L,3-Dioxolan-2-ylium ions

Cyclic cations such as 4-oxo-l,3-dioxolane-2-ylium ion 313 and 4-oxo-l,3-dioxane-2-ylium ion 314 can be prepared in intramolecular reaction of a-acetoxyacyl chlorides616 or p-acetoxyacyl chlorides617 with SbCl5. Meerwein s... 

An acetal group readily participates in hydride transfer reactions because the resulting carboxonium ion is stabilized by electron pairs of the two neighboring oxygen atoms. Thus if compounds capable of acting as hydride ion acceptors, such as (triphenylmethyl)carbenium ions, are used as initiators, initiation of DXL polymerization proceeds through intermediate l,3-dioxolan-2-ylium ion, which is a real initiator, as shown in Scheme... 

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],... 

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. 

Surprisingly, the equilibration studies consistently show a substantial proportion of the -D-arabino ion (75), as indicated by a corresponding proportion of tetra-0-acetyl-/3-D-arabinopyranose obtained after hydrolysis and subsequent analysis of the product mixture. The /3-D ion (75) cannot be formed through acetoxonium rearrangement of 72, but must arise through subsequent anomerization of the a-D ion (74) formed initially, because the acetoxonium salt functions as an effective catalyst for anomerization If solutions of anomerically pure aldopentopyranose tetraacetates in nitromethane are treated with 2-metiiyl-l,3-dioxolan-2-ylium hexachloroantimonate (2,R=Me), a rapid onset of anomeric equilibration is observed. Under these conditions, tetra-O-acetyl-D-arabinopyranose gives an a /3 ratio of 9 91, and tetra-O-acetyl-D-xylopyranose, an a /3 ratio of 96 4. 

CH2CI2, 25 °C), while an attack of another monomer molecule on an oxonium ion is apparently slow. Thus, instead of simple initiation by addition, as shown in Scheme 13, hydride transfer takes place and l,3-dioxolan-2-ylium salt and triphenylmethane are formed quantitatively as shown in Scheme 8. 1,3-Dioxolan-2-ylium salts, which are relatively stable and can be isolated as pure crystalline compounds, are thus the tme initiators, formed in situ in the DXL-PhsC A system. 

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