Dialkoxycarbonium ions

Other reagents which approach or exceed the oxonium salts in alkylating ability include dialkoxycarbonium ions,35 alkyl... 

The catalysts generally used are sulfuric, hydrochloric, andp-toluenesulfonic acids, and the reaction probably involves dialkoxycarbonium ion intermediates [RC(XR )2],... 

The experimental evidence obtained (116, 117) indicates that the central carbon of the acetal function becomes positively charged during the oxidation step. Furthermore, a rather high primary isotope effect (k /k = 6.5) has been measured (121). These results indicate that the reaction mechanism proceeds either via a direct hydride transfer yielding a dialkoxycarbonium ion and a hydrotrioxide ion which would collapse to the hydrotrioxide intermediate (162 - 163 165) (Fig. 20), or via an insertion of ozone in a 1,3-... 

There is no direct experimental evidence to show the importance of the secondary electronic effects in the ester function except for the relative stability of the Z over the form. However, the relative stability of the different forms of dialkoxycarbonium ions can be explained by considering these electronic effects. Since dialkoxycarbonium ions are alkylated derivatives of esters, the result can be used as evidence to support the importance of the secondary electronic effects in the ester function. It is known from X-ray evidence and supported by calculations (8, 9) that dialkoxycarbonium ions like esters are planar and that they can exist theoretically in three different forms, the ZZ ( 0), the EZ (VO, and the EE (12) forms. The two oxygens of 10 each have one non-bonded electron pair antiperiplanar to a polar C-0 bond, 0 has one, whereas 2 has none. Thus, 10, 11, and 12 have respectively two, one and zero secondary electronic effects, and on that basis, their relative stability should follow in this order. In the ZZ form ( ] 0), there is a severe steric repulsion between the two R groups thus, with the exception of cases where the two R groups are part of a ring, this form must be eliminated. The EZ form (VO must therefore represent the most stable form of dialkoxycarbonium ions. 

These results demonstrate that conformer F is definitely not involved in the course of the hydrolysis reaction. For example, if conformer F, i.e. 83 of orthoester 78 is examined on the basis of the stereoelectronic principle, it must yield a mixture of the two hydroxy-esters 81 and 82. Indeed, conformer 83 must produce first the open-chain dialkoxycarbonium ion 84 which after hydration would give the acyclic tetrahedral intermediates 85 and 86. Since internal rotation is allowed in 85 and 86, they would then give a series of different conforrners which should fragment to give a mixture of the hydroxy-esters 81 and 82. 

The authors (73) have first observed that cyclic dialkoxycarbonium ions 153 and 154 (RsCHj or CgHg) as well as acyclic dialkoxycarbonium ion 155 (R=CH3 or CgHg) reacted with sodium hydrosulfide (NaSH) to give monothioesters 159 and 160 and thionobenzoate 161 respectively. These results show that the hemi-orthothiol esters 156, 157, 158 must be formed as intermediates in these reactions. The salts 153 and 154 (R=H) behave in a similar fashion (nmr and tic analyses), but isolation of the products (159 and 160, (R=H)) was thwarted by their high reactivity. 

Synthesis of Derivatives of Thionocarboxylic Acids.—Thiono-esters are obtained by the reaction of dialkoxycarbonium ions with NajS or anhydrous NaHS in acetonitrile (Scheme 6). Cyclic carbonium salts from 1,2- or 1,3-diols give monothiono-esters of these diols. The tetrahedral intermediates in this process may be alkylated to give... 

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