sharpless asymmetric dihydroxylation of olefins

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Chemical and Electrochemical Asymmetric Dihydroxylation ofOlefins in I 2-K2CO3-K2OsO2(OH)4 and I2-K3PO4/

K2HPO4-K2OsO2(OH)4 Systems with Sharpless Ligand

Sigeru Torii,*, P in g L iu , N a r a y a n a s w a m y B h u v a n es w a r i , Christ ian Amatore, a n dAnny J uta nd

Depart ment of A ppl i ed Chemi st ry, Facul t y of E ngi neeri ng, Okayama Uni versi t y, Okayama 700, Japan,and E col e Norm al e S uperi eur e, Depart ement de Chi m i e, U RA CN RS 1679, 24 rue L homond,

75231 P ari s Cedex 05, Fr ance

Received December 1, 1995X

Iodine-assisted chemical and electrochemical asymmetric dihydroxylation of various olefins in I 2-K 2C O3-K 2Os O2(OH)4a n d I 2-K 3P O4/K 2H P O4-K 2Os O2(OH)4systems with Sha rpless ligand providedth e optically a ctive glycols in excellent isolat ed yields and h igh ena ntiomeric excesses. Iodine (I 2)was used stoichiometrically for the chemical dihydroxylation, and good results were obtained withnonconjugat ed olef ins in contra st to th e case of pota ssium ferr icya nide as a co-oxidant . Thepotentiality of I 2as a co-oxida nt under s toichiometric conditions h a s been proven to be effective asa n oxidizing mediator in electrolysis systems. Iodine-a ssisted a symm etric electro-dihydr oxylationof olefins in either a t-B uO H /H 2O(1/1)-K 2C O3/(DH QD )2P H A L -(Pt) or t-B uO H /H 2O(1/1)-K 3P O4/K 2H P O4/(DH QD )2P H A L-(P t) system in th e presence of pota ssium osmat e in an und ivided cell wa sinvestigat ed in detai l . Irrespective of the substi tution pat tern, a l l the olef ins a f forded the diols in

high yields an d excellent ena ntiomeric excesses. A plausible mecha nism is discussed on the ba sisof cyclic voltam mograms as well a s experimenta l observat ions.

Introduction

As y m m et r i c i n d u ct i on h a s b ee n f r eq u e n t ly u s ed i nsyntheses of optically active natural products and biologi-cal ly a ctive compounds since t he a symmetric centerspresent in t hese compounds m ay be provided by chiraltransfer to readily avai lable prochiral moieties.1 Thes y n t h e t i c s t r a t e g y i s m o s t l y l e f t t o t h e i m a g i n a t i o n o fchemists a nd not restr icted by the a vailabil i ty of certa insta rting ma terials. Among the most prominent examplesof asymmetric reactions, the Sharpless process for the

asym metric dihydroxylation of olef inic compounds iswidely recognized in recent years. 2 Our ef forts in thisf ie ld h a v e l ed t o t h e e s t a b li s h m en t of ch e m ica l a n delectroca ta lytic a symmetric dihydroxylat ion of olef insfeatured by (1) novel co-oxidant systems for conjugatedolefins as well a s nonconjugat ed termina l olefins, (2) th euse of less a mount of the co-oxida nt, a nd (3) performingthe electrolysis in an undivided cell.

Criegees pioneering work3 s h o w e d t h a t o s m i u m t e t -raoxide in a stoichiometric am ount could be used foreffective ci s-d i h y d rox y la t i on of ol ef in s a n d t h a t t h i smethod is more reliable tha n other diol syntheses despitei t s h ig h cos t a n d t ox ici t y . C a t a l y t ic h y d rox yl a t i on 4

e m er g ed i n t h e f ie ld d u e t o t h e g r ow i n g e con om i ca l

requirement. Inorganic co-oxidant s such a s sodium or

pota ssium chlorat e5a an d hydrogen peroxide5b w e r e t h ef irst to be introduced, but in some cases th ese reagentsled to diminished yields of diols due to overoxidation.Subseq uently, some orga nic co-oxida nts such a s alka linetert-buty l hydr operoxide6 a n d N-ethylmorpholine N-ox-id e7 were introduced, which afforded comparatively betterresults. Recently, pota ssium ferricya nide in combina tionwith potassium carbonate has been demonstrated to bea p ow e r f ul s y s t em f or os m i um -ca t a l y z ed a s y m m e t r i cdihydr oxylat ion of olefins.8-10 The effective chira l liga ndsfor enantioselective dioxyosmylation of olefins have been

investigated in detail.11 Among the reported a symmetr icd i h y dr ox y la t i on s y s t e m s, a n H 2O/t-BuOH -K 3Fe(CN)6/

* Te l: 8 1-8 6-2 51 -8 07 2. F a x : 8 1-8 6-2 55 -3 42 4. E -m a i l :[email protected].

O kay ama U ni ve rsi ty . URA C NRS 1679.X Abstract published in A dvance ACS Abstracts, April 1, 1996.(1) For a r eview, see: (a)Asymmetric Catalysis in Organic Synthesis;

Noyori, R .; Wiley-Int erscience P ublications: New York, 1993. (b)Catalytic Asymmetric Synthesis; Ojima, I ., Ed.; VCH P ublications: NewYork, 1 99 3. (c ) Be rri sford, D. J . ; B olm, C . ; Sha rpl ess, K. B. Angew.Chem., I n t . E d . E n g l . 1995, 34, 1059.

(2) Kolb, H. C.; VanNieuwenhze, M. S.; S ha rpless, K. B. Chem. Rev.1994, 94, 2483.

(3) (a) Criegee, R. J u s t u s L i e b i g s An n . C h e m . 1936, 522, 75. (b)Criegee, R. Angew. Chem.1937, 5 0, 153. (c) Criegee, R. Angew. Chem.1938, 51, 5 19 . (d) C ri e g ee , R. ; Marc hand, B. ; Wannowi s, H. J u s t u s Liebigs Ann. Chem. 1942, 550, 99.

(4) Schroder, M. Chem. Rev. 1980, 80, 187.

(5) (a) H ofmann, K. A.Chem. Ber. 1912, 4 5, 3329. (b) Milas , N. A.;Trepagnier, J . H .; Nolan, J . T., J r.; I l iopulos, M. I . J. Am. Chem. Soc.1959, 81, 4730.

(6) Sharpless, K. B.; Akashi, K. J. Am. Chem. Soc. 1976, 98, 1986.(7) (a ) Schneider, W. P.; McIn tosh, A. V. U.S . P a tent 2,769,824, Nov

6, 1956; Chem. Abstr.1956, 51, 8822e. (b) Van Rheenen, V.; Kelly, R.C . ; C h a , D . Y . Tetrahedron Lett . 1976, 1973.

(8) Minat o, M.; Yama moto, K.; Tsuji, J .J. Org. Chem.1990,5 5, 766.(9) (a) Gao, Y.; Charles, M. Z. U.S. Patent 5,302,257, Apr 12, 1994;

Chem. A bstr. 1994, 1 20, 119437j. (b) Amun dsen, A. R.; B a lko, E. N . J .Appl. Electrochem. 1992, 22, 810.

(10) Torii , S.; Liu, P .; Tan aka , H . Chem. L ett . 1995, 319.(1 1) (a) He ntg e s, S . G. ; Sha rpl ess, K. B. J . Am . C h em . So c . 1980,

102, 4 26 3. (b) Sha rpl ess, K. B. ; Ambe rg, W.; B e ll er, M.; C he n, H.;Ha rturg , J . ; Ka wa nami , Y. ; L ubbe n, D . ; Ma noung , E . ; O g ino, Y. ;Shi bata , T.; U ki ta , T. J. Org. Chem. 1991, 56, 4585. (c) Sharpless, K.B.; Amberg, W.; Bennani, Y. L.; Crispino, G. A.; Hartung, J .; J eong,K.-S.; Kw ong, H.-L.; Morikaw a, K.; Wang, Z.-M.; Xu, D.; Zha ng, X.-L.J. Org. Chem. 1992, 57, 2768. (d) Andersson, P. G.; Sharpless, K. B.J . Am . C h e m . So c . 1993, 11 5, 7047. (e) Wang, S.-M.; Kakiuchi, K.;Sha rpl ess, K. B. J. Org. Chem. 1994, 5 9, 6895. (f) Becker, H.; King, S.B.; Taniguchi, M.; Vanhessche, K. P . M.; Sha rpless, K. B .J . Org. Chem.1995, 6 0, 3940. (g) Tomioka, K.; Nakajima, M.; Koga, K. J. Am. Chem.Soc.1987, 1 09, 6213. (h) Tomioka, K.; Na kajima, M.; Iita ka, Y.; Koga,K. Tetrahedron Lett.1988, 2 9, 573. (i) Yama da, T.; Nara saka , K. Chem.Lett. 1986, 131. (j) Tokles, M.; Snyder, J . K. Tetrahedron Lett . 1986,27, 3 9 5 1 . ( k) Hi rama, M.; O i shi , T.; I to , S . J. Chem. Soc., Chem.C o m m u n . 1989, 6 65 . ( l) Annunz i ata , R. ; C i nq ui ni , M.; C oz zi , F. ;Raimondi, L.; Stefanelli, S. Tetrahedron L ett.1987,2 8, 3139. (m) Corey,E. J .; J ardin e, P. D .; Virgil, S.; Yuen, P.-W.; Connell, R. D. J . Am. Chem.Soc. 1989, 111, 9 2 4 3 . ( n) Ki ng , S. B. ; Sharpl e ss, K. B. T etrah edronLett. 1994, 35, 5611.

(12) Mukaiyama, T.; Imagawa, K.; Yamada, T.; Takai, T. Chem. L ett.1992, 231.

3055J . O r g . C h em . 1996, 61 , 3055-3060

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K 2C O3system with chiral l igands such as dihydroquini-dine (DHQD ) and /or dihydr oquinine (DHQ) derivat iveshas been found to be superior .11c

In the preceding paper,10 we r eported the electrochemi-cal a symmetric dihydroxylation of olef ins using S ha rp-l es s l ig a n d i n t h e p r es en c e o f p ot a s s iu m os m a t e b yrecycling a catalytic amount of potassium ferr icyanideb y e le ct r o ch e m ica l ox id a t i o n . We a l s o m e n t ion e d apossibility of the choice of other co-oxidants as well as

bases in the electrolytic conversion.In continua tion of our research in chemical an d elec-

trocat alyt ic asymmetric dihydroxylat ion rea ctions, now,w e f ou n d t h a t I 2 a n d i o d i n e-iodide combinations areefficient co-oxida nts for the osmat e recycling. Na mely,the novel co-oxida nt -b a s e s y st e m s i nv ol vi n g a n I 2-K 2C O3-K 2Os 2(OH)4a n d I 2-K 3P O4/K 2H P O4-K 2Os 2(OH)4combina tion, wh ich can realize efficient ena ntioselectived i ox y os m y l a t i on of ol ef in s , w e r e p r ov ed t o b e m or eeffective both under chemical and electrochemical condi-tions.

Results and Discussion

I n i t ia l ly , w e s t u d ie d t h e p ot e n t ia l i t y of I 2 a s a na l t e rn a t i v e co-ox id a n t i n t h e K 2C O 3-K 2Os O 2(OH)4-(D HQD )2PHAL system for asymmetric dihydroxylationof styrene. Ha lides ha ve been found to be an ef ficientcatalyst in mediatory systems, which spured us to gatherp r ec is e d a t a i n t h e i s ol a t e d a n d e n a n t i om e r ic e x ce s syields taking advantage of the halide-assisted electro-dihydroxylation of styrene (Table 1, entries 2 -7). Ap-parently, most co-oxidant systems provided the diols inhigh yields of %ee. However, different efficiencies werenoticed in conversion yields. In compa rison with t he ca seof potassium ferricyanide (Table 1, entry 1), the potas-sium halides (entries 2, 3, and 4) were less effective evenw h e n t h e y w e r e u s e d i n e x c e s s e s m o r e t h a n 1 0 t i m e slarger tha n tha t of pota ssium ferr icyan ide. This situa -tion, however, seems to be improved by addit ion of I 2(e nt r ie s 5 a n d 6). Th e m os t e xci t in g r es u lt s w e r eo b t a i n e d w h e n I 2 a l on e w a s u s ed i n a n I 2-K 2C O3-K 2-Os O2(OH)4 s y st e m a s a co-ox id a n t (e nt r y 7) u n d erelectrochemica l conditions. The correla tion of the iso-lated and enantiomeric excess yields in respect to theamount of I 2 is demonstra ted in Figure 1. Appar ently,the use of 0.3 equiv of I 2 still afforded sufficiently highyield of the chiral diol f rom st yrene. Without passingelectricity, the isolated yield sharply decreased in propor-tion to the am ount of I2, being less tha n 1.5 equiv. Thesetrends were not changed with dif ferent systems.

Th e ch e m ic a l a s y m m e t r i c d i h y d r ox y la t i o n w i t h I 2smoothly proceeded in 96.6% isolat ed yield together

wit h 96.2%ee when t he molar ra tio of styrene a nd I 2w a s1.5. How ever, th e isolat ed yield diminished in proportionw i t h t h e a m o un t of I 2. I n t e r es t i n g ly , u n d e r t h e s e c ir -cumsta nces, the va lue of enant iomeric excess wa s a lwa ysaround 96%. Next, w e investigared electrochemical

os m i um -c a t a l y z e d a s y m m e t r i c d i h y d ro xy l a t i on i n t h esame sy stem by recycling use of a cata lytic amount of I 2(electrochemica l procedure I). As show n in Figure 1, itis clea r t h a t t h e I 2-K 2C O 3-K 2Os O2(OH)4-(D HQD )2-P HAL syst em opera ted w ell under electrolysis conditionsa n d t h a t t h e I 2 played t he expected role a s a recyclableco-oxidant for potassium osmat e. Actually, more than90%of isolated yields (96%ee) could be attained in thepresence of 0.3 equiv of I 2. Under a similar electrolysiscon d i t ion , a l a r g e a m ou n t of s t y r e n e (5 e q u i v ) ca n b eoxidized by changing the electrolysis t ime and currentdensity to a f ford 3 in 89%yield (91.9%ee).

Encoura ged by our ini t ial results, we sought an al ter-na tive co-oxida nt -base combina tion for t he K 2OsO2(OH)4-(D HQD )2P HAL system active at pH va lues around 11.0-11.5 which is required for the hydrolysis of the chiralmonoglycolat e intermediate 2. Th e I 2-K 3P O4/K 2H P O 4com b i n a t i on w a s p r ov en t o b e i d e a l f or t h e p r es e ntp ur pos e, a n d t h e r es u lt s a r e i nd ica t e d i n F i gu r e 2 .Electrolysis of 1 i n a I 2(0.5 equiv)-K 3P O4/K 2H P O4(1.88/

Table 1. Electro-dihydroxylationa of Styrene withVarious Co-Oxidants

e nt r y co-ox id a n ta m ou n t /

mmolconversion

yi eld /%enantiomeric

exces s/%e e

1 K 3Fe(CN)6 0.1 95.0 97.32 K C l 1.0 48.0 92.03 K B r 1.0 65.3 92.84 K I 4.0 41.4 86.85 K I /I 2 2/1 87.6 96.56 K I /I 2 2/0.5 91.6 95.07 I 2 0.5 93.7 97.9

a Carried out in a mixed solution of t-BuOH (5 mL) and H 2O (5mL) in the presence of (DH QD)2PHAL (0.01 mmol), K 2Os O2(OH)4(0.002 mmol), and K 2C O3 (3.0 mm ol) under passage of 2.5-5.0F/mol of elect ricit y for 25-30 h.

Figure 1. E lectrochemical asy mmetric dihydroxylation ofstyrene in an I 2-K 2C O3-K 2Os O2(OH)4system in t he presenceof (DHQD)2PH AL. Isolat ed yield, %(b), obtained by changingthe a mount of iodine under the following conditions: styrene(1.0 mm ol), (DH QD)2PHAL (0.01 mmol), K 2Os O2(OH)4 (0.002mmol), K 2C O3(3.0 mmol) in t-BuOH (5 mL)-H 2O (5mL) at 0 C in a n u ndivided cell with 5.11 F/mol electricity (1.5-3.0 V,5 mA/cm 2). Enantiomeric excess, % ee (4), determined byHPLC analysis of the diol (CHIRALCEL OB-H column, 10%i-P rOH in h exan e, 0.5 mL/min).

3056 J. Or g. Chem., Vol. 61, N o. 9, 1996 Torii et a l.


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1.12 equiv)-K 2Os O2(OH)4-(D HQD )2P H A L s y s t em a f -forded the diol 3 in 96.2%yield (96.5%ee) (see electro-ch em ica l p roce du r e I I ) . As s h ow n i n F i gu r e 2 , t h epresence of 0.2 equiv of I 2 could afford the diol in more

tha n 81.8%yields in isolated y ield w ith 95.2%ena ntio-meric excess.

Table 2 further i l lustrates the results obtained fromthe asymmetric dihydroxylation of a variety of olef insusing iodine-b a s e com b in a t i on s y s t em s u n d er b ot hchemical and electrochemical conditions in comparisonwith the previous results.11 I t i s ob v iou s t h a t b ot h t h eI 2-K 2C O3a n d I 2-K 3P O4/K 2H P O4systems a re well suitedfor va rious olef ins under t he a bove conditions. Espe-cial ly, terminal as well as di- and tr isubsti tuted olef insbearing alkyl and aryl groups were good candidates forthe a symmetric dihydroxylation. Some exceptions w eree n cou n t e r ed w h e n t h e p r es e nt I 2-K 2C O3 s y s t e m w a se xt e n d ed t o t h e a s y m m e t r i c d i h y d r ox y la t i o n of R,-

unsaturated esters (entries 4, 11, and 12), which exhib-ited relat ively lower conversion yields probably due tohydrolysis of the est er groups under the employed condi-tions. However, the use of K 2C O3/K H C O3(2.5/0.5 m m ol)in place of K 2C O3(3 mmol) wa s proven t o be effective forthe improvement of the conversion yields. The scale-upelectrolysis performed in a microflow cell (ElectroCell AB,Sweden) provided the same results obtained in a glassappara tus (EOS-22, J apa n). The diols obta ined fromolefins (entries 5, 13, and 14) are well-known as chiralsynthons for the synthesis of chiral -blockers,13 chiralauxil iar ies,11n a n d a v a r i e t y o f o p t i c a l l y a c t i v e n a t u r a l

products.14 A successful a pplicat ion of the present pro-cedures for the synthesis of chiral l-shikonin has beencarried out which will be published soon elsewhere. 15

Electrochemical Measurement of the Redox Sys-tem K2Os(VI)O2(OH)4/I2 in Water. The cyclic voltam-mogram of a solution of K 2Os O2(OH)4 (2 mM) in w at erco nt a i n in g K 2C O3 (0 .3 M ) a s s u pp or t i n g e le ct r o ly t eexhibits tw o oxidat ion pea ks (Figure 3). The first elec-tron transfer is controlled by the diffusion and takes placea t Epa (O1) ) -0.115 V (eq 1, at a platinum electrode at

the scan ra te of 0.2 V s -1). The first oxidat ion producesa species that is adsorbed at the electrode surface, andtherefore the second electron transfer at Epa (O2) ) +0.225V (eq 3) is not controlled by t he diffusion. The speciesresulting from the first electron transfer is also reduceda t Epc(R1) ) -0. 225 V (e q 2), a n d t h e s h a p e o f t h ereduction peak is also characteristic of a species adsorbedon the electrode surface.

Ad d i t ion o f 1 e q u iv o f I 2 r es u lt s i n a d eca y of t h eoxida tion peak O 1w hile two new oxidat ion peaks a ppeara t m or e p os i t iv e p ot e n t i a l a t Epa (O3) ) +0.295 V andEpa (O4) ) +0 .5 06 V (e q s 4 a n d 5 ; F i g u r e 3 ). Th e s eo x i d a t i o n p e a k s a r e c h a r a c t e r i s t i c o f t h e o x i d a t i o n o fiodide anions (I -) as evidenced by the increase in t heirrespective magnitude when potassium iodide is added tothe solution. Therefore we a ssume tha t I 2 oxidizes theosmium(VI) sal t in w at er a nd i tself is reduced t o I -.

Electrochemical Measurement of the Redox Sys-tem K2Os2(OH)4/I2in a Mixture (1/1) of Water andt er t-Butyl Alcohol. A m ix t u r e of w a t e r a n d tert-butyla lcohol (1/1) conta ining K 2C O 3 (0.3 M) a s a supportinge le ct r ol y t e r es u lt s i n t h e f or m a t i on of t w o p h a s es .K 2Os(VI)O2(OH)4 (2 mM) is only soluble in the aqueousphase a s a t tested by i ts pale violet color . No oxidation

peak is detected when voltametry is performed in theorganic phase. In th e aqueous pha se, the two oxidationp e a k s o f K 2Os(VI)O2(OH)4 a r e d e t e c t e d a t Epa (O1) )+0 . 0 4 0 V a n d Epa (O2) ) +0.205 V, i.e., a t p o t e n t i a l ssimilar t o that mentioned above in pure wa ter . When I 2is added to this solution, the pale violet color character-istic of K 2Os(VI)O2(OH)4 disappears and a yellow colorcharacteristic of Os(VIII)O4appears in the organic phase.H o w e ve r , n o r e d uc t ion p ea k t h a t c ou l d ch a r a ct e r iz eOs(VIII)O4 i s o b s e r v e d ( c a r r i e d o u t w i t h a n a u t h e n t i csa mple of Os(VII I)O4 alone). In t he presence of 5 equivof the ligand, th e oxidat ion peaks of K 2Os(VI)O 2(OH)4a re

(1 3) Rama Rao, A. V.; G urjar, M. K.; J oshi, S . V. Tetrahedron:Asymmetry1990, 1, 697.

(14) Dumount, V. R.; Phander, H. H el v . C h i m . Ac t a 1983, 66, 814.(15) Torii, S.; Akiyama, K., Chem. L ett . 1996, in press.

Figure 2. E lectrochemical asym metric dihydroxylation of

styrene in an I 2-K 3P O4/K 2H P O4-K 2Os O2(OH)4system in thepresence of (DHQD)2P HAL. Isolat ed yield, %(b), obtained bychang i ng t he am ount of i odi ne und e r t he fol low ing c ond i -t i on s : s t y r en e (1 .0 m m ol ), (D H Q D )2P H AL (0.01 m m ol),K 2Os O2(OH)4 (0.002 mmol), K 3P O4/K 2H P O4 (1.88/1.12 mm ol)in t-BuOH (5 mL)-H 2O (5m L ) at 0 C i n an und i vi d e d c e l lwit h 5.11 F/mol electr icity (1.5-3.0 V, 5 mA/cm2). Enantio-meric excess, %ee (4), d e t e r m ine d b y H P L C ana l y sis of t hediol (CHIRALCEL OB-H column, 10% i-P rOH in hexane, 0.5mL /mi n).

Os(VI)O2(OH)42-98

-e

Os(VII)O3(OH)32-

+ H + O1 (1)

Os(VII)O3(OH)32-98

H +,e

Os(VI)O2(OH)42-

R 1 (2)

Os(VII)O3(OH)32-98

-e

Os(VIII)O4(OH)22-

+ H + O2 (3)

I 2 + K 2Os(VI)O2(OH)4 f

2I - + K 2Os(VIII)O4(OH)2 + 2H+ (4)

or 2KI + Os(VIII)O4 + 2H 2O (5)

Asymmetric Dihydroxylation of Olefins J . Org. Chem., Vol. 61, N o. 9, 1996 3057


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detected in the a queous phase. Addition of I2 results inthe oxidat ion of K 2Os(VI)O2(OH)4as at tested by the decayof t h e ox id a t i on p ea k s of K 2Os(VI)O2(OH)4 a n d t h eappearance of oxidation peaks of I - (Epa (O3) ) +0.325 Va nd Eoa (O4) ) +0. 535 V). B u t n o r ed u ct i on p ea k scharacteristic of the new complex Os(VIII)O 4L * c a n b ed e t ect e d i n t h e o r g a n i c p h a s e . Th e r e d u ct i on o f t h eligan d a lone can be observed in THF. Its cyclic voltam -m og r a m e xh i bi t s t w o i r r ev er s ib l e r e d u ct i on p ea k s a t-2.00 a nd -2.03 V and a third reversible one at -2.275V. The reduction of the liga nd cann ot be observed in them i xt u r e o f w a t e r /tert-butyl alcohol since in this systemthe reduction of the solvent ta kes place a t ar ound -0.9V (carried out with an authentic sample of Os(VIII)O 4 +l igan d (5 equiv) in w at er/tert-but yl alcohol). Therefore,t h e ox id a t i on of t h e os m iu m (VI ) s a l t b y I 2 a n d t h erecycling of I - t o I 2 by oxidation at the anode proceed inthe aqueous phase whereas the osmium(VIII) complexproduced by oxidation of the osmium(VI) salts is trans-ferred t o the organic phase.

Optimal Experimental Conditions for the Elec-trochemical Process. Although K 2Os(VI)O2(OH)4ca nb e ox id i ze d a t t h e a n o d e, i t s d i r ect e le ct r o ch e m ic a l

oxidation leads t o adsorption on th e electrode tha t m ays t op t h e p r oc es s . I t i s w h y a m e d ia t or i s n e ce s sa r y t operform t he oxidat ion of K 2Os(VI)O2(OH)4in the solution.Iodine pla ys t his role, a nd t he resulting iodide a nions (I -)a r e o x i d i z e d a t t h e a n o d e b a c k t o I 2. S i nce K2Os(VI)-O2(OH)4is more ea sily oxidized tha n I -, it is not possibleto oxidize I - t o I 2in th e presence of K2Os(VI)(OH)4. Thiscan explain w hy performing th e reaction using a mixtur eof K

2Os(VI)O

2(OH)

4a nd potassium iodide lead s t o lower

yield than starting from the mixture of K 2Os(VI)O2(OH)4a n d I 2. Indeed, it is necessary to sta r t th e electrolysiswith a mixture of K 2Os(VI)O2(OH)4 and an excess of I 2.Thus, I 2 is able to convert quanti tat ively the K 2Os(VI)-O2(OH)4 to Os(VIII ). This depletes th e Os(VI) concentr a -t i on i n t h e d i ff u si on l a y e r a n d r e pl en i s h es i t w i t h aconcentration of iodide ions which can then be freelyoxidized t o regenerat e I 2. The process could only w orkif I - is alone in the diffusion layer (or with Os(VIII) butwit hout th e Os(VI)). This implies tha t I 2 should oxidizet h e O s(VI ) b y a v er y f a s t r ea c t ion com pa r e d t o t h eelectrolysis.

In the a queous electrolysis, th e osmat e Os(VI)O2(OH)42-

unambiguously undergoes a two-electron oxidation with

Table 2. Asymmetric Dihydroxylation of Olefins Using a Variety of Iodine-Base Systems

a Ca rried out in a mixture oft-BuOH (5 mL) and H 2O (5 mL) in t he presence of K 2C O3(3.0 mmol) or K 3P O4/K 2H P O4 (1.88/1.12 m mo l),(DHQD)2P HAL (0.01 mmol), I 2 (1.5 mmol) and K 2Os O2(OH)4 (0.002 mmol) based on 1 mmol of olefin at 0 C or rt for 30 h. b Performedby use of substrate (1 mmol), t-BuOH (5 mL), H 2O (5 mL), K 2C O3 (3.0 mm ol) or K 3P O4/K 2H P O4 (1.88/1.12 mm ol), (DH QD )2P HAL (0.01

mmol), I 2(0.5 mmol), K 2Os O2(OH)4(0.002 mmol) in a n un divided cell at 0 C or rt for 5.11 F/mol of electricity (1.5-3.0 V). c

Reported bySharpless et al., w i t h ( D H Q D )2P H A L .11c d En an tiomeric excess, ee %, determ ined by 19F-NMR a nd 1H-NMR a nalyses of mono- and bis-MTPA esters of the corresponding diols. e The yields r eported here a re isolated yields. f Determined by comparison of optical rota t ionvalues wit h reported ones. g Ee %determined by HP LC a na lysis of the diols (CH IRALCE L OB -H column, 10%i-P rOH in hexane). h Insteadof K 2C O3, a mixed base of K 2C O3/KH CO 3 (2.5/0.5 mm ol) wa s u sed.



5/6

electrochemically produced active iodine-oxidizing speciesto form Os(VII I)O6(OH)22- in a ba sic buffering conditions(eq 6). The octa va lent Os species would chan ge into OsO4by eliminat ing hydr oxy anions (eq 7). This process seemsto be reversible; osmium tetraoxide, however, is moresoluble in an organic phase. The combinat ion of OsO4

w i t h n i t r og e n -c on t a i n in g ch i r a l l ig a n d s L * p r oce ed spromptly to provide a rather t ight complex which mayprovide a bin ding pocket for olefin (eq 8). Subseq uently,olefin would be trapped by the binding pocket to affordthe olefin -Os -L* complexes a s a precursor of the chira ldiol (eq 9). The hy drolysis of t he complexes ma y givechiral diols, chiral ligands, and osmate(VI) (eq 10).

Experimental Section

General. S t a r t i ng ole fi nic sub st r at e s, unl ess spec ial l yst at e d , w e r e pur c hase d fr om c om m e r ci al sour ce s and notfurt her purified prior to use. All reactions were monitored bythin layer chromat ography (TLC) a nd judged complete wh ensta rting ma terial wa s no longer detected in TLC. The enan-tiomeric excesses of the chiral diols were determined by H P LCan alysis of diols on a CHI RALCEL OB -H column (Daicel) or

NM R anal y si s vi a Mosher derivatives (mono-, or bis-MTPAesters) prepar ed by sta nda rd procedures.16 1H NMR, 13C NMR,a nd 19F NMR spectra were r ecorded in deuteriochloroform(CDCl3) a t 200 and 500 M H z. O pt i cal r ot at i ons w e r e t ake nat 25 C at t he sodi um l ine .

Electrolysis Apparatus. U n l es s ot h e rw i s e n o t ed , a nundivided cell (EOS -V22, TEC HNOS IG MA, J a pan , 2.2 cm indiameter a nd 6 cm in height (ca. 20 mL)) fit ted w ith a st irringbar a nd a gas outlet pipe or a flow cell (MICRO FLOW CELL,E l e ct r oC el l AB , S w e d e n) w a s used . I n t he E O S -V22, t w oplatinum foil electrodes were placed parallel to each other 10

mm apa rt . The vessel wa s immersed in a ice-w at e r b at h orkept at room temperature.

Electrochemical Setup and Electrochemical Proce-dure for Cyclic Voltammetry. C y c l i c vol t am m e t r y w asper for m e d w i t h a hom em ad e pot ent i ost at and a w a ve for mg e ner at or , P AR M od e l 175. C y c li c volt am m og r am s w e r erecorded with a Nicolet 3091 digita l oscilloscope. Experim entsw e r e c ar r i ed out i n a t hr e e-e le ct r od e c el l c onnect e d t o aSchlenk l ine. The counterelectrode wa s a platinum w ire ofca. 1 cm 2 apparent surfa ce area ; the reference wa s a sa tura tedcalomel electrode (Tacussel) separa ted fr om th e solution by abridge (3 mL) filled with a 0.3 M K2C O3solution in a mixtureo f w a t e r a n d tert -buty l a lcohol (1/1). A 12 mL volume of am i xt ur e of w at e r a nd tert -but yl a lcohol (1/1) cont a inin g 0.3 MK 2C O3wa s poured into the cell . A 8.84 mg a mount (2 10-3

M) of K 2Os(VI)O 2(OH)4 (commercial from Aldrich) wa s then

ad d e d . C y c li c vol t am m e t r y w as per for m e d i n t he aq ue ousphase a t a sta tionary disc electrode (a plat inum disc made froma cross section of wire, 0.5 mm in diameter, sealed into glass)a t a s ca n r a t e of 0 .2 V s -1. S ui t ab l e am ount s of r eq ui r edr e ag ent s (e.g., 5.6 mg amount (2 1 0-3 M) of I 2 an d/or 93. 4mg a mount (10-2 M) of the l igan d (DHQD)2P HAL) were thenadded to the cell , and the cyclic voltammetry was performeda g a i n .

Electrochemical Procedure I with the I2-K2CO3-K2OsO2(OH)4 System. To a st irred solution of tert -butylalcohol (5 mL), I 2 (127 mg, 0.5 mmol), K 2C O3(410 mg, 3.0mmol), (DHQD )2PHAL (7.8 mg, 0.01 mmol), and K 2Os O2(OH)4(0.74 mg, 0.002 mmol) at 0 C in a n un divided cell (EOS-V22)w a s a d d ed s t y r en e ( 1 m m ol ) a t on ce . Th e m i x t ur e w a selectrolyzed u nder a consta nt current of 5 mA/cm2 undervigorous stirring (applied voltage 1.0-3.0 V). After passa ge

of 5.11 F/mol of elect ricit y for ca. 27 h, the rea ction (monitoredb y TL C ) w as q ue nche d w i t h a sat ur a t e d sod ium t hi osulfat esolution. Usua l workup followed by column chroma tographyg a v e (-)-(R)-1-phen yl-1,2-eth a nediol (3, 129.3 mg ; 93.7%);[R]25D ) -62.03 (c ) 1.98 (CD Cl 3)). The ena ntiomeric excessof theR-diol wa s determined by H PLC an alysis (CHI RALCELOB -H ; D a icel) to be 97.9%.

Electrochemical Procedure II with the I2-K3PO4/K2HPO4-K2OsO2(OH)4 System. To a stir red solution of tert-butyl alcohol (5 mL), water (5 mL), I 2 (127 mg, 0.5 mmol),K 3P O4 (398 mg, 1.88 mmol), K 2H P O4 (196 mg, 1.12 mmol),(DHQD)2P HAL (7.8 mg, 0.01 mmol), a nd K 2Os O2(OH)4 (0.74mg, 0.002 mmol) at 0 C in an undivided cell (EOS-V22) wasad ded styrene (1 mmol) at once. The mixture wa s electrolyzed(5.0 mA/cm 2) under vigorous st irring (applied voltage 1.0-3.0V). After pas sa ge of 5.11 F/mol of electricity for ca. 27 h, the

mixture (monitored by TLC) wa s worked up in a usu al ma nneran d chromatographed to give (-)-(R)-1-phenyl-1,2-ethanediol(3, 130 mg ; 96%); [R]25D ) -61.8 (c ) 2.46 (CDC l3)). Theenant iomeric excess of th e R-diol was determined by HPLCa na lysis (CHI RALCEL OB -H, D a icel) to be 97.1%.

Electrochemical ProcedureI II in a Flow Cell System.A mixture of tert -butyl alcohol-wa ter (25/25 mL), meth a ne-sul fonam i de (5 m m ol ), K 2C O3 (15 mmol), I 2 (2.0 mmol),K 2Os O2(OH)4 (0.01 mmol), (DHQD)2PHAL (0.05 mmol), andt r a n s -stilbene (5 mmol) wa s electrolyzed in an undivided flowc el l (MI C R O FL O W C E L L ) e q ui pped w i t h t w o plat i numelectrodes (3 4 cm 2). After pa ssa ge of 2.33 F/mol of electricit yat room temperature (progress was monitored by TLC), 5 g of

(16) Dale, J . A.; Du ll, D. L.; Mosher, H. S . J. Org. Chem. 1969, 34,2543.

Figure 3. (s) Cyclic volta mmetry of K2Os(VI)O2(OH)4(2 mM)i n w at e r c ont ai ni ng K 2C O3 (0.3M) at a s t at i onar y plat i numdisk electrode (d ) 0.5 mm) with a scan ra te of 0.2 V s-1 at 20 C. (- - -) Cy clic voltammetry of K 2Os(VI)O 2(OH)4 (2 mM) inthe presence of I 2(2 mM) performed under the sam e conditionsas above.

Os(VI)O2(OH)42-fOs(VIII)O 4(OH)2

2-+ 2e - + 2H +

(6)

Os(VIII)O 4(OH)22-fOs(VIII)O 4 + 2OH

- (7)

Os(VIII)O4 + L * fOs(VIII)O 4L * (8)

Os(VIII)O4L * + olefin folefin -Os(VIII)O 4L * (9)

olefin -Os(VIII)O 4L * + OH-f

diol* + L * + Os(VI)O2(OH)42- (10)

Asymmetric Dihydroxylation of Olefins J . Org. Chem., Vol. 61, N o. 9, 1996 3059


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solid sodium sulfi te was added and the solution was stirredfor 40 min. The two phases were separa ted, and t he aqueousp h a s e w a s t h e n e x t r a ct e d ( 3 25 m L ) w i t h E t O Ac. Thecom bi ned or g ani c phase s w e r e w a she d w i t h 2 M NaO H anddried over MgSO 4. C once nt r at i on and f l ash c hr om at og r aphyaffor de d t he (+)-(1R,2R)-1,2-phenyl-1,2-ethanediol in 82.4%yield, [R]25D ) +91 (CDCl3). The enan tiomric excess of theRR-d i ol w as d e t er m i ned b y H P L C ana l y sis [C H I R AL C E LOB-H column (Daicel), 10%i-P rOH in h exane, 0.5 mL /min]to be 99.5%. The a lka liod liga nd w a s recovered in 84.0%yield.

Chemical Procedure I with the I2-K2CO3-K2OsO2-

(OH)4 System. To a stirr ed solution oftert-but yl alcohol/w a ter(50/50 mL ), K 2C O3(30 mmol), I 2 (15 mmol), methanesulfona-mide (10 mmol), K 2Os O2(OH)4 (0.02 mmol), and (DHQD)2-PH AL (0.1 mmol) was added t r a n s -stilbene (10 mmol) at onceat 0 C, the mixture was stirred vigorously for more than 30h (monitored by TLC), and the reaction was quenched with10 g of solid sodium sulfite. The tw o phases were separa ted,and t he a q ue ous phase w a s t he n e xt r ac t e d (3 50 mL) withEtOAc. The combined organ ic pha ses were wa shed with 2 MN a OH a n d d r ie d ov er M gS O4. C o nce nt r a t i on a n d f la s hch r o m a t og r a p h y a f f or d e d t h e (+)-(1R,2 R)-1,2-phenyl-1,2-ethanediol in 98.4%yield, [R]25D ) +91 (CDCl3). The ena n-tiomeric excess of t he RR-d i ol w as d e t er m i ned b y H P L Canalysis [CHIRALCEL OB-H column (Daicel), 10% i-PrOH in

hexane, 0.5 mL/min] to be 99.5%. The a lkaloid l igand wa srecovered in 88.0%y ield.

Chemical P rocedure II with the I2-K3PO4/K2HPO4-K2OsO2(OH)4 System. To a st irred solution of tert -butylalcohol (5 mL), I 2 (382 mg, 1.5 mmol), K 3P O4 (1.88 mmol),K 2H P O4(mg, 1.12 mmol), (DHQD)2PHAL (7.8 mg, 0.01 mmol),a n d K 2Os O2(OH)4 (0.74 mg, 0.002 mmol) at 0 C was addedsty rene (1 mmol) a t once. The mixture wa s stirred vigorouslyfor ca. 30 h (progress was monitored by TLC and GLC), andthe reaction was quenched with a saturated sodium thiosulfatesolution (5 mL). Usua l workup followed by column chroma-

t o g r a p h y g a v e (-)-(R)-1-phen yl-1,2-eth a nediol (3) in 96.5%yield, [R]25D ) -61.5 (c ) 1.98 (CDCl3)). The ena nt iomerice xce ss of t he R-d iol w a s d et e rm in e d b y H P L C a n a l y s is(CHIRALCE L OB -H; Da icel) to be 96.2%.

Acknowledgment. I t is a pleasure to a cknowledgesupport of this investigat ion by the G ran ts-in-Aid forScientific Resea rch 05235107 a nd 05403025 for theMinistry of Educat ion, Science a nd C ulture, J a pan . Wea re g ra t e f u l t o t h e 500 M H z SC- N M R la bo ra t o ry o fOkayama University for the NMR experiments.

J O952137R


sharpless asymmetric dihydroxylation of olefins

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