Silicon 4-membered heterocycles

Silicon protection is also commonly used to direct lithiation chemistry in five-membered heterocycles. For example, oxazoles , thiazoles and Ai-alkylimidazoles ° ° lithiate preferentially at C-2, where the inductive effect of the heteroatoms is greatest. If C-2 is blocked, lithiation occurs at C-5, where there is no adjacent lone pair to destabilize the organolithium. Functionalization of these heterocycles at C-5 can therefore be achieved by first silylating C-2, reacting at C-5 and then removing the silyl group. The synthesis of 666 illustrates this sort of sequence (Scheme 258) °. ... 

Figure 1 Silicon-containing five-membered heterocycles.

Figure 1 Silicon-containing five-membered heterocycles with three or four heteroatoms.

The molecular structures of several substituted l,2-diaza-3-silacyclopentenes 13 have been reported. The X-ray structure analysis of all investigated compounds reveals an envelope conformation with the silicon positioned out of the plane of the nitrogen and carbon atoms. The most important geometrical parameters of the five-membered heterocycle are (Si-C) = 187.4-190.6 pm, z/(C-C) 152.0-152.8 pm, z/(C=N) 128.0-129.5 pm, 4N-N) =... 

No systematic studies on vibrational spectroscopy of five-membered heterocycles with three or more heteroatoms and at least one tetracoordinated silicon atom were reported. 

No systematic experimental studies on thermodynamic properties of five-membered heterocycles with three or more heteroatoms and at least one tetracoordinated silicon atom were reported. The aromaticity of the fully conjugated germadisiloleanion 21, a heavy congener of the cyclopentadienyl anion, was deduced mainly from its NMR spectroscopic and structural parameters <2005JA13143>. 

In eight-membered heterocycles containing atoms of silicon and donor (D) in positions 1 and 5, a coordinate bond Si<-D can be formed to yield two condensed five-membered rings. Such diptych structures (largely with transannular interaction... 

Silylphosphines, (R3Si)3P, are well known. On the basis of electronegativity (Table 1) they are weaker n acids than organophosphines. With the isolation of R2Si=PR compounds, an increased variety of structmes is available. Six-membered silicon-phosphorus heterocycles (1) have been proposed as ligands for catalyst candidates, owing to the steric accessibility of the transition metal. 

Summary The reaction of two equivalents of lithium phosphinomethanides with di- or trifunctional chlorosilanes yields novel five- and six-membered heterocycles by multistep rearrangements or transmetallation reactions. Silaethene intermediates and hypervalent intermediates are likely to be involved. The reaction of one or two equivalents of lithium phosphinomethanide Li[C(PMe2)2(SiMe2Ph)] TMEDA with pTolSiCb and ci QHbcii iCb yields novel penta- and hexacoordinated silicon complexes. Both are the first examples of truly hypervalent organosilicon species with phosphorus donors characterized by X-ray structure determination. 

Four-membered heterocyclic systems have been postulated to be involved as intermediates along the reaction pathway in a number of systems allowing olefin synthesis. This is the case of the Wittig reaction (phosphorus) and a family of related reactions the boron-Wittig (boron), the Peterson (silicon) 44 and the Peterson-type reactions (germanium, tin, lead). 5,l46 11 these systems,... 

Tin, germanium and silicon five-membered heterocycles have been treated with arynes. Stannole 298 (X = Sn) gave naphthalene 300, presumably via the norbomadiene 299... 

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