Tertiary Oxonium Ions

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

Cationic ring-opening polymerization is the only polymerization mechanism available to tetrahydrofuran (5,6,8). The propagating species is a tertiary oxonium ion associated with a negatively charged counterion ... 

Initia.tlon. The basic requirement for polymerization is that a THF tertiary oxonium ion must be formed by some mechanism. If a suitable counterion is present, polymerization follows. The requisite tertiary oxonium ion can be formed in any of several ways. 

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 ... 

Propa.ga.tlon, The tertiary THF oxonium ion undergoes propagation by an S. mechanism as a result of a bimolecular colHsion with THF monomer. Only colHsions at the ring a-carbon atoms of the oxonium ion result in chain growth. Depropagation results from an intramolecular nucleophilic attack of the penultimate chain oxygen atom at the exocycHc a-carbon atom of the oxonium ion, followed by expulsion of a monomer molecule. 

The relationship between 9 and its predecessor 10 is close. Oxidation of the allylic C-3 methylene group in 10 and elimination of the methoxy group could furnish enone 9. Retrosynthetic disassembly of ring E in 10 furnishes tertiary alcohol 11 as a viable precursor. That treatment of 11 with a catalytic amount of acid will induce the formation of a transient oxonium ion at C-12 which is then intercepted by the appropriately placed C-4 tertiary hydroxyl group is a very reasonable proposition. As we will see, the introduction of the requisite C-4 hydroxyl group is straightforward from intermediate 12. 

Chojnowski and co-workers have studied the polymerization of octamethyltetrasila-l,4-dioxane, a monomer more basic than cyclosiloxanes, which is capable of forming more stable oxonium ions, and thus being a useful model to study the role of silyloxonium ions.150-152 In recent work, these authors used Olah s initiating system and observed the formation of oxonium ion and its transformation to the corresponding tertiary silyloxonium ion at the chain ends.153 The 29Si NMR spectroscopic data and theoretical calculations were consistent with the postulated mechanism. Stannett and co-workers studied an unconventional process of radiation-initiated polymerization of cyclic siloxanes and proposed a mechanism involving the intermediate formation of silicenium ions solvated by the siloxane... 

The thermochemical analysis for reaction (III), written for the general tertiary oxonium ion R30+, gives us ... 

A closely related method was used by Jaacks and his collaborators [22] to determine the concentration of tertiary oxonium ions (IV) in the polymerisation of dioxolan. They did this by killing the reaction with EtONa, hydrolysing the polymer and determining the EtOH derived from the killing agent by gas-chromatography ... 

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 ... 

The short-stop method of measuring [Pn ] was developed to produce an easily determinable end-group in systems in which the natural end-groups are unsuitable for accurate, quantitative determination. It was first used by Jaacks et al., (1968) to determine the concentration of tertiary oxonium ions during the polymerisation of 1,3-dioxolan, and by Saegusa et al., (1968) for similar studies on tetrahydrofuran. For the polymerisation of alkenes it has only been used on two occasions. 

Another possibility is that the ion (II) reacts with an oxygen of the main chain to give a tertiary oxonium ion which is part of a macrocycle, and that the linear fragment forming part of this tertiary oxonium ion is then transferred as a new ion of type (II) to a monomer molecule. 

Jaacks and co-workers [17], however, maintain that in these polymerisations the propagating species is a tertiary oxonium ion (VII), and they consider the propagation to be essentially the same as in the polymerisation of tetrahydrofuran, as is shown in Reaction (C). 

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 object of the work described was to discriminate between the two principal rival theories concerning the polymerisation of 1,3-dioxacycloalkanes by anhydrous perchloric acid, the Mainz theory and the Keele theory . By means of Jaaks s method for determining tertiary oxonium ions we found that in polymerisations under the driest conditions the concentration of these is negligibly small. This was done with 1,3-dioxolane (1), 4-methyl-l,3-dioxolane (4), and 1,3-dioxepane (5), and the findings are supported by determinations of the content of hydroxy groups of polymers prepared and killed under different conditions. 

Water causes the formation of tertiary oxonium ions and this probably explains why previously other workers and ourselves had reported their presence, and sometimes their dominance, in reaction mixtures prepared under much less stringent conditions. 

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. 

Since it is now evident that extremely rigorous drying of all reagents is essential to prevent the formation of tert-mm in these systems, all previous work done under less stringent conditions, which purported to prove that tert-oxonium ions are essential features of the polymerisation initiated by anhydrous perchloric acid, must be discounted. In other words, we see no reason to deny the existence of tert-oxon um ions in those systems in which Jaacks and his collaborators alleged that they found them indeed, considering their experimental conditions in the light of what we have found out since then, it is entirely credible that in some of their studies the tertiary ions were dominant. 

Upon hydrolysis of the product or of the analogous compounds formed from DCA oligomers, only the ethyl group attached to the formal group is released as ethanol. Thus the presence of ethanol in the hydrolysate signals the presence both of tertiary oxonium ions and of hemiformal hydroxyls. Evidently the number of tert.-oxonium ions cannot exceed the number of anions in the system, i.e., the acid concentration, but there is no... 

The Polarography of Some Oxonium Ions in Methylene Dichloride. Part I Experimental Techniques and Results for Two Tertiary Ions. P.H. Plesch and F.G. 

The Polarography of Oxonium Ions, Part II. The Half-wave Potentials of Five Tertiary Ions. G.E. Holdcroft, Kabir-ud-Din, and P.H. Plesch, Journal of Chemical Research, 1980, 390-391. 

In the initial step of the polymerization, a cyclic oxonium ion is formed by transfer of an alkyl group from the initiator to the cyclic ether. Propagation occurs by SN2 attack of a monomer molecule at a ring a-methylene position of the cyclic tertiary oxonium ion, followed by opening of the oxonium ring and formation of a new cyclic oxonium ion. 

We observe the a-methylene carbons of the methyl tetrahydro-furanium ion, the a-carbons of the two types of propagating chain heads, the macroion and the macroester (17). The observation of the a-methylene carbon resonances of the acyclic tertiary oxonium ion provides a direct proof of chain transfer reaction in THF polymerization. 

Propagation in the cationic polymerization of cyclic ethers is generally considered as proceeding via a tertiary oxonium ion, for example, for the polymerization of 3,3-bis(chlor-omethyl)oxetane (R = CH2C1)... 

A variety of initiator systems, of the types used in the cationic polymerization of alkenes (Sec. 5-2a) can be used to generate the tertiary oxonium ion-propagating species [Dreyfuss and Dreyfuss, 1969, 1976 Inoue and Aida, 1984 Penczek and Kubisa, 1989a,b]. 

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. 

Since the tertiary oxonium ion is the propagating species, preformed oxonium ions such as triethyloxonium tetrafluoroborate can be used for initiation (Eq. 7-28) [Meerwein et al., I960] ... 

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. 

Initiation consists of protonation of monomer followed by subsequent reaction with monomer to form the tertiary oxonium ion LXXXI (Eq. 7-109). Propagation for the ring-opening... 

THF can be polymerized only with cationic initiators, for example, boron trifluoride or antimony pentachloride. The initial step consists of the formation of a cyclic oxonium ion one of two activated methylene groups in the a-position to the oxonium ion is then attacked by a monomer molecule in an S 2-reaction, resulting in the opening of the ring. Further chain growth proceeds again via tertiary oxonium ions and not, as formerly assumed, via free carbonium ions ... 

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