Synthesis, spectroscopic, and structural properties of spirocyclopropanated bicyclobutylidenes and their radical cations.
Spirocyclopropanated bicyclobutylidenes have been prepd. by McMurry coupling of the corresponding spirocyclopropanated cyclobutanone, Staudinger-Pfenniger reaction, oxidative coupling of a Wittig ylide or Wittig olefination of perspirocyclopropanated cyclobutanone. The structure of the parent I and the perspirocyclopropanated bicyclobutylidene II was detd. by X-ray crystallog. which disclosed considerable steric congestion around the double bond. As a result II did undergo addn. of dichlorocarbene, epoxidn. with meta-chloroperbenzoic acid, and cyclopropanation with CH2I2/ZnEt2, but did not add the more bulky dibromocarbene. The reaction of II 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, resp., for each pair of b- and a-spirocyclopropane groups. In the He(I)-photoelectron spectra of these bicyclobutylidenes, the effect of spirocyclopropanation upon their p-ionization energies (p-IEv) was found to be almost additive, leading to a lowering of 0.05 eV per any addnl. b-spirocyclopropane, and 0.28-0.22 eV per addnl. a-spirocyclopropane group; this indicates an increasing nucleophilicity of the double bonds. Following the radical cations of the three sym. bicyclobutylidenes without and with six spiroannelated cyclopropane rings, the radical cations of two sym. bicyclobutylidenes with two and four such rings were studied by ESR spectroscopy. Whereas radical cations of the 2,2',4,4'-substituted dicyclobutylidenes could be generated by electrolytic oxidn. of the corresponding hydrocarbons in soln., the spectra of those with unsubstituted 2,2',4,4'-positions were obsd. upon radiolysis of their neutral precursors in a Freon matrix. Computations by means of the d. functional theory (DFT) at the B3LYP/6-311+G*//B3LYP/6-31G* level reproduce well the exptl. hyperfine data.
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