The Pummerer rearrangement

The Pummerer reaction of sulfinyl compounds involves the formation of an o-functionalized sulfide [244,245] from a sulfoxide. Acetic anhydride is commonly used as the electrophile, which adds to the sulfoxide to yield a sulfonium salt, and the rearrangement occurs through successive formations of an ylide (rate-determining step) and an alkylidene sulfonium, trapped by a nucleophile, or stabilized by a proton loss with formation of a vinyl sulfide. 

Good precursors of carbonyl compounds are formed in these processes and many applications deal with such syntheses. A comprehensive review has appeared in a 1991 issue of Organic Reactions [246] (479 references). The synthesis of n-octanal from n-octyl phenyl sulfoxide is described [246], 

Repeated use of the Pummerer reaction has been made in the systematic syntheses of all L.-hcxoscs [247]. The accompanying scheme summarizes the steps yielding the protected erythro- and threo-aldehydes from 4-benzhydryloxy-( )-2-butene-l-ol (Ph2CHOCH2-CH=CH-CH2OH) (1) via (2) and (3). 

New examples of application appear regularly. In the following one [248] phenylsulfenylation of nopinone (4) was followed by oxidation to the sulfoxide. Pummerer rearrangement of the latter led directly to the a-phenylthio substituted a-unsaturated ketone (6) (overall yield 94%), starting material for further elaboration of natural sesquiterpene synthesis via 4,4-disubstituted nopinones (7). 

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All of these ehimnddon reacdons contain fi-carbonyl groups in the nltro compounds Of course, masked carbonyl groups are also frequently employed for such fi-elimination of HNO, as shown in Eq 7131, Eq 7 133, and Eq 7 133In these cases, the sulfinylmethyl or hydroxymethyl group is converted into the carbonyl group by the Pummerer rearrangement or by simple oxidation... 

Recently, the Pummerer rearrangement has been employed also in the 2//-selenopyran synthesis (93CC577). 

The differing nucleophilicity of acetate and trifluoroacetate anion determined the manner in which naphtho[l,8-/yt]-l,5-dithiocinc sulfoxide 127 rearranged on treatment with acetic and trifluoroacetic anhydrides. In both cases, the reaction proceeded through formation of a disulfonium dication 128, but the final products were different. When acetic anhydride was used, the reaction afforded the corresponding a-acetylsulfide 130, a normal product of the Pummerer rearrangement, while trifluoroacetic anhydride caused isomerization with formation of dithioacetal 132 (see Scheme 16) <1995HAC559>. 

Monosulfoxide 13 undergoes the Pummerer rearrangement when treated with acetic anhydride in the presence of sodium acetate.85 The experiments with tetradeuterated and 180-labeled sulfoxide confirm intermediate formation of a dication.86 The ratio of 2,8,8-trideuteriated to 4,4,6,6-tetradeuteriated product 37 is equal to the intramolecular isotope effect ku k0 =1.7 (Scheme 21).85... 

Reaction of -dimethylthiobenzene sulfoxide 56 with trifluoroacetic anhydride results in a mixture of sulfide 65 and the corresponding mono- and disubstituted products of the Pummerer rearrangement 63, 64 via intermediate disulfonium dication 62 (Scheme 23).88... 

Can the organic chemists associated with so-called Named Reactions make the same claim as supermodel Heidi Klum Many scholars of chemistry do not hesitate to point out that the names associated with name reactions are often not the acmal inventors. For instance, the Arndt-Eistert reaction has nothing to do with either Arndt or Eistert, Pummerer did not discover the Pummerer rearrangement, and even the famous Birch reduction owes its initial discovery to someone named Charles Wooster (first reported in a DuPont patent). The list goes on and on... 

Oxindoles are prepared using a solid-phase Pummerer reaction. Thus, the immobilized amide 182 undergoes the Pummerer rearrangement on treatment with TFAA and boron trifluoride diethyl etherate (BF3 Et20) (Equation 118). Cleavage of the oxindole from the linker is achieved using samarium iodide and 1,3-dimethyl-3,4,5,6-tetrahydro-2(lH)-pyrimidinone (DMPU) <2003CC2380>. 

Thieno[2,3-c]furans have also been prepared in situ by the Pummerer-rearrangement cyclization reaction (96JOC6166). For transient generation of thieno[2,3-c]furans see also Kuroda et al. [91JCS(CC)1635]. These compounds proved to be reactive intermediates for inter- and intramolecular Diels-Alder reactions (see Section IV). 

The Pummerer rearrangement has also been used to transform the 4-tolylsulfinyl group, which is an effective chiral auxiliary, into other functionalities via the aldehyde. Thus, chiral /> -(ben-7yloxy)-/J-(fluoroalkyl) sulfoxides 11 have been transformed into aldehydes 12 in a two-step process.7,8 The 3,3-difluoro-substituted aldehydes 12 can be converted directly into a variety of other functional groups without isolation. The chiral center at C2 is derived from the corresponding chiral / -oxo sulfoxide, which can be reduced diastereoselectively due to the presence of the chiral 4-tolylsulfinyl group.7,8 Diastereoselective reduction of fluorine-free fi-oxo sulfoxides has been intensively studied, especially in the laboratories of Solladie.9... 

Methods for the construction of the thieno[2,3-c]pyridine skeleton based on the formally simultaneous formation of both the pyridine and thiophene rings were documented. Under the Pummerer rearrangement conditions, ( -s ul liny lain idc 222 underwent a cascade transformation into 223, which was oxidized to fused lactam 224 in low yield (1999JOC2038). Data on the use of cascade transformations, including the Pummerer rearrangement - cycloaddition sequence, in the synthesis of complex heterocyclic systems were summarized in a review (1997S1353). 

One generally applicable method of oxidation for alkyl bromides involves the Pummerer rearrangement. This gives aldehydes upon rearrangement of the alkyl aryl sulfoxide formed by displacement of halide by thiolate followed by oxidation (equation 41)358. 

The chemical reactivity of sulfoxides as compared with sulfides is much greater. The effect of the sulfinyl group on adjacent methylene protons allows chlorination and the Pummerer rearrangement to take place. The chlorination is stereospecific, resulting in cis products. The Pummerer rearrangement results in two possible isomers (Scheme 2) (55). The Nuphar sulfoxides can be epimerized on carbon C 7 by thermal rearrangement (see Section IV, Scheme 1). 

Thioethers of azines are generally oxidizable to sulfones. By analogy (466) can be oxidized to its sulfone and the intermediate sulfoxide may be isolated. The latter, when subjected to the Pummerer rearrangement, yields the 2-acetoxy derivative (467), which on acid catalysis eliminates acetic acid to form the thiazole (79CPB1207). 

The first step is just the SN2 displacement of Cl- by RS that you have already seen. The second step actually involves chlorination at sulfur (you have also seen that sulfides are good soft nucleophiles for halogens) to form a sulfonium salt. Now a remarkable thing happens. The chlorine atom is transferred from the sulfur atom to the adjacent carbon atom by the Pummerer rearrangement. 

There are many variations on the Pummerer rearrangement but they all involve the same steps a leaving group is lost from the sulfur atom of a sulfonium ylid to create a cationic intermediate that captures a nucleophile at the a carbon atom. Often the starting material is a sulfoxide. 

The Pummerer rearrangement of sulfoxides with acid anhydrides has been extensively utilized as a method for the synthesis of a-substituted sulfides. When a,(3-unsaturated sulfoxides are used, the initial formed oxysulfonium ion may undergo two different pathways the additive Pummerer reaction or the vinylogous Pummerer reaction. The following sections will consider examples from both pathways. 

The cyclization of the Pummerer rearrangement product 30 derived from o-methylsulfinyl-difluorostyrene 29 leads to 2-fluorobenzo[ ]thiophene 31 (Scheme 7) <1997CC1537>. Treatment of thiol 32 with NaH affords 2-fluoro-4,5-dihydrothiophene 33 by a cyclization <2000CC1887>. 

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