The [4.4.4.4]Fenestranes and[2.2.2.2]paddlane. Prospects for the realization of planar tetracoordinate carbon?

Semi-empirical (MINDO/3 and UNDO) MO calculations on highly strained planar tetracoordinate carbon candidates indicate the central carbons in $\text{cis}$- 4.4.4.4] fenestrane ($\text{1}$) to have pyramidal ($\text{1a}$) and in trans-4.4.4.]fenestrane ($\text{2}$) to have distorted tetrahedral ($\text{2a}$) geometries. In 2.2.2.2]paddlane ($\text{3}$), the two central carbons are pentacoordinate. Each is nearly coplanar with four carbon neighbours; additionally, the two bridgehead carbons are connected by a single bond ($\text{3a}$).