Oxonium ions, secondary tertiary

With secondary and tertiary alcohols Ihis slage is an 8 1 reaclion m which Ihe alkyl oxonium ion dissociates to a carbocalion and water... 

A protonic acid derived from a suitable or desired anion would seem to be an ideal initiator, especially if the desired end product is a poly(tetramethylene oxide) glycol. There are, however, a number of drawbacks. The protonated THF, ie, the secondary oxonium ion, is less reactive than the propagating tertiary oxonium ion. This results in a slow initiation process. Also, in the case of several of the readily available acids, eg, CF SO H, FSO H, HCIO4, and H2SO4, there is an ion—ester equiUbrium with the counterion, which further reduces the concentration of the much more reactive ionic species. The reaction is illustrated for CF SO counterion as follows ... 

Thus the quantity of EtOH in the hydrolysate is equivalent to the number of tertiary oxonium ions any secondary oxonium ions react with EtO to give EtOH which is removed before hydrolysis of the polymer. 

Hetero-cations, such as secondary or tertiary oxonium ions can be formed easily by the addition of a proton or a carbenium ion to an ether ... 

Although this work is still incomplete, it shows that some tertiary oxonium ions are formed in the reaction, but that by far the greater part of the active species are secondary oxonium ions. The origin of the tertiary oxonium ions, which yield the involatile phenyl ether by reaction with C6H5CT, is not at all clear at present. Some may be formed from an impurity in the monomer and others may arise from a slow side-reaction. 

The results obtained by Jaacks s ethoxide method, shown in Tables 1 and 2, prove that for all systems the concentrations of tert-oxonium ions are very considerably smaller than those of the perchloric acid. In view of the correlations shown above, they must also be much smaller than the concentrations of ions in the polymerising solutions. We conclude, therefore, that the principal growing ions are not tertiary, and that they must, therefore, be secondary. 

This type of initiation is limited hy the nucleophilicity of the anion A derived from the acid. For acids other than the very strong acids such as fluorosulfonic and triflic acids, the anion is sufficiently nucleophilic to compete with monomer for either the proton or secondary and tertiary oxonium ions. Only very-low-molecular-weight products are possible. The presence of water can also directly dismpt the polymerization since its nucleophilicity allows it to compete with monomer for the oxonium ions. 

Combinations of a Lewis acid, protogen or cationogen, and a reactive cyclic ether (e.g., oxirane or oxetane) have been used to initiate the polymerization of less reactive cyclic ethers such as tetrahydrofuran [Saegusa and Matsumoto, 1968]. Initiation occurs by formation of the secondary and tertiary oxonium ions of the more reactive cyclic ether, which then act as initiators for polymerization of the less reactive cyclic ether. The reactive cyclic ether, referred to as a promoter, is used in small amounts relative to the cyclic ether being polymerized and increases the ability of the latter to form the tertiary oxonium ion. 

Resonance, similar to that in pyrylium salts, was shown594,595 to exist between oxonium ion (299a) and carbenium ion (299b) forms in alkylated ketones, esters, and lactones that were obtained via alkylation with trimethyl- or triethyloxonium tetra-fluoroborates596 [Eq. (3.78)]. Ramsey and Taft597 used H NMR spectroscopy to investigate the nature of a series of secondary and tertiary carboxonium ions (300-302). 

Cyclic ethers, acetals and cyclic amines belong to the group of hard bases while cyclic sulfides can be considered as soft bases. On the other hand proton is the hardest acid whereas alkyl cations are much softer. Thus, in good agreement with the concept of hard and soft acids and bases (the alike ones react faster giving more stable products), secondary oxonium ions react slower with monomer than their tertiary oxonium counterparts. The same is observed for cyclic amines whereas secondary sulfonium ions apparently react faster with cyclic sulfides than the corresponding tertiary sulfonium ions do l. 

Chemical shifts of protons in secondary oxonium ions differ substantially ftom chemical shifts of protons in the primary hydroxy groups. One can expect a fast proton exchange between these two spwies. However, if the individual chemical shifts are known, then the observed chemical drift (due to exchange) permits the determination of the actual proportions of the condary oxonium ion IV.l and tertiary oxonium ion IV.4. 

The simultaneous presence of secondary and tertiary oxonium ions in the polymerization of 1,3-dioxolane initiated with trifluoromethanesulfonic acid was alas shown by trapping a proton (from the secondary oxonium ion) and a carbenium ion (from either the tertiary oxonium ion or the alkoxycarbenium ion) by the ion-trap-ping technique. 

Propagation may proceed by both secondary and tertiary oxonium ions but the latter are known to be much mote reactive. The respective rate constants have not yet been determined. 

Polymerization initiated by protonic acids additionally involves an equflibrium between secondary and tertiary oxonium ions, with proporttons of both kinds de-pendii on the chain length. For the longer chains, the proportion of the tertiary oxonium ions increases. 

In the polymerization of several cyclic acetals and ethers autoacceleration was observed. There might be a number of reasons for this behaviour, and some of these have already been discussed, namely the preinitiation equilibria and the inequality ki enhanced reactivity of the hydroxy end group in polyacetals toward active species (in comparison with acetal groups) is another, not yet considered, reason for induction periods. When the pol5mierization d ee increases with conversion the proportion of the active tertiary oxonium ions also increases, at the cost of the less reactive secondary oxonium ions (cf. Ref. 164), because the b k-biting or intermolec-ular transfer become more important than the end-biting. Thus, there b no need to make speculative assumptions about the nature of the active species and to propose the two stage polymerization of acetals in order to explain the induction... 

A variety of initiator systems of the types used in the cationic polymerization of alkenes (Chapter 8) can be used to generate the tertiary oxonium ion prpoagating species. Strong protonic acids such as sulfuric, trifiuoroacetic, fluorosulfonic, and trifluoromethanesulfonic (triflic) acids initiate polymerization via the initial formation of a secondary oxonium ion ... 

Protonic acids are efficient initiators for the polymerization of both sulfides and amines. The polymerization of thiiranes initiated with perchloric acid proceeds without induction periods. Induction periods are present, however, with methyl fluorosulfonate initiator 11). Secondary sulfonium salts are more reactive than tertiary ones (the opposite is true with oxonium ions)12) and induce rapid polymerization ... 

The relative contribution of the specific chain growth mechanism (i.e., activated monomer vs. oxonium ion addition) may depend on ring strain of monomer, nucleo-philicity of anion and solvating power of solvent (ability to stablize ions). Many of these factors have been quantiatively determined in the polymerization of cyclic ethers and acetals, where the concentrations of the tertiary and secondary oxonium ions were simultaneously determined by the phosphine cation-trapping method (cf. Adv. Polymer Sci. 37). This method seems to be also applicable in the polymerization of siloxanes, but has not yet been evaluated. 

With secondary and tertiary alcohols, this stage is an S l reaction in which the aUcyl-oxonium ion dissociates to a carbocation and water. 

Acid-catalysed addition of primary, secondary, and tertiary alcohols to 3,4-dihy-dro-2//-pyran in dichloromethane at room temperature is the only general method currently in use for preparing THP ethers and the variations cited below concern the choice of acid. The reaction proceeds by protonation of the enol ether carbon to generate a highly electrophilic oxonium ion which is then attacked by the alcohol. Yields are generally good. Favoured acid catalysts include p-toluenesulfonic acid or camphorsulfonic acid. To protect tertiary allylic alcohols and sensitive functional groups such as epoxides, the milder acid pyridinium p-toluenesulfonate has been employed (Scheme 4.316]. A variety of other acid catalysts have been used such as phosphorus oxychloride, iodotrimethylsilane- and bis(trimethylsilyl)sulfate. but one cannot help but suspect that in all of these cases, the real catalyst is a proton derived from reaction of the putative catalysts with adventitious water. Scheme 4.317 illustrates the use of bis(trimethylsilyl)sulfate in circumstances where other traditional methods failed. - For the protection of tertiary benzylic alcohols, a transition metal catalyst, [Ru(MeCN)2(triphos)](OTf)2 (0.05 mol%) in dichloromethane at room temperature is effective. ... 

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