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

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solubilizing properties and miscibility, together with the high selectivities of the

transformations and the advantageous properties of hydrogen peroxide make these

systems almost ideal. Mechanistic studies have revealed that the catalytic effects of

ﬂuorinated alcohol solvents hinge mostly on their pronounced hydrogen bond donor

ability, together with low nucleophilicity. In some cases, a detailed picture of how

these supramolecular solvent-substrate interactions operate could be obtained. As

an appealing next step, it may be envisaged that synthetic chemistry may provide

second generation ﬂ uorinated solvents/catalysts that incorporate these features

and do not depend on the statistics of supramolecular chemistry for the formation

of active aggregates.

References

1 Earlier review on ﬂuorinated alcohols as

reaction medium: B



egu



e, J.-P., Bonnet-

Delpon, D., and Crousse, B. (2004) Synlett,

18–29.

2 Imperiali, B. and Abeles, R.H. (1986)

Tetrahedron Lett., 27, 135–138.

3 (a) Thaisrivongs, S., Pals, D.T., Kati, W.M.,

Turner, S.R., Thomasco, L.M., and Watt,

W. (1986) J. Med. Chem., 29, 2080–2087;

(b) Fearon, K., Spaltenstein, A., Hopkins,

P.B., and Gelb, M.H. (1987) J. Med. Chem.,

30, 1617–1622.

4 (a) Arends, I.W.C.E. and Sheldon,

R.A. (2004) Modern Oxidation Methods

(ed. J-.E. B

€

ackvall), Wiley-VCH,

Weinheim, pp. 83–118; (b) Brinksma, J.,

de Boer, J.W., Hage, R., and Feringa, B.L.,

ibid. 295–326.

5 Arends, I.W.C.E. and Sheldon, R.A. (2002)

Top. Catal., 19, 133–141.

6 Strukul,G. (2003)Catalytic Oxidations with

Hydrogen Peroxide as Oxidant, Kluwer.

7 Berkessel, A. and Etzenbach-Effers, K.

(2009) Hydrogen Bonding in Organic

Synthesis (ed. P. Pihko), Wiley-VCH,

Weinheim, pp. 15–38.

8 Berkessel, A. and Etzenbach-Effers, K.

(2010) Top. Curr. Chem., 291,1–27.

9 Legros, J., Crousse, B., Bourdon, J.,

Bonnet-Delpon, D., and B



egu



e, J.-P.

(2001) Tetrahedron Lett., 42, 4463–4466.

10 Legros, J., Crousse,B., Bonnet-Delpon, D.,

and B



egu



e, J.-P. (2002) Tetrahedron, 58,

3993–3998.

11 Eberson, L., Hartshorn, M.P., and

Persson, O. (1995) J. Chem. Soc., Perkin

Trans 2, 1735–1744.

12 (a) Kamlet, M.J., Abboud, J.M.L.,

Abraham, M.H., and Taft, R.W. (1983)

J. Org. Chem., 48, 2877–2887;

(b) Middleton, W.J. and Lindsey, R.V.

(1964) J. Am. Chem. Soc., 86,

4948–4952.

13 (a) Schadt, F.L., Bentley, T.W., and

Schleyer, P.v.R. (1976) J. Am. Chem.

Soc., 98, 7667–7674; (b) Allard, B.,

Casadevall, A., Casadevall, E., and

Largeau, C. (1979) Nouv. J. Chim., 3,

335–342; (c) Allard, B., Casadevall, A.,

Casadevall, E., and Largeau, C. (1980)

Nouv. J. Chim., 4, 539–545; (d) Leonard,

N.J. and Neelima, A. (1995) Tetrahedron

Lett., 36, 7833–7836.

14 (a) Murto, J., Kivinen, A., and

Strandman, L. (1971) Suomen Kemistilehti

B, 44, 308; (b) Schall, H., H

€

aber, T., and

Suhm, M.A. (2000) J. Phys. Chem. A, 104,

265–274.

15 (a) Reichard, C. (1994) Chem. Rev., 94,

2319–2358; (b) Reichardt, C. (2002)

Solvents and Solvent Effects in Organic

Chemistry, 3rd edn, Wiley-VCH,

Weinheim.

16 Berkessel, A., Adrio, J.A., H

€

uttenhain, D.,

and Neud

€

orﬂ, J.M. (2006) J. Am. Chem.

Soc., 128, 8421–8426.

17 Maiti, N.C., Zhu, Y., Carmichael, I.,

Serianni, A.S., and Anderson, V.E. (2006)

J. Org. Chem., 71, 2878–2880.

18 Berkessel, A. and Adrio, J.A. (2006) J. Am.

Chem. Soc., 128, 13412–13420.

19 Shrine, T.M. and Kim, L. (1977)

US 4024165 (Patent to Shell Oil

Company).

References

143

20 Romanelli, M.G. (1983) Exxon EP

0096130.

21 Neimann, K. and Neumann, R. (2000)

Org. Lett., 2, 2861–2863.

22 van Vliet, M.C.A., Arends, I.W.C.E., and

Sheldon, R.A. (2001) Synlett, 248–250.

23 Berkessel, A. and Adrio, J.A. (2004)

Adv. Synth. Catal., 346, 275–280.

24 Dryuk, V.G. (1976) Tetrahedron, 32,

2855–2866.

25 de Visser, S.P., Kaneti, J., Neumann, R.,

and Shaik, S. (2003) J. Org. Chem., 68,

2903–2912.

26 Cleland, W.W., Frey, P.A., and

Gerlt, J.A. (1998) J. Biol. Chem., 273,

25529–25532.

27 Johnstone en Francsicsn



e-Czinege,

R.A.W.,WO 98/17640, 30 April 1998.

28 van Vliet, M.C.A., Arends, I.W.C.E., and

Sheldon, R.A. (1999) Tetrahedron Lett., 40,

5239–5242.

29 Jacobson, S.E., Mares, F., and Zambri,

P.M. (1979) J. Am. Chem. Soc., 101,

6946–6950.

30 Berkessel, A. and Andreae, M.R.M. (2001)

Tetrahedron Lett., 42, 2293–2295.

31 For a review on selenium-promoted

oxidations of organic compounds, see:

Mlochowski, J., Brzaszcz, M., Giurg, M.,

Palus, J., and Wojtowicz, H. (2003) Eur. J.

Org. Chem., 4329–4339.

32 Syper, L. and Mlochowski, J. (1987)

Tetrahedron, 43, 207–213.

33 Grieco, P.A., Yokoyama, Y., Gilman, S.,

and Nishizawa, M. (1977) J. Org. Chem.,

42, 2034–2036.

34 ten Brink, G.-J., Fernandes, B.C.M.,

van Vliet, M.C., Arends, I.W.C.E., and

Sheldon, R.A. (2001) J. Chem. Soc., Perkin

Trans. 1, 224–228.

35 For a report on selenium-catalyzed

epoxidations with hydrogen peroxide in

perﬂuroalkanes (not ﬂuorinated alcohols),

see: Betzemeier, B., Lhermitte, F., and

Knochel, P. (1999) Synlett, 489–491.

36 Adolfsson, H. (2004) Modern Oxidation

Methods (ed. J-.E. B

€

ackvall), Wiley-VCH,

Weinheim, pp. 21–49.

37 Romao, C.C., K

€

uhn, F.E., and

Herrmann, W.A. (1997) Chem. Rev., 97,

3197–3246.

38 van Vliet, M.C.A., Arends, I.W.C.E., and

Sheldon, R.A. (2001) Synlett, 1305–1307.

39 van Vliet, M.C.A., Arends, I.W.C.E., and

Sheldon, R.A. (1999) Chem. Commun.,

263–264.

40 Adam, W., Saha-M

€

oller, C.R., and

Ganeshpure, P.A. (2001) Chem. Rev., 101,

3499–3548.

41 Herrmann, W.A., Fischer, R.W., and

Marz, D.W. (1991) Angew. Chem. Int. Ed.,

30, 1638–1641.

42 Herrmann, W.A., Fischer, R.W., Scherer,

W., and Rauch, M.U. (1993) Angew. Chem.

Int. Ed., 32, 1157–1160.

43 Rudolph, J., Reddy, K.L., Chiang, J.P., and

Sharpless, K.B. (1997) J. Am. Chem. Soc.,

119, 6189–6190.

44 Cop



eret, C., Adolfsson, H., and Sharpless,

K.B. (1997) Chem. Commun.,

1565–1566.

45 Herrmann, W.A., K

€

uhn, F.E.,

Fischer, R.W., Thiel, W.R., and Ramao,

C.C. (1992) Inorg. Chem., 31,

4431–4432.

46 Herrmann, W.A., Kratzer, R.M., and

Fischer, R.W. (1997) Angew. Chem., Int. Ed.

Engl., 36, 2652–2654.

47 van Vliet, M.C.A., Arends, I.W.C.E., and

Sheldon, R.A. (2000) J. Chem. Soc., Perkin

Trans. 1, 377–380.

48 Ravikumar, K.S., B



egu



e, J.P., and

Bonnet-Delpon, D. (1998) Tetrahedron

Lett., 39, 3141–3144.

49 Ravikumar, K.S., Zhang, Y.M., B



egu



e, J.P.,

and Bonnet-Delpon, D. (1998) Eur. J. Org.

Chem., 2937–2940.

50 Keasvan, V., Bonnet-Delpon, D., and



egu



e, J.-P. (2000) Tetrahedron Lett., 41,

2895–2898.

51 Ravikumar, K.S., Kesavan, V.,

Crousse, B., Bonnet-Delpon, D., and



egu



e, J.P. (2003) Org. Synth., Coll., 80,

184–186.

52 Kesavan, V., Bonnet-Delpon, D.,

and B



egu



e, J.P. (2000) Synthesis,

223–225.

53 Berkessel, A., Andreae, M.R.M.,

Schmickler, H., and Lex, J. (2002) Angew.

Chem. Int. Ed., 41, 4481–4484.

54 Berkessel, A. and Vogl, N. (2006)

The Chemistry of Peroxides, vol. 2

(ed. Z. Rappoport), Wiley, Chichester,

pp. 307–596.

55 Dilthey, W., Inckel, M., and Stephan, H.

(1940) J. Prakt. Chem., 154, 219–237.

144

4 Catalytic Oxidations with Hydrogen Peroxide in Fluorinated Alcohol Solvents

56 Jacobson, S.E., Mares, F., and Zambri,

P.M. (1979) J. Am. Chem. Soc., 101,

6938–6946.

57 Syper, L. (1989) Synthesis, 167–172.

58 ten Brink, G.-J., Vis, J.-M., Arends,

I.W.C.E., and Sheldon, R.A. (2001) J. Org.

Chem., 66, 2429–2433.

59 ten Brink, G.-J., Vis, J.-M., Arends,

I.W.C.E., and Sheldon, R.A. (2002)

Tetrahedron, 58, 3977–3983.

60 Miyake, Y., Nishibayashi, Y., and

Uemura, S. (2002) Bull. Chem. Soc. Jpn.,

75, 2233–2237.

References

145

Modern Oxidation of Alcohols using Environmentally

Benign Oxidants

Isabel W.C.E. Arends and Roger A. Sheldon

5.1

Introduction

The oxidation of primary and secondary alcohols to the corresponding carbonyl

compounds plays a central role in organic synthesis [1]. However, standard organic

textbooks [2] still recommend classical oxidation methods using stoichiometric

quantities of inorganic oxidants, notably chromium(VI) reagents [3] or ruthenium

or manganese salts [4], which are highly toxic and environmentally polluting. Other

classical non-green methods are based on the use of high-valent iodine compounds

(notably the Dess Martin reagent) or involve the stoichiometric use of DMSO (Swern

oxidation) [4]. However, the state of the art in alcohol oxidation nowadays is far better.

Numerous catalytic methods are now known which can be used to oxidize alcohols

using either O

or H

as the oxidant. These oxidants are to be preferred because

they are inexpensive and produce water as the sole by-product. In this review we focus

on the use of metal catalysts to mediate the selective oxidation of alcohols using O

as the primary oxidant. Mainly homogeneous catalysts are described, but,

where relevant, heterogeneous catalysts, in particular metal nanoparticles, are

covered as well. For an excellent review on heterogeneous oxidation of alcohols and

carbohydrates we refer to the publications by Gallezot et al. [5]. Before turning to

metal-mediated oxidation of alcohols, we ﬁrst describe the recent developments in

catalytic oxoammonium-mediated oxidation of alcohols.

5.2

Oxoammonium based Oxidation of Alcohols – TEMPO as Catalyst

A very useful and frequently applied method in the ﬁne chemical industry to convert

alcohols into the corresponding carbonyl compounds is the use of oxoammonium

salts as oxidants, as shown in Figure 5.1 [6]. These are very selective oxidants for

alcohols, operate under mild conditions, and tolerate a large variety of functional

groups. The oxidation proceeds in acidic as well as alkaline media.

147

The oxoammonium is generated in situ from its precursor, 2,2

,6,6

-tetramethyl-

piperidine-N-oxyl (TEMPO), or derivatives thereof, which is used in catalytic quan-

tities (see Figure 5.2). Various oxidants can be applied as the ﬁnal oxidant [7–12]. In

particular, the TEMPO-bleach protocol using bromide as co-catalyst introduced by

Anelli et al. is ﬁnding wide application in organic synthesis [7]. TEMPO is used in

amounts as low as 1 mol% relative to the substrate, and full conversion of substrates

can commonly be achieved within 30 min.

The major drawbacks of this method are the use of NaOCl as the oxidant, the need

for addition of 10 mol% bromide as a cocatalyst, and the necessity to use chlorinated

solvents. Recently a great deal of effort has been devoted toward a greener oxoam-

monium-based method, for example, by replacing TEMPO by heterogeneous varia-

tions or replacing NaOCl by a combination of metal as cocatalyst and molecular

oxygen as oxidant. Examples of heterogeneous variants of TEMPO are anchoring

TEMPO to solid supports such as silica [13, 14] and the mesoporous silica,

+ RCH

- H

alkaline conditions

acidic conditions

+ RCHO

Figure 5.1 Mechanism for the oxoammonium-catalyzed oxidation of alcohols.

OCl

OBr

oxid.

Figure 5.2 TEMPO-catalyzed oxidation of alcohols using hypochlorite as the oxidant.

148

5 Modern Oxidation of Alcohols using Environmentally Benign Oxidants

MCM-41, [15] or by entrapping TEMPO in sol-gel [16]. Alternatively, in our group we

developed an oligomeric TEMPO (Figure 5.3) derived from Chimassorb 944 [17].

This new polymer-immobilized TEMPO, which we refer to as PIPO (polymer-

immobilized piperidinyl oxyl), proved to be a very effective catalyst for the oxidation of

alcohols with hypochlorite (Figure 5.3) [17]. Under the standard conditions, PIPO is

dissolved in the dichloromethane layer. In contrast, in the absence of solvent, PIPO

was a very effective recyclable heterogeneous catalyst. Furthermore, the enhanced

activity of PIPO compared to TEMPO made the use of a bromide cocatalyst

redundant. Hence, the use of PIPO, in an amount equivalent to 1 mol% of nitroxyl

radical, provided an effective (heterogeneous) catalytic method for the oxidation of a

variety of alcohols with 1.25 equiv. of 0.35 M NaOCl (pH 9.1) in a bromide-free and

chlorinated hydrocarbon solvent-free medium (Figure 5.4). Under these environ-

mentally benign conditions, PIPO was superior to the already mentioned hetero-

geneous TEMPO systems and homogeneous TEMPO [18]. In the solvent-free

system, primary alcohols such as octan-1-ol gave low selectivities to the correspond-

ing aldehyde owing to overoxidation to the carboxylic acid. This problem was

circumvented by using n-hexane as the solvent, in which PIPO is insoluble, affording

an increase in aldehyde selectivity from 50 to 94%.

Various approaches have been used to replace the hypochlorite oxidant with the

greener molecular oxygen or air. For example, Neumann and coworkers showed that

a heteropolyacid, a known redox catalyst, was able to generate oxoammonium ions in-

situ with 2 atm of molecular oxygen at 100



C [19]. In another approach, a combi-

nation of manganese and cobalt (5 mol%) was able to generate oxoammonium ions

under acidic conditions at 40



C [20]. Results for these two methods are compared in

Table 5.1. Although these conditions are still susceptible to improvement, both

processes use molecular oxygen as the ultimate oxidant and are chlorine free, and

therefore constitute valuable examples of progress in this area. Later, we discuss

examples where the use of TEMPO in combination with a Ru or Cu catalyst

results in even more active catalytic systems. However, the mechanism in these

cases is metal-based instead of oxoammonium-based and is therefore listed in the

(CH

)

NH(

tert

-octyl)

PIPO (2.5 mg)

0°C, no solvent, 1.25 eq. aq. NaOCl

45 min, 99% conv.

Figure 5.3 PIPO as heterogeneous catalyst for alcohol oxidation.

5.2 Oxoammonium based Oxidation of Alcohols – TEMPO as Catalyst

149

100

1209060300

time

(

min

)

conv. (%)

Figure 5.4 Bleach oxidation of octan-2-ol

under chlorinated hydrocarbon solvent-

and bromide-free conditions using 1 mol%

of nitroxyl catalyst: (~) PIPO (3.19 mmol/g;

amine linker) [17]; (

) MCM-41

TEMPO (0.60 mmol/g; ether linker)

[15]; (

l ) SiO

TEMPO (0.87 mmol/g,

amine linker) [13]; (&) SiO

TEMPO

(0.40 mmol/g, amide linker) [14];

(^) TEMPO.

Table 5.1 Aerobic oxoammonium-based oxidation of alcohols.

Aldehyde or ketone yield

Substrate 2 mol% Mn(NO

)

2 mol% Co(NO

)

1 mol%

[PMo

]

Laccase (5.9 U mL

1

);

Water pH 4.5;

15 mol% TEMPO;



C, 1 atm air

10 mol% TEMPO

acetic acid, 40



3 mol% TEMPO

acetone, 100



1 atm O

2 atm O

n-C

OH 97% (6 h)

n-C

OH 98% (18 h)

n-C

CH(CH

)OH 100% (5 h)

n-C

CH(CH

)OH 96% (18 h)

PhCH

OH 98% (10 h)

100% (6 h) 87% (7 h)

PhCH(CH

)OH 98% (6 h)

67% (5.5 h)

cis-C

CH¼

CHCH

100% (10 h)

PhCH¼CHCH

OH 99% (3 h) 94% (24 h)

a) GLC Yields.

b) Cecchetto et al. [20].

c) Daniel et al. [19].

d) Arends et al. [22].

e) Reaction performed at 20



C with air.

150

5 Modern Oxidation of Alcohols using Environmentally Benign Oxidants

appropriate section. Another approach to the generation of oxoammonium ions

in situ is an enzymatic one. Laccase, an abundant highly potent redox enzyme, is

capable of oxidizing TEMPO to the oxoammonium ion [21]. Although this method

requires large amounts of TEMPO (up to 30 mol%) and long reaction times (24 h), it

demonstrates that a combination of laccase and TEMPO is able to catalyze the aerobic

oxidation of alcohols. In a more recent study [22] this method was optimized, and

10 mol% TEMPO was sufﬁcient to give good conversion and excellent selectivity (see

Table 5.1). The combinations of TEMPO with NaNO

and 1,3-dibromo-5,5-dimethyl-

hydantoin [23] or Fe(III) salts [24] as cocatalysts have also been shown to catalyze the

aerobic oxidation of alcohols in water at 80



5.3

Metal-Mediated Oxidation of Alcohols – Mechanism

Noble metal salts, for example, of Pd(II) or Pt(II), undergo reduction by primary and

secondary alcohols in homogeneous solution. Indeed, the ability of alcohols to reduce

Pd(II) was described as early as 1828 by Berzelius, who showed that K

PdCl

was

reduced to palladium metal in an aqueous ethanolic solution [25]. The reaction

involves a b-hydride elimination from an alkoxymetal intermediate and is a com-

monly used method for the preparation of noble metal hydrides (Eq. (5.1)). In the

presence of dioxygen this leads to catalytic oxidative dehydrogenation of the alcohol,

for example, with palladium salts [26–30].

ð5:1Þ

The aerobic oxidation of alcohols catalyzed by low-valent late transition metal ions,

particularly those of Group VIII elements, involves an oxidative dehydrogenation

mechanism. In the catalytic cycle (see Figure 5.5) a hydridometal species, formed by

b-hydride elimination from an alkoxymetal intermediate, is reoxidized by dioxygen,

presumably via insertion of O

into the M–H bond with formation of H

Alternatively, an alkoxymetal species can decompose to a proton and the reduced

form of the catalyst (see Figure 5.5), either directly or via the intermediacy of a

hydridometal intermediate. These reactions are promoted by bases as cocatalysts,

which presumably facilitate the formation of an alkoxymetal intermediate and/or

b-hydride elimination. Examples of metal ions that operate via this pathway are Pd

(II), Ru(II) and Rh(III).

Metal-catalyzed oxidations of alcohols with peroxide reagents can be conveniently

divided into two categories involving peroxometal and oxometal species, respectively,

as the active oxidant (Figure 5.6). In the peroxometal pathway the metal ion remains

in the same oxidation state throughout the catalytic cycle, and no stoichiometric

oxidation is observed in the absence of the peroxide. In contrast, oxometal pathways

involve a two-electron change in the oxidation state of the metal ion, and a

stoichiometric oxidation is observed, with the oxidized form of the catalyst, in the

5.3 Metal-Mediated Oxidation of Alcohols – Mechanism

151

absence of, for example, H

. Indeed, this is a test for distinguishing between the

two pathways.

Peroxometal pathways are typically observed with early transition metal ions with a

conﬁguration, for example, Mo(VI), W(VI), Ti(IV), and Re(VII), which are

relatively weak oxidants. Oxometal pathways are characteristic of late transition

elements and ﬁrst row transition elements, for example, Cr(VI), Mn(V), Os(VIII),

Ru(VI), and Ru(VIII), which are strong oxidants in high oxidation states. Some

metals can operate via both pathways depending, among other things, on the

substrate; for example, vanadium(V) operates via a peroxometal pathway in

alkene epoxidations, but an oxometal pathway is involved in alcohol oxidations [31].

In some cases, notably ruthenium, the aerobic oxidation of alcohols is catalyzed by

both low- and high-valent forms of the metal (see later). In the former case the

reaction involves (see Figure 5.5) the formation of a hydridometal species (or its

equivalent), while the latter involves an oxometal intermediate (see Figure 5.6), which

is regenerated by reaction of the reduced form of the catalyst with dioxygen instead of

OOHM

+ H

Figure 5.5 Hydridometal pathways for alcohol oxidation.

Peroxometal

pathway

Oxometal

ROM

HOM

(n-2)+

ROH

ROOH

OOR

ROOH

+ ROH

Figure 5.6 Oxometal versus peroxometal pathways in metal-catalyzed alcohol oxidations.

152

5 Modern Oxidation of Alcohols using Environmentally Benign Oxidants