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Inorganica Chimica Acta 360 (2007) 1230–1234

Note

Photochemical carbon–sulfur bond cleavagein some alkyl and benzyl sulfides

Sergio M. Bonesi a,b, Maurizio Fagnoni b, Daniele Dondi b, Angelo Albini b,*

a CHIDECAR-CONICET, Departmento de Quımica Organica, Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires,

Ciudad Universidaria, 1428 Buenos Aires, Argentinab Department of Organic Chemistry, University of Pavia, v. Taramelli 10, 27100 Pavia, Italy

Received 7 June 2006; accepted 6 July 2006Available online 21 July 2006

Dedicated to Professor Vincenzo Balzani.

Abstract

Irradiation (254 nm) of five alkyl and benzyl ethyl sulfides causes efficient (Ur 0.27–0.90) homolytic cleavage of the C–S bond. Of theresulting fragments, thiyl radicals mainly couple, while alkyl radicals abstract hydrogen, disproportionate or couple when stabilized (ben-zyl). Selective trapping of either of the two types of radicals occurs in the presence of nucleophilic (methyl vinyl ether and 1-hexene) and,respectively, electrophilic (acrylonitrile) alkenes. When an easily oxidized radical is formed, e.g. cumyl, secondary electron transfer leadsto the corresponding cation.� 2006 Elsevier B.V. All rights reserved.

Keywords: Photochemistry; Sulfides; Thioethers; Radicals; Cleavage

1. Introduction

The photochemistry of thioethers has been somewhatsparsely studied over the years. In general, aliphatic thioe-thers undergo homolytic dissociation, both in solution andin the gas phase. The end products obtained result fromcoupling or disproportionation of the alkyl or thiyl radicalsformed [1–7]. With aryl thioethers, photohomolysis may befollowed by in cage electron transfer to give an ion pair[8,9]. However, systematic investigations or basic data suchas quantum yield measurements are scarce at present.

Photolysis may be a way for the desulfurization of fuel,though photocatalyzed oxidation reactions have been mostoften considered for this application. On the other hand,interest in the photochemical C–S bond cleavage in thesemolecules has been increased by important applicationsin fields such as UV photocurable compositions [10],

0020-1693/$ - see front matter � 2006 Elsevier B.V. All rights reserved.

doi:10.1016/j.ica.2006.07.022

* Corresponding author. Tel.: +39 0 382 987316; fax +39 0 382 987323.E-mail address: angelo.albini@unipv.it (A. Albini).

solid-phase synthesis [11,12], and preparation on monolay-ers of metals [13].

2. Results and discussion

As a contribution to the understanding of the photo-chemistry of the C–S moiety, we presently report some dataabout the irradiation of a series of ethyl sulfides RSEt,where the second S-bonded group is an alkyl or benzylgroup. The irradiations were carried out at 254 nm andthe photoproducts formed were recognized by analysis oftheir MS/GC characteristics and comparison with thoseof model compounds. With a dialkyl derivative such as 1,the main products were the alkane RH (accompanied bya lower amount of the alkene and 2% of the aldehyde)and the three possible disulfides (along with some of mer-captan RSH). The product distribution differed little incyclohexane, acetonitrile and benzene. The phenethyl sul-fide 2 likewise gave the alkane and the disulfides as themain products, accompanied by a small amount of 1,2-bis(ethylthio)-1-phenylethane (see Scheme 1a, Table 1).

R-CH2SEthν

1 R = CH3(CH2)10-2 R = PhCH2-

R-CH3 + R-CH2SH + R-CH2S2Et + (R-CH2)2S2

+ Et2S2 + C10H21CH=CH2 + C11H23CHO + PhCH(SEt)CH2SEt

a

bPh-C-SEt

R2

R1hν Ph-CR1R2H + (Ph-CR1R2)2 + (Ph-CR1R2)2S2 + Et2S2

3 R1 = R2 = H4 R1 = R2 = Me5 R1 = H; R2 = Ph

+ PhCMe2OH + PhCOMe + Ph2CHOH + Ph2CO

Scheme 1.

Table 1Products from the photolysis of sulfides RSEt (1 · 10�2 M)

Sulfide Solvent Products, % yield

RH R2 RSH RS2Et RS2R EtS2Et

1, C12H25SEt MeCN 25 4 20 2 2 C10H21CH@CH2, 9 C11H23CHO, 2C6H12 20 7 9 2 1 15 2C6H6 23 5 9 1 1 10

2, PhC2H4SEt MeCN 29 12 5 3 PhCH(SEt)CH2SEt, 1

3, PhCH2SEt MeCN 7 24 11MeOH 23 11C6H6 10 21 10

4, PhCMe2SEt MeCN 3 13 3 7 PhCMe2OH, 20 PhCOMe, 3

5, Ph2CHSEt MeCN 3 31 6 Ph2CHOH, 2 Ph2CO, 2

Table 2Energetic and photochemical parameters for sulfides RSEt

Sulfide ES

(kcal/mol)ET

a

(kcal/mol)C–S BDEb

(kcal/mol)Ur

C12H25SEt ca. 106 71 0.27PhC2H4SEt 104 84 71 0.90PhCH2SEt 104 82 58 0.79PhCMe2Set 102 76 57 0.74Ph2CHSEt 102 74 53.5 0.41

a From the onset of the phosphorescence emission.b From thermochemical data, see Section 3. B3LYP calculations gave

somewhat larger values (see Section 3).

S.M. Bonesi et al. / Inorganica Chimica Acta 360 (2007) 1230–1234 1231

The other sulfides examined were benzyl derivatives.Benzyl ethyl sulfide 3 gave 3.5 times as much bibenzyl astoluene, along with diethyl disulfide. Likewise, bicumyland tetraphenylethane were conspicuous products fromcumyl ethyl 4 and diphenylmethyl ethyl sulfides 5, alongwith diethyl disulfide. Furthermore, the correspondingalcohols and ketones (acetophenone and benzophenone,respectively, see Scheme 1b) were also formed. These wereminor product from 5 but cumyl alcohol was the mostabundant sulfur-free product from 4.

The quantum yield of decomposition was measured inexperiments at low (<20%) conversion. The valuesobtained ranged from 0.27 and 0.90 and are reported inTable 2. The present sulfides were weakly fluorescent, whilecompounds 2–5, viz. those containing a benzene ring,exhibited a marked phosphorescence. The shape and theposition of the latter emission were quite similar to thatof the corresponding benzenes (i.e. toluene, tert-butylben-zene, and diphenylmethane). Thus, it seems safe to assumethat the lowest triplet has consistently a pp* character inthese compounds.

All of the above products can be rationalized as result-ing from homolytic C–S bond (see below). In view of thelack of fluorescence, it seems likely that C–S bond dissoci-

ation occurs from the high energy excited singlet state. Itshould be noted that the cleavage is exothermic also fromthe triplet state (see Table 2) and may occur also from thisstate. This is likely the case with sulfide 1, which wasdecomposed at about half the rate (for equal adsorbed flux)by irradiation in benzene (>95% light absorbed by the sol-vent, ET benzene 84 kcal/mol) as by direct irradiation at254 nm. However, with benzyl sulfide 3, photosensitizationby benzene causes a slow decomposition (10 times as slowas direct irradiation), while leading to the same productdistribution and thus may result, at least in part, fromthe fraction of light directly absorbed by the sulfide.

1232 S.M. Bonesi et al. / Inorganica Chimica Acta 360 (2007) 1230–1234

This fact, along with the lower value of Ur with thediphenylmethyl sulfide 5 – which shows the strongest phos-phorescence in the series – suggests that in benzyl sulfides,T1 is a pp* state localized on the aromatic moiety andpoorly contributes to C–S bond cleavage. This again sup-ports that the reaction originates from S1 (if the tail atthe red edge of the absorption corresponds to a pr* state)or possibly from vibrationally excited levels of T1 formedin the intersystem crossing (as it has been shown to bethe case, e.g. for C–H bond cleavage in diphenylmethane)[14]. The situation is similar to that of phenyl alkyl sulfides,where T1 is likewise a pp* state [15] and the cleavage hasbeen found to be efficient for bulky alkyl groups (Ur 0.6for the adamantyl derivative) [8].

The selectivity of the photocleavage and the end-prod-uct distribution depends on the nature of the alkyl andthiyl radicals that are the primary products (see Scheme2). Thus, when both substituents are simple alkyl radicals,as in 1 and 2, the cleavage is unselective. The main processfrom the alkyl radical R� is hydrogen abstraction to give thealkane, or, in the case of the phenethyl radical, dispropor-tionation to alkane and alkene. On the contrary, the morestabilized thiyl radicals mainly couple to the disulfides. In

R-S-Et

hν

R. + EtS. Et. + RS.R-H

R-R EtS2EtRS2Et RSH

RS2R

R+ + EtS-H2OR-OH

Disproportionation

Scheme 2.

R-S-Ethν

R. + EtS.

CN

CN

R .

R = C12H25

CN

C12H25 H

R=

PhCH2

CN

PhH2C CH2Ph+

OEt

PhH2C

R = Ph

C4H9R = PhCH2

PhCH2C

C4H9

PhH2C .

Scheme

the case of 2, some hydrogen abstraction from the benzylicposition occurred and gave radical PhCH�CH2SEt, asrevealed by the isolation of a small amount of the couplingproduct with EtS�. Finally, the formation of small amountof dodecylaldehyde is probably due to the addition ofresidual oxygen to the dodecyl radicals.

With benzyl derivatives 3–5, the cleavage was regioselec-tive and the main process from the more persistent benzylradical was coupling rather than hydrogen abstraction.With the two last sulfides, a ionic path also operated. Thus,in the case of 4, cumyl alcohol was obtained in a largeramount with respect to dicumyl, and benzhydrol was like-wise obtained from 5, though in a much smaller yield thantetraphenylethane. In analogy with previous work, in par-ticular with the detailed analysis of the photocleavage ofbenzyl ethers [8,9,16], this result is explained by electrontransfer between the first formed radicals (see Scheme 3).The feasibility of such a process can be assessed by meansof Eq. (1).

DG� ¼ E�ðR�=RþÞ�E�ðEtS�=EtS�Þþ ½ð2:6=eÞ� 0:13� ðin eVÞð1Þ

On the basis of the literature data (see Section 3), theprocess result to be endothermic in all of the cases(+22.8 kcal/mol for the benzyl, +9.7 for the cumyl and+14.1 for the diphenylmethyl radical). Considering theuncertainty of the evaluation of E� from voltammetricmeasurements on nonreversible oxidation, this result mustbe considered a quantitative evaluation. Thus, it appearsjustifiable that the predicted easiest process – the one lead-ing to cumyl cation – has an important role, taking alsointo account that oxidation is followed by nucleophilicaddition (by traces of moisture) and that coupling betweenthe two bulky radicals is less competitive.

CN

PhH2C SEt

CN

PhH2CCN+

OEt

OEt

.OEt

PhH2C H

OEt

PhH2C SEt+

CH2

OEt

EtS HC4H9

EtS-C6H13

H=CH2C4H9

3.

Table 3Products from the photolysis of some sulfides in the presence of various alkenes

Sulfide Alkene, 0.1 M Products

From R� From RS� EtS2Et Trapping products

C12H25SEt CH2@CHCN C12H26, 8; C10H21CH@CH2, 16 C12H25SH, 8 C10H25CH2CH2CN, 20PhCH2SEt CH2@CHCN PhCH3, 4; PhCH2CH2Ph, 2 PhCH2CH2CH(CH2Ph)CN, 5; PhCH2CH2CH(SEt)CN,

11; PhCH2CH2CH(CN)CH2CH2CN, 5 + 5a

CH2@CHCN PhCH3, 12; PhCH2CH2Ph, 4 PhCH2CH2CH(CH2Ph)CN, 6; PhCH2CH2CH(SEt)CN,10; PhCH2CH2CH(CN)CH2CH2CN, 2 + 2a

CH2@CHOEt PhCH3, 4; PhCH2CH2Ph, 19 2 PhCH2CH2CH2OEt, 3; EtSCH2CH2OEt, 15;PhCH2CH2CH(SEt)OEt, 3

CH2@CHC4H9 PhCH3, 7; PhCH2CH2Ph, 22 3 PhCH2CH@CHC4H9, 17; EtSC6H13, 30

a Two diastereoisomeric 1–2 adducts present.

S.M. Bonesi et al. / Inorganica Chimica Acta 360 (2007) 1230–1234 1233

In the second part of this work, we irradiated a dialkylsulfide 1 and the benzyl derivative 3 in the presence of somealkenes, in order to further characterize the role of theintermediate radicals. As it appears in Scheme 3 and Table3, in the presence of 0.1 M acrylonitrile, the yield of dode-cane from 1 decreased by 2/3rds, and pentadecacarbonitrilebecame the main product. Likewise, with benzyl sulfide 3,the main products in neat solvent (dibenzyl and diethylsul-fide) gave place to trapping products resulting from theaddition of two benzyl, or one benzyl and a sulfide radical.

This is expected, in view of the nucleophilicity of alkyland benzyl radicals, which are trapped by an electrophilicalkene such as acrylonitrile. The more persistent a-cya-noalkyl radical then couples with a benzyl radical or a thiylradical or, to a small extent, adds a further alkene moleculegiving the diastereoisomeric 1–2 adducts (for analogy, seeRef. [16]). Just as the irradiation in neat solvent, the prod-uct distribution in the presence of the trap does not changeon irradiation in benzene, though the required irradiationtime is much larger, suggesting again that cleavage fromthe triplet is inefficient.

When a nucleophilic alkene is used as the trap, the pat-tern changes completely. Thus, with both ethyl vinyl etherand 1-hexene, the yield of bibenzyl is only partiallydecreased, while it is the ethylsulfide radical that is prefer-entially trapped. The results fit with the known electro-philic character of thiyl radicals that add to alkenes andvinyl ethers. [17–19]. Thus, it appears that the differentcharacter of the two types of radicals generated can beexploited for selective trapping.

The above result shows that dialkyl sulfides undergo effi-cient homolytic cleavage yielding alkyl and thiyl radicals.Direct photolysis of these compounds may be consideredon one hand, as a possible desulfurization method andon the other hand as a convenient source of radicals fora synthetic purpose via intermolecular addition (cases ofintramolecular addition had been previously reported)[5,6]. The test investigations carried out in the presence ofalkenes suggest that both alkylation and alkylthiylationcan be conveniently obtained, in the latter case via a pat-tern alternative to the thermolysis or photolysis of thiols.When an easily oxidized alkyl radical is formed, secondaryelectron transfer leads to the corresponding carbocation.

3. Experimental

3.1. Materials

The sulfide 1 was prepared from dodecyl sulfide andethyl bromide, the analogues 2, 3, and 5 from benzyl bro-mides and ethyl sulfide, and the cumyl derivative 4 froma-methylstyrene and ethyl sulfide [20–24].

3.2. Photochemical reactions

The photochemical reactions were carried out by irradi-ating 0.01 M solutions of the sulfides (3 mL samples inquartz tubes) after flushing with Argon for 15 min by usinga 15 W low pressure mercury arc.

The product distribution was investigated by HPLC,GC, and GC/MS experiments. Cyclododecane and biphe-nyl were added as standards. Among the authentic samplesfor the identification of the photoproducts, some were ofcommercial origin (dodecene, dodecanaldehyde, the alco-hols, and ketones). The disulfides were prepared by oxida-tion of the thiols [25,26] except for dodecyl ethyl disulfide,the formation of which was proposed on the basis of thefragmentation pattern in GC/MS. The structure of thealkene trapping products was likewise attributed basedon the comparison of the GC/MS pattern with that ofauthentic samples (pentadecacarbonitrile [27], phenylpro-pyl ethyl ether [28], 1-phenyl-2-octene [29]) or on analogywith such model compounds. Most of the trapping prod-ucts have been previously reported: benzyl-acrylonitrileadducts [30]; benzyl-ethylthio-acrylonitrile adduct [31]; 2-ethoxyethyl ethyl sulfide [32]; 1-phenyl-2-hexene [33]; ethylhexyl sulfide [34].

The light flux was determined by using the photoconver-sion of acetanilide into 2-aminoacetophenone (0.06 incyclohexane [35]). Fluorescence and phosphorescene (indiethylether/pentane/ethyl alcohol glass at 77 K) were mea-sured by means of a Perkin–Elmer instrument.

3.3. Bond dissociation energies

The data gathered in Table 2 correspond to the thermo-chemical evaluation as reported in the literature, with

1234 S.M. Bonesi et al. / Inorganica Chimica Acta 360 (2007) 1230–1234

extensions based on analogy. The bond dissociation energyis DG(RX) 1 = 71 [36], 2 @ 71, based on the hypothesis thatb-phenyl substitution has a minor effect (@0.5 kcalmol) onthe BDE, as it happens, e.g. with the iodide [37], 3 = 58 asfor the previous term, 4 = 57 based upon the hypothesisthat a,a-dimethyl substitution lowers the BDE by about1.3 kcal/mol, as it does with benzyl chlorides [38],5 = 53.5 kcal/mol on the hypothesis that a-phenyl substitu-tion lowers the BDE by 4.5 kcal/mol, as it does with alk-anes [39]. B3LYP calculations were carried out by usingthe B2 version of the GAUSSIAN 2003 program package[40] and gave the following values (after subtracting10 kcal/mol from the H values obtained to allow for theentropic aspect of the fragmentation): 4 = 67, 5 =64 kcal/mol.

3.4. Electron transfer

The likelihood of electron transfer between the two ini-tially formed radicals was evaluated according to Eq. (1)(see text). The solvation term is �1.4 kcal/mol in acetoni-trile. E� (EtS�/EtS�) = � 0.32 V versus SCE [41], (R�/R+) = 0.99, 0.3, 0.16, and 0.35 for ethyl (and presumablydodecyl), benzyl, cumyl, and diphenylmethyl [42],respectively.

Acknowledgement

Partial support of this work by MURST, Rome, isgratefully acknowledged.

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