Metalloles reduction

In the course of the synthesis and the study of several group 14 metalloles (silacyclopentadienes and germacyclopentadienes) their reduction potentials were recorded (vs SCE), with 0.1 M TBAP in dimethoxyethane on DME by polarography and by hanging mercury electrode voltammetry176. In (r 4-gcrmacyclopcnladicnc)C.o(C.O)212... 

Reaction with an alkali metal gives substitution instead of reduction if the metallole... 

To highlight what one would expect in reactions of the diphosphazirco-nole 37, it is instructive to examine the rj4-l,3-diphosphacyclobutadiene complex (38) (94,95), whose X-ray structure is compared in Fig. 15 with that of the isoelectronic rj4-cyclobutadiene complex 39 (96). Compound 38 is readily obtained from reaction of (Cp)Co(T/2-C2H4)2 and 2 equiv of Bu CP. The same reaction with a pure alkyne does not stop at a cyclodimer but leads to cyclotrimerization (97). In fact, transition metal-cyclobutadiene complexes normally form only at temperatures above 80°C, presumably from a metallole intermediate, by a double reductive elimination process. It is noteworthy how readily this cyclodimerization to complex 38 takes place with phosphaalkynes. 

The unique photophysical properties of metalloles (mainly siloles and germoles) forced the development of new preparation methods of these compounds, some of which will be addressed here again. Thus, the reductive cyclization of dialkyn-l-ylsilanes 142 gives first 2,5-dilithiosiloles 143 which possess an enormous synthetic potential, also opening the way to different 2,5-(bisaryl)metalloles (via cross-coupling reactions), known to show outstanding optical properties. 

Recent years have witnessed renewed interest in the problems of chemical bonding in Group IVa metallole mono- and dianions Several metallole anions have been synthesized by reductive dehalogenation of dihalometalloles as reported by the groups of Joo [1], Boudjouk [2], Tilley [3] and West [4]. 

Oxidative coupling of two alkynes to give the metallole, followed by CO insertion and reductive elimination. 

Intermediates with one coordinated alkene are often seen (e.g., Eq. 6.49), but the bis-alkcne species is probably the immediate precursor of the coupled product." The products from alkynes are often stable and are known as metalloles (Eq. 6.51) but they can also reductively eliminate to give cyclobutadiene complexes (Eq. 6.47). 

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