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1Recent Developments in the Osmium-catalyzed Dihydroxylationof OlefinsUta Sundermeier, Christian Döbler, and Matthias Beller

1.1Introduction

The oxidative functionalization of olefins is of major importance for both organicsynthesis and the industrial production of bulk and fine chemicals [1]. Among thedifferent 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 perannum, 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 chemicalindustry. 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 twostep 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 processthe dihydroxylation of C=C double bonds constitutes a more atom-efficient andshorter 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 themost 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 stoichiometricamounts various olefins, including mono-, di-, and trisubstituted unfunctionalized,as well as many functionalized olefins, can be converted into the correspondingdiols. OsO4 as an electrophilic reagent reacts only slowly with electron-deficient ole-fins, and therefore higher amounts of catalyst and ligand are necessary in thesecases. Recent studies have revealed that these substrates react much more efficientlywhen the pH of the reaction medium is maintained on the acidic side [5]. Here, citricacid appears to be superior for maintaining the pH in the desired range. On theother hand, in another study it was found that providing a constant pH value of 12.0leads 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].

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Modern Oxidation Methods. Edited by Jan-Erling BäckvallCopyright � 2004 WILEY-VCH Verlag GmbH & Co. KGaA,WeinheimISBN: 3-527-30642-0

While the problem of enantioselectivity has largely been solved through extensivesynthesis and screening of cinchona alkaloid ligands by the Sharpless group, somefeatures of this general method remain problematic for larger scale applications.Firstly, the use of the expensive osmium catalyst must be minimized and an efficientrecycling of the metal should be developed. Secondly, the applied reoxidants for OsVI

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 Et4NOH [11] and a range ofN-oxides, such as N-methylmorpholine N-oxide (NMO) [12] (the Upjohn process) andtrimethylamine N-oxide. K3[Fe(CN)6] gave a substantial improvement in the enantios-electivities in asymmetric dihydroxylations when it was introduced as a reoxidant forosmium(VI) species in 1990 [13]. However, even as early on as 1975 it was alreadybeing described as an oxidant for Os-catalyzed oxidation reactions [14]. Today the “AD-mix”, containing the catalyst precursor K2[OsO2(OH)4], the co-oxidant K3[Fe(CN)6],the base K2CO3, and the chiral ligand, is commercially available and the dihydroxyla-tion reaction is easy to carry out. However, the production of overstoichiometricamounts 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 theosmium catalyst.

1.2Environmentally Friendly Terminal Oxidants

1.2.1Hydrogen Peroxide

Ever since the Upjohn procedure was published in 1976 the N-methylmorpholineN-oxide-based procedure has become one of the standard methods for osmium-cata-lyzed dihydroxylations. However, in the asymmetric dihydroxylation NMO has not

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

Scheme 1.1 Osmylation of olefins

been fully appreciated since it was difficult to obtain high ee with this oxidant. Someyears ago it was demonstrated that NMO could be employed as the oxidant in the ADreaction to give high ee in aqueous tert-BuOH with 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 actuallyused until recently. When using hydrogen peroxide as the reoxidant for transitionmetal catalysts, very often there is the big disadvantage that a large excess of H2O2

is required, implying that the unproductive peroxide decomposition is the majorprocess.

Recently Bäckvall and coworkers were able to improve the H2O2 reoxidation pro-cess significantly by using N-methylmorpholine together with flavin as co-catalystsin the presence of hydrogen peroxide [16]. Thus a renaissance of both NMO andH2O2 was induced. The mechanism of the triple catalytic H2O2 oxidation is shownin Scheme 1.2.

The flavin hydroperoxide generated from flavin and H2O2 recycles the N-methyl-morpholine (NMM) to N-methylmorpholine N-oxide (NMO), which in turn reoxi-dizes the OsVI to OsVIII. While the use of hydrogen peroxide as the oxidant withoutthe electron-transfer mediators (NMM, flavin) is inefficient and nonselective, variousolefins were oxidized to diols in good to excellent yields employing this mild triplecatalytic system (Scheme 1.3).

By using a chiral Sharpless ligand high enantioselectivities were obtained. Here,an increase in the addition time for olefin and H2O2 can have a positive effect on theenantioselectivity.

31.2 Environmentally Friendly Terminal Oxidants

Scheme 1.2 Osmium-catalyzed dihydroxylation of olefins usingH2O2 as the terminal oxidant

Scheme 1.3 Osmium-catalyzed dihydroxylation of �-methylstyreneusing H2O2

Bäckvall and coworkers have shown that other tertiary amines can assume the roleof 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)2PHAL both as anelectron-transfer mediator and chiral ligand in the osmium-catalyzed dihydroxylationare comparable to those obtained employing NMM together with (DHQD)2PHAL.The proposed catalytic cycle for the reaction is depicted in Scheme 1.4.

The flavin is an efficient electron-transfer mediator, but rather unstable. Severaltransition 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 H2O2 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 reoxidizesOsVI to OsVIII. This system gives vicinal diols in good yields and high enantiomericexcess up to 99%.


Scheme 1.4 Catalytic cycle for the enantioselective dihydroxylationof olefins using (DHQD)2PHAL for oxygen transfer and as a sourceof chirality

1.2.2Hypochlorite

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 onlybeen applied in the presence of osmium complexes in two patents in the early 1970sfor 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 wasdescribed by us [21]. Using �-methylstyrene as a model compound, 100% conversionand 98% yield of the desired 1,2-diol were obtained (Scheme 1.5).

Interestingly, the yield of 2-phenyl-1,2-propanediol after 1 h was significantlyhigher using hypochlorite compared with literature protocols using NMO (90 %) [22]or K3[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 K3[Fe(CN)6]as the reoxidant. The lower enantioselectivity can be explained by some involvementof the so-called second catalytic cycle with the intermediate OsVI glycolate being oxi-dized to an OsVIII 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)2PHAL a good enantioselectivity of91% ee is observed for �-methylstyrene. Using tert-butylmethylether as the organicco-solvent instead of tert-butanol, 99% yield and 89% ee with only 1 mol%(DHQD)2PHAL 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% aqueousNaCl solution showed a similar positive effect. Table 1.1 shows the results of theasymmetric dihydroxylation of various olefins with NaOCl as the terminal oxidant.

Despite the slow hydrolysis of the corresponding sterically hindered OsVI 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.


Scheme 1.5 Osmium-catalyzed dihydroxylation of �-methylstyreneusing sodium hypochlorite


Scheme 1.6 The two catalytic cycles in the asymmetric dihydroxylation

Tab. 1.1 Asymmetric dihydroxylation of different olefins using NaOCl as terminal oxidant a

Entry Olefin Time Yield Selectivity ee ee (%)(h) (%) (%) (%) Ref.

1 1 88 88 95 99

2 2 93 99 95 97

3 1 99 99 91 95

4 1 92 94 93 97

5 1 84 84 91 97

6 2 88 94 73 88

a Reaction conditions: 2 mmol olefin, 0.4 mol% K2[OsO2(OH)4], 5 mol% (DHQD)2PHAL, 10 mL H2O,10 mL tBuOH, 1.5 equiv. NaOCl, 2 equiv. K2CO3, 0 �C.

1.2.3Oxygen or Air

In the past it has been demonstrated by several groups that in the presence of OsO4

and oxygen mainly non-selective oxidation reactions take place [25]. However, in1999 Krief et al. published a reaction system consisting of oxygen, catalytic amountsof OsO4 and selenides for the asymmetric dihydroxylation of �-methylstyrene underirradiation with visible light in the presence of a sensitizer (Scheme 1.7) [26]. Here,the selenides are oxidized to their oxides by singlet oxygen and the selene oxides areable to re-oxidize osmium(VI) to osmium(VIII). The reaction works with similaryields and ee values to those of the Sharpless-AD. Potassium carbonate is also used,but only one tenth of the amount present in the AD-mix. Air can be used instead ofpure oxygen.

The reaction was extended to a wide range of aromatic and aliphatic olefins [27]. Itwas shown that both yield and enantioselectivity are influenced by the pH of the re-action medium. The procedure was also applied to practical syntheses of natural pro-duct derivatives [28]. This version of the AD reaction not only uses a more ecologicalco-oxidant, it also requires much less matter: 87 mg of matter (catalyst, ligand, base,


Tab. 1.1 (continued)

Entry Olefin Time Yield Selectivity ee ee (%)(h) (%) (%) (%) Ref.

7 2 87 93 80 b

8 2 97 97 73

9 2 94 96 34 b

10 2 97 >97 80 b 92

b 5 mol% (DHQD)2PYR instead of (DHQD)2PHAL.

Scheme 1.7 Osmium-catalyzed dihydroxylation using 1O2 andbenzyl phenyl selenide

reoxidant) are required to oxidize 1 mmol of the same olefin instead of 1400 mgwhen the AD-mix is used.

Also in 1999 there was the first publication on the use of molecular oxygen with-out any additive to reoxidize osmium(VI) to osmium(VIII). We reported that the os-mium-catalyzed dihydroxylation of aliphatic and aromatic olefins proceeds efficientlyin the presence of dioxygen under ambient conditions [29]. As shown in Table 1.2the new dihydroxylation procedure constitutes a significant advancement comparedwith other reoxidation procedures. Here, the dihydroxylation of �-methylstyrene iscompared using different stoichiometric oxidants. The yield of the 1,2-diol remainsgood to very good (87–96%), independent of the oxidant used. The best enantioselec-tivities (94–96% ee) are obtained with hydroquinidine 1,4-phthalazinediyl diether[(DHQD)2PHAL] as the ligand at 0–12 �C (Table 1.2, entries 1 and 3).

The dihydroxylation process with oxygen is clearly the most ecologically favorableprocedure (Table 1.2, entry 5), when the production of waste from a stoichiometricreoxidant is considered. With the use of K3[Fe(CN)6] as oxidant approximately 8.1 kgof iron salts per kg of product are formed. However, in the case of the Krief (Ta-ble 1.2, entry 3) and Bäckvall procedures (Table 1.2, entry 4) as well as in the pre-sence of NaOCl (Table 1.2, entry 6) some byproducts also arise due to the use of co-catalysts and co-oxidants. It should be noted that only salts and byproducts formed


Tab. 1.2 Comparison of the dihydroxylation of �-methylstyrene in the presence of different oxidants

Entry Oxidant Yield Reaction conditions ee TON Waste (oxidant) Ref.(%) (%) (kg/kg diol)

1 K3[Fe(CN)6] 90 0 �C 94 a 450 8.1 c [7b]K2[OsO2(OH)4]tBuOH/H2O

2 NMO 90 0 �C 33b 225 0.88d [22]OsO4

acetone/H2O

3 PhSeCH2Ph/O2 89 12 �C 96a 222 0.16e [26a]PhSeCH2Ph/air 87 K2[OsO2(OH)4] 93a 48 0.16e [26a]

tBuOH/H2O

4 NMM/flavin/H2O2 93 RT – 46 0.33f [16a]OsO4

acetone/H2O

5 O2 96 50 �C 80a 192 – [29]K2[OsO2(OH)4]tBuOH/aq. buffer

6 NaOCl 99 0 �C 91a 247 0.58g [21]K2[OsO2(OH)4]tBuOH/H2O

a Ligand: Hydroquinidine 1,4-phthalazinediyl diether. b Hydroquinidine p-chlorobenzoate.c K4[Fe(CN)6]. d N-Methylmorpholine (NMM). e PhSe(O)CH2Ph. f NMO/flavin-OOH. g NaCl.

from the oxidant have been included in the calculation. Other waste products havenot been considered. Nevertheless the calculations presented in Table 1.2 give arough estimation of the environmental impact of the reaction.

Since the use of pure molecular oxygen on a larger scale might lead to safety pro-blems it is even more advantageous to use air as the oxidizing agent. Hence, all cur-rent bulk oxidation processes, e. g., the oxidation of BTX (benzene, toluene, xylene)aromatics or alkanes to give carboxylic acids, and the conversion of ethylene intoethylene oxide, use air and not pure oxygen as the oxidant [30]. In Table 1.3 the re-sults of the dihydroxylation of �-methylstyrene as a model compound using air asthe stoichiometric oxidant are shown in contrast to that with pure oxygen (Scheme1.8; Table 1.3) [31].

The dihydroxylation of �-methylstyrene in the presence of 1 bar of pure oxygen pro-ceeds smoothly (Table 1.3, entries 1–2), with the best results being obtained atpH 10.4. In the presence of 0.5 mol% K2[OsO2(OH)4]/1.5 mol% DABCO or 1.5 mol%(DHQD)2PHAL at pH 10.4 and 50 �C total conversion was achieved after 16 h or 20 hdepending on the ligand. While the total yield and selectivity of the reaction are excel-lent (97% and 96%, respectively), the total turnover frequency of the catalyst is com-paratively low (TOF = 10–12 h–1). In the presence of the chiral cinchona ligand


Tab. 1.3 Dihydroxylation of �-methylstyrene with air a

Entry Pressure Cat. Ligand L/Os [L] Time Yield Selectivity ee(bar)c (mol%) (mmol L–1) (h) (%) (%) (%)

1 1 (pure O2) 0.5 DABCOd 3 :1 3.0 16 97 97 –2 1 (pure O2) 0.5 (DHQD)2PHALe 3 : 1 3.0 20 96 96 803 1 0.5 DABCO 3.1 3.0 24 24 85 –4 1 0.5 DABCO 3.1 3.0 68 58 83 –5 5 0.1 DABCO 3 : 1 0.6 24 41 93 –6 9 0.1 DABCO 3 : 1 0.6 24 76 92 –7 20 0.5 (DHQD)2PHAL 3 : 1 3.0 17 96 96 828 20 0.1 (DHQD)2PHAL 3 : 1 0.6 24 95 95 629 20 0.1 (DHQD)2PHAL 15 : 1 3.0 24 95 95 83

10b 20 0.1 (DHQD)2PHAL 3 : 1 1.5 24 94 94 6711b 20 0.1 (DHQD)2PHAL 6 : 1 3.0 24 94 94 7812b 20 0.1 (DHQD)2PHAL 15 : 1 7.5 24 60 95 82

a Reaction conditions: K2[OsO2(OH)4], 50 �C, 2 mmol olefin, 25 mL buffer solution (pH 10.4), 10 mL tBuOH. b 10 mmololefin, 50 mL buffer solution (pH 10.4), 20 mL tBuOH. c The autoclave was purged with air and then pressurized to thegiven value. d 1,4-Diazabicyclo[2.2.2.]octane. e Hydroquinidine 1,4-phthalazinediyl diether.

Scheme 1.8 Osmium-catalyzed dihydroxylation of �-methylstyrene

(DHQD)2PHAL an ee of 80% is observed. Sharpless et al. reported an enantioselec-tivity of 94% for the dihydroxylation of �-methylstyrene with (DHQD)2PHAL as theligand using K3[Fe(CN)6] as the reoxidant at 0 �C [32]. Studies of the ceiling ee at 50 �C(88% ee) show that the main difference in the enantioselectivity stems from thehigher reaction temperature. Using air instead of pure oxygen gas gave only 24% ofthe corresponding diol after 24 h (TOF = 1 h–1; Table 1.3, entry 3). Although the reac-tion is slow, it is important to note that the catalyst stays active, as shown by the factthat 58% of the product is obtained after 68 h (Table 1.3, entry 4). Interestingly thechemoselectivity of the dihydroxylation does not significantly decrease after a pro-longed reaction time. At 5–20 bar air pressure the turnover frequency of the catalystis improved (Table 1.3, entries 5–11).

Full conversion of a �-methylstyrene is achieved at an air pressure of 20 bar in thepresence of 0.1 mol% of osmium, which corresponds to a turnover frequency of40 h–1 (Table 1.3, entries 8–11). Thus, by increasing the air pressure to 20 bar, itwas possible to reduce the amount of osmium catalyst by a factor of 5. A decrease ofthe osmium catalyst and the ligand leads to a decrease in the enantioselectivity of from82% to 62% ee. This is easily explained by the fact that the ligand concentration deter-mines the stereoselectivity of the dihydroxylation reaction (Table 1.3, entries 7 and 9).

While the reaction at higher substrate concentration (10 mmol instead of 2 mmol)proceeds only sluggishly at 1 bar even with pure oxygen, full conversion is achievedafter 24 h at 20 bar of air (Table 1.3, entries 10 and 11, and Table 1.4, entries 17 and18). In all experiments performed under air pressure the chemoselectivity of the di-hydroxylation remained excellent (92–96%).

Table 1.4 shows the results of the osmium-catalyzed dihydroxylation of various ole-fins with air.

As depicted in Table 1.4 all olefins gave the corresponding diols in moderate togood yields (48–89%). Applying standard reaction conditions, the best yields of diolswere obtained with 1-octene (97%), 1-phenyl-1-cyclohexene (88%), trans-5-decene(85%), allyl phenyl ether (77%) and styrene (76%). The enantioselectivities variedfrom 53 to 98% ee depending on the substrate. It is important to note that the chemo-selectivity of the reaction decreases under standard conditions in the following sub-strate order: �-methylstyrene = 1-octene > 1-phenyl-1-cyclohexene > trans-5-decene >n-C6F13CH=CH2 > allyl phenyl ether > styrene >> trans-stilbene. A correlation be-tween the chemoselectivity of the reaction and the sensitivity of the produced diol to-wards further oxidation is evident,with the main side reaction being the oxidative clea-vage of the C=C double bond. Aromatic diols with benzylic hydrogen atoms are espe-cially sensitive to this oxidation reaction. Thus, the dihydroxylation of trans-stilbenegave no hydrobenzoin in the biphasic mixture water/tert-butanol at pH 10.4, 50 �Cand 20 bar air pressure (Table 1.4, entry 9). Instead of dihydroxylation a highly selec-tive cleavage of stilbene to give benzaldehyde (84–87% yield) was observed. Interest-ingly, changing the solvent to isobutyl methyl ketone (Table 1.4, entry 12) makes itpossible to obtain hydrobenzoin in high yield (89%) and enantioselectivity (98%) atpH 10.4.

The mechanism of the dihydroxylation reaction with oxygen or air is presumed tobe similar to the catalytic cycle presented by Sharpless et al. for the osmium-cata-


lyzed dihydroxylation with K3[Fe(CN)6] as the reoxidant (Scheme 1.9). The additionof the olefin to a ligated OsVIII species proceeds mainly in the organic phase. De-pending on the hydrolytic stability of the resulting OsVI glycolate complex, the ratedetermining step of the reaction is either hydrolysis of the OsVI glycolate or the reoxi-dation of OsVI hydroxy species. There must be a minor involvement of a second cata-lytic cycle, as suggested for the dihydroxylation with NMO. Such a second cyclewould lead to significantly lower enantioselectivities, as the attack of a second olefinmolecule on the OsVIII glycolate would occur in the absence of the chiral ligand. Theobserved enantioselectivities for the dihydroxylation with air are only slightly lowerthan the data previously published by the Sharpless group, despite the higher reac-tion temperature (50 �C vs. 0 �C). Therefore the direct oxidation of the OsVI glycolateto an OsVIII glycolate does not represent a major reaction pathway.


Tab. 1.4 Dihydroxylation of various olefins with aira

Entry Olefin Cat. Ligand L/Os [L] Time Yield Selectivity ee(mol%) (mmol L–1) (h) (%)b (%)b (%)

1 0.5 (DHQD)2PHAL 3 : 1 3.0 24 42 42 872 0.5 (DHQD)2PHAL 3 : 1 3.0 16 66 66 863 0.5 (DHQD)2PHAL 3 : 1 3.0 14 76 76 87

4 0.5 (DHQD)2PHAL 3 : 1 3.0 24 88 88 89

5 0.5 (DHQD)2PHAL 3 : 1 3.0 24 63 63 676 0.5 (DHQD)2PHAL 3 : 1 3.0 18 68 68 687 0.5 (DHQD)2PHAL 3 : 1 3.0 14 67 67 668 0.5 (DHQD)2PHAL 3 : 1 3.0 9 77 77 68

9 0.5 – – – 24 0 (84) 0 (84) –10c 1.0 DABCO 3 : 1 1.5 24 4 (77) 5 (87) –11c, d 1.0 (DHQD)2PHAL 3.1 1.5 24 40 (35) 48 (42) 8612c, e 1.0 (DHQD)2PHAL 3 : 1 1.5 24 89 (7) 89 (7) 98

13d 1.0 (DHQD)2PHAL 3 : 1 6.0 24 85 85 82

14 0.5 (DHQD)2PHAL 3 : 1 3.0 18 96 96 6315 0.1 (DHQD)2PHAL 3 : 1 0.6 24 95 95 4416 0.1 (DHQD)2PHAL 15 : 1 3.0 24 97 97 6217 f 0.1 (DHQD)2PHAL 3 : 1 1.5 24 94 94 4718 f 0.1 (DHQD)2PHAL 6 : 1 3.0 24 95 95 62

19 2.0 (DHQD)2PYRg 3 : 1 12.0 24 55 – 68

a Reaction conditions: K2[OsO2(OH)4], 50 �C, 2 mmol olefin, 20 bar air, pH = 10.4, 25 mL buffer solution, 10 mL tBuOH;entries 9–12: 15 mL buffer solution, 20 mL tBuOH, entries 17–18: 50 mL buffer solution, 20 mL tBuOH. b Values in par-entheses are for benzaldehyde. c 1 mmol olefin. d pH = 12. e Isobutyl methyl ketone instead of tBuOH. f 10 mmol olefin.g Hydroquinidine 2,5-diphenyl-4,6-pyrimidinediyl diether.

1.3Supported Osmium Catalyst

Hazardous toxicity and high costs are the chief drawbacks to reactions using os-mium tetroxide. Besides the development of procedures where catalytic amounts ofosmium tetroxide are joined with a stoichiometrically used secondary oxidant con-tinuously regenerating the tetroxide, these disadvantages can be overcome by theuse of stable and nonvolatile adducts of osmium tetroxide with heterogeneous sup-ports [33]. They offer the advantages of easy and safe handling, simple separationfrom the reaction medium, and the possibility to reuse the expensive transition me-tal. Unfortunately, problems with the stability of the polymer support and leachingof the metal generally occur.

In this context Cainelli and coworkers had already reported, in 1989, the prepara-tion of polymer-supported catalysts: here, OsO4 was immobilized on several aminetype polymers [34]. Such catalysts have structures of the type OsO4�L with theN-group of the polymer (= L) being coordinated to the Lewis acidic osmium center.Based upon this concept, a catalytic enantioselective dihydroxylation was establishedby using polymers containing cinchona alkaloid derivatives [35]. However, since theamine ligands coordinate to osmium under equilibrium conditions, recovery of theosmium using polymer supported ligands was difficult. Os-diolate hydrolysis seemsto require detachment from the polymeric ligand, and hence causes leaching.


Scheme 1.9 Proposed catalytic cycle for the dihydroxylation of olefinswith OsO4 and oxygen as the terminal oxidant

Herrmann and coworkers reported on the preparation of immobilized OsO4 onpoly(4-vinyl pyridine) and its use in the dihydroxylation of alkenes by means of hy-drogen peroxide [36]. However, the problems of gradual polymer decomposition andosmium leaching were not solved.

A new strategy was published by Kobayashi and coworkers in 1998: they used mi-croencapsulated osmium tetroxide. Here the metal is immobilized onto a polymeron the basis of physical envelopment by the polymer and on electron interactionsbetween the �-electrons of the benzene rings of the polystyrene based polymer anda vacant orbital of the Lewis acid [37]. Using cyclohexene as a model compound itwas shown that this microencapsulated osmium tetroxide (MC OsO4) can be usedas a catalyst in the dihydroxylation, with NMO as the stoichiometric oxidant(Scheme 1.10).

In contrast to other typical OsO4-catalyzed dihydroxylations, where H2O-tBuOH isused as the solvent system, the best yields were obtained in H2O/acetone/CH3CN.While the reaction was successfully carried out using NMO, moderate yields wereobtained using trimethylamine N-oxide, and much lower yields were observed usinghydrogen peroxide or potassium ferricyanide. The catalyst was recovered quantita-tively by simple filtration and reused several times. The activity of the recovered cata-lyst did not decrease even after the fifth use.

A study of the rate of conversion of the starting material showed that the reactionproceeds faster using OsO4 than using the microencapsulated catalyst. This is as-cribed to the slower reoxidation of the microencapsulated osmium ester with NMO,compared with simple OsO4.

Subsequently acryronitrile/butadiene/polystyrene polymer was used as a supportbased on the same microencapsulation technique and several olefins, includingcyclic and acyclic, terminal, mono-, di-, tri-, and tetrasubstituted, gave the corre-sponding diols in high yields [38]. When (DHQD)2PHAL as a chiral source wasadded to the reaction mixture enantioselectivities up to 95% ee were obtained.However, this reaction requires slow addition of the olefin. After running a100 mmol experiment, more than 95% of the ABS-MC OsO4 and the chiral ligandwere recovered.

Recently Kobayashi and coworkers reported on a new type of microencapsulatedosmium tetroxide using phenoxyethoxymethyl-polystyrene as the support [39]. Withthis catalyst, asymmetric dihydroxylation of olefins has been successfully performedusing (DHQD)2PHAL as a chiral ligand and K3[Fe(CN)6] as a cooxidant in H2O/acet-one (Scheme 1.11).

131.3 Supported Osmium Catalyst

Scheme 1.10 Dihydroxylation of cyclohexene using microencapsulatedosmium tetroxide (MC OsO4)

In this instance the dihydroxylation does not require slow addition of the olefin,and the catalyst can be recovered quantitatively by simple filtration and reused with-out loss of activity.

Jacobs and coworkers published a completely different type of heterogeneous os-mium catalyst. Their approach is based on two details from the mechanism of the cis-dihydroxylation: (1) tetrasubstituted olefins are smoothly osmylated to an osmate(VI)

ester, but these esters are not hydrolyzed under mild conditions, and (2) an OsVI

monodiolate complex can be reoxidized to cis-dioxo OsVIII without release of the diol;subsequent addition of a second olefin results in an Os bisdiolate complex. Thesetwo properties make it possible to immobilize a catalytically active osmium com-pound by the addition of OsO4 to a tetrasubstituted olefin that is covalently linked toa silica support. The tetrasubstituted diolate ester which is formed at one side of theOs atom is stable, and keeps the catalyst fixed on the support material. The catalyticreaction can take place at the free coordination sites of Os (Scheme 1.12) [40].

The dihydroxylation of monosubstituted and disubstituted aliphatic olefins andcyclic olefins was successfully performed using this heterogeneous catalyst and


Scheme 1.11 Asymmetric dihydroxylation of olefins usingPEM-MC OsO4

Scheme 1.12 Immobilization of Os in a tertiary diolate complex, andproposed catalytic cycle for cis-dihydroxylation

NMO as the cooxidant. With respect to the olefin, 0.25 mol% Os was needed and theexcellent chemoselectivity of the homogeneous reaction with NMO is preserved.However, somewhat increased reaction times are required. The development of anasymmetric variant of this process by addition of the typical chiral alkaloid ligands ofthe asymmetric dihydroxylation should be difficult since the reactions performedwith these heterogeneous catalysts are taking place in the so-called second cycle.With alkaloid ligands high ee values are only achieved in dihydroxylations occurringin the first cycle. However, recent findings by the groups of Sharpless and Adolfssonshow that even second-cycle dihydroxylations may give substantial ee results [41].Although this process must be optimized, further development of the concept of anenantioselective second-cycle process offers a perspective for a future heterogeneousasymmetric catalyst.

Choudary and his group reported, in 2001, on the design of an ion-exchange tech-nique for the development of recoverable and reusable osmium catalysts immobi-lized on layered double hydroxides (LDH), modified silica, and organic resin forasymmetric dihydroxylation [42]. An activity profile of the dihydroxylation of trans-stilbene with various exchanger/OsO4 catalysts revealed that LDH/OsO4 displays thehighest activity and that the heterogenized catalysts in general have higher reactivitythan K2[OsO2(OH)4]. When trans-stilbene was added to a mixture of LDH/OsO4,(DHQD)2PHAL as the chiral ligand (1 mol% each), and NMO in H2O/tBuOH, thedesired diol is obtained in 96% yield with 99% ee. Similarly, excellent ee results areobtained with resin/OsO4 and SiO2/OsO4 in the same reaction. All of the preparedcatalysts are recovered quantitatively by simple filtration and reused for five cycleswith consistent activity. With this procedure, various olefins ranging from mono- totrisubstituted and from activated to non-activated are transformed into their diols. Inmost cases, the desired diols are formed in higher yields, albeit with almost similaree values as reported in homogeneous systems. Slow addition of the olefin to the re-action mixture is warranted to achieve higher ee. This LDH/OsO4 system presentedby Choudary and coworkers is superior in terms of activity, enantioselectivity andscope of the reaction in comparison with that of Kobayashi.

Although the LDH/OsO4 shows excellent activity with NMO, it is deactivatedwhen K3[Fe(CN)6] or molecular oxygen is used as the co-oxidant [43]. This deactiva-tion is attributed to the displacement of OsO4

2– by the competing anions, which in-clude ferricyanide, ferrocyanide, and phosphate ions (from the aqueous buffer solu-tion). To solve this problem resin/OsO4 and SiO2/OsO4 were designed and preparedby the ion-exchange process on the quaternary ammonium-anchored resin and si-lica, respectively, as these ion-exchangers are expected to prefer bivalent anionsrather than trivalent anions. These new heterogeneous catalysts show consistent per-formance in the dihydroxylation of �-methylstyrene for a number of recycles usingNMO, K3[Fe(CN)6] or O2 as reoxidant. The resin/OsO4 catalyst, however, displayshigher activity than the SiO2/OsO4 catalyst. In the presence of Sharpless ligands var-ious olefins were oxidized with high enantioselectivity using these heterogeneoussystems. Very good ee results were obtained with each of the three co-oxidants. Equi-molar ratios of ligand to osmium are sufficient to achieve excellent ee results. This isin contrast to the homogeneous reaction in which a 2–3 molar excess of the expen-

151.3 Supported Osmium Catalyst

sive chiral ligand to osmium is usually employed. These studies indicate that thebinding ability of these heterogeneous osmium catalysts with the chiral ligand isgreater than the homogeneous analogue.

Incidentally, this forms the first report of a heterogeneous osmium-catalystmediated AD reaction of olefins using molecular oxygen as the co-oxidant. Underidentical conditions, the turnover numbers of the heterogeneous catalyst are almostsimilar to the homogeneous system.

Furthermore, Choudary and coworkers presented a procedure for the applicationof a heterogeneous catalytic system for the AD reaction in combination with hydro-gen peroxide as co-oxidant [44]. Here a triple catalytic system composed of NMMand two heterogeneous catalysts was designed. A titanium silicalite acts as the elec-tron transfer mediator to perform oxidation of NMM that is used in catalyticamounts with hydrogen peroxide to provide in situ NMO continuously for AD of ole-fins, which is catalyzed by another heterogeneous catalyst, silica gel-supported cinch-ona alkaloid [SGS-(DHQD)2PHAL]-OsO4. Good yields were observed for various ole-fins. Again very good ee results have been achieved with an equimolar ratio of ligandto osmium, but slow addition of olefin and H2O2 is necessary. Unfortunately, recov-ery and reuse of the SGS-(DHQD)2PHAL-OsO4/TS-1 revealed that about 30% of theosmium had leached during the reaction. This amount has to be replenished in eachadditional run.

1.4Ionic Liquids

Recently ionic liquids have become popular as new solvents in organic synthesis [45,46]. They can dissolve a wide range of organometallic compounds and are misciblewith organic compounds. They are highly polar but non-coordinating. In generalionic liquids exhibit excellent chemical and thermal stability along with ease of re-use. It is possible to vary their miscibility with water and organic solvents simply bychanging the counter anion. Advantageously they have essentially negligible vaporpressure.

In 2002 olefin dihydroxylation by recoverable and reusable OsO4 in ionic liquidswas published for the first time [47]. Yanada and coworkers described the immobili-zation of OsO4 in 1-ethyl-3-methylimidazolium tetrafluoroborate [47a]. They chose1,1-diphenylethylene as a model compound and found that the use of 5 mol% OsO4

in [emim]BF4, 1.2 equiv. of NMO�H2O, and room temperature were the best reac-tion conditions for good yield. After 18 h 100% of the corresponding diol was ob-tained. OsO4-catalyzed reactions with other co-oxidants such as hydrogen peroxide,sodium percarbonate, and tert-butyl hydroperoxide gave poor results. With anhy-drous NMO only 6% diol was found. After the reaction the 1,2-diol can be extractedwith ethyl acetate and the ionic liquid containing the catalyst can be reused forfurther catalytic oxidation reaction. It was shown that even in the fifth run the ob-tained yield did not change. This new method using immobilized OsO4 in an ionicliquid was applied to several substrates, including mono-, di-, and trisubstituted ali-


phatic olefins, as well as to aromatic olefins. In all cases, the desired diols were ob-tained in high yields.

The group working with Yao developed a slightly different procedure. They used[bmim]PF6 (bmim = 1-n-butyl-3-methylimidazol)/water/tBuOH (1 : 1 : 2) as the sol-vent system and NMO (1.2 equiv.) as the reoxidant for the osmium catalyst [47 b].Here 2 mol% osmium are needed for efficient dihydroxylation of various olefins.After the reaction, all volatiles were removed under reduced pressure and the pro-duct was extracted from the ionic liquid layer using ether. The ionic liquid layer con-taining the catalyst can be used several times with only a slight drop in catalyst activ-ity. In order to prevent osmium leaching, 1.2 equiv. of DMAP relative to OsO4 haveto be added to the reaction mixture. This amine forms stable complexes with OsO4,and this strong binding to a polar amine enhances its partitioning in the more polarionic liquid layer. Recently, Song and coworkers reported on the Os-catalyzed dihy-droxylation using NMO in mixtures of ionic liquids (1-butyl-3-methylimidazoliumhexafluorophosphate or hexafluoroantimonate) with acetone/H2O [48]. They used1,4-bis(9-O-quininyl)phthalazine [(QN)2PHAL] as the chiral ligand. (QN)2PHAL willbe converted into a new ligand bearing highly polar residues (four hydroxy groupsin the 10,11-positions of the quinine parts) during AD reactions of olefins. The useof (QN)2PHAL instead of (DHQD)2PHAL afforded the same yields and ee resultsand, moreover, resulted in drastic improvement in recyclability of both catalyticcomponents. In another recent report Branco and coworkers described theK2OsO2(OH)4/K3Fe(CN)6/(DHQD)2PHAL or (DHQD)2PYR system for the asym-metric dihydroxylation using two different ionic liquids [49]. Both of the systemsused, [bmim][PF6]/water and [bmim][PF6]/water/tert-butanol (bmim = 1-n-butyl-3-methylimidazol), are effective for a considerable number of runs (e.g., run 1, 88%,ee 90 %; run 9, 83%, ee 89%). Only after 11 or 12 cycles was a significant drop in thechemical yield and optical purity observed.

In summary, it has been demonstrated that the application of an ionic liquid pro-vides a simple approach to the immobilization of an osmium catalyst for olefin dihy-droxylation. It is important to note that the volatility and toxicity of OsO4 are greatlysuppressed when ionic liquids are used.

17References

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