Reactions of radical anions. Part XIII. Electron spin resonance study of the radical anions of 1,4-bistrimethylsilyl- and 1,4-di-t-butylbuta-1,3-diyne

Abstract

We have studied the radical anions of the diacetylenes (CH₃)₃SiCC–CCSi(CH₃)₃ and (CH₃)₃CCC–CCC(CH₃)₃ in tetrahydrofuran by e.s.r. spectroscopy. These radical anions have only one type of proton present, the 18 methyl protons. E.s.r. experiments show clearly that with the silyl radical anion, unpaired electron density occurs not only on the silicon atoms but also on the methyl groups, and this we attribute to d_π–p_π interaction. In the radical anion of 1,4-di-t-butylbuta-1,3-diyne, however, no delocalisation of the electron through to the methyl groups occurs. There is a marked difference in reactivity between the radical anions of the two diacetylenes. The silyl radical anion is very stable whereas the di-t-butyl radical anion is very reactive. This difference in reactivity is discussed in terms of electron distribution and steric hindrance.