Control of Chemoselectivity of SET-Promoted Photoaddition Reactions of Fullerene C₆₀ with α-Trimethylsilyl Group-Containing N-Alkylglycinates Yielding Aminomethyl-1,2-dihydrofullerenes or Fulleropyrrolidines

Abstract

Knowledge about factors that govern chemoselectivity is pivotal to the design of reactions that are utilized to produce complex organic substances. In the current study, single-electron transfer (SET)-promoted photoaddition reactions of fullerene C60 with both trimethylsilyl and various alkyl group-containing glycinates and ethyl N-alkyl-N-((trimethylsilyl)­methyl)­glycinates were explored to evaluate how the nature of N-alkyl substituents of glycinate substrates and reaction conditions govern the chemoselectivity of reaction pathways followed. The results showed that photoreactions of C60 with glycinates, performed in deoxygenated conditions, produced aminomethyl-1,2-dihydrofullerenes efficiently through a pathway involving the addition of α-amino radical intermediates that are generated by sequential SET-solvent-assisted desilylation of glycinate substrates to C60. Under oxygenated conditions, photoreactions of glycinate substrates, except N-benzyl-substituted analogues, did not take place efficiently owing to quenching of 3C60* by oxygen. Interestingly, N-benzyl-substituted glycinates did react under these conditions to form fulleropyrrolidines through a pathway involving 1,3-dipolar cycloaddition of in situ formed azomethine ylides to C60. The ylide intermediates were formed by regioselective H-atom transfer from glycinates by singlet oxygen. Furthermore, methylene blue (MB)-photosensitized reactions of C60 with glycinates under oxygenated conditions took place efficiently to produce fulleropyrrolidines independent of the nature of N-alkyl substituents of glycinates.

Visible-Light-Induced Selective C–C Bond Cleavage Reactions of Dimeric β-O-4 and β-1 Lignin Model Substrates Utilizing Amine-Functionalized Fullerene

“…Notably, we indicated that the efficiencies of degradation reactions can be varied by both structural features and the arene ring substitution patterns of lignin model compounds. As part of our continuing interest in developing mild and eco-friendly photochemical techniques for lignin fragmentation, in this study, we prepared dimeric β-O-4 and β-1 lignin model compounds containing varying numbers of methoxy substituents on the arene rings and carried out visible-light-mediated photocatalytic degradation reactions of these substrates using amine-functionalized fullerene-based photocatalysts (Figure ) because these fullerene-based photocatalysts can absorb a wide range of visible-light wavelengths and serve as efficient electron acceptors (see Table S1 and Figures S1 and S2). The results showed that, compared to the lignin model photodegradation mediated using common photoredox catalysts such as methylene blue (MB), rose bengal (RB), and eosin Y (EY), the photoreactions of lignin model compounds using the amine-functionalized fullerene derivatives can lead to more efficient lignin fragmentation through a pathway involving selective C α –C β bond cleavage, and in addition, the C α - hydroxyl moiety in the lignin model compounds plays a significant role in the success of the C α –C β bond cleavage reaction of lignin model substrates.…”

Reaction-Environment-Dependent Photoaddition Reactions of N-Phenyl Amino Acid Esters Possessing a Silyl Group with Fullerene C60: Selective Formation of Aminomethyl-1,2-dihydrofullerenes vs Fulleropyrrolidines

“…With prepared N-phenylglycinates 14a−14j and N-phenylalaninates 15a−15e in hand, photochemical reactions of them with C 60 were performed in conditions identical to those employed in our earlier photoreaction using N-alkylglycinates. 14 Specifically, 10% EtOH−toluene (v/v) solutions containing C 60 (0.28 mmol, 1 equiv) and AAEs (0.56 mmol, 2 equiv) were irradiated for a fixed time period by a 450 W Hanovia medium pressure mercury lamp equipped with flint glass filter (>310 nm) under either a deoxygenated (N 2 -purged) or oxygenated (O 2 -purged) environment. The photolyzates were then purified to determine the resulting photoproducts.…”

“…As far as reaction efficiency and photoproduct profiles are concerned, it is noteworthy to compare the results of photoreactions using silyl-group-containing N-alkylglycinates. 14 As shown in Table 4, first, photoreactions of N-alkyl substituted glycinates 17a−17e with C 60 took place much more efficiently (i.e., a much shorter irradiation time was required to bring about high yields of photoproducts as well as the high conversion of C 60 ) than those of N-phenyl analogs. More importantly, photoreactions of 17a−17e with C 60 led to the formation of desilylated amine-incorporated fullerenes 18a−18e as sole products rather than decarboxymethylated products.…”

“…Indeed, the photoreactions of silyl-tether-containing N-alkyl substituted glycinates 19a−19e with C 60 in oxygenated solutions failed to produce photoproducts, even with the longer photoirradiation. 14 However, albeit somewhat inefficient, photoreactions of N-phenylglycinates 14a−14j with C 60…”

“…As a result, photoreactions of AAEs with C 60 under oxygenated conditions would not occur. Indeed, the photoreactions of silyl-tether-containing N -alkyl substituted glycinates 19a – 19e with C 60 in oxygenated solutions failed to produce photoproducts, even with the longer photoirradiation . However, albeit somewhat inefficient, photoreactions of N -phenylglycinates 14a – 14j with C 60 under oxygenated conditions took place to produce pyrrolidine ring-fused fullerenes (fulleropyrrolidines) 22a – 22j rather than aminomethylated adducts 16a – 16e .…”