Bredt-Olefin

The Ramberg-Backlund reaction of compound 384 gave the Bredt olefin 388, which was actually isolated . By reaction of chlorosulfones 385-387 and 392-394 (n = 2) with potassium t-butoxide the corresponding oxaproperanes 392-397 were obtained by the Ramberg-Backlund reaction, but only in low yields . In contrast, the reaction of the chlorosulfoxides 392, 393 and 394 n = 1) gave oxaproperanes 395, 396 and 397 under analogous conditions in 85-90, 25 and 85% yields, respectively . 

Compounds with at least one stereogenic center and one stereogenic axis (this also includes anti-Bredt-olefins that exist in two stable atropisomeric forms). 

The ionization potential, 8.69 eY, is lower than in the case of -cyclooctene (8.98 eV) or cyclohexene (9.12 eV), as expected. The highly strained anti-Bredt olefin, 11-bromo-e/ -9-chloro-7-ethoxybicyclo[5.3.1]undec-l(ll)ene has been synthesized and its struc-... 

Anomeric effect, 82, 310-311, 305 Antarafacial, 163 examples, 164 sigma bonds, 167 Anti-Bredt olefin, 102 Approximations of MO theory Born-Oppenheimer, 22 Hartree-Fock, 222 Huckel, 35, 86 independent electron, 35 LCAO, 229 nonrelativistic, 22 SHMO, 87... 

The structures of regioisomers, represented by the partial formulas 77 and 78 and formed by addition of organic azides to Bredt olefins, can be assigned unambiguously by lH- and 13C-NMR spectra. In structure 77 the proton a to the N=N double bond appears at lower field than the proton a to the /V-Ph group with approximately AS = 0.8 ppm. In the13C-NMR spectra a chemical... 

The sila- /r-Bredt olefins (olefins with bridgehead double bond) were obtained starting from 59, as a key intermediate. Thus, addition of dibromo- or chlorofluorocarbene to 59 gave the crude 60, which was converted in ethanol to 4-silabicyclo[5.3.1]undec-l(ll)-ene 61 (Scheme 12). Molecular structure of 61 was confirmed by X-ray crystallographic studies <2001JOC1216>. 

Twist as well as pyramidalization are the typical deformations of bridgehead double bonds in bicyclic compounds of type 44 (Table 4). Structures of the type 44 comprise Bredt-olefms (formerly called anti-Bredt-olefins) and bridgehead olefins. In Bredt-olefins, the double bond is located at the bridgehead of a bicyclo[m.n.o]alkane skeleton with m,n,o 1, whereas... 

The size of the torsional angle in the Bredt-olefins 45-48 increases with decreasing ring size and by substitution of a CH2 group with an oxygen atom. This may be due to the observation that the average C-O-C bond... 

The rather low frequencies for C=C bond stretching in the Bredt-olefins 69, 176,258, and 259 may be an indication of reduced 7t-bond order (96). Whereas the in-plane distortions of the trisubstituted olefins 257a and 257b hardly affect... 

In suitable cases such as cis/trans-isomers, the difference in strain energies can be determined from equilibrium constants or heats of hydrogenation. It is apparent from Table 20 that both methods give similar results. The strain energy of the Bredt-olefin 69 has been estimated from the heat of reaction with acetic acid, with the calculated AHR of the corresponding reaction of 2-methyl-2-butene (257a) as standard (219). 

For the hydronium-ion-catalyzed hydration of bicyclo[4.2.1]non-l-ene (177) and bicyclo[4.2.1]non-l(8)-ene (176), appreciable solvent isotope effects have been observed. Since these correspond to those found for reactions of unstrained olefins, it was concluded that the hydration proceeds as with unstrained alkenes by a two-step mechanism protonation of the double bond is followed by addition of the nucleophile (152b). The strained olefin with its distorted double bond is higher in energy and more reactive than an unstrained alkene. Hence, the transition state for protonation of Bredt-olefins is expected to be an early one (95). 

The rates of hydration of the Bredt-olefins 69 and 176 (Table 27) have been used to estimate strain effects in electrophilic reactions of these strained olefins. It was estimated that the 105-fold rate increase of 69 over 2-methyl-but-2-ene (257a) is due to about 50% strain release in the transition state of protonation. 

Relative Rates of Electrophilic Addition to Bredt Olefins (152b)... 

Cycloadditions. The Bredt-olefins 69, 176, and 177 and trans-1-methylcyclooctene (37c) react with l,l-dichloro-2,2-difluoroethene at elevated temperature with low regioselectivity and high stereoselectivity favoring the... 

The regioselectivity in reactions of Bredt-olefins has been studied. Typical electrophilic and nucleophilic reactions, the latter supported by only two examples, show high regioselectivity and syn-stereoselectivity. With the exception of diphenylketene, the [2 + 2] and [2 + 3]cycloadditions of Bredt-olefins have negligible regioselectivity. 

More than one of the three possible oop bond angle deformations are usually apparent in the distorted olefins. Nevertheless, it is possible to detect in most cases the dominant type of distortion. The kinetic stability of olefins with distorted double bonds is enhanced by steric shielding. Hence, distorted double bonds in bicyclic and polycyclic structures may not display the reactivity to be expected if the dominant type of the oop distortion is the selectivity-controlling factor. The strong preference of syn- over anti-addition in Bredt-olefins may be due to this steric shielding. 

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