Germylenes rearrangements

Keywords Bis(hypersilyl)silylene / Bis(hypersilyl)germylene / Rearrangement / Three-Membered Rings... 

Another type of thermal rearrangement has been observed with dimeth-ylgermene 1526 leading to germylene 48 and then, probably via a germirane intermediate, to germylene 4926 [Eq. (12)]. 

Photochemical rearrangements of transient germadienes 3136 and 3236 have been evidenced from the corresponding germole or germylene (see Section II,A,6). 

Intramolecular Lewis base complexation of the germanium atom also did not appear to be a valuable alternative to overcome the lability of these elusive intermediates, since the germanones expected from oxidation of the base-stabilized germylenes 140131 (with Me3NO) and 141132 (with 02, DMSO, or pyridine oxide) either rearrange as already mentioned [Section VI,A,4, Eq. (33)]131 or dimerize to digermadioxetane 160132 [Eq. (35)]. 

SCHEME 1. Putative rearrangement paths of germylenes and silylenes bearing silyl or germyl substituents. Proposed intermediates are given in brackets... 

Quantum-chemical calculations have been used to probe all the characteristic chemical reactions of CAs (at least in the case of silylenes and germylenes). The theoretical studies cover intramolecular rearrangements, insertions into 0-bonds, additions to double and triple bonds and dimerizations. Note that experimental data on the mechanisms of these reactions are still scarce and the results of theoretical studies are needed to understand the main trends in the reactivity of germylenes, stannylenes and plumbylenes. 

Intramolecular rearrangement of germylenes, stannylenes and plumbylenes into a doubly bonded isomer can be another cause of their intrinsic instability. For example, in the matrix IR study of l-germacyclopent-3-en-l,l-ylidene, direct experimental evidence was obtained for a photochemical germylene-germene isomerization105,357. 

In the case of the FpGeMe2SnMe3, this compound rearranges prior to loss of the group 14 carbene analog to form FpSnMe2GeMe3. This illustrates the more or less equal stability of the stannylene and germylene intermediates, as may be expected. 

The platinum germylene complex 67 has been extensively studied and an array of interesting chemistry has been developed. The chemistry, outlined in equation 58, is predominantly addition to the M=Ge bond, e.g. to give 68-71. The rearrangement of the 1,2-dioxametallocycle in 68 to the 1,3 isomer 72 is understood in terms of forming two M—O bonds at the expense of the relatively weak 0—0 bond136-139. 

The reaction of the stable germylene 32 with a triarylmethylium ion gave not the expected germylium ion 33, but in a series of unexpected rearrangements, the germy-lium ion 34, which is intramolecularly stabilized by interaction of the electron-deficient germylium center with a remote aryl group (Scheme 16).80... 

The lower heats of formation of silylenes compared with carbenes translate into somewhat lower, albeit similar, reactivity. There is a general lesson here that slowing down the reactions of carbene-like species makes possible processes like dimerization that are more or less impossible for carbenes. We have already seen that slowing down their rearrangements allows the observation of intermolecular reactions of alkylsilylenes not observed for alkylcarbenes. The dimerization of silylenes to disilenes, discovered in the gas phase [36], was employed to brilliant effect by West and coworkers in the condensed phase synthesis of isolable disilenes [37]. While these were not the first disilenes, silylene dimerization opened the way for the preparation of many new molecules, including digermenes from the dimerization of germylenes [38]. 

Silylenes are very similar to germylenes, and can be prepared in the same way. The reaction involves a carbene-to-carbene rearrangement, as does the similar germylene reaction. 

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