The Second Cyclopropannulene: Cycloprop-annulene
journal contributionposted on 2005-02-02, 00:00 authored by Matthew K. Kiesewetter, Richard C. Reiter, Cheryl D. Stevenson
Reacting (at 0 °C) a mixture of CH2Cl2 and monobromoannulene (C8H7Br) with potassium tert-butoxide in hexamethylphosphoramide (HMPA) and following with exposure to potassium metal led to the formation of the anion radical of an HMPA-[6.1.0]bicyclononatetraene condensation product, in which two HMPA fragments are geminal and attached to the number 9 carbon. When the reaction sequence is carried out in THF, the dianion of cyclopropannulene is predominantly formed. Neutral cyclopropannulene can be isolated via the I2 oxidation of the THF solution. The NMR analysis reveals that the eight-membered ring is nearly planar, and the three-membered ring is more like a dimethylenecyclopropane than it is like a cyclopropene. Further, the chemical shifts due to the protons on the eight-membered ring are nearly 2 ppm further upfield than are those for annulene itself, suggesting a paratropic ring current.