076 J. Chem. SOC. (B), 1968

Neighbouring-Group Participation in Pyrolysis of Aryl Azides By L. K. Dyall * and J. E. Kemp, Department of Chemistry,The University of Newcastle, New South Wales, 2308,

Australia

Kinetic studies of the pyrolysis of a series of substituted phenyl azides in decalin or di-n-butyl phthalate solutions have revealed substantial anchimeric assistance when a phenylazo-, nitro-, acetyl, or benzoyl group is the ortho- substituent. E,, Values for these four azides are respectively 22.4, 26.2, 25.8, and 27.2 kcal. mole-l, while for 4-nitrophenyl azide, it is 40.6. Corresponding 10- llA values are 3.0, 19, 5.8, 5.3, and 690,000. Driving-force values are calculated, and constitute the first set of quantitative data available on neighbouring-group abilities in reactions involving concerted n- bond reorganization.

EaCt Values for 4-methyl-, 3-methyl-, and 6-methyl-2-nitrophenyl azide are 27.3, 29.0, and 28.8 kcal. mole- l, with corresponding 10- llA values of 42, 44, and 10. These results demonstrate the requirement that the azido- and nitro-groups both lie in the plane of the aromatic ring for ready cyclization.

MANY phen yl azides with unsaturated ortho-subst it uent s can be pyrolysed to obtain cyclic products (see Scheme 1). Examples are known where the ortho-substituent is nitro,l-s carbonyl,*-12 phenylazo,13J4 phenyl,5J5yl6 thio- phenyl,12 phenylsulphonyl,12 and a~ometh ine ,~J~ and the cyclic pyrolysis products are respectively furoxans, ant hranils, benzo t riazoles, carbazoles, pheno t hiazines, phenot hiazine dioxides, and benzimidazoles or indazoles.

M. A. Forster and J. H. Schaeppi, J . Chem. SOC., 1912, 101,

0. Turek, Chimie at lusdustrie, Special Number, 1933, 883. P. A. S. Smith and B. B. Brown, J . Amer. Chem. SOC., 1951,

* T. F. Fagley, J. R. Sutter, and R. L. Oglukian, J . Amer.

E. A. Birkhimer, B. Norup, and T. A. Bak, Acta Chenz.

E. Anderson, E. A. Birkhimer, and T. A. Bak, Acta Chem.

S. Patai and Y . Gotshal, J . Chem. SOC. (B) , 1966, 489. A. R. Katritzky, A. J. Boulton, and P. B. Ghosh, J . Chem.

1359.

73, 2435.

Chern. SOC., 1957, 78, 5567.

Scand., 1960, 14, 1894.

Scand., 1960, 14, 1899.

SOC. (B) , 1966, 1011.

While many of these cyclizations are believed to occur after a molecule of nitrogen has been evolved to form an intermediate nitrene,l6# l7 it is widely recognized that the nitro-group in 2-nitrophenyl azide participates in the displacement of the molecule of Our own qualitative workl8 on the pyrolysis of 2-sub- stituted phenyl azides indicated that acetyl, benzoyl,

E. Bamberger and E. Demuth, Chem. Ber., 1901, 34, 3874. lo A. Schearschmidt, -4. Constandachi, and M. Thiele, Chevn.

l1 L. Gattermann and R. Ebert, Chem. Ber., 1916, 49, 2117. 12 P. A. S. Smith, B. E. Brown, R. K. Putney, and R. F.

1s T. Zincke and A. T. Lawson, Chem. Ber., 1887, 20, 1176. l 4 T. Zincke and H. Jaenke, Chem. Ber., 1888, 21, 546. 15 P. A. S. Smith, J. H. Clegg, and J. H. Hall, J . Org. Chein..

l6 G. Smolinsky, J . Anter. Chem. SOC., 1961, 83, 2489. l7 L. Krbecheck and H. Takimoto, J . Org. Chem., 1964, 29,

l8 L. K. Dyall and J. E. Kemp, Austral. J . Chem., 1967, 20,

Be),., 1916, 49, 1632.

Reinisch, J . Amer. Chenz. SOG., 1957, 75, 6335.

1955, 23, 524.

1150, 3630.

1625.

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Phys. Org.

and phenylazo Likewise functioned as a neighbouring group in the displacement of nitrogen from the aryl azide. We now report a kinetic study of the pyrolyses of aryl azides in which a nitro-, methoxycarbonyl,

977

SCHEME 1

li ydrosyme t hyl, acetyl, benzo yl, or phen ylazo-group was the ortho-substituent. The results augment the scant information available on neighbouring-group abilities in conjugated systems.

EXPERIMENTAL MateriaZs.-4-Acetylphenyl azide and 4-methyl-2-nitro-

phenyl azide were each prepared by Procedure A of Smith and Brown.19 The former azide crystallized from light petroleum as colourless needles m.p. 43' (lit., 44O) and the latter as yellow needles, m.p. 35-36'. Z-Hydroxymethyl- phenyl azide was likewise prepared and then purified by vacuum sublimation.20 We have previously described the preparation and purification of the remaining phenyl azides .21

Di-n-butyl phthalate was washed with 5% sodium hydroxide solution and water, dried (MgSO,), and then fractionally distilled under reduced pressure ; 22 nD20 1.4930 (lit., 1.4926). Decalin was distilled under reduced pressure under nitrogen.

Kinetic Measureinents.-The weighed sample of the aryl azide (20-30 mg.) was added in a small Teflon boat to the stirred solvent [decalin or di-n-butyl phthalate (10 ml.)] in a reaction vessel surrounded by the vapour of a liquid boiling under reflux in a lagged glass vessel. The tem- perature of the reaction mixture could normally be held constant to f0-1' by this method, and the run was dis- carded if atmospheric pressure fluctuations varied the temperature outside these limits. .

The volume of evolved nitrogen was recorded a t atmos- pheric pressure by collection in a 5 ml. gas burette housed within the heating vessel. Runs were usually followed for three to four half-lives, and never for less than two. It was established for each run that the data fitted a first-order kinetic plot. The first-order rate constant K , was then calculated by the graphical procedure of G~ggenheim.~~ The Arrhenius pre-exponential factor A , and values of Eact and AS,t, were calculated by the usual meth0ds.2~ The results are presented in Table 1.

At temperatures above llOo, the pyrolyses in decalin solution exhibited occasional short-lived fluctuations in rate ; the kinetics were otherwise first-order. This erratic behaviour was attributed to peroxides in the solvent, and was eliminated in many instances by addition of 2,6-di-t- butyl-4-methylphenol (30 mg. per run). Rates obtained in the presence of this antioxidant matched those obtained

l9 P. A. S. Smith and B. B. Brown, J . Amer. Chela. Soc., 1951, 73, 2438.

2 l L. K. Dyall and J. E. Kemp, Austral. J . Ckem., 1967, 20,

22 A. Weissberger, ' Techniques of Organic Chemistry,' Inter-

G. Smolinsky, J . Org. Chem., 1961, 26, 4108.

1395.

science, New York, 1955, vol. VII.

TABLE 1 Kinetic data for pyrolysis of %substituted phenyl azides

(Decalin solution unless otherwise noted) Substituent in phenyl

azide 2-NzNPh

2 - A C

2-BZ

2-NO2

2-N02-3-Me

Arrhenius 104k, A

Temp. (sec.-l) factor I

40-1 56.5 4.34, 4-45 68.9 14.9, 15.1 80-4 43.8, 44.9

44.5 * 87-6 99.7 2.64, 2-69

111.0 7.35, 7-42 118.0 11.9, 12.7

110.8 1-70, 1.66 117.6 3.26, 2.99 132.3 10.4, 11.0

0.667, 0.718 3.0 X 10"

0.719, 0.752 5.8 x lo1'

99.5 0.526, 0.580 5.3 X 1Ol1

80.5 1-38, 1.18 1.9 X 1OI2 87.0 2.40, 2-19 99.4 8.08, 7.82

110.4 22.0, 22-0 21.9 *

80.7 87.5 1.22, 1.32 99.5 4.29, 4.57

99.5

0.639, 0.637 4.2 X 10"

110-6 12.4, 13.2

110.7 1.49, 1-55 117.5 2.94, 2.85 132.2 11.1, 10.4

0.436, 0.422 4.4 x 1Ol2

Eact (kcal.

nole-l) 22-4 k 0 . 1

25.8 k 0 . 3

27-2 f 0.2

26-2 - + 0.3

27.3 & 0.2

29.0 & 0.2

2-N02-6-Met: 117.8 0.816, 0.892 1.0 x 1012 28.8 126.5 1.51, 1.53 & 0.8 132.0 2-58, 2.93 144.6 8.90, 8-75

2-C02Me t 161-6 5-3, 8.1

4-AC 161.6 5.9, 5-3 2-CH2OH t 161.6 4.9, 4.3, 3.2

161.6 3-6,t 3.6 4-NOZ t $ 155.1 1.26, 1.46 6.9 x 10l6 40.6

162-3 3.24, 3.40 f 1.2 168.1 4.56, 4-99 176.0 12.8, 13.5

A s a c t (e.u.)

f 0.2 - 8.3

- 7.1 h 0 . S

- 7.4 & 0.5

- 4.9 f. 0.8

- 3.1 3 0.5

- 3.2 & 0.5

-6 f 2

16.0 5 2.8

* Antioxidant added. t Obtained with di-n-butyl phthal- ate solutions. $ For comparison, the A , Ead, and AS,,, figures (averages of two sets of literature values 4*6) for 2-nitro- phenyl azide in di-n-butyl phthalate solution are respectively (2-0 f 0.7) x loza, 26.0 & 0-3, and -4.9 & 1.

from the well-behaved portions of runs where i t was absent. The antioxidant did not affect the rates of those pyrolyses carried out below 110" (see Table 1).

A t temperatures above 140°, all pyrolyses in decalin solution behaved erratically, even with antioxidant present. The nitrogen evolution alternately ceased and surged des- pite rapid stirring of the solution. Some of the azides pyrolysed smoothly in di-n-butyl phthalate solution with- out requiring antioxidant. The pyrolyses of 4-acetyl- and 4-nitro-phenyl azide deviated from their initial first- order kinetics towards the end of the first half-life. In these instances, k , was evaluated for the early stages of reaction by use of the experimental volume of nitrogen evolved at complete reaction. We confirm the observation of Smith and Hall 25 that 4-substituted phenyl azides liberate more than one mole of nitrogen per mole of pyrolysed azide.

23 E. A . Guggenheim, Phi l . Mag., Series 7, 1926, 2, 538. 24 A. Weissberger, S. L. Friess, and E. S. Lewis, ' Investigation

of Rates and Mechanisms of Reactions,' Interscience, New York, 1961, 2nd edn.

25 P. A. S. Smith and J. H. Hall, J . Amev. Cheriz. Soc., 1962, 84, 480.

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978 J. Chem. SOC. (B), 1968

2-Methoxycarbonylphenyl azide pyrolysed erratically even in di-n-butyl phthalate solution, and the rate constants obtained for i t should be regarded as rough estimates.

Pyrolysis Products.-The products of pyrolysis, in decalin solution, of most of these aryl azides have been reported by us previously.l* We now report that 6-methyl-2-nitro- phenyl azide in di-n-butyl phthalate solution yields 4-methyl- benzofuroxan (8 1 %, estimated spectrophotometrically in the ester solution a t 366 mp). The pyrolysis of azides in which there is no formation of cyclic product is known to lead to mixtures of tars, amines, azo-compounds, and other minor product~.~8,~5-2~

1.r. Sfiectra.-These were measured on carbon tetra- chloride solutions (20 mg. azide per ml.) with a Hitachi EPI-G2 spectrophotometer, using 0.1 mm. rock-salt cells. Spectra were calibrated with the polystyrene spectrum and are accurate to & l cm.-l.

DISCUSSION

Identijcation of Neighbouring Groups iiz Aryl Azide PyroZyyses.-Our kinetic results (see Table 1) establish quite clearly that phenylazo-, nitro-, acetyl-, and benzoyl- groups function as neighbouring groups in the displace- ment of nitrogen from the corresponding 2-substituted phenyl azides. The energies of activation for the pyroly- sis of these four aryl azides range from 22.4 to 27.2 kcal. mole-l, whereas the values for aryl azides whose pyrolyses proceed without anchimeric assistance range from 32-5 to 40.6 kcal. mole-l (Table 1 and references 25, 26, and 28).

The entropies of activation for our ortho-substituted phenyl azides are not constant (Table l), but the vari- ations are too small to permit detailed interpretation. The negative values are consistent with the formation of a cyclic transition-state involving the neighbouring g r o ~ p , ~ ~ ~ - ' but it must be noted that small negative entropies are observed for pyrolyses of 2-azidobiphenyls which show no evidence for anchimeric a s~ i s t ance .~~ The large positive entropy change for 4-nitrophenyl azide (16 e.u.) indicates that the loosing of the molecule of nitrogen (whose standard entropy is 45-8 units 29) has made more substantial progress in the transition state than is the case for the anchimerically assisted pyrolyses.

Identification of effective neighbouring groups is most conveniently made on the basis of comparative rates of pyrolysis. These rates, drawn from the literature and the present work, are collected in Table 2. The non- anchimeric substituent effects are small, so that rate increases beyond ten-fold with respect to phenyl azide are clearly diagnostic of neighbouring-group participation. On this basis, the phenylazo-, nitro-, acetyl, and benzoyl substituents give considerable anchimeric assistance to the pyrolysis, whereas arsonic acid, phenyl, phenyl- sulphinyl, methoxycarbonyl, and hydroxymethyl are ineffective. The phenyl substituent does become in- corporated into a cyclic product, but the rate-determining

26 I?. Walker and W. A. Waters, J . Chem. SOC., 1962, 1632. 27 J. H. Hall, J. W. Hill, and H. Tsai, Tetvaliedron Letters, 1966,

28 K. E. Russell, J . Amer. ClLena. SOC., 1955, 77, 3487. 2211.

step is believed to be the formation of a nitrene inter- mediate which subsequently effects the cy~ l i za t ion .~~

The failure of the ortho-hydroxymethyl substituent to assist pyrolysis is significant. Obviously, the driving force is not provided merely by a lone pair of electrons on the ortho-substituent, but by the concerted x-bond reorganization leading to the new heterocyclic ring (see Scheme 2). An equivalent scheme using an alter- native canonical structure for the azido-group may be written,30 as also can one using a lone pair of electrons on the atom Y rather than its x-bond.

SCHEME 2

The energy of activation for formation of the cyclic transition-state will reflect steric strains, changes in delocalization energies of the initial x-bond systems, and the extent to which the delocalization energy of the new heterocycle has become available. The failure of several potential neighbouring groups can be understood in terms of these factors. Participation by an ortho- phenyl group would lead to the unstable tautomer (I) of carbazole, and the nett loss of delocalization energy would account for the absence of neighbouring-group participation. Delocalization energy considerations also predict that methoxycarbonyl will be very much less effective than acetyl as a neighbouring group, for the former must lose a substantial internal resonance energy in order to participate. We attribute the failure of the phenylsulphinyl group to prohibitive angle strains in the required transition state.

H

Steric E$ects on Neighbouring-group Participation.- The concerted x-bond rearrangement (see Scheme 2) requires that both the azido-group and the neighbouring group be essentially in the plane of the aromatic ring. In the 3-methyl-2-nitrophenyl azide molecule this condition is not met, and a decreased degree of anchi- meric assistance (by comparison with 2-nitrophenyl azide or 4-methyl-2-nitrophenyl azide) is indicated by the higher energy of activation and lower rate of re- action. The twisting of the nitro-group from the aromatic ring plane in the 3-methyl compound is clearly indicated by the i.r. spectrum (Table 3). The

z9 F. D. Rossini, D. D. Wagman, W. H. Evans, S. Levine, and I. Jeffe , ' Selected Values of Chemical Thermodynamic Proper- ties,' U.S. Govt. Printing Office, 1952.

30 A. J. Boulton, A. C. G. Gray, and A. R. Katritzky, J . Chem. SOC., 1965, 5958.

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Phys. Org. 979

increased symmetrical N-0 stretching frequency con- stitutes classical evidence for reduced c ~ n j u g a t i o n , ~ ~ * ~ ~ whereas the asymmetric N-0 stretching frequency shows the usual insensitivity to ortho-steric effects.

TABLE 2

Rates of pyrolysis of aryl azides at 161' (Decalin solutions unless otherwise noted)

Substituent 104k, Substituent 104k1 in phenyl (sec.-l) in phenyl (sec.-l)

azide * at 161.2" azide at 161.6" Krel. t H ............... 2-5 2-N=NPh ...... 16,710 6680

4-OMe ......... 11 2-Bz ............ 11 3 45.1 (5) i 2-Ac ............ 634 254

4-NO2 ......... 3.5 2-N02 ......... 1343 537

4-Ac ............ 5.3 2-N02-4-Me ... 808

2-AsO3H2 ... 12 2-C02Me ...... 6.7 1.3

(2.6) : (1470) : (;it) (3.6) $ 2-NO,-3-Me ... 119 47.6

4-Ph ............ 5.7 2-N02-6-Me ... 35 $ 7.0

2-SOPh ...... 8.5 2-CH20H ...... 4.1 : 0.82 2-Ph ............ 4.3

* The results for decalin solutions in this column are those of Smith and Hall ( J . Amer. Chem. Soc., 1962, 84, 480), except those for 4-acetylphenyl azide which are our own. t Rates relative to that of phenyl azide in the appropriate solvent. $ Values for di-n-butyl phthalate solutions. The figure for 2-nitrophenyl azide is calculated from the data of Patai and Gotshal.'

TABLE 3

N-0 Stretching frequencies in aryl azides (carbon tetrachloride solution)

Substituent in Asym Y(N-0) Sym v(N-0) 2-nitrophenyl azide (cm.-l) (cm.-l) 4-Me ..................... 1534 1345 3-Me ..................... 1538 1366 6-Me ..................... 1533 1344

There is only slight evidence for anchimeric assistance in the pyrolysis of 6-methyl-2-nitrophenyl azide (Table 2), and the cyclic product (81% yield) was accompanied by tars, which suggests the intrusion of other reaction mechanisms. The i.r. spectrum of this azide shows that the nitro-group is coplanar with the aromatic ring (see Table 3) but yields no stereochemical information on the azido-group since the vibrations of this group are not simple.21 Stereomodels indicate that the azido-group in this compound cannot lie in or near the ring plane, and in these circumstances the adjacent nitro-group cannot effectively assist the pyrolysis.

The qualitative evidence obtained by Katritzky and his co-workers 30933 on the ease of pyrolysis of 3- or 6-sub- stituted-2-nitrophenyl azides demonstrates that sub- stituents other than methyl can also cause steric hind- rance to the pyrolysis.

Quantitative Treatments of Anchimeric Assistance.- Neighbouring-group abilities in carbonium ion reactions are defined 34 in terms of the driving force L = RT In ( k ~ / k , ) , where kA is the rate constant for anchimerically- assisted solvolysis via a bridged ion and k , is the rate

31 V. van Veen, P. E. Verkade, and B. M. Wepster, Rec.

31 J. Trotter, Canad. J . Chem., 1959, 37, 1487. Trav. chim., 1957, 76, 801.

constant for the hypothetical unassisted sdvolysis. By analogy, neighbouring-group abilities in these azide pyrolyses can be defined in terms of assisted pyrolysis via a ' bridged nitrene ' and the unassisted pyrolysis via a free nitrene intermediate. We have equated k , with the rate of pyrolysis of the corresponding 4-substituted phenyl azide. The L-values for phenylazo-, nitro-, acetyl, and benzoyl groups are given in Table 4. The value for the nitro-group is demonstrated to be only very slightly sensitive to change of reaction temperature or solvent.

TABLE 4 Values of driving force ( L = RT In K A / K , ) in aryl azide

pyrolysis ortho- 1 0 4 1 ~ 1 0 4 ~ ~ L

Substituent Solvent Temp. (sec.-l) (sec.-l) kA/K, (kcal.) N=NPh Decalin 161.6 16,710 5 * 3342 6.99 Ac Decalin 161.6 634 5.5 115 4.10 Bz Decalin 161.6 113 5 *

Decalin 161.6 1343 3.6t 373 5-10 Decalin 141.3 256 0.657 394 4.91 DNBP 161.6 1470 2-7 544 5.43

22-6 2.69 NO2

* This value is the average for the two ---type para- substituents used in this work. ? These values are those of Smith and Hall ( J . Amer. Chem. SOC., 1962, 84, 480). : Di-n- butyl phthalate.

These L-values inevitably reflect steric acceleration in some of the azides. When the ortho-substituent is a carbonyl group COR, it can give rise to two conformations with respect to the azido-group.

The conformer (11) involves lone-pair repulsions which are not present in (111) for some identities of the group R. When R = OMe, both conformations involve this un- favourable energy term. The i.r. spectrum (carbon tetrachloride solution) of 2-methoxycarbonylphenyl azide, measured under high resolution, shows two v(C=O) bands (at 1736 and 1726 cm.?) of equal intensity which attribute to these two conformers. We could not resolve two such bands when R was Me or Ph, and we interpret this result to mean that conformer (111) is heavily favoured over (11) in these latter two azides. The energy difference between the reactive pyrolysis species (I) and the unreactive species (111) must, of course, form part of the energy of activation for pyrolysis.

When the ortho-substituent is a nitro-group, the repulsion between the lone pairs on azido-nitrogen and nitro-oxygen is necessarily present in the ground state. By comparison with the acetyl- and benzoyl-substituted azides, the nitrophenyl azide will be sterically accelerated.

[8/220 Received, January Sth, 19681

33 A. J. Boulton, A. C. G. Gray, and A. R. Katritzky, J . Chem.

34 B. Capon, Quart. Rev., 1964, 18, 45. SOC. (B) , 1967, 909.

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