Metalloles stability

III. ORGANIC CHEMICAL PROPERTIES OF GROUP 14 METALLOLES A. Stability and Isomerizations... 

The kinetic stability of group 14 metalloles towards [4 + 2] dimerization is dependent on the nature of the heteroatom and on the substituents bonded to it and the ring carbons. Like cyclopentadiene, the silole 2 is unstable as a monomer. The silicon methylated derivatives (3 and 4) are also unstable, but two mesityl groups provide stabilization by steric hindrance of the [4 + 2] dimerization transition state15. The same effect has been observed for 1,1-dimesitylgermole24. 

Aromatic groups on the ring carbons stabilize strongly the C-phenylated group 14 metalloles, as in the case with phospholes109, but the reactivity of the dienic system (i.e. Diels-Alder cycloadditions, formation of transition metal complexes) decreases3,4. 

This chapter will concentrate on the chemistry of metal-14-centered anions (Ge, Sn, Pb). These compounds and their silyl analogues are ionic or polarized alkaline and alkaline earth metal-14 compounds, as well as delocalized molecules such as metalloles. Ammonium metallates Mi4 R4N+ or metal-14-centered anion radicals are also considered. The subject was explored during the 1960s and 1970s and thoroughly reviewed in 1982 and 1995 in Comprehensive Organometallic Chemistry, Vols. I and and for silicon species in a previous volume of this series . By that time the main routes to metal-14 anions were known. Since then, the subject has been developed in the topics of particular syntheses, stabilization using steric hindrance, electronic effects and complexation, spectroscopic and structural analyses "... 

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