Metalloles—

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... 

Ab initio calculations indicate that there is an increased pyramidality at tin in CaEUSnH and ChfTSnHLi, resulting in reduced aromaticity. The metallole dianions G/iH-tSn2 (M = C, Si, Ge, Sn, or Pb) are more aromatic, the aromaticity staying remarkably constant down the group.240... 

Sohn H, Sailor MJ, Magde D, Trogler WC (2003) Detection of nitroaromatic explosives based on photoluminescent polymers containing metalloles. J Am Chem Soc 125 3821-3830... 

Group 14 metalloles, ionic species and coordination compounds... 

With regard to group 14 metalloles, two reviews were published in 1990, the first on their synthesis and their properties3, the second on their ionic species and their coordination compounds4. Here, we present a simplified review by concentrating particularly on the last five years work. 

Much sought-after by several laboratories, C-unsubstituted group 14 metalloles and parent metalloles, such as the silole (2), have been the subject of one of the most interesting challenges in organometallic chemistry over more than three decades. 

These compounds form the largest class of group 14 metalloles known today30. We will review again the more outstanding results concerning the long-established synthetic methods for these heterocycles, as well as the more recent ones. 

Many substrates have been used to prepare C-substituted group 14 metalloles by this method, in particular those which generate analogues of carbenes S2E 2315. 

Group 14 metalloles, ionic species and coordination compounds 1979 Wrackmeyer and coworkers using trialkylboranes56-58 (equation 13). 

These reactions are not chemospecific, but the formation of other heterocycles can be avoided, and the yields of metallole are generally high. Some heteroatomic groups attached to the ring carbons can be substituted by hydrogen (e.g. in protodeborylation or protodestannylation reactions)560 d. 

Though this method is still limited to 3,4-diphenylsiloles, it represents a distinct advance in group 14 metallole synthesis. 

Since the dehydrohalogenation method could not be extended to other group 14 metalloles and in particular to the simple metalloles65, except for 1,1,3,4-tetramethylsilole66, it seems clear today that Gilman s synthesis is specific to 2,5-diphenylsiloles owing to the benzylic positions involved. 

The catalytic dehydration of metallacyclopentenols is not a convenient method for the synthesis of C-methylated metalloles since ... 

In the case of 3,4-dimethylmetalloles 58, which are stable in their monomeric form (see Section III.A.l), the method proposed was the thermolysis of iV-phcnylcarbamates 57, the decomposition of which is regioselective (equation 22). Intracyclic ft C—H elimination giving the metallole 58 is the main process (80-90%), and the transoid isomer 59 is always the minor product. Tertiary /V-phenylcarbamates 57 decompose at about 70 °C in common solvents. In carbon tetrachloride, one can follow easily the progress of the reaction by NMR spectroscopy. An interesting one-pot synthesis is possible with two equivalents of phenyl isocyanate, the side products are gaseous carbon dioxide and diphenylurea which precipitates (equation 23)14,22,67c. 

S -Methylxanthates 62 derived from the same alcohols 50 are thermally less stable than the corresponding carbamates 57. They decompose during their synthesis according to two competitive elimination pathways, ft C—H and ft C—Si (3/2) eliminations14 (Scheme 12), giving rise to a lower metallole yield than from carbamates 57. 

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