Triplet Energy Transfer (EnT)-Promoted 1,3-Dipolar Cycloaddition Reactions of N-(Trimethylsilyl)methylphthalimide with Electron-Deficient Alkynes and Alkenes.

Triplet energy transfer (EnT)-promoted photochemical pathways, arisen by visible light and its photosensitizers, have gained significant attention as a complementary strategy for initiating organic transformations in photochemical reactions that are unlikely to occur through a single electron transfer (SET) process. In the present study, we investigated the triplet EnT-promoted 1,3-dipolar cycloaddition reactions of N-(trimethylsilyl)methylphthalimide with electron-deficient alkynyl and alkenyl dipolarophiles. The triplet excited state of N-(trimethylsilyl)methylphthalimide, promoted by the triplet EnT from thioxanthone (TXA) photosensitizer, underwent sequential intramolecular SET and carbon-to-oxygen migration of the silyl group to form azomethine ylide. This generated ylide cycloadded to alkynes or alkenes to regioselectively and stereospecifically produce a nitrogen-containing benzopyrrolizidine scaffold with multiple stereogenic centers. Crucially, the stereoselectivity of these cycloaddition reactions (i.e., endo versus exo addition) was influenced by the nature of the dipolarophiles.

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.

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

Finding a selective and efficient fragmentation process under ambient conditions is pivotal for the generation of fuels and chemical feedstocks from lignoceullosic biomass. In the present study, visible-light and amine-functionalized fullerene-based photocatalyst-promoted photodegradation reactions of dimeric ß-O-4 and ß-1 lignin model compounds, containing varying numbers of methoxy substituents on the arene ring, were explored to find and develop mild, eco-friendly photochemical techniques for efficient delignification. The results showed that, in contrast to well-known organic photoredox catalysts, amine-functionalized fullerene photocatalyst promoted photochemical reactions of lignin model compounds could lead to more efficient lignin fragmentation reactions through a pathway involving a selective Cα-Cß bond cleavage process, and in addition, Cα-hydroxyl moiety in lignin model compounds played a significant role in the success of the Cα-Cß bond cleavage reaction of lignin model substrates.

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.

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.

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

Photoreactions between C60 and secondary N-trimethylsilylmethyl-N-benzylamines were explored to evaluate the feasibility of a new method for secondary aminomethylation of electron acceptors. The results show that photoreactions of C60 with these secondary amines in 10% EtOH-toluene occur to form aminomethyl-1,2-dihydrofullerenes predominantly through a pathway involving single electron transfer (SET)-promoted formation of secondary aminium radicals followed by preferential loss of the α-trimethylsilyl group. The aminomethyl radicals formed in this manner then couple with C60 or C60(•-) to form radical or anion precursors of the aminomethyl-1,2-dihydrofullerenes. In contrast to thermal and photochemical strategies developed previously, the new SET photochemical approach using α-trimethylsilyl-substituted secondary amines is both mild and efficient, and as a result, it should be useful in broadening the library of substituted fullerenes. Moreover, the results should have an impact on the design of SET-promoted C-C bond forming reactions. Specifically, introduction of an α-trimethylsilyl group leads to a change in the chemoselectivity of SET-promoted reactions of secondary amines with acceptors that typically favor aminium radical N-H deprotonation, leading to N-C bond formation. Finally, symmetric and unsymmetric fulleropyrrolidines are also generated in yields that are highly dependent on the electronic properties of arene ring substituents in amines, irradiation time, and solvent.

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.

Method for the synthesis of amine-functionalized fullerenes involving SET-promoted photoaddition reactions of α-silylamines.

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.

Effects of alkoxy groups on arene rings of lignin ß-O-4 model compounds on the efficiencies of single electron transfer-promoted photochemical and enzymatic C-C Bond Cleavage Reactions.

To gain information about how alkoxy substitution in arene rings of ß-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 ß-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 ß-O-4 radical cations decrease as the degree of alkoxy substitution decreases. In SET reactions of tetrameric compounds consisting of two ß-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 ß-O-4 moiety. However, regioselective C1-C2 cleavage in the more alkoxy-substituted ß-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 ß-O-4 units would be more efficiently cleaved by SET mechanisms.

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.

Photoaddition reactions of N-benzylglycinates containing α-trimethylsilyl group with dimethyl acetylenedicarboxylate: competitive formation of pyrroles vs. ß-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 ß-enamino-esters. The distributions of pyrrole and ß-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.