Modern oxidation methods - Backvall.pdf - Redox - ziolek6661

Recent Developments in the Osmium-catalyzed Dihydroxylation

of Olefins

Uta Sundermeier, Christian Döbler, and Matthias Beller

1.1

Introduction

The oxidative functionalization of olefins is of major importance for both organic

synthesis and the industrial production of bulk and fine chemicals [1]. Among the

different oxidation products of olefins, 1,2-diols are used in a wide variety of applica-

tions. Ethylene- and propylene-glycol are produced on a multi-million ton scale per

annum, due to their importance as polyester monomers and anti-freeze agents [2].

A number of 1,2-diols such as 2,3-dimethyl-2,3-butanediol, 1,2-octanediol, 1,2-hexa-

nediol, 1,2-pentanediol, 1,2- and 2,3-butanediol are of interest in the fine chemical

industry. In addition, chiral 1,2-diols are employed as intermediates for pharmaceuti-

cals and agrochemicals. At present 1,2-diols are manufactured industrially by a two

step sequence consisting of epoxidation of an olefin with a hydroperoxide or a pera-

cid followed by hydrolysis of the resulting epoxide [3]. Compared with this process

the dihydroxylation of C=C double bonds constitutes a more atom-efficient and

shorter route to 1,2-diols. In general the dihydroxylation of olefins is catalyzed by os-

mium, ruthenium or manganese oxo species. The osmium-catalyzed variant is the

most reliable and efficient method for the synthesis of cis -1,2-diols [4]. Using os-

mium in catalytic amounts together with a secondary oxidant in stoichiometric

amounts various olefins, including mono-, di-, and trisubstituted unfunctionalized,

as well as many functionalized olefins, can be converted into the corresponding

diols. OsO 4 as an electrophilic reagent reacts only slowly with electron-deficient ole-

fins, and therefore higher amounts of catalyst and ligand are necessary in these

cases. Recent studies have revealed that these substrates react much more efficiently

when the pH of the reaction medium is maintained on the acidic side [5]. Here, citric

acid appears to be superior for maintaining the pH in the desired range. On the

other hand, in another study it was found that providing a constant pH value of 12.0

leads to improved reaction rates for internal olefins [6].

Since its discovery by Sharpless and coworkers, catalytic asymmetric dihydroxyla-

tion (AD) has significantly enhanced the utility of osmium-catalyzed dihydroxylation

(Scheme 1.1) [7]. Numerous applications in organic synthesis have appeared in re-

cent years [8].

2004 WILEY-VCH Verlag GmbH & Co. KGaA,Weinheim

ISBN: 3-527-30642-0

Modern Oxidation Methods. Edited by Jan-Erling Bäckvall

2 1 Recent Developments in the Osmium-catalyzed Dihydroxylation of Olefins

Scheme 1.1 Osmylation of olefins

While the problem of enantioselectivity has largely been solved through extensive

synthesis and screening of cinchona alkaloid ligands by the Sharpless group, some

features of this general method remain problematic for larger scale applications.

Firstly, the use of the expensive osmium catalyst must be minimized and an efficient

recycling of the metal should be developed. Secondly, the applied reoxidants for Os VI

species are expensive and lead to overstoichiometric amounts of waste.

In the past several reoxidation processes for osmium(VI) glycolates or other os-

mium(VI) species have been developed. Historically, chlorates [9] and hydrogen per-

oxide [10] were first applied as stoichiometric oxidants, however in both cases the dihy-

droxylation often proceeds with low chemoselectivity. Other reoxidants for os-

mium(VI) are tert -butyl hydroperoxide in the presence of Et 4 NOH [11] and a range of

N -oxides, such as N -methylmorpholine N -oxide (NMO) [12] (the Upjohn process) and

trimethylamine N -oxide. K 3 [Fe(CN) 6 ] gave a substantial improvement in the enantios-

electivities in asymmetric dihydroxylations when it was introduced as a reoxidant for

osmium(VI) species in 1990 [13]. However, even as early on as 1975 it was already

being described as an oxidant for Os-catalyzed oxidation reactions [14]. Today the “AD-

mix”, containing the catalyst precursor K 2 [OsO 2 (OH) 4 ], the co-oxidant K 3 [Fe(CN) 6 ],

the base K 2 CO 3 , and the chiral ligand, is commercially available and the dihydroxyla-

tion reaction is easy to carry out. However, the production of overstoichiometric

amounts of waste remains as a significant disadvantage of the reaction protocol.

This chapter will summarize the recent developments in the area of osmium-cata-

lyzed dihydroxylations, which bring this transformation closer to a “green reaction”.

Hence, special emphasis is given to the use of new reoxidants and recycling of the

osmium catalyst.

1.2

Environmentally Friendly Terminal Oxidants

1.2.1

Hydrogen Peroxide

Ever since the Upjohn procedure was published in 1976 the N -methylmorpholine

N -oxide-based procedure has become one of the standard methods for osmium-cata-

lyzed dihydroxylations. However, in the asymmetric dihydroxylation NMO has not

1.2 Environmentally Friendly Terminal Oxidants

been fully appreciated since it was difficult to obtain high ee with this oxidant. Some

years ago it was demonstrated that NMO could be employed as the oxidant in the AD

reaction to give high ee in aqueous tert- BuOHwith slow addition of the olefin [15].

In spite of the fact that hydrogen peroxide was one of the first stoichiometric oxi-

dants to be introduced for the osmium-catalyzed dihydroxylation it was not actually

used until recently. When using hydrogen peroxide as the reoxidant for transition

metal catalysts, very often there is the big disadvantage that a large excess of H 2 O 2

is required, implying that the unproductive peroxide decomposition is the major

process.

Recently Bäckvall and coworkers were able to improve the H 2 O 2 reoxidation pro-

cess significantly by using N -methylmorpholine together with flavin as co-catalysts

in the presence of hydrogen peroxide [16]. Thus a renaissance of both NMO and

H 2 O 2 was induced. The mechanism of the triple catalytic H 2 O 2 oxidation is shown

in Scheme 1.2.

Scheme 1.2 Osmium-catalyzed dihydroxylation of olefins using

H 2 O 2 as the terminal oxidant

The flavin hydroperoxide generated from flavin and H 2 O 2 recycles the N -methyl-

morpholine (NMM) to N -methylmorpholine N -oxide (NMO), which in turn reoxi-

dizes the Os VI to Os VIII . While the use of hydrogen peroxide as the oxidant without

the electron-transfer mediators (NMM, flavin) is inefficient and nonselective, various

olefins were oxidized to diols in good to excellent yields employing this mild triple

catalytic system (Scheme 1.3).

Scheme 1.3 Osmium-catalyzed dihydroxylation of -methylstyrene

using H 2 O 2

By using a chiral Sharpless ligand high enantioselectivities were obtained. Here,

an increase in the addition time for olefin and H 2 O 2 can have a positive effect on the

enantioselectivity.

4 1 Recent Developments in the Osmium-catalyzed Dihydroxylation of Olefins

Bäckvall and coworkers have shown that other tertiary amines can assume the role

of the N -methylmorpholine. They reported on the first example of an enantioselec-

tive catalytic redox process where the chiral ligand has two different modes of opera-

tion: (1) to provide stereocontrol in the addition of the substrate, and (2) to be respon-

sible for the reoxidation of the metal through an oxidized form [17]. The results ob-

tained with hydroquinidine 1,4-phthalazinediyl diether (DHQD) 2 PHAL both as an

electron-transfer mediator and chiral ligand in the osmium-catalyzed dihydroxylation

are comparable to those obtained employing NMM together with (DHQD) 2 PHAL.

The proposed catalytic cycle for the reaction is depicted in Scheme 1.4.

The flavin is an efficient electron-transfer mediator, but rather unstable. Several

transition metal complexes, for instance vanadyl acetylacetonate, can also activate hy-

drogen peroxide and are capable of replacing the flavin in the dihydroxylation reac-

tion [18].

More recently Bäckvall and coworkers developed a novel and robust system for os-

mium-catalyzed asymmetric dihydroxylation of olefins by H 2 O 2 with methyltrioxo-

rhenium (MTO) as the electron transfer mediator [19]. Interestingly, here MTO cata-

lyzes oxidation of the chiral ligand to its mono- N -oxide, which in turn reoxidizes

Os VI to Os VIII . This system gives vicinal diols in good yields and high enantiomeric

excess up to 99%.

Scheme 1.4 Catalytic cycle for the enantioselective dihydroxylation

of olefins using (DHQD) 2 PHAL for oxygen transfer and as a source

of chirality

1.2 Environmentally Friendly Terminal Oxidants

1.2.2

Hypochlorite

Apart from oxygen and hydrogen peroxide, bleach is the simplest and cheapest oxi-

dant that can be used in industry without problems. In the past this oxidant has only

been applied in the presence of osmium complexes in two patents in the early 1970s

for the oxidation of fatty acids [20]. In 2003 the first general dihydroxylation proce-

dure of various olefins in the presence of sodium hypochlorite as the reoxidant was

described by us [21]. Using -methylstyrene as a model compound, 100% conversion

and 98% yield of the desired 1,2-diol were obtained (Scheme 1.5).

Scheme 1.5 Osmium-catalyzed dihydroxylation of -methylstyrene

using sodium hypochlorite

-methylstyrene. Using tert -butylmethylether as the organic

co-solvent instead of tert -butanol, 99% yield and 89% ee with only 1 mol%

(DHQD) 2 PHAL are reported for the same substrate. This increase in enantioselectiv-

ity can be explained by an increase in the concentration of the chiral ligand in the or-

ganic phase. Increasing the polarity of the water phase by using a 10% aqueous

NaCl solution showed a similar positive effect. Table 1.1 shows the results of the

asymmetric dihydroxylation of various olefins with NaOCl as the terminal oxidant.

Despite the slow hydrolysis of the corresponding sterically hindered Os VI glyco-

late, trans -5-decene reacted fast without any problems. This result is especially inter-

esting since it is necessary to add stoichiometric amounts of hydrolysis aids to the di-

hydroxylation of most internal olefins in the presence of other oxidants.

With this protocol a very fast, easy to perform, and cheap procedure for the asym-

metric dihydroxylation is presented.

Interestingly, the yield of 2-phenyl-1,2-propanediol after 1 h was significantly

higher using hypochlorite compared with literature protocols using NMO (90%) [22]

or K 3 [Fe(CN) 6 ] (90%) at this temperature. The turnover frequency was 242 h –1 ,

which is a reasonable level [23]. Under the conditions shown in Scheme 1.5 an enan-

tioselectivity of only 77% ee is obtained, while 94% ee is reported using K 3 [Fe(CN) 6 ]

as the reoxidant. The lower enantioselectivity can be explained by some involvement

of the so-called second catalytic cycle with the intermediate Os VI glycolate being oxi-

dized to an Os VIII species prior to hydrolysis (Scheme 1.6) [24].

Nevertheless, the enantioselectivity was improved by applying a higher ligand con-

centration. In the presence of 5 mol% (DHQD) 2 PHAL a good enantioselectivity of

91% ee is observed for

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