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Current Organic Chemistry, 2007, 11, 1053-1075 1053 Photooxygenation of Non-Aromatic Heterocycles

M. Rosaria Iesce*, F. Cermola, M. Rubino

Dipartimento di Chimica Organica e Biochimica, Università di Napoli Federico II, Complesso Universitario Monte S. Angelo, via Cinthia 4, I-80126 – Napoli, Italy

Abstract: Photooxygenation of non-aromatic heterocycles and cyclic compounds containing non-usual heteroatoms, namely silicon, germanium and tellurium has been reviewed. All three types of photooxygenation (Types I-III) can take place. Moreover the heteroatom can be frequently involved endorsing electron-transfer reactions which turn out to be the main pathways, even in singlet oxygen oxygenation. A vast collection of novel and unexpected products are often formed, sometimes in a stereocontrolled manner.

1. INTRODUCTION sources; aromatic ketones, as benzophenone, or cyanoaro- Since the first experiments by enthusiastic Ciamician [1] matic compounds, as 9,10-dicyanoanthracene (DCA), are on the roof of his institute in Bononia (Bologna) University, used in photooxygenation of Type I or III, respectively and a pletora of papers and books have been made regarding in- mercury-lamps (preferably UV filtered) are used as light teraction of light with matter, and in this context photooxy- sources [2, 4c, 4d]. genation, combination of light and oxygen generally in the In 2005 we published a review on the photooxygenation presence of a suitable conjugated molecule (sensitized pho- of heterocyclic aromatic compounds with the aim to bring tooxygenation), has been widely used to introduce oxygen- up-to-date on the latest twenty-year results [2]. In this review ated functions in organic molecules. we focus our attention on the photooxygenation of non- aromatic heterocycles (unsaturated where bonds to the het- Three common mechanisms are invoked for the sensi- tized photooxygenation which differ essentially by the dif- eroatom are directly involved or saturated) [11] and have ferent role of the sensitizer [2, 3]. So, the photoexcitated expanded the discussion to cyclic compounds containing sensitizer can interact with the molecule by extracting hy- non-usual heteroatoms, namely silicon, germanium and tellu- drogen (Type I) or an electron (Type III), and the radical or rium [12]. the radical cation of the substrate so formed react with triplet oxygen or superoxide anion to give the oxygenated products. 2. PHOTOOXYGENATION OF UNSATURATED HET- In Type II an energy transfer from the excited sensitizer to EROCYCLES triplet ground state oxygen can occur generating singlet The nucleophilicity of a double bond is enhanced by the 1 oxygen ( O2), a highly reactive species, whose behaviour presence of the heteroatom so that partially saturated deriva- towards organic molecules is continuously under investiga- tives as cyclic enol ethers or enamines react easily with sin- tion due to the chemical [4] and biochemical [5] implica- glet oxygen by [2+2] cycloaddition and afford the character- tions. The electrophilicity of this species and its alkene-type istic cleavage products from thermally unstable dioxetanes. character promote addition reactions to unsaturated systems The dioxetane-mode however competes with ene mode in ([4+2] cycloaddition [6], [2+2] cycloaddition [6], or ene-like the presence of adjacent allylic hydrogens. The nature of reaction [7]). Singlet oxygen may also react at electron pair heteroatom, ring size, substitution as well as environmental bearing heteroatom centers, e.g. sulfur, to give the corre- factors influence the product distribution. So, in the pho- sponding oxide [4]. Sometime reactions with electron-rich tooxygenation of dihydropyran 1, both dicarbonyl compound substrates, such as amines [8], sulfides [9] and phenols [10], 4, the product expected from cleavage of the dioxetane 2, may proceed by electron transfer from the electron-rich sub- and dihydropyrone 5, the dehydration product from the hy- strate to singlet oxygen to give a cation radical-superoxide ion pair or charge-transfer complex. Coupling reaction of the 1 1 ion pair could give the oxygenated product or a back- O O2 O2 electron transfer could occur producing triplet oxygen and O ene OOH the starting compound. The latter route is particularly impor- O [2+2] OO tant for nitrogen-containing molecules, and some suitable H derivatives, e.g. diazabicyclo[2.2.2]octane (DABCO), are 2 1 3 specifically used as singlet–state oxygen inactivators [2, 4]. - H2O Typical sensitizers for singlet oxygen reactions are dyes as methylene blue (MB) or Rose Bengal (RB) or tetraphenyl- O porphine (TPP) and tungsten-halogen lamps are used as light

O O OO 4 5 Address correspondence to this author at the Dipartimento di Chimica Or- ganica e Biochimica, Università di Napoli "Federico II" Complesso Univer- 4/5 ratio increases up to 58-fold from C H to CH CN sitario Monte S. Angelo, Via Cinthia, 4, I-80126 Napoli Italy; Tel: +39-081- 6 6 3 674-334; Fax: +39-081- 674-393; E-mail: [email protected] Scheme 1.

1385-2728/07 $50.00+.00 © 2007 Bentham Science Publishers Ltd. 1054 Current Organic Chemistry, 2007, Vol. 11, No. 12 Iesce et al. droperoxide 3, are formed and the product ratio 4/5 varies the preferred products [15]. It is interesting to note that hy- over a 58-fold as the solvent changes from benzene to aceto- droperoxides 10 (for n=1) undergo straightforward H2O2 nitrile with polar solvent favouring the 1,2-addition (Scheme elimination to give furans [14]. 1) [13]. The solvent effect however seems to be associated A different behaviour is observed in the singlet oxygena- with the unsymmetrical character of the enol ether substrate tion of thio-analogues. Indeed, they essentially lead to dicar- [13a]. bonyl derivatives via dioxetanes. Less than 5% of allylic Small rings and electron-donor substitution appear to fa- hydroperoxides is formed starting from five-membered de- vour the formation of dioxetanes at the expense of allylic rivatives as dihydrothiophene 13 while dioxetane mode is the hydroperoxides [14, 15]. Scheme 2 and Table 1 report the unique route in six-membered derivatives as in 3,4-dihydro- trend observed in oxygenated systems (dihydrofurans and 2H-thiopyran 16 which gives exclusively compound 18 dihydropyrans). (Scheme 3) [16]. The nature of heteroatom plays a role also Significant is the strong substituent effect in the oxygena- in the thermal stability of dioxetane intermediate. S- tion of ethyl 3,4-dihydro-6-methyl-2H-pyran-5-carboxylate substituted dioxetanes result more thermally unstable than O- (Table 1) and 5-acetyl analogue [15]. These compounds un- analogues, and have been spectroscopically detected at very dergo the exclusive ene-reaction due to the presence of the low temperatures (< -70 °C), if any [16]. electron-withdrawing ester and acetyl groups which further The selective formation of dicarbonyl derivatives via di- address the formation of the conjugated hydroperoxides as oxetanes is obviously observed in the presence of two het-

R O O O O R n O Me n O Me 8 7 R OOH

n CH2 1 O O2 9

R R R

1O n= 1 2 OOH n O Me n O Me O Me

H2O2 6 10 11

CH2 R= Me OOH n O Me 12 Scheme 2.

Table 1.

CCl4 CH3CN

entry n R Products (%)a

8 9 10 12 8 9 10 12

a14 1 Me 80 - 13 7 87 - 7 6

14 b 1 CO2Me 3 14 83 - 44 6 50 -

c14 2 Me 28 52 - 20 35 55 - 10

15 d 2 CO2Et - 17 83 - - 65 35 - a By 1H NMR. Photooxygenation of Non-Aromatic Heterocycles Current Organic Chemistry, 2007, Vol. 11, No. 12 1055

CO Me CO2Me 2 O O , h
, Sens Me SCO2Me 2 O + < 5% of ene products Me S S Me O O 13 14 15 (>90%)

O O , h
, Sens S 2 S Ph S Ph CH2Cl2, -78 °C Ph Ph O Ph Ph OO 18 (quant.) 16 17 Scheme 3. O 1O X Y 2 X Y Ph X Y Ph CH2Cl2, -78 °C Ph Ph O Ph Ph OO 21 19 20 a; X= Y= O b; X= Y= S c; X= O, Y= S d; X= Y= NMe Scheme 4. eroatoms on the double bond as in 19 (Scheme 4) [17]. Once usual decomposition of dioxetanes. However peculiar rear- more the nature of the heteroatoms affects the thermal stabil- rangements have also been observed. So, oxygenation of ity of the corresponding dioxetanes. A mechanism involving compound 22 sometime leads to diketone 24 derived from an intramolecular electron-transfer process has been pro- the decomposition of dioxetane 23 via C-S bond cleavages posed for the cleavage of unstable S- and N-substituted di- [18], as also observed in bis-, tris- and tetrakis-1,2- oxetanes [17]. This mechanism requires that the stability of ethylthioethylenes (Scheme 5) [19]. dioxetanes is related to the oxidation potential of the heteroa- The presence of suitable substituents at appropriate posi- tom substituents. So, dioxetanes bearing easily oxidized tions may induce peculiar rearrangements. So, in the oxy- groups such as N- or S-substituents are dramatically less genation of 5,6-dihydro-1,4-oxathiins 25 (X=O) [20] and stable than a similar dioxetane with an O-group possessing a 5,6-dihydro-1,4-dithiins 25 (X=S) [21] the formation of keto much higher oxidation potential [17]. sulfoxides 28 and/or 30 has been observed in the presence of As observed in above Schemes, fragmentation to dicar- an electron withdrawing group at the double bond (Scheme bonyl compounds via O-O and C-C bond breakage is the 6). It has been suggested that in the dioxetanes 26 the in-

O S 1 S H O2 O H
OMe O OMe S S O

22 23 24 Scheme 5.

O 1 O X S 1 X S 2 R X SR2 R1 R2 O R1 R2 OO 27 25 26

i

XS for X= O OS O X S O O 1 2 R1 R2 R1 R2 R R O O O 29 28 30 X =O, S i; only for R2 = electron-withdrawing group CO2Me, CON Scheme 6. 1056 Current Organic Chemistry, 2007, Vol. 11, No. 12 Iesce et al.

Me N

O OH O But

33

a

R1 R1 Me N R2 N R2 1 N But O2 b O O t Bu O OH But O O O

32 31 34

c

a; R1= R2= Me NH2 COBut b; R1= H, R2= Me O c; R1= R2= H O

35 Scheme 7.

1 1 R R H2 C R1= Me N CH3 N H 33 O O O O But But

O O

36 32 R1= H

34 Scheme 8. creased electron demand by the O-O bond favours the in- with N-methylamino or N,N-dimethylamino groups at the tramolecular nucleophilic attack of the ring sulfur leading to meta or para position [22]. final products 28 and/or 30 via the labile undetected sulfox- Interestingly, the oxygenation of similar enol ether 37 ide epoxides 29 The selective formation of compounds 28 leads exclusively to the diasteromeric mixture of endoper- for X=S is evidently due to the major migratory aptitude of oxides 38 (Scheme 9) [23]. In this case [4+2]cycloaddition sulphur than that of sulfoxide or oxygen moiety [21]. of singlet oxygen to diene system prevails on [2+2] addition Heterocycles 33 and 34 have been obtained in high yields to the double bond, albeit activated, and this trend is gener- from the oxygenation of 2-(o-aminophenyl)-4,5-dihydro- ally observed whenever a
-conjugated system is present in furans 31a and 31b via unusual decomposition of dioxetanes the substrate (see below). 32 (Scheme 7) [22]. Sometime dehydrogenation has been observed in the These peculiar rearrangements would be induced by in- oxygenation of five-membered unsaturated heterocycles [14, tramolecular nucleophilic attack of o-methylamino and o- 16, 24, 25]. So, pyrroles 41 are formed from dihydropyrroles dimethylamino groups at O-O bonds of the dioxetanes 32a,b 39 and, as above reported for compounds 10 (Scheme 2) (Scheme 8) [22]. In particular, the zwitterion 36a should [14], it would be due to the decomposition of the intermedi- cause Stevens-like rearrangement by abstraction of a methyl ate hydroperoxides 40 (Scheme 10) [24]. The substrates 39 proton by the close oxy anion leading to 33. It is to be noted (N-aryl cyclic enamines) have been found to be sensitizers of that the oxygenation of 31c affords normal carbonyl product singlet oxygen [24]. 35 (Scheme 7) as do derivatives bearing a phenyl substituted Photooxygenation of Non-Aromatic Heterocycles Current Organic Chemistry, 2007, Vol. 11, No. 12 1057

O O O H H R R O R O N But N O N O hν, O , TPP t t 2 Bu + Bu

-78 °C, CH2Cl2 O O O

37 cis-38 (65%) trans-38 (35%) R= PhCH2 Scheme 9.

CO2Et CO2Et CO2Et hν, O 2 Me Me Me N with or N OOH N H O Ar without TPP Ar 2 2 Ar 39 40 41

Ar= Ph, 4-MeC6H4, 4-ClC6H4 Scheme 10.

Dehydrogenation also occurs in the oxygenation of 1- simply O2 addition-double bond migration. For example, the phenyl-2-pyrazolines 42 (Scheme 11) [25]. Some deriva- photooxygenation of 2,5-dimethyl-4-hydroxy-3-(2H)-fura- tives, e.g. 1-phenyl-3-[p-(dimethylamino)-phenyl]-2-pyra- none 47, a caramel-like, sweet, fruity flavour, leads in abso- zoline and 3-[p-(diethylamino)-phenyl]-2-pyrazoline how- lute ethanol to a variety of products (Scheme 13) [28]. The ever are stable under the reaction conditions and capable of key intermediate has been suggested to be the hydroperoxide 1 quenching O2 efficiently. The authors suggest that the 48 formed by the attack of singlet oxygen to the double bond quenching mechanism would involve a charge-transfer proc- at C-5 position. Ring opening to 5-hydroxy-2,3,4-hexa- ess or weak molecular complexes, because these 2- netrione intermediate 49, hydrolysis, fragmentations, re- pyrazoline derivatives have relatively high electron densities arrangements and esterification would be the events leading on their rings as supported by their low oxidation potentials to the observed products. [25]. Products 53 and/or 54 (the latter together with 53 using methanol as solvent) have been isolated in the oxygenation R R of isoquinolinones 50 (Scheme 14) [29]. The authors propose

hν / O2 / MB that, despite the presence of the highly activated enol- N N Ar Ar enamine C=C double bond, an ene-type electrophilic attack N CH2Cl2 N of singlet oxygen to C-6 of the substrates would occur giving Ph Ph the zwitterionic intermediates 51. The latter would afford 42 43 endoperoxides 52 by a transannular nucleophilic attack of Ar the peroxidic anion to the para-carbonyl group. Homolytic R= Ar, cleavage of the O-O bond and heterolytic C-N bond scission Scheme 11. result in the formation of 53 while products 54 derive from the nucleophilic trapping of the iminium cation in 51 or 52 Pyridones 46 (X=CH) and pyrimidones 46 (X=N) are ob- by methanol (Scheme 14) [29]. tained in the RB-sensitized photooxygenation of 1,4- An interesting RB-sensitized photooxygenation reaction dihydropyridines 44 (X=CH) [26] and 1,4-dihydro- has been observed starting from 1,6-diazaphenalene 55 pyrimidines 44 (X=N) [27] via elimination of H2O from the which leads in dilute solution to product 56 in 50 % yield related hydroperoxides 45 (Scheme 12). The choice of ace- (Scheme 15) [30]. It is formed via uptake of oxygen at the 7- tone as oxygenating solvent is essential to obtain 46. position of 55 as an ene-like reaction, facilitated by electron Peculiar ene-type reactions have also been described release from the nitrogen at position-1, followed by dehydra- which involve an allylic hydrogen linked to heteroatom or tion. Although electron availability at C-3 might render this

Ph R Ph R Ph R HOO O 1O X X 2 H - H2O X

Me2CO PhN Ph PhN Ph PhN Ph

44 45 46

X = CH, N Scheme 12. 1058 Current Organic Chemistry, 2007, Vol. 11, No. 12 Iesce et al.

O O HO O 1 O2 EtOH HOO O O 48 47

O a O O O O b O O O O O HO 49

OH O O O O O O O

O O

O Scheme 13.

OO H O HO O X O X O 1O X N 2 X N N O N O R O 52 50 R 53 51 R R

CH3OH CH3OH

HOO H O OCH3 OCH3 X X

N N

O O X= O, S R R Scheme 14. 54 H N N N h
, O , RB HOO - H O O 2 2 H CHCl3/EtOH

N N N

55 56

H O N

O

N Scheme 15. Photooxygenation of Non-Aromatic Heterocycles Current Organic Chemistry, 2007, Vol. 11, No. 12 1059 position a competitive site for attack by the electrophilic been confirmed by trapping experiments using typical singlet oxygen, the reaction at C-7 has the advantage of a favourable oxygen acceptors [33b]. In particular, heating of an equi- 6-membered transition state for C-O bond formation coinci- molecular solution of endoperoxide 63 and tetraphenylfuran dent with the breaking up of the N-H bond. in dichloromethane affords cis-dibenzoylstilbene oxide and Reactivity of 2,3-dihydropyrazines towards singlet oxy- the starting pteridone 62 in 82 % and quantitative yields, gen depends on the substitution. So alkyl-substituted 2,3- respectively (Scheme 17) [33b]. dihydropyrazines 57 afford 1-isocyano-2-(acylamino)ethanes Photooxygenation of tetramethyl-4H-pyrazole 64a pro- 58 and aldehydes 59 (Scheme 16) [31]. 5,6-Diphenyl deriva- duces diketone 66 and N2 presumably via the corresponding tive is inert [31b]. The unstable hydroperoxide 61 derived endoperoxide (Scheme 18) [34]. The same intermediate has from a perepoxide intermediate 60 has been proposed as key been proposed in the photosensitized oxygenation of the intermediate in the formation of the observed products drug acetazolamide 64b [35]. It however rearranges as re- (Scheme 16) [31, 32]. ported for thiophene endoperoxides [36] and gives sulfine 67 (Scheme 18) [35]. It is interesting to note that this product NR O has also been obtained by unsensitized photooxygenation of 3O , h , Sens. 2 ν acetazolamide and that the drug is able to oxidate 2,5- CN-CH2-CH2-NH-CO-R + 1 XH dimethylfuran (efficient acceptor for O2) so showing that it N CH2X 58 59 posseses Type II photodynamic activity [35]. 57 1O An endoperoxide intermediate 69, formed by oxygen ad- 2 dition to the conjugated diene system, has also been invoked NR NR in the self-sensitized photooxidation of isoquinolin-3-one 68 OH (Scheme 19). The opening of endoperoxide 69 by the solvent O (methanol) would lead to compound 70 which by methanol N CH2X N O elimination followed by irradiation under basic conditions HX O X=H, Me affords norpontevedrine 71a, key intermediate for ponteve- 60 61 drine 71b, a 4,5-dioxoaporphine alkaloid. A “one pot” con- version of 68a into 71a is achieved by carrying out the irra- Scheme 16. diation in ethanolic alkali solution under oxygen atmosphere [37]. The presence of 1,4-dicarbosubstituted
-conjugated sys- tem in pyrazin-2-ones [33a] or condensed derivatives, as 3. PHOTOOXYGENATION OF SATURATED HET- pteridin-2,4,7-trione 62 [33b], addresses the reaction to [4+2] EROCYCLES cycloaddition and, hence, 1,4-endoperoxides are formed Photooxygenation of acyclic sulfides as well as that of which lead to fragmentation products or undergo retro- cyclic analogues has received much attention owing to the cycloaddition, respectively. Liberation of singlet oxygen has

O O Ph Ph Me N Ph Me N Ph 1O N 2 N Ph O Ph Ph O Ph O O + 62 PhCO COPh O N N O O N N O Δ Me Me 1 Me Me O2

62 63 Scheme 17.

1 O O 1 X 2 O2 Me Me R R X a R1 R2 N N Me Me N N O O N2 64 65 66 b

O SO

MeCOHN SO2NH2 NN a; R1=R2=Me X= C(Me) 2 67 1 2 b; R = NHCOMe R =SO2NH2 X= S Scheme 18. 1060 Current Organic Chemistry, 2007, Vol. 11, No. 12 Iesce et al.

MeO O MeO O O NR h
, O2 MeO O NR MeO OMe OMe

Br OMe Br OMe 68 69 h
, O2, ethanolic alkali solution CH3OH O MeO O

O NR MeO O MeO

OMe NR MeO MeO OMe MeO

OMe Br a; R= H 71 70 b; R= Me Scheme 19. 

OOH A H O R 71 71 O S 2 x 74 (ROH) R S OCH3 R R 75 71 3 + O2 c d CH OH e 3 HA (ROH) O O O O 1O O 71 S 2 2 RR R S R S S a b RR 71 R R f 74 72 73 h OO CH g S R = R'S(O)R' RR OO 74 S 76 OOH + R' R' C S R 77

SR ' l i 2 j 71 k or alkenes HOO SOR ' m 2 2 x 74 76 cleavage products or epoxides C SR

78 Scheme 20. synthetic potential of the reaction [4a, 12, 38, 39] and further formed peroxidic intermediates [4a, 12, 38, 42-44]. The to the role of many natural sulfur-containing compounds in widely accepted mechanism is that the reagents initially form the activity of some enzymes and to the deactivation of the a weakly bound persulfoxide 72 which collapses to thiadiox- latter by active oxygen species [40]. Schenck first reported irane 73 (a) and this, in turn, reacts with sulphide substrate that dialkyl sulfides undergo photooxidation to give two 71 to give two sulfoxides 74 (b) (Scheme 20) [45]. The reac- moles of sulfoxides per mol of absorbed oxygen [41]. Since tion is instead very complex as proven by ab initio calcula- then great efforts have been devoted to the knowledge of this tions [43, 46], isotopic effects [47, 48], alcohol [49, 50] or reaction, mainly to the identity and reactivity of initially protic medium effects [51]. Depending on sulphide structure, Photooxygenation of Non-Aromatic Heterocycles Current Organic Chemistry, 2007, Vol. 11, No. 12 1061

SO

1 i O2 80 S ii MeO OMe OMe 79 S + S SOMe

O i, aprotic solvents; ii, MeOH 81 82

Scheme 21. O

1 S O O S O2 O Ad Ad + + + Ad Ad + SO2 Ad Ad Ad Ad Ad Ad 83 84 85 86 87

1 O2 Ad =

Scheme 22. substituents and reaction conditions (solvent, concentration, solvents while in methanol sulfinic esters are found at low temperature) the persulfoxide may undergo a myriad of in- substrate concentration and thiirane oxides at high concentra- ter- and intramolecular reactions (Scheme 20). tion [59]. This trend is observed also in the oxygenation of In protic solvents stabilization occurs by hydrogen bond- condensed thiiranes as 79 (Scheme 21). Disulfides as 82 are ing (c) [45, 49-51] (or in MeOH by formation of a sulfurane sometime found particularly in the presence of an acid. intermediate 75 (d) [45]) and attack to a second molecule of Diphenyl-substituted thiirane is inert even after prolonged sulphide is promoted leading to two sulfoxides. Under these irradiation [59]. conditions the reaction is very efficient. In aprotic medium Strained thiiranes as biadamantylidene sulfide 83 give more debated is the question of intermediates as well as less sulfoxides nearly quantitatively, even in methanol (Scheme predictable the efficiency of the reaction. The persulfoxide 22). In apolar solvent, in addition to the oxide 84, desulfuri- 72 partitions between decomposition to sulfide and ground zation products, through SO2 elimination, as dioxetane 85, oxygen (e) (physical quenching) [45] and chemical proc- oxirane 86 and alkene 87 have also been evidenced, the first esses. In particular, it may 1) convert into sulfoxide via two deriving from singlet oxygenation of alkene 87 so thiadioxirane (a,b) [45]; 2) rearrange to sulfone 76 (f) [9d, formed [60]. 48, 52], sometime favoured by low temperature and low con- The four-membered ring sulphide, thietane 88, gives the centration [9d]; 3) be trapped by sulfoxides R’2SO (h) [45, sulfoxide product 91, even in aprotic medium [61]. A de- 50, 53]; 4) rearrange to a S-hydroperoxysulfonium ylide 77 tailed kinetic study has indicated that it is due to a self- (g). The successful formation of the S-hydroperoxy- catalyzed mechanism, i.e. thietane itself apparently stabilizes sulfonium ylide 77 appears to be governed by a variety of the first formed intermediate (as in A), and thus S-oxidation factors including accessibility and acidity of the
-hydrogen competes significantly with the quenching [61]. The unique [47]. In some cases the ylide may undergo a 1,2-OOH shift ability of 88 to catalyze its own oxidation is a result of a to a
-hydroperoxysulfide 78 (Pummerer rearrangement) (l) [54], and this one in some cases can lead to cleavage prod- ucts (m) [46, 55]. The ylide 77 may be trapped by sulfides or O O O O O alkenes (i, j) [56] or rearrange to sulfones (k) [57]. Recently 1 O S O2 S a conformationally induced electrostatic stabilization (CIES) S S of the persulfoxide by a remote electron rich functional S 90 group has been proposed as yet another mechanism to in- 88 89 crease the efficiency of the sulphide oxidation (see below) 88 [58]. A O In cyclic sulfides formation of products depends signifi- S cantly on the structure of the starting compounds and, in 2 x many cases, they display reactivity differences relatively to acyclic sulfides. Thus, in singlet oxygenation of thiiranes, the 91 primary products are the thiirane oxides in non-nucleophilic Scheme 23. 1062 Current Organic Chemistry, 2007, Vol. 11, No. 12 Iesce et al. small C-S-C angle which allows an unencumbered approach The alcohol 96 is stereospecifically formed with 4,5- to the sulfonium sulphur [61]. trans configuration, and the optically active isomer gives the Cleavage products or oxyfunctionalization
to sulfur optically active alcohol as only one diastereomer [56, 63]. have been observed in five-membered ring sulfides having Significant is the cooxidation of alkenes to epoxides in the 1
-hydrogens [54, 56, 62, 63]. Indeed, thiolane 92 gives in presence of alkenes inert toward O2 (Scheme 25) [56]. Con- addition to sulfoxide and sulfone, the disulfide 93 corre- trol experiments have shown that hydroperoxide 95 is not sponding to oxidation of the
-carbon (Pummerer rear- responsible of this oxidation, so key intermediate would be rangement, path l in Scheme 20) and ring opening of the the hydroperoxysulfoxide 97 which undergoes a 1,2-shift to hydroperoxide intermediate (Scheme 24) [62]. hydroperoxide 95 (Pummerer rearrangement) or transfers oxygen to alkenes (Scheme 26) [56]. 1 The photooxygenation of thiazolidines 98 in the presence O2 + + OHC S of meso-tetraphenylporphine followed by reduction of the S S S 2 intermediate hydroperoxides with triphenylphosphine (Ph3P) O OO or dimethyl sulphide (Me2S) represents a mild procedure for 92 93 a selective and stereospecific hydroxylation
to sulfur (Scheme 27) [54]. 1 O2 HOO S R HO R 1. hν, O2, TPP 2. Me2S or Ph3P ' SNR' Scheme 24. SNR

Me Me C-S Bond cleavage has also been observed for substi- Me Me tuted or condensed thiolanes while the six- or seven- 98 99 (90-97%) membered sulfides lead only to sulfoxides and sulfones [62]. R = CO2Me, CH2OH; R'= COPh, COMe C-S Bond cleavage has been explained on the basis of the acidity of
-proton which is significant in a five-membered Scheme 27. ring as result from kinetic data on acidity of
-proton of cy- clic sulfides [64]. Interesting applications of this reaction to the synthesis of In the singlet oxygenation of thiazolidine 94, Ando et al. ,-unsaturated D-
-amino acids are depicted in Schemes 28 have found that the
-hydroperoxysulfide 95 is stable at 0°C and 29 [65]. The N-acylthiazolidines 100, derived from L- and capable of oxidizing sulfides and phosphines (Scheme cystein, are oxidized to hydroperoxides 101 which are con- 25) [54]. verted in 5-thiazolidinones 102.

CO Me CO Me 2 HOO 2 HO CO2Me h
, TPP, O2 THF, < 0 °C Me2S or Ph3P S NCOPh S NCOPh S NCOPh

Me Me Me Me Me Me 94 95 96 R3 R2 1 O2 R4 R1 R3 R2 O R3 R2 96 + = norbornene, cyclohexene R4 R1 R4 R1 Scheme 25.

94 95

1 O2 CO2Me CO2Me CO2Me

O S NCOPh HO S NCOPh HO S NCOPh O O O Me Me Me Me 97 Me Me

O

Scheme 26. 96 Photooxygenation of Non-Aromatic Heterocycles Current Organic Chemistry, 2007, Vol. 11, No. 12 1063

HO O O Singlet oxygenation of 1,3-dithianes 106 leads to the cor- R R R responding 1-oxides 107 in good yields and good stereose- a) b) 4 5 lectivity, the latter being governed by steric factors (Scheme S S S N N N 31) [67]. The methodology represents an important comple- Boc Boc Boc ment to conventional oxidizing methods. Indeed, m- chloroperbenzoic acid procedure gives unreacted starting 100 101 102 reagent or, under exhaustive conditions (warming up and longer duration of reaction) leads to side products. Similar a)TPP/ O2 / h
(125 W, halogen) / THF, -78°C good yields have been obtained from the same dithiolanes by b) Ac O / Et N / THF, -78°C 2 3 photosensitised electron transfer oxidation (oxygen saturated Scheme 28. mixture of CH3CN/H2O; 1-cyanonaphtalene as sensitizer) [68]. However substitution patterns as well as the reaction Compounds 102 upon suitable simple reactions afford conditions can affect the reaction course and carbonyl com- amino acids of high enantiomeric purity [65]. pounds and sulfones can be obtained starting from aryl- Some interesting applications starting from 102a (R = substituted 1,3-dithiolanes in addition to sulfoxides [68b]. CH=CHMe) are reported in Scheme 29. A triplet sensitizer as benzophenone induces photodethi- The presence of a good -leaving group induces an oxi- oketalization as reported in Scheme 32 for thioketal 108 [69]. The reaction has been successfully employed on thi- dative elimination reaction leading to
,-unsaturated sul- foxides. So, in the reaction of chlorothiane 103, in addition oketals of steroids and tetrahydrosantonine [69]. of sulfoxides cis- and trans-104, compound 105 is found, Highly stereoselective is the oxygenation of anancomeric derived from the corresponding hydroperoxysulfonium ylide (conformationally fixed) 1,3-dithianes 109 [47]. Compounds as depicted in Scheme 30 [66]. 109a,c,d give exclusively (> 98%) the equatorial sulfoxides

O O O a) b) Me R S RO N Me Me Boc X NH NHBoc 3

102a R = CH R = OH (80%) 3 R = H (95%) R = NHCH2C6H5 (57%) c) O

Me S N H.HBr

a) LiOH/ H2O-THF (96 - 99%)

b) CH3OH/ (Me)3SiBr or (Me)3SiCl c) (Me)3SiBr/phenol/CH2Cl2

Scheme 29.

Cl Cl Cl ++ S S S S O O O 103 cis-104 trans-104 105

104 1 O2 -HCl

Cl 103 Cl

S H Cl S H S O O O O O H O

Scheme 30. 1064 Current Organic Chemistry, 2007, Vol. 11, No. 12 Iesce et al.

(CH2)n (CH2)n 1 O2 S S S S MeOH O 1 2 R1 R2 R R

106 107

1 2 n=2 R , R = (CH2)4 (78%) n= 2 R1 , R2 = (68%) (CH2)5 n= 2 R1 = Ph, R2 = H (72%) n= 2 R1 = Ph, R2 = Me (75%, stereoisomer ratio 7:1) (32%, stereoisomer ratio 4:1) n= 2 R1 = Ph, R2 = Et (72%) n= 3 R1 = Ph, R2 = H

Scheme 31.

R' R' R' S O 1 1O S S O2 2 S R S R S R i S S S O O 110 109 111 108 a; R = CH , R' = H 3 b; R = H, R' = CH3 i; benzophenone, O2, high pressure Hg-lamp (Pyrex filter) c; R = CO2CH3, R' = H d; R = Ph, R' = H Scheme 32. e; R = H, R' = Ph 111a,c,d (Scheme 33). Axial oxidation suffers from destabi- Scheme 33. lizing steric and electronic reactions between the oxidant and the axial hydrogen and with the axial lone pair on the remote The key intermediate is assumed to be the captodatively sulphur. It has been observed in derivatives 109b,e and has stabilized radical 115 which should be formed by in- been explained through the addition of singlet oxygen to tramolecular electron transfer from the carbon-centered an- sulfur in energetically accessible twist-boat conformations. ion to the peroxy linkage in the hydroperoxysulfonium ylide More complex is the reaction of derivative 109c which in (Scheme 35). addition to 111c affords compounds 112-114 (Scheme 34) It is interesting to note that S-oxidation is completely [47]. overcome in the presence of a double bond as in compound

S CO Me S 2 109c

1 O O2 S S MeO2C CO Me S 2 S CO Me S O + 2 + + S O SS SS 111c 112 O 113 114 Scheme 34.

O OH 1 O2 S CO Me 109c 109c 2 S CO2Me 2 x 111c S O O S

OH

O OH S CO Me dimerization + 113 2 S 112 HO CO2Me O O 115

Scheme 35. 114 Photooxygenation of Non-Aromatic Heterocycles Current Organic Chemistry, 2007, Vol. 11, No. 12 1065

S

S 116

O2, -78°C TPP / hν

OOS S O + O + other products S S 100% < 46% Scheme 36.

116 which reacts with singlet oxygen by [2+2] cycloaddition give a 86/14 mixture of the cis and trans-bissulfoxides 122 leading to dioxetane cleavage products (Scheme 36) [70]. (Scheme 38) [71]. At longer reaction times and lower con- A highly stereoselecive oxidation is observed in the reac- centrations of 120 the sulfoxide 121 also continues to react tion of 1,4-dithiane 117 which leads exclusively to cis- to produce cleavage products 123-125 presumably via an
disulfoxide 119 when its concentration is 10-3 M or lower hydroperoxysulphide intermediate 126 [71]. The mixture of (Scheme 37) [9d]. This is consistent with an intramolecular products is formed and derives from two competing reac- transfer of an oxygen atom in a persulfoxide intermediate at tions, S-oxidation and C-S bond cleavage. low concentration. Recent kinetic studies by Clennan’s group have shown that compound 120, in comparison to either 1,4-dithiane or thiacyclohexane, exhibits an enhanced ability to chemically S S react than physically quench singlet oxygen [52,58b]. A con- formationally induced electrostatic stabilization (CIES) has 117 been suggested as yet another mechanism to increase the efficiency of these reactions. In particular, a transannular 1 O2 interaction occurs leading to formation of a S-S bond in both the radical cation and dication (Scheme 39). The electronic OO O O interaction that lowers the energy of the transition state for S S S S its formation also increases the stability of the persulfoxide O S SO and suppresses physical quenching. Successively the con-

118 formationally induced electrostatic stabilization (CIES) sul- 119 phide photooxygenation mechanism has been computation- Scheme 37. ally examined and extended to oxygen and NH groups. In- vestigation has confirmed the role of remote functional The sensitized photooxygenation of 1,5-dithiacyclo- groups in stabilizing the related persulfoxide [58a]. octane 120 is complex depending on the solvent and concen- A good chemoselectivity is observed in the singlet oxy- tration [52, 58, 71]. Indeed it leads at high concentration to genation of dithiocin 127 which leads essentially to sulfoxide sulfoxide 121 as the primary product that further reacts to

1 O2

1 H O2 S S S S O + OOSS + + + O S S O

O O 120 121 122 123 124 125

1 O2 S S O

HOO

Scheme 38. 126 1066 Current Organic Chemistry, 2007, Vol. 11, No. 12 Iesce et al.

cation and superoxide anion. Physical quenching by a re- verse ET is a significant reaction in the dye-sensitized pho- -e -e S S S S S S tooxygenation of these compounds with the quenching effi- ciency decreasing in the order tertiary amines> secondary amines > primary amines, so tertiary amines are employed as 120 typical singlet oxygen quenchers [2, 4c, 4d, 12, 73]. Chemi-

cal reactions occur, often via either by Type I or III mecha- Scheme 39. nisms, and furnish carbonyl compounds and amines, result- 128, as expected due to the presence of activated
- ing from N-dealkylation [74] or -oxidation products [75] or hydrogens (Scheme 40) [72]. The same result has been ob- dehydrogenation products [75, 76]. Starting from five- tained in the DCA-sensitized oxygenation involving the reac- membered systems oxygenation can lead to the related aro- tion between the cation radical and superoxide anion [72]. matic compounds as does compound 129 which converts to

S S h
, O2, MB, CH2Cl2 S S or h
, O2, DCA, CH3CN

O 127 128

Scheme 40.

h
, sens, O2

HOO OOH N N N H H H

129 130 Scheme 41.

h
, O2, RB (CH2)n MeN O (CH2)n OHCN O

131 a; n= 1 Scheme 42. b; n= 0

i pyrrole 130, presumably via the intermediate hydroperoxide

O (Scheme 41) [76b]. N O O N N-formyl derivatives resulting from
-oxidation have R R been found, e.g in the photooxygenation of some bicyclic

132 133 amines as pseudopelletierine 131a and tropinone 131b or analogues (Scheme 42) [74].

Oxygenation using 150 W medium-pressure Hg lamp i O (Pyrex filter), oxygen-saturated solution in t-BuOH, benzo- N N phenone as sensitizer allows lactams 132 to be transformed

to the imides 133 (Scheme 43) [77]. Similar trend has been R O R O observed also starting from amides 134 (Scheme 43) [77a]. 134 Oxidation is suggested to be initiated by abstraction of i; h
, Ph2C=O, O2, t-BuOH or C6H6 the hydrogen
to the amide nitrogen by the triplet benzo- Scheme 43. phenone (Scheme 44). Lactams which are substituted
to the nitrogen atom give
-hydroperoxy or hydroxyamides The reaction of nitrogen containing compounds with sin- [77b]. glet oxygen often involves an electron transfer from the sub- Activation of a double bond by heteroatom is the key fea- strate to singlet oxygen with formation of substrate radical ture of the easy dye-sensitized photooxygenation of certain

h
* Ph COH Ph2CO Ph2CO 2

O2 132 133 N O Scheme 44. R Photooxygenation of Non-Aromatic Heterocycles Current Organic Chemistry, 2007, Vol. 11, No. 12 1067

2 O R O O HO NHR1 O i + O N 2 N R = H R1 R1 O O O 135 136 137 (for R1= H) R1 and R2= H i

R2 2 OOH R OH R2 OH O O

+ O N + 1 N R R1 O O O 138 139 140

i; hν, O2, RB /MeOH or TPP/benzene-pyridine Scheme 45.

R2 R2 OH OH 135 N N R1 R1 O O

141

ene-like 1 [4+2] 1O O2 2

R2 OOH O O R2 O OH O OH NHR1 139 N 1 N 1 R R = H O R1 2 R = H O O O 138 142 H2O H O 2 1 2 R and R = H 2 R = H R1NCO R2= H 137

136

O

R2 140 OH

Scheme 46. O lactams via their enol forms and can be correlated to the under basic conditions (benzene-pyridine) using TPP as sen- oxygenation of enols. Indeed, it is known that singlet oxygen sitizer. Under these conditions the oxygenation leads to may react with enolic forms, e.g. of 1,3-diketones, and the 1,3,4-isoquinolinetriones 136 and 3-hydroxybenzoisofuran- reaction is favoured under suitable conditions (when the 1-one-3-carbamides 137 for R2=H [79a] and compounds equilibrium is shifted toward the enol side or the electron 138-140 (for 4-alkylated derivatives) [79b] (Scheme 45). density at the enolic C=C is increased) [78]. The oxygena- The authors explain the results via the intermediacy of tion of 1,3-isoquinolinediones 135 represents an interesting hydroperoxides 138, formed by singlet oxygen addition to example [79]. Indeed, while under typical singlet oxygen the electron rich enol bond, and the intermediacy of the en- reaction conditions with TPP as sensitizer in benzene solu- doperoxides 142, derived from 138 or directly by addition of tion they are unreactive even on prolonged irradiation, pho- singlet oxygen to the diene system of 141 (Scheme 46) [79]. tooxygenation occurs in methanol using RB as sensitizer or The role of pyridine or Rose Bengal, the latter via its anionic 1068 Current Organic Chemistry, 2007, Vol. 11, No. 12 Iesce et al. structure, is to act as hydrogen bond acceptors and, hence, to Ar 1 shift the keto-enol tautomerism equilibrium toward the enol 143 + O2 N N + O2 side. H n Interesting examples of photooxidation of nitrogen- 147 containing compounds which does not lead to addition of oxygen but involves oxygen, have been recently reported for Ar N-aminopiperidine and pyrrolidine derivatives [80, 81]. In- Ar deed, the reaction of compounds 143 in the presence of MB N N or N N + HO2 and oxygen leads to acyclic aminoaldehydes 144 or amino- H n n dialkylacetals 145 using CH3CN/H2O or CH3CN/ROH (9:1), respectively, as solvent [80]. Ring-opening of unsymmetrical derivatives occurs selectively at the bond between the nitro- Ar Ar gen atom and the less substituted carbon. The reaction to N N + HO2 compounds 144 proceeds smoothly with good to excellent N N n n yields (46-98%) and the resulting acyclic compounds con- H serve the high diastereomerical purity of the starting sub- 148 strates (Scheme 47). Oxidation of compounds 143 in the Nu presence of trimethyl silyl cyanide (TMSCN) leads regio- and stereoselectively to
-hydrazinonitriles 146 with good Ar yields [81]. The
-hydrazinonitrile obtained corresponds to N N the attack of the more substituted -C-H bond and in 4- n
H methylpiperidine the substituent adopts trans-relationship Nu with the CN group in the axial plane and the methyl group in 149 the equatorial position. Scheme 48.

H A useful application of the photooxidation by single elec- Ar N tron transfer (SET) of N-arylaminopiperidines and N- N O n arylaminopyrrolidines 143 is the “one-pot” synthesis of the R corresponding lactams 150 (Scheme 49) [82]. Indeed, irra-

144 diation by visible light in the presence of catalytic amount of methylene blue (MBcat) and TMSCN gives cyanoproducts 146, which are photooxidated in the presence of water to h , O , MB
2 lactams 150. CH3CN, H2O

R Ar Ar h
, O , MB h , O , MB OR 2 Ar
2 N N N N CH CN, ROH Ar N n n N N 3 H CH3CN, TMSCN H N n OR H n R= H 143 NC 145 143 n= 1, 2 146

h , O , MB h
, O2, MB
2 R = H CH CN, H O CH3CN, TMSCN 3 2

Ar Ar N N N N n H H n NC O 146 150 Scheme 49. Scheme 47.

The key step is the formation of a hydrazinium alky- Rose Bengal-sensitized photooxygenation of strained cy- lidene cation 148 which undergoes addition of a nucleophile clic amines as aziridines occurs but it leads to a variety of products depending on the nature of the substituents present giving 149 [81]. When H2O or ROH are used, ring-opening of the intermediate 149 leads to the final aldehyde 144 or in the ring [83]. It has been suggested that an azomethine acetal 145, respectively. The radical cation 147 would be ylide as 154 (Scheme 50) is formed under photochemical formed by electron transfer from the substrate to singlet oxy- conditions, and then it adds singlet oxygen to give the cyclic gen (SET). The combination between hydrazinium alky- peroxide whose decomposition leads to the observed prod- -. ucts. So, 1,2,3-triphenylaziridine 151 on photooxygenation lidene radical cation 147 and the superoxide anion O2 could lead to oxidation products as reported in Scheme 48. gives benzoic acid 152 and benzanilide 153 by fragmenta- Photooxygenation of Non-Aromatic Heterocycles Current Organic Chemistry, 2007, Vol. 11, No. 12 1069

Ph

HHN h
, O2, RB PhCOOH+ PhCONHPh Ph Ph C6H6/Acetone 152 153 151

h
[O]

Ph Ph Ph 1 N H PhN Ph O2 H H N H Ph PhOO Ph Ph 154 O O

155 156 Scheme 50. tion of the 1,2,4-dioxazolidine intermediate 155 into the toward singlet oxygen, can be photooxygenated to form diradical intermediate 156 and subsequent oxidations [83a]. ozonides [85], and the efficiency of the reaction can be im- Starting from the bicyclic derivative 157, 2-cyclohexyl-3- proved by using biphenyl (BP) [a’), Scheme 52] [86]. The hydroxy-3-phenylphthalimidine 158 has been obtained in key step in these reactions involves electron transfer fluores- 51% yield through decomposition of the intermediate perox- cence quenching of the sensitizer by the substrate with for- ide followed by hydrolysis and cyclization (Scheme 51) mation of azomethine (X=N) or carbonyl ylides (X=O) 159 [83b]. as intermediates (Scheme 52). DCA-sensitized photooxy- genations are, therefore, limited to easily oxidized substrates with oxidation potential of less than 2V vs. SCE in MeCN [87]. The efficiency of the reaction of less reactive oxiranes Ph HO can be effected by using biphenyl (BP) which acts as a non- Ph N light-absorbing cosensitizer in conjuction with DCA; its ac- h
, O , RB 2 N tion is explained considering that it is more easily oxidized H C6H6/MeOH than the substrate and therefore quenches singlet excited DCA more efficiently [86a]. N O Scheme 53 reports the results of oxygenation of com- 157 158 pounds 160a-d. 1,2,4-Dioxazolidines 161 are obtained in 39- 83% yields [84a]. The cis/trans ratio of the peroxides ap- Scheme 51. pears to depend on the bulkiness of the substituent on the nitrogen atom which sterically destabilizes the azomethine 1 4 More controlled is the oxygenation of aziridines in the ilide intermediate (159 with R =R =Ph, X=N in Scheme 52). presence of cyano-substituted aromatic hydrocarbons such as Investigation of the stereochemistry in the oxygenation of 9,10-dicyanoanthracene (DCA) [84]. Under these typical cis- and trans-162 has shown that both isomers are converted electron-transfer conditions oxiranes, which are unreactive to cis-ozonide 163 in 65% yield (Scheme 54) [86a].

3 X 2 4 1 R R h
R X R a) DCA + DCA + R4 R1 CH3CN R3 R2

h
BP + DCA DCA + BP

4 1 a') R3 O R2 R O R BP + BP + R4 R1 R3 R2

DCA + O2 DCA + O2

R4 1 X R 4 1 1 R X R O2 + O2 + R3 R2 3 2 R R 159

X= O, NR R4 X R1

BP = biphenyl 3 2 Scheme 52. R OOR 1070 Current Organic Chemistry, 2007, Vol. 11, No. 12 Iesce et al.

R R R N hν, DCA N N O Ph Ph Ph Ph + 2 + CH CN Ph Ph 3 OO OO

161 160 a; R= H > 99 < 1 b; R= Me 85 15

c; R= CH2Ph 40 60 d; R= t-Bu < 1 > 99

Scheme 53.

O years due to the role of this species in the oxidation and deg- radation of polysilanes, which have potential technological Ph Ph usefulness [89]. Light-induced oxidations have also been studied mainly to gain mechanistic information. Less inves- cis-162 O hν, DCA, O2 Ph Ph tigated has been the photochemistry of germanium- containing molecules or analogues [90]. A peculiarity of BP, CH3CN O OO compounds that contain group 14 to group 14 or group 14 to

Ph Ph cis-163 carbon bonds is that they can act as excellent electron do- nors. These bonds indeed are subject to cleavage by various trans-162 electrophiles, and due to their low ionization potential these compounds undergo efficient electron transfer reactions [90]. BP=biphenyl This trend is generally observed in the photooxygenation reactions, too. Scheme 54. The classical [4+2] addition of oxygen to the diene sys- tem appears to be involved in photooxygenation of silacy- The stereoselective formation of cis-163 from both iso- clopentadiene 164 which leads to dicarbonyl compound 166 mers has been explained assuming that equilibration occurs presumably via the related endoperoxide 165 with extrusion to afford the most stable E,E-conformer of carbonyl ylide of the metal (Scheme 55) [91]. A peculiar bicyclic product 159 (X=O, Scheme 52) which undergoes a concerted 1,3- 167 has also been obtained and has been suggested to form addition of singlet oxygen generated by a second electron via 168 (Scheme 55) [91a]. transfer [86a]. It is interesting to note that high stereoselec- The presence of a Si-Si
bond entails bond cleavage and tivity has been observed successively in other electron trans- oxygen addition, even in unsaturated molecules. Rearrange- fer reaction of oxiranes in the presence of TCNE although in ment or decomposition induced by silicon may lead to char- this case neither an oxygen radical anion nor singlet oxygen acteristic final products. So, dioxygen insertion has been is generated [88]. observed in the photooxygenation of 1,2-disiletene 169 in acetonitrile/methylene chloride in the presence of DCA [92]. 4. PHOTOOXYGENATION OF SILICON, GERMA- The reaction affords the corresponding 1,2,3,6-dioxadisilin NIUM AND TELLURIUM-CONTAINING COM- 170 in moderate yield together with 1,2,5-oxadisilolene 171 POUNDS (Scheme 56). Much attention has been drawn to the reaction of or- Similar results are obtained in the presence of methylene ganosilicon compounds with molecular oxygen in the recent blue (MB) or 2,4,6-triphenylpyrilium perchlorate while

Ph Ph Ph Ph

+ Ph Ph Ph Ph Me Me O O O O Si Ph Ph Si Ph Ph 166 Me Me hν, O2 O 167 Ph Ph Ph O Si Ph Me Me 165 Ph Ph

164 Ph Ph O Si O Me Me

168 Scheme 55. Photooxygenation of Non-Aromatic Heterocycles Current Organic Chemistry, 2007, Vol. 11, No. 12 1071

PPh3

Ph Ph Ph hν, O2, Sens. R Si SiR 2 2 + R2Si SiR2 R2Si SiR2 CH3CN/CH2Cl2 (1:1) OO O

169 170 171

R= Mesityl Δ Ph Sens.= DCA or MB or 2,4,6-triphenylpyrilium R Si SiR2 O RO 172 Scheme 56.

3 Sens O2 169

h

169 Sens. O2

O2 170

Ph Ph

3 e O2 169 R2Si SiR2 R2Si SiR2 171 O O O O Ph Ph

3 CH3CN O2 e R2Si SiR2 170 R2Si SiR2 O N N O CCH CCH 3 3 Scheme 57. compound 169 appears stable under singlet oxygen oxygena- Oxygen insertion prevails also in the RB-sensitized pho- tion accounted conditions, i.e in the presence of a typical tooxygenation of 173 despite the presence of a
-conjugated singlet oxygen sensitizer as TPP [92]. The reaction is there- system usually highly reactive under these irradiation condi- fore suggested to involve a photo-induced electron transfer tions (Scheme 58) [94]. Only a cyclic disiloxane 174 has in from disilitene 169 to the excited singlet state of the sensi- fact been obtained. tizer and attack of superoxide anion (or ground oxygen) to +. the silyl radical cation 169 as reported in Scheme 57 [92]. Ph Ph Ph Ph An increasing ratio of 170 to 171 is observed on increas- hν, O , RB ing acetonitrile in the solvent composition, and this has been Ph Ph 2 Ph Ph interpreted assuming that acetonitrile acts as the nucleophile THF Me2Si SiMe2 Me2Si SiMe2 to stabilize the cation radical 169+. and assists the ring clo- O sure to 170 (Scheme 57). Peroxide 170 reacts easily with 173 174 triphenylphosphine (PPh3) to give 171 while it decomposes gradually at room temperature to afford siloxane 172 via Scheme 58. Criegee type rearrangement (Scheme 56). A very similar Similar trend has been observed in the irradiations of behaviour has been observed in electron-transfer oxygena- benzene-benzonitrile solutions of compounds 175 which in tion of 1,2-digermetenes confirming the involvement of the presence of C60 lead to compounds 176 (Scheme 59) cation radical-initiated cycloaddition [93]. [95]. Processes of both Type II and III have been proposed 1072 Current Organic Chemistry, 2007, Vol. 11, No. 12 Iesce et al.

Ph Ph Ph Ph

hν (>420 nm), O2 Ph Ph + C60 Ph Ph C6H6/C6H5CN R2EER2 R2E ER2 (1:4) O 175 176 175 3 * 3 1 C60 ++O2 C60 O2 Type II

hν 1 * C60 C60 176

3O a; E,E = Ge, R = Me 3 * 2 Type III C60 + C60 + b; E, E = Ge, R = Et 175 175 c; E, E = Si, R = Et d; E = Ge, E = Si, R = Et

Scheme 59.

X X h
, O2, TPP X X R2Si SiR2 R2Si SiR2 + R Si SiR R2Si SiR2 O 2 2 C6H6 O O O 177 O 179 178 180

PPh R'2SO 3

180 + R'2SO2

a; 96a X = O, R= 2,6-diisopropylphenyl; 2,6-diethyl- or dimethylphenyl; mesityl

96b b; X = CH2, R= mesityl Scheme 60.

X X X h , O , TPP Ph P
2 R Ge GeR 3 2 2 R Ge GeR + Ph3PO R2Ge GeR2 2 2 C6H6 O O O 181 182 183 a; X= CH2, R= 2,6- diethylphenyl b; X= N-Ph, R= mesityl

Scheme 61.

(Scheme 59). In the first case, singlet oxygen is generated by gous silicon compounds (182a has been structurally charac- 3 an energy transfer reaction between the photoexcited C60* terized by X-ray analysis) according with a trend well docu- and triplet oxygen, while in the second pathway the radical mented. Both peroxides 182a,b are quantitatively reduced to +. cation 175 reacts with triplet oxygen. Assumption has been compounds 183 with triphenylphosphine (PPh3) [97]. 1 confirmed by suitable experiments using a typical O2 sensi- Heavier organochalcogen compounds as seleno- or tellu- tizer as Rose Bengal, and an electron-transfer sensitizer as rium-compounds have been little investigated if compared DCA [95]. with sulphur-containing molecules. Oxidations with metal Strained disiliranes as 177 give the corresponding 1,2- elimination have sometime been observed in the photooxida- peroxides 179 by TPP-sensitized oxygenation [96]. It has tion of diverse acyclic and cyclic compounds [98]. In par- 1 been suggested that O2 may approach perpendicularly to the ticular, tellurophenopyridazine 184 decomposes under the Si-Si bond to afford peroxonium ion 178. The nucleophilic influence of light and oxygen into 4,5-dibenzoylpyridazine oxygen-atom transfer capability of 178 has been evidenced 186 presumably via the peroxide 185, derived from Diels- towards sulfoxides (Scheme 60). Compounds 180, which are Alder reaction of singlet oxygen with 184, with extrusion of sometime found in little amounts, can be obtained by the metal (Scheme 62) [99]. Analogously, 4,5-dibenzoyl-2,7- triphenylphosphine (PPh3) treatment of 179. diphenyltropone 188 is formed in 43% yield by visible light Singlet oxygenation of digermirane 181a and azadiger- photooxidative detelluration of compound 187 (Scheme 62) miridine 181b proceeds similarly and affords 1,2-peroxides [99]. 182a and 182b in good yields (Scheme 61) [97]. The latter Oxidation at the atom site has been observed in tellu- compounds are remarkably stable if compared with analo- rapyrylium dyes such as 189 (Scheme 63) [100]. These com- Photooxygenation of Non-Aromatic Heterocycles Current Organic Chemistry, 2007, Vol. 11, No. 12 1073

Ph Ph PhCO N hν, O2 O N - Te N Te Te N CHCl O N 40% N 3 PhCO Ph 184 Ph 185 186

Ph Ph Ph PhCO hν, O2 Te O O CHCl 3 PhCO Ph Ph Ph 187 188

Scheme 62.

O O

But Te But But Te But HO OH But Te But O

But Te But O , W-bulb H O 2 189 2 But or H2O But X O O BF4 t Te But X BF But Bu 4

But 189

X=Te, Se 190

Scheme 63. pounds are able to produce singlet oxygen and react with it. ber of reports dealing with this question [5a]. Moreover, light The authors suggest intermediates similar to persulfoxide 72 and oxygen play a significant role in the fate of organic or thiadioxirane 73 intermediates (Scheme 20) to explain the molecules in the environment and their action can be influ- oxidation. Dihydroxides 190 are hydrated forms of tellurox- enced by highly conjugated molecules as dyes or polycyclic ides. aromatic hydrocarbons (PAH), which either can accelerate It is interesting to note that the presence of tellurium in the phototransformations or promote the formation of singlet these dyes enhances significantly the quantum efficiency for oxygen [102]. the generation of and the rate of reaction with singlet oxygen [100]. REFERENCES [1] Ciamician, G.; Silber, P. Chem. Ber., 1912, 45, 1842. 5. CONCLUSION [2] Iesce, M. R.; Cermola, F.; Temussi, F. Curr. Org. Chem., 2005, 9, 109. It should be evident from the above survey and preceding [3] Albini, A.; Freccero, M. In Handbook of Organic Photochemistry review that the field of photooxygenation of heterocycles is a and Photobiology; Horspool, W. M., Song, P-S., Eds.; CRC Press: very active area of research and offers a lot of interesting Boca Raton (FL), 1995; pp. 346-357. applications. In addition to the initial synthetic interest, due [4] For reviews: a) Clennan, E. L.; Pace A. Tetrahedron, 2005, 61, to the widespread use and presence of heterocycles, the re- 6665. b) Gorman, A. A.; Rodgers, M. A. J. Chem. Soc. Rev., 1981, 10, 205. For books: c) Foote, C. S.; Clennan, E. L. In Active Oxy- sponse of these systems to photooxygenation is of high im- gen in Chemistry; Foote, C. S.; Valentine, J. S.; Greenberg, A.; portance in various fields of science (from environmental Liebman, J. F. Eds.; Chapman & Hall: London, 1995; pp. 105-140. chemistry to biochemistry, from engineering to medicine). d) Frimer, A. A. Singlet Oxygen, CRC Press: Boca Raton, 1985; The use of photodynamic therapy (PDT) in diseases as pso- Vol. 1-IV. riasi or some sorts of cancers is of current application [101]. [5] a) Quintero, B.; Miranda, M. A. Ars Pharm., 2000, 41, 27. b) Bur- rows, C. J.; Muller, J. G. Chem. Rev., 998, 98, 1109. c) Pratviel, G.; On the other hand the importance of the photosensitization Bernadou, J.; Meunier, B. Angew. Chem. Int. Ed. Engl., 995, 34, processes induced by drugs, usually heterocyclic compounds 746. d) Sies, H. Angew. Chem. Int. Ed. Engl., 1986, 25, 1058. is easily understood taking into account the increasing num- 1074 Current Organic Chemistry, 2007, Vol. 11, No. 12 Iesce et al.

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