Photochemistry of 1-phenyl-4-(2-pyridyl)-1,3-butadiyne:photoreaction with some olefins

Abstract

1-Phenyl-4-(2-pyridyl)-1,3-butadiyne (2-PPB) was prepared successfully with moderate yields by proper coupling methods from commercially available material in order to investigate the effect of pyridyl substituent on photochemical properties of butadiynes. Irradiation of 2-PPB with dimethyl fumarate (DMFu) in deaerated dichloromethane solution at 300 nm yields 1:1 photoadducts ($\underline{2}$, $\underline{3}$, $\underline{4}$ and $\underline{5}$) and 1:2 photoadducts ($\underline{1}$, $\underline{6}$, and $\underline{7}$). The 1:1 photoadducts (2 and 3) is initially formed and prolonged irradiation of the solution results in the formation of 1:2 photoadducts (1, 6, and 7). Photolysis of pure $\underline{2}$/$\underline{3}$ isolated with DMFu yields $\underline{1}$ and $\underline{6}$/$\underline{7}$, respectively, indicating that $\underline{2}$ and $\underline{3}$ are the primary photoadducts and $\underline{1}$, $\underline{6}$, and $\underline{7}$ are the secondary photoadducts. Triplet quenching effect on the photoreaction with oxygen shows that $\underline{2}$ and $\underline{3}$ are produced from triplet excited state of 2-PPB. Photoreaction of 2-PPB with 2,3-dimethyl-2-butene (DMB) in dichloromethane yields $\underline{8}$-$\underline{10}$, and photoreaction in neat DMB yields $\underline{8}$, $\underline{10}$, $\underline{11}$ and $\underline{12}$. All of these photoreactions proceeded from triplet excited states. 2-PPB in the photoreaction with some olefins is more reactive than 1,4-diphenyl-1,3-butadiyne (DPB) because intersystem crossing is enhanced by introducing $n-π^*$ state. Differently from DPB, photoreaction of 2-PPB with DMFu and DMB yields $\underline{6}$, $\underline{9}$, and $\underline{12}$. The formation of $\underline{6}$ may be affected by electron withdrawing effect of pyridyl group for excited state of $\underline{2}$. On the formation of $\underline{9}$, it looks as though the triplet state lifetime of 2-PPB-DMB were longer than t...