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

Abstract

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 2a 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 17nm, respectively, for each pair of beta- and alpha-spirocyclopropane groups. In the He(I)-photoelectron spectra of these bicyclobutylidenes, the effect of spirocyclopropanation upon their pi-ionization energies (pi-IE,) was found to be almost additive, leading to a lowering of 0.05 eV per any additional beta-spirocyclopropane, and 0.28-0.22 eV per additional alpha-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 (2a, 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 2b.+, 3.+, and 5.+ could be generated by electrolytic oxidation of the corresponding hydrocarbons in solution, the spectra of 2a.+ and 4.+, with unsubstituted 2,2',4,4'-positions, were observed upon radiolysis of their neutral precursors in a Freon matrix. On going from 2a.+ to 4.+, the coupling constant [aH] of the eight beta protons in the 2,2',4,4'-positions of bicyclobutylidene increases from 2.62 to 3.08 mT, and that of the four gamma protons in the 3,3'-positions changes from 0.27 to 0.049 to 0.401 mT on passing from 2a.+ via 2b.+ 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.

Noncovalent interactions in crowded olefinic radical cations

“…Since α-hyperconjugation affects mostly the structures of highly twisted forms, accounting dispersions has only little influence on the geometry of 3b •+ . Note that for bis-2,2,4,4-tetramethylcyclobutylidene, the cyclobutane analog of 5, where the methyl groups are more distant, the twisting in the radical cation is only 29 -30° [32], i. e. essentially the same as in ethylene.…”

Three-Membered-Ring-Based Molecular Architectures

“…All of these alkenes possess electron-rich double bonds, which is mirrored in their physical properties and their chemical behavior. The ionization energies as measured by He(I)-photoelectron spectroscopy steadily decrease on going from 1,1-dicyclopropyl- ( 382 , 8.80 eV) via 1,2-dicyclopropyl- ( 517 , cis 8.50 eV, trans 8.40 eV) to tricyclopropylethene ( 519 , 8.00 eV) and finally tetracyclopropylethene ( 525 , 7.90 eV) . Studies of the rate constants of carbene additions onto the double bonds of these compounds show two mutually counteracting influences by an increasing number of cyclopropyl substituents.…”

BiotecVisions 2014, February

“…Cyclobutylidene cyclobutane ( 45 ) has been prepared by different routes,147, 148 one of which uses triphenylphosphonium cyclobutylide as a precursor 148. 149 Its structure has been determined by X‐ray diffraction and shown to feature a central CC bond length of 1.315 Å with the six core carbon atoms roughly in a common plane, but with puckered four‐membered rings (14° as the dihedral angle) 150…”