PHOTOADDITION REACTIONS OF ACENAPHTHYLENEDIONE WITH ALPHA-SILYL N-ELECTRON DONORS VIA TRIPLET SINGLE ELECTRON-TRANSFER DESILYLATION AND TRIPLET HYDROGEN-ATOM ABSTRACTION PATHWAYS

Studies have been conducted to explore single electron transfer (SET) induced photoaddition reactions of acenaphthylenedione (ACND) with the n-electron donors Et2NCH2SiMe3, n-PrSCH2SiMe3, EtOCH2SiMe3, EtCO2CH2N(CH2Ph)CH2SiMe3, and EtCO2CH2N(CH2Ph)CH3. Photoaddition of alpha-silyl amine Et2NCH2SiMe3 to ACND occurs in CH3OH and CH3CN to produce 2-hydroxy-2-[(diethylamino)methyl]acenaphthylen-1-one. In contrast, photoaddition of n-PrSCH2SiMe3 to ACND generates two photoadducts, 2-hydroxy-2-[(n-propylthio)methyl]acenaphthylen-1-one and 2-hydroxy-2-[(n-propylthio)(trimethylsilyl)methyl]acenaphthylen-1-one, along with a ACND photoreduction dimer. Photoaddition of EtOCH2SiMe3 to ACND produces two diastereomers of 2-hydroxy-2-[ethoxy(trimethylsilyl)methyl]acenaphthylen-1-one along with the reduction dimer. The formation of all photoproducts in these photoreactions is quenched by oxygen, indicating that the triplet of ACND is the reactive excited state. Based on a consideration of the oxidation potentials of the alpha-silyl n-electron donors, and the nature of photoproducts, mechanisms for these photoadditions involving triplet SET-desilylation and triplet H atom abstraction pathways are proposed. Photoaddition of EtCO2CH2N(CH2Ph)CH2SiMe3 to ACND provides two major products, 2-hydroxy-2-[[N-benzyl-N-(carbethoxymethyl)amino]methyl]acenaphthylen-1-one and 2-hydroxy-2-[[N-benzyl-N-[(trimethylsilyl)methyl]amino]carbethoxymethyl]acenaphthylen-1-one along with several minor products. The formation of the major products via sequential SET-deprotonation pathways shows that the electron-withdrawing carbethoxy substituent serves to control the regioselectivity for deprotonation of the amine radical cation intermediate. Results obtained from the study of the photoaddition of the non-silicon-containing amino ester, EtCO2CH2N(CH2Ph)CH3, also demonstrate the effect of electron-withdrawing carbethoxy substituent on amine radical cation deprotonation regiochemistry.