last three compounds, the Sussex group proposes this reaction scheme:
RESEARCH
Trimethylsilyl Radical Chemistry Unfolds Decomposition of bis(trimethylsilyl)jriercury permits direct study of radical's reactions Bis (trimethylsilyl)mercury, when it's heated or exposed to light, decomposes to form trimethylsilyl radicals, three chemists from the University of Sussex (Brighton, England) have found. The radicals can then react with organic substrates, Sussex's Dr. Colin Eaborn said at the Second International Symposium on Organometallic Chemistry, at the University of Wisconsin (Madison). Chemists have generally believed that several important reactions in organosilicon chemistry involve silyl radicals, Dr. Eaborn points out. But direct study of the properties of these radicals has had to await the availability of a suitable source. Thé work of Dr. Eaborn's group gives a clue to mechanisms involved in the reactions of these radicals. Also, their work may point toward other synthetic reactions involving silyl radicals. In addition, the Sussex research indicates that free radicals of elements (such as silicon) other than carbon may not behave in the same way as carbon free radicals. Dr. Eaborn and his co-workers, Dr. R. A. Jackson and R. W. Walsingham, make bis ( trimethylsilyl ) mercury in
conventional glass equipment by treating mercury amalgam with trimethylsilyl chloride. This is a simplification of the original preparation involving the less readily available trimethylsilyl bromide. The earlier technique was developed by Dr. Egon Wiberg and his associates at University of Munich's Institute for Inorganic Chemistry. Bis (trimethylsilyl) mercury is a yellow solid that melts at 102° to 104° C. (without decomposing) to an orange liquid. The compound decomposes thermally in cyclohexane with a half life of about three days at 190° C. In laboratory light, it decomposes rapidly in cyclohexane, but much more slowly in aromatic solvents. Decomposition. When bis (trimethylsilyl) mercury decomposes in cyclohexane, it produces mercury and trimethylsilyl radicals. Most of the radicals combine to give hexamethyldisilane, but some trimethylsilane also forms. Heating the compound in toluene at 190° C. for two weeks gives not only mercury and hexamethyldisilane, but also trimethylsilane, benzyltrimethylsilane, and bibenzyl. For forming the
RADICALS. Dr. Colin Eaborn (left) and R. W. Walsingham discuss trimethylsilyl radicals. Several important organosilicon reactions involve these radicals 60
C&EN
SEPT.
13,
1965
When bis ( trimethylsilyl ) mercury decomposes in chlorobenzene under conditions similar to those with toluene as the solvent, it yields trimethylsilane, phenyltrimethylsilane, and chlorotrimethylsilane. Apparently, Me 3 Si· radicals can abstract chlorine atoms from the aromatic ring to give a phenyl radical—a process that may result from the high energy of the Si—CI bond, Dr. Eaborn says. Thus, decomposition in chlorobenzene also produces a little biphenyl and some mono- and dichlorobiphenyl. In benzene, bis (trimethylsilyl)mercury decomposes to give trimethylsilane and phenyltrimethylsilane, but no biphenyl. This indicates that hydrogen abstraction, which would give phenyl radicals, doesn't occur. Dr. Eabom believes a possible sequence for the reactions in benzene is: H Me3Sj# SiMe3 / j ^ S i M e 3 + Me3SiH
