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

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275

Selective Oxidation of Amines and Sulfides

Jan-E. B

€

ackvall

8.1

Introduction

Heteroatom oxidation is of great importance in organic synthesis, amines and

sulﬁdes being the compounds most commonly involved in such reactions. These

compounds can be oxidized to a number of different products, and various reagents

have been developed for these transformations. This chapter deals with selective

oxidations of sulﬁdes (thioethers) to sulfoxides and of tertiary amines to N-oxides.

8.2

Oxidation of Sulfides to Sulfoxides

The oxidation of sulﬁdes has been reviewed previously [1–5], one of these reviews

being focused on asymmetric sulfoxidation [5]. Organosulfur compounds such as

sulfoxides and sulfones are useful synthetic reagents in organic chemistry. In

particular, sulfoxides are important intermediates in the synthesis of natural pro-

ducts and biologically signiﬁcant molecules [6]. Optically active sulfoxides are also of

importance in medicinal and pharmaceutical chemistry [5], and chiral sulfoxides

have been extensively used in asymmetric synthesis [7]. They have also been

employed as ligands in asymmetric catalysis [8] and as oxotransfer reagents [9]. The

synthesis and utilization of chiral sulfoxides has been reviewed [8a]. A large number

of methods are available for the oxidation of sulﬁdes to sulfoxides, and an important

issue is to obtain high selectivity for sulfoxide over sulfone. Usually there is a

reasonably good selectivity for oxidation to sulfoxide, since the sulﬁde is much more

nucleophilic than the sulfoxide and hence reacts faster with the electrophilic

reagent/catalyst. However, there are large variations between the different oxidation

systems.

In this review, oxidations of sulﬁdes to sulfoxides have been divided into

three s ubsections: (i) stoichiometric react ions, (ii) chemocatalytic reactions, and

(iii) biocatalytic reactions.

277

8.2.1

Stoichiometric Reactions

A large number of methods are known in the literature for the oxidation of sulﬁdes

(thioethers) to sulfoxides by electrophilic reagents. These include the use of per-

acids [10], NaIO

, hypochlorites, MnO

,CrO

, SeO

, and iodosobenzene. The use of

these reagents has been reviewed up to 2005 [3, 4]. Hydrogen peroxide can also be

used as a direct oxidant, although this reaction is slow in the absence of catalyst.

Its use in catalytic reactions is discussed below in Section 8.2.2.

8.2.1.1 Peracids

Peracids are commonly used oxidants in the oxidation of sulﬁdes to sulfoxides [10].

The reaction proceeds at a good rate at room temperature, giving the sulfoxide

together with some sulfone. Usually the selectivity for sulfoxide over sulfone is good

enough for synthetic purposes, since the oxidation of sulfoxide is considerably slower

than that of sulﬁde.

8.2.1.2 Dioxiranes

Dioxiranes have been successfully used for the selective oxidation of sulﬁdes to

sulfoxides (Eq. (8.1)) [11].

ð8:1Þ

These reactions are rapid, and the sulﬁde is efﬁciently oxidized to sulfoxide as

the only product with no overoxidation to sulfone. The reaction is thought to proceed

via direct oxygen transfer from the oxirane to the sulﬁde. In a mechanistic study [12],

it was found that a hypervalent sulfurane is an intermediate in the reaction, being in

equilibrium with the electrophilic zwitterionic intermediate formed from electro-

philic attack by the peroxide on the sulﬁde.

The oxirane oxidation of sulﬁdes has found applications in organic synthe-

sis [13, 14]. For example, sulfoxidation of disulﬁde 1 with dioxirane afforded

disulfoxide 2 in 98% yield (Eq. (8.2)) [13].

oxidant

MCPBA

0%98%

49%9%

ð8:2Þ

278

8 Selective Oxidation of Amines and Sulfides

The corresponding sulfoxidation of 1 with MCPBAwas unsuccessful and gave only

9% of disulfoxide 2 together with substantial amounts of degraded dicarbonyl

compound (29%) and monosulfoxide 3 (49%). Other applications of sulfoxidation

with the use of dioxirane are given in Ref. [14].

8.2.1.3 Oxone and Derivatives

Oxone, which is commercially available as a 2 : 1 : 1 mixture of KHSO

, KHSO

and K

, has been used for the oxidation of sulﬁdes to sulfoxides [15]. It shows a

tendency to overoxidation, and was originally used for the oxidation of sulﬁdes to

sulfones [16]. More recently, some improvements were obtained with surface-

mediated oxone oxidations, the oxone being bound to a silica gel surface (Eq. (8.3)).

However, some sulfone was still formed.

Bu Bu

OXONE

on silica

88%

ð8:3Þ

In a recent modiﬁcation of the oxone-type oxidation, the quaternary salt benzyl-

triphenyl phosphonium peroxymonosulfate PhCH

HSO



was employed

(Eq. (8.4)). No overoxidation to sulfone was detected, according to the authors.

PhCH

HSO

MeCN, 80

R = Me, 91%

R = n-Bu 92%

R = Bn 88%

ð8:4Þ

8.2.1.4 H

in Fluorous Phaseand Related Reactions

Oxidation of sulﬁdes to sulfoxides by H

in hexaﬂuoro-2-propanol has been

reported to occur with an exceptionally high rate and selectivity [17]. Reaction of

ethyl phenyl sulﬁde with 2 equiv. of 30% aqueous H

in hexaﬂuoro-2-propanol at



C was complete within 5 min and gave the corresponding sulfoxide in 97% yield.

No sulfone was formed; in a control experiment the sulfoxide was stirred with 2 equiv.

of 30% H

for 8 h without any formation of sulfone. The normally slow-reacting

sulﬁdes 4, 5, and 6, reacted fast, and the procedure tolerates double bonds (Table 8.1).

Sulﬁdes having double bonds also underwent a selective sulfoxidation (e.g., 7 and 8,

entries 4 and 5). This seems to be an excellent method for efﬁcient and highly

selective oxidation of sulﬁdes (thioethers) to the corresponding sulfoxides, the only

limitation being the toxicity of the hexa ﬂuoro-2-propanol.

A highly selective oxidation of sulﬁdes to sulfoxides takes place by 30% aqueous

in phenol at room temperature [18]. Methyl phenyl sulﬁde was oxidized to the

sulfoxide within 0.5 minutes in 99% yield. No sulfone could be detected. In a control

experiment methyl phenyl sulfoxide was allowed to react for 14 h with 30% aqueous

in phenol at room temperature; the sulfoxide was recovered unchanged.

8.2 Oxidation of Sulfides to Sulfoxides

279

A range of sulﬁdes including diphenyl sulﬁde were oxidized to sulfoxides in high

yield (93–99%) in less than 5 min.

The high rates of the H

-based sulfoxidation in hexaﬂuoro-2-propanol and

phenol are ascribed to hydrogen-bonded H

(see Chapter 4).

8.2.2

Chemocatalytic Reactions

In almost all catalytic reactions reported for oxidation of sulﬁdes to sulfoxides a

peroxide compound or molecular oxygen is employed. The most common oxidant is

, usually as a 30% aqueous solution.

8.2.2.1 H

as Terminal Oxidant

A large number of catalysts have been reported in the literature for the H

oxidation of sulﬁdes to sulfoxides. These include various metal complexes (of

transition metals and lanthanides), ﬂavins, and benzthiazoles.

Transition Metals as Catalysts Oxidation of various sulﬁdes to sulfones by H

mediated by TiCl

was reported by Watanabe [19]. Quantitative yields of sulfoxide

were obtained within 5–15 min with no overoxidation to sulfone. However, a

sevenfold excess of hydrogen peroxide and 2 equivalents of TiCl

per mole of

substrate were employed.

Tungsten-based catalytic systems for H

oxidations of sulﬁdes have attracted

considerable interest, and some early reports include the use of H

[20]. More

recently, various tungsten-catalyzed methods have been used [21–25].

Table 8.1 Hydrogen peroxide oxidation of sulfides to sulfoxides in fluorous phase.

Entry Substrate Reaction time (min) % Yield of sulfoxide

599

10 93

20 97

599

15 94

280

8 Selective Oxidation of Amines and Sulfides

The Venturello-type peroxo complex Q

{(PO

)[W(O)(O

)

]

}, where Q ¼ N-(n-

) pyridinium, was employed as catalyst for the oxidation of sulﬁdes to

sulfoxides and sulfones by hydrogen peroxide [21]. The selectivity for sulfoxides

was low with this catalyst, which gave only sulfone. The corresponding molybdenum

complex Q

{(PO

)[M(O)(O

)

]

} and Q

PMo

as catalyst in the H

oxidations

gave mixtures of sulfoxide and sulfone that ranged from 3 : 1 to 1 : 3 depending on

the substrate. Finally, Q

as catalyst in the H

oxidations gave a good

selectivity [21]. Related peroxo-tungstates and -molybdates Ph

)[MO(O

)

]



(M ¼ W or Mo) were studied as catalysts for H

-oxidation of sulfoxides. The

selectivity for sulfoxide was low for these catalysts [22].

Polyoxometalates WZnMn

(ZnW

)

2

were employed to oxidize sulﬁdes to

sulfones in moderate selectivity (85–90% selectivity with 10–15% of sulfone) [23].

The catalytic effect was strong for the oxidation of aromatic sulﬁdes but weak for the

oxidation of aliphatic sulﬁdes. Recently, Mizuno has reported that by using

the selenium-contaning peroxotungstate [SeO

{WO(O

)

}]

2

, various sulﬁdes were

efﬁciently oxidized to sulfoxides by H

[26]. The selectivity and yield of these

reactions were very high. A larger-scale oxidation (100 mmol) of thioanisole was

demonstrated, which afforded 12.73 g (91%) of the corresponding sulfoxide (99%

pure). The reaction gives very high turnover numbers.

(91%)g12.73

H+

mmol100

(TBA)

[SeO

{WO(O

)

}]

ml)(40CN

4hC,

mol%)(0.005

Noyori has reported on a tungsten-based halogen-free system for the oxidation to

sulfoxides with 30% aqueous H

that gives sulfoxides with good to moderate

selectivity [24]. The reactions were run without organic solvent, and the catalyst

employed was Na

together with PhPO

and CH

(n-C

)NHSO

.For

example, thioanisole gave sulfoxide and sulfone in a ratio of 94 : 6 after 9 h at 0



Cwith

the use of a substrate-to-catalyst ratio of 1000 : 1.

Choudary and coworkers [25] subsequently extended the tungsten system to the

use of layered double hydroxides (LDHs) with water as solvent and 30% H

as the

oxidant. Under these conditions the new catalysts LDH-WO

2

gave good turnover

rates; however, the selectivity of the oxidation of thioanisole was moderate, with a

sulfoxide:sulfone ratio of 88 : 12 (Eq. (8.5)). Other sulﬁde oxidations also gave a

moderate selectivity (Eq. (8.5))

LDH-WO

[24]ref6:94Me=R

[25]ref12:88Me=R

[25]ref29:71vinyl=R

ð8:5Þ

(8.5)

8.2 Oxidation of Sulfides to Sulfoxides

281

The advantage with the latter system, in spite of the moderate selectivity for

sulfoxide over sulfone, is that the immobilized catalyst can be recovered and reused.

The catalyst showed consistent activity and selectivity for six recyclings.

The catalytic cycle with the WO

2

catalysts is thought to involve the generation of

tungsten peroxy complexes (Scheme 8.1). These react fast with the sulﬁde with

transfer of an oxygen to give the sulfoxide.

Feringa investigated various nitrogen ligands for the selective Mn-catalyzed

oxidation of sulﬁdes to sulfoxides with 30% aqueous H

[27]. The use of Mn

(OAc)

2H

O with bipyridine ligand 9 in the oxidation of thioanisole gave sulfoxide

free from sulfone in 55% yield. With ligand 10 the same yield was obtained, but now

with the sulfoxide in 18% ee.

N N

10 R = Me

9 R = H

Titanium-catalyzed oxidations with 35% aqueous H

using Schiff-base (salen)

titanium oxo complexes as catalysts showed very high activity [28]. The oxidation of

methyl phenyl sulﬁde required only 0.1 mol% of catalyst. The use of chiral salen

complexes gave low enantioselectivity (<20% ee).

Vanadium-catalyzed H

oxidations of sulﬁdes to sulfoxides have been re-

ported by several groups. These reactions have been shown to work well for

asymmetric sulfoxidation. In 1995 Bolm and Bienewald reported on the use of

vanadium-chiral Schiff base complexes as catalysts for asymmetric sulfoxidation by

30% H

[29]. Oxidation of thioanisole using ligand 11 afforded the correspond-

ing sulfoxide in 70% ee in high yield without any signiﬁcant overoxidation to

sulfone (Eq. (8.6)).

Scheme 8.1

282

8 Selective Oxidation of Amines and Sulfides