Silicon compounds sterically overcrowded

Bulky substituents at tetravalent silicon centers are the structural key element of sterically overcrowded compounds with extraordinary properties Inter- and intramolecular interactions of the bulky ligands can be studied in detail and allow an assessment of the ligands influence on structure and reactivity. 

Three additional rings systems containing endocyclic Si=Si double bonds were obtained from the tetrasilabutadiene 139. Since Diels-Alder products of this compound have as yet not been synthesized, probably because of the steric overcrowding and the large 1, 4-separation of the terminal silicon atoms, it was surprising to find that the action of sulfur on 139 resulted in a formal [4 + 1] cycloaddition to furnish the thiatetrasilacyclopentene 154 in high yield (equation 39)142. 

Hagele, G.. Goudetsidis. S., Wilke, E., Seega, J., Blum, H., and Murray, M., Synthesis and properties of compounds related to l-terf-butylacetylene-2-phosphonic acid and l-teri-butylethane-l,2,2-triph-osphonic acid. Sterically overcrowded phosphoms compounds. Part 1, Phosphorus, Sulfur Silicon Relat. Elem., 48, 131, 1990. 

Cycloadditions. Although 6 exists in the solid state in the s-gauche form, which should favor [4+2] cycloadditions of the Diels-Alder type, to date all attempted reactions with alkenes, acetylenes, and the C=0 bonds of ketones have remained unsuccessful. The reasons for this are probably the steric overcrowding in 6 as well as the large 1,4-separation of 540 pm between the terminal silicon atoms. Thus, it was at first surprising to find that compound 6 reacted with maleic anhydride through a formal [4+4] cycloaddition to furnish the tetracyclic compound 7. ... 

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