J . Org. C h m . , Vol. 58, No. 8, 1975 1559
PHOTOCHEMISTRY OF AROMATIC THIOLESTERS
The Photochemistry of Aromatic Thiol Esters JOHN R. GRUNWELL,*~ NEIL A. MARRON, AND SALIBA I. HANHAN Hughes Laboratories, Miami University, Oxford, Ohio 46066 Received August 81, 1976 The photolysis of a variety of aromatic thiol esters WLM investigated in cyclohexane using a 254-nm light source. The photochemical reaction proceeds initially by cleavage of the 8-acyl bond, giving rise to the corresponding aromatic thiyl radical and an acyl radical which sometimes decarbonylates. The resulting radicals recombine to give disulfides, sulfides, and hydrocarbons. Acyl radicals which are less prone to decarbonylation abstract hydrogen to form aldehydes. No photo-Fries rearrangement and a minor amount of Norrish type I1 cleavage were observed. Sulfide formation appears to be intermolecular. SCHEME I1
We found that the photolysis of 4-tolyl thiolacetate gave 4-tolyl disulfide and methyl 4-tolyl sulfide but no 4-toluenethiol nor any photo-Fries rearrangement.2 Subsequently, Bradshaw and coworkers3 reported the major products of the photolysis of phenyl thiolacetate t o be phenyl disulfide, methyl phenyl sulfide, and thiophenol, which arises from secondary photolysis of phenyl disulfide without the intervention of s o l ~ e n t ,plus ~ minor amounts of the corresponding photo-Fries products. The purpose of this paper is t o report the photochemistry of several aryl thiol esters in an effort t o uncover partially the nature of the excited state responsible for the photoreaction, t o discover whether or not sulfide formation is intramolecular, and t o explain the lack of photo-Fries rearrangement in thiol eaters.
Results Irradiation of approximately 1% solutions of 4substituted phenyl thiolacetates 1 in cyclohexane for 3 hr using a 254-nm light source produced the corresponding diphenyl disulfides 2 and methyl phenyl sulfides 3 as shown in Scheme I. In no case were the
hv
CBH, 4
FPr
e
5 (6%)
6 (9%)
6 +6+JQfSH To determine whether or not sulfide formation is intramolecular, photolysis of the esters phenyl 4toluenethiolacetate (10) and 4’-tolyl benzenethiolacetate (11) was examined (Table I) because the expected TABLE Ia PRODUCT YIELDSFOR hv OF ESTERS 10 A N D 11 hu of ArlSC(=O)CHzArz
SCHEME I Products
la, X = CH,
b,X = (CHJLN c, X = NO,
2a, 77%
3a, 7%
b,77%
b,7%
c, 90%
c, 8%
4-substituted thiophenol or thc photo-Fries rearrangement products found, a result which is in striking contrast t o the reported photolysis of phenyl t h i ~ l a c e t a t e . ~ While esters l a and l b were 75 and 78% photolyzed after 3 hr, the nitro ester IC was only 47% gone because precipitation of the disulfide 2c coated the vessel wall. Irradiation of 4’-tolyl thiol-4-phenylbutyrate (4) with 254-nm light in cyclohexane solution (Scheme 11) gave propylbenzcne (5), styrene (6),4-toluenethiol (7), 4-phenylbutanal (8), 3-phenylpropyl 4’-tolyl sulfide (9), 4-tolyl disulfide @a), and a trace of 4-tolyl thiolacetate (la). Clearly, the Norrish type I1 reaction is relatively inefficient as compared to cleavage of the #-acyl bond. (1) Summer Faculty Research Fellow, Miami University, 1971. (2) J. R. Grunwell, Chem. Commun., 1437 (1969). (3) E. L. Loveridge, B. R. Beck, and J. S. Bradshaw, J . Ore. Chem., 86, 221 (1971). (4) Y. Schoafsma, A. F. Bickel, and E. Y. Kooyrnan, Tetrahedron, 10, 76 (1960).
7 (7%)
(ArlCHdz (AraCHzh ArlCHzCHtAra (Ark32 (ArtSh ArlSSArZ ArlCHtSArz ArtCHZSArl AnCHtSAn ArtCH2SArZ Ar&Ha ArlSH H Art
\
/c=c\
a
/
12 14 22 16 2a 23 13
hu of AnSC(=O)CHzAn
6 3 2 16
35
40 5 5
25
13 3
8 18
19
19
+
28 18
15 20 21 17 18
hu of 10 11
2 6 11
Art H Yields are given in per cent; Arl = CBHs;Art = 4-CHaCeH4.
acyl radicals readily decarbonylate. The photolysis of 10 gave l12-diphenylethane (12), benzyl 4-tolyl sulfide (13), and the disulfide 2a. Thiol ester 11 formed not only lj2-di(4’-tolyl)ethane (14), 4-tolyl phenyl sulfide (15), and diphenyl disulfide (16), but also 4xylene (17), thiophenol (18)) and trans-4,4’-dimethylstilbene (19). After 3 hr the esters 10 and 11 have photolyzed 85 and 87%, respectively. However, when an equimolar solution of 10 and 11 was photolyzed for
J. Org. Chem., Vol. 38, No. 8, 1873
1560
GRUNWELL, MARRON, AND HANHAN
3 hr in cyclohexane, 10 disappeared about twice as fast as 11. In addition t o products 12-16, the mixed sulfides 20 and 21, the mixed hydrocarbon 22, and the mixed disulfide 23 were also formed. Sulfide formation appears t o be intermolecular for these esters. Laarhoven and coworkers have shown that benzyl sulfides photodissociate t o benzyl and thiyl radicals.6a~bHowever, this reaction is slower than the photodissociation of the thiol esters. For instance, the sulfide 15 disappeared 40% after 3 hr of photolysis under identical conditions as 10 and 1 1 a 6 ~Therefore, the “cross” sulfides arise only partially from secondary photolysis of 13 and 15. The product ratios for the photolysis of 10 and 11 are shown in Table I1 and are all be-
thiol 7, as shown by Bradshaw.a Therefore, sulfide formation appears to be intermolecular. Further, the acetyl radical has sufficient stability and lifetime t o diffuse away from its original partner and then combine with another thiyl radical to generate a new ester. It seems possible that the photo-Fries rearrangement observed by Bradshaw may be intermolecular.
Discussion The products of the photolyses seem best accounted for by homolytic cleavage of the S-acyl bond of the excited thiol ester followed by a series of dark reactions typical of the radicals produced. The proposed mechanism is summarized in Scheme 111. The
TABLE I1 PRODUCT RATIOS Product
Ratio
lO/ll 12/14 2a/16 13/15 21/20
2.22 2.00 2.48 2.50 1.88
SCHEME I11 0
0
”
hv
ArSCR 2ArS.
hv
ArS-SAr
(2)
+ CGO 7 ArSR
(4)
1I
RO
---t
ArS.
+
R. R.
a
24
+
+ Ar,SH
Ar,SH
18
7
(la) and an equimolar amount of thiophenol (18)dissolved in cyclohexane were photolyzed for 3 hr, phenyl thiolacctate (24), methyl phenyl sulfide (25), and 4tolenethiol (7) in addition to the disulfides 16, 2a, and 23 were formed. Unfortunately, we were unable to observe the photo-Fries products. When phenyl thiolacetate (24) and 4-toluenethiol (7)were photolyzed, the ester la was formed in addition to the other products with the exception of the sulfides 3a and 25. The formation of methyl phenyl sulfide (25) from la and 18 does not arise from secondary photolysis of 24, since we found more 25 than 24, and a t no time does the sulfide 25 exceed the amount of ester 24 when 24 is irradiated in the absence of the
W. H.
Laarhoven and Th. J. H. M. Cuppen, Tetrahedron Lett.,
41, 5003 (1966); (b) W. H. Laarhoven, Th. J. H. M. Cuppen, and R. J. F. Nivard, R e d . Trau. Chzm. Pays-Bas, 86, 821 (1967); ( 0 ) D. L. Foerst and
J. R. Grunwell, unpublished results.
+
ArS
+
Ar,SC(=O)CHs
ArlSH 18 4 8 ArzSH 7 5 6 ArlSCOCHa 24 3 28 ArzSCOCH3 l a 9 11 ArlSCH3 25 5 0 ArlSCHa 3a
