Five membered strained cyclic allene: the synthesis of 2-dehydro-3A,4,5,6,6A-pentahydropentalene

Algı, Fatih
The equilibrium geometry for an ailene is linear with orthogonal pairs of substituents. An ailene incorporated into a carbocyclic ring of nine or more carbon atoms is relatively unstrained. However, if the ring size is decreased, the linear perpendicular ailene will be twisted and bent until, at some point, the energy gained by 7U bonding in the two double bonds will be insufficient to offset the increased strain. Furthermore, ring constraints will exert torsion toward a planar arrangement of ligands. The electronic structure of allenes, their ability to form stabilized intermediates does render them highly reactive intermediates. Therefore, one of the fundemantal question is the influence of ring size on the barrier to 7t bond rotation. The synthesis, isolation, and trapping of these strained compounds is a field of interest over the last three decades. inIn this thesis, the smallest member of strained cyclic allenes, cyclopenta- 1,2-diene (5), hitherto of which none derivative could have been synthesized due to its high strain energy (ca 30 kcal) was investigated. To achieve the synthesis of cyclopenta-l,2-diene derivative 282, cycloheptatriene (283) was used as the starting material. The selective reduction of cycloheptatriene (283) to cyclohepta-l,3-diene (284) followed by the photolysis afforded bicyclo[3.2.0]hept-6-ene (285). Then the carbene chemistry was applied. Dibromocarbene addition to olefin 285 resulted in the formation of 4,5-dibromo- l,2,3,3a,4,6a-hexahydro-pentalene (303). On the other hand, fluorobromocarbene addition led to the formation of both 3-bromo-3-fluoro-tricyclo['4]octane (316) and 4-bromo-5-fluoro-l,2,3,3a,4,6a-hexahydro-pentalene (317). The formation of 2-bromo-l,3a,4,5,6,6a-hexahydro-pentalene-l-ol (304) and 2-fluoro- l,3a,4,5,6,6a-hexahydro-pentalene-l-ol (318) as by products was explained on the basis of the interaction of allylic bromines with the column materials in the purification step. The cyclopropane 316 was treated with MeLi at low temperatures in the presence of furan to give one isomer of [244] cycloadduct 13-oxatetracyclo [S^.l.O^.O^trideca-S.ll-diene (321), which clearly indicated the formation of 2- dehydro-3a,4,5,6,6a-pentahydropentalene (282) as a reactive intermediate via cyclopropylidene- ailene rearrangement. The structure of trapped isomer 321a was elucidated on the basis of both spectroscopic methods and theoretical calculations. Consequently, the title ailene 282 was synthesized and characterized spectroscopically by being the first direct observation for the formation of the smallest and the most strained member of cyclic allenes, cyclopenta-l,2-diene (5).
Citation Formats
F. Algı, “Five membered strained cyclic allene: the synthesis of 2-dehydro-3A,4,5,6,6A-pentahydropentalene,” Middle East Technical University, 2002.