Allylic sulfinates

Allyl sulfones formed from allyl sulfinates (cf. equation 1) can easily tautomerize to give a, /J-unsaturated sulfones26 in cases for which R1, R2 are part of an (hetero) aromatic system, this tautomerization occurs spontaneously. Similarly, sulfinic acid esters from jV-phenylhydroxamic acids as reactive intermediates rearrange to give o-(major part) and p-sulfonylanilines (minor part)27 ... 

Baldwin and coworkers73 have used the allylic sulfinate to sulfone rearrangement (13 to 14) for the development of a convenient route to the dihydroxyacetone unit, an... 

As a continuation to the studies by Darwish and Braverman on the [2,3]-sigmatropic rearrangement of allylic sulfinates to sulfones, and in view of its remarkable facility and stereospecificity (see Chapter 13), Braverman and Stabinsky investigated the predictable analogous rearrangement of allylic sulfenates to sulfoxides, namely the reverse rearrangement of that attempted by Cope and coworkers . These authors initiated their studies by the preparation of the claimed allyl trichloromethanesulfenate using the method of Sosnovsky . This method involves the reaction between trichloro-methanesulfenyl chloride and allyl alcohol in ether at 0 °C, in the presence of pyridine (equation 6). 

Asymmetric Rearrangment and Kinetic Resolution of Allylic Sulfinates... 

Scheme 2.1.4.12 Synthesis and Pd(0)-catalyzed rearrangement of allylic sulfinates to allylic sulfones.

The bisphosphane BPA was selected as a ligand for the Pd atom because of the high enantioselectivities which had been recorded in the allylation of sulfinate ions with racemic allylic carbonates (vide supra). Besides the variations of the carbon skeleton, the substituent on the S atom of the racemic allylic sulfinates was varied in order to see whether both aryl and alkyl sulfinates are amenable to a highly selective rearrangement. 

Scheme 2.1.4.14 Pd(0)/BPA-catalyzed enantioselective rearrangement of allylic sulfinates.

The Pd-catalyzed allylic alkylation of sulfinate ions, thiols, and thiocarboxylate ions with racemic cyclic and acyclic allylic esters in the presence of bisphosphane BPA generally provides for an efficient asymmetric synthesis of allylic sulfones, sulfides, and thioesters. The Pd-catalyzed rearrangements of allylic sulfinates and allylic O-thiocarbamates, both of which proceed very efficiently in the presence of BPA, are attractive alternative ways to the asymmetric synthesis of allylic sulfones and allyUc thioesters also starting from the corresponding racemic alcohols. 

This topic has been discussed in a review on rearrangements of sulfones129. The first attempts to thermally convert allylic sulfinates to sulfones were reported as early as 1950130. Later, it was demonstrated that this rearrangement is a general reaction occurring under thermal or solvolytic conditions and the mechanistic and stereochemical features of this conversion were elucidated. 

The rearrangement of unsymmetrically substituted allylic sulfinates involves simultaneous isomerization of the allylic group. Full inversion of configuration is observed in the conversion of (4/0-4-(2,4-dimethylphenylsulfinyIoxy)-2-pentene to the (4S)-product l131. This conversion is also the first example of a C - C 1,3-chirality transfer accompanied by C—S bond formation. 

A report on the chirality transfer from sulfur to carbon in the allylic sulfinate to sulfone rearrangement has appeared in which a series of 5-configurated (E)- and (Z)-l-(4-methylphen-ylsulfinyloxy)-2-alkenes 2 were converted to the sulfones 3132. 

Scheme 16 Sulfur to carbon chirality transfer with enantioenriched allylic sulfinates...

Scheme 38 Mixed allylic sulfinates via S-alkylation of sulfoxylates...

Scheme 39 Mixed allylic sulfinates by alcoholysis of dialkoxy disulfides...

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... 

When the allylic sulfinamides are hydrolyzed under special conditions (classical methods like alkali-or acid-catalyzed hydrolysis did not work properly) the obtained allylic sulfinic acids are unstable and will fragment smoothly again with allylic rearrangement into terminal alkenes. 

This transformation probably proceeds through an allylic sulfinic acid which loses S02 via a retro-ene process. The mechanism of this reaction was first investigated by Mock and Nugent50 and was examined further by Weinreb et a/.5152... 

A number of allylic compounds, including allyl vinyl ethers, allyJic azides, allylic thiocyanates, and allylic sulfinates, undergo unimolecular thermal isomerization reactions. Most of these reactions are concerted, four-center processes and in most of them the allylic substituent, as well as the allylic carbon skeleton, undergoes alteration. 

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