Single Electron Transfer-Promoted Photochemical Reactions of Secondary N-Trimethylsilylmethyl-N-benzylamines Leading to Aminomethylation of Fullerene C60

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

The current study investigates SET-promoted photoaddition reactions of the silyl-group-containing N-phenylglycinates and N-phenylalaninates, N-((trimethylsilyl)methyl)-N-phenyl-substituted glycinates and alaninates, respectively, with fullerene C60 to explore how the types of amino acid esters (AAEs) and molecular oxygen affect the photoaddition reaction efficiencies and chemoselectivity of in situ formed radical cations of AAEs. The results showed that under deoxygenated (N2-purged) conditions, photoreactions of N-phenylglycinates with C60 produced aminomethyl-1,2-dihydrofullerenes through the addition of α-amino radicals arising by sequential SET and desilylation processes from initially formed secondary anilines to C60. In oxygenated conditions, photoreactions of N-phenylglycinates with C60, albeit less efficient, took place to form fulleropyrrolidines through a pathway involving 1,3-dipolar cycloaddition of azomethine ylides to C60 assisted by in situ formed 1O2. The same types of photoproducts were observed with N-phenylalaninates, though the reactions were less efficient. The use of methylene blue (MB) as a photosensitizer in the photoreactions under oxygenated conditions was especially effective in enhancing the efficiency of fulleropyrrolidine formation. These results demonstrate that photoaddition reactions of silyl-tether-containing N-phenyl AAEs with C60 can be governed by the reaction conditions and the presence or absence of a photosensitizer employed.

Silyl Tether-Assisted Photooxygenation of Electron-Deficient Enaminoesters: Direct Access to Oxamate Formation

Photooxygenation reactions of electron-deficient enaminoesters bearing an oxophilic silyl tether at the α-position of the nitrogen atom using methylene blue (MB) were explored to develop a mild and efficient photochemical strategy for oxidative C-C double bond cleavage reactions via singlet oxygen (¹O₂). Photochemically generated ¹O₂, through energy transfer from the triplet excited state of MB (³MB*) to molecular oxygen (³O₂), was added across a C-C double bond moiety of enaminoesters to form perepoxides, which rearranged to form dioxetane intermediates. The cycloreversion of the formed dioxetane via both C-C and O-O bond cleavage processes led to the formation of oxamates. Importantly, contrary to alkyl group tether-substituted electron-deficient enaminoesters that typically disfavor photooxygenation, the silyl tether-substituted analogues undergo this photochemical transformation efficiently with the assistance of a silyl tether, which facilitates formation of the perepoxide. The observations in this study provide useful information about photosensitized oxygenation reactions of unsaturated C-C bonds, and, moreover, this photochemical strategy can be utilized as a mild and feasible method for the preparation of diversely functionalized carbonyl compounds including oxamates.

Fullerene C60 promoted photochemical hydroamination reactions of an electron deficient alkyne with trimethylsilyl group containing tertiary N-alkylbenzylamines

C60-promoted photoaddition reactions of both trimethylsilyl- and a variety of alkyl group containing tertiary benzylamines (i.e., N-α-trimethylsilyl-N-alkylbenzylamines) with dimethyl acetylenedicarboxylate (DMAD) were carried out to explore the synthetic utility of trimethylsilyl group containing tertiary amines as a substrate in the photochemical hydroamination reactions with dimethyl acetylenedicarboxylate (DMAD). The results showed that photoreactions of all the trimethylsilyl containing N-alkylbenzylamines with DMAD, under an O2-purged environment, produced non-silyl containing enamines efficiently through a pathway involving addition of secondary amines to DMAD, the former of which are produced by hydrolytic cleavage of in situ formed iminium ions. Exceptionally, five-membered N-heterocyclic rings, pyrroles, could be produced competitively in photoreaction of bulky alkyl (i.e., tert-butyl) group substituted benzylamines through a pathway involving 1,3-dipolar cycloaddition of azomethine ylides to DMAD. Furthermore, C60-sensitized photochemical reactions of non-silyl containing benzylamines with DMAD under oxygenated conditions took place in a less efficient and non-regioselective manner to produce enamine photoadducts. The observations made in this study show that regioselectivity of C60-promoted photochemical reactions of N-α-trimethylsilyl-N-alkylbenzylamines, leading to formation of secondary amines, can be controlled by the presence of the trimethylsilyl group, and that these trimethylsilyl containing tertiary amines can serve as a precursor of secondary amines for hydroamination reactions with a variety of electron deficient acetylenes.

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

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.

Metal-Free Synthesis of N-Alkyl-2,5-Unsubstituted/Monosubstituted Fulleropyrrolidines: Reaction of [60]Fullerene with Paraformaldehyde and Amines

A series of scarce N-alkyl-2,5-unsubstituted/monosubstituted fulleropyrrolidines were synthesized in moderate to excellent yields by the simple one-step thermal reaction of [60]fullerene with primary/secondary amines in the presence of paraformaldehyde without the addition of valuable metal salts. Intriguingly, the reaction with primary amines unexpectedly afforded N-alkyl-2,5-unsubstituted fulleropyrrolidines instead of the anticipated 2,5-monosubstituted fulleropyrrolidines. A plausible reaction pathway is proposed to elucidate the above-mentioned reaction process based on the experimental results.

Photoaddition reactions of N-benzylglycinates containing α-trimethylsilyl group with dimethyl acetylenedicarboxylate: competitive formation of pyrroles vs. β-enamino esters

A study was conducted to gain insight into the preparative potential of photosensitized reactions of acyclic N-benzylglycinates containing an α-trimethylsilyl group with dimethyl acetylenedicarboxylate (DMAD). The photosensitizers employed in the reactions include 9,10-dicyanoanthracene (DCA), 1,4-dicyanonaphthalene (DCN), rose bengal (RB) and fullerene C60. The results show that photoirradiation of oxygenated solutions containing the photosensitizers, glycinates and dimethyl acetylenedicarboxylate leads to competitive formation of pyrroles and β-enamino-esters. The distributions of pyrrole and β-enamino-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.

Photochemical Approach for the Preparation of N-Alkyl/Aryl Substituted Fulleropyrrolidines: Photoaddition Reactions of Silyl Group Containing α-Aminonitriles with Fullerene C60

The photochemical reactions of C₆₀ with N-(trimethylsilyl)methyl substituted and N-alkyl/aryl substituted α-aminonitriles were explored to evaluate the scope and reaction efficiency depending on the structural nature of amine substrates. The results showed that photoreactions of C₆₀ with trimethylsilyl group containing N-alkyl amines produced predominantly both trimethylsilyl and cyano group containing trans-pyrrolidine ring fused fulleropyrrolidines in a chemo- and stereoselective manner. Interestingly, photoreactions of C₆₀ with N-branched alkyl substituted amines led to exclusive formation of non-silyl containing cycloadducts. In contrast to those of N-alkyl substituted α-aminonitriles, photoreactions of N-(trimethylsilyl)methyl and N-aryl substituted α-aminonitriles gave rise to the formation of both trans- and cis-isomeric fulleropyrrolidines with an inefficient and non-stereoselective manner. The feasible mechanistic pathways leading to generation of fulleropyrrolidines are 1,3-dipolar cycloaddition of the azomethine ylides, generated by either a single electron transfer (SET) (under N₂-purged conditions) or H atom abstraction (under O₂-purged conditions) process, to fullerene C₆₀. The stereoselectivities of photoproducts depending on the nature of amines are likely to be associated with conformational stabilities of in situ generated azoemthine ylides.

Pyreneacyl sulfides as a visible light-induced versatile ligation platform

We report a visible light responsive moiety capable of generating highly reactive thioaldehydes.

Pd-Catalyzed thiophene directed regioselective functionalization of arenes: a direct approach to multiply-substituted benzyl amines

A thiophene directed ortho-C–H functionalization via a palladium catalyst in the synthesis of multiply-substituted benzyl amines has been developed.

Single electron transfer promoted photoaddition reactions of α-trimethylsilyl substituted secondary N-alkylamines with fullerene C60

Single electron transfer (SET) promoted photoaddition reactions of secondary N-α-trimethylsilyl-N-alkylamines to C₆₀ were explored to gain a deeper understanding of the mechanistic pathways followed and to expand the library of novel types of organofullerenes that can be generated using this approach. The results show that photoreactions of 10% EtOH-toluene solutions containing C₆₀ and N-α-trimethylsilyl-N-alkylamines produce either aminomethyl-1,2-dihydrofullerenes or symmetric fulleropyrrolidines as major products depending on the nature of alkyl substituents. In contrast, photoreactions of 10% EtOH-ODCB solutions of these amines with C₆₀ mainly lead to the formation of symmetric fulleropyrrolidines. Based on the analysis of product distributions and the results of earlier studies, two feasible mechanistic pathways are proposed for these processes. One route is initiated by SET from the amine substrates to the triplet-excited state of C₆₀ to form the corresponding aminium radicals and C₆₀ anion radicals. EtOH-promoted desilylation of the aminium radicals then takes place to produce aminomethyl radicals which can either add to C₆₀ or couple with the C₆₀ radical anions to form respective radicals or anion precursors of aminomethyl-1,2-dihydrofullerene products. The competing pathway leading to the generation of symmetric fulleropyrrolidines also involves the formation of aminomethyl radicals by using the sequential SET-desilylation process. In this route, the aminomethyl radicals are oxidized by SET to C₆₀ to form iminium ions, which are then transformed to azomethine ylides by a pathway involving a second molecule of the secondary amine. Dipolar cycloaddition of the azomethine ylides to C₆₀ forms the symmetric fulleropyrrolidine cycloadducts. Importantly, the observation that symmetric fulleropyrrolidines are the sole products formed in photoreactions between N-α-trimethylsilyl-N-alkylamines and C₆₀ in 10% EtOH-ODCB has synthetic significance.

Radical–radical cross coupling reactions of photo-excited fluorenones

Radical–radical cross coupling reactions of photoexcited 9-fluorenones have been accomplished for the first time, leading to the synthesis of 9-alkyl, pyrollidinyl and spiro-THF derivatives of 9-fluorenones.