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

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.