Methanesulfonate anion

The pATa of ethanol is 16, and that of acetic acid is 4.8. The increased acidity of acetic acid relative to ethanol can be rationalized in terms of delocalization of charge in the acetate anion, whereas in ethoxide the charge is localized on oxygen. Even more delocalization is possible in the methanesulfonate anion, and this is reflected in the increased acidity of methanesulfonic acid (pAla — 1-2). 

We have also shown some representations of acetate and methanesulfonate anions that have been devised to emphasize resonance delocalization these include partial bonds rather than double/single bonds. Although these representations are valuable, they can lead to some confusion in interpretation. It is important to remember that there is a double bond in these systems. Therefore, we prefer to draw out the contributing resonance structures. 

The reaction of 2,3-0-isopropylidene-5-0-mesyl-D-lyxofuranose (8) with sodium methoxide proceeds similarly. The main product, methyl 2,3-0-isopropylidene-y3-L-ribofuranoside (10), is formed (with its an-omer) by opening of the furanose ring, elimination of the primary, 5-methanesulfonate anion (inversion not observable) by 0-4 alkoxide ion to form epoxide 9, and attack at C-l by methoxide to form an 0-1 alkoxide ion which attacks C-4 with inversion. Some attack at C-5 gives minor proportions of methyl 2,3-0-isopropylidene-/3-i>lyxo-pyranoside and its anomer (11). The formation of l,5-anhydro-2,3-0-isopropylidene-/J-D-lyxofuranose (l,4-anhydro-2,3-0-isopropylidene-/3-D-lyxopyranose), 12, occurs to only a minor extent55,56 (see Scheme 2). 

The expected relative ordering of the reactivity of methyl sulfonic acid and dimethyl sulfone may presumably be accounted for in terms of inductive effects a more detailed study of the influence of solution pH on the rate of reaction of OH with the methanesulfonate anion might be of use here. 

Figure 11.7. Crystal structure of the [Ag4(2)4]4+ cage with the inclusion of one methanesulfonate anion inside the cage. Hydrogen atoms and anions outside the cage have been omitted for clarity.

Hsiao et al. (1992) patented a preparation of bis-methy-lene ether-linked bispyridiniiun compoimds (Figure 66.11). Bis(methanesulfonoxymethyl)ether was used to form a linker between pyridine rings, and methanesulfonate anions were replaced by chloride anions. 

Table I. Experimental and Simulated Values for the Percentages of the Labeled Methanesulfonate Anions...

The methanesulfonate anion (H3C—s—o ) is less basic than the methoxide... 

The synthesis of 5-arylthianthrenium salts, 8, [CRI 02a, CRI 04] can be carried out by the condensation of thianthrene-S-oxide with various aromatic compounds bearing electron-donating substituents, R, as depicted in equation [2.1], A mixture of methanesulfonic acid and phosphorous pentoxide serves as an acid catalyst and dehydrating agent for this reaction. Replacement of the methanesulfonate anion by a metathetical reaction with NaMtXn affords the desired active photoinitiator, 8. Depending on the specific R substituent, 5-arylthianthrenium photoinitiators have UV absorption maxima in the range of 300-400 nm. 

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