A new Rotane family

Abstract

The synthesis, structure, conformation, and dynamics of a new rotane family consisting of four-membered rings are described. All syntheses are based on bicyclobutylidene (9):  [2+1] cycloaddition of cyclobutylidene yields [3.4]rotane (5) (9−5), [2+2] cycloaddition of trimethyleneketene followed by spiroalkylation of the resulting trispiroketone 10 yields [4.4]rotane (6) (9−10−13−14−6), homologization of 10 via β-hydroxy selenides gives access to tetraspiroketone 11 and pentaspiroketone 12 (10−15−11−16−12), and further elaboration directed toward a cyclopropylcarbene−cyclobutene rearrangement yields [5.4]rotane (7) and [6.4]rotane (8) [11(12)−18(24)−19(25)−20(26)−21(27)−22(28)−23(29)−7(8)]. The structures of 6, 7, and 8 were determined by high-precision low-temperature X-ray analyses and by force field calculations using the search routine HUNTER in connection with MM3(92). The following special features were observed:  From 5 to 8, the bond angles of the central ring increase while the bond angles at the spiro center of the spiroannelated cyclobutane rings decrease. As a consequence, the cyclobutane rings change their geometry from a regular trapezoid in 6 to a kite with the smallest angle at the spiro center in 7 and 8. At the same time their folding decreases, until in 8 they are close to planar. At room temperature, all hexaspiranes (8, 25−29) are conformationally stable. This phenomenon allowed a stereoselective synthesis of axially labeled [1-13C]8' and, via a high-temperature equilibration with equatorially labeled [1-13C]8, a determination of the free energy of activation for the chair to chair interconversion as ΔG⧧487 = 156.8 ± 1.1 kJ/mol. This is the highest barrier of inversion ever reported for a cyclohexane. Promising candidates for even higher barriers are [6.5]rotane (32) and [6.6]rotane (33).