Oxonium from cyclic acetals

On the other hand, when highly stabilized carbenium ion results from unimolecular ring opening, the carbenium active species may be present in the system in detectable concentrations. Indeed, in the polymerization of cyclic acetals —O -Ct + active species exist in equilibrium with tertiary oxonium ion active species (cf., Section lI.B.6.b.). 

Equilibria like (62) are strongly shifted to the side of oxonium ions e. g. the calculated heat of formation of the triethyloxonium ion from ethyl cation and diethyl ether is as high as 128 kcal mole" in the gas phase. This would give no chance for a single carbenium ion to exist at the usual polymerization conditions of the majority of heterocycles cyclic sulfides and amines are even stronger nucleophiles than cyclic ethers (cf., however, Ihe discussion on cyclic acetals below). 

If our hypothesis was correct, the oxonium ion 99, derived from dimethyl acetal 98, would rearrange more readily than oxazoline 100 to give 102. The bond between the 6-carbon and 6 -oxonium carbon in 99 can rotate freely around the axis this would facilitate the approach of the 14-OH group and its overlap with the jt orbital of the carbonyl carbon in 99 to produce the cyclic acetal intermediate 105. Then 105 could rearrange to 102 via the oxonium 106 (Scheme 34). Furthermore, we had shown that the three aforementioned general ketones (TV-benzylpiperidone (78), acetophenone (79), and 4-phenylcyclohexanone (80)) without OH groups... 

Meerwein was the first to succeed in obtaining dioxolanylium ions of type 2, sufficiently stabilized as salts with non-polarizable anions that they could be isolated crystalline. The compounds can be prepared by splitting out of an anion from cyclic ortho esters or acetals wherein the required ring-system is already present. The ortho ester 1 reacts with antimony pentachloride or boron trifluoride, with splitting out of OR, to give 2. Acetals (3) from aldehydes can be converted, by hydride abstraction with triphenylmethyl or triethyl-oxonium fluorohorate, into salts (2) this reaction proceeds well only with acetals of the 1,3-dioxolane type (3) that have little steric hindrance. With acetals of the 1,3-dioxane type, formed from aldehydes, the reaction of hydride abstraction is not, as a rule, possible. In all such reactions, the anion involved is either SbClg or BF4 . 

Cyclic acetals like this are more resistant to hydrolysis than acyclic ones, and easier to make—they form quite readily even from ketones. One explanation for this is that whenever the second oxonium ion in this mechanism forms, the hydroxyl group Is always held close by, ready to snap shut and give back the dioxolane water gets less of a chance to attack It and hydrolyse the acetal. We will discuss In entropic terms why cyclic acetals and hemlacetals are more stable in Chapter 12. 

The cationic pohmierizations of cyclic acetals are different from the polymerizations of the rest of the cyclic ethers. The differences arise from greater nucleophilicity of the cyclic ethers as compared to that of the acetals. In addition, cyclic ether monomers, epirane, tetrahydrofuran, and oxepane, are stronger bases than their corresponding polymers. The opposite is true of the acetals. As a result, in acetal polymerizations, active species like those of 1,3-dioxolane may exist in equilibrium with macroalkoxy carbon cations and tertiary oxonium ions. By comparison, the active propagating species in polymerizations of cyclic ethers, like tetrahydrofuran, are only terdaiy oxonium ions. The properties of the equilibrium of the active species in acetal polymerizations depend very much upon polymerization conditions and upon the structures of the individual monomers. 

It follows from the preceding discussion that in the polymerization of cyclic acetals (at least DXL) small but definite concentrations of alkoxycarbenium active species exist in equilibrium with oxonium active species. The equilibrium constant, measured for a model system, indicates that alkoxycarbenium ions constitute ca. 10 % of all active species in the polymerization of DXL at -78 °C in CH2CI2. This proportion may vary substantially with the conditions applied and, of course, depends on the stmcture of the monomer. To estimate to what extent the alkoxycarbenium ions participate in propagation, the rate constants of model reactions have been measured (Scheme 20). 

However, for a variety of reasons it seems extremely unlikely that the same mechanism is applicable to the polymerisation of cyclic formals and acetals. One reason is that these compounds cannot be co-polymerised with cyclic ethers another is that the polymers are predominantly cyclic, with the number of end-groups far smaller than the number of growing chains. One mechanism which has been proposed and which accounts for most of the observations involves formation of an oxonium ion (X) from the initiator and the monomer, and a subsequent propagation by a ring-expansion reaction (see 13). 

Cyclic mixed acetals with pendant diazo ketone side-chains undergo rearrangement to ether-bridged cycloheptane ring systems on treatment with Cu(hfacac)2.118 A Stevens [1,2]-shift of an oxonium ylide gives the major product (62), in some cases accompanied by minor amounts of a product (63) resulting from a [l,2]-shift of a sulfonium ylide. 

The first example of methoxyl participation reported in carbohydrate chemistry was the migration454 of a methoxyl group from C-l to C-4 during an attempted benzoate displacement reaction with 2,3,5-tri-O-benzyl-4-O-p-tolylsulfonyl-D-ribose dimethyl acetal (185). Instead of the 4-O-benzoyl-L-lyxose derivative expected, the isomeric l-0-benzoyl-2,3,5-tri-0-benzyl-4-0-methyl-L-lyxose methyl hemi-acetal (187) was obtained, presumably by way of the cyclic, oxonium ion 186. Solvolysis of the related aldehydo sugar, namely, 2,3,5-tri-O-benzyl-4-O-p-tolylsulfonyl-D-ribose, readily gives 2,3,5-tri-O-benzyl-L-lyxofuranose, and the reaction probably involves participation by the free aldehyde group.455... 

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