Metalloles reactions

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. 

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. 

These three metalloles are the lower C-alkyl substituted Si—H bond-containing siloles which were isolated. The possible substitution of hydrogen bonded to silicon by another atom or a functional group appeared very attractive. Scheme 13 shows the functionalization reactions of 1,3,4-trimethylsilole (63) described by Dubac and coworkers75. 1 -Fluoro-1,3,4-trirncthylsilolc (66) has been identified spectroscopically and chemically,... 

The cycloaddition reactions involving group 14 metalloles have been extensively reported in the previous review123. Here we summarize the main points, and give new results. 

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 synthesis600 (equation 52). By contrast, substitution of exocyclic substituents with conservation of the metallole structure has been discovered by Zuckerman and coworkers... 

The action of acids on siloles results in the cleavage of the two endocyclic Si—C bonds. Boiling concentrated hydrochloric acid, hydrogen bromide or glacial acetic acid promote the reaction, and the substituted butadienes, in which the geometry of the parent metallole is retained, are produced in high yield34,155 (equation 55). 

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

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. 

Stannoles differ significantly from other group 14 metalloles in their ability to undergo exchange reactions with boron, arsenic or antimony halides. Boroles563,76, arsoles and... 

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. 

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. 

One of the M-Si bonds in l,l-bis(trimethylsilyl)metalloles 17 (M = Si, Ge) is cleaved by the reaction with benzyl lithium to give the monoanions 18 which are further converted to the respective hafnocenes 2 (Figure 3)... 

Reactions with alkali metals usually proceed by substitution of one or two substituents at the heteroatom of the metallole with the formation of mono- or dianionic species, respectively. Thus, 1-chlorosilole 30 reacts with Li in tetrahydrofuran (THF) at room temperature to give the monoanion 31 which can be alkylated to 32 or undergoes dimerization to the tricyclic dianion 33 <1997OM2770>. 

Metalloles easily react with dienophiles to give [4+2] cycloaddition products. A computational study of the cycloaddition reaction of silole with acetylene was performed <2002JMT(589)291>. It is shown that the silole has the lowest activation energy among five-membered conjugated heterocycles containing C, N, P, O, and S. 

Trimethylsilyl groups as substituents in metalloles can also be substituted by halogens. Reaction of compounds 89 with PyHBt3 or with ICl/AgBF4 affords 2,5-dibromo- and 2,5-diiodosiloles 90, respectively <19980M5133>. 

The reaction between 1,4-dilithiobutadienes or 2,2 -dilithiodiphenyl is a well-known and successful route to metalloles, although the choice of substituents in the 2-5 positions is somewhat limited. However, various substituents can be introduced at the metal. Thus, dilithiated butadienes 166 and 168 react with halo- or methoxy-derivatives of Si and Ge to give siloles 167 and germoles 169 in high yield <2000JCD1049, 1998JOM(559)73>. 

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. 

Organoboration reactions (8.4) give the possibility to obtain metalloles with up to six different substituents, starting from readily available alkynes (Scheme 7). 

The reactivity in Diels-Alder reactions of some mono- and polycyclic metalloles containing silicon, germanium and tin was calculated <2006NJC1149>. 

The dehydrogenation reaction of Ph2SiMc2 with Pd/C catalyst at 400° to 500° under hydrogen flow afforded biphenyl as the only isolated reaction product 312), A later successful investigation of the thermal dehydrogenation of Ph2SiMe2 involved reaction in a sealed evacuated tube at similar temperatures 90). The metallole system is evidently thermally stable but reacts in presence of a metal catalyst. 

Hi. Reactions involving l,n-diynes. If the two acetylenic groups are borne by the same carbon chain, cyclization on a E2E fragment can give a bicyclic metallole as shown in... 

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