Kinetics and mechanism of the reaction of photoinduced 9-aryloxy-1

18. 19. 20. 21. 22.

B. Wojcik and S. H. Adkin, J. Am. Chem. Soc., 56, 2424 (1934). Zh. A. Krasnaya and T. S. Stytesenko, Izv. Akad. Nauk SSSR, Ser. Khim., 821 (1987). D. M. Grant and B. V. Cheney, J. Am. Chem. Soc., 89, 5315 (1967). B. V. Cheney and D. M. Grant, J. Am. Chem. Soc., 89, 5319 (1967). M. O. Dekaprilevich, L. G. Vorontsova, and O. S. Chizhov, Zh. Strukt. Khim., 18, 328

23.

M. O. Dekaprilevich, L. G. Vorontsova, and O. S. Chizhov, Izv. Akad. Nauk SSSR, Ser. Khim. (1980), pp. 402, 2050. J. Schreurs, S. A. H. van Noorden-Mudde, L. J. M. van de Ven, and J. W. de Haan, Org. Magn. Reson., 13, 354 (1980).

(1977). 24.

KINETICS AND MECHANISM OF THE REACTION OF PHOTOINDUCED 9-ARYLOXY-I,10-ANTHRAQUINONES WITH ALCOHOLS L. S. Klimenko, N. P. Gritsan, and E. P. Fokin

UDC 541.124:541.127:541.14:542.91: 547.673.6:547.26

A stepwise mechanism for the reaction of 9-aryloxy-l,10-anthraquinones with alcohols has been established, starting with nucleophilic 1,4-addition of a molecule of alcohol. A quantitative reactivity study has been carried out. Introduction of acceptor substituents into the anthraquinone nucleus substantially increases the reaction rate, while donor substituents reduce it. A qualitative correlation between the rate constants and the electron density ~ at the C and O atoms of the reaction site has been established.

Under irradiation by light, some l-aryloxy-9,10-anthraquinones undergo reversible migration of the aryl group to the peri-oxygen atom to give the corresponding 9-aryloxy-l,10anthraquinones (AOA) [I]. The cyclic nature of this photochromic reaction is restricted by degradative processes resulting from the high reactivity of photoinduced 9-aryloxy-l,10anthraquinones toward nucleophiles [2]. It has already been shown [2-4] that in the reactions of 1,10-anthraquinones nucleophilic attack is directed primarily toward the 9-position. In the presence of water, hydrogen sulfide, amines, or the anions of CH acids, replacement of the group present in the 9-position occurs, whereas with alcohols and hydrogen halides, both substitution and addition occur, to give the 9,9-disubstituted l-hydroxy-10-anthrones~ The aim of the present investigation was to make a quantitative study of the reactivities of some AOA containing donor and acceptor substituents in the quinoid nucleus toward alcohols, and to elucidate the mechanism of this reaction. RESULTS AND DISCUSSION This study of the reactions of 1,10-anthraquinones with alcohols was initially carried out with stable 2-alkylamino-9-p-tert-butylphenoxy-l,10-anthraquinones (I) and (II), obtained photochemically [2]. Unlike the photochromic 2- and 4-acylamino-derivatives, these compounds do not undergo the reverse reaction under irradiation, and can be isolated in the pure state. However, quinones (I) and (II) display properties inherent in the highly reactive l,lO-anthraquinone system. For example, they react with alcohols (methanol and n-butanol) at room temperature. By varying the exposure, it was possible to isolate and characterize both the

Novosibirsk Institute of Organic Chemistry, Siberian Branch, Academy of Sciences of the USSR. Institute of Chemical Kinetics and Combustion, Siberian Branch, Academy of Sciences of the USSR, Novosibirsk. Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 2, pp. 366-370, February, 1990. Original article submitted February 15, 1989.

306

0568-5230/90/3902-0306512.50

9

1990 Plenum Publishing Corporation

TABLE i. Rate Constants for the Nucleophilic Addition of Methanol at 25~ in Toluene (k), and the Calculated Values of the Charges on the Oxygen (GO) and Carbon (G C) of the Reaction Site in AOA Derivatives ArO

0

I

l!

l] O

I R~

ArO ~/

R1

II 0

(vlI)--(xIII) Compound (VII) (vln) (ix) (x) (xI) (Xli) (xiIi) *

OMe OH +/ R ~

(VIIa)--(XlIIa)

R,

k.10a , liter. mole"19 1

R2

300 4200 40

H H H H

H

NO2 NHCOCH3 NH~ H

t R2

t

H

OCH3 NHCOCH3

H

H

6,6 0,4 220

GC

Go

0,14

-0,48

0.13 0,t2 0.t| OA4

-0,5 -0,49 -0,47 -0.44

*Pentafluorophenoxy group in the 9-position. mixed 9-alkoxy-9-(p-tert-butylphenoxy)-l-hydroxy-10-anthrones (III) and (IV), and the 9,9dialkoxy-l-hydroxy-10-anthrones (V) and (VI).

x_ 9

7 o/I

x _ 9-

NHR,

k),,)

\//o.

+

k),,A/d

II 0

II 0

(I), (II)

(III), (IV) R20

OR2 OH

___+~ \ / \ (

-~-NHR~

AAr 0 (V), (VII R ~ = CH3 (I), CHiC6H~ (II); R 1 = R 2 = CH3 (III); R ~--- Cff3, RI=R~=CH3(V); RI=CH2CeHs, R i = C H 3 ( V I ) ; X = t - B u .

R~-----n-C4H~ (IV);

It was thus possible to establish the stepwise mechanism of the reaction of AOA with alcohols. The first step is the nucleophilic 1,4-addition of a molecule of the alcohol, as in the case of =,~-unsaturated carbonyl compounds. The second step involves nucleophilic replacement of the aryloxy group in the 9-position by the alkoxy group. The resulting anthrones (III)-(VI) are unstable, readily losing a molecule of alcohol on heating. Thus, the mass spectrum of (V) shows a peak for the ion with m/z 267, formed by loss of a molecule of methanol (299 - 32). Cleavage of a molecule of alcohol can also take place photochemically. The yellow solutions of (V) and (Vl) in nonpolar organic solvents become blue on irradiation (Xma x 603 nm) with the formation of compounds the electronic spectra of which are fully analogous to those of 2-alkylamino-l,10-anthraquinones. It was not possible, however, to isolate the 2-alkylamino-9-methoxy-l,10-anthraquinones in the free state, since they readily underwent conversion to the l-hydroxy-2-alkylamino-9,10-anthraquinones. To assess the effects of substituents in the anthraquinone nucleus on the reaction rate for the nucleophilic addition of alcohols, we examined several amino- and hydroxy-9aryloxy-l,10-anthraquinones (VII)-(XIII) (Table i)o

307

1,0 O

~s

I

I

3O v. li3_, cm-1 Fig. i. Absorption spectrum of 2-methylamino-9(p-tert-butylphenoxy)-l,10-anthraquinone in toluene at 25~ (i), and its change during the dark reaction with methanol (C = 2 mole/liter) after 30 min (2), 3.5 h (3), and 14 h (4).

5

1,0 D

~,1 a,/'~"1,,

b

/,0

2

K'I~,

sec-1

.../ 20

JO

v. i0-, J cm-I

ION30H_~,mole/liter

Fig. 2. a) Absorption spectrum of l-(p-tert-butylphenoxy)-2acetylamino-9,10-anthraquinone (IX) in toluene at 25~ (i), and the changes O n irradiation for one minute in the absence of added alcohol (2), and in the presence of methanol (C = 0.i mole/liter) (3). Change in spectrum (3) in the dark reaction after 3.5 min (4) and 40 min (5). b) Plot of the pseudofirst order rate constant for the reaction of 2-acetylamino9-(p-tert-butylphenoxy)-l,10-anthraquinone (IXa) versus methanol concentration. Compounds (VII)-(XIII) were obtained photochemically, by irradiating the appropriate l-aryloxy-9,10-anthraquinones (C = 10 -4 mole/liter) in toluene with the addition of methanol (10-3-1 mole/liter). The AOA formed on photolysis reacted with the methanol to give compounds which absorbed strongly in the near UV (Ima x = 350-390 nm; Fi~s. i, 2a). The similarity of the spectra of the products (VIIa)-(XIIIa) to those of the previously reported anthrones (III)-(VI) indicates that these 1,10-anthraquinones (VII)-(XIII) also react with methanol to give 9-methoxy-9-(p-tert-butylphenoxy)-l-hydroxy-10-anthrones. The anthrones (VIIa)-(XIIIa) are however less stable, and on isolation are converted into the l-hydroxy9,10-anthraquinones. The addition reaction was examined using a large excess of methanol, so that the kinetic plots are described by a pseudo-first order equation (Fig. 25). Table 1 shows the second order rate constants for (VII)-(XIII) obtained from similar plots. Introduction of a nitro group into the anthraquinone nucleus results in an increase in the rate constant by an order of magnitude, while donor substituents (amino and methoxy) reduce the rate constant by 2-3

308

orders of magnitude. Note that replacement of the p-tert-butylphenyl phenyl has virtually no effect on the rate constant.

group by pentafluoro-

The course of the addition, and the high values of the rate constants obtained for this reaction, may be rationalized in terms of the charge distribution in the AOA molecule obtained by quantum chemical calculations. PhO

0

--0.48

l+O,lCJ

fp +0.05 0 --0.40

As will be seen from the diagram, the greatest electron deficiency is at the carbon in the 9-position, with a considerable negative charge on the quinoid oxygen attached to the C in the 1-position. The positive charge at the 4-position and the negative charge on the second quinoid oxygen are considerably smaller, and have values similar to those in the original l-aryloxy-9,10-anthraquinone. Attack of the alcohol on the 10-position is possible only in the case of 2,3,4,5,8-pentamethyl-l,10-anthraquinone, as a result of the screening of the 9-position by the 8-methyl group [ 5 ] . It will be seen from Table 1 that the introduction of substituents results in changes in the charges both on the carbon, and the oxygen of the reaction site, although these changes are not regular. Qualitative examination of the data shows that the rate constants for nucleophilic addition decrease with increased electron density on the carbon atom, and with decreased electron deficiency on the oxygen. The influence of the electron density on the oxygen atom on the reaction rate appears to show that heterolytic cleavage of the O-H bond precedes the transition state. R

H

\/..

ArO 0

b 0-

/\/\ I +I l We also examined the effect of the type of alcohol on the nudleophilic addition rate constants (k.103, liter.mole-Z.sec-1), these being 40 (methanol), 20 (ethanol), 16 (benzyl alcohol), 4 (propan-2-ol), and