Pogodin Sergey, Cohen Shmuel, Biedermann P Ulrich, Agranat Israel
Department of Organic Chemistry, The Hebrew University of Jerusalem, Jerusalem 91904, Israel.
Enantiomer. 2002 Nov-Dec;7(6):261-9. doi: 10.1080/10242430215701.
X-ray crystallographic and semiempirical PM3 and AM1 studies of 1,1'-dichlorobi-9H-fluoren-9-ylidene (5) are reported. The X-ray molecular structure of (Z)-5 indicated an approximately C2 symmetric conformation with pure twist around C9 = C9' of 40.4 degrees. The fjord regions are somewhat overcrowded: r(C8...C8') = 315.3 pm, r(Cl(1)...Cl(1') = 341.7 pm, r(C(8)...H(8')) = 259.0 pm. The four chlorine atoms of two neighboring molecules of (Z)-5 form a chain. The PM3 calculations showed that the global minimum of 5 is the C2 symmetric twisted conformation t(E)-5, which is 2.4 kJ/mol more stable than its diastereomer C2-t(Z)-5. The corresponding AM1 relative stability is reversed: C2-t(Z)-5 is 1.1 kJ/mol more stable than C2-t(E)-5. The pure twists of t(Z)-5 and t(E)-5 are 37.0 degrees and 37.2 degrees (PM3) and 40.5 degrees and 39.1 degrees (AM1). The corresponding (E) --> (Z) (PM3) and (Z) --> (E) (AM1) energy barriers of diastereomerization are 80.6 kJ/mol (PM3) and 75.8 kJ/mol (AM1). Two anti-folded local minima conformations C2-a(Z)-5 and C(i)-a(E)-5 were found to be 21.2 and 29.5 kJ/mol (PM3) and 25.8 and 35.2 kJ/mol (AM1) less stable than t(E)-5. The syn-folded conformations C(S)-s(Z)-5 and C2-s(E)-5 are transition states for the enantiomerization processes of C2-tz-5 and C2-tE-5, respectively, and lay 79.8 and 94.1 kJ/mol (PM3) and 108.3 and 107.4 kJ/mol (AM1) higher in energy than their corresponding twisted conformations. An alternative pathway for enantiomerization of C2-t(E)-5 via the anti-folded achiral intermediate C(i)-a(E) has a barrier of 56.0 kJ/mol (PM3) and 68.5 (AM1). An alternative pathway for enantiomerization of C2-t(Z)-5 via C2-t(E) and C(i)-a(E) has a barrier of 80.6 (PM3) and 75.8 (AM1) kJ/mol.
报道了1,1'-二氯联-9H-芴-9-亚基（5）的X射线晶体学以及半经验PM3和AM1研究。（Z）-5的X射线分子结构表明其具有近似C2对称构象，C9 = C9'处的纯扭转角为40.4度。峡湾区域有些拥挤：r(C8...C8') = 315.3皮米，r(Cl(1)...Cl(1') = 341.7皮米，r(C(8)...H(8')) = 259.0皮米。（Z）-5的两个相邻分子的四个氯原子形成一条链。PM3计算表明，5的全局最小值是C2对称扭转构象t(E)-5，它比其非对映异构体C2-t(Z)-5稳定2.4 kJ/mol。相应的AM1相对稳定性则相反：C2-t(Z)-5比C2-t(E)-5稳定1.1 kJ/mol。t(Z)-5和t(E)-5的纯扭转角分别为37.0度和37.2度（PM3）以及40.5度和39.1度（AM1）。非对映异构化的相应（E）→（Z）（PM3）和（Z）→（E）（AM1）能垒分别为80.6 kJ/mol（PM3）和75.8 kJ/mol（AM1）。发现两个反折叠局部最小值构象C2-a(Z)-5和C(i)-a(E)-5比t(E)-5不稳定21.2和29.5 kJ/mol（PM3）以及25.8和35.2 kJ/mol（AM1）。顺折叠构象C(S)-s(Z)-5和C2-s(E)-5分别是C2-tz-5和C2-tE-5对映异构化过程的过渡态，其能量比相应的扭转构象高79.8和94.1 kJ/mol（PM3）以及108.3和107.4 kJ/mol（AM1）。C2-t(E)-5通过反折叠非手性中间体C(i)-a(E)进行对映异构化的另一条途径的能垒为56.0 kJ/mol（PM3）和68.5（AM1）。C2-t(Z)-5通过C2-t(E)和C(i)-a(E)进行对映异构化的另一条途径的能垒为80.6（PM3）和75.8（AM1）kJ/mol。
