Sulfinic acid elimination

Primarily, Michael addition and intramolecular aldol addition gives rise to pyrrolidine-2-carboxylic esters 52, which are converted into pyrrole-2-carboxylates 53 by successive H2O and sulfinic acid eliminations. 

Zwanenburg and Wagenaar have reported the rather unusual rearrangement of sulfone 81 to 82, after standing overnight at 0°, and suggested an elimination-addition mechanism, via initial isomerization of A to the A -thiazoline-oxide with subsequent elimination and readdition of sulfinic acid, followed by spontaneous loss of water in a Pummerer-type aromatization reaction. 

Os(II) complexes of tosylmethylisocyanide react with aldehydes and ketones in the presence of sodium methoxide, producing cyclic carbene complexes (66). Compound 32 undergoes a rapid reaction with benzal-dehyde and NaOMe, yielding the oxazol-2-ylidene complex 33 and eliminating p-toluene sulfinic acid (66) ... 

Scheme 36. Elimination of sulfinic acid from 2-deoxy-2-C-p-tolylsulfonyl-/i-D-ara moHexopyranosyl p-tolyl sulfones promoted by silica.

A variant that eliminates the production of water and that has proved effective for esterification of hydroxy and aromatic amino acids involves the use of thionyl chloride instead of acid. At a low temperature, the alcohol reacts with the chloride, generating methyl sulfinyl chloride, which produces the ester, probably through the mixed carboxylic acid-sulfinic acid anhydride (Figure 3.18, B). p-Toluenesulfonyl chloride added to the acid and benzyl alcohol serves the same purpose in the preparation of benzyl esters. 

Many examples are found in the literature in which the reaction of an a-sulfonylcarbanion with an electrophile is followed by a base-induced elimination of a sulfinic acid. 

These are good dienophiles. and aryl vinyl sulfones have found use as equivalents of ethylene and ketene through functional modifications of their adducts. However, as the base-induced elimination of a sulfinic acid to yield an olefin occurs only with difficulty, they are not direct precursors of acetylene equivalents, unless suitably modified as in ( )-l-phenyl-sulfonyl-2-trimethylsilyl ethylene (PhS02-CH=CH-TMS). In its cycloadducts the elimination to an alkene is smoothly realized by the fluoride ion. If an alkylation step is previously carried out on the adduct, the overall process realizes an indirect addition of a terminal acetylene, as in the examples given here [533]. 

Elimination of sulfinic acid from ketosulfones was also a key step in an expedient synthesis of a,0-unsaturated ketones via the Michael adducts of sulfonyl carbanions to nitroalkenes. Conversion of the resulting nitronates into ketones was most conveniently realized by ozonolysis. DBU-promoted elimination of sulfinic add completed the process. Overall yields range from 71 to 88% for the examples reported [425]. 

Elimination (for the term see Section 4.1.1) of ortho-nitrobenzene sulfinic acid (F) leads from D to the N=N double bond of compound E. Because of the functional group C-N=N-H E is called a diazene. Diazenes assume a middle position between azo compounds, which contain a C-N=N-C group, and the NH compound diimide (sometimes called chi-mine H-N=N-H). 

Unlike many other type of radical addition reactions, the product is most often an alkyl-cobalt(III) species capable of further manipulation. These product Co—C bonds have been converted in good yields to carbon-oxygen (alcohol, acetate), carbon-nitrogen (oxime, amine), carbon-halogen, carbon-sulfur (sulfide, sulfinic acid) and carbon-selenium bonds (equations 179 and 180)354. Exceptions to this rule are the intermolecular additions to electron-deficient olefins, in which the putative organocobalt(III) species eliminates to form an a,/ -unsaturated carbonyl compound or styrene353 or is reduced (under electrochemical conditions) to the alkane (equation 181)355. 

The tosyl compound reacts with aldehydes in the presence of potassium carbonate to yield 5-alkyl- or 5-aryl-oxazoles, the intermediate dihydrooxazoles (which can be isolated) eliminating toluene-p-sulfinic acid (Scheme 30). Use of acyl chlorides in place of aldehydes leads to 4-tosyloxazoles (288). Furthermore, alkylation of tosylmethyl isocyanide with an alkyl halide RfX, followed by treatment with an aldehyde R2CHO, yields a 4,5-disubstituted oxazole (289). A related reaction is that of A-tosylmethyl-iV -tritylcarbodiimide with aromatic aldehydes under phase-transfer catalysis to yield 2-tritylaminooxazoles which are readily converted into 2-amino-5-aryloxazoles (equation 117) (81JOC2069). 

The reaction of arylsulfonyl-substituted sulfines (175) with diazomethane gives A3-l,3,4-thiadiazoline 1-oxides (176) which, however, are unstable and rearrange via an isomerization of the A3- to the A-thiadiazoline oxide. This is followed by an elimination and readdition of sulfinic acid and loss of water in a Pummerer-type aromatization to give the rearranged thiadiazole (177 Scheme 24) (73TL5009). 

Thione groups can often be eliminated by oxidation probably the sulfinic acid is the intermediate. Sometimes the sulfinic acid can be isolated (e.g., 740 741), but more often it spontaneously loses SO2. In this way, thiazoline-2-thiones give thiazoles, l,2-dithiole-3-thiones 742 are converted into 1,2-dithiolylium salts 743, l,3-dithiole-2-thiones 744 into 1,3-dithiolylium salts 745, 1,5-disubstituted imidazole-2-thiones into imidazoles <2003JHC229>, and 3-mercapto-l,2,4-triazoles into the parent triazole <2006S156>. In the pyrazole series, 746 also loses an A-methyl group to yield 747. 

Lactols and their acetals can be transformed easily into their 2-arylsulfonyl derivatives 337 by reaction with a sulfinic acid under Lewis acid activation. The corresponding organolithiums are prepared by deprotonation with n-BuLi or LDA and, after reaction with electrophiles, a /(-elimination of sulfinic acid afforded a cyclic a-substituted enol ether514,547,548. 2-Lithio-2-(arylsulfonyl)tetrahydropyrans equilibrated to give mainly the anomer with the lithium atom at the equatorial position549. 

Removal of the mercapto (or thione) function is most commonly achieved using Raney nickel, although concentrated HCl at high temperatures or nickel boride have been employed. Thiohydantoins are transformed into imidazolidin-4-ones by sodium in amyl alcohol (70JHC439). Oxidative desulfurization procedures too are often used the reactions probably proceed through the unstable sulfinic acid species which very readily eliminates sulfur dioxide. The high resistance of sulfonic acids to acid hydrolysis renders unlikely the possibility that they are reaction intermediates. 

Until recently, dihydrothiazine oxides had not found much use in synthesis. Recently, Weinreb and coworkers have exploited some of the known reactions of these adducts, along with some new transformations, in stereoselective preparation of some complex nitrogen-containing molecules. One useful transformation of these adducts is the hydrolysis/retro-ene elimination of sulfur dioxide shown in equation (53). Thus dihydrothiazine oxide (120), prepared from ( , )-tetramethylbutadiene, underwent hydrolysis to allylic sulfinic acid (121) which suffered a retro-ene reaction via the chair-like conformation... 

The kinetics of chromium(III)-catalysed oxidation of formic acid by Ce(IV) in aqueous H2SO4 can be rationalized in terms of initial formation of an outer-sphere complex involving oxidant, catalyst, and substrate (S), Ce(IV)(S)Cr(III), followed by an inner-sphere complex Ce(III)(S)Cr(IV). It is proposed that electron transfer occurs within this complex from substrate to Cr(IV) (with elimination of H+) followed by fast reaction to give CO2 (again with elimination of H+). " In contrast, there was no kinetic evidence for the accumulation of a corresponding inner-sphere intermediate in the osmium(VIII)-catalysed Ce(IV) oxidation of DMSO to dimethyl sulfone here, the observed rate law was rationalized in terms of rate-determining bimolecular electron transfer from DMSO to Os(VIII) in an outer-sphere step. The kinetics of oxidation of 2-hydroxy-1-naphthalidene anil by cerium(IV) in aqueous sulfinic acid have been... 

In a separate study of the decomposition of sulfinic acids in the absence of solvent at 200 °C the major products were sulfur dioxide and alkenes . Minor products were water, carbon dioxide, carbon monoxide, carbonyl sulfide and sulfur. The reaction is considered to proceed by a unimolecular free radical mechanism, although kinetic evidence is lacking. Olefin formation results from transfer reactions followed by elimination and one plausible pathway is... 

Toluene-p-sulfonylmethyl isocyanide (32) ( Tosmic ) reacts in a base-induced cycloaddition (in protic medium) with aldimines. Toluene-p-sulfinic acid is eliminated to yield 1,5-disubstituted imidazoles (33 = H) in... 

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