Radical cyclobutylcarbinyl

Either way, ether analog 25 was expected to be more reactive, and indeed, 28 was found to cyclize quite efficiently with a rate constant for cyclization of 3.8 ( 0.3) X 105 s 1 [140]. Only the exo-mode of cyclization was observed, in contrast to Piccardi s results. Further studies of fluorinated 4-pentenyl and cyclobutylcarbinyl radical systems will hopefully provide eventual definitive insight into those factors which govern rates and equilibria in this system. 

Crimmins, M. T. Dudek, C. M. Cheung, W.-H. A fragmentation-rearrangement sequence of cyclobutylcarbinyl radicals. Tetrahedron Lett. 1992, 33, 181—184. 

What, then, about the cyclobutylcarbinyl radical Eq. 3.108 shows the treatment of tricyclic cyclobutylmethyl iodide (268) with Bu3SnH in the presence of AIBN in refluxing benzene solution to generate bicyclic exo-methylene compound (269), via 13-cleavage of the cyclobutylcarbinyl radical. This bicyclic exo-methylene (269) is the skeleton of a terpenoide natural product, guaiane alismol [277-280]. 

In a similar way to the cyclobutylcarbinyl radical, cyclobutylaminyl and cyclopropy-laminyl radicals also induce rapid (3-cleavage reactions as shown in eq. 3.112 [288-292]. 

Ishii and coworkers developed a Mn(OAc)2-catalyzed hydrophosphonation of alkenes 40 (Fig. 47) [271]. The active Mn(III) catalyst is generated by reaction of Mn(OAc)2 with oxygen. Hydrogen abstraction from diethyl phosphite 169 forms a phosphonyl radical, which adds to 40. The resulting alkyl radical is reduced by 169 to continue the chain reaction. Alkylphosphonates 170 were isolated in 51-84% yield. With (3-pinene a cyclobutylcarbinyl radical ring opening was observed in 32% yield, while 1,5-cyclooctadiene underwent a tandem radical addition/ transannular 5-exo cyclization (cf. Fig. 38). 

The Re(III) complex Re(PPh3)2(MeCN)Cl3 (2 mol%) catalyzes the ATRA of tetrachloromethane or bromotrichloromethane to terminal alkenes in 39-76% yield [303]. p-Pinene suffered a cyclobutylcarbinyl radical ring opening, thus supporting the free radical mechanism. With l, -dienes double addition was found, while 1,3-dienes gave the 1,4-addition product. Internal alkenes were almost inert under the reaction conditions. 1,6-Dienes 158 underwent a tandem radical addition/ cyclization reaction to cycles 159 in 64—87% yield with 3-6 1 c/s-diastereos-electivity (cf. Fig 43). This compares well to the results obtained with the most frequently used catalyst Ru(PPh3)3Cl2 (see Part 2, Sects. 3.3.1 and 3.3.2). 

Small-ring radical opening is a facile process. This process has been incorporated in the zero bridge cleavage of strained bicycles for one- and two-carbon ring expansions (Scheme 14). Ring expansions of cyclopropyloxy and cyclobutyloxy radicals have been discussed in the previous section. This section focuses on cyclopropyl-carbinyl, ot,j8-epoxyalkyl and cyclobutylcarbinyl radicals. 

Beckwith, A. L. J. Moad, G. The kinetics and mechanism of ring opening of radicals containing the cyclobutylcarbinyl system. J. Chem. Soc. Perkin Trans. 2 1980, 1083—1092. 

Baker, J. M. Dolbier, W. R. Density functional theory calculations of the effect of fluorine substitution on the cyclobutylcarbinyl to 4-pentenyl radical rearrangement. J. Org. 

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