“…For example, tetramethylethylene (IP 8.30)334 reacts three times faster than 1,1-dlcylopropyl-2-methylpropene (IP 7.82). 335 Were electron or charge transfer the rate-determining step, one would have expected the vinylcyclopropane to react several orders of magnitude faster.…”
“…For example, tetramethylethylene (IP 8.30)334 reacts three times faster than 1,1-dlcylopropyl-2-methylpropene (IP 7.82). 335 Were electron or charge transfer the rate-determining step, one would have expected the vinylcyclopropane to react several orders of magnitude faster.…”
“…The steric effect of the four three‐membered rings adjacent to the double bond in 5 is evident from the outcome of its reaction with tetracyanoethene (TCNE) in comparison to that of the bicyclobutylidene 6 with only two α ‐cyclopropane rings. While 6 yielded the normal [2+2] cycloadduct 20 , the sterically encumbered 5 was solely attacked at one of the α ‐spirocyclopropane groups, as was observed for tetracyclopropylethene,2a, 25 to give 21 (see also 26). Bromination and acetoxymercuration of 5 also occurred with ring opening of one of the proximal spirocyclopropane groups, according to the NMR spectra of the not fully characterized products (see 5).…”
Section: Resultsmentioning
confidence: 85%
“…The cyclopropyl substituent is well known to be the best nonheteroatom containing donor group for electron‐deficient centers,1 and as such it enhances the nucleophilicity of alkenes1c by efficiently stabilizing the incipient carbenium ion upon attack of any electrophile on the double bond. The ultimate nucleophilicity would thus be achieved for tetracyclopropylethene ( 1 ),2a if all four cyclopropyl groups were in a bisected orientation with respect to the central double bond. Yet, in the ground state, at least one of the four cyclopropyl groups in 1 is in a non‐bisected (synclinal) orientation, a conclusion which can be drawn, for example, from its π‐ionization energy derived from photoelectron spectra, in comparison to those of tri‐, di‐, and monocyclopropylethene 2b,2c.…”
Section: Introductionmentioning
confidence: 99%
“…The ultimate nucleophilicity would thus be achieved for tetracyclopropylethene ( 1 ),2a if all four cyclopropyl groups were in a bisected orientation with respect to the central double bond. Yet, in the ground state, at least one of the four cyclopropyl groups in 1 is in a non‐bisected (synclinal) orientation, a conclusion which can be drawn, for example, from its π‐ionization energy derived from photoelectron spectra, in comparison to those of tri‐, di‐, and monocyclopropylethene 2b,2c. By proper bridging of the two pairs of cyclopropyl groups in tetracyclopropylethene ( 1 ), however, all four cyclopropyl groups can be rigidly held in a synplanar conformation and thus in a parallel orientation with respect to the π‐orbital axis of the double bond, as realized in the tetraspirocyclopropanated bicyclobutylidene 3 .…”
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.
“…As early as 1970, Nishida et al reported the formation of tetracyanocyclobutanes such as 547 from 519 . Shortly thereafter, the same group found a markedly different mode of reaction for tetracyclopropylethene ( 525 ) with insertion of TCNE into one of the cyclopropyl rings leading to the tetracyanovinylcyclopentane derivative 548 (Scheme ). , 89 …”
Section: Branched Aggregates Of Three-membered
Ringsmentioning
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