Baeckvall J.-E. (ed.) Modern Oxidation Methods

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1 Recent Developments in Metal-catalyzed Dihydroxylation of Alkenes

Transition Metal-Catalyzed Epoxidation of Alkenes

Hans Adolfsson

2.1

Introduction

The formation of epoxides via metal-catalyzed oxidation of alkenes in the presence of

molecular oxygen or hydrogen peroxide represents the most elegant and environ-

mentally friendly route for the production of this compound class [1, 2]. This is of

particular importance considering that the conservation and management of re-

sources should be the main focus of interest when novel chemical processes are

developed. Thus, the introduction and improvement of catalytic epoxidation methods

where molecular oxygen or hydrogen peroxide are employed as terminal oxidants are

highly desirable. However, one of todays industrial routes for the formation of simple

epoxides (e.g., propylene oxide) is the chlorohydrin process, where alkenes are reacted

with chlorine in the presence of sodium hydroxide (Scheme 2.1) [3]. This process

produces 2.01 tonne NaCl and 0.102 tonne 1,2-dichloropropane as by-products per

tonne of propylene oxide. These signiﬁcant amounts of waste are certainly not

acceptable in the long run, and efforts to replace such chemical processes with

greener epoxidation processes are under way. When it comes to the production of

ﬁne chemicals, non-catalyzed processes with traditional oxidants (e.g., peroxyacetic

acid and meta-chloroperoxybenzoic acid) are often used. In these cases, though,

transition metal-based systems using hydrogen peroxide as the terminal oxidant have

several advantages. This chapter covers the development of efﬁcient and selective

metal-catalyzed epoxidation methods starting with early transition metals (Ti, V) and

ending with group X metals (Pt). The scope and focus will be to highlight some novel

approaches to alkene oxidation using environmentally benign oxidants.

+ 2 NaOH

2 NaCl + H

Scheme 2.1

2.2

Choice of Oxidant for Selective Epoxidation

Several terminal oxidants are available for the transition metal-catalyzed epoxida-

tion of alkenes (Table 2.1). Typical oxidants compatible with a majority of

metal-based epoxidation systems include various alkyl hydroperoxides, hypochlo-

rite, or iodosylbenzene. A problem associated with these oxidants is their low active

oxygen content (Table 2.1). Considering the nature of the waste produced, there are

further drawbacks using these oxidants. From an environmental and economical

point of view, molecular oxygen should be the preferred oxidant in view of its high

active oxygen content and the fact that that no waste product or only water is

formed. One of the major limitations, however, in using molecular oxygen as

terminal oxidant for the formation of epoxides is the poor product selectivity

obtained in these processes [4]. In combination with the limited number of catalysts

available for direct activation of molecular oxygen, this seriously limits the use of

this oxidant. On the other hand, hydrogen peroxide displays much better properties

as terminal oxidant. The active oxygen content of H

is about as high as that in

typical applications of molecular oxygen in epoxidations (since a reductor is

required in almost all cases), and the waste produced by employing this oxidant

is plain water. As in the case of molecular oxygen, the epoxide selectivity using

can sometimes be relatively poor, although recent developments have led to

transition metal-based protocols where excellent reactivity and epoxide selectivity

can be obtained [5]. The various oxidation systems available for the selective

epoxidation of alkenes using transition metal catalysts and hydrogen peroxide will

be covered in the following sections.

Table 2.1 Oxidants used in transition metal-catalyzed epoxidations, and their active oxygen

content.

Oxidant Active oxygen content (wt. %) Waste product

Oxygen (O

) 100 Nothing or H

Oxygen (O

)/reductor 50 H

47 H

NaOCl 21.6 NaCl

H 21.1 CH

t-BuOOH (TBHP) 17.8 t-BuOH

KHSO

10.5 KHSO

BTSP

9 Hexamethyldisiloxane

PhIO 7.3 PhI

a) Bistrimethylsilyl peroxide.

2 Transition Metal-Catalyzed Epoxidation of Alkenes

2.3

Epoxidations of Alkenes Catalyzed by Early Transition Metals

High-valent early transition metals like titanium(IV) and vanadium(V) have been

shown to efﬁciently catalyze the epoxidation of alkenes. The preferred oxidants using

these catalysts are various alkyl hydroperoxides, typically tert-butyl hydroperoxide

(TBHP) or ethylbenzene hydroperoxide (EBHP). One of the routes for the industrial

production of propylene oxide is based on a heterogeneous Ti

/SiO

catalyst, which

employs EBHP as the terminal oxidant [6].

The Sharpless-Katsuki asymmetric epoxidation (AE) protocol for the enantiose-

lective formation of epoxides from allylic alcohols is a milestone in asymmetric

catalysis [7]. This historically highly important and classical asymmetric transforma-

tion is catalyzed by a dimeric complex formed from titanium isopropoxide in

combination with a tartrate ester (1). TBHP is the preferred terminal oxidant, and

the reaction has been widely used in various synthetic applications. There are several

excellent reviews covering the scope and utility of the AE reaction [8]. As a complement

to the Sharpless-Katsuki AE-reaction of allylic alcohols, Yamamoto recently presented

some highly interesting results using novel vanadium catalysts (2 and 3) based on C

symmetric bis-hydroxamic acid ligands [9]. The newly designed catalysts were indeed

superior to previously reported vanadium systems, and excellent enantioselectivities

were achieved in the epoxidation of a series of differently substituted allylic alcohols.

In particular, the AE reaction on cis-substituted alkenes, which normally tend to give

lower enantioselectivities, resulted in excellent ees employing catalyst 3. In addition,

this epoxidation protocol tolerates the use of aqueous TBHP, and does not suffer from

the ligand deceleration effects normally observed for vanadium catalysts.

O O

OR'

OR'O

V O

CPh

V O

On the other hand, the use of hydrogen peroxide as an oxidant in combination with

early transition metal catalysts (Ti and V) is rather limited. The reason for the poor

reactivity can be traced to severe inhibition of the metal complexes by strongly

coordinating ligands like alcohols, in particular, water. The development of the

heterogeneous titanium(IV)-silicate catalyst (TS-1) by chemists at Enichem repre-

sented a breakthrough for reactions performed with hydrogen peroxide [10]. This

hydrophobic molecular sieve demonstrated excellent properties (i.e., high catalytic

activity and selectivity) for the epoxidation of small linear alkenes in methanol. The

substrates are adsorbed into the micropores of the TS-1 catalyst, which efﬁciently

prevents the inhibition by water observed using the Ti

/SiO

catalyst. After the

2.3 Epoxidations of Alkenes Catalyzed by Early Transition Metals

epoxidation reaction, the TS-1 catalyst can easily be separated and reused. To enlarge

the scope of this epoxidation method and thereby allow for the oxidation of a wider

range of substrates, several different titanium-containing silicate zeolites have been

prepared. Consequently, the scope has been somewhat improved, but the best

epoxidation results using titanium silicates as catalysts are obtained with smaller,

nonbranched substrates.

The problems encountered with high oxophilicity of early transition metal catalyst

systems have severely limited the use of aqueous hydrogen peroxide as terminal

oxidant.However, recentlythegroupof Katsukireported onasigniﬁcantbreakthrough

in the ﬁeld of titanium-catalyzed asymmetric epoxidation of nonfunctionalized

alkenes [11– 13]. The successful catalysts used in this system are based on a combi-

nation of titanium and reduced salen-type ligands (also known as salenan and salan,

respectively). The oxidation of terminal alkenes such as styrene and 1-octene using

catalyst 4 resulted in good yields of the epoxides (90 and 70% respectively) in high

enantioselectivity (93 and 87% ee respectively). Most strikingly, only 1 mol% of the

catalyst and 1.05 equivalents of 30% aqueous hydrogen peroxide were required in the

epoxidations. When this catalytic system was employed for the epoxidation of 1,2-

dihydronaphthalene, the corresponding epoxide was obtained in 99% yield and 99%

ee. The dimeric m-oxo-bridged catalyst 4 with its half-reduced salen ligands has a rather

elaboratestructureandasomewhat complicatedpreparationprocedure.Nevertheless,

its stability is remarkable since the homochiral complex keeps its dimeric structure in

methanol for more than 24 h. For comparison, the corresponding salen-type complex,

which is completely inactive as an epoxidation catalyst under these conditions,

dissociates immediately into monomeric titanium-salen species in methanol. The

difference in catalytic activity observed between 4 and the corresponding [Ti(salen)]

complex is believed to be due to the presence of an intramolecular hydrogen bonding

betweenthe amineNHanda peroxo-grouponthe metal,whichactivates thecomplex

for the transfer of oxygen to the substrate. A synthetically less challenging and more

ﬂexible analog of 4 has also been evaluated as an epoxidation catalyst in the presence of

aqueous hydrogen peroxide. Complex 5a (Ar ¼ Ph), easily prepared directly from

titaniumisopropoxide and the salan ligand, is,however, less activeandselectivethan4.

N N

O O

Ph Ph

N N

Ar Ar

O O

Ph=Ar

5b =Ar

-MeOC

5c =Ar

-CF

2 Transition Metal-Catalyzed Epoxidation of Alkenes

Moreover, a structural modiﬁcation of the initial salan ligand used in the study led

to the development of catalysts which are able to form epoxides with a high degree of

stereocontrol. As shown in Table 2.2, catalysts based on ligands 5b and 5c (Ar ¼ o-

MeOC

and o-CF

, respectively) are able to epoxidize a range of alkenes in

good yields and with excellent selectivity. As in the case of the corresponding

manganese-salen systems, cis-substituted alkenes are the preferred substrates. The

epoxidation reaction is reported to be stereospeciﬁc, since no trans-epoxides are

formed starting from cis-alkenes.

The oxidation of simple alkenes like cyclohexene using monovanadium complexes

and hydrogen peroxide as terminal oxidant predominantly results in allylic oxidations

instead of epoxide formation. However, when vanadium is incorporated into poly-

oxometalates the results are signiﬁcantly improved. Mizuno and coworkers recently

reported that the tetra- n-butylammonium salt of a polyoxometalate with the com-

position [c-1,2-H

SiV

]

4

acts as a highly active and selective epoxidation

catalyst employing aqueous H

as terminal oxidant [14, 15]. This polyoxometalate,

which has a bridging di-VO-core, VO-(m-OH)

-VO, catalyzes the stereospeciﬁc

epoxidation of a wide range of unfunctionalized alkenes, dienes, and hydroxyl-

functionalized alkenes, in the presence of only one equivalent of oxidant per

unsaturation (Scheme 2.2). The limitation of this system appears to be trans-

disubstituted alkenes, which showed poor reactivity (trans-2-octene gave only 6%

yield of the epoxide under the conditions presented in Scheme 2.4). Mechanistic

studies and DFT calculations indicate severe steric interactions between the sub-

stituents of such substrates and the polyoxometalate framework [16].

Table 2.2 Asymmetric titanium-catalyzed epoxidation.

H30%

equiv)1.5-(1.01

mol%)(1-5Ti-catalyst

solvent, r.t.

-C

Ti catalyst Yield (%) ee (%) Yield (%) ee (%) Yield (%) ee (%) Yield (%) ee (%)

93 87

99 64

88 70

d),f)

47 82 72 95 69 90 25 55

68 89 86 98 84 94

80 87 77 98 92 96

a) 1 mol% catalyst, reaction time 24 h.

b) 5 mol% catalyst, reaction time 24 h.

c) Catalyst prepared in situ from 5 mol% titanium isopropoxide and 6 mol% of the corresponding

ligand, reaction time 9 h.

d) Reaction performed in dichloromethane.

e) Reaction performed in ethyl acetate.

f) Reaction time 48 h.

2.3 Epoxidations of Alkenes Catalyzed by Early Transition Metals

2.4

Molybdenum and Tungsten-Catalyzed Epoxidations

Epoxidation systems based on molybdenum and tungsten catalysts have been

extensively studied for more than 40 years. The typical catalysts, Mo

-oxo or W

oxo species do, however, behave quite differently depending on whether anionic or

neutral complexes are employed. Whereas the former catalysts, especially tungstates

under phase-transfer conditions, are able to efﬁciently activate aqueous hydrogen

peroxide for the formation of epoxides. Neutral molybdenum or tungsten complexes

do react with hydrogen peroxide, but better selectivities are often achieved using

organic hydroperoxides (e.g., t-butyl hydroperoxide) as terminal oxidants [17, 18].

2.4.1

Homogeneous Catalysts – Hydrogen Peroxide as the Terminal Oxidant

Payne and Williams reported in 1959 on the selective epoxidation of maleic, fumaric,

and crotonic acids using a catalytic amount of sodium tungstate (2 mol%) in

combination with aqueous hydrogen peroxide as the terminal oxidant [19]. The key

to success was careful control of the pH (4–5.5) in the reaction media. These electron-

deﬁcient substrates were notoriously difﬁcult to selectively oxidize using the standard

techniques (peroxy acid reagents) available at the time. Previous attempts to use

sodium tungstate and hydrogen peroxide led to the isolation of the corresponding

diols because of rapid hydrolysis of the intermediate epoxides. Signiﬁcant improve-

ments of this catalytic system were introduced by Venturello and coworkers [20, 21].

They found that the addition of phosphoric acid and the introduction of quaternary

ammonium salts as PTC-reagents considerably increased the scope of the reaction.

The active tungstate catalysts are often generated in situ, although catalytically active

peroxo-complexes like (n-hexyl

[PO

(W(O)(O

)

] have been isolated and char-

acterized (Figure 2.1) [22].

In more recent work, Noyori and coworkers found conditions for the selective

epoxidation of aliphatic terminal alkenes either in toluene, or using a completely

solvent-free reaction setup [23, 24]. One of the disadvantages with the previous

24hC,

equiv.)1aq,(30%

[

-1,2-H

SiV

][Bu

mol%)(5

92%

99%

90%

99%

88%

99%

87%

12:88)(syn:anti95%

yield:

selectivity:

Scheme 2.2

2 Transition Metal-Catalyzed Epoxidation of Alkenes