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On the Formation of Alkenylaryl Nitroxide Free Radicals in the Photolysis of 2-Nitrobenzaldehyde-olefin Systems

W . G. F i l b y and. K . G ü n t h e rIn s titu t für Radiochemie, Kernforschungszentrum Karlsruhe

(Z. Naturforsch. 32 b, 693-697 [1977]; received February 22, 1977)

Nitrosobenzoic Acids, ESR, Alkenylaryl Nitroxides, Decay Kinetics

The origin and nature of the radicals arising during the photolysis of 2-nitrobenzaldehyde in the presence of olefin quenchers is discussed. The radicals, assigned an alkenylaryl nitroxide structure, arise via a dark Diels-Alder reaction involving product nitroso benzoic acid and the olefine.

We have recently reported1 on the form ation and assignment of the nitroxide free radical (1)

O'\

^ rco'2i

observed during the photolysis of 2-nitrobenzalde- h y d e2 both in organic solvents and in the melted state.

During the course of a series of photochemical experim ents involving olefinic quenchers in benzene solution we were surprised to observe no t the usual radical 1 bu t a species not previously observed in olefine free systems. A literature survey and further experim ents soon revealed th a t the only possible origin could be a dark reaction between product acid (2-nitrosobenzoic acid) and the olefine quencher. Thus in order to delineate the two possible effects of olefine in this system i.e. photochemical quenching3 and radical form ation via dark reaction(s) it was im portan t to understand more of the la tte r process. Thus we present here, as complement to the work of S u l l i v a n 4, the results of a brief study we undertook with this aim in mind. The work is comprised of an investigation of the E S R spectra observed when some nitro substitu ted (4 nitro; 6 n itro ; 4,6 dinitro; unsubstituted) 2-nitrosobenzoic acids are allowed to react in the dark, w ith some linear and cyclic mono-olefines. For the large part

R equests for reprin ts should be sent to Dr. W. G. F i l b y , In s titu t für Radiochemie, Gesellschaft für Kernforschung, Postfach 3640, D-7500 Karlsruhe 1.

radical properties and E S R param eters parallel those for the alkenylalkylnitroxy radicals reported by S u l l i v a n 4. Mechanistically we prefer the processes described by this author as the source of the free radical interm ediates

2

Experimentala) Materials

All solvents employed in this work were purissum grade and were not further treated . All olefines contained trace am ounts of im purities by gas chromatographic analysis (generally < 0 .05% ) and were similarly not purified. Blank experiments employing olefines from different sources however revealed no alteration in chemical or spectral behaviour.

Ortho-nitrosobenzoic acid and its n itro-substituted derivatives were all prepared by photolysis of the corresponding ortho nitrosubstitu ted aldehyde in benzene solution5. There was no necessity to degas the solutions and the acids precipitated very rapidly as the sole product. They were recrystallised from boiling methanol to chrom atographic purity .

b) E S R spectrometerDetails of the ESR spectrom eter are presented

elsewhere6. Frem y salt (aN = 13.01 G) was used as a reference standard for the measurem ent of hyper- fine coupling constants.c) Preparation of radicals

We found, similar to S u l l i v a n 4, th a t no single procedure served to provide optim al ESR signals

694 W . G. F ilb y -K . G ün ther • Photo lysis of 2 -N itrobenzyldehyde

for all substances. The two procedures outlined below, however yielded solutions containing suffi­cient free radical to be observed w ith instrum ent m odulations of 0.1-0.5 G. A stock solution of nitroso compound (10-2 M in benzene, 5% methanol added to aid solution) was prepared and employed in the two most successful procedures described below.

Procedure (a ): To 10.0 ml of stock nitroso com­pound in an open beaker 0.01 ml of olefin was added. In all cases a rapid yellowing set in resulting in a strongly param agnetic solution. Subsequent degas­sing by freeze-thaw and/or warming the solution lead to no im provem ent in either the intensity or resolution of the spectra.

For air sensitive or otherwise unstable radicals procedure (b) was preferred.

Procedure (b ): Here the two reactants (in concen­tra tion ratios as in procedure 1) were separately degassed in the same specially designed cell and the frozen solutions held separately until immediately before recording the spectra.

Procedure (a) was employed for reactions of2-nitrosobenzoic, 2-nitroso-4,6 dinitrobenzoic, 2 nitroso-4 nitrobenzoic and 2 nitroso-6 nitrobenzoic acids w ith 2,3-dimethylbutene, while the reaction of the same nitroso compounds with cyclohexene,1-methylcyclohexene required procedure (b) in order to provide optim al conditions for ESR ob­servation. Even so the substances varied widely in their reactiv ity and in the stability of the derived radicals.

A ttem pts to induce a radical forming reaction with some linear diolefines (butadiene, hexadiene) failed in all cases a lthough a colour change signifying reaction set in rapidly.

d) Kinetic measurementsOnly procedure (a) was employed for the prepara­

tion of radicals in these experiments. The standard for concentration measurements was solutions of D P P H in benzene and comparisons were made by double integration of first derivative spectra obtained under identical instrum ent conditions. The decay kinetics were followed by autom atically scanning the spectra for 3-4 half-lives starting after achievem ent of the maxim um signal intensity i.e. after radical form ation was essentially complete.

Results and Discussion

1) General features of the E S R spectraIn all the spectra observed except th a t arising

from the reaction between 2 nitroso- 4 nitrobenzoic acid and cyclohexene spectral features arising from interaction of the unpaired electron with the N 14 nucleus (1 = 1) is the dom inant feature (aN 9.4 to11.5 G). Smaller trip le t or quarte t hyperfine split­

tings arising from ring or chain protons lie usually in the range 0.2-3 G.

The former product appears not to follow the p attern shown by the other reactants in th a t a large double proton splitting (11.8 G) dominates the spectrum. This taken in conjunction with the low au value (7.5 G) suggests a radical centre bearing an N -H group. The ra tio aN/aH ° f 0.64 is typical for ortho bonded groups bearing a hydrogen bond to the N -H group. I t is thus likely to be analogous in structure to the m inor radical (1) discussed under heading 2 i. e. it appears th a t no reaction with define has taken place.

Generally the num ber of observable, assignable protons is however very much lower than those present in the structure postulated for the re­sponsible param agnetic species. This is a feature also noted by S u l l i v a n , who accounted for it by suggesting th a t the combined effect of an ortho substituent and a bulky alkyl residue on the nitrogen could lead to considerable redistribution of spin density. This is an effect which could be ex­pected to be, if anything, aggravated in this system, where both alkene and ortho group used are larger than anything employed previously. This may well be the origin for the deceiving simplicity of some of the spectra. Presum ably much could be revealed by ENDOR studies of the more in tractable radicals.

We begin by comparing the aN values of the radicals studied here w ith those of alkenyl alkylnitro- xides and some stable ortho substituted aryl-t-butyl- nitroxides reported by P e t e r s e n et a l.10 and F o r r e s t e r et a l.7>u . Several points are evidenti) the unsubstitu ted phenyl nitroxide ArNHO- and its 2-COOH substitu ted derivative both have the low aN values characteristic of yr-nitroxides in which the nitroxy group is in conjugation w ith the ring. The la tte r is anomalous in the 2-substituted aryl nitroxides in th a t the carboxyl group helps m aintain conjugation by means of an intram olecular hydrogen bond, ii) introduction of bulky groups around the radical centre either in the ring a t the 2 position or in the side chain raises the aN value 4-5 G.

The same tendency is m aintained in the radicals investigated here where comparison of the 2 carb- oxyalkenylnitroxide (Table II) w ith the H, 2 carb­oxyl substitu ted nitroxide (Table I) shows an increase in aN value of 4.3 G. Introduction of a ferJ-butyl group to phenyl nitroxide causes however an aN difference of 4.80. Thus the 2,3 DMB residue

W . G. F ilb y -K . G ü n th e r • Photo lysis o f 2 -N itrobenzyldehyde 695

Table I. ESR spectra of some typical nitroxide radicals X • A r-NO • R.

Substituents (R, X) aN aia a 2 a 3 a*4 a 5 aß Reference

tB u, H 13.35 _ 1.70 0.75 0.75 1.70 8

H, H 8.5 1 1 . 2 2.85 0.96 2.85 0.96 2.85 9H, 2 carboxyl6b 7.93 11.54 - 0.98 2.98 0.98 2.98 1 0

tB u, 2,4 dinitroc 11.4 - - - - - 1 0

tB u, 2,4,6 trin itroc 1 2 . 0 - - - - - 1 0

tB u, 2-chlorod 13.9 — 0.25 - 0.47 — 0.29 0.82 — 0.74 1 1

tB u, 2-bromod 13.9 — 0.26 - 0.43 — 0.27 0.87 — 0.76 1 1

-CMe2 -C(Me)(CH2), 2 -chloroe 12.7 - - - - - - 4-CMe2-C(Me)(CH2), 2-methyl^ 12.3 - - - - - - 4

a ai Represents the hyperfine splitting constants to protons in the N -R side chain (where resolved).Signs, where given, were measured by NMR spectroscopy,

b generated photochemically in te trahydrofuran solution, c measured in m ethylene chloride a t — 50 °C, d measured in carbon tetrachloride.

Table II . ESR coupling param eters for the free radicals resulting from addition of substitu ted 2 carboxynitroso-benzenes to some olefinic solvents.

Ringsubstituent aNi

Coupling constants in Gaussa ai a 2

0 1 efinicbreactant

2 carboxyl 1 2 . 2 2 1.57 (3) 0.82(1)

2 carboxyl, 5 nitro 1 1 . 2 1 2.4 (2) 2,3 DMB10.4 - - - 1 MCH

7.5 2.94(2) 0.95(1) 1 1 . 8 CH

2 carboxyl, 3 nitro 11.4 1.95(2) 2,3 DMB10.03 2.42(3) 1 MCH10.4 - CH

2 carboxyl, 3,5 dinitro 1 1 . 0 2 2 . 1 (2 ) 2,3 DMB9.79 2.52(3) 1 MCH9.38 - CH

a Num ber of equivalent protons in brackets,b 2,3 DMB = 2,3 dim ethylbutene, 1 MCH = 1 methylcyclohexene, CH = cyclohexene, c a second hyperfine splitting due to nitrogen was also observed (0.2 G), d in benzene solution.

seems to be about as effective as a tert-butyl group in steric inhibition of conjugation. U nfortunately d a ta enabling a comparison of the effectiveness of cyclohexyl or 1-methylcyclohexyl is no t available. I t is interesting th a t in none of the n itro derivatives do the aN values i) differ from one another, ii) differ much from the 2 COOH substitu ted case. An observation not unexpected since one would expect an electron w ithdrawing effect in aN only when the ring conjugation rem ains in tact.

I I ) i) The reaction between 2-nitrosobenzoic acid and defines

Here of the olefines used 2,3 dim ethylbutene, cyclohexene, I-methylcyclohexene the la tte r tw o

yielded, on warming, only strong spectra of the species 1 previously reported. No new reaction became apparent even on warming w ith a hundred fold excess of each olefine. W ith 2,3 dim ethylbutene (2,3 DMB) however procedure (a) led to rapid formation of an extremely intense ESR signal, assignable to two radical species. The minor radical is, to judge from the general structure and assign­able hyperfine splittings again the species I, while the m ajor radical, present in 90% concentration, is assigned an alkenylarylnitroxide structure 2. The differing nature of the radicals is reflected in their vastly differing stabilities, for whereas after ten m inutes warming a t 55 °C the low yield radical had completely disappeared the m ajor radical remained

696 W . G. F ilb y -K . G ün ther • Photo lysis of 2 -N itrobenzyldehyde

Table I I I . Second order decay rate constants for nitro substitu ted alkenylalkylnitroxides (X = ring substituent, R = corresponding olefinic residue)f. Tem perature = 28 °C.

Substituents(R ,X )

Initial radical concentration (X 10-5 M/lt)

Decay ra te constant (1 • mole- 1 sec-1)X 10- 2

2,3 DMB, 2 COOHa-e 3.2 28.410.5 31.730.5 30.1

2,3 DMB, (2 COOH, 5 N 0 2) 2 . 6 26.49.4 26.9

2 2 . 6 26.21 MCH (2 COOH, 5 N 0 2) 0 .0 2 2 b 2282,3 DMB (2 COOH, 3 N 0 2) 2.98 24.9

1 0 . 2 25.229.6 24.7

CH (2 COOH, 3 N 0 2) 0.027b 2561 MCH (2 COOH, 3 N 0 2) 0.025b 268d2,3 DMB (2 COOH, 3,5 dinitro) 50.6 v. slowcCH (2 COOH 3,5 dinitro) 0.029b 2 0 0 d1 MCH (2 COOH; 4,6 diNOa) 0.034b 180d

a No reaction observable w ith CH and 1 MCH, b it was not possible to reach essentially higher concentrations, c see tex t, d complex kinetics appear after ~ 2 half lives, e the radical 2-COOH • Ar NHÖ decays w ith a second order rate constant of 30 1 mole- 1 sec- 1 a t 30 °C (unpublished results), f num bering begins a t n itroxy bearing carbon.

sensibly unaltered in concentration. This observa­tion implies th a t i) radical 1 is not a precursor of 2, ii) the two radicals do not react efficiently with one another, iii) 2 possesses an unusual degree of stability, indeed kinetic experiments showed th a t the species generated in benzene solution in the absence of excess olefine decayed with good second order kinetics with a half life of more than 3 months a t 32 °C. Thus the species would appear to be a t least as stable as the structurally similar mesityl £er£-butyl nitroxides reported by F o r r e s t e r et a l .1 (see however below).

E S R concentration measurements of radical yield showed th a t the reaction proceeded to approxim ately 30% completion. This is somewhat lower than the value observed (40%) by S u l l i v a n in his study of nitrosobenzene and 2,3-dimethylbutene.

i i ) The reaction between the mono-nitro derivatives of 2-nitrosobenzoic acid and olefines

Here for both mononitro derivatives studied there was a wide variety of behaviour with the different olefines used. For, while both derivatives gave, in an instantaneous reaction approxim ately equal radical yields of 30-35% in the case of 2,3 DMB reaction with other olefines either yielded no stable radical a t all or only a one of approxim ately 1 % the

intensity used in the unsubstitu ted case. Decay kinetics become more complex changing to a non­integral value a t > 2 half lives (Table II).

The reaction between the 4-nitro derivative and cyclohexene deserves special m ention in this respect, as the doublet proton hyperfine splitting constant (a3 — 11.8 G, Table II) suggests a radical of differing nature to the others, e.g. an N -H containing radical. An “anom alous” kinetic is thus not completely unexpected.

i i i ) The reaction between 2-nitroso, 4,5.dinitrobenzoic acid (3) and olefines

The reaction of 3 w ith 2,3 DMB took place yielding the most stable radical of the series in­vestigated here. The reaction yields the highest radical yield (65%) presum ably because of the extrem ely low removal ra te during its formation. While decay was so slow th a t it was not a ttem pted to measure an accurate decay rate it was observed th a t negligible decrease in concentration occurred on m aintaining a sample a t 45° for six months. Shaking with air produced only line broadening and re-degassing restored the ESR signal to its original intensity. The reactions w ith cyclohexene and 1- methylcyclohexene proceeded only slowly a t room tem perature and warming was generally employed

W . G. F ilb y -K . G ü n th e r • Photo lysis of 2-N itrobenzyldehyde 697

in order to produce sufficient radical for measure­m ent. Even so the steady sta te concentration was approxim ately 1/100 of th a t for the 3/2,3 DMB case and the decay ra te constants derived the highest in the series.

I t is difficult to understand this phenomenon unless other decay pathw ays are open to the cyclo- hexyl based radicals than the familar C -0 para coupling7. I t m ay be significant th a t pathways involving intram olecular hydrogen abstraction from alkyl 2-substituents in 2-methyl alkyl phenyl nitroxides have recently been shown to take place7.

1 W. G. F i l b y and Iv. G ü n t h e r , Z. N aturforsch. 28b, 810 [1973]; Z. Phys. Chem. (Neue Folge) 95, 289[1975].

2 N itro and chloro-substituted 2-nitrobenzaldehydes also undergo th is well docum ented rearrangem ent. N itroxide radicals of varying degrees of stability appear in each case. W. G . F i l b y and K. G ü n t h e r , unpublished resnlts.

3 A. A. L a m o la , “Energy Transfer in Solution” in Energy Transfer and Organic Photochem istry, Interscience Publishers, New York 1969.

4 A. B. S u l l i v a n , J . Org. Chem. 81, 2811 [1966]. This author assigned the radicals in nitrosobenzene/olefine (mainly 2,3 DM B) by means of reductive techniques and standard nitroxide radical synthesis.

5 For a comprehensive discussion of neighboringgroup interactions in ortho-substituted nitrobenzenederivatives see P . N. P r e s t o n and G . T e n n a n t ,Chem. Rev. 72, 627 [1972].

ConclusionsAlkenylaryl nitroxide radicals of varying degrees

of stability result not only from the reaction of nitrosobenzene with alkenes4 but also in the case of heavily substituted nitrosobenzoic acids and alkenes or cycloalkenes. Care m ust thus be exercised in interpreting experiments involving quenching of nitroarom atic photochem istry by olefinic reagents wrhere the above radicals could easily be formed by coupling of nitroso product with olefine quencher12. Stable nitroxide radicals have recently been shown to be exceptionally efficient as trip let quenchers13.

6 W. G. F i l b y and K. G ü n t h e r , J . Chem. Phys. 60, 3355 [1974]; J . Phys. Chem. 78, 1521 [1974],

7 A. R. F o r r e s t e r and S. P. H e p b u r n , J . Chem. Soc. C, 1970, 1277.

8 G. C h a p e l e t - L e t o u r n e u x , H. L e m a ir e , and A. R a s s a t , Bull. Soc. C h im . F r . 1965, 3282.

9 A. L. B u c h a c h e n k o , I z v . AN SSR, OKLN, b, 1120[1963], cited in “ Stable R adicals” , Consultants Bureau, New York 1965.

1 0 J . P e d e r s e n and K. T o r s b l l , Acta Chem. Scand. 25, 3151 [1971].

1 1 A. C a l d e r , A. R. F o r r e s t e r , and S. P. H e p b u r n , J . Chem. Soc., Perkin I, 1973, 456.

1 2 J . H . B o y e r , in H. F e u e r : The Chemistry of the Nitro Nitroso Groups, Cliapt. 5, Interscience Publ., N.Y. 1969.

1 3 C. H. D e p u y and O. L. C h a p m a n , Molecular R eac­tions and Photochem istry, p. 90, Prentice-Hall, Englewood Cliffs 1972.