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

• Published in: Chemistry : a European journal 2001-12, Vol.7 (24), p.5382-5390
• Main Authors: de Meijere, Armin, Wenck, Horst, Zöllner, Stephan, Merstetter, Pascal, Arnold, Anton, Gerson, Fabian, Schreiner, Peter R., Boese, Roland, Bläser, Dieter, Gleiter, Rolf, Kozhushkov, Sergei I.
• Format: Article
• Language: English
• Subjects: bicyclobutylidenes; conjugation; density functional calculations; radical ions; small ring systems
• ISSN: 0947-6539; 1521-3765
• DOI: 10.1002/1521-3765(20011217)7:24<5382::AID-CHEM5382>3.0.CO;2-Y

The spirocyclopropanated bicyclobutylidenes 3–7 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