Onium compounds, function catalysts

A new class of compounds, macrocyclic polyethylenoxysilanes, called silacrowns have been prepared which demonstrate phase transfer catalytic properties. An alkoxy functional silacrown has been immobilized in a single-step reaction on a siliceous support. The immobilized silacrown also demonstrates phase transfer catalytic properties. A functionalized onium phase transfer catalyst was also prepared that reacts directly with a siliceous support and is catalytically active. 

Crivello has published extensively on onium compounds and their function as catalysts (3,4). Generally, such compounds are photocatalysts, but are stable in the absence of light. Irradiation at the proper wavelength leads to a complex photodecomposition of the onium ion in which reactive cation radicals and neutral radicals are formed as short lived intermediates. Also formed are acids, corresponding to the anion of the onium compound, which catalyze the epoxy polymerization. Equations 2 and 3 show the decomposition of iodonium cations. 

While most of the initial studies have involved the transition metal-catalyzed decomposition of a-carbonyl diazo compounds and have been reviewed [3-51], it appears appropriate to highlight again some milestones of these transformations, since polycyclic structures could be nicely assembled from acyclic precursors in a single step. Two main reactivities of metalo carbenoids derived from a-carbonyl diazo precursors, namely addition to a C - C insaturation (olefin or alkyne) and formation of a ylid (carbonyl or onium), have been the source of fruitful cascades. Both of these are illustrated in Scheme 27 [52]. The two diazo ketone functions present in the same substrate 57 and under the action of the same catalyst react in two distinct ways. The initially formed carbenoid adds to a pending olefin to form a bi-cyclop. 1.0] intermediate 58 that subsequently cyclizes to produce a carbonyl ylide 59, that is further trapped intramolecularly in a [3 + 2] cycloaddition. The overall process gives birth to a highly complex pentacyclic structure 60. 

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