Stereochemistry of ( E )- and ( Z )-1,1′-Dichlorobifluorenylidenes, Substituted Overcrowded Fullerene Fragments

X-ray crystallographic and semiempirical PM3 and AM1 studies of 1,1'-dichlorobi-9 H -fluoren-9-ylidene ( 5 ) are reported. The X-ray molecular structure of ( Z )- 5 indicated an approximately C 2 symmetric conformation with pure twist around C 9 = C 9 ' of 40.4. The fjord regions are somewhat overcrowded: r(C 8 C 8 ') = 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 C 2 symmetric twisted conformation t E - 5 , which is 2.4 kJ/mol more stable than its diastereomer C 2 - t Z - 5 . The corresponding AM1 relative stability is reversed: C 2 - t Z - 5 is 1.1 kJ/mol more stable than C 2 - t E - 5 . The pure twists of t Z - 5 and t E - 5 are 37.0 and 37.2 (PM3) and 40.5 and 39.1 (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 C 2 - 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 C 2 - s E - 5 are transition states for the enantiomerization processes of C 2 - t Z - 5 and C 2 - t E - 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 C 2 - 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 C 2 - t Z - 5 via C 2 - t E and C i - a E has a barrier of 80.6 (PM3) and 75.8 (AM1) kJ/mol.

Keywords:

• Bistricyclic Aromatic Enes
• Crystal Structure
• Enantiomerization
• ( E ) $\rightleftharpoons$ ( Z )-isomerization
• Semiempirical Calculations