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 that of DPB-DMB and the rate of hydrogen abstraction were increased to compete with the rate of spin-flip by introducing pyridyl group.
부타디인들의 광화학적 성질에 피리딜 치환체가 미치는 효과를 조사하기 위하여 1-페닐-4-(2-피리딜)-1,3-부타디인(2-PPB)을 상업적으로 이용 가능한 물질로부터 적당한 결합방법을 이용하여 비교적 좋은 수율로 합성하였다.
디클로로메탄에서 디메칠퓨마레이트(DMFu)와 2-PPB를 조사하면 1:1 광생성물($\underline{2}$, $\underline{3}$, $\underline{4}$, $\underline{5}$)과 1:2 광생성물($\underline{1}$, $\underline{6}$, $\underline{7}$)이 생성된다. 처음에는 1:1 광생성물이 형성되고, 이 용액에 빛을 더 조사하면 1:2 광생성물이 형성된다. 깨끗한 $\underline{2}$/$\underline{3}$을 DMFu와 광반응시키면 각각 $\underline{1}$,$\underline{6}$/$\underline{7}$을 생성시키 는데, 이것은 $\underline{2}$와 $\underline{3}$이 1차 광생성물이고 $\underline{1}$, $\underline{6}$, $\underline{7}$이 2차 광생성물임을 암시한다. 산소 소광 실험은 2-PPB의 삼중항 여기 상태에서 $\underline{2}$와 $\underline{3}$이 생성됨을 보여 준다. 디클로로메탄에서 2,3-디메칠-2-부텐(DMB)과 광반응을 시키면 $\underline{8}$-$\underline{10}$이 생성되고, 순수한 DMB용액에서는 $\underline{8}$, $\underline{10}$, $\underline{11}$, $\underline{12}$가 생성된다.
위의 모든 반응은 삼중항 여기 상태에서 진행된다. $n-π^*$ 상태를 도입하므로써 계간전이가 증가하게되므로 몇가지 올레핀들과의 광반응에서 2-PPB는 1,4-디페닐-1,3-부타디인(DPB)보다 더 반응성이 좋아진다. DPB와는 달리 DMFu, DMB와 2-PPB의 광반응은 $\underline{6}$, $\underline{9}$, $\underline{12}$를 생성한다. $\underline{6}$의 형성은 $\underline{2}$의 여기 상태에서 피리딜기의 전자 끌기 효과에 영향을 받은 것이다. 피리딜기의 도입에 의해 2-PPB-DMB의 삼중항 상태 수명이 DPB-DMB와 비교해서 길어지고 수소전이속도가 스핀플립속도와 경쟁할 수 있게 되어서 $\underline{9}$가 생성되는것 같다.
