To gain information about how alkoxy substitution in arene rings of β-O-4 structural units within lignin governs the efficiencies/rates of radical cation C1-C2 bond cleavage reactions, single electron transfer (SET) photochemical and lignin peroxidase-catalyzed oxidation reactions of dimeric/tetrameric model compounds have been explored. The results show that the radical cations derived from less alkoxy-substituted dimeric β-O-4 models undergo more rapid C1-C2 bond cleavage than those of more alkoxy-substituted analogues. These findings gained support from the results of DFT calculations, which demonstrate that C1-C2 bond dissociation energies of β-O-4 radical cations decrease as the degree of alkoxy substitution decreases. In SET reactions of tetrameric compounds consisting of two β-O-4 units, containing different degrees of alkoxy substitution, regioselective radical cation C-C bond cleavage was observed to occur in one case at the C1-C2 bond in the less alkoxy-substituted β-O-4 moiety. However, regioselective C1-C2 cleavage in the more alkoxy-substituted β-O-4 moiety was observed in another case, suggesting that other factors might participate in controlling this process. These observations show that lignins containing greater proportions of less rather than more alkoxylated rings as part of β-O-4 units would be more efficiently cleaved by SET mechanisms.

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A novel method for the preparation of structurally diverse fullerene derivatives, which relies on the use of single electron transfer (SET)-promoted photochemical reactions between fullerene C60 and α-trimethylsilylamines, has been developed. Photoirradiation of 10% EtOH-toluene solutions containing C60 and α-silylamines leads to high-yielding, regioselective formation of 1,2-adducts that arise through a pathway in which sequential SET-desilylation occurs to generate α-amino and C60 anion radical pair intermediates, which undergo C-C bond formation. Protonation of generated α-aminofullerene anions gives rise to formation of monoaddition products that possess functionalized α-aminomethyl-substituted 1,2-dihydrofullerene structures. Observations made in this effort show that the use of EtOH in the solvent mixture is critical for efficient photoproduct formation. In contrast to typical thermal and photochemical strategies devised previously for the preparation of fullerene derivatives, the new photochemical approach takes place under mild conditions and does not require the use of excess amounts of substrates. Thus, the method developed in this study could broaden the scope of fullerene chemistry by providing a simple photochemical strategy for large-scale preparation of highly substituted fullerene derivatives. Finally, the α-aminomethyl-substituted 1,2-dihydrofullerene photoadducts are observed to undergo photoinduced fragmentation reactions to produce C60 and the corresponding N-methylamines.

A study was conducted to gain insight into the preparative potential of photosensitized reactions of acyclic N-benzylglycinates containing an a-trimethylsilyl group with dimethyl acetylenedicarboxylate (DMAD). The photosensitizers employed in the reactions include 9,10-dicyanoanthracene (DCA), 1,4dicyanonaphthalene (DCN), rose bengal (RB) and fullerene C 60 . The results show that photoirradiation of oxygenated solutions containing the photosensitizers, glycinates and dimethyl acetylenedicarboxylate leads to competitive formation of pyrroles and b-enamino-esters. The distributions of pyrrole and benamino-ester products formed in these reactions are highly influenced by the electronic nature of the phenyl ring substituent on the benzylglycinates and the photosensitizer used. These photoaddition reactions take place via mechanistic pathways involving competitive formation of azomethine ylides and secondary amines, generated by a mechanistic routes involving initial SET from the benzylglycinates to photosensitizers.Scheme 1 Photoinduced single electron transfer (SET) reaction pathways opened for amine substrates. Scheme 2 SET-promoted 1,3-dipolar cycloaddition reactions of ethyl 2-(3,4-dihydroisoquinolin-2(1H)-yl)acetate 6 with dipolarophile. Scheme 3 SET-promoted 1,3-dipolar cycloaddition reactions of tertiary N-benzylglycinates 10 (E ¼ H, SiMe 3 ) with fullerene C 60 . 5640 | RSC Adv., 2019, 9, 5639-5648 This journal is a 220 mL of MeCN solution containing glycinate (3.2 mM), acetylene (3.2 mM) and RB (0.32 mM). b Isolated yields. 5642 | RSC Adv., 2019, 9, 5639-5648 This journal is a 220 mL of MeCN or toluene solutions containing N-benzylamines (3.2 mM), acetylene (3.2 mM) and photosensitizer (DCA (0.27 mM), DCN (0.32 mM), RB (0.32 mM) and C 60 (0.16 mM)). b Isolated yields. This journal isScheme 7 Non-regioselective formation of iminium ions 39-41 from non-silyl amine analog 21.This journal is