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[116, 117]. As seen in Scheme 2.21, a reasonably high enantioselectivity can be

obtained with sterically hindered substrates, whereas simple alkenes gave signiﬁ-

cantly lower ees. Interestingly, products of opposite stereochemistry were obtained in

the epoxidation of trans-stilbene using the two different (S,S)-diamine ligands shown

in Scheme 2.21.

An interesting chiral bimetallic iron complex was recently presented by Kwong and

coworkers [118]. Starting from the enantiomerically pure sexipyridine ligand 24 and

iron(II) chloride, the bimetallic iron complex [Fe

O(24)Cl

] was obtained

(Scheme 2.22).

N N

[Fe

)Cl

mol%)(2]

mol%)(20AcOH

min.3C,

Hequiv.1.5

CHaq),(35%

[Fe

O(24)Cl

]

yield95%

(

)ee43%

yield100%

(

)ee30%

yield90%

(

)ee42%

yield85%

)ee37%

yield62%

)ee40%

yield29%

ee31% 2

)

Scheme 2.22

Using 2 mol% of complex [Fe

O(24)Cl

] as the catalyst for the epoxidation of a

series of aromatic alkenes resulted in the formation of the corresponding epoxides in

good yields and in moderate enantioselectivity.

Even though only moderate enantiomeric excesses were obtained using the

chiral iron-containing catalysts described above, these examples are of particular

interest, since further elaboration along these lines could produce more selective

catalysts.

2.7 Iron-Catalyzed Epoxidations

2.8

Ruthenium-Catalyzed Epoxidations

High-valent ruthenium oxides (e.g., RuO

) are powerful oxidants and react readily

with alkenes, mostly resulting in cleavage of the double bond [119]. If reactions are

performed with very short reaction times (0.5 min) at 0



C it is possible to obtain

better control of the reactivity and thereby obtain cis-diols. On the other hand, the use

of less reactive low-valency ruthenium complexes in combination with various

terminal oxidants has been described for the preparation of epoxides from simple

alkenes [120]. In the more successful earlier cases, ruthenium porphyrins were used

as catalysts, especially in combination with N-oxides as terminal oxidants [121–123].

Two examples are shown in Scheme 2.23, where terminal alkenes are oxidized in the

presence of catalytic amounts of Ru porphyrins 25 and 26 employing the sterically

hindered 2,6-dichloropyridine N-oxide (2,6-DCPNO) as the oxidant. The use of this

particular oxidant was crucial for high epoxide yield, since simple pyridine, generated

from reaction with pyridine N-oxide, signiﬁcantly lowered the epoxidation rate by

coordination to the ruthenium catalyst. Interestingly, aerobic epoxidation of alkenes

under 1 bar of molecular oxygen without any additional reducing agent has been

reported with catalyst 26 [124].

Nishiyama and coworkers reported that the combination of pyridine-2,6-di-

carboxylic acid (H

pydic) and terpyridine ligated to ruthenium(II) resulted in a

complex with reasonably high catalytic activity for the epoxidation of trans-stilbene in

the presence of various high-valent iodine compounds or TBHP as terminal

=Ar

2625

mol%)(0.25

equiv.)(12,6-DCPNO

65,

1hC,

26 mol%)(0.6

equiv.)(1.12,6-DCPNO

30benzene,

h6C,

yield100%

yield96%

Scheme 2.23

2 Transition Metal-Catalyzed Epoxidation of Alkenes

oxidants [125]. Based on these ﬁndings, Beller and coworkers initially reported that

RuCl

and H

pydic acted as an efﬁcient catalyst for the epoxidation of alkenes using

30% aqueous hydrogen peroxide as the terminal oxidant [126]. With RuCl

loadings

ranging from 0.01–5 mol% and slow addition of the oxidant to avoid ruthenium-

catalyzed decomposition of H

, a number of different alkenes were successfully

converted into epoxides in yields up to 99%. Further developments using the

Nishiyama ligand combination (H

pydic and terpyridine) allowed for an even more

efﬁcient catalytic process [127]. Selected examples using the latter protocol are

exempliﬁed in Scheme 2.24.

Asymmetric epoxidation of alkenes has been reported using ruthenium catalysts

based on either chiral porphyrins or on pyridine-2,6-bisoxazoline (pybox) ligands.

Berkessel et al. reported that catalysts 27 and 28 were efﬁcient catalysts for the

enantioselective epoxidation of aryl-substituted alkenes [128]. Enantioselectivity up

to 83% was obtained in the epoxidation of 1,2-dihydronaphthalene using catalyst 28

and 2,6-DCPNO. Simple alkenes like 1-octene reacted poorly and gave epoxides of low

enantioselectivity. The mixed-ligand ruthenium complex 29, containing a chiral

tridentate Schiff base ligand derived from

D-glucose and two triphenylphosphines

ligands, was recently prepared, and the catalytic properties of this complex were

evaluated in the epoxidation of styrene and styrene derivatives [129]. The catalytic

activity was rather poor, giving epoxides in modest yields, although very promising

enantioselectivities were obtained. In the epoxidation of 4-chlorostyrene with 1 mol%

catalyst loading, the corresponding epoxide was formed in 45% yield and in 94% ee

using one equivalent of aqueous TBHP as the terminal oxidant. A certain degree of

over-oxidation was observed using this catalytic system, and signiﬁcant amounts of

aldehydes (8–18%) were formed along with the epoxides.

Hequiv.3

aq),(30%

-AmylOH

addition)(slow

[Ru(pydic)(terpyridine)]

mol%)(0.5[Ru(pydic)(terpyridine)]

yield95%yield71%

cat.mol%0.1

yield99% yield88%

yield95%

yield97%OH:=X

yield94%:Cl=X

yield84%

Scheme 2.24

2.8 Ruthenium-Catalyzed Epoxidations

OMe CF

=Ar

PPh

The use of pybox ligands in ruthenium-catalyzed asymmetric epoxidations was

ﬁrst reported by Nishiyama et al., who used catalyst 31 in combination with either

iodosyl benzene, bisacetoxyiodo benzene [PhI(OAc)

], or TBHP for the oxidation of

trans-stilbene [130]. In the best result, using PhI(OAc)

as oxidant, they obtained

trans-stilbene oxide in 80% yield and 63% ee. More recently, Beller and coworkers

have reexamined this catalytic system and found that asymmetric epoxidations could

be performed using ruthenium catalysts 30 and 31 and 30% aqueous hydrogen

peroxide [131–133]. A development of the pybox ligand led to ruthenium complex 32,

which turned out to be the most efﬁcient catalyst for asymmetric alkene epoxidation.

Thus, using 5 mol% of 32 and slow addition of hydrogen peroxide, a number of aryl

substituted alkenes were epoxidized in yields up >99% and enantioselectivity up to

84% (Scheme 2.25).

2.9

Epoxidations Using Late Transition Metals

The use of transition metal complexes from group IX as catalysts or reagents for

alkene epoxidation is so far rather limited. Cobalt is the main metal to have been

investigated, since a number of cobalt complexes are known to directly activate

molecular oxygen, which thereafter can be used in further oxidative processes.

Mukaiyama and coworkers reported that cobalt(II)-Schiff base complexes catalyzed

the epoxidation of various alkenes using molecular oxygen as the terminal oxidant

and ketones or aldehydes as reductants.[134, 135] Isolated yields up to 98% were

reported for some substrates, but typically these protocols resulted in lower epoxide

yields. When it comes to epoxidation processes mediated by group X metals, the

situation is similar to that for group IX metals. There are a number of reports which

discuss the use of nickel complexes as catalysts for the epoxidation of alkenes. From a

synthetic point of view, no efﬁcient and selective nickel-based systems have been

2 Transition Metal-Catalyzed Epoxidation of Alkenes

discovered [136–139]. On the contrary, Strukul and coworkers recently reported on a

highly interesting epoxidation system based on cationic electron-deﬁcient platinum

(II) complexes, which in the presence of aqueous hydrogen peroxide catalyzed the

formation of epoxides from simple terminal alkenes [140]. The catalyst precursor in

the Strukul epoxidation system is the organometallic Pt(II) complex 33, containing a

pentaﬂuorophenyl ligand along with a bidentate diphosphine and a coordinated

water molecule. The catalytic activity using a number of different diphosphine

ligands was investigated, and reasonable yields were obtained in the epoxidation

of terminal alkenes using 2 mol% catalyst.

The introduction of chiral bidentate diphosphines allowed for asymmetric epox-

idations of terminal alkenes. Screening a number of different chiral diphosphines in

the epoxidation of 4-methylpentene using 2 mol% of catalyst and 1 equivalent of

aqueous hydrogen peroxide as the terminal oxidant revealed that the catalyst contain-

ing S,S-Chiraphos (34) allowed for an efﬁcient chirality transfer from the catalyst to

the substrate [141]. With this particular catalyst combination, the R-isomer of 4-

methylpentene oxide was obtained in 60% yield and 75% enantiomeric excess. The

R R

O O

N N

O O

ArAr

Ph=R

2-naphthyl=Ar

Hequiv.3

aq),(30%

-AmylOH

h)12overaddition(slow

Ru-catalyst

mol%)(0.5

yield85%

ee59%

yield95%

ee72%

yield91%

h)(26ee84%

yield100%

ee54%

Scheme 2.25

2.9 Epoxidations Using Late Transition Metals

activity and selectivity of the platinum complex 34 turned out to be quite general, and

a number of different terminal alkenes were efﬁciently and selectively converted to

their corresponding epoxides (Scheme 2.26).

In addition to the outstanding selectivities obtained in the epoxidation of terminal

alkenes, a particularly interesting feature of this catalytic system is the inherent

reactivity of the catalyst. Most epoxidation systems known favor the oxidation of the

most electron-rich double bond; however, when catalyst 34 was employed in the

epoxidation of dienes containing both internal and terminal alkenes, only the

terminal position reacted [142]. Hence, this catalyst shows unique selectivity prop-

erties. In further studies of this catalytic system, Strukul and coworkers developed an

environmentally benign enantioselective epoxidation protocol in which reactions

were performed in a water-surfactant medium [143]. The best conditions were

obtained using 1 mol% of 34 and one equivalent of aqueous hydrogen peroxide in

a mixture of water and the surfactant Triton-X100. Under these reaction conditions, a

number of terminal alkenes were converted into their corresponding epoxides in

good yields and with moderate to good enantioselectivity within a short reaction time.

This protocol is still in its immature stage, and we will surely see further develop-

ments along these lines in the near future.

R R

O O O

O O

Yield:

ee:

Yield:

ee:

98%

58%

99%

74%

88%

79%

75%

64%

87%

66%

98%

equiv.)1aq.(35%

mol%)(2

-10

h24-48C,

Scheme 2.26

2 Transition Metal-Catalyzed Epoxidation of Alkenes

Regarding the epoxidation activity of group XI metals, the industrial formation of

ethylene oxide is a classic example of a silver-catalyzed epoxidation process [144]. In

this oxidation reaction using molecular oxygen as the terminal oxidant, the active

catalyst consists of silver particles supported on a-alumina. In order to moderate the

activity of the catalyst and thereby further increase epoxide selectivity, the catalyst

system is treated with ppm levels of gaseous chlorocarbons and controlled amounts

of alkali. Nevertheless, a problem when using these silver-based catalytic systems is

the high activity of the heterogeneous catalyst, which often leads to ethylene

combustion instead of formation of ethylene oxide. For higher alkenes, for example,

propene, this is the major reaction, and therefore Ag catalysts are poor mediators for

alkene epoxidation of substrates other than ethylene. Since propene oxide is such an

important commodity chemical, great effort is being put into ﬁ nding new and more

selective catalysts for the epoxidation of propene. Some of the latest developments

even involve supported gold catalysts [145, 146].

2.10

Concluding Remarks

The epoxidation of alkenes using transition metal-based catalysts is certainly a well-

studied reaction. There are, however, only a few really good and general systems that

work with environmentally benign oxidants (e.g., aqueous hydrogen peroxide). A

comparison of the efﬁciency ﬁgures obtained with the catalysts described in this

chapter is presented in Table 2.9.

A striking feature of the various catalytic epoxidation methods presented in this

chapter in general and in Table 2.9 in particular is the poor substrate-to-catalyst ratio

observed for most systems. In the best cases, the catalyst loading is as low as 0.01 mol

%, but most often 0.5–1 mol% of the catalyst needs to be added to ensure full

conversion of the starting alkene. In comparison to the corresponding highly efﬁcient

catalysts existing for the reduction of alkenes using molecular hydrogen, there is a

long way to go before oxidation catalysts reach the same level of efﬁciency. When it

comes to substrate scope, it is evident from the content of this chapter that there are

advantages and limitations with almost all available epoxidation systems. The

environmentally attractive TS-1 system is highly efﬁcient, but is restricted to linear

substrates. The various tungsten systems available efﬁciently produce epoxides from

simple substrates, but acid-sensitive products undergo further reactions, thus

effectively reducing the selectivity of the process. MTO is a highly active epoxidation

catalyst, and when it is combined with the most appropriate heterocyclic additives,

even hydrolytically sensitive products are obtained in good yield and selectivity. Most

of the MTO-catalyzed reactions are, however, performed in chlorinated or ﬂuorinated

solvents. Regarding asymmetric processes using hydrogen peroxide as the terminal

oxidant, there are only a few reported systems that produce epoxides with respectable

enantioselectivity values. The ruthenium- and iron-based catalysts developed by

Beller and coworkers are promising candidates, and further developments along

these lines may produce more selective systems.

2.10 Concluding Remarks

In conclusion, the above summary of oxidation methods shows that there is still

room for further improvement in the ﬁeld of selective alkene epoxidation. The

development of active and selective catalysts which are capable of oxidizing a broad

range of alkene substrates employing aqueous hydrogen peroxide as terminal oxidant

in inexpensive environmentally benign solvents remains a continuing challenge.

Table 2.9 Transition metal-catalyzed epoxidation of alkenes using H

as the terminal oxidant.

Catalyst S/C Solvent Temp. (



C) 1-Alkene

yield (%)/

TOF (h

1

)

Cyclooctene

yield (%)/

TOF (h

1

)

Ref.

100 MeOH 25 74/108

— 10

50/500 Toluene 90 91/12 98/122 24

4000/10000 CH

CN 25 94/3760 80/24000 31

1000 CH

CN/

0 90/600

95/1600 54

200 CH

25 99/14 89/8900 76

1000 CF

OH 25 97/48 99/990 87

33 CH

CN 4 85/337 86/341 104

50 DCE

0 81/10 — 143

a) TS-1.

b) Na

,NH

,(C

)

NCH

HSO



c) [PPh

][Mo(O)(O

)

(QO)].

d) [Mn(OTf)

(pyTACN)].

e) MTO, pyrazole.

f) 19.

g) 34.

h) 1-Decene.

i) 1-Octene.

j) DCE ¼ dichloroethane.

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2 Transition Metal-Catalyzed Epoxidation of Alkenes