Onium compounds, function

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. 

Virtually all of the reported work to date has been with organic species such as amines and amino-acids, which can be dissolved in the form of salts and then form onium compounds by ion-exchange with cations on the clay surfaces. The main variables are the number of chains attached to the nitrogen of the amine, (i.e., is it a primary, secondary, tertiary or quaternary amine ), the length distribution and branching of these chains, whether they are simple hydrocarbons or carry some other functionality such as carboxylic acid or unsaturation and finally, their concentration on the filler surface. 

Some compounds show meso-phases between the solid and liquid phases. These phases are classified into two kinds, namely liquid crystals in which the molecules have orientational order and disorganized position in one or more dimensions, and plastic crystal in which the molecules have organized positions and orientational disorder. Although the component ions in ILs are largely disordered, the appearance of liquid crystalline or plastic crystal phases could be the function of ion structures, when component ions have a tendency towards orientational or positional ordering by alignment of the ions and/or interaction among ions. Onium salt-type plastic crystals have been reported by MacFarlane [12,39]. 

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. 

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. 

A task specific ionic liquid (TSIL) is defined as an ionic liquid which incorporates a covalently linked reactive functionality. This function confers to the assembly additional properties (chemical, optical, magnetic, physical or biological) which can then be employed to perform a specific task. If the melting points of these salts are above 100 °C they are called task specific onium salts (TSOSs) [17]. The latter constitutes a much larger family of compounds that include TSILs and all associations of cations and anions, regardless of their melting points. 

The rate at which various onium salts undergo photolysis is a function of the cationic portion of the molecule. Manipulation of the absorption characteristics of these compounds through the introduction of various substituents can exert a profound effect on their apparent photosensitivity. These effects can be clearly seen in... 

The compound has functional groups that support dimerization type crosslinking and cationic polymerization upon UV exposure (A, = 300—360 nm).. Photodimerization of the chalcone-epoxy compound was confirmed by UV-visible and IR absorbance changes of the C=C double bond of the chalcone unit. Additions of small amounts of onium salts will also photoinitiate cationic polymerization of the epoxy groups present in the above chalcone-epoxy compound by exposine to UV. This ultra-violet light cured chalcone-epoxy compound was reported to possess excellent thermal stability and compares well with conventional UV-cured Bisphenol A type epoxy resins. (see Chapter 3)... 

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