Cation Radical Chain Cyclopropanation

A cation radical chain cyclopropanation reaction has also been described using ethyl a-diazoacetate (2), to give reasonable yields of useful three-membered rings. A full account of the Lewis acid catalysed [2++4] polar cycloaddition reaction of the methylthio chloride (3) with a 1,3-diene is of interest/ as is a study of the... 

As was noted in Scheme 12, distonic radical cations obtained from cyclopropane bond cleavages add oxygen rapidly, producing products with two CO bonds. So do some alkene radical cations. Addition of O2 to an alkene radical cation is formally a nucleophilic attack by the single alkene n electron on O2, and oxidizes both carbon atoms (an alkene radical cations has formally two -f carbons, and the adduct a 1+ and an oxygen-bound carbon). The oxygenation of the radical cation of bia-damantylidine (96) leads to dioxetanes such as 98 in chain reactions (see Scheme 21) [110]. The reactions may be initiated electrochemically or photochemically, but tris(o,p-dibromophenyl)amine hexafluoroantimonate, 97, is a superior catalyst for the dark reaction of certain tetraalkylalkenes, with turnovers up to ca. 800 at... 

The chain addition of cation radicals of conjugated dienes and electron rich styrenes to ethyl diazoacetate has been found to be a relatively eiiicient method of generating cyclopropane derivatives (Scheme 50) [90],... 

As an alternative to a radical chain mechanism for this bromination, a cationic mechanism has been proposed for the reaction between 48 and A-bromosuccinimide. It involves attack of bromine at C6 of 48 leading ultimately to the cyclopropylmethyl cation A. This cation is a bromo derivative of tricyclic cyclopropylmethyl cation, which has been shown to be the common intermediate in the solvolysis of esters of tricyclo[3.2.1.0 ]octan-3-ol, endo- and exo-tricyclo[3.2.1.0 ]octan-4-ol and of cxo-bicyclo[3.2.1]oct-2-en-7-ol. It has been shown that under long-lived ion conditions at — 78 C such cations are the most stable species that are formed from bicyclo[3.2.1]oct-2-en-3-ol and from bicyclo[3.2.1]octa-2,6-dienes. In kinetically controlled reactions, which are postulated to proceed via cyclopropylmethyl cations a tendency can be seen towards formation of products retaining the cyclopropane ring. This case is achieved through loss of one of the protons at C4 of A. 

Metallic bronze fh(N 11, ), can, furthermore, be prepared from calcium-ammonia solution by evaporation of the ammonia. A bronze sohd is obtained, which in TH F at -30 °C behaves as a sohd surface in its reaction with alkyl hahdes (Scheme 4.17) [39]. Its apphcation in the reduction of 61 b produces a mixture of cyclopropane derivative 62 (81%) and straight-chain diphenylbutane 64 (8%). The process involves the formation of a tight anion radical-cation radical as an intermediate. Carbon-halogen bond cleavage occurs on the surface of the metalhc cluster. 

SET induced cyclisations have been carried out on a series of non-conjugated dienes one component of which is a silyl ether. Irradiation in acetonitrile of dienes such as (330, R = H) using DCA as the electron accepting sensitiser results in the formation of the radical-cation of the diene which on cyclisation affords the cyclic ketone (331) in 25% yield. The reaction is solvent sensitive and in a mixture of acetonitrile/propan-2-ol yields three products which are identified as (331, 30%) and two minor products (332, 11%) and (333, 9%). The reaction is suggested to have some considerable synthetic potential and the effect of chain length and substituents on the reaction has been evaluated. Thus (334) is converted into (335, 11%) while (330, R = Me) affords an isomeric mixture of products (336). Other studies with silyl ether derivatives have examined the electron transfer-induced ring opening processes encountered with the cyclopropane derivatives (337). ... 

The reverse Q—reaction is thermally forbidden by the Woodward-Hoffmann rules, but this transformation can be readily accomplished via a radical-cation chain reaction pathway initiated by chemical, electrochemical, or photosensitized single-electron oxidations. The metastable Q structure contains a highly strained cyclobutane and two cyclopropane rings, and the enthalpy of the 2—>1 isomerization in the pure liquid state has the value of AH = -89 kj mob, as obtained from direct calorimetric measurement. Results of other experiments and computed values vary in the range of -81 to -112 kJ mol". - - ... 

Tamai, T., Ichinose, N., Tanaka, T, Sasuga, T., Hashida, 1., and Mizuno, K., Generation of polyphenylene radical cations and their cosensitization ability in the 9,10-dicyanoanthracene-sensitized photochemical chain reactions of l,2-i s(4-methoxyphenyl)cyclopropane,/. Org. Chem., 63, 3204, 1998. 

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