Si=Ge double bond

Very recentiy, it has been reported that silaaromatic and germaaromatic compounds, stabilized by steric protection, react with Ss to afford the corresponding trithiametallolanes and/or pentathiametallepanes via the addition across the M-C (M=Si, Ge) double bond [94-96] (Scheme 33). 

Under photolysis of the benzene solution of 22 with a high pressure Hg lamp (X > 300 nm) for 4 h, a migration of the silyl substituent with the formation of an endocyclic Si=Ge double bond system takes place (Scheme 10). The reaction proceeds quite cleanly... 

The reaction of the mixed cyclotrimetallenes 22 and 23 with phenylacetylene also proceeds through the initial [2 + 2] cycloaddition of the first molecule of phenylacetylene across the E=Si (E = Si, Ge) double bond with the formation of the three- and four-membered bicyclic compounds 30 and 3122. But the reaction does not stop at this stage valence isomerization of the bicyclic compound takes place to form the silole type structures 32 and 33 with one Si=C and one Ge=C double bond, which in turn... 

The initially formed bicyclic compound 38 has a highly strained and very reactive bridgehead endocyclic Si—Si bond. It can easily react with a second molecule of benzaldehyde by the insertion pathway to form a new bicyclic compound 40 with a norbornane type skeleton (Scheme 14 and Figure 12). Although this last reaction closely resembles the previous case of phenylacetylene22, the mechanism is evidently different in the case of phenylacetylene the final product 36 is a result of [2 + 2] cycloaddition of the second molecule of phenylacetylene across the new Si=Ge double bond, whereas in the case of benzaldehyde the final norbornane 40 is a result of the insertion of the second molecule of benzaldehyde into the strained Si—Si single bond. Apparently, the reactions of disi-lagermirenes with phenylacetylene and benzaldehyde have the same initial steps to form bicyclic compounds, but then the reaction pathways become different due to the different nature of these intermediate bicyclic compounds. 

This fundamental discovery dramatically affected the whole chemistry of main-group elements. Subsequently, a series of new compounds with silicon element multiple bonds has been introduced. Within only a few years, stable silenes (silaethenes with a Si = C double bond) [8-11], silaimines Si = N [12-14], and silaphosphenes Si = P [15] were synthesized. As a pacemaker, silicon chemistry has exerted a strong influence on further areas of main-group chemistry a variety of stable molecules with Ge = Ge [16], P = P [17], As = As [18], P = C and P = C [19-22] bonds were subsequently isolated, and systems with cumulated double bonds P = C = P [23-25] are also known today. 

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