Cyclopropyl radical and

The value might be for a mixture of the cyclopropyl radical and its acyloxyl precursor see text. 

Interconversions of acychc carbon-centered radicals between n and a types are low-energy processes. The methyl radical is planar, but increasing alkyl substitution at the radical center results in an increasing preference for pyramidalization. The ferf-butyl radical is pyramidalized with the methyl groups 10° from planarity (the deviation from planarity for a tetrahedral atom is 19°) and a barrier to inversion of 0.5 kcal/mol. When a radical center is in a carbocycle, a planar radical is favored for all cases except the cyclopropyl radical, and the barrier for inversion in cyclopropyl is only 3 kcal/mol. ... 

The absence of intramolecular H-abstraction in cyclopropyl diketones reflects a combination of conformational factors, difficulty in formation of cyclopropyl radicals, and strain in the cyclization product. 

As the most electronegative element, fluorine would be expected to have the greatest effect on the stereochemical stability of the cyclopropyl radical and it does. When comparing ESR spectra of cyclopropyl radicals with 1-fluoro cyclopropyl radicals Kawamura and coworkers found the inversion frequencies of 1, 2 and 3 at — 99°C comparable to that found for the parent cyclopropyl radical, 10 s at — 175°C. In contrast, the inversion frequency of the a-fluorocyclopropyl radicals 4,5 and 6 is estimated to be lowered to 10 s at — 108°C. 

The results of these limited experiments suggest that an a-methoxyl group is not very effective in stabilizing the configuration of the cyclopropyl radical" and indicate that delocalization of the radical by the methoxyl group may be making a significant contribution to the stabilization of the 7c-radical intermediate or transition state. 

By these simple rules Woodward and Hoffmann predicted a disrota-tory course for the opening of the cyclopropyl cation in its ground state to the corresponding allyl cation, while the thermal opening of cyclopropyl radical and anion to allyl radical and anion is conrotatory. A glance at Fig. 2 clearly shows the reason. Reverse predictions can be made for photochemically induced reactions. 

The product distribution evidently reflects pre-equilibration of the syn- and anti-cyclopropyl radicals and the relative rates of their trapping with metal to form metal alkyls. That the metal alkyls are relatively stable once formed was shown by the conversion of the anti-bromide (857 X = H, Y = BrX on treatment with butyl-lithium followed by deuteriolysis, exclusively into the anti-deuterioalkane (857 X = H,... 

Computational studies on the ring openings of cyclopropyl radical and cyclopropyl cation ... 

As noted in Section V.2.C, opening of the cyclopropyl radical and closing of the allyl radical are slow processes. Nevertheless, this possibility has been used as a working hypothesis (Scheme 135) to explain the formation of products resulting from the treatment of 355 with sodium in liquid ammonia. ... 

Write out the ehitrge density diagTiiiiis for the positive ion. free radical, and negative ion of the cyclopropyl system. 

Below — 140°C, the EPR spectrum observed was that of the cyclopropylmethyl radical. If the photolysis was done above — 140°C, however, the spectmm of a second species was seen, and above — 100°C, this was the only spectmm observed. This second spectmm could be shown to be that of the 3-butenyl radical. This study also established that the 3-butenyl radical did not revert to the cyclopropylmethyl radical on being cooled back to — 140°C. The conclusion is that the ring opening of the cyclopropyl radical is a very facile process and that the lifetime of the cyclopropyl radical above — 100°C is very short. Even though the equilibrium favors the 3-butenyl radical, the reversible ring closure can be detected by isotopic labeling experiments, which reveal the occurrence of deuterium migration ... 

The results obtained show that radicals can be divided into two classes depending on the signs of a and p. The methyl, ethyl, and cyclopropyl radicals... 

The allyl radical [115] trapped in an argon matrix can be photolytically (A = 410 nm) converted into the cyclopropyl radical [116] (Holtzhauer er a/., 1990). Dicyclopropane and cyclopropane were formed when the photolysed matrix was warmed from 18 to 35 K. The intermediate [116] was shown to be a cr-type (Cs symmetry) and not a rr-type symmetry) radical. Normal coordinate analysis of the radical [116] has been carried out and the IR band at 3118 cm has been assigned to the stretching vibration of the C—H bond at the radical centre. 

Carbon-centered organic radicals are highly reactive trivalent species with only one nonbonding electron. While most known radicals have their unpaired electron in a pure p- or a delocalized Ji-orbital, there are also examples of radicals centered in s/t" hybrid o-orbitals, such as the well known phenyl and cyclopropyl radicals. The first radical reported in the literature is credited to Gomberg s landmark paper in 1900 when he postulated the formation of triphenylmethyl radical 36, also known as tri-fyj 99,100 jj-jjyj j-adical is an example of a persistent radical that exists in equilibrium... 

They argue that this and related enone photorearrangements do not take place via an a-cleavage (8.67), inasmuch as bonding to the central carbon atom of the allylic radical (47) or the rearrangement of an allyl radical to a cyclopropyl radical is the least favorable path available to the radical species ... 

FIGURE 8.23 One electron oxidation of a cyclopropyl amine leading to ring opening and the formation of a carbon-centered free radical and an iminium ion. 

Aryl radicals are much more reactive than alkyl radicals (k ca. 2 x 106M-1 s 1), and rather more reactive than cyclopropyl radicals (ca. 8 x 107) or vinyl radicals (ca. 3 x 10s). Fluoroalkyl radicals are also about 100 times more reactive than alkyl radicals, because of contributions from structures Rf 6 - H -4+SnR3 to the transition state. Singlet-excited acetone is twice as reactive as triplet-excited acetone, which shows a similar reactivity to that of the /-butoxyl radical (k ca. 2 x 108M-1 s-1). 

E. Electrocyclic Ring Opening of Cyclopropyl Ions and Radicals... 

Cyclopropyl ions and radicals (23) can undergo conversion to allyl (24) by typical electrocyclic ring opening processes we have carried out calculations for ring opening by both conrotatory and disrotatory paths. Table 5 shows calculated activation energies for the various processes. 

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