Radical cyclopropyl

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 acyl selenide 19 affords the decarbonylated )S-lactam in good yield. A N-hydroxypyridine-2-thione ester 20 is used in the key step to construct the chiral cis-cyclopropane structure in compounds designed as antidopaminergic agents. The observed high cis selectivity is due to the hydrogen abstraction from the sterically demanding (TMSlsSiH, which occurs from the less-hindered side of the intermediate cyclopropyl radical. 

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... 

A similar rearrangement but where the allylic radical closes to yield a cyclopropyl radical can be seen in the following examples 44,46 ... 

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 ... 

The partial loss of configuration often observed during reduction of cyclopropyl halides may actually occur via the corresponding cyclopropyl radicals, which lose configuration rapidly °>. In that event, their behavior would resemble that of vinyl halides, as exemplified by the 3-iodo-3-hexenes 16>. Occasional cases of partial inversion could be associated with shielding of the cyclopropyl carbanion by the electrode surface, with concomitant protonation on the other face of the carbanion S8>. 

The study of the decomposition of optically active 1-methyl-2,2-diphenylcyclopropanoyl peroxide proved the retention (37%) of the product of the geminate radical pair recombination [90]. The radical center in the formed cyclopropyl radical is so strained that the racemization rate is unusually slow. 

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). 

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

We note that the rate constants for reaction of the cyclopropyl radical with... 

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. ... 

Figure 4.1. The (a) ethyl (shown in a staggered conformation) and (b) cyclohexyl radical are n radicals, and the (c) trifluoromethyl radical and (d) cyclopropyl radical are a radicals.

For the vinyl radical, the hyperfine coupling for the a-carbon is 107.6 G, which would suggest 10% s character in the hybrid orbital. The vinyl radical clearly has the odd electron in a c-type orbital because the (3 protons of the vinyl radical have distinct hyperfine couplings with a = 37 G for the c/i-H, and a = 65 G for the trans-M. The cyclopropyl radical also is a c radical with an a- C a value of 98 G, whereas the cyclohexyl radical, which has a nearly planar radical center, has an a- C a value of 41 G. 

In the area of reaction energetics. Baker, Muir, and Andzehn have compared six levels of theory for the enthalpies of forward activation and reaction for 12 organic reactions the unimolecular rearrangements vinyl alcohol -> acetaldehyde, cyclobutene -> s-trans butadiene, s-cis butadiene s-trans butadiene, and cyclopropyl radical allyl radical the unimolecular decompositions tetrazine -> 2HCN -F N2 and trifluoromethanol -> carbonyl difluoride -F HF the bimolecular condensation reactions butadiene -F ethylene -> cyclohexene (the Diels-Alder reaction), methyl radical -F ethylene -> propyl radical, and methyl radical -F formaldehyde -> ethoxyl radical and the bimolecular exchange reactions FO -F H2 FOH -F H, HO -F H2 H2O -F H, and H -F acetylene H2 -F HC2. Their results are summarized in Table 8.3 (Reaction Set 1). One feature noted by these authors is... 

For a review of cyclopropyl radicals, see Walborsky Tetrahedron 1981,37.1625-1651. Sec also Bochc, Walborsky. Ref. 102. 

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