Metalloles synthesis

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

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... 

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. 

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. 

Dihalostannoles are very useful starting materials for the synthesis of many substituted stannoles153. They also give rise to the only known group 14 metalloles having a five- or six-coordinated heteroatom in the form of anionic or cationic species152,153. 

LiAlLEt is widely used for the synthesis of metalloles or benzometalloles with M—H bonds167. This hydride does not react by attacking the diene ring. Lithium hydride formed in situ may, however, give an addition reaction to the diene system160. 

In addition to the problem of the chemical bonding in metallole anions, these species may offer the promise of rich chemistry and lead to unusual structures. One example is provided by the synthesis and characterization of a novel trisgermole complex of lithium 140199 (eqUation 83). 

Studies in the chemistry of group 14 metalloles over the past few years have been numerous. Mention should be made of the synthesis of the C-unsubstituted l//-silole the structures of the product itself, as well as of its tautomeric forms and dimer, have been established. Progress has also been made in developing new synthetic routes. By way of a transmetallation reaction from zirconacyclopentadiene, the synthesis of group 14 hete-rocyclopentadiene derivatives was considerably facilitated. On the other hand, though the method is limited to 3,4-diphenylsiloles, a further noticeable contribution was a general and versatile synthesis of the corresponding 2,5-difunctional derivatives. 

The ring systems dealt with in this chapter were reviewed in Chapter 1.20 of the first edition of Comprehensive Heterocyclic Chemistry (CHEC-I) as part of a much wider brief. Since 1985, the synthesis, organic chemistry, and physicochemical properties of these metalloles has been extensively surveyed <90CRV215>, as well as their coordination chemistry <90CRV265>. Their chemistry has also been featured in Comprehensive Organometallic Chemistry. 

The reaction of iodine with l,l-di-n-butyl-2,5-dimethylstannole leads to (Z,Z)-2,5-diiodohexa-2,4-diene, which is an interesting precursor for stibole and bismole synthesis (equation 52). By contrast, substitution of exocyclic substituents with conservation of the metallole structure has been discovered by Zuckerman and coworkers... 

Aluminacyclopentadiene as a metallole compound was not widely studied, and synthesis toward aluminacyclopentadiene via transmetallation of organolithium reagent and aluminum base compound was few. In 1977, Hoberg and... 

Dubac J, Laporterie A, Manuel G (1990) Group 14 metalloles. 1. Synthesis, organic chemistry, and physicochemical data. Chem Rev 90 215-263... 

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