Cycloaromatization Initiated by External Radicals

After initiation of the radical process using BujSnH, enediynes 3.630 cycloaromatize producing a mixture of two benzofulvene stereoisomers 3.631 in 57% yield. The tetrafluoropyridinyl substituents are unchanged 

Theoretical studies of free-radical initiated cyclization of enediynes predicted that electron-deficient acetylenic substituents increases the Bergman cyclization barrier. The radical cyclization of enediynes containing electron-withdrawing substituents (such as acetyl and ethoxy-carbonyl groups) has been investigated. Heating a solution of l,2-bis(3-ethoxycarbonyl-l-butynyl)ethene 3.652 in the presence of stable radicals 

2-Diethynylbenzene has a half-life of 29 minutes in its thermal cyclization to produce naphthalene in the yield of about 99% (162°C, 100%, 1,4-cyclohexadiene). On the other hand, the half-lives of compounds 3.652 and 3.657 under the same conditions is much longer (tj/2 

481 and 660 minutes, respectively), confirming the decrease in the reactivity of enediynes bearing electron-acceptor substituents [317]. 

Thermal cycloaromatization of enediynes to fulvenes, apparently, is not compatible with systems bearing electron-withdrawing substituents. Thus calculations did not reveal a stable transition state probably due to the high energy required for its formation, hence fulvenes cannot be prepared thermally from the substituted enediynes considered in these calculations. Analysis of the data (BLYP/6G ) showed that the change in reactivity of carbonyl- and carboxyl-containing enediynes is due to the formation of free radicals, which are responsible for the observed cyclizations [7, 316], 

---