1.3- dioxolan-2-ylium ions from 1,3-dioxolane

The initiation with some carbocations, especially trityl, does not involve direct addition to monomer. The carbocation abstracts a hydride ion from the a-carbon of monomer and the newly formed carbocation initiates polymerization [Afsar-Taromi et al., 1978 Kuntz, 1967]. This hydride ion abstraction is so facile with 1,3-dioxolane that it is used to preform stable l,3-dioxolan-2-ylium salts (XII) that can be used subsequently as initiators [Jedlinski et al., 1985],... 

When a strong electrophile, such as a triphenyl methylium ion, abstracts a hydride ion from the 2-position of 1,3-dioxolan, the product is the 1,3-dioxolan-2-ylium or (for short) dioxolenium ion as shown in Equation (5.3) ... 

An interpretation of the oxidation of acetals by chromium trioxide is summarized in Scheme 1 the reagent could initially remove a proton from the acetalic carbon atom, to give a dioxolan-(or dioxan-)2-ylium ion, rapidly hydrolyzed to an ester (formyl, acetyl, or benzoyl) of an a-alcohol, which would be further oxidized to a ketone [see Scheme 1, path (a)] in the case of methylene acetals, in the presence of acetic anhydride (which acts as a water scavenger), the intermediate is further oxidized, probably through a chromic ester. 

Further evidence for the intervention of l,2-dioxolan-2-ylium ions has been given. First, a rather stable, tribromide aroyl ion was isolated from the reaction of the p-nitrobenzylidene acetal of norbomanediol... 

More recently Penczek and his collaborators (134) have used l,3-dioxolan-2-ylium salts to initiate the polymerisation of THF in a variety of solvents. In these systems initiation is clean and efficient, and in the case of carbon tetrachloride as solvent, the only significant ionic species present are ion pairs. Rate constants /c for propagation in this solvent were obtained directly from conversion/time data and are a measure of the reactivity of ion pairs. At 25° C (tTHF]0 = 8.0 M) the value of /c (4.0 x 10 2 M-1 sec-1) was independent of the counter-ion employed, in the series AsFg, PFg, and SbFg. 

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