Cyclobutyl radicals

Cyclopropyl and Cyclobutyl Radicals. Ab initio calculations on the transformation of tetrahedrane into the bicyclobiityl biradical (856) indicate that the former is actually an energy minimum on the surface and that there is a barrier of at least 18 kcal moP for this allowed transition. Semiempirical MO calculations on the cyclo-propenyl radical have also been reported.  

Carsky, and R. Zahradnik, Coll. Czech. Chem. Comm.. 1974, 39, 2175. 

Efforts to produce the cyclopropylamino-radical for e.s.r. studies gave only ring-opened imine radicals. Ring-openings of cyclopropyl, cyclobutyl, and bicyclo[n,l,0]alkyl radicals have been reported and the disrotatory nature of the ring-opening of cyclopropyl radicals has been established by examination of ring-fused systems.  

Whereas photosensitized bromination of cyclopropane affords 1,3-dibromo-propane, bromination with NBS gives cyclopropyl bromide. Thus, whereas Br reacts exclusively by attack on carbon to give ring-opening, the succinimidyl radical abstracts hydrogen to give a cycloprop radical, which rapidly combines with a bromine atom.  

Generation of cyclobutyl radicals in tribromomethane affords a mixture of cyclobutyl, cyclopropylmethyl, and allylmethyl bromides. Perfluorocyclobutane suffers S 2 attack by fluorine atoms when heated with fluorine, generating thermally excited radicals which afford C-1—C-4 perfluoroalkanes.  

Cyclopropyl and Cyclobutyl Radicals. The reaction of 1,1-dichloro-cyclopropane with photochemically produced chlorine atoms at 0°C in carbon tetrachloride gives 1,3,3-trichloropropane (which may react further with chlorine). The products of the reactions of chlorine atoms with the specifically labelled cyclopropanes (616a) and (616b) have been shown to be (617a) and (617b) respectively, by 250 MHz n.m.r. spectroscopy. This stereospecificity corresponds to initial radical substitution with inversion of configuration. 

It was reported last year that the stereospecificity of tri(n-butyl)tin hydride reduction of cyclopropyl halides is dependent upon the other substituent present at the reaction site, i.e. the ability of the substituent to stabilize the radical or affect its geometry. Norcarane derivatives (618) have been made by standard methods and reduced with HSnBu3 under a variety of experimental 

Wakabayashi, H. Yamanaka, and W. Funasaka, BtdL Chem. Soc. Japan, 1972, 45, 1576. 

HSnBuj to give ring-opening, and the reaction rates increase with electron-withdrawing substituents in the aryl groups.  

Decarboxylation of cyclopropanecarboxylic acids may be achieved anodi-cally or with lead tetra-acetate cyclopropyl radicals are involved in both reactions. Only the simple decarboxylation products and dimers of the initially formed cyclopropyl radicals are obtained by anodic decarboxylation of 2,3-di(methoxycarbonyl)cyclopropanecarboxylic acids. Clearly, there can be no further oxidation of the cyclopropyl radical to the cation in this process. The cyclopropyl radical obtained by lead tetra-acetate oxidation of (621) gives homolytic substitution of the aromatic ring to yield benzonorcaranes. The yields in this reaction were affected by the experimental conditions but were generally rather modest. 

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Cyclobutane (35) and its analogs were not described in literature so far. They can be regarded as an easy source both of a cyclobutyl radical [239] (by cleavage of C—S bond under electron transfer) and a dienophile [240]. Thus, (35) readily yields the corresponding cyclobutene (36) in the presence of electrogenerated bases (EGB)... 

Thus the first electron transfer to Pb relates to the reaction (a) in Section 7.4.3.1.1, and the second involves the oxidation of the cyclobutyl radicals either by electron transfer/deprotonation with Cu" in equation (17) or by ligand transfer of chlorine with PlAci in equation (18). When the product of a generic reaction is itself a radical cation (such as in Sections 7.4.3.1.8 and 7.4.3.1.9), an electron-transfer chain or ETC process can ensue, as in the hole-catalyzed cycloadditions and autoxidations of dienes,The electron-transfer propagation sequence for the latter is simply given as in equations (19) and (20). 

The stereochemistry of the second step is of special interest. Studies by Szeimies and coworkers have shown that in the system 60, both H and PhS are trapped by the cyclobutyl radical preferentially from the axial direction (equation 86). The ratios of axial vs. 

The selectivity in addition reactions of cyclobutyl radicals to alkenes has been investigated in reactions of /1-lactam derivatives31-33. 6-Bromopenicillanic acid esters were used as precursors in reductive addition reactions with alkenes31,32 or with allylstannanes30,31. Addition to the intermediate penicillanic acid-6-yl radical occurred exclusively from the a-face of the /1-lactam ring. 

Inversion of the radical center in cyclobutyl radicals is fast when compared to their cyclopropyl analogs. The steric outcome in hydrogen-abstraction reactions is therefore mainly determined by the substitution pattern at the / -carbon atoms in the radical7. 

Step. Pigou utilized the BS model in order to determine the likelihood of cyclization and selectivity during the ring closure in some substituted cyclobutyl radicals 25 (Scheme 3) [12], While high selectivity as well as Thorpe-Ingold rate enhancement was predicted, this route proved not to be synthetically viable due to other competing rearrangement processes [12]. 

A cydization/Barbier-type reaction was reported by Curran and coworkers [21] in 2004 in the total synthesis of penitrem D 46. In this work, the aryl radical generated from the iodoarene 41 and Smij proceeded to attack the tethered cyclobutene to from a cyclobutyl radical 42. Subsequently, reaction with Smij led the organo-samarium species 43, which underwent a Barbier-typie reaction with acetone to give the tertiary alcohol 44 in 40% yield. The product contains the BCD ring system of penitrem D 46 (Scheme 5.12). 

Homolytic aromatic substitution with cyclopropyl and cyclobutyl radicals has been reported. ... 

A Barton-type radical decarboxylation was used to generate a cyclobutyl radical which was captured by heterocycles to give carbocyclic C-nucleosides of type 183 reference 169 above describes similar chemistry in pentofuranosyl compounds. 

Extensive studies have been devoted to these radicals in connection with their structure (classical or nonclassical) and their reactivity (homoallylic rearrangement and 1,2-vinyl migration) and the main results have been reviewed.For instance, the classical nature of the allylcarbinyl, cyclopropyl carbinyl, and cyclobutyl radicals now seems well established. 

From an esr study of 1-aziridylcarbinyl radical 230 at low temperature (Scheme 96), it was concluded that ring opening to 231 is a very fast process (which is not unexpected in this Cy3/Cy4 case) at temperatures above — 130°C when R = H, and under all conditions when R = CHj in this latter case only the open radical 231 was observed and there was no trace of the azetidinyl (Cy 4) radical. An analogous result was observed when the cyclopropylamino radical 232 was generated only the opened radical 233 was observed with no evidence for the 2-azetidyl (Cy 4) radical. A reaction involving opening of a 2-azetidyl radical has recently been advocated to explain the nature of the products but the evidence is so indirect that this interpretation must be questioned on the basis of the stability of cyclobutyl radicals (see Section V.2.B). 

In many cases, particularly for the study of fast reactions (ky> key in Scheme 164), the opening of the cyclopropylmethyl radical to the isomeric allyl-carbinyl radical, one thousand times faster than the cyclization of the 5-hexenyl radical, is a useful complementary tool. It has been used mainly by Kochi for the study of cage reactions of alkyl radicals in solution as well for the study of ligand and electron transfer oxidations of alkyl radicals. Furthermore, in the last three studies the knowledge of the fate of the isomeric cyclobutyl intermediate was very useful in distinguishing between a homolytic (no rearrangement of the cyclobutyl radical) and a heterolytic pathway (fast... 

Stein and Rabinovitch have studied the ring-opening and isomerization of chemically activated cycloalkyl radicals. The cyclobutyl radical, formed by the addition of H to the olefin gives straight-chain products. The threshold energy for the reaction is between 31 and 33kcalmol, quite close to that for the reaction of unstrained species. 

Norrish type-ll-cleavage, p-orbital of cyclobutyl radical is almost perpendicular for maximum ovelapping to give cyclic product)... 

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