148 Gas-phase thermolysis of tert-butyl hydroperoxide in a nitrogen atmosphere

Recl. Trav. Chim. Pays-Bas 103, 148-152 (1984) 0034-186X/84/05148-05$1.75

Gas-phase thermolysis of tert-butyl hydroperoxide in a nitrogen atmosphere. The effect of added toluene

Peter Mulder and Robert Louw*

Gorlaeus Laboratories, P.O. Box 9502, 2300 RA Leiden, The Netherlands (Received November 23rd, 1983)

Abstract. Gas-phase thermolysis of tert-butyl hydroperoxide 1, 5 1 % m in nitrogen (pyrex reactor, atm. pressure, 200-300°C), gives methane and acetone as the main products. The homo- lytic mechanism involves 0-0 bond dissociation (eqn. 1) and induced decomposition of 1 by hydroxyl and methyl radicals (eqn. 3). In the presence of a 2 5 0 fold excess of toluene, reaction (3) is suppressed, ‘OH and part of the *Me radicals abstracting hydrogen from toluene (eqn. 9). Values for k,, and k,, (abstraction by ‘Me) are derived and discussed. Arrhenius parameters for k l , log A/s-’ = 15.3, EA = 41.4 kcal/mol, are in agreement with the thermochemistry and with the results of the VLPP studies of Benson et al.

Introduction

In the course of our studies on thermal gas-phase reactions of hydroxyl radicals with benzene and its derivatives’, we became interested in using tert-butyl hydroperoxide (1) as a thermal source of *OH (eqn. 1).

A

A ( I ) Me,CO-OH -+ Me3CO* + ‘OH

(2) Me3CO* -+ ‘Me + Me,C=O

(3) 1 + Y’ -+ Me3COO’ + Y-H (Y = alkyl, alkoxy, OH)

starting hydroperoxide. No mention was made of acetone as a product. In view of the very unsatisfactory material balance, major questions remain unanswered as to the mechanism(s) involved. In the present paper we describe and discuss rates and products of the thermolysis of 1 in nitrogen at atmospheric pressure, with and without a large excess of toluene, employing a Pyrex-glass tank flow reactor of 620 ml (21O-28O0C, residence time 1-3 min).

Table I nitrogena.

Thermolysis of Me,COOH (1) in an atmosphere of

Thermal decomposition of hydroperoxides - important intermediates in autoxidation - has already been amply studied, although mainly in solution’. Even in the absence of catalysts such as bases or metal ions, the reactions are mostly very complex. When restricted to homogeneous, free-radical reactions, the rate of 0-0 bond homolysis (eqn. 1) appears to be assisted “by practically anything” (ref. 2a, p. 96). Apart from further splitting of Me3CO’ (eqn. 2), induced decomposition (eqn. 3) often over- shadows reaction (I ) , and can be important even in highly diluted solution3. There are only a few reports on gas-phase thermolysis of 14. Kirk and K ~ o x ~ ~ , using a pyrex or quartz flow reactor, have pyrolyzed 1 in a ca. 50-fold excess of benzene at 10-20 mm pressure between 280-380OC. Biphenyl was found to be an important product, in addition to H,, O,, CO, CH,, C2H6 and C,- and C,-hydrocarbons. The reaction was probably partly heterogeneous. Under VLPP conditions (fused silica reactors, 300-950°C), neat 1 underwent both

unimolecular and chain-type reactions’. Acetone was a major product. Assuming that acetone in only formed via steps (1) and (2), treatment of kinetic data led to high- -pressure-limit rate constants obeying log k , / s - ’ = 15.6 & 0.5 - (42.2 2 kcal/mo1)/(2.3 RT), a value consistent with the thermo~hemis t ry~ . In their study on the autoxidation of isobutane, Lucyuin et aL6 also dealt with the thermolysis of 1. In a flow system (quartz reactor), using CO, as a carrier gas, complete conversion of 1 was achieved in 6 min at 25OoC. Products reported were CH, (1273, C,H, (2.2%), isobutane (4.5 %) and five other C,-C4 compounds comprising ca. 3% of

R u n n o

1-1 1-2 1-3 1-4 1-5‘

21 I 228 255 277 277 __

Tcs, Conver s ion of 1

t ime T ( 5 )

204 I97 187 I80 I14

20 0.21 38 0.40 81 0.85

2 9 5 1.05

Products (mmolih) I

a Intake: N, = 280, 1 = 1.05 mmol/h. XCH, = (CH,) + 2 (C,H,). ‘ Intake: plus acetone = 162 mmol/h. Ace- tone: ca. 0.9 mmol/h. ‘ Not determined; when assuming primary dissociation (eqn. I ) only, with k , = 0.032 s - ’ (see Fig. 3), calculated degree of conversion = 85 %.

l a P. Mulder, forthcoming thesis, Leiden; b P . Mulder and R. LOUW, Tetrahedron Lett. 23, 2605 (1982).

2aR. Hiarr in “Organic Peroxides”, Vol. 11, Ed. D. Swern,

b R . C . P. Cubhon in “Progress in Reaction Kinetics”, Vol. V, Wiley, New York, 1970, Ch. I ;

Ed. G. Porter, Pergamon Press, Oxford, 1970, p. 29. R. Hiatt and K . C . Irwin, J. Org. Chem. 33, 1436 (1968).

4a A . D. Kirk and J . H. Knos, Trans. Faraday SOC. 56, 1296 ( 1 960) ; N . A . Milas and D. M . Surgenor, J. Am. Chem. SOC. 68, 205 (1 946) ; ‘ E. R. Bell, J . H. Raley, F. H . Seubold and W. E. Vaughan,

Disc. Faraday SOC. 10, 246 (1951). S. W. Benson and G . N . Spokes, J. Phys. Chem. 12, 1182 (1968).

ti J . P . Sawerysyn, L.-R. Sochet, M . earlier and M. LucyuE, Bull. SOC. Chim. Fr. 199 (1977).

Recueil, Journal of the Royal Netherlands Chemical Society, 103/5, may 1984

In take C,H,Me (rnrnol/h) (4

149

Products (rnrnol/h)

CH'4 (C6H5CH2)2 I C,H,Et I C,H,CHO

Results

111-1" 101 57

III-2h 95 I I4 III-3h 72 285

A useful flow of 1 was obtained by passing a stream of dry nitrogen through a set of two washing bottles containing liquid 1, maintained at 22°C. Results of calibration, by iodometric titration, are shown in Fig. I .

0.900 0.146 0.469 0.408 0.088 0.97 1 0.045 0.563 0.302 0.070 1.070 0.019 0.770 0.254 0.096

400.

m L 'r- 0 E 300 E u N z

1 20c

100.

0 0.5 1 .o 1:5

- tBuOOH [mmol /h]

Fig. I . Calibration of intake tert-build hydroperoxide (I).

( i ) Thertnolysis in the absence o f toluene Decomposition of 1, about 0.4% m in N,, increased from 20 to nearly 100% between 211 and 277°C (average residence time ca. 3 min.) (Table I) . Yields of CH, and C2H, were quantitatively measured by GLC on samples of exit gas. Unconverted 1 was analyzed by titration of aliquots of condensates collected in a liquid-nitrogen trap. As can be seen from the date presented in Table I, CCH, equals the amount of I decomposed. Acetone was found to be an important product throughout, as corro- borated by a quantitative GLC analysis (run 1-4). tert- -Butanol could not be detected.

( i i ) Thermolysis of Iltoluene

The results of a series of runs involving a large excess of toluene are summarized in Table 11. Apart from methane, bibenzyl and ethylbenzene, together with smaller pro- portions of benzoldehyde, are important products. Benzyl alcohol and benzenc are formed in ca. I yield. C I 4 H l 4 (isomers of bibenzyl) were formed in trace amounts only; phenol or cresols could not be detected. A graphic re- presentation (Fig. 2) includes plots for CCH, (the sum of CH, fragments) as well as for the total of toluene-derived products. The effect of further increasing the [toluene]/[l] intake ratio at 275°C (a temperature at which 1 is largely decom- posed) is exemplified in Table 111.

Trrhle 111 Efkr.1 of the C , H , M e / l intake rulio (27S'C)

Table 11 Thermolysis of 1 in the presence of toluene".

R u n T T no. ("c) (s)

1 1 - 1 196 117 11-2 2 1 1 1 1 3 11-3 221 I I I 11-4 230 109 11-5 240 107 11-6 249 105 11-7 261 102 11-8 270 101 11-9 281 99 I I - l O h 278 95 1 1 - 1 1 288 97

I

- 0. I44

0.336

0.548 0.65 I

0.939

-

-

-

-

0.008

0.025

0.064 0.085

0.132

-

-

-

Products (mmol ih)

0.006 0.016 0.055 0.092 0.154 0.224 0.31 I 0.371 0.408 0.515 0.41 1

C,HsEt

0.01 I 0.020 0.045 0.063 0.088 0.129 0.165 0.195 0.209 0.397 0.217

0.004 0.004 0.013 0.057 0.058 0.062 0.046 0.052 0.039 0.084 0.051

' Intake N2: 440, C,H,Me: 57, I: 0.78 rnrnol/h. 465, C,H,Me: 57, 1: 1.17 mrnol/h.

Intake N,:

c '1 0 E E

l

I I

I

D

200 25 0 300 - T[OC] Fig. 2. Thermolysis of I in the presence of toluene; com- position of outflow as a ,function of temperature (cf. Tables I/ and I V ) .

2 (C,H,CH,CH,C,H,) f C,H,C,H, + C,H,CHO; c: CH,; D: C,H,CH,CH,C,H,; E: C,H,C,H,; F: C,H,; G: C,H,CHO.

A : 1; B: CCH3 = Z (CzH,) + CH4+ C,H,C,H, ECBZ =

R u n n o

" Intake N,: 435, 1: 1.13 mniol /h . Intake N2: 410, I : 1.02 rnmol/h.

Gas-phase thermolysis of tert-butyl hydroperoxide in a nitrogen, atmosphere

the intermediate can decompose via two pathways (see reaction scheme). However, path (a), formation of 2 Me3CO' and 0,, is the

1.9 2.0 2.1 -- 1000 C K - ' J T

k Fig. 3. 0 :pyrolysis o f 1 in an excess of toluene. A : neat pyrolysis of 1. Benson's line' ( - - - - -).

Arrhenius plot for tBuOOH 2 tBuO' + 'OH.

Such an increase leads to a further suppression of produc- tion of C,H, and to a decrease in the ratio [C,H,Et]/ [bibenzyl], from 0.87 (run 111-1) to 0.33 (run 111-3). The yield of benzaldehyde is fairly constant.

Discussion

Rates and products justify a mechanistic discussion in terms of homolytic reactions. With reaction (1) as the obvious first step, Me3CO', under the present conditions, will quantitatively split off 'Me (eqn. 2)', as is evidenced by the fact that tBuOH was not present to any measurable extent. In "neat" pyrolysis of 1, at moderate degrees of conversion, reactions (3a) and (3b) will be major pathways for the conversion of 'OH and 'Me, respectively.

(3a) *OH + 1 -+ H,O + Me3COO' (3b) 'Me + 1 -+ CH, + Me3COO'

At 211°C (run 1, Table I) , not only is the [CH,]/[C,H,] product ratio large, but the amount of CH, is close to that of 1 decomposed. In this case most of the methyl radicals abstract hydrogen from 1. In other words, the major part of 1 reacts via induced decomposition. The Me3COO' radicals thus formed are likely to react with each other (eqn. 4), which leads to regeneration of one Me' per tert- -butyl fragment.

(4) 2 Me,COO' ---* (tetraoxide) +

-+ 0, + 2 Me,C=O + 2 Me'

Reversible formation of tetraoxide is well documented for liquid-phase reactions at room temperature and below';

(a) - 2RO' + 0,

2 ROO' s (ROO),= - (b) R O O R + O ,

predominating step even at room temperature. Further, under our conditions, reaction (b), even if occurring, will be followed by homolysis, RO-OR bond splitting being much faster than dissociation of RO-OH (vide irzfra). Kirsch and Parkes' observed, while studying the recom- bination reaction of tBuOO', formation of some t-butyl peroxide in the gas phase at 60"C, but this may be the re- sult of combination of two Me3CO' radicals. At higher temperatures (and hence, higher degrees of conversion of l), reaction (3) is less predominant. At 277°C (run 1-4), the fraction of methyl radicals forming CH, has decreased by 50%. Under these conditions the chemistry will be rather complex; for example, alkyl radicals can interact with the 0, produced in step (4) and products such as acetone will undergo free-radical attack. At 228°C (run I-2), 38% decomposition of 1 corresponds to an overall first-order rate constant, kexp N 3.3 x s-'. Judging from the amount of CH, produced (0.32 mmol/h or 80% on 1 decomposed) primary bond disso- ciation (eqn. 1) will account for 20% of the overall decom- position. Hence, k, N 6.6 x lo-, s- ' . This value is somewhat lower than that calculated from Benson's parameters', uiz. 14 x lo-, s - ' . A corresponding treat- ment for run 1-3 (255°C) leads to k, = 0.35 x kexp N

0.35 x 2.3 x lo-' = 8.0 x s- ' (calculated': 16 x

Hydroxyl radicals are so reactive towards molecules that they will play a negligible role in termination. Since radical-radical combinations involving Me3COO' (eqn. 4) will regenerate reactive methyl radicals, it seems justi- fiable to consider eqn. (5) as the major termination step, at least at moderate temperatures.

(5) 2'Me + C,H,

The rate of formation of ethane is then a measure of the rate of of 0-0 bond homolysis (eqn. 1). Indeed, produc- tion of ethane corresponds to the proportion of 1 reacting as described above (runds I-2,3). For reaction at 211°C (run I-l), C,H, accounts for (2 x 0.014/1.05) x 100 = 2.7% of primary decomposition, leading to k , N 1.65 x lo-, s - ' . This value is in line with the data obtained at the other temperatures mentioned above. On the basis of this mechanism, values for k3b can also be calculated. At 228°C (run 1-2), the rate of production of C,H, is 2.1 x lo-* M . s - ' . Using l o g k , / M - ' . s - ' = 10.3 (ref. 9), ['Me] is calculated to be 1.0 x M. The rate of formation of CH,, 1.44 x lo-' M ' s - ' , equals k3b x ['Me] x [l]. The stationary concentration of 1 is 5.74 x lo-' M in this case, with the result that k Jb N

2.5 x 10, M - ' ' s - ' . Analogously, k3b N 6.0 x 10, at 255°C (run 1-3). These values are close to those obtained for H abstraction from formaldehyde", but about a

1 0 - 3 s-1).

' L. Blutt and G. N. Robinson, Int. J . Chem. Kinet. 14, 1053 ( 1 982). P.S. Nungru and S. W. Benson, Int. J . Chem. Kinet. 12, 29

L. J. Kirsch and D. A. Parkes, J . Chem. SOC. Faraday Trans. I 77, 293 (1981). J . A. Kerr in "Free Radicals", Vol. I, Ed. J . K. Kochi, Wiley, New York, 1973, Ch. I.

( 1980) ;

l o S. Toby and K. 0. Kutschke, Can. J . Chem. 37, 672 (1959).

Recueil, Journal of the Royal Netherlands Chemical Society, 103/5, may 1984 151

hundred-fold larger than those derived for 'Me + neo- pentane" or acetone" (3 x lo4 M - ' . s - ' at 250°C). This underscores the proposition that reaction (3b') will play a negligible role and that only large proportions of (added) acetone can compete with 1 for 'Me radicals (eqn. 6b).

(3bl)

(6a)

(6b)

'Me + 1 -+ CH, + 'CH,-CMe,OOH

'OH + Me,C=O -, H,O + MeC(O)CH,'

'Me + Me,C=O -+ CH, + MeC(O)CH,'

In run 1-5, reactions (6a,b) will trap all the 'OH and nearly all the 'Me radicals which arise from step (1). The observation that the yield of CH, + C,H6 is over 150% shows that acetonyl radicals give rise to additional CH,. Likely candidates are reactions (7) and @)I3.

(7) MeC(O)CH,' + Me,COO' -+ (peroxide) -+

'Me + Me,C=O + MeC(O)CH,O'

MeC(O)CH,O' -+ CH,O + CO + 'Me

(8) MeC(O)CH,' + 0, - MeC(O)CH,OO' 3 CH,O + CO, + 'Me

In exp. 1-5, the rate of formation of C,H, is cu. 2.5 x M . s- ' , leading to ['Me] N 1 .1 x lop9 M. With u(CH4) = k,, x ['Me] x [acetone] = 6.4 x lo-' M.s - ' , k6, is found to be 7.3 x lo-, M-I ' s - I , in excellent agreement with the literature" value of 7.8 x lo-, M - ' . s - ' .

Reactions in an e x c e . ~ of toluene

When using intake ratios [C6H5Me]/[l] 2 50, reaction (9a) will replace (3a), since k,, is as high as lo9 M- ' . s - ' (ref. 14). Reaction (9b) will compete with (3b). Important termination reactions, in addition to reaction (5 ) , are now reactions ( 1 0) and ( I 1)

(9a) 'OH + C,H,CH, -+

(9b) 'Me + C6H,CH, ---t

(10) 'Me + C,H,CH,' -+

( 1 1) 2C6H,CH,' -,

Assuming, as a first approximation, that methane is only formed via reaction 9b, the data given in Table 111 allow a calculation of kgb, via u(CH,) = k g b x ['Me] x [C,H,CH,]. As rates of formation of C,H, lead to [.Me] = 1.81 x lo-', 1.01 x lo-, and 6.54 x 1O-IoM for runs III-I,2,3, respectively, the corresponding values for k, , are 8.62 x lo4, 8.91 x lo4 and 7.97 x lo4 M-'. s - I . From Mulcuhy's activation parameters", k, , at 275OC is calculated to be 5.5 x lo4 M - ' . s - ' . Our number may be somewhat higher due to our neglecting the contribution via reaction (3b). Since k3, /k9, is around lo2, 1 will compete with toluene for methyl radicals, especially at lower degrees of conversion. That reaction (3b) remains important, even at high [C6H5Me]/[l] (intake) ratios, is seen in Table 111. At 275"C, when reaction ( I ) would involve CQ. 80% decomposition of 1, an increase of the [C,H,Me]/[l] intake ratio from ca. 50 to ca. 280 leads to an increase in the ratio [CBz]/[l] [CBz = 2 C,H,CH,CH,C,H, + C,H,Et + C,H,CHO] from 1.27 (run 111-I), via 1.46 to 1.85 (run 111-3). At higher temperatures, it is seen from Fig. 3 that X H , , as well as CBz, comprise cu. 130% of intake 1. Additional CH, may arise from toluene via reaction ( 12)',. Although there is a likely source of H atoms (uide infra, reactions 13,

(12) 14), the amount of benzene which we observed (despite experimental difficulties due to its relatively high volatility) appears to be too low to make eqn. (12) a fully satisfactory rationale. Since both Me,COO' and C,H,CH; are present, reaction (13) is likely to occur. The intermediate peroxide will have a half-life of only 2 s at 230OC" and hence will decompose

C,H,Me + 'H -+ C,H, + 'Me

(13) Me,COO' + C,HSCH,' -,

[Me,COOCH,C,H,] -,

-+ Me' + Me,C=O + C6HsCH,0'

to yield Me' and C,H,CH,O'. The latter radical will lose a H atom (eqn. 14) rather than react by abstraction of hydrogen from toluene or 1. AHl4 is only cu. 16 kcal/mol*

(14) C6H,CH,0' -+ H' + C6H,CH0

( A H , , N 16 kcal/mol) and hence the energy of activation for decomposition will be around 20 kcal/mol, making C6H,CH,0' only a little more stable than Me,CO'. Reaction (14) not only offers a rationale for the production of benzaldehyde but also entails formation of H atoms (uide supra). The rate of formation, above ca. 23OoC, is only slightly temperature-dependent (Fig. 2), although (C6H,CH,') is likely to increase with increasing tempera- ture, while formation of Me,COO' will be less significant at higher degrees of conversion of 1. The data shown in Tables I1 and I l l also allow a calculation of the value of cross-combination ratio for Me' and C6H,CH,*; [C,H,Et]/([bibenzyl] x [C,H,])* is cu. 1.3 0.2 between 220 and 270°C, close to the statistical value of 2. M u l ~ a h y ' ~ reported a value of 1.3 0.3 (220-260°C).

Induced decomposition of 1 (eqn. 3), as well as radical- -radical reactions (4) and (1 3), regenerate free radicals; hence, the rate of production of radicals (eqn. 1) should be equal to the sum of the rates of the termination steps, uiz. (5), (10) and ( I l ) , as measured by the sum of C,H,, C,H,Et and bibenzyl. Taking the amount of benzaldehyde as a measure of the induced decomposition of 1, overall rates of decomposition of 1 and subsequently k , can be calculated (Table IV).

* AHF(Me-CH,O'), = - 5 kcal/mol18; the increment for Me-CH,Z + C6H,CH,Z is 32 2 kcal/mol for Z = Me, H, OMe, SMe, CI, Br, acetyl, etc.Ig. Hence, AH; (C,H,CH,O'), is cu. 27. With AHP(C,H,CHO)g = -'9, AHl4 is CN. 16 kcal/ mol. J . A. Kerr and D. Tirnliri, J. Chem. SOC. A 1241 (1969).

l 2 N . L. Arthur, P . Grciv and A. A . Herocl, Can. J. Chem. 41, 1346

l 3 J . A. Burritii.d, Adv. Chem. Scr. 76, 98 (1968). l 4 F. P. Tully, A . R . Ravishankara, R. L. Thompson, J . M.

Nicovich, R . C. Shah, N . M . Kreurer and P . H. Wine, J. Phys. Chem. 85, 2262 (1981).

l 5 M . F. R. Mulcahy, D . J . W i l l i a m and J . R. Wilmshursr, Austr. J. Chem. 17, 1329 (1964).

l 6 A . A m m o , 0. Horie and N . H . Nank, Int. J. Chem. Kinet. 8, 321 (1967). T. Koeniyin in "Free Radicals", Vol. I , Ed. J . K. Kochi, Wiley, New York, 1973, p. 122. J . K. Kochi in "Free Radicals", Vol. I I , Ed. J. K. Kochi, Wiley, New York, 1973, p. 680.

1 9 ' D . R. Stull, E. F. Westrum, Jr. and G . C. Sinke, "The Chemical Thermodynamics of Organic Compounds", Wiley, New York, 1969.

bS. W . Benson, F. R. Cruickshunk, D . M . Golden, G. R. Haugen, H. E. O'Neal , A . S. Rorlyers, R. Shuw and R. Wulsli, Chem. Rev. 69, 279 ( I 969).

( 1069).

152 Photochemical reaction of some aromatic thiones with (methy1thio)acetylenes

Table I V Rate constants .for Me,CO-OH bond dissociation"

T (" C)

210 220 230 240 250 260

1 I3 I l l 109 107 I05 I03

------I log k , / s - l Conversion of 1 (%)

Induced" 0-0 homolysis'

0.5 4.6 3.7 11.5 7.3 20.5 7.4 34.0 7.5 50.4 5.9 69.2

Total

- 3.37

- 2.27 - 1.94 - 1.57

Data interpolated from Fig. 2 and Table 11. Based on benzaldehyde. Based on Z(C,H,CH,CH,C,H, + C,H,Et + C2H,). k l is calculated from I&,,; for example, at 220°C

k,,,, = - - = 1.61 1 0 - 3 ~ - l , k , = x 1.61 x

or l o g k , = -2.91.

1 15.2 11.5 1 1 I 84.8 11.5 + 3.7

= 1.22 x

These values of k , are plotted in Fig. 3; a good straight line is obtained (r = 0.998), with Arrhenius parameters: log A / s - ' = 15.3, E, = 41.4. In this Figure, the three values for k l for thermolysis of neat 1 are also represented. Given the rather different approach to k l , and recalling that in the neat pyrolysis reaction ( I ) , 0-0 bond homo- lysis constituted less than 35 % of the overall conversion of 1, the difference between the two sets of data of 50.4 log k units is acceptable. Note that the line pertaining to reac- tions involving toluene (where induced decomposition is much less important) is very close to that inferred from Benson's parameters5. Altogether we are convinced that undcr our condi t ions - atmospheric pressure, pyrex

reactor, 5 1 % concentrations of hydroperoxide, 21& 280°C - the primary reaction of 1 involves 0-0 bond homolysis, to give *OH and *Me radicals, their fate depending upon the composition of i, the pyrolyzing mixture.

Experimental

Chemicals

Me,COOH (1) - Fluka, Technical grade, 80% - was distilled twice in uucuo (b.p. 32OC/18 mm). Iodometric titration - method I of ref. 20 - showed a purity of 98 f 1 %. Toluene (Merck p.a.) and thank nitrogen (Loosco 99.99 %) were used as such. The apparatus and method of thermolysis have been previously described2'.

Analysis

Methane and ethane were determined on a Carbosieve B column (Supelco) at 190°C (N2 as a carrier gas), using a Becker gas chromatograph equipped with a FID detector, by injecting 0.50 ml samples of exit gas. Peak surface were calibrated by com- parison with data on gas mixtures of known composition. Other organic (liquid) products were analysed using a HP 5700 gas chromatograph using a 40 m SE-30 column. Conditions: [H2] = 0.4 atm. (flow ca. 1 ml/min. Tnj = qe, = 220°C. Temp. prog.: IOOOC (8 min.), 8OC/min., 2OOOC (10 min.). The relative concentrations of products were calculated from integrated area ratios using p-ClC,H,CI as an internal standard.

2 o R . D. Mair and A . J . Graupner, Anal. Chem. 36, 194 (1964). 2 1 R. LOUMJ and H . J . Lucas, Recl. Trav. Chim. Pays-Bas 92, 55

(1973).

Recl. Trav. Chim. Pays-Bas 103, 152-164 (1984) 0034- 186X/84/05 152- 13$3.75

Photochemical reaction of some aromatic thiones with aryl- and alkyl-substituted (methy1thio)acetylenes. The (2 + 2)-cycloaddition products and the occurrence of the equilibrium thiete a, P-unsaturated thioxo compound

A. C. Brouwer and H. J. T. Bos

Laboratory for Organic Chemistry, Rijksuniversiteit, Croesestraat 79, 3522 AD Utrecht, The Netherlands (Received June 21st, 1983)

Abstract. Upon irradiation (h > 525 nm), xanthenethione 20 and thioxanthenethione 2s react in a non-regiospecific manner with the triple bond of the acetylenes 6a (Ph-C-C-SCH,) and 6b [(2-thienyl)-C=C-SCH3] to produce in each case the two possible (2 + 2)-cycloaddition prod- ucts, which were isolated as thietes or a,P-unsaturated thioxo compounds. In a solution of each of these reaction products, spectroscopic results confirmed the occurrence of the equilibrium thiete a,P-unsaturated thioxo compound. With acetylene 6c (t-Bu-CEC-SCH,), the photochemically excited thiones 20 and 2s gave, in each case, only the (2 + 2)-cycloaddition product 8. In solution these thietes 8 do not equilibrate with the corresponding a,P-unsaturated thiones. Surprisingly, no products could be isolated or detected after irradiation of thiobenzophenone 2h in the presence of acetylenes 6a or 6b.

General introduction

For some time, we have been interested in the photo- chemical reactions of thiones with acetylenes'-'. We have previously reported that, upon long-wavelength irradia- tion (h > 525 nm), the aromatic thiones 2 react with bis- (alky1thio)acetylenes 1 in a (2 + 2)-cycloaddition reaction to give thietes 3 which rearrange into a,P-unsaturated dithioesters 4.

* Photochemistry of Acetylenes. Part XI11. Part of the dis- sertation of A . C. Brouwer, University of Utrecht, 1979. A . C. Brouwer and H . J . T. Bos, Tetrahedron Lett. 209 (1976). A . C. Brouwer, A . V . E. George, D. Seijkem and H . J . T. Bos, Tetrahedron Lett. 4839 (1978). A . C. Brouwer, A . V. E. George and H. J . T. Bos, Recl. Trav. Chim. Pays-Bas 102, 83 (1983). A . C. Brouwer and H . J . T. Bos, Recl. Trav. Chim. Pays-Bas 102, 91 (1983). A . C. Brouwer and H. J . T. Bos, Recl. Trav. Chim. Pays-Bas 102, 103 (1983).