. Chim. Fr.
1992, 129, 85. (c) Wessel, J.; Crabtree, R. H. J. Mol. Catal. A:
Chem. 1996, 113, 13.
1
Florisil and the filtrate concentrated. H NMR analysis of
the residue indicated the presence of cis and trans epoxides
in a 9.4:1 ratio. The residue was purified by flash chroma-
tography (eluent n-hexane–ethyl acetate) to afford the cis
epoxide in a 66% yield (containing a trace of the trans
isomer). The ee of the cis epoxide was determined to be
¨
16. Pietikainen, P. Tetrahedron Lett. 1999, 40, 1001.
17. (a) Campestrini, S.; Di Furia, F.; Labat, G.; Novello, F.
J. Chem. Soc., Perkin Trans. 1 1994, 2175. (b) Campestrini, S.;
Di Furia, F.; Ghiotto, P.; Novello, F.; Travaglini, C. J. Mol. Catal.
A: Chem. 1996, 105, 17.
18. (a) Jacobsen, E. N.; Zhang, W.; Gu¨ler, M. L. J. Am. Chem. Soc.
1991, 113, 6703. (b) Palucki, M.; Finney, N. S.; Pospisil, P. J.;
Gu¨ler, M. L.; Ishida, T.; Jacobsen, E. N. J. Am. Chem. Soc. 1998,
120, 948.
1
89% by H NMR analysis in the presence of Eu(hfc)3. The
absolute configuration was determined to be (1R,2S) by
measuring the optical rotation: [a]2D0 Ϫ36.4 (c 0.5, CHCl3,
89% ee) [lit.:30 [a]D20 ϩ47.5 (c 1.17, CHCl3) for (1S,2R)-
1
isomer]. H NMR: d 1.08 (3H, d, Jˆ5.5 Hz), 3.34 (1H, dq,
¨
19. Pietikainen, P.; Brunow, G. Spring Meeting in Synthetic
Jˆ4.3 and 5.5 Hz), 4.06 (1H, Jˆ4.3 Hz), 7.2–7.4 (5H, m).
Chemistry, Turku, Finland, 11–12 May 1995, p 43. See also Ref.
3b for similar results.
¨
20. Pietikainen, P.; Pajunen, A.; Mutikainen, I. Work in progress.
Acknowledgements
21. (a) Rispens, M. T.; Meetsma, A.; Feringa, B. L. Recl. Trav.
Chim. Pays-Bas 1994, 113, 413. (b) Pospisil, P. J.; Carsten, D. H.;
Jacobsen, E. N. Chem. Eur. J. 1996, 2, 974 and references therein.
This work was partially supported by the Technology

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P. Pietikainen / Tetrahedron 56 (2000) 417–424
424
˚
22. (a) Norrby, P.-O.; Linde, C.; Akermark, B. J. Am. Chem. Soc.
1995, 117, 11 035. (b) Hamada, T.; Fukuda, T.; Imanishi, H.;
Katsuki, T. Tetrahedron 1996, 52, 515. (c) Houk, K. N.; DeMello,
N. C.; Condroski, K.; Fennen, J.; Kasuga, T. In Electronic Conference
on Heterocyclic Chemistry (ECHET96); Rzepa, H. S., Snyder, J. P.,
Leach, C., Eds.; The Royal Society of Chemistry: London; 24 June–
22 July 1996, http://www.ch.ic.ac.uk/ectoc/echet96/.
mechanism is the so called spin crossover effect: Jin, N.; Groves,
J. T. J. Am. Chem. Soc. 1999, 121, 2923; Linde, C.; Akermark, B.;
˚
Norrby, P.-O.; Svensson, M. J. Am. Chem. Soc. 1999, 121, 5083.
25. Harwood, L. M. Aldrichimica. Acta 1985, 18, 25.
26. Rosevear, J.; Wilshire, J. F. K. Aust. J. Chem. 1985, 38,
1163.
27. Deng, L.; Jacobsen, E. N. J. Org. Chem. 1992, 57, 4320.
28. Shulgin, A. T. J. Org. Chem. 1962, 27, 3868.
29. Jiang, Y.; Gong, L.; Feng, X.; Hu, W.; Pan, W.; Li, Z.; Mi, A.
Tetrahedron 1997, 53, 14 327.
23. (a) Hosoya, N.; Hatayama, A.; Yanai, K.; Fujii, H.; Irie, R.;
Katsuki, T. Synlett 1993, 641. (b) Hamada, T.; Irie, R.; Katsuki, T.
Synlett 1994, 479.
24. Other phenomenon introduced recently that may affect the
30. Witkop, B.; Foltz, C. M. J. Am. Chem. Soc. 1957, 79, 197.