Cyclopropylcarbinyl—butenyl

Cu(II) EPR signal in nitriles as solvent as well as by polarographic measurements 144>. Similarly, the EPR signal disappeared when Cu(OTf)2 was used for catalytic cyclo-propanation of olefins with diazoesters 64). In these cases, no evidence for radical-chain reactions has been reported, however. The Cu(acac)2- or Cu(hfacac)2-eatalyzed decomposition of N2CHCOOEt, N2C(COOEt)2, MeCOC(N2)COOEt and N2CHCOCOOEt in the presence of cyclopropyl-substituted ethylenes did not furnish any products derived from a cyclopropylcarbinyl - butenyl rearrangement128. These results rule out the possible participation of electron-transfer processes and radical intermediates which would arise from interaction between the olefin and a radical species derived from the diazocarbonyl compound. 

The radical cations 15 + and 16 + add methanol exclusively from the exo face forming exo-methoxynorbornyl and nortricyclyl free radicals, which undergo rapid cyclopropylcarbinyl-butenyl interconversions (119 120 121 see... 

This is less common than rearrangement of carbocations, but it does occur (though not when R = alkyl or hydrogen see Chapter 18). Perhaps the best-known rearrangement is that of cyclopropylcarbinyl radicals to a butenyl radical. The rate constant for this rapid ring opening has been measured in... 

Cyclopropylcarbinyl radicals (5) are alkyl radicals but they undergo rapid ring opening to give butenyl radicals." The rate constant for this process has been measured by picosecond radical kinetic techniques to be in the range of 10 M s for the parent to lO Af s for substituted derivatives. This process has been observed in bicyclo[4.1,0]heptan-4-ones. ... 

Cyclopropylcarbinols. Treatment of cyclopropylcarbinols 15 (R = Ph, C-C3H5) with trifluoroacetic acid in dichloromethane leads to the rapid formation of ring-opened 4-substituted 3-butenyl-l-trifluoroacetate esters 16 (Eq. 20).130 Cyclopropylcarbinyl trifluoroacetates are not formed. Ring opening is facilitated by phenyl substituents. Addition of organosilicon hydrides to the reaction mixture favors the formation of cyclopropylmethanes 17 and suppresses the formation of the ring-opened esters.130... 

The dominant contributor to the reactivity of vinylcyclopropanes in any radical reaction is the form (4a), the cyclopropylcarbinyl radical system. The opening of a cyclopropylcarbinyl radical to a butenyl radical is among the fastest radical processes known, with a rate constant of 1.3 x 10 sec". - The various stereoelectronic effects of this rearrangement have been reviewed. The structure of (4a), deduced from its ESR spectrum - and in agreement with calculations (STO-36 basis set), is in the bisected conformation shown, predicted to be 1.4 kcal mol more stable than its perpendicularly oriented counterpart. Above -KX) T only the butenyl radical (4b) can be detected. Substituent efiects do not seem to operate here when the substituents are on the cyclopropane (i.e. product stabilization). The cy-clopropylcaibinyl cation and anion have structures similar to (4a), bisect conformations (5) and (6), respectively. A concise summary of solvolytic and mechanistic data for system (5) has recently appeai Reviews of cyclopropylcarbinyl anions and carbenes are also available. - ... 

Bicyclo[1.1.0]butane also reacts with the halogens. With I2, the main product is 1,3-diiodocyclobutane (5 1 cis. trans). With Br2 and CI2, cyclopropylcarbinyl and butenyl products are also formed. The initial attack occurs from the endo face of the molecule and the precise character of the intermediate appears to be dependent on the halogen. 

Below we lay out the key carbocationic intermediates that could be involved in these solvolysis reactions. Cyclobutyl, cyclopropylcarbinyl, and 3-butenyl structures are conventional carbenium ions. Less conventional are the bicyclobutonium and tricyclobutonium ions. Although part of the earlier analyses of the solvolysis reactions, the tricyclobutonium... 

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