Reductive desulfonylation, of vinyl sulfone

The reductive desulfonylation of vinylic sulfones has been mainly carried out with sodium and aluminum amalgams, Sml2, and Mg in M,Al-dimethyl-formamide (DMF) in the presence of chlorotrimethylsilane (TMSC1). The configuration of the double bond is not necessarily preserved in the desulfonylation of vinylic sulfones when Na/Hg is employed. This reagent is the most widely used when there is no need for stereochemical control as shown in Eqs. 2139 and 22.40... 

Table 7. Reductive Desulfonylation of Vinyl Sulfones (Continued)...

Reductive Desulfonylations by Tin Hydrides. Reductive desulfonylation of allyl, vinyl, and a-functionalized sulfones can be carried out employing tin hydrides. This radical reaction is usually promoted thermally or photochemically and provides organotin derivatives as intermediates which are finally subjected to protonolysis (Eq. 32). Both steps can be carried out in one pot employing catalytic amounts of tin. 

A highly stereoselective synthesis of 1,2-disubstituted alkenes can be also accomplished via reductive desulfonylation of 1,2-disubstituted vinylic sulfones since both E- and Z-vinylic sulfones can be stereoselectively prepared by several methods.8 Although different reducing agents have been used for this purpose (Eqs. 24-28), the reduction of 1,2-disubstituted alkenyl sulfones by Na2S2C>4 is particularly efficient and highly stereoselective (Eq. 168). 

Moreover, two important variations of the Julia reaction were reported during the 1990s. As both procedures afford the final alkene product directly from the sulfone and carbonyl components, they shall be discussed in greater detail in a subsequent part of this chapter (Section 3.5). In this section, attention is focused on the reductive desulfonylation of various 8-oxygenated sulfones and of vinyl sulfones. Insights into the advantages and disadvantages of the various protocols, some mechanistic considerations, and the stereochemical outcome of the reductive elimination step are provided. 

From a mechanistic viewpoint, the reduction of vinyl sulfones and the reductive elimination of /f-acetoxy sulfones with Na(Hg) or Sml2 are the only procedures to have been investigated to any great extent. Recent work published by Keck et al. [105] has shown that different mechanisms are operative when Na(Hg)/MeOH or Sml2/HMPA are employed. They set out to study the reductive desulfonylation of several vinyl sulfones under two sets of reaction conditions, specifically 8 equiv. Sml2, N,N-dimethylpropyleneurea (DMPU), MeOD, 60 min. and 5% Na(Hg), Na2HP04, THF/MeOD (4 1), 0 °C, 60 min (Table 3.5). 

As a group, annulation reactions have been exceedingly valuable to the synthetic organic chemist. Unfortunately, processes of this type involving simple alkenes and cycloalkenes are few. However, the facility with which unactivated olefins can be transformed into vinyl sulfones, the high degree to which a, 8-unsaturated sulfones are captured regioselectively by unsymmetrical dienes such as those developed by Danishefsky, and the ease with which reductive desulfonylation can be effected, combine to permit convenient synthetic entry to substituted cyclohexenones. Several representative examples can be found in Table 1. 

Site and stereocontrol of the reductive desulfonylation reactions of allylic and vinylic sulfones depend on the choice of the reducing agents and reaction conditions. With allylic sulfones migration of the double bond to the most stable position is usually observed with all the reagents investigated (Eq. 19).37 The migration of the double bond takes place with little or no stereocontrol, and different results can be obtained depending on the method employed as depicted in Eq. 20 for vinylic sulfones.38... 

Use of Lithium Naphthalenide. Lithium arene radical anion complexes are mild and highly effective reagents for the reductive desulfonylation process of functionalized sulfones. These reagents have only rarely been used with vinylic and allylic sulfones. In addition to high yields and their operational simplicity, metal arene radical anion complexes demonstrate high chemoselectivity (Eq. 67).123... 

Use of Samarium(II) Iodide. In recent years, samarium(II) iodide has become a popular choice as a single-electron-transfer reagent for mild and selective reductive desulfonylations. This reagent is employed for the reduction of a wide variety of functionalized sulfones, principally 3-keto and vinylic sulfones (Eq. 89).149... 

Desulfonylation. A convenient and economical preparation of phenyl vinyl sulfone involves the reduction of (Z)-l,2-bis(benzenesulfonyl)ethene with Bu.SnH. 

Some years later, further developments by Alexakis et al. have shown that prolinol derivative 22a (10 mol%) is a better catalysts for the reaction [89]. The results vary from good, for conjugate addition of linear aldehydes to l,l-bis(benzenesulfonyl) ethylene (77-90% yields 76-93% ee), to moderate (12-91% ee) when using a-branched aldehydes as nucleophiles. Catalyst 22a has been also used by Palomo et al. for the enantioselective conjugate addition of linear and 3-branched aldehydes to E-a-ethoxycarbonyl vinyl sulfones and -a-cyano vinyl sulfones [90], derivatives that after further transformations, which usually involve a reductive desulfonylation process [91], have made possible the synthesis of different interesting chiral building blocks. 

Use of Sodium Hydrogen Telluride. Certain a-functionalized-a,(3-unsaturated sulfones are desulfonylated using sodium hydrogen telluride in ethanol at room temperature.164-1663 u - Me thy I (hu Mi, j3-unsaturated sulfones are reduced to give vinylic thioethers in good yields albeit with moderate selectivities (Eq. 97).164 On the other hand, a-methylsulfonyl chalcones suffer tandem reduction-desulfonyla-tion in the presence of DMF as cosolvent.165,166a... 

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