Electron transfer-sensitized irradiation

Yasuda et al. have used steady-state kinetics to investigate the reaction of the phenanthrene cation radical with ammonia and several primary amines. Rale constants were observed to increase from 0.3 x 10 M s for ammonia to 8.9 x I0 M s for /-butylamine. Correlation of these rates with the Taft o parameter (Table 4) indicates that there is substantial development of positive charge on nitrogen in the transition state for bond formation. 

---

A photochemical synthesis of isoquinoline and benzazepine derivatives in good preparative yields is shown in Scheme 23 [127, 128]. Upon electron-transfer-sensitized irradiation, the primary aminoethyl and aminopropyl stil-... 

Electron transfer sensitized irradiation of acyclic dienes gives rise to mixtures of [4 + 2] and [2 + 2] dimers for example, an 8 1 mixture was obtained from 2,4-dimethyl-l,3-pentadiene [137]. On the other hand, [4 + 2] dimers were obtained exclusively upon oxidation with aminium radical cation salts [138]. This difference has led some workers to question the key role of electron transfer in these reactions... 

The electron transfer sensitized irradiation of allyl- or benzylcyclopropene systems (68) results in ring enlargement with dehydrogenation (->70) [245]. Formally, these structures can be considered as substituted hexadiene systems (vide infra). Although mechanistic details have not been established, the reaction very likely proceeds via bifunctional intermediates (69). 

Mariano and co-workers also reported the photochemistry of aUenes using an electron-transfer sensitizer.Irradiation of allenes 12,49, and 50 in the presence of 2-phenylpyrrohnium perchlorate affords adducts 51 and 52, 53 to 55, and 56, respectively. In these reactions, the radical cation of allene 57 and pyrrolidinyl radical 58 are assumed to be the key intermediates. Mangion and co-workers reported the photochemical reactions of allenes with cyanoarenes. For example, irradiation of 1,2,4,5-tetracyanobenzene 59 and tetramethylallene 50 in acetonitrile-methanol (3 1) through a Pyrex filter leads to 1 1 1 arene-aUene-methanol adduct 60. Similar reaction of 1,1-dimethylaUene 49 also affords adducts 61 and 62. 

C)S-Diphenylcyclopropane is achiral while the trans-isomer is chiraL Energy- or electron-transfer sensitized irradiation of cis-diphenylcydopropane in isotropic solvents provides the trans-isomer as a... 

Fig. 20. Proposed photochemical mechanisms for the generation of acid from sulfonium salt photolysis. Shown ate examples illustrating photon absorption by the onium salt (direct irradiation) as well as electron transfer sensitization, initiated by irradiation of an aromatic hydrocarbon.

Photochemical addition of ammonia and primary amines to aryl olefins (equation 42) can be effected by irradiation in the presence of an electron acceptor such as dicyanoben-zene (DCNB)103-106. The proposed mechanism for the sensitised addition to the stilbene system is shown in Scheme 7. Electron transfer quenching of DCNB by t-S (or vice versa) yields the t-S cation radical (t-S)+ Nucleophilic addition of ammonia or the primary amine to (t-S)+ followed by proton and electron transfer steps yields the adduct and regenerates the electron transfer sensitizer. The reaction is a variation of the electron-transfer sensitized addition of nucleophiles to terminal arylolefins107,108. 

The presence of hetero-atoms within the system, remote from the alkene double bonds, does not have an adverse influence on the SET processes that occur. Thus irradiation of the diene 33 in benzene solution with 1,4-dicyanonaphthalene as the electron-transfer sensitizer affords the cyclobutane 34 in 78% yield. Various examples of the reaction were described giving cyclobutane derivatives in 54-69% yield. Benzene, or an arene solvent, is vital for the success of the reaction. When acetonitrile is used, allylation of the sensitizer (akin to the photo-NOCAS reaction) results in the formation of the three products 35-3718. (2 + 2)-Cyclization of this type described for 33 is also seen with the dialkenyl ether 38. When 38 is irradiated using X > 350 nm or X > 450 nm in acetonitrile... 

The DCA-sensitized irradiation of 107a for 13 hr affords, after column chromatography on silica gel, the rrans -cyclopropane derivative 108a (10%) as a 1 1 mixture of C=N bond fiZ-isomers. Similarly, irradiation of the oxime acetate 107b under these conditions for 2.5 hr affords, after chromatography, the rrans -cyclopropane derivative 108b (12%). These results show that the novel 1-ADPM rearrangement promoted by electron-transfer sensitization can be extended to other C—double-bond derivatives. 

Cyclobutane Fragmentations, irradiation of oxidative electron transfer sensitizers in the presence of aryl cyclobutanes causes ring fragmentation, in a formal retrocyclo-addition sense. For example, Mukai has shown, eq. 33 (98),... 

Other oxygenated compounds also participate in electron exchange. Lewis and co-workers have shown, for example, that charge transfer irradiation of metal complexes of tetrahydro-furan initiates polymerization through an intervening cation radical (139). Phenols can act as excited state donors either directly (140) or via electron transfer sensitization (141). 

The relative stability of benzylic radicals and cations makes photoinduced C-X cleavage of benzylic derivatives a widespread occurrence, and in fact according to different modes. These are i) fragmentation of an excited state, formed by direct irradiation or via energy transfer sensitization ii) atom abstraction by a chemical sensitizer iii) fragmentation of a benzylic radical cation or anion generated via electron transfer sensitization or quenching (Sch. 2). 

A Ci-symmetric 1,2,3,4-adduct with a Cs-symmetric bridge102 between C(2) and C(3) (( )-167, Figure 1.39) has been reported as minor product resulting from the reaction between 2-methylpropan-2-ol and C6o +, generated by irradiation in the presence of the PET-(photoinduced electron transfer) sensitizer 2,4,6-triphenylpyrylium tetrafluoroborate.323 Its formation can be understood in terms of a further transformation of the initially formed monoadduct l-([60]fullerenc-l (2//)-yl)-2-mcthylpropan-2-ol. 

Examples of intramolecular trapping of carbonyl ylide dipoles by alkenes have now been reported.These include, for example, the conversion of the oxirane (172) into the tetrahydrofuran (173). Carbonyl ylides have also been prepared by irradiation of 2,3-bis-(p-methoxyphenyl)oxirane in the presence of dicyanoanthracene as electron-transfer sensitizer direct or triplet-sensitized irradiation, however, leads mainly to rearrangement via carbon-oxygen bond cleavage. In contrast, cyclohexene oxide and styrene oxide, on naphthalene-sensitized irradiation in alcohols, undergo solvolysis via oxide anion-radical intermediates. ... 

Direct photolysis of 50-53 in 02-saturated acetonitrile solution also leads to the corresponding carbazoles with a quantum yield of ca 0.64 in all cases (equation 13)161. Apparently, substituents have only a little effect on the chemical yield of carbazole produced by steady-state irradiation in aerated acetonitrile. However, an attempt to carry out such a photocyclization reaction by using photoinduced electron-transfer sensitization has failed, presumably due to fast back electron transfer that quenches the net reaction. It is also interesting to note that chemical oxidation and electrochemical oxidation of 50-53 does not result in carbazoles. Instead, benzidine products are formed. These results are consistent with the AMI calculations, which suggest that the cyclization reaction is both kinetically and thermodynamically more favorable from the triplet state than from the cation radical or dication. 

Sensitized photolysis of both salts resulted in the formation of the same major products as was observed in direct irradiation, iodobenzene, or phenylsulfide and acid. Quantum yields of organic and acidic products were measured using nine sensitizers. Product quantum yields were relatively high (< 1) under conditions where electron transfer sensitization was expected to occur. Quantum yields >2 were obtained in solvents likely to be substrates for hydrogen abstraction, indicating that a chain reaction was involved. Triplet energy sensitization resulted in low product quantum yields. 

Cycloadditions only proceeding after electron transfer activation via the radical cation of one partner are illustrated by the final examples. According to K. Mizono various bis-enolethers tethered by long chains (polyether or alkyl) can be cyclisized to bicyclic cyclobutanes using electron transfer sensitizer like dicyanonaphthalene or dicyano-anthracene. Note that this type of dimerization starting from enol ethers are not possible under triplet sensitization or by direct irradiation. Only the intramolecular cyclization ci the silane-bridged 2>. s-styrene can be carried out under direct photolysis. E. Steckhan made use of this procedure to perform an intermolecular [4+2] cycloaddition of indole to a chiral 1,3-cyclohexadiene. He has used successfully the sensitizer triphenylpyrylium salt in many examples. Here, the reaction follows a general course which has been developed Bauld and which may be called "hole catalyzed Diels-Alder reaction". 

Wiest and co-workers have demonstrated that indole will participate in electron-transfer sensitized Diels-Alder reactions [14c, 41]. Irradiation of the electron-transfer sensitizer triphenylpyrylium tetrafluoroborate 21 in a methylene chloride solution containing indole, 1,3-cyclohexadiene and acetyl chloride yields the adduct 22 as an exo and endo mixture in a 1 3.3 ratio and a combined yield of 70% (Scheme 9) [14c,41a], Experiments are reported [14c, 41a] which demonstrate that the reaction proceeds by electron transfer from the indole to the excited sensitizer the indole radical cation produced then attacks the diene to give an intermediate 23 (Ri = = H) which, in conjunction with back-electron transfer from the... 

Laser irradiation at 266 nm of the 2,5-dimethylhexa-2,4-diene in aerated acetonitrile affords the corresponding radical cation. Other less heavily substituted dienes do not undergo this process on direct irradiation and electron transfer sensitization has to be used. Typical of this process is the generation of the radical cation of piperylene by irradiation with dicyanobenzene as the sensitizer and biphenyl as the co-sensitizer. The presence of the co-sensitizer permits cage escape of the radical cation of the diene. A study of the photochemical ring... 

The ring-opening reactions of (40) and (41), in the presence of chloranil and (42) as electron-transfer sensitizers, has been studied. Enone (40) reacts only under chloranil sensitization and affords the two products (43) and (44) after 10 min irradiation in methylene chloride. Compound (41) affords (45) with chloranil and (46) under sensitization with (42). It is clear from these studies and from the types of products formed that radical cation intermediates are involved. 

Using cresyl violet perchlorate (CVP) as the electron-transfer sensitizer, photo-induced electron transfer of stilbene was also studied (Fig. 6.13). As expected, isomerization of franx-stilbene in ethanol resulted in fran -isomer as main product (Table 6.4). Interestingly, in aqueous media containing dendrimers, CM-isomers were obtained as main products. Additionally, like direct irradiation, dendrimers with smaller cavity size favored c/x-stilbene formation. This observation can confirm the role of dendrimer cavity size. 

TABLE 6.4 Photo-Isomerization of frans-Stilbene Encapsulated Within C2C3-C5G3 Dendrimers, in the Presence of CVP as Electron-Transfer Sensitizer Dye, Using a 380 nm Filter and Irradiation for 2 h... 

---