Synthesis, Spectroscopic, and Structural Properties of Spirocyclopropanated Bicyclobutylidenes and Their Radical Cations



The spirocyclopropanated bicyclobutylidenes 37 have been prepared by McMurry coupling of the corresponding spirocyclopropanated cyclobutanone (3 and 5), Staudinger–Pfenniger reaction (4), oxidative coupling of a Wittig ylide (4) or Wittig olefination of perspirocyclopropanated cyclobutanone (6 and 7). The structure of the parent 2 a and the perspirocyclopropanated bicyclobutylidene 5 was determined by X-ray crystallography which disclosed considerable steric congestion around the double bond. As a result 5 did undergo addition of dichlorocarbene, epoxidation with meta-chloroperbenzoic acid, and cyclopropanation with CH2I2/ZnEt2, but did not add the more bulky dibromocarbene. The reaction of 5 with tetracyanoethene proceeded smoothly, but led to a formal [3+2] cycloadduct across the proximal single bond of one of the inner cyclopropane rings. The consecutive spirocyclopropanation of bicyclobutylidene led to a bathochromic shift in the UV spectra of 12 and 17 nm, respectively, for each pair of β- and α-spirocyclopropane groups. In the He(I)-photoelectron spectra of these bicyclobutylidenes, the effect of spirocyclopropanation upon their π-ionization energies (π-IEv) was found to be almost additive, leading to a lowering of 0.05 eV per any additional β-spirocyclopropane, and 0.28–0.22 eV per additional α-spirocyclopropane group; this indicates an increasing nucleophilicity of the double bonds in the order 1<4<3<5. Following the radical cations of the three symmetrical bicyclobutylidenes without (2 a, b) and with six (5) spiroannelated cyclopropane rings, the radical cations of two symmetrical bicyclobutylidenes with two (4) and four (3) such rings were studied by ESR spectroscopy. Whereas 2 b.+, 3.+, and 5.+ could be generated by electrolytic oxidation of the corresponding hydrocarbons in solution, the spectra of 2 a.+ and 4.+, with unsubstituted 2,2′,4,4′-positions, were observed upon radiolysis of their neutral precursors in a Freon matrix. On going from 2 a.+ to 4.+, the coupling constant |aH| of the eight β protons in the 2,2′,4,4′-positions of bicyclobutylidene increases from 2.62 to 3.08 mT, and that of the four γ protons in the 3,3′-positions changes from 0.27 to 0.049 to 0.401 mT on passing from 2 a.+ via 2 b.+ to 3.+. Computations by means of the density functional theory (DFT) at the B3LYP/6-311+G*//B3LYP/6-31G* level reproduce well the experimental hyperfine data.