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Cobalt-Schiff Base Complex Catalyzed Dehydrogenation of Amines with T-Butyl Hydroperoxide
L.I. Simindi (Editor 1, Dioxygen Activation and Homogeneous Catalytic Oxidation 0 1991 Elsevier Science Publishers B.V., Amsterdam
489
COBALT-SCHIFFBASE COMPLEX CATALVZED DEHYDROGENATION OF ANIINES WITH t-BUTYL HYDROPEROXIDE KAZUSIGE IvIARUYAMA, TAKAHIRO KUSUKAWA. YOSHIHIKO HIGUCHI and AKIFU NISHINAGA* Department of AppZied Chemistry, Osaka Institute of Technology. Ohrniga 5, Asahi-ku, Osaka 535, Japan summary
Cobalt(I1)-Schiffbase complexes [Co11(SB)] catalyzed the oxidation of amines with t-butyl hydroperoxide (TBHP) as the terminal oxidant resulting selectively in dehydrogenation to imines. In the oxidation of substituted N-benzylanilines. the reaction rate was independent of the p-substituent in the benzyl moiety but dependent on the p-substituent in the aniline part with a p value of -1.1 against the O+ constant. p-Substituted N-t-butyl benzyl amines also undergo the dehydrogenation by the TBHP-CO~WB)system, where no substituent effect was observed on the reaction rate. Addition of 2,4.6-tri-t-butylphenol.a radical scavenger, to the oxidation system accelerated the reaction unexpectedly. Proposed mechanism involves electron transfer from the nitrogen atom in the substrate to t-butyloxyl or t-butylperoxyl radical generated by the decomposition of t-butylperoxocobalt(II1)complex formed In situ Introduction Metal-catalyzed oxidations are very important in connection with biological as well as synthetic processes [l]. Cobalt(I1)-Schiff base complexs [Coil(SB)] are of paticular interest because they function as artificial metal containing oxidoreductases including dioxygenases, monooxygenases, and peroxidases : in aprotic solvents, where Co11 (SB) complexes bind molecular oxygen reversibly, dioxygenations take place with organic molecules related to substrates for some dioxygenase reactions [21. In alcohols, on the other hand, CoII(SB) complexes are irreversibly oxidized with molecular oxygen to give hydroxocobalt(II1) or alkoxocobalt(II1) species [3].where monooxygenation of phenols [4], hydrazones [51. nitroalkanes 161. and alkenes [7,8]. Peroxydase-type activity of Col1(SB)can be seen in the CoII(SB) catalyzed dehydrogenation of alcohols with t-butyl hydroperoxide (TBHP) as the terminal oxidant 191. Metalcatalyzed dehydrogenation of amines is also important area in organic synthesis 110.111. The present paper deals with the Co11(SB) catalyzed oxidation of amines using TBHP as the terminal oxidant. I t has been
490
found that the TBHP-CoII(SB) system can oxidize m i n e substrates which are unsusceptible to the Oz-CoII(SB) system [ 111. Electron transfer from the nitrogen atom in the substrate to the t-butyloxyl or t-butyl peroxyl radical produced in situ is suggested to the present dehydrogenation process on the basis of kinetic data. Results and Discussion A solution of TBHP (4.8 mmol), amine (1) (4.0 mmol), and Co(L1) (0.8 mmol) in dimetyl sulfoxide (DMSO) (20 ml) was warmed at 4OoC for 8h. The reaction mixture was then diluted with a saturated brine (200 ml) and extracted with ether. After the ether solution was washed with brine, dried over sodium sulfate and evaporated, the residue was chromatographed on a Florisil column with ether as the eluent. Products were purified by vaccum distillation. The structures of the products were identified by comparison with authentic samples (1H NMR, IR). The results obtained in the oxidation of substituted benzyl anilines (1) are summarized in Table 1.
1
3
2
a;X=Y=H d ;X = CI, Y = H g ;X = H, Y = Me j ; X = Y = Me
b ; X = OMe, Y = H
e ;X = H,Y = NO, h ; X = H, Y = CI k;X=Y=CI
c ;X = Me, Y = H f ; X = H, Y = OMe I ; X = Y = OMe
Co"( L)
Co"(L2) ;
z=
Co"(L3) ;
z=
a
h e he CO"(L9 ; Z = C(CH&C(CH,), CO"(L6) ; Z = (CH,),NH(CH,),
As seen from Tablel, no reaction took place without the cobalt catalyst (Run 1). and the dehydrogenation of 1 was generally catalyzed by Co(L1). The p-substituent X in the aniline moiety influenced the reaction rate (Run 3, 41, whereas nearly no substituent effect was observed with the substituent Y in the benzylic group ( Run 6 - 9). Similar results were obtained in the stoichiometric reaction of 1 with Co(Ll)(t-BuOO) 1121. indicating that the reactive species in the catalytic reaction should be
491
TABLE 1 Oxidation of amine (1)with TBHP catalyzed by CO(L').~
1 2 3 4 5 6 7 8 9 10
11 12 a
1
Ic01 /I1]
la
0 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2
la
Ib Ic Id le If
lg Ih li
Ij
Ik
Product
Conversion Reaction time (h)
(%I
0 91 100 98 97 97 93 96 97 98 99 94
8 8 0.75 4 8 8 8 8 8 0.25 4 8
0.2 0.2
(%)b
2
3
0 94 95 94 96 90 93 96 98 100
0 6 5 6 4 10 7 4 2 0
96 97
4 3
Amine (1) (4mmol). TBHP (4.8mmol). Co(L') (0.8mmol). DMSO (20ml). 40°C
Determined by 'H NMR
(?-I3 Ph-CH-NH-Ph
Ph-CH2-NH-CH2-Ph
Y-
5
6
Ph-C=N-Ph
Ph-CH=N-CHZ-Ph
Ph-CH=N-Buf
7
8
4 ?33
YH3
10
9
YHO
Ph-N-CH2-Ph
Ph-N-CHy Ph
11
12
the t-butylperoxocobalt (111) complex, which is rapidly formed by the reaction of CoI1(L1)with TBHP 1121. In the dehydrogenation process using the 02/Co(Ll) system, a sterically hindered aniline derivatives 4 and N-benzyl aliphatic amines (5. 6) were unsusceptible to the dehydrogenation (no reaction in 24h). although
492
N-benzylanilines (1)were effectively oxidized to give 2 and 3 1111. On the contrary, the present TBHP/Co(Ll) system was found to be effective for the dehydrogenation of these compounds 4,6 and 6 (Y = H) selectively to give 7 (37% in 8h), 8 (85% in 4h), and 9 (95% in 1Oh). Compound 10. on the other hand, was not reactive against the TBHP/Co(L11 system. Therefore, a benzylidene type conjugation system in the products seemes to be important for the dehydrogenation taking place. Furthermore, compound 11, which is absolutely stable against the 02/Co(L1) system, was oxidized effectively by the TBHP/Co(L1) system (93% conversion in 6h) to give la (12%). 2a (28%). 3a (17%). and Nbenzyl formanilide 1 2 (70). The results resemble those obtained in the cytochrome p-450 oxidation system [ 131. Since no reaction took place when 1 2 was treated with the TBHP/Co(Ll) system under similar conditions for the oxidation of 11, compounds 2a and 12 were the products competitively formed. The competition reaction may be rationalized by assuming an electron transfer from the nitrogen atom of 11 to the oxidant, probably a t-butyloql radical produced by the decomposition of Co(L1)(OOBut), resulting in the formation of radical cation 13. The radical cation 13 can undergo further deprotonation and succeeding one-electron oxidation eventually to give iminium cation 14, into which TBHP and water are competitively added. The decomposition of the resulting intermediates 15 and 16 would give 12 and 2a, respectively.
11
-e
-H+,-e
4 7
+
Ph-N-CH2-Ph ' II CH2 14
Ph-N-CH2-Ph 15
7H2OH Ph-N-CH2-Ph 16
Hammett plots for the oxidation of 1 (Y = H) against G+ values gave a straight line ( P = -1.1) (Fig.1). The results support the mechanism involving electron transfer from the nitrogen atom of the substrate. Similar results are also obtained with the 02/Co(Ll) system ( P = -1.8 against G+) [ 111. Figures 2 and 3 show time dependent dehydrogenations of p-substituted benzylaniline 1 (X = H) and N-t-butyl-p-substituted benzylamine 6. As seen from these Figures, almost no substituent effects were observed. These data also support the electron transfer mechanism. Interestingly, when the Co(L1) catalyzed oxidation of lb (4 mmol) with TBHP (4.8 mmol) (the same conditions shown in Table 1) was carried out in the presence of an equivalent amount (4 mmol) of 2.4,6-tri-tbutylphenol, the reaction was completed within 3 minutes. In other word, the reaction was accelerated by the addition of the phenol, which is known well as a radical scavenger. It is. therefore, obvious that the
493
1.5 x-@H-CH,-O-Y -
;=" 0
4
''O
-
- P-OMe p = -1.1
0.5 -
P-CI
0.0 I
I
I
I
I
I
100
80
,S 60
2 5
u
40 A
0
20
0
A
0
2
4 Time/h
6
8
0
2
4
6
: Me0 :Me :H
:c1
:NO, 8
Time/h
Fig. 2 Time dependent dehydrogenation Fig. 3 Time dependent dehydrogenation of of p-substituted benzylaniline 1 (X=H) N-t-butyl-p-substituted benzyl amines 6
present oxidation of amines involves no hydrogen abstraction by any radical species. Since it has been known that the TBHP/Co(Ll) system readily oxidizes 2.4.6-tri-t-butylphenol to the 2.4.6- tri-t-butylphenoxyl radical 1141. the acceleration of the dehydrogenation of l b by the addition of 2,4,6-tri-t-butyl phenol can be reasonably explained in terms of electron transfer from l b to the 2.4.6-tri-t-butylphenoxylradical. Actu-
494
ally, when a solution of 2.4,6-tri-t-butylphenoxylradical in benzene sepratery prepared was mixed with a solution of lb in DMSO without the cobalt catalyst under nitrogen atmosphere, very rapid formation of 2b was observed. From all these findings, the present dehydrogenation of amines using the TBHP/Co(L1) system is rationalized by the mechanizm depicted in
-
2 co"(L) + 3 t-BuOOH 2 Co"'(L)(OOBut) + t-BuOH + H2O 2 CO"'(L)(OOBU~) 2 Co"(L) + 2 t-BuO6 2 t-BuOt) c2t-Bub +02 Arm-CH+' + t-Bu6 Ar'h-CH2Ar' + t-BuOor ArNH-CH&' + t-BuO6 k Ar+%-I-CH$r'+ t-BuOOw Arfi-CHW + H+ Arh-CH2Ar' ArkCH&-' + t-Bu6 .fastArN=CHAr' + t-BuOH or ArrfT-CHW + t-BuO6 ArN=CHAr' + t-BuOOH fast t-BuO- + H+ t-BuOH fast o r t-BuOO- + H+ * t-BuOOH
-
( 1)
(2) (3) (4) (4')
(5) (6) (6')
(7) (7')
Scheme 1 100
80
-s
60
v
s
8
40
20
0
1
2
3 4 Time(h)
5
6
Fig. 4. Catalytic activity of Co(L)in the oxidation of la with TBHP (inDMSO at 4OOC) . W') c a y (rr). w3)rn). 6).con7 (01. (A).
m),
w?
coo
495
Scheme 1. Reactions (1) and (2)occur rapidly [14].The following electron transfer reaction (4)or (4')should be the rate controlling step. There is no evidence to specify which one. t-butylperoxyl radical or tbutyloxyl radical, is the real oxidizing species. After the formation of m i n e radical cation, the succeeding reactions should be rapid processes. A small value of deuterium isotope effect k~/k~=1.3 observed for the oxidation of PhNHCDzPh also supports the mechanism shown in Scheme 1. Reactivity of CobaltO-Schlff Base Complexes
Figure 4 shows the catalytic efficiency of Cobalt(I1)-Schiff base complexes [CoII(L)I.The reactivity of CoII(L) was in the order : C011(L5) > CoII(L1) > Co"(L2) > Co"(L3) > Co"(L4) > Co"(L6). The observed reactivity of Co"(L) may'be attributed to the ability of homolytic cleavage of the Co-0 bond in Co(L)(OOBut). which should be correlated to the reC0111 couple. Detailed mechanism of the present dox potential of Co" dehydrogenation is currently investigated.
=
References 1 (a)R.A.Sheldon and J.K.Kochi, "Metal-Catalyzed Oxidations of Organic Cornpounds",Acadernic Press,(1 981). ; (b) Sharpless, Aldrichimica Acta, 12 (1979)63. 2 A.Nishinaga, Protain, Nucleic Acid, Enzymes. 26 (1983)214 (ISCN 0371-9565). 3 A.Nishinaga, T.Kondo and T.Matsuura, Chem Lett., (1985)905. 4 A.Nishinaga. H.Tomita and T.Matsuura, Tetrahedron Lett.. 21 (1980) 1261. 5 A-Nishinaga, S.Yamazaki and T.Matsuura, Tetrahedron Lett.. 27 (1986) 2649. 6 A.Nishinaga, S.Morikawa, K.Yoshida and T.Matsuura, Nippon Kagaku Kaishi, (1988)487. 7 D.E.Hmilton. R.S.Drago and A.Zombeck. J. Am, Chem Soc., 109 (1987)374. 8 ANishinaga. T.Yarnada. H.Fujisawa, K.Ishizaki. HJhara and T.Matsuura, J. Mol. Catal., 48 (1988)249. 9 A.Nishinaga, 1.Sugimoto and T.Matsuura. Nippon Kagaku Kaishi, (1988)495. 10 S.Murahashi, T.Naota, and H.Takai. J. Chem Soc., Chem Commun., (1985)613. 1 1 A.Nishinaga. S.Yamazaki and T.Matsuura, Tetrahedron Lett., 29 (1988)4115. 12 A.Nishinaga, H.Ohara and H.Tomita. Chern. Lett., (1983)1751. 13 M.W.Nee, T.C.Bruice, J. Am Chem Soc.,l06 (1984)1514. 14 A.Nishinaga. H.Nogusa, T.Yamashita and T.Matsuura. Nippon Kagaku Kaishi. (1985)378.
489
COBALT-SCHIFFBASE COMPLEX CATALVZED DEHYDROGENATION OF ANIINES WITH t-BUTYL HYDROPEROXIDE KAZUSIGE IvIARUYAMA, TAKAHIRO KUSUKAWA. YOSHIHIKO HIGUCHI and AKIFU NISHINAGA* Department of AppZied Chemistry, Osaka Institute of Technology. Ohrniga 5, Asahi-ku, Osaka 535, Japan summary
Cobalt(I1)-Schiffbase complexes [Co11(SB)] catalyzed the oxidation of amines with t-butyl hydroperoxide (TBHP) as the terminal oxidant resulting selectively in dehydrogenation to imines. In the oxidation of substituted N-benzylanilines. the reaction rate was independent of the p-substituent in the benzyl moiety but dependent on the p-substituent in the aniline part with a p value of -1.1 against the O+ constant. p-Substituted N-t-butyl benzyl amines also undergo the dehydrogenation by the TBHP-CO~WB)system, where no substituent effect was observed on the reaction rate. Addition of 2,4.6-tri-t-butylphenol.a radical scavenger, to the oxidation system accelerated the reaction unexpectedly. Proposed mechanism involves electron transfer from the nitrogen atom in the substrate to t-butyloxyl or t-butylperoxyl radical generated by the decomposition of t-butylperoxocobalt(II1)complex formed In situ Introduction Metal-catalyzed oxidations are very important in connection with biological as well as synthetic processes [l]. Cobalt(I1)-Schiff base complexs [Coil(SB)] are of paticular interest because they function as artificial metal containing oxidoreductases including dioxygenases, monooxygenases, and peroxidases : in aprotic solvents, where Co11 (SB) complexes bind molecular oxygen reversibly, dioxygenations take place with organic molecules related to substrates for some dioxygenase reactions [21. In alcohols, on the other hand, CoII(SB) complexes are irreversibly oxidized with molecular oxygen to give hydroxocobalt(II1) or alkoxocobalt(II1) species [3].where monooxygenation of phenols [4], hydrazones [51. nitroalkanes 161. and alkenes [7,8]. Peroxydase-type activity of Col1(SB)can be seen in the CoII(SB) catalyzed dehydrogenation of alcohols with t-butyl hydroperoxide (TBHP) as the terminal oxidant 191. Metalcatalyzed dehydrogenation of amines is also important area in organic synthesis 110.111. The present paper deals with the Co11(SB) catalyzed oxidation of amines using TBHP as the terminal oxidant. I t has been
490
found that the TBHP-CoII(SB) system can oxidize m i n e substrates which are unsusceptible to the Oz-CoII(SB) system [ 111. Electron transfer from the nitrogen atom in the substrate to the t-butyloxyl or t-butyl peroxyl radical produced in situ is suggested to the present dehydrogenation process on the basis of kinetic data. Results and Discussion A solution of TBHP (4.8 mmol), amine (1) (4.0 mmol), and Co(L1) (0.8 mmol) in dimetyl sulfoxide (DMSO) (20 ml) was warmed at 4OoC for 8h. The reaction mixture was then diluted with a saturated brine (200 ml) and extracted with ether. After the ether solution was washed with brine, dried over sodium sulfate and evaporated, the residue was chromatographed on a Florisil column with ether as the eluent. Products were purified by vaccum distillation. The structures of the products were identified by comparison with authentic samples (1H NMR, IR). The results obtained in the oxidation of substituted benzyl anilines (1) are summarized in Table 1.
1
3
2
a;X=Y=H d ;X = CI, Y = H g ;X = H, Y = Me j ; X = Y = Me
b ; X = OMe, Y = H
e ;X = H,Y = NO, h ; X = H, Y = CI k;X=Y=CI
c ;X = Me, Y = H f ; X = H, Y = OMe I ; X = Y = OMe
Co"( L)
Co"(L2) ;
z=
Co"(L3) ;
z=
a
h e he CO"(L9 ; Z = C(CH&C(CH,), CO"(L6) ; Z = (CH,),NH(CH,),
As seen from Tablel, no reaction took place without the cobalt catalyst (Run 1). and the dehydrogenation of 1 was generally catalyzed by Co(L1). The p-substituent X in the aniline moiety influenced the reaction rate (Run 3, 41, whereas nearly no substituent effect was observed with the substituent Y in the benzylic group ( Run 6 - 9). Similar results were obtained in the stoichiometric reaction of 1 with Co(Ll)(t-BuOO) 1121. indicating that the reactive species in the catalytic reaction should be
491
TABLE 1 Oxidation of amine (1)with TBHP catalyzed by CO(L').~
1 2 3 4 5 6 7 8 9 10
11 12 a
1
Ic01 /I1]
la
0 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2
la
Ib Ic Id le If
lg Ih li
Ij
Ik
Product
Conversion Reaction time (h)
(%I
0 91 100 98 97 97 93 96 97 98 99 94
8 8 0.75 4 8 8 8 8 8 0.25 4 8
0.2 0.2
(%)b
2
3
0 94 95 94 96 90 93 96 98 100
0 6 5 6 4 10 7 4 2 0
96 97
4 3
Amine (1) (4mmol). TBHP (4.8mmol). Co(L') (0.8mmol). DMSO (20ml). 40°C
Determined by 'H NMR
(?-I3 Ph-CH-NH-Ph
Ph-CH2-NH-CH2-Ph
Y-
5
6
Ph-C=N-Ph
Ph-CH=N-CHZ-Ph
Ph-CH=N-Buf
7
8
4 ?33
YH3
10
9
YHO
Ph-N-CH2-Ph
Ph-N-CHy Ph
11
12
the t-butylperoxocobalt (111) complex, which is rapidly formed by the reaction of CoI1(L1)with TBHP 1121. In the dehydrogenation process using the 02/Co(Ll) system, a sterically hindered aniline derivatives 4 and N-benzyl aliphatic amines (5. 6) were unsusceptible to the dehydrogenation (no reaction in 24h). although
492
N-benzylanilines (1)were effectively oxidized to give 2 and 3 1111. On the contrary, the present TBHP/Co(Ll) system was found to be effective for the dehydrogenation of these compounds 4,6 and 6 (Y = H) selectively to give 7 (37% in 8h), 8 (85% in 4h), and 9 (95% in 1Oh). Compound 10. on the other hand, was not reactive against the TBHP/Co(L11 system. Therefore, a benzylidene type conjugation system in the products seemes to be important for the dehydrogenation taking place. Furthermore, compound 11, which is absolutely stable against the 02/Co(L1) system, was oxidized effectively by the TBHP/Co(L1) system (93% conversion in 6h) to give la (12%). 2a (28%). 3a (17%). and Nbenzyl formanilide 1 2 (70). The results resemble those obtained in the cytochrome p-450 oxidation system [ 131. Since no reaction took place when 1 2 was treated with the TBHP/Co(Ll) system under similar conditions for the oxidation of 11, compounds 2a and 12 were the products competitively formed. The competition reaction may be rationalized by assuming an electron transfer from the nitrogen atom of 11 to the oxidant, probably a t-butyloql radical produced by the decomposition of Co(L1)(OOBut), resulting in the formation of radical cation 13. The radical cation 13 can undergo further deprotonation and succeeding one-electron oxidation eventually to give iminium cation 14, into which TBHP and water are competitively added. The decomposition of the resulting intermediates 15 and 16 would give 12 and 2a, respectively.
11
-e
-H+,-e
4 7
+
Ph-N-CH2-Ph ' II CH2 14
Ph-N-CH2-Ph 15
7H2OH Ph-N-CH2-Ph 16
Hammett plots for the oxidation of 1 (Y = H) against G+ values gave a straight line ( P = -1.1) (Fig.1). The results support the mechanism involving electron transfer from the nitrogen atom of the substrate. Similar results are also obtained with the 02/Co(Ll) system ( P = -1.8 against G+) [ 111. Figures 2 and 3 show time dependent dehydrogenations of p-substituted benzylaniline 1 (X = H) and N-t-butyl-p-substituted benzylamine 6. As seen from these Figures, almost no substituent effects were observed. These data also support the electron transfer mechanism. Interestingly, when the Co(L1) catalyzed oxidation of lb (4 mmol) with TBHP (4.8 mmol) (the same conditions shown in Table 1) was carried out in the presence of an equivalent amount (4 mmol) of 2.4,6-tri-tbutylphenol, the reaction was completed within 3 minutes. In other word, the reaction was accelerated by the addition of the phenol, which is known well as a radical scavenger. It is. therefore, obvious that the
493
1.5 x-@H-CH,-O-Y -
;=" 0
4
''O
-
- P-OMe p = -1.1
0.5 -
P-CI
0.0 I
I
I
I
I
I
100
80
,S 60
2 5
u
40 A
0
20
0
A
0
2
4 Time/h
6
8
0
2
4
6
: Me0 :Me :H
:c1
:NO, 8
Time/h
Fig. 2 Time dependent dehydrogenation Fig. 3 Time dependent dehydrogenation of of p-substituted benzylaniline 1 (X=H) N-t-butyl-p-substituted benzyl amines 6
present oxidation of amines involves no hydrogen abstraction by any radical species. Since it has been known that the TBHP/Co(Ll) system readily oxidizes 2.4.6-tri-t-butylphenol to the 2.4.6- tri-t-butylphenoxyl radical 1141. the acceleration of the dehydrogenation of l b by the addition of 2,4,6-tri-t-butyl phenol can be reasonably explained in terms of electron transfer from l b to the 2.4.6-tri-t-butylphenoxylradical. Actu-
494
ally, when a solution of 2.4,6-tri-t-butylphenoxylradical in benzene sepratery prepared was mixed with a solution of lb in DMSO without the cobalt catalyst under nitrogen atmosphere, very rapid formation of 2b was observed. From all these findings, the present dehydrogenation of amines using the TBHP/Co(L1) system is rationalized by the mechanizm depicted in
-
2 co"(L) + 3 t-BuOOH 2 Co"'(L)(OOBut) + t-BuOH + H2O 2 CO"'(L)(OOBU~) 2 Co"(L) + 2 t-BuO6 2 t-BuOt) c2t-Bub +02 Arm-CH+' + t-Bu6 Ar'h-CH2Ar' + t-BuOor ArNH-CH&' + t-BuO6 k Ar+%-I-CH$r'+ t-BuOOw Arfi-CHW + H+ Arh-CH2Ar' ArkCH&-' + t-Bu6 .fastArN=CHAr' + t-BuOH or ArrfT-CHW + t-BuO6 ArN=CHAr' + t-BuOOH fast t-BuO- + H+ t-BuOH fast o r t-BuOO- + H+ * t-BuOOH
-
( 1)
(2) (3) (4) (4')
(5) (6) (6')
(7) (7')
Scheme 1 100
80
-s
60
v
s
8
40
20
0
1
2
3 4 Time(h)
5
6
Fig. 4. Catalytic activity of Co(L)in the oxidation of la with TBHP (inDMSO at 4OOC) . W') c a y (rr). w3)rn). 6).con7 (01. (A).
m),
w?
coo
495
Scheme 1. Reactions (1) and (2)occur rapidly [14].The following electron transfer reaction (4)or (4')should be the rate controlling step. There is no evidence to specify which one. t-butylperoxyl radical or tbutyloxyl radical, is the real oxidizing species. After the formation of m i n e radical cation, the succeeding reactions should be rapid processes. A small value of deuterium isotope effect k~/k~=1.3 observed for the oxidation of PhNHCDzPh also supports the mechanism shown in Scheme 1. Reactivity of CobaltO-Schlff Base Complexes
Figure 4 shows the catalytic efficiency of Cobalt(I1)-Schiff base complexes [CoII(L)I.The reactivity of CoII(L) was in the order : C011(L5) > CoII(L1) > Co"(L2) > Co"(L3) > Co"(L4) > Co"(L6). The observed reactivity of Co"(L) may'be attributed to the ability of homolytic cleavage of the Co-0 bond in Co(L)(OOBut). which should be correlated to the reC0111 couple. Detailed mechanism of the present dox potential of Co" dehydrogenation is currently investigated.
=
References 1 (a)R.A.Sheldon and J.K.Kochi, "Metal-Catalyzed Oxidations of Organic Cornpounds",Acadernic Press,(1 981). ; (b) Sharpless, Aldrichimica Acta, 12 (1979)63. 2 A.Nishinaga, Protain, Nucleic Acid, Enzymes. 26 (1983)214 (ISCN 0371-9565). 3 A.Nishinaga, T.Kondo and T.Matsuura, Chem Lett., (1985)905. 4 A.Nishinaga. H.Tomita and T.Matsuura, Tetrahedron Lett.. 21 (1980) 1261. 5 A-Nishinaga, S.Yamazaki and T.Matsuura, Tetrahedron Lett.. 27 (1986) 2649. 6 A.Nishinaga, S.Morikawa, K.Yoshida and T.Matsuura, Nippon Kagaku Kaishi, (1988)487. 7 D.E.Hmilton. R.S.Drago and A.Zombeck. J. Am, Chem Soc., 109 (1987)374. 8 ANishinaga. T.Yarnada. H.Fujisawa, K.Ishizaki. HJhara and T.Matsuura, J. Mol. Catal., 48 (1988)249. 9 A.Nishinaga, 1.Sugimoto and T.Matsuura. Nippon Kagaku Kaishi, (1988)495. 10 S.Murahashi, T.Naota, and H.Takai. J. Chem Soc., Chem Commun., (1985)613. 1 1 A.Nishinaga. S.Yamazaki and T.Matsuura, Tetrahedron Lett., 29 (1988)4115. 12 A.Nishinaga, H.Ohara and H.Tomita. Chern. Lett., (1983)1751. 13 M.W.Nee, T.C.Bruice, J. Am Chem Soc.,l06 (1984)1514. 14 A.Nishinaga. H.Nogusa, T.Yamashita and T.Matsuura. Nippon Kagaku Kaishi. (1985)378.