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Synthesis, Spectroscopic, and Structural Properties of Spirocyclopropanated Bicyclobutylidenes and Their Radical Cations
Author(s) -
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.
Publication year - 2001
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/1521-3765(20011217)7:24<5382::aid-chem5382>3.0.co;2-y
Subject(s) - cyclopropane , chemistry , wittig reaction , cyclopropanation , ylide , dichlorocarbene , steric effects , double bond , medicinal chemistry , nucleophile , lone pair , photochemistry , crystallography , stereochemistry , ring (chemistry) , organic chemistry , molecule , catalysis
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 CH 2 I 2 /ZnEt 2 , 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‐IE v ) 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 | a H | 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.