- Email: [email protected]
Recent advances in asymmetric copper allylic oxidation of olefins
C. R. Acad. Sci. Paris, t. 2, SCrie II c, p. 19-23, SynthPse organique et organom&allique/Organic
1999 and
organometallic
synthesis
Recent advances in asymmetric allylic oxidation of olefins Jean-Michel BRUNEL *, 01 ivier LEGRAND,
Gerard BUONO *
!%ole nationale suptrieure de spnthkses, UMR CNRS 65 16, facuk de St-Jt%me, E-mail: [email protected]
de procPd& et d’in&ierie chimiques a~. Escadrille-Normandie-Niemen,
(Received
after revision
14 September
13?8,
accepted
copper
12 November
d’Aix-Marseille 13377 Marseille
(ENSSP1C.W).
cedex 20, France
1778)
Abstract - This review deals with the recent advances in asymmetric copper allylic oxidation of olefins. An exhaustive analysis of the results has been realized showing the limitation of the different asymmetric catalytic systems used. 0 Acadtmie des sciences/Elsevier, Paris allylic
esters
/ allylic
oxidation
/ asymmetric
catalysis
/ chiral
copper
complexes
Version frangaise abrCgCe - DCveloppements rkents dans I’oxydation allylique asymbtrique catalytique des okfines par des complexes chiraux du cuivre. La fonctionnalisation stMos&lective des okfines non activkes reprksente B I’heure actuelle un challenge important pour Ie chimiste organicien. En effet, cette mkthodologie reprtsente une voie d’accks i I‘obtention d’une grande classe de mokcules d’un grand ink&t synthetique. Une des mkhodes les plus d&eloppkes fair intervenir un caralyseur ?I base de sels de cuivre (Cu(OAc):, Cu(OTf), Cu(OTf),...) et d’un oxydant (H:O,, hydroperoxyde de tert-butyl, peroxyester). Les cycloalcknes sont ainsi oxydk en esters dont l’hydmlyse conduit aux alcools cyclaniques u-allyliques. En presence d’un ligand chiral, les hydrogPnes knantiotopiques allyliques sont discriminks et substituk par un carbon+ oxy. La grande difficult6 de cette mkhode &side dans le contrdle de I’CnantiosklectivitP de la reaction. Plusieurs sysckmes catalytiques ont kemment hit I’objet d’itudes, et des auxiliaires chiraux tels que les aminoacides, ou bien les oxazolines, ont 6rP utilis& en tant que l&and du cuivre. Ces derniers se sont ainsi r&&s &re les meilleurs vecteurs de chiralitt, les excks Pnantiomkiques restant nCanmoins modestes dans la plupart des autres cas er n’atteignant que trPs raremenr 90 %I e.e. Cette mise au point relate ainsi Ies principaux developpements r&&s& B ce jour dans l’oxydation allylique asymktrique des okfines catalyke par des complexes chiraux du cuivre. Sur la base de I’analyse des rkultats exptrimentaux, un mkanisme rendant compte de cette reaction d’oxydation allylique a pu stre tgalement propok. 0 Academic des sciences/Elsevier, Paris esters
allyliques
/ oxydation
allylique
/ catalyse
asymktrique
Stereoselective functionalization of unactivated olefins is a very active and challenging area of research with many potential synthetic applications [ 11. Thus, metal-induced allylic oxidation of alkenes has appeared to be one of the best methods of synthesis and has been the subject of severalinvestigations in literature 121. Apart from radical-initiated reactions [3], reac-
Communicated
* Correspondence
1251~8069/99/00020019
by
Henri
/ complexes
chiraux
du cuivre
tions based on selenium [4] or palladium [5] systems have attracted considerable interest. Among all these methods, Kharasch and Sosnovsky were the first to report the allylic oxidation of olefks with tert-butyl perbenzoate and a caralytic amount of a copper(I) salt [6]. On the other hand, except enzymatic methods [7], allylic esterswere obtained in low enanti-
KAGAN.
and reprints.
0 Acadkmie
des sciences/Elsevier,
Paris
19
J.-M.
Brunei
et al.
omeric excess (e.e.). Nevertheless, new catalytic enantioselective allylic oxidation methods of olefins with chiral copper catalyst and peroxyesters as oxidants have been recently developped. In 1965, Denney et al. describe the use of a stoichiometric amount of a chiral cuprous salt prepared from copper acetate (Cu(OAc),) and a-ethyl camphorate [8]. In this case, only low enantiomeric excesses for cyclic alkenes and no asymmetric induction for acyclic substrates have been detected. On the basis of these results, Muzart et al. have improved the enantioselective copper catalyzed acetoxylation of cyclic alkenes using a tert-butyl hydroperoxide (TBHP)/acetic acid mixture as oxidant. In this system, the chiral copper catalyst was prepared by mixing CU(OAC)~ and L-proline 1 (8 equiv. with respect to the copper source) [9]. Under these conditions, cyclohexenyl acetate hasbeen obtained with an enantiomeric excess up to 30 % e.e. This system hasbeen improved using tert-bury1 perbenzoate (TBPB) and copper oxide (CuZO) instead of respectively TBHP and copper acetate, increasing the enantioselectivity up to 59 % e.e. (scheme1) [lo].
cyclic alkenesusing a catalytic system(2.5 mol% with respect to the substrate) prepared from Cu(OA&, L-p ro 1ine and copper bronze (ratio 1:6:2O) [ 121. In this case, metallic copper acts as a reducing agent of copper(I1) and under these conditions, cyclohexenyl propionate has been prepared in 70 % yield and 57 % e.e.. In the sameway, acetic acid led to the formation of cyclohexenyl acetate in 71 % yield and 53 % e.e. (scheme2). The same group has demonstrated that the use of TBHP asoxidant in the presence of 4 equiv. of anthraquinone (with respect to the copper source) increases the enantioselectivity (60 % e.e.) while the yield remains quite the same [ 131. Anderson et al. have extended this catalytic system using rigid amino acids 1 and 2 (scbeme3). The results are slightly better than those previously obtained but the amounts of the chiral catalyst used are more important
[141. Recently, Pfaltz et al. were the first to describe the use of a catalytic amount of a chiral copper complex generated from cuprous triflate and a bisoxazoline 4a-b or 5a ligand (scheme4) [ 151. E.e.‘s up to 82 % have been obtained at 7 “C using tert-butylperbenzoate as oxidant in acetonitrile. In the caseof cyclopentene, a decrease of temperature from 7 to -20 “C increasesthe enantioselectivity from 79 % to 84 % e.e. but in detriment to the reaction rate (scheme S)
The importance of a copper(I) salt is not well established. Thus, the use of a bis(prolinato) copper(I1) complex, has no significant influence on the outcome of the reaction in terms of conversion and enantioselectivity [ 1 I]. In the same way, Feringa et al. have described the preparation of chiral allylpropionates from
5
OCOR p
mol%catalytic system
0
[la.
b
(“1 n
Cu(OAc), / 1 = 1 / 8 Solvent : AcOH n=2 ee = 28% Yield = 52% ($0 n=l n=2
/ 1 = I/ 2.5 ee = 59% ee = 45%
Solvent : CH,CN
Oxidant : TBHP
Oxidant : TBPB
Yield = 47% Yield = 59% Scheme 1. 9COR _
Cu(OAc), (2.5 mol%) 1 (6 eq./Cu) 0.10 (20 equiv./Cu) TBPB , RCOOH CJrl$N
+3
Scheme 2.
20
R = Et
conv. = 70% ee = 57%
R = Me
conv. = 71% ee = 53%
Recent
Cu(OAc),
advances
in asymmetric
(5 mol%)
&w
2 or 3 (5 equiv./Cu) pCoph Cue(10 equiv./Cu) -I 0 TBPB , PhCOOH
2
(9 n
n=l n=2
Benzene Scheme
copper
allylic
‘H
‘H
3
Yield = 54% ee= 60% Yield = 64%
ee = 40%
3.
a x=i-Pr 4
of olefins
&cop
a Y=H b Y=Ph
b X=t-Bu x
oxidation
x Scheme 4.
Catalytic enantioselective allylic oxidation of 1-methylcyclohexene as substrate led to a mixture of three regioisomers6,7,8 with e.e. lower than for non-substituted cycloalkens, except for regioisomer 6 obtained with an e.e. up to 90 % but in a low chemical yield (scheme6). Andrus et al. have developped a similar useof chiral bisoxazolines 4b for the preparation of optically active allylic benzoate [ 171. Thus, cycloalkenes led to the corresponding allylesters with high e.e. varying from 70 to 80 %. Moreover, allylbenzene and l-octene were regioselectively converted into the terminal allylic ester with e.e. up to 36 % (scheme7). The use of substituted peresters and new bisoxazoline ligand 9 at -20 “C led to identical results in terms of enantiomeric excess and
chemical yield but a significant decreaseof the reaction
time
In all these cases,the most effective catalysts are those generated from cuprous salt. Nevertheless, Katsuki et al. have reported that the copper catalyst prepared from Cu(OTf), and chiral trioxazoline is an effective catalyst for the enantioselective catalytic allylic oxidation of cycloalkenes by tert-butyl perbenzoate [ 191. The corresponding allylic ester of cyclopentene hasbeen obtained in 76 % e.e. and 83 % yield at room temperature. A decreaseof temperature led to an increase of the enantiomeric excess at the expense of the chemical yield. Moreover, the useof molecular sieveshasa beneficial effect on the outcome of the reaction increasing the reaction rate and the enantioselectivity (scheme9).
CuOTf (5 mol%)
n ee(%) yield (%)
L*
1
4b 4b 4a Sa 5b
2 Scheme
(scheme 8) [ 181.
79
65
81
64 44 80 58
5.
CuOTf(5 mol%) 4a (6
mol%)
l
TBPB, acetone, O°C Yield : @Z:
r
&c-
&ocoph
6
7
8
9% 90%
50% 63%
41% 13%
Scheme 6.
21
J.-M.
Brunel
et al.
CuOTf (5 mol%) 4b (6
R-H-------*
by copper(I) to give a copper(I1) benzoate intermediate and tert-butoxy radical This radical may abstract an allylic hydrogen atom to give tert-butanol and an allylic radical. Rapid addition of copper(I1) to the ally1 radical to generate copper(M) benzoate with the bound ally1 fragment then occurs, appearing as the key step for the induction of chirality. The final step of the chain mechanism is rearrangement of the copper(II1) intermediate to give the product and regenerate the copper(I) catalyst.
OCOPh
mol%)
TBPB, benzene, 55°C R: phenyl ee= 36% yield=34% n-pentyl ee = 30% yield = 50% Scheme 7.
OBz-p-NO,
z
9
In summary, the synthesis of optically active allylic esters may be achieved by direct allylic oxidation of alkenesusing various chiral copper catalysts by transferring the carboxylate group to one of the diastereotopic allylic carbons. Nevertheless, only the cyclic olefins led to good results, the acyclic alkenes providing the corresponding allylic estersin poor yields and selectivities. Thus, although the enantioselective Karash-Sosnovsky reaction is competitive compared to the results obtained with other transition-metal catalyst, it is expected that a more thorough understanding of the enantiodiscriminating event in theseprocesseswill result in the development of more efficient catalysts.
5 days Yield = 76%
-2OT,
ee = 73%
Scheme 8.
All these results clearly indicate that the copper catalyst is able to transfer the chirality during the formation of the carbon-oxygen bond. A catalytic cycle may be envisioned for the catalytic asymmetric copper allylic oxidation of olefins (scheme10). The mechanism has been shown to involve homolysis of the perester oxygen-oxygen bond
WOTf), (5 moI%)
0I
9 (7.5
d
mol%)
OCOPh
T (“C)
Yield (%) Ee (%) 83
76 *
68
74
30
93 *
11
88
20
TBPB, acetone, MS 4A
-20 N 9
3
* Reaction performed in presence of MS 4w Scheme 9.
ROOCOR’ R = Terr-butyl R’=Ph
Cu(II)(OCOR’)
RO’
ROH
22
Recent
advances
in asymmetric
111(a)
[21 (a)
Walling C., Zavitsas A.A., J. Am. Chem. Sot. 85 (1963) 2084; (b) Kochi J.K., J. Am. Chem. Sot. 84 (1962) 774; (c) Goering H.L., Mayer U., J. Am. Chem. Sot. 86 (1964) 3753; (d) Bulman Page PC., McCarthy T.J., in: Trost B.M., Fleming 1. (Eds.), Comprehensive Organic Synthesis, Pergamon Press, Oxford, 1991, vol. 7, pp. 83-l 17.
[31 Stephenson L.M., Grdina M.R.. Chem. Res. 13 (1980) 4 19.
Orfanopoulor
M.. Act.
M.A., Sharpless K.B., J. Am. Chem. Sot. [41 (a) Umbreit 99 (1977) 5526; (b) Stevenson L.M., Speth D.R., J. Org. Chem. 44 (1979) 4683: (c) Sharpless K.B., Latter R.F., J. Am. Chem. Sac. 94 (1972) 7154; (d) Sharpless K.B., Latter R.F., J. Org. Chem. 39 (1974) 429; (e) Reich H.J., J. Org. Chem. 39 (1974) 428; (0 Engman L., J. Org. Chem. 54 (1989) 889: (g) Wirth T., Hauptli S., Leuenberger M., Tetrahedron: Asymmetry 9 (1998) 547. H., BackvaIl J.E., in: Balm C.. Be&r M. J51 (a) Greenberg (Eds.), Transition Metals for Fine Chemicals and Organic Synthesis, VCH, Weinheim, 1998; (b) Heumann A., Akermark B.: Angew. Chem., Int. Ed. Engl. 23 (1984) 453; (c) Principato B., Maffei M., Siv C., Buono G., Peiffer G.. Tetrahedron 52 (1996) 2087.
allylic
oxidation
of olefins
(a) Kharasch M.S., Sosnovsky G., J. Am. Chem. Sot. 80 (1958) 756; (b) Kharasch M.S., Sosnovsky G., Yang N.C., J, Am. Chem. SOC. 81 (1959) 5819; (c) Sosnovsky G., Rawlinson D.J., in: Svern D. (Ed.), Organic Peroxides, Wiley, New York, 1970, vol. 1.
References Fukazana T., Hashimoto T., Tetrahedron: Asymmetry 4 (1995) 2323; (b) Gupta A.K., Jaszlauskas R.J., Tetrahedron: Asymmetry 4 (1993) 879; (c) Asami M., Ishizaki T., Inoue S., Tetrahedron: Asymmetry 5 (1994) 793; (d) Araki M., Nagase T., Chem. Abstr. 86 (1977) 120886r.
copper
PI
Wong C.H., Whitesides G.H., Enzymes in Synthetic Organic Chemistry, Pergamon Press. New York, 1994.
[81 Denney
D.B., Napier R., Cammarata A., J. Org. Chem. 30 (1965) 3151. [‘)I (a) Muzart J., Bull. Sot. Chim. Fr. (1986) 65; (b) Muzart J., J. Mol. Caral. 64 (1991) 381. Synth. Commun. 2j (1995) 1789. [lOI Ievina A., MuzartJ.. Asymmetry 6 (1995) [ill Levina A., Muzarr J., Tetrahedron: 147. C., Feringa B.L., Tetrahe1121Rispens M.T., Zondervan dron: Asymmetry 6 (1995) 661, C., Feringa iI31 Zondervan (1996) 1895. M.J., [I41 Sodergren (1996) 7577.
B.L., Tetrahedron:
Anderson
PG.,
Tetrahedron
A.S., Minidis A.B.E., [I51 (a) Cokhale hedron Lett. 36 (1995) 1831; (b) Sekar G., Gupta A.D., Singh VK.. (1998) 2961.
[161 Gupta
Asymmetry
Pfaltz
Lett.
37
A., Terra-
J. Org. Chem.
A.D., Singh V.K., Tetrahedron Lett. 2633. [171 (a) Andrus M.B., Argade A.B., Chen X., M.G., Tetrahedron Lett. 36 (1995) 2945; (b) Andrus M.B., Chen X.. Tetrahedron 16229. [I81 Andrus M.B., Asgari D., Sclahni J.A., J. Org. (1997) 9365. I191 (a) Kawasaki K., Tsumura S., Katsuki T., (1995) 1245; (b) Kohmura Y., Kawasaki K., Katsuki T., (1997) 1456; (c) Kawasaki K., Karsuki T., Tetrahedron 6337.
7
63
37 (1996) Pamment 53 (1397) Chem.
62
Synlett
12
Synlett
12
53 (1997)
23
1999 and
organometallic
synthesis
Recent advances in asymmetric allylic oxidation of olefins Jean-Michel BRUNEL *, 01 ivier LEGRAND,
Gerard BUONO *
!%ole nationale suptrieure de spnthkses, UMR CNRS 65 16, facuk de St-Jt%me, E-mail: [email protected]
de procPd& et d’in&ierie chimiques a~. Escadrille-Normandie-Niemen,
(Received
after revision
14 September
13?8,
accepted
copper
12 November
d’Aix-Marseille 13377 Marseille
(ENSSP1C.W).
cedex 20, France
1778)
Abstract - This review deals with the recent advances in asymmetric copper allylic oxidation of olefins. An exhaustive analysis of the results has been realized showing the limitation of the different asymmetric catalytic systems used. 0 Acadtmie des sciences/Elsevier, Paris allylic
esters
/ allylic
oxidation
/ asymmetric
catalysis
/ chiral
copper
complexes
Version frangaise abrCgCe - DCveloppements rkents dans I’oxydation allylique asymbtrique catalytique des okfines par des complexes chiraux du cuivre. La fonctionnalisation stMos&lective des okfines non activkes reprksente B I’heure actuelle un challenge important pour Ie chimiste organicien. En effet, cette mkthodologie reprtsente une voie d’accks i I‘obtention d’une grande classe de mokcules d’un grand ink&t synthetique. Une des mkhodes les plus d&eloppkes fair intervenir un caralyseur ?I base de sels de cuivre (Cu(OAc):, Cu(OTf), Cu(OTf),...) et d’un oxydant (H:O,, hydroperoxyde de tert-butyl, peroxyester). Les cycloalcknes sont ainsi oxydk en esters dont l’hydmlyse conduit aux alcools cyclaniques u-allyliques. En presence d’un ligand chiral, les hydrogPnes knantiotopiques allyliques sont discriminks et substituk par un carbon+ oxy. La grande difficult6 de cette mkhode &side dans le contrdle de I’CnantiosklectivitP de la reaction. Plusieurs sysckmes catalytiques ont kemment hit I’objet d’itudes, et des auxiliaires chiraux tels que les aminoacides, ou bien les oxazolines, ont 6rP utilis& en tant que l&and du cuivre. Ces derniers se sont ainsi r&&s &re les meilleurs vecteurs de chiralitt, les excks Pnantiomkiques restant nCanmoins modestes dans la plupart des autres cas er n’atteignant que trPs raremenr 90 %I e.e. Cette mise au point relate ainsi Ies principaux developpements r&&s& B ce jour dans l’oxydation allylique asymktrique des okfines catalyke par des complexes chiraux du cuivre. Sur la base de I’analyse des rkultats exptrimentaux, un mkanisme rendant compte de cette reaction d’oxydation allylique a pu stre tgalement propok. 0 Academic des sciences/Elsevier, Paris esters
allyliques
/ oxydation
allylique
/ catalyse
asymktrique
Stereoselective functionalization of unactivated olefins is a very active and challenging area of research with many potential synthetic applications [ 11. Thus, metal-induced allylic oxidation of alkenes has appeared to be one of the best methods of synthesis and has been the subject of severalinvestigations in literature 121. Apart from radical-initiated reactions [3], reac-
Communicated
* Correspondence
1251~8069/99/00020019
by
Henri
/ complexes
chiraux
du cuivre
tions based on selenium [4] or palladium [5] systems have attracted considerable interest. Among all these methods, Kharasch and Sosnovsky were the first to report the allylic oxidation of olefks with tert-butyl perbenzoate and a caralytic amount of a copper(I) salt [6]. On the other hand, except enzymatic methods [7], allylic esterswere obtained in low enanti-
KAGAN.
and reprints.
0 Acadkmie
des sciences/Elsevier,
Paris
19
J.-M.
Brunei
et al.
omeric excess (e.e.). Nevertheless, new catalytic enantioselective allylic oxidation methods of olefins with chiral copper catalyst and peroxyesters as oxidants have been recently developped. In 1965, Denney et al. describe the use of a stoichiometric amount of a chiral cuprous salt prepared from copper acetate (Cu(OAc),) and a-ethyl camphorate [8]. In this case, only low enantiomeric excesses for cyclic alkenes and no asymmetric induction for acyclic substrates have been detected. On the basis of these results, Muzart et al. have improved the enantioselective copper catalyzed acetoxylation of cyclic alkenes using a tert-butyl hydroperoxide (TBHP)/acetic acid mixture as oxidant. In this system, the chiral copper catalyst was prepared by mixing CU(OAC)~ and L-proline 1 (8 equiv. with respect to the copper source) [9]. Under these conditions, cyclohexenyl acetate hasbeen obtained with an enantiomeric excess up to 30 % e.e. This system hasbeen improved using tert-bury1 perbenzoate (TBPB) and copper oxide (CuZO) instead of respectively TBHP and copper acetate, increasing the enantioselectivity up to 59 % e.e. (scheme1) [lo].
cyclic alkenesusing a catalytic system(2.5 mol% with respect to the substrate) prepared from Cu(OA&, L-p ro 1ine and copper bronze (ratio 1:6:2O) [ 121. In this case, metallic copper acts as a reducing agent of copper(I1) and under these conditions, cyclohexenyl propionate has been prepared in 70 % yield and 57 % e.e.. In the sameway, acetic acid led to the formation of cyclohexenyl acetate in 71 % yield and 53 % e.e. (scheme2). The same group has demonstrated that the use of TBHP asoxidant in the presence of 4 equiv. of anthraquinone (with respect to the copper source) increases the enantioselectivity (60 % e.e.) while the yield remains quite the same [ 131. Anderson et al. have extended this catalytic system using rigid amino acids 1 and 2 (scbeme3). The results are slightly better than those previously obtained but the amounts of the chiral catalyst used are more important
[141. Recently, Pfaltz et al. were the first to describe the use of a catalytic amount of a chiral copper complex generated from cuprous triflate and a bisoxazoline 4a-b or 5a ligand (scheme4) [ 151. E.e.‘s up to 82 % have been obtained at 7 “C using tert-butylperbenzoate as oxidant in acetonitrile. In the caseof cyclopentene, a decrease of temperature from 7 to -20 “C increasesthe enantioselectivity from 79 % to 84 % e.e. but in detriment to the reaction rate (scheme S)
The importance of a copper(I) salt is not well established. Thus, the use of a bis(prolinato) copper(I1) complex, has no significant influence on the outcome of the reaction in terms of conversion and enantioselectivity [ 1 I]. In the same way, Feringa et al. have described the preparation of chiral allylpropionates from
5
OCOR p
mol%catalytic system
0
[la.
b
(“1 n
Cu(OAc), / 1 = 1 / 8 Solvent : AcOH n=2 ee = 28% Yield = 52% ($0 n=l n=2
/ 1 = I/ 2.5 ee = 59% ee = 45%
Solvent : CH,CN
Oxidant : TBHP
Oxidant : TBPB
Yield = 47% Yield = 59% Scheme 1. 9COR _
Cu(OAc), (2.5 mol%) 1 (6 eq./Cu) 0.10 (20 equiv./Cu) TBPB , RCOOH CJrl$N
+3
Scheme 2.
20
R = Et
conv. = 70% ee = 57%
R = Me
conv. = 71% ee = 53%
Recent
Cu(OAc),
advances
in asymmetric
(5 mol%)
&w
2 or 3 (5 equiv./Cu) pCoph Cue(10 equiv./Cu) -I 0 TBPB , PhCOOH
2
(9 n
n=l n=2
Benzene Scheme
copper
allylic
‘H
‘H
3
Yield = 54% ee= 60% Yield = 64%
ee = 40%
3.
a x=i-Pr 4
of olefins
&cop
a Y=H b Y=Ph
b X=t-Bu x
oxidation
x Scheme 4.
Catalytic enantioselective allylic oxidation of 1-methylcyclohexene as substrate led to a mixture of three regioisomers6,7,8 with e.e. lower than for non-substituted cycloalkens, except for regioisomer 6 obtained with an e.e. up to 90 % but in a low chemical yield (scheme6). Andrus et al. have developped a similar useof chiral bisoxazolines 4b for the preparation of optically active allylic benzoate [ 171. Thus, cycloalkenes led to the corresponding allylesters with high e.e. varying from 70 to 80 %. Moreover, allylbenzene and l-octene were regioselectively converted into the terminal allylic ester with e.e. up to 36 % (scheme7). The use of substituted peresters and new bisoxazoline ligand 9 at -20 “C led to identical results in terms of enantiomeric excess and
chemical yield but a significant decreaseof the reaction
time
In all these cases,the most effective catalysts are those generated from cuprous salt. Nevertheless, Katsuki et al. have reported that the copper catalyst prepared from Cu(OTf), and chiral trioxazoline is an effective catalyst for the enantioselective catalytic allylic oxidation of cycloalkenes by tert-butyl perbenzoate [ 191. The corresponding allylic ester of cyclopentene hasbeen obtained in 76 % e.e. and 83 % yield at room temperature. A decreaseof temperature led to an increase of the enantiomeric excess at the expense of the chemical yield. Moreover, the useof molecular sieveshasa beneficial effect on the outcome of the reaction increasing the reaction rate and the enantioselectivity (scheme9).
CuOTf (5 mol%)
n ee(%) yield (%)
L*
1
4b 4b 4a Sa 5b
2 Scheme
(scheme 8) [ 181.
79
65
81
64 44 80 58
5.
CuOTf(5 mol%) 4a (6
mol%)
l
TBPB, acetone, O°C Yield : @Z:
r
&c-
&ocoph
6
7
8
9% 90%
50% 63%
41% 13%
Scheme 6.
21
J.-M.
Brunel
et al.
CuOTf (5 mol%) 4b (6
R-H-------*
by copper(I) to give a copper(I1) benzoate intermediate and tert-butoxy radical This radical may abstract an allylic hydrogen atom to give tert-butanol and an allylic radical. Rapid addition of copper(I1) to the ally1 radical to generate copper(M) benzoate with the bound ally1 fragment then occurs, appearing as the key step for the induction of chirality. The final step of the chain mechanism is rearrangement of the copper(II1) intermediate to give the product and regenerate the copper(I) catalyst.
OCOPh
mol%)
TBPB, benzene, 55°C R: phenyl ee= 36% yield=34% n-pentyl ee = 30% yield = 50% Scheme 7.
OBz-p-NO,
z
9
In summary, the synthesis of optically active allylic esters may be achieved by direct allylic oxidation of alkenesusing various chiral copper catalysts by transferring the carboxylate group to one of the diastereotopic allylic carbons. Nevertheless, only the cyclic olefins led to good results, the acyclic alkenes providing the corresponding allylic estersin poor yields and selectivities. Thus, although the enantioselective Karash-Sosnovsky reaction is competitive compared to the results obtained with other transition-metal catalyst, it is expected that a more thorough understanding of the enantiodiscriminating event in theseprocesseswill result in the development of more efficient catalysts.
5 days Yield = 76%
-2OT,
ee = 73%
Scheme 8.
All these results clearly indicate that the copper catalyst is able to transfer the chirality during the formation of the carbon-oxygen bond. A catalytic cycle may be envisioned for the catalytic asymmetric copper allylic oxidation of olefins (scheme10). The mechanism has been shown to involve homolysis of the perester oxygen-oxygen bond
WOTf), (5 moI%)
0I
9 (7.5
d
mol%)
OCOPh
T (“C)
Yield (%) Ee (%) 83
76 *
68
74
30
93 *
11
88
20
TBPB, acetone, MS 4A
-20 N 9
3
* Reaction performed in presence of MS 4w Scheme 9.
ROOCOR’ R = Terr-butyl R’=Ph
Cu(II)(OCOR’)
RO’
ROH
22
Recent
advances
in asymmetric
111(a)
[21 (a)
Walling C., Zavitsas A.A., J. Am. Chem. Sot. 85 (1963) 2084; (b) Kochi J.K., J. Am. Chem. Sot. 84 (1962) 774; (c) Goering H.L., Mayer U., J. Am. Chem. Sot. 86 (1964) 3753; (d) Bulman Page PC., McCarthy T.J., in: Trost B.M., Fleming 1. (Eds.), Comprehensive Organic Synthesis, Pergamon Press, Oxford, 1991, vol. 7, pp. 83-l 17.
[31 Stephenson L.M., Grdina M.R.. Chem. Res. 13 (1980) 4 19.
Orfanopoulor
M.. Act.
M.A., Sharpless K.B., J. Am. Chem. Sot. [41 (a) Umbreit 99 (1977) 5526; (b) Stevenson L.M., Speth D.R., J. Org. Chem. 44 (1979) 4683: (c) Sharpless K.B., Latter R.F., J. Am. Chem. Sac. 94 (1972) 7154; (d) Sharpless K.B., Latter R.F., J. Org. Chem. 39 (1974) 429; (e) Reich H.J., J. Org. Chem. 39 (1974) 428; (0 Engman L., J. Org. Chem. 54 (1989) 889: (g) Wirth T., Hauptli S., Leuenberger M., Tetrahedron: Asymmetry 9 (1998) 547. H., BackvaIl J.E., in: Balm C.. Be&r M. J51 (a) Greenberg (Eds.), Transition Metals for Fine Chemicals and Organic Synthesis, VCH, Weinheim, 1998; (b) Heumann A., Akermark B.: Angew. Chem., Int. Ed. Engl. 23 (1984) 453; (c) Principato B., Maffei M., Siv C., Buono G., Peiffer G.. Tetrahedron 52 (1996) 2087.
allylic
oxidation
of olefins
(a) Kharasch M.S., Sosnovsky G., J. Am. Chem. Sot. 80 (1958) 756; (b) Kharasch M.S., Sosnovsky G., Yang N.C., J, Am. Chem. SOC. 81 (1959) 5819; (c) Sosnovsky G., Rawlinson D.J., in: Svern D. (Ed.), Organic Peroxides, Wiley, New York, 1970, vol. 1.
References Fukazana T., Hashimoto T., Tetrahedron: Asymmetry 4 (1995) 2323; (b) Gupta A.K., Jaszlauskas R.J., Tetrahedron: Asymmetry 4 (1993) 879; (c) Asami M., Ishizaki T., Inoue S., Tetrahedron: Asymmetry 5 (1994) 793; (d) Araki M., Nagase T., Chem. Abstr. 86 (1977) 120886r.
copper
PI
Wong C.H., Whitesides G.H., Enzymes in Synthetic Organic Chemistry, Pergamon Press. New York, 1994.
[81 Denney
D.B., Napier R., Cammarata A., J. Org. Chem. 30 (1965) 3151. [‘)I (a) Muzart J., Bull. Sot. Chim. Fr. (1986) 65; (b) Muzart J., J. Mol. Caral. 64 (1991) 381. Synth. Commun. 2j (1995) 1789. [lOI Ievina A., MuzartJ.. Asymmetry 6 (1995) [ill Levina A., Muzarr J., Tetrahedron: 147. C., Feringa B.L., Tetrahe1121Rispens M.T., Zondervan dron: Asymmetry 6 (1995) 661, C., Feringa iI31 Zondervan (1996) 1895. M.J., [I41 Sodergren (1996) 7577.
B.L., Tetrahedron:
Anderson
PG.,
Tetrahedron
A.S., Minidis A.B.E., [I51 (a) Cokhale hedron Lett. 36 (1995) 1831; (b) Sekar G., Gupta A.D., Singh VK.. (1998) 2961.
[161 Gupta
Asymmetry
Pfaltz
Lett.
37
A., Terra-
J. Org. Chem.
A.D., Singh V.K., Tetrahedron Lett. 2633. [171 (a) Andrus M.B., Argade A.B., Chen X., M.G., Tetrahedron Lett. 36 (1995) 2945; (b) Andrus M.B., Chen X.. Tetrahedron 16229. [I81 Andrus M.B., Asgari D., Sclahni J.A., J. Org. (1997) 9365. I191 (a) Kawasaki K., Tsumura S., Katsuki T., (1995) 1245; (b) Kohmura Y., Kawasaki K., Katsuki T., (1997) 1456; (c) Kawasaki K., Karsuki T., Tetrahedron 6337.
7
63
37 (1996) Pamment 53 (1397) Chem.
62
Synlett
12
Synlett
12
53 (1997)
23