Hydride Ion Shift and Transfer Reactions

According to well-established facts the hydride anion can only be shifted or transferred to the carbenium ion the onium ions themselves are no hydride anion acceptors. Nevertheless, some of the onium ions exist in equilibrium with the correspondii carbenium ions (cf. Sect. 3.1) and in this isomeric form can abstract hydride anions. The classical experiments were performed by Jaacks in the polymerization of 1,3-diox-olane initiated by methoxymethylium perchlorate  

The reaction between the methoxycarbenium ion and 1,3-dioxolane has already been discussed in Sect. 3.2. On the basis of these recent studies, the results quoted above can be explained by the following equilibria and competiiig reactions of chain transfer and chain propagation  

at lower [DXLJo, the concentration of methoxycarbenium tons is h ter than at higher [DXLJo because practically no polymerization occurs ([DXLJo [DXL]e). Eventually, all the initiator is converted into the dimethyl ether. Starting from sufficiently high [DXLJo the slow hydride transfer to the methoxycarbenium ton (kn- is probably much lower than both k and ks7, although it has not yet been measured) vanishes because the initiator is rapidly converted into the polymer end group. 

At polymer-monomer equilibrium, there may still be a small concentration of polymeric alkoxycarbenium ions existing at equilibrium with the polymeric oxonium ions. Indeed, the hydride anion transfer was documented on the basis of the slowly 

The hydride anion transfer reactions can almost be completely avoided in the polymerization of 1,3-dioxolane at lower temperature and polymers with polymerization degrees DP equal to DP (calc,)= ([MJq- [M]e)/[IIo could be obtained (rf. Sect. 4.1). Model studies of the interaction between CHsOJHj cation and cyclic and linear acetals have indeed shown that hydride anion transfer can hardly be observed below -30 Clt was also suggested that the original hydride transfer can be decreased if stronger nucleophiles (e.g. ethers) are added . 

---