Chapter 5 Oxidation of phenols

126

CHAPTER 5

OXIDATION OF PHENOLS 5.1 Introduction The oxidation of phenols has many different aspects. In this account, oxidative coupling, the formation of cyclohexadienones, conversion to quinones and other carbonylic and transformation products (other than those obtained by electrophilic substitution or rearrangement of phenyl esters) are considered. Particular attention has been paid to work carried out in the last decade. An early review has described initial work (ref.1) and that carried out later is available in the same source (ref.1, Ch.2). To a certain extent this present chapter inevitably impinges on the chemistry of alkylphenols (Ch.6), although oxidation has been excluded in that section except in the case of hindered phenols which are only briefly referred to in the following examples.

5.2 Oxidative Coupling 5.2.1 Phenols Under mild conditions the oxidation of thymol has been studied extensively in

i P r OH ~ ~ l i P r ~0 ~_iPr _~iPrA -Me L Me ,.~ "Me~ --,if---Me 0 0 ,'" HV iPr~ #," iPr ~ Me M iPr~ lie • 0

0

127

ethanolic solution at 60~ by dropwise addition to anhydrous cupric chloride and gassing of the stirred solution with oxygen over 48 hour to give the thymylthymoquinone shown (65%), the more completely oxidised diphenoquinone (10%) as well as some thymoquinone (25%) (ref.2). 2-Methylphenol under similar conditions afforded a cresylcresoquinone analogue of the major product from the oxidation of thymol. From 2,6-dimethylphenol and 2,6-di-tert-butylphenol 3,5,3',5'-diphenoquinones resulted as the major products in 51% and 70% yields respectively with, in the former case, some 2,6-dimethylbenzo-l,4-quinone (32%). The general mechanism shown on the preceding page seems operative although the phenylbenzoquinone pathway is speculative. By contrast with the above conditions, the oxidation of 2,6-di-tert-butylphenol in 50% aqueous potassium hydroxide containing 50% aqueous methyl tri-n-butylammonium chloride in a sealed reactor at 200~ and 300 psig. for 1 hour with 50%hydrogen peroxide has been reported to give 4,4'-bis(2,6-di-tertbutylphenol) the precursor of the diphenoquinone in 97.5% yield (ref.3).

tBu

tBu

tBu

tBu/

tBu

tBu

A comparison of phenolic coupling on solid potassium permanganate and on potassium manganate surfaces has been made to emulate the natural biological process (ref.4). Diphenoquinone formation in more than 90% yield was observed with the oxidation of 2,6-dimethyl, 2,6-di-isopropyl, and 2,6-di-tertbutyl phenol (R = Me,i-Pr,t-Bu) in chloroform solution on (a) solid potassium permanganate whereas under the same conditions (b) potassium manganate,

R

OH ~R

R

R

R

R

a

b

R

LR

_In R

128 a milder oxidant, gave exclusively a poly(2,6-diaryl-1,4-phenyleneoxide). By the addition of powdered potassium hydroxide to the potassium permanganate system, the polyphenylene oxide (85%) and the diphenoquinone (15%) resulted. It has also been reported that solid iron(Ill) chloride is more effective for the oxidative coupling of phenols than when used in solution (ref.5). Cobalt(Ill) acetate (2.5 moles) with 2,6-dimethylphenol in acetic acid reacted at 70~ during 5.5 hours afforded a 75% yield of 3,3',5,5~ methyl-4,4'diphenoquinone together with a 23% yield of 2,6-dimethyl1,4-benzoquinone ( ref. 6).

OH M e n , Me ~

Me Me 0 = ~ / ~ Me

Biological methods have been extended by the use of mushroom tyrosinase which with 0.01M 2,6-dimethylphenol in stirred phosphate buffer (pH 6.8) gave after 9 hours the diphenoquinone in 96% yield. Dissolved oxygen was necessary for the oxidation. Only o-blocked electron-donating phenols were oxidised since 2,6-dichlorophenol proved unreactive in this system. The co-solvent acetonitrile afforded bis-phenols as by-products (ref.7). The previous examples have invariably involved the 4-position but where this is substituted as in the case of 4-methoxyphenol, oxidative coupling can take place at the 2-position probably via an intermediate phenoxide ion without 1,4-benzoquinone formation. Thus I mole of aluminium chloride in nitromethane solution treated with 4-methoxyphenol and, after 1 hour, anhydrous ferric chloride in nitromethane gradually introduced, led after reaction during 5 hours and acidic work-up to 2,2'-dihydroxy-5,5'-dimethoxybiphenyl in 78% yield (ref.8).

OH

<> _. OMe

OH

C1

-* C, CMe _]

MeO

OH

\

OMe

Oxidative cross-coupling has been effected moderately selectively. For example, by the addition of aluminium chloride in nitromethane to a nitromethane solution of 4-methylphenol and 4-methoxyphenol under nitrogen followed by gradual treatment with with dichlorodicyano-1,4-benzoquinone and reaction at ambient

129 temperature during 1 hour 2.2'-dihydroxy-5'-methyl-5-methoxybiphenyl was obtained in 70% yield (ref.9).

OH

OH

Me

OMe

OH

OH

OMe

5.2.2 Naphthols With the cupric chloride/oxygen system used for phenols, 2-naphthol in ethanol solution did not give a coupled product but only 4-ethoxy-l,2-naphthoquinone (65%) together with 1-chloro-2-naphthol (30%) was isolated. Finely powdered 2-naphthol with ferric chloride hexahydrate maintained at 50~ during 2 hours followed by acidic hydrolysis gave racemic 2,2'-dihydroxy-l,l'-binaphthyl, (1, l'-dinaphthalene-2,2'- diol) in 95% yield without quinone formation (ref. 10). By the use of the S(+)-amphetamine copper(ll) complex, the enantiomer S(-)-[1,1'-binaphthalene]-2,2'-diol has been synthesised in 85% yield and high optical purity (up to 95%) with 80% recovery of the amphetamine salt (ref.11).

The absolute configuration of the monohydroxy analogue, 2-hydroxy-l,l'binaphthyl reported (ref.12), has been revised to the R-(+) form. Asymmetric cross coupling of the Grignard reagent from 2-methoxy-l-bromonaphthalene with menthyl 1-(-)menthoxy-2-naphthoate in ether/benzene (1:1) by reaction at ambient temperature for 3 hours followed by gentle refluxing for 2 hours, gave the product shown in 81% yield with an optical purity of 98% (ref. 13).

130

r

OMe

+

~

r

O-Ment C02-Ment

[(-)-menthol = ment] Asymmetric oxidative dimerisation of S(+)-3,4,8-trimethyl-5,6,7,8-tetrahydro2-naphthol in ether with potassium ferricyanide in aqueous 0.2M sodium hydroxide was effected by stirring at ambient temperature for 2 hours to afford the S,S-(+)-trans dimer in 62% yield (ref.14).

.o-.

.eMe

5.3 Formation of Cyclohexadienones Cyclohexadienones can be considered structurally as hemi-quinones and they have been synthesised in recent years by a variety of simple approaches in which invariably a substituent becomes attached to the 2- or 4- position and the structure becomes locked. 2,6-Dimethylphenol in hexane or benzene converted at 0~ to the anion with n-butyllithium in hexane, gradually warmed to ambient temperature, stirred for 1 hour, and then alkylated at 0~ with chloromethyl methyl ether afforded after 2-4 hours reaction 2,6-dimethyl6-methoxymethylcyclohexa-2,4-dienone in 85% yield without formation of the 2,5-isomer. Chloromethyl methyl thioether and 2-trimethylsilylethoxymethyl chloride were also used (ref. 15).

131

OH

0 ------

2,6-di-isopropylphenol interacted with paraformaldehyde and 40% aqueous dimethylamine in i s o p r o p a n o l to give the e x p e c t e d 4-hydroxy-N,N-dimethylbenzylamine which, probably by way of the quinone methide, has been reacted then with buta-l,3-diene by heating in a pressure vessel at 200~ for 7 hours to give the spiro compound, 2,4-di-isopropylspiro[6,6]undeca-l,4,8-trien-3-one in 82% yield (ref.16).

OH iPr~~iPr__~L

r

iPr

iP

,

0

iPr

,

The oxime of 4-hydroxybenzyl methyl ketone by treatment in refluxing acetonitrile solution with phenyliodosodi(trifluoroacetate) afforded the spiro product 3-methyl-l-oxa-2-azaspiro[4.5]deca-2,6,9-trien-8-one in 63% yield (ref.17). OH

0

The cyclohexadienone formed from carvacrol, (5-isopropyl-2-methylphenol), by potassium periodate oxidation, or with iodic acid in ethanol, has been used to synthesise 3,10-dihydroxydielmentha-5,11-diene-4,9-dione, a monoterpene dimer from the plant, Cailitris macleayana (ref. 18).

132

The iodic acid oxidation of thymol and of isothymol also afforded similar compounds (ref. 18). A number of alternative routes have been investigated for the conversion of 2,4,6-trimethylphenol to a number of different cyclohexadienones. These are summarised in Table 5.1 (refs.19-24). By contrast the electrolytic oxidation at 8~ and a current of 0.7A with a graphite anode and a stainless steel cathode of a solution of mesitylene in aqueous sulphuric acid/acetonitdle (4:1) gave, over 5.5 hours, a 43% yield of the same product, namely 4-hydroxy-2,4,6-trimethylcyclohexa-2,5-dienone (ref.25) as obtained with the oxidants chlorine, manganese dioxide or sodium hypochlorite. 0

Men,Me

M

,Me

Me

A number of halogenated cyclohexadienones have been prepared. 4,4-Difluorocyclohexa-2,5-dienone was obtained in 60% yield by the addition over 15 min. of phenol and lead dioxide to a stirred mixture at 35^C of 70% hydrogen fluoride in pyridine and dichloromethane followed by reaction for 30 min. (ref.26). OH

0

F

F

2,6-Dibromo-4-methylphenol in methanolic dichloromethane upon treatment with phenyliodosoacetate in dichloromethane at ambient temperature and reaction with stirring for I hour afforded 2,6-dibromo-4-methoxy-4-methylcyclohexa-2,5dienone in 63% yield (ref.27).

Br

Br

Br

MeO

Br

Z

= .1

a;

S

O t~

c~

O

d

"E: ~j

o ,J=

/

o

X

O

O O o~-~

~

v~

o~o rj~

~.'2.

P4

o~..~

O

C'4 C'4

(3O

o

O~ O~

/

~)

o

<

~-

~'~

I

~,

c,I

O

o,-~

~)

.1.-~

133

~J

O

i

o~

.~-.4

""

~

o~

0~

~j O

~J

~J

9

"E:

~'.=.

oq

OC)

G

134

4-Methyl-4-nitro-2,3,5,6-tetrabromocyclohexa-2,5-dienone, a regiospecific nitrating agent was obtained in 88% yield by the addition at 100~ over 10 mins. of nitric acid (d 1.52) to 4-methyl-2,3,5,6-tetrabromophenol in acetic acid followed by reaction for 2 hours at 5~ (ref.28). Br

Br

Br~OH

/ ~ " "Br Br

r

O2N

Br

With an oxidisable group in the 2-position to the phenolic hydroxyl group an o-quinonimine structure can be derived as in the oxidation of 4- metho xy- 2-( [2-( 2- metho xy etho xy )- 5-nitrophenyl]s ulphonamido )-5-methylphenol in acetone by manganese dioxide at ambient temperature over 3 hours to give 4- m etho xy- N( [2-(2-methoxyethoxy)- 5-nitrophenyl]s ulphonyl)- 5-methyl-o- benzo quinonimine in 59% yield (ref.29).

~

O~/~(~ e

O2N-~/I~ = A r

NHSO2A~

Production of an anionic centre by the use of potassium tert-butoxide in tert-butanol accompanied by an S,2 intramolecular cyclisation has enabled a 2,5-cyclohexadienone to be used in a novel synthetic way as shown in the following reaction which after a 7 hour refluxing period gave 6,7,8,9-tetrahydro-4a, 7-methano-4a, (H)- benzocyclohepten-2, (5H)-one in 57 % yield (ref.30).

H~CH2OSO2

Me

An electrolytic procedure facilitated the conversion of a 1% methanolic potasium hydroxide solution of 4,4'-dimethoxybiphenyl, in a single cell with a platinum

135 gauze anode and a constant current of 2A over 8 hours, to the dimethyl acetal of a cyclohexadienone system in 86% yield (ref.31).

M e O ~ O M e

,

.~._

M

e ~/

~

OMe

/ \--! MeO

CtMe

Spiro-annulated 2,5-cyclohexadienones have been obtained by the oxidation of 4-phenylphenols with phenyliodosodiacetate. Thus 1-(4-hydroxyphenyl)-2-(3methoxyphenyl)ethane in acetonitrile afforded the 1-spirotetralin compound illustrated upon refluxing with the reagent during 4 hours (ref.32). Unsubstituted, 4-methoxy and 3,4-dimethoxyphenyl analogues have also been synthesised. O

OH

The structurally related spiroindane (A),(R = alkyl) together with the cyclohexadienylanisole (B) by-product, in proportion dependent upon the position of a methoxyl substituent, resulted from oxidation of 2'-alkenyl-4phenylphenols by treatment with the same reagent in methanolic solution (ref. 33). For example with a 4-methoxy group, as shown, A(56%) and B(34%) were produced while a 3-methoxy reactant afforded only B(63%).

R R2

R2

OMe

"

/~k/OMe

o•M A

7" OMe

e ~ O M e RI/

B

Naphthalenic compounds such as 4,8-dimethoxy-6-methyl-l-naphthol can be oxidatively coupled essentially by way of a cyclohexadienone. The naphthol

136 shown in chloroform containing 0.2% triethylamine was oxidised with silver oxide and the extent of reaction monitored chromatographically until formation of 4,4',8,8'-tetramethoxy-6,6'-dimethyl-2,2'-dinaphthylidene-l,l'-dione was complete with a 95% yield (ref.34).

OMe

OMe

OMe OH

OMe 0

% ~ " ~ M e

OHe

5.4 Quinone Formation 5.4.1 Benzoquinones Under more drastic conditions than for formation of cyclohexadienones, 1,4-quinones are produced sometimes even when the the 4-position is substituted as in the case of 2,4,6-trichlorophenol which gives 2,6-dichloro- 1,4-benzoq uinone. Table 5.2 (refs.35-44) summarises the conditions employed for a variety of alkyl-substituted phenols. In many cases homologous and other substituted compounds gave equally satisfactory results. The compounds listed in the table all afforded high yields of 1,4-benzoquinones even in the examples of thymol, carvacrol and the 3- and 4-methylphenols. 1,4-Benzoquinones were derived from the oxidation of electron-rich methoxyarenes by magnesium monoperoxyphthalate with a water-soluble iron porphyrin as catalyst (ref.45). Primin (2-methoxy-6-n-pentyl-1,4-benzoquinone) has been synthesised from the oxidation of 2-methoxy-6-n-pentylphenol with Fremy's salt (the Teuber reaction) by the route indicated (ref. 46). OH 0

OH OH

OH

0

O (i) 2BuLi, H30§ (ii) Pd-C, H2 (iii) oxidn.

@,

N

f~

e,1

.<

r..q

D 0

z 0

tr

/

i~

~ ~

~

~.~ m

9

,~ ~ ~ c-g

o,I

0

',,D

O~

-'-

O

0,1

-X

o~~

0

o,I

e,i

~,

~=

-~

r.~

~

~.~

,

c, ~ -e,

~ ~.~~ ~~.~ ~ ~' ~

~

N~2

.~.~ o

0

q ',,O o,I

o0

0

o',

0

<

0~

< ~

0,1 ,.~

~~

0

0

137

~ / 0

c

! ~

,q. "..q

o

~o

~~

eq[.~

138

.-,

=

e,l

r.z.l

F-,

D D 0

o

Z

o

F-~

2: <

ox

I'~

c,.)

~

0

~

~,

~ 6 ~ '~

~.~ *~ .-~ ~

~

,.~ ca

"~,-

~o~ <

~.""

c.

d

0

o,I

I"--

= e~

o . ,.4' ~ Z

. ,...~

~ =

~o

--=~

~-,

-~

,.c: "~. ~.~

~

~

,-.,

. ,,.,-I

o

~"~ "~ ,.~ M~ c~

O~

0 0

o

.= p_,N "-~

o~~ ..~ ~.~

9"-d ~,'~, 0

,,

,.~ c~

~'~ "a 0

~ ~ ~

~

~

r---

0

"a.~,

}-

t: ,.e

" a '-a c~

~

.~~o

~"

,'-a

-~

o

9

"~'~

0

~,-~

00

~~ 9

~"~

~

0

9

~

o

:~

~

0

m

0

~g~.~ e "~

o

i

139

By the use of iodoxybenzene in the presence of a small amount of trichloroacetic at ambient temperature o-quinones have been obtained (ref.47). Dimethyldioxirane oxidises phenols to o-quinones (ref.48) while catechols are cleaved to Z,Z-muconic acid (ref.49). In the tocopherol series, 2,3,6-trimethyl-5-(3'-hydroxy-3',7', 11 ', 15'-tetramethyl hexadecany)phenol in benzene was added to a slurry of Fremy's salt and aqueous sodium carbonate containing tri-caprylmethylammonium chloride in benzene to give, after 2.5 hours reaction tocopherylquinone in 93% yield (ref.50).

5.4.2 Naphthoquinones and Polycyclic Quinones Some new reagents have been introduced for the preparation of 1,4- and 1,2-naphthoquinones. These have been summadsed in Table 5.3 (refs. 51-57). 2,3-Naphthoquinones have been generated by a non-oxidative route involving bromination with N-bromosuccinimide in the 1-position of the respective 2-keto tautomer as the mono t-butyldimethylsilyl derivative, treatment of the brorno compound with tetra-n-butylammonium fluoride and trapping of the product as a norbornadiene adduct, in a yield of 64% in the case of the 1,4-diphenyl member (ref. 58). Ph

~

Ph

"OSiMe 2tBu Ph

Br

~'~ ~T~ "OSiMe 2tBu Ph

Ph

ph

(i) NBS, CH2CI2, (ii)Bu4NF, CH2CI2,-50~ 2,7-Phenanthrenequinones have been produced from the coupling of cis4,4'-dihydroxystilbenes, (in one example obtained through a Wittig reaction with 3,4,5-trimethoxybenzaldehyde and 2,3,4-trimethoxybenzylphosphoniumbromide), followed by selective demethylation and oxidation with the silver(I) cation, (Ag20) (ref.59).

140

r~

0

0

0 r~

0

E .<

0 0 .<

0

L,)

D D 0

z 0

0

re3

p--

t,ki e ' l

~

o

~o

0 0

re3

0 0

>

~

~

0

~.~

"~"~

z~

~'-~ - ~

~ ~.~

0

t~

~

~.,

.~. ~ ,~

9

~

~.~ "~

~

~

~o~

>

~,_

'

o

,~~

0

o

o

~_~

z,

._~ ~

~.~

z

~,

t~

0

~

o0 ~0o

o , r/~ ~

\

~

~

0

,

-~

m

~

~.~

,-...i

0

tf3

0

6~

..-~

o ~

~ ~~

~

Z~ ~=

~

0

r..)

0 c~

Z 9

9 r..)

9

E

[-~

0

i

E~

~

"S

O

I

o

o !

0

0

0

t~

x

"t::)

[-'4

9 O

9

'~

P

9

0

~

141

142

OMe

MeO

OH Me~OMe

MeO~ YOMe OMe

0

MeO~ i~OMe OH

MeO-"~,r"~OMe 0

5.4.3 Bicyclic Heterocydic Quinones Specific oxidative treatments have enabled 1,4-quinonesystems to be obtained. 1-Hydroxy-3-methylcarbazole in dichloromethane treated very briefly with pyridinium chlorochromate at ambient temperature afforded a 40% yield of 3-methylcarbazole-1,4-quinone (ref.60).

<

~

M

e

j,.

<

l~I OH

~

M

O e

~I 0

A hemiquinone intermediate rather than an o-quinone resulted from the oxidation of methyl 6-hydroxyindole-2-carboxylate in dimethoxyethane with activated manganesedioxide. By reaction with benzylaminein dimethoxyethane solution the intermediate formed methyl 2-phenylpyrrolo[2,3,e]-benzoxazole-5-carboxylate in 61% yield (ref.61).

•••--C02M e

HO/ ~

~%

~

C02Me

CO2 --~ Ph

143

5.5 Other Carbonylic and Transformation Products The versatility of the phenolic molecule towards oxidation is demonstrated by the formation of other oxidation derivatves than the coupling, quinone and cyclohexadienone series. 2,4,6-Trimethylphenol has been converted in 98% yield to 3,5-dimethyl-4-hydroxybenzaldehyde by aerial oxidation in dimethylformamide/methanol solution (1:5) containing cuprous chloride by passage of air through the solution during 8 hours (ref.62).

;

OH

Formation of a trimethyltropone has been described by the transformation of the cyclohexadienone intermediate formed in the Reimer-Tiemann reaction in 59% yield from 2,4,6-trimethylphenoi, chloroform and a little cetyltrimethylammonium bromide by treatment dropwise at 50~ over 10 mins. with aqueous sodium hydroxide. Refluxing of the cyclohexadienone for 4 hours with tributyitin hydride in benzene containing azoisobutyronitrile (AIBN) afforded 2,4,7-trimethyltropone in quantitative yield (ref.63).

OH

0

0

The photochemical rearrangement products of thymol in trifluoromethanesulphonic acid by UV irradiation at 300nm during 40 hours at ambient temperature have been studied in detail and ten compounds isolated of which eight have been characterised. (ref.64). The conversion of the hydroxyl group in three of the products to a keto group is of interest in this section on oxidation. The irradiated acidic reaction mixture was quenched at 0~ in sodium hydrogen carbonate and dichloromethane and after isolation of the organic material it was separated chromatographically, in this one-step process one of the important products was umbellone (10%) formed by a regioselective type A rearrangement. Three other processes were suggested to be operative. Firstly, a C2-C3 migration (type A rearrangement) and ring-opening to give the principal product, 3-isopropyl-5-methylphenol (17%), secondly intermolecular

144

transalkylation giving three isomeric di-isopropylphenols (17%) and thirdly, formation of pipedtone (5%) initiated by hydrogen abstraction.

OH

Me

Me~ 5~ OH iPr

~

+ O

+

iPr

Me

~0%

5%

Pr 17%

OH +

iPr~iPr ~7%

Although outside the scope of the present chapter, another transformation of interest is the conversion of the fully hydrogenated product from phenol, namely cyclohexanol, to cyclohexanone in 100% yield by addition of a dichloromethane solution to bis(quinuclidine)bromine fluoroborate and silver fluoroborate in dichloromethane followed by reaction for 30 mins.at ambient temperature (ref.65).

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145

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