Model Catalytic oxygenations with Co(II)—Schiff base complexes and the role of cobalt-oxygen complexes in the oxygenation process

Journal of Molecular Cataly&r. 7 (1980) 179 0 metier Sequoia S.A., Lwsarme - printed

Ii9

- 199

in the pdeth=lancls

MODEL CATALYTIC OXYGEPITATLONS WITH Co(H)-SCHIFF BASE COMPLEXES AND TH_E ROL& OF CO-BALT-OXYGEN COMPLEXES IN THE OXYGENATiON E’Ft9CESS

AKIRA

NISHINAGA

Department (Japan)

and HARUO

of Synthetic

ChemLhy.

TOM?iTA Fucdty

of Engineering,

Kyoto

University,

Kyoto

Summary Catalytic activity of Co(H)-Schiff base complexes toward selective oxygenation of organic molecules related to biological systems has been reviewed. The complexes dispiay (i) dioxygenase-type activity (selective oxygenolyses of 3-substitu’ted indoles and flavonols, and regioselective dioxygen incorporation into Z,&di-t-butylphenols yielding peroxyquinolato Co(III) complexes), (ii) monooxygenaseand/or phenolase-type activity (selective formation ofp-quinols and benzoquinones from phenols), and (iii) peroxidase-type activity (one electron oxidation of phenols and oxidative cleavage of ether bond of p-phenoxyphenols) Preliminary kinetic studies on the regioselective peroxyquinolato Co(IEI) complex formation show that the reaction involves a ratedetemGning hydrogen abstraction from the phenol substrate by a superoxo Co(III) complex initially formed, followed by rapid reduction of the resulting phenoxy radical with Co(H) species leading to the formation of a phenolato Co(III) complex, to which dioxygen is incorporated. This mechanism may be supported by the reaction of phenols and phenoxy radicals with other superoxo Co(IIII) complexes, [(CN)sCo(III) _ In all the selective oxidations, the Pdl 3- and [(34VeOSalen)Co(III)(Oz)] role of cobalt-dioxygen complexes is to activate substrates, and the selectivity results from the coordination of thus activated substrates to Co(HI) species to form key intermediates, into which dioxygen is incorporated.

Introduction Biological oxidations are mostly catalyzed by iron and copper complexes situated in the reaction center of relevant enzymes [ 11. Heme complex participates in the catalytic center of dioxygenases (trrtophan 2,3dioxygenase and indoleamine 2,3dioxygenase; oxygenolysis of indole ring), monooxygenase (cy-tochrome P-450; partial incorporation of diosygen into organic substrates), and peroxidases (oxidation of organic substrates including dehydrogenation). Non-heme iron complexes are involved in pyro-

180

catechase (intradiol oxygenolysis of catechols) metapyrocatechase (extradiol oxygenolysis of catechols), and other di- and monooxygenases Cl]. Copper complexes participate in the reaction center of phenolase, lactase, quercetinase (oxygenolysis of 3hydroxyflavones). and SC on. The catalysis by these iron and copper complexes in enzymes is considered to be initialed by the activation of O2 in such a way that reactive dioxygen co.mplexes of types Fe-O-O and Cu-0-0-Cu are formed_ These dioxygen complexes are formally equivalent with those of heme and copper complexes in the natural oxygen carriers, hemoglobin (Hb) and hemocyanin (Hc). Hb;

(Heme)Fe

+ 0,

Hc;

zCu++o~

=

=+ (Heme)Fe-O-O [Cu-o-0-Cu]

a+_

The electronic structure of FeOa complexes is considered to be a supero.xo Fe(II1) comp!ex [ 2J _ However, the diamagnetic property of tne FeO, complex obscures its true electronic structure_ Many Co(I11) complexes coordina’%d with N-bases. on the other hand, have been demonstrati to reversibly intiract with 0, to form CoOz and Co,O, complexes depending on the natuze of the ligand, displaying a model of the two types of the natural O,-carriers [2 - 41 _ T$pical Co(H) cornplexes which are able to interact reversibly with O2 are Schiff base cornplexes: Co(Salen), Co(Salpr), and their derivatives_ Since COO, md Co,O, complexes from Co(II) species are well known as superoxo and peroxo complexes, respectively, it is therefore

Co(Salen);

R=H

Co(3-MeOSalen);

R=OMe

a; Rl,R2,Ra,H;

Co(SaIpr)

b; R1 =Me, R2 =R3 =H;

Co(NMeSalpr)

c; Rr =Me, R2 =OMe, R3 =H;

Co(3MeONMeSalpr)

d; R1 =Me, R2=R3 =Cl;

Co(3,&C12NMeSalpr)

e; R1 =Me, R”=H, R3 =N02 ;

_

Co(5-NOaNMeSalpr)

181

For Co(Salen)

seris

141

N”o L

\I

ca+o~= -t_ I\

NO

L -t

/N~_o,o :c

co(m)

. 0;

I\

1

NO

A

- o-o-_-co

“\I” I_

;

I\ f ON

Ccotm,

-

c$’

co(m)

1

L; Donor ligand For Co(Salpr) ,o-

series [5] x

0 ,H.ZH,-R’

interesting to know whether these Co(H) complexes can catalyze the oxygenation of organic molecules related to biological systems_ van Dart and Guersen 161, and Yogt eL al. [7,8] have reported the oxygenation of some phenols giving p-benzoquinones and diphenoquinones catalyzed by Co(Salen) and its derivatives. We have found that these e(H) complexes 1 and 2 display activities leading to highly selective oxidations similar to those of dioxygenases (catalytic oxygenolysis of S-substituted indoles and flavonols, regioselective dioxygen inco~oration into phenols), monooxygenase or phenoIase (formation of quinols a?d quinones from phenols), and peroxidase (oxidative coupling of phenols, oxidative clavage of ether bond), as summarized in the following diagrams.

Dionygenase (i) Cdtafytic

type activity Oxygenation

of indoles with Co(Saten)

19,

IO]

182 Reaction

rate: R = Me > CT& CHB NHAc

> CH2 CE&COOMe

>

> CH2CH(NHA6)COCJMe Solvent effect:

Reaction

rate; CH,Cl,

No reactim; Catalyst effect:

> MeOH

DMF, Fy, AcOH

No reaction;

Co(Salpr),

[Co(CN),]

Co(3_MeOSalen)Py,

H

H

R1

=R2

=H,

R3

cc_-;

R1

=R3

R’=R’=H,

R3=CHZCH,~OH,

R1 =R’=H,

R3 =CH* CHCWH;

=R3

3-_. Cb(Ac),

Me

=M~;

Rl=Rz,H, R1 =Rz=H,

R3zcH,

R3 =CHO_

-H;

, Co(acac),

183 (ii)

~ygenolysis

Ccctdytic

of~ji%cmnob with Co(dk~)

[ll]

0

6

5 SeIeetivity Substitution

effect-. Reaction R’ =OMe,

HO

ruclion

;

- 100%

rate; R1 =R2 =OH > RI =OH, R2 =H > R’ =X-I, R”=OMe

R2 =H >

> R1 =R* =H

OH

:

Solvent effect:

Reaction rate; DMF = DMSO > Fy No reaction; MeOH, CH, Cl,, AcOH

Catalyst effect:

Squti planar complexes and CuCZ2 are effective. No reaction; C&I,, Co(Ac)*, Co(acac),

(iii) RegloseCectiue chxygen incorpomtior. info phenols_ Peroxy compkx for-mattin with Co(sulpr) ?znd its deriuatiues 112, 131 OH

0

in

7

Co(lIi)

CH2 Cl,.

THF.

8

a; R=t-En

selectiviky;

b; R=i-Pr

Isolated yield; >60%

c; R=Et

Co(fII);

d; R=Me

Co(3-MeONMeSaIpr), C0(3,5-C~~NMeSalpr), CCB( 5-NO2 NMapr)

cai IGO% Co(Salpr),

Co(NMeSaIpr)

0

‘/

‘O-

0

-ComI)(salpr)

‘I 8

R 10 Selectivity; 100%

a; R=&MeO

Others; R=3-MeO, 4-Me, 4-C!,

3-Me,

Z-Me

H

and/or

Monooxygenase

Isolated yield; >60%

2-M&,

phenolas+hype

activity

(i) Catalytic hydmxylation of pheno!.s with Co(salpr) and its derivatives_ p-quinol formation [14] and aromatization [IS] 0

OH

in

f&l +$

:,“D”,“F”

HcOH OH

R

11 a; R=

t-Bu;

b; R=i-E’r,

c; R=Et,

d; R=Me

R=Et,

Me

R= t-Bu,

The overall reaction from 7 to 12 provides monooxygenation of phenols with the NW-shift UK&13, which is isolated with R=t-Bu and i-Pr.

(ii) Catalytic

quinone fonnafion

from phenols

Selectivity;>90%

i-J%

17

16

Selectivity ; > 8 5%

,Peroxidase-type (i)

Oxidative

activity

Coupkzg

of Pkenols

R

7

UC

18

R=f-Bu,

in CH,

Cl2

Et, AI

OH

OH

OH

i-R,

.MdH

OH

21 SelecGtity;

100%

186

Mechanism

of regioselective

dioxygen

incorporation

into phenols

The regioselective peroxy complex formation, especially 10, is of particular interest, because an analogous o-peroxy complex, 2.3, has beer. postulated for the dioxygenase reaction with phenolic subsBates 1171. OH

0

x

O-O

02 Fr

-

Fc(Enz)

(Enz)

d

d

R

23 Table 1 shows relations between 02-uptake and the formation of peroxy complexes 8 and 10 [POOCo(III)] _ The time course of the oxygenation of phenol 7d with Co(NMeSalpr) (2b) shows that the formation of peroxy complex 8 corresponds to the consumption of 2b by 1:l with double Oa-uptake (Fig. 1) [12].

TABLE

1

Relation Phecol

7a a, 7d 7d 7d 7d ?d

between 2

2a 2a 2a 2e 2d 2c 2b

02-uptake

and the formation

(tin)

O,-upt3ke/ (mol/mol)

14 10 10 11 10

1.25 125 125 1.25 1 _o 1 .o 1 .o

t-v;

2

of POOCo(LLl)*

FOOCo(m)/2 (mol/mol)

Oz-uptake/POOCo(IIi)*** (mol/mol)

1 .o 1 .O 1 .o (1 _O)? (OS)+ 0.5 0.5

1.25 125 1.25 (1.25) i2.0) 2.0 2.0

*Ref. 12; at 5 ‘C under a mnstant 0, p--ure (780 mm%). **Half conversion time_ ‘**Calculated from the data of 02-uptike/2 and POOcO(II1)/2. ‘Not determined experimentiy.

No hydrogen peroxide is found in the oxygenation mixture. Howmer, where double 02-uptake is obtained against the formation of 8, acidification of the oxygenation mixture leads to the formation of hydrogen peroxide, indicating the formation of Co(III)OOH. From these resu%, the following stoichiometries are depicted. For 02-uptake/POOCo(IE) = 2.0,

2Co(II)

+ 202

f PH

-,

POOCo(III)

+

Co(III)OOH

(1)

187

Fig. 1. Time course oE the osygenation of 7d with Co(NMeSalpr) (26). Conditions: [2b] [7d] = 2.5 X lo-%l in CH~Cl2 (40 ml), 5 “C. 0, PCOCo(IQ) (9d); 0.02~uptake; l , PH (7d).

and for O,-uptake/POOCo(IlI) Co(II)

+ 5/40,

= 1.25,

f PH --t PoOCo(IIi)

-i l,r2H20

(2)

where PH denotes phenoi 7 or ,9_ The stability of Co(III)OOH is dependent on the nature of the ligand and decomposes to give Co(III)OH and O2 (eqn_ (3)) [lS]. Further oxygenation Co(IJI)OOH

+ Co(III)OH

of PH with Co(III)OH PH f Co(ILI)OH

f L/20a

(3)

(eqn. (4)) gives the stoichiometzy f O2

-,

PQOCo(lII)

f Hz0

(2). ,In fact, the (4)

oxygenation of 7d and 9a in the presence of an equimolar amount of Co(K) (Salpr)OH in CH2C12 gives tne corresponding POOCo(III) in quantitative yield. Phenols 7 and 9 are quite stable against O2 without a catalyst. Obviously, therefore, the present oxygenation require the activation of 0, by Co(Salpr) and iB derivatives (2). The dioxygen z&iv&ion by 2 results from the reversible formation of the superoxo complex (GO*) or the peroxo complex (Co,02)_ These species are therefore included in the 02-bubbled solution of 2, from which COO2 can actually be detected by e.sr. even at room temperature. Concentraiion of Coo, is dependent on the natures of the solvent used and the ligand ‘m 2. A high concentration of Coo2 is obtained in DMF and pyridine and a low ctmcenkation in methanol and C&Cl, as judged from the magnitide of e.sr_ signals. Co(Salpr) gives the highest concentration of CoOz among its derivatives 2, whereas Co(5-NO2 NMeSalpr) (2d) gives tie least, refkting a substitution effect.

=

188 The peroxo Co,O, complex may not be the reactive species in the dioxygen incorporation into the phenols because the interaction of this species with PH is sterically hindered as judged by a molecular model_ A possibility that the superoxo Coo2 complex acts as a base to accept a proton from PH may alsobe negligible, taking account of its nucleophilic inactivity and also of the fact that the rate of the oxygenation of PH with 2 is faster than that with [Co(CN)sOz] 3--, a stable superoxo Co(W) complex obtained irreversibly (vide infin) _ It is therefore considered reasonable, that the dioxygen incorporation into PH is initiated throl-lgh hydrogen abstraction by CoOa from the phenol. Actually, when a small amount of PH is added to the solution of Co(Salpr) in CH2 Cl2 containing CoOg under interception of 02, the signal of COO2 is diminished with the simultaneous appearance of a signal attributed to the phenoxy radical of PH. Upon 02-bubbling through the resulting solution, the signal of the phenoxy radical disappeared and that of the Co& reappeared_ Such an e_sr_ okervation can be repeated several times, and POOCo(II1) is obtained from the final solution. The e.s.r. observations seem to suggest that the formation of PCOCo(III) results from a radical combination between the phenoxy radical of PH and the Coo, complex. However, it may not explain the regiose1ectivit.y in the formation of POCCo(II1) with phenols 9, because the reaction of the phenoxy radical of 9 prefers the para position to the ortho position [ 19]_ Recently, on the other hand, it has been found that the oxygenation of phenolate anions of PH leads to the regioselective clioxygen incorporation similar to the POOCo(III) complex formation_ This would strongly suggest that the phenoxy radical may be reduced to phenolat anion by Co(D) species that exist in great excess as opposed to the phenoxy radical formed during the course of the reaction_ It is actually found that phenoxy radicals of PH are rapidly reduced to the corresponding phenolate anions by 2 when it is in excess (eqn. (5)) and subsequent 02bubbling through the resulting solution leads to the quantitative regioselective formation of POOCo(III). 00

O-

Co(NMeSrlpr)

N? in CH?

Cl2

OIDUF

R=t-Bu,

poocOuQ’

(5)

4-MeOPh

The phenolate anion thus formed should not be in a free state but in a coordinated form (24) (eqn. (5)), because the oxygenation of free phenolate anion 7 gives only epoxy-pquinols (25) (eqn. (6)) and free anion 9 is quite stable against Oa (eqn. (7))_ These free anions are formed in DMP. The phenolate anion 9- is oxygenated to give o-hydroperoxide (26) oniy when it is associated with metal ions (eqn. (8)) [20,21] _

1) *

(61

21 H,O

4

Ha

ruclioo

(7)

P

‘- 4 2. i-y

0‘I’ I

0

(8)

R

26 M’=K’,

Na+

The rate of form&ion of POOCo(II~) depends on the concentretion of PH. The reaction with tenfold excess of 7d follows pseudo first order kinetics witi respect to Co(NMeSalpr) (2b) up to more than 90% conversion with a rate constant of 7.8 X IO-” s-l at 5 “c. These results are rationalized by the following reaction mechanisms involving rate determining steps (9) and (10). & coo, [co(m)-co-,] co(D) -+ 02 = (9) 2 kl

coo2

f PI-I -

kz

Co(WWH P-f

Co(lIIj

“3,. -

co(m)OOH

Co(rn)OH F-co(m)

+ P-

-c-I/2(3,

(W (11) CI2)

190

PH f Co(III)OH P-Co(III)

+ 0,

-

P-Co(III)

+ Ha0

k6 -

POOCo(III).

(13) (14)

Since the concentration of CoOi i&m Co(NMeSaIpr) (2b) in CH2Cln is estimated to be much less than 1% as judged from Oauptake (ca. l%,), a steady-state approximation is applicable for reactions (9) and (18). An assumption that these steps are fast reactions is inconsistent With the 02uptake curve (Fig. 1). Reaction (11) is negligible with 2b (uide supra). Therefore, under the conditions where PH exists in excess, the foIlowing kinetic law can he derived_ k,

Rate = k ohs

k ohs where

k;

[a(

k; I+ [ PH] = fr-,

k,

=

CCo(Wl IPW ikI + k2 [HI] CO,

= k; _

+ k, [PHI

I

(15)

k-,&b, kz

(14%

IPHI

=klWzl-

at the different Therefore, a plot Of kobs against k,,,/[~~] of PH should give a linear relationship, which is represented

1.0

2.0

3.0

L ,,,_/clw

Fig_ 2. Plot of R,,

4.0

5.0

6.0

COnCentitiOi-&

in Fig. 2.

7.0

(I IO3SK-’ 1“)

LG. k,,/[PH]

_

Frcm the intercept k; is estimated as 9.95 X ‘LO-’ s-l and from the slope k-,/k2 x3-59x lo-‘M. A rate constant, kG , with a PCo(HI) complex from P- (R=t-Bu) and 2b is estimated as cc_ 5-O x 10-l s-l M-l (0 ‘c), which is about 23 times larger than the rate ccnstant of dioxygen incorporation into free phenohte :. anions [20] _

191 Ek2ctions

of--. -=P&F.O ~~~,

~Co(~&feOsde~)(Pji)-t+] .,. ;

s- a+

colllP+$s E-u=-)5 - 021 w-iF phetiolsakd phknoxy radicals

~With the~intentiion'&f'~~&~gtte mechanisni bj; which dioxygen is incorpoiatedinti the phenolk’with i,:the reactions tif [C~(CBJ)~G~ 1 3- ., and [Co(3-MeOSalen)CPy)(O=)] , supemxo Co{III) complexes, with phenols and ph&nox$ kdicA.s have.beeri iniiestig&ed_ [cO(CN),(02)Jsis keversibly formed by the o~ygenatick df [Co(CN), 1 (NEt& in DMF [23]. The structure (27) of this tiperoxo complex has been determined by X-ray analysis [24] _ [iPy) - Co(~)(3-M~Sden)- (O,‘)]’ (28) is ftzHy obtained by the oxygtination of Co(3-MeOSalen) in py-ridke [25] _ The 5orrnation of 28 is rever&Ie. S-

28 Some characteristics Table 2_

‘Ref_ **Ref.

26_ 27_

of sup~oxo

Co(EII)

complexes

are summarized

in

192

The hyperfine splitting constant, aCo , of COO, Eflects the h2sicity of the 0, mmoiety in the CoOa complex. The COO, complex (27) has the strongest basicity among the three CoOz complexes listed in Table 2. Little information has been available so far about the reactivity of the superoxo Ca(H1) complex itself, except for a report on hydrogen abstraction by 28 from hydroquinone [ 28]_ Our preliminary experiments shcw that complexes 27 and 28 also lead to dioxygen incorporation into phenols 7 and 9. It has also been found that neither 27 ncr 28 couple with the phenoxy radicals of 7 and 9, but reduce them rapidly to the corresponding phenolate anions. The reaction of 27 with phenols (PH) in DMF gives products 29 - 33, the yields of which are dependent on the reaction conditions (Table 3). The reactivity of 27 towards 7a is much slower than that of 2: 20% conversion is observed at 0 % in 2 h (Table 3), under which conditions, on the other hand, the reaction with 2 is complete. Therefore, an assumption that the COO, complex from 2 may act as a base to initiate the dioxygen incorporation in’& the phenols PH is inconsistent with the fact that the reaction of 2 containing a low concentration of COO, is faster than that of CoOa itself. TABLE

3

Reaction

of [CO(CN),O,]~-

PH

R=t-Bu R=Me

*Two *

phenols

Reaction temp_

Reaction time

(“C)

(h)

(PI-X) in DMF*

Conversion (Z)

Pzoduct 29

yield 30 _

0

2

20

100

0

14

100

88

tracr

88 100

91 37’f

-

0 20

R=4 -MeOPh

with

2 1.5

(%) 31

32

-

-

-

8

-

-

9 35

28 -

0

19

0

_

_

-

_

20

96

60

_

-

-

100

equimolar amounts of 27 towards is mainly contained _

33

_

PH are used.

‘pJ$Cnol

*eFc+j391 * +L++o+ q-JcH+o+o R

F! PH

29

30

OH

R

31

0

R

OH

R

32

Phenoxy radicals (P -) are rapidly reduced by 27 in DMF to the corresponding phenolate anions (P), which can be oxygenated giving products 29 - 32 (Table 4).

33

193

TABLE

4

Reaction

of 27 with

temperature

Reaction time

(“C)

(tin)

R2aCtiOn

P-

R=t-Bu

0 (N,) 0 (o,)** 20 (O#*

R=2-MeOPh

9

(N2)

0

(02)=*

20

R=2-MePh

*An **COO,

pheroxy radicals (P -)f

-20 -20

(02)**

Conversion

(%)

Product PH

yield 29 -

2 1_5h 13 h

100 100 LOO

100

5 2h 48h

100 100 100

5 1

100 100

(N.2) (o,)+*

(%) 30

31

32

_

_

_ 15

67 -

-

25 85

100 80

_ _

13

_

_

35

-

_

_

65

100 53

_

31

_

16

0 0

8

equimolzr arnou~~C of 27 towards and P - are mired

under

P- is used in E)MF. N2 then 0, was bubbled_

The dioxygen incorporation into 9 in DMF (Table 4) skongly suggests that 9- is not in a free state but is coordinated to the resulting Co(iI1) species. This also provides additional evidence for the PCo(ILI) complex formation (eqn. (17)). The finding that P- isnot combined directly with 27 but reduced quantitatively to P is of particular interest. This reaction is quite analogous to that of Pmand KO;. [29] _

Pm i

co(fIr)o,-

+

P-Co(LIi)

1

H+

Pooco(rrr)

-

(17)

PH

The reaction of Cc(3-MeOSalen)(Py)(O,) with the phenol PH gives POCCo(III) complexes quantitatively similar +A that of 2. It also reduces the phenoxy radicals Pmrapidly. The results are summarized in Tables 5 and 6. The dioxygen incorporzkion into the phenols 7 and 9 with CoOa complexes 27 and 28 should also involve P-Co(III) int.ermediate. It is, however, obscure whether this intermediate results from the action of these CoOa cornplexes as base or by a process invoking hydrogen abstraction from the phenols, because phenoxy radicals are rapidly reduced by the Coo, cornpIes-_

194 TABLE

6

Formation

of POOCo(II1)

complex-

in the reaction

df phenols

(PH) with Co(3-Me0

SaIen)(PyHW* PH

Conversion

Reaction

Reactkin

Product

yield

(%)

temperature CC)

time &)

(%a)

p-Poocojm)

7a

0 0

2 15

70 96

100 100

-

7d

5 5

2 15

67 85

100 100

_

9a

5 5

2 15

67 70

-

100 100

10, (50

was bubbled ml).

TABLE

through

a solution

of Co(H)

(1.74

mmol)

o-POOGJ(nI)

and PIi (2.0-01)

in Py

6

Reaction

of phenoxy

radi&

(P -) with C0(3-MeOSalen)(Py)(O~) Conversion

Reaction temp. (“C)

Reaction time

R=t-Bu

-78 (Nz) -20 (0,)s 20 (%)*

4 34 3.7 h

100 100 100

r=4-MeOPh

-78 (N2) -20 (0,)s 20 (O#

@.5 2 h 15 h

100 100 lC!O

P-

_P- (O-4 -1) (25 ml) urder

Product PH

W)

yield

(%a) POOCo(III)

(tin)

in benzene (10 ml) was added to COO, nitrogen, then 0, was bubbled.

67 30 13

33 70 87

100 37 15

0 63 85

(0.4-01)

in PyCQC12

(1:l)

Characteristics for the reaction with 27 lie in the slow reaction of 9 and the formation of epoxyquinol 31 and 32. The yield of these epoxyquinols increases with increase of the reaction temperature. These facts smug& that the coordination of F with the Co(II1) species is fairly weak and F is in a free state to some extent. This may result from a steric hindrance due to the bulky counter cation (NEta)+.

Mechanism

found

of Co( saIpr)+zataIyzed

The Co(Salpr)catalyzed to involve POOCo(III)

pquincl

p-quir:ol complex

formation

fkom pheno!s

7

formation from 7 in MeOH has been 8 at the initial Skye_ The r-tion at a

195

gives only 8, even in MeOH. Therefore, the peroxy 8 should be the intermediak in this p-quinol formation. In fact, &I [Co(Sdpr)] is quantitatively decomposed by MeOH at higher temperatures to give pquinol. ZPd, and Co(Ill)(Salpr)OH. The results clearly indicak that the POOCo(IIl) complex is formed in the fast initial step and reduced in the following slow step.

reduced

terrperzture

complek

/ Q OH

6

07/caEalPr) ‘==I

R

a

7

e f

“y”

OH

a

o-o-ca(m)

Co(IZ)OH

11

8

A kinetic study shows that the reaction of Sd [Co(Salpr)] in MeOH follows first order kinetics with respect to SC with a rate constant of 4-6 X IN6 se1 (10 “C)_ Since the oxygenation of 7 with Co(lli)OH gives 8 (uide supra), it is considered that the Co(III)OH complex is the real reactive species in the catalytic cycle for the p-qumol formation, and that the dioxygen activation by Co(Salpr) takes part in the initiation of the reaction as shown in the following diagram (Fig. 3). The formation of formaldehyde in the catalysis has not been confirmed, but from the catalysis in CH2C12 or benzene in the presence of benzyl alcohol, benzaldehyde has been isolated (about 35% yield) ]14]_

~pooco~~~‘;(_,.., r

‘,”

--co(m)

Fig.

3.

Initiation

and ~talytic

* HCHO

Co(mlOH

cycle

for the fornztioo

of

~-quinok

from 7.

The selective reduction of POOCo(III) complex also takes place with other alcohols and the rate of reduction depends on the acidity of the alcohol (Table 7). The stronger the alcohol acidity generally, the faster the reaction, suggesting that a hydrogen bond may play an important role for the reduction of the peroxy bond in POOCo(IlI) intermediate, which then undergoes the reduction probably by a concerkd mechanism (34). A possibility of reductive.cleavage of the peroxy lxmd involving a homolytic process may be ruled out, muse the dtiug quiuoxy ELI%& normally

196 TABLE

7

Xak constant

for the reduction

02 Sd[Co(Ealpr)]

Alcohol

R( x lo6

CCl,CH,OH CH,OH CH3CH20H (CH&CHOH (CH&aCOH CBH,CH,OH

99 4.6 19 OS 0 5s

(C6H5)2CH0H

13

*POOCo(III) “C.

disolved

in

a

mixture

6th

various

~CO~O~S*

a’)

of the appropriate

alcohol

and CH2Cl2

(6r4)

at 10.5

(34) an efficient ring expansion [30]_ No such expanded prcduct is found in the reduction of PQOCo(III) ccmplex. Contra!! to 8, peroxy complexes 10 do not give the corresponding o-quiriols selectively.

undergoes

Mechanistic flavonols

considerations

on the catalytic oxygenolysis

of indoles and

The electronic spectrum of Co(Salen) in CH,Cl, is not affected by the presence or ahence of the substrate under O,-free conditions, indicating that no significzlt coordination of the substrate to Co(Salen) ties place without 0,. The effects of solvent, substitution, and catalyst on the oxygenolysis of tidoles 3 [S] strongly suggest that the reaction should be initiated by a concerted coordination of the subskate and dioxygen to Co(Salen). As the reaction rate is lineariy correlated to their ability for a mdonor-acceptor charge-transfer complex formation with 2,4, Gtrinitrobenzene [S] , the concerkcl coordination and the following electron transfer

from the subsWa@

to the Co center should he the rate determining

The following mccbanism fied by a avitable kinetic

is reasonably

suggesk3,

although

it DILL&

steps. be veri-

study.

Recently, Ricroch and Gaudemer 1311 f and Uchida ef a!!_ 1321 have reportA the cafdytic oxygenolysis of indoles with Co(TPP)-and Mn(Pc). respectively.

197

SH

--

Co(X)

-

so-

corn,

o2

SH---Co(n)---02---X

-

-.

~ocm 5

-

.I

&MJEl

SOOCO(9)

SC-I

A

may

HOOX

SH

+

sn

SDOH

4

SOOH

SOOH

soocom~

&kdIU)

-

+

I 02 1

02

Moco(ml

~-CO(D)

rrlcr

IO

02 ,’

OoH

The solvent effect on the oxygenolysis of flavonols 5 strongly suggests that the peroxo-bridged complex [LCo(III)O$-Co(III)L] is the reactive species and acts as a b.ase to accept a proton from the substrate_ Dioxygen may then be incorporated into the resulting flavonolate anion speciesActually, t-BuOKcatiyzed oxygenolysis of fIavonols in DMF easily leads to the sekctive formation of 6 [33]. The following mechanistic diagram is reasonably proposed for the ca’dytic onygenolysis of flavonok The eatalytic cycle may be supported by the fact that peroxy Co(III)(Salpr) complex 23 can catalyze the oxygenolysis of the flavonols giving the depsides 6.

vhcrc

FIH

I flaronol=

6;

L E

IJMF.

DMSO

i FIOOH

i-nay

dcnatc

198 Notes

for the other reactions

The selectivity resulting from the oxygenation of phenolic substances with Co(Salen) strongly depends on the nature of the solvent used. In solvents such as CH2 Cl2 and MeOH, which are less coordinative to Co(Salen), one electron oxidation is predominant, whereas in a coordinative solvent such as DMF, dioxygen incorporation mainly occurs. Thus, the oxygenation of phenols 7 and 2,4-di-t-butylphenol in CHzCI, or MeOH gives phenoxy radical coupling products including peroxide 18, and di-t-butylphenols 14 and 16 in DMF gives quinones 15 md 1'7selectively. The formation of these quinones is reasonably assumed to involve peroxy Co(III) complex intermediates of types 8 or 10. The oxidativvc coupling of 4-t-butyl-2,6diiodophenol with Co(Salpr)O, Dbviously involves a phenoxy radical intermediate, because Co(Salpr) itself and the Gethyl ether of this phenol are not reactive_ A direct cleavage of the C-I bond by the Co(I1) species 1341 is not the case. This is an additional support for the fact that the COO, complex from Co(Salpr) can abstract hydrogen from phenols. Strangely, hcwever, if the t-butyl.group of 20 is replaced by another alkyl group, the oxidation no longer occurs.

20

Conclusion The present oxygenation with Co(II)-Schiff base complexes displays selectivity depending on the nature of the ligand and the reaction medium (environmental control!). The reactions require the activation of dioxygen under the formation of cobalt-dioxygen complexes. The reactive species of five coordination complexes is superoxo Co(III) species. The reactivity of square planar complexes depends on the solvent used. With coordinative solvents, a peroxo-bridged Co(II1) complex may be the reactkJe species. The role of the cobalt--dioxygen complexes in the present catalytic oxygenation is to initiate the reaction through the activation of substrate, which may be summarized as follows. Su_peroxo Co(II1) complekes activate phenolic substances by a hyckogen ahskaction, whereas peroxo-bridged Co(III) complexes +&e part in the activation as a base. Concerted coordination of suhstrzte and dioxygen to square planar Co(II)-Schiff base complexes also activates the substrate through an electron transfer process. For the catalytic process, Co(II1) species fcrmed in the initial step play an essential role.

199

The seiectivity obseerved in the pwnt oxygenation may result mostly horn the coordination of the thus activated substrate t-o the Co(III) species, into which dioxygen is incorporated.

References (ed.), Molecuior Mechanism of Oxygen Activation. Academic Press, New 1 0. Hay&i York and London, 1974, p?_ 1 - 678. 275. 2 L. Vaska, Act. Chem. Ees., 9 (1975) and S. E. Wiberley, Chem. Reu., 63 (1963) 269. 3 L. H. Vogt, Jr., H. M. Faigenbaum A&J. Chem. Ser., 100 (1971) 111. 4 R. G. mlkins. W. P. Schaefer snd R. E. Marsh. Acta Crystoilogr.. B27 (1971) 1 461. 5 L. A. Lindblom, Receuic 86 (1967) 5206 H. M. w.n Dart and H_ J_.Geurxn, and H_ L Finkheiner, J. Org. Chem.. 34 (1969) 273. 7 L. H. Vogt, Jr., G.Wirth Org. Chem.. 35 (1970) 2 029. 6 D. L Tornaja, L. H. Vogt and J. G. Wirth,J. Chem Left., (1975) 273. 9 A. Nishinaga, T. Itahara and T. Matsu-ura, unpublished data. 10 A_ Ni&inaga, H. Tomi& T. Tojo and T. Matslrura, Chem. Commun.. (1974) 896. 11 A. Nishinaga, and T. Matsuura, Tetruhedron Left.. (1979) 2 893. 12 A. Nishinagz, H_ Tomiti K_ Nishizawa, H. Tomita and T. Maoiuura, J. Am. Chem. Sot.. 99 13 A_ Nishw, (1977) 1 287 _ Lett_. (1374) 1 291_ 14 A. Nkhinaga, K. Watanabe and T. Matsuura, Tetrahedron T. Itahara, T. M&suura, S. Berger, G. Hens and A_ Rieker, Chem. Ber., 15 A_ Nishtiga, 109 (1976) 1 530. and T. Matsuurn, Chem_ Lett., (1972) ill_ 16 A. Nishinaga, T. Nagamachi 1’7 G. A. Hzruilion, in 0. Yeyaishi (ed.), Jfolecrrlar Mechtwzism of Oxygen Activation. Academic Press, New York and London. 1972, Ch_ IO, p_ 405. 18 G_ Pregaglia, D. Morelli, F. Conti, G. Gregorio and R. Ugo, Discuss_ Fa~adoy Sot.. 46 (1968) 110. da’&. 19 A. Nishinaga, T. Shirnizu and T. zMatsuura, unpublished 20 A. Nishinaga, T. Itahara, T. Shimizu and T. Matsuura, J. Am. Chem. Sot_. 100 (1978) 1820. T. Mahuura, A. Rieker, D. Koch, K. Albert and P. B_ 21 A. Nishinaga, T. E&a, Hitchcnck,J. ilm. Chem. Sot., 100 (1978) 1 826. Org_ Chem.. 44 (1979) 2 983. 22 A. Nishinaga, T. Shimizu and T. Mat.suura,J. and M. Baker, fnorg. Chem.. ll(l972) 2 160. 23 D. A. White, A. S. Solo&r (1974) 470. J. Chem. Sot., Chem. Commun.. 24 L. D. Brown and K. N. Raymond, J. Ckem. Sot. A, (1969) 946. 26 C. Florianj and F. Calde-, B. H. Hoff+ and F. Basolo, Chem. Commun., (1970) 467. 26 D. Diemente, S. R. Pickens and A. E. Martell, Inorg. Chem., 16 (197711 551. 27 G. Mdendon, J. Am. Chem. Sot.. 96 (1974) 26 E. W. Abel, J. M. Prati. R. Whelan and P. J. wlkin.son, 7 113. T. Shimizu and T. MaD;uura, Chem. Lett_, (2977) 547. 29 A. Ni&inaga, and T. Matsuura, Tefr&edron Letf., (1978) 3 557. 30 A. Nishinaga,. K. Nakamura Tetrnhedron Left_. (1976) 4 079_ 31 M_ N. Dufoour-FZicroch and A_ Gaudemer, Chem. Letl., (1978) 471. 32 K. Ucbida, M. Some and K. wu, 33 A. Nishinaga, T. Toio, H. Tomita and T. Matnrura, J_ Chem_ So-c., Perkin Trans. fr. (1979) in press; A. Nishinaga and T. M~tsuura, Chem. Commun.. (1973) 9. 34 J. Halpem and J. P. IM.aher, d. Am_ Chem_ Sot.. 87 (1965) 5 361.