NNNS Chemistry blog
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24 June 2019 - Orgo
In chemistry silicon to oxygen chemical bonds are nothing special, just look at siloxanes. But whereas a single bond is common, a double bond would require some counseling between the two. In a so-called silanone R(R)Si=0 a large difference in electronegativity is in the way and orbital mixing is generally considered just not that good. But an impossible bond to one sometimes means an excellent research opportunity to another. In this case Kobayashi et al. report (DOI) they have cracked it and made an actual stable and uncoordinated silanone. The new silanone is an improvement from an earlier prototype synthesized in 2015 that was stable at -80°C for several hours at best. The strategy is one of kinetic stabilization and in the 2015 effort a cyclic silanone was handled 4 bulky substituents (SiMe2iPr) but apparently not bulky and certainly not inert enough as an isomerization reaction takes place.
In the 2019 iteration the 4 bulky groups are 1,3-di-tert-butylaryls that do the job much better. In the final reaction step the precursor was a silylene (the silicone carbene analogue) and the oxidizing agent nitrous oxide. Measured Si=O length 1.518 A compared to 1.63 A for a single Si-0 bond. The introduction of the aryls had one distinct disadvantage that of overcrowding the spectroscopy. The compound is stable at RT but heating resulted in head-to-tail dimerisation.