Sulfonate tethers

Both the sulfone-tethered enyne 426 and diene 427 undergo RCM to give 5,6-dihydro-2//-thiopyran 1,1-dioxides when treated with a second generation Grubbs mthenium catalyst 425 tagged with an ionic liquid (Scheme 136) <2005JOM(690)3577>. 

Scheme 10-22 A sulfonate tether affords cycloadducts stereo- and regioselectively.

Metz et al. have also demonstrated the use of sulfonate tethers in the preparation of the bicyclic sesquiterpene lactone, ivangulin 68 [25f]. The sulfonate derived from vinyl sulfonyl chloride and diene 69 underwent smooth IMDA reaction affording a mixture of exo cycloadducts 70 and 71 in a 1 1.4 ratio. Heating this mixture in toluene at reflux allowed... 

Scheme 10-23 A sulfonate tether strategy was used in Winteifeldt s synthesis of myltaylenol.

Scheme 10-36 Sulfoxide and sulfone tethers can also be used in [5 + 2] cycloadditions.

Skrydstrup, Beau and co-workers122 have adapted Stork s method to the SmI2-reduction of glycosyl pyridyl sulfones bearing a silicon-tethered unsaturated group at HO-C(2). An example is shown with the synthesis of methyl a-C-zso-maltoside 172 from alkyne 170 via the 5-exo-dig radical cyclization of 171 (Scheme 56).144... 

Allenes as versatile synthons including Diels-Alder reactions and especially intramolecular cycloadditions of this type were reviewed by Aso and Kanematsu [338], In some cases of intramolecular Diels-Alder reactions of open-chain starting materials such as 340 [339], 342 [339] and similar acceptor-substituted allenes [156], the formation of two new six-membered rings seems to be favorable if possible (Scheme 7.48). The non-activated cumulated C=C bond of 340 takes part in the [4+ 2]-cycloaddition and hence the necessary reaction temperature is high. On the other hand, the progressive truncation of the tether and the electron deficiency of the allenic C=C bond involved give rise to a remarkable Diels-Alder reactivity of the sulfone 346 generated in situ from sulfoxide 345 [339]. 

In the case of PEMs, the situation is more complicated because the sulfonate counter-ions (in the case of a PEM such as Nafion ) are bound to the polymer chain and are thus relatively immobile, in contrast to the free counter-ion in a small molecule acid such as sulfuric or acetic acid. Tethering of the sulfonate group can be considered to be an impediment to the mobility of the proton as it traverses the membrane. Proton mobility is also affected by the effective mean-free path of connectivity of the conduction pathway as shown in Figure 3.2. In situation (a), the increased number of dead ends and tortuosity of the aqueous domains through which proton transport occurs over the situation in (b) leads to lower overall mobility. This has been demonstrated by Kreuer and will be discussed later in this section. 

Catalysts 75c and 76a also indnce cyclization of a variety of aliphatic snbstrates for the construction of tetrasubstitnted carbon centers in good yields and high enantioselectivities (Scheme 12). Despite the snccess of carbon, nitrogen and oxygen tethers, sulfide side chains have proven resistant to cyclization nnder optimized conditions. By changing the linker to a sulfone 87, cyclization was accomplished in 98% yield, albeit 80% ee. 

A similar process is possible using samarium diiodide. With this reagent carbohydrate radicals are generated through a one-electron reduction of, for example, anomeric phenyl sulfones. The so-induced cyclization of the tethered sugar 59 gives compound 60, which has been transformed to a C-disaccharide [31]. 

The high acidity of the unique sulfonic acid function of Nafion can also be utilized by tethering perfluoroalkanesulfonic acid groups to the surface of various silicas. Two approaches have been described to prepare such hybrid organic-inorganic materials. The grafted materials 3 have been prepared by reacting preformed silica materials (MCM-41, SBA-15) with sultone 2 [Eq. (2.31)].165 Co-condensation of tetraethoxy-silane with 4 in the presence of dodecylamine template, in turn, furnished the HMS-based material 5 [Eq. (2.32)].166,167... 

Furthermore, a four-component cycloaddition reaction (7]6-thiepine I,l-dioxide)tricarbonylchromium(0) 52 with tethered diynes under photoactivation afforded pentacyclic adducts formally derived from a sequential [671+271]/ [67l+27t]/[2(T+27i] cycloaddition process <1999OL507>. Photocycloaddition of the complex 52 with excess 1,7-octadiyne 66a or 1,8-nonadiyne 66b (C1CH2CH2C1, hv (Pyrex filter)) afforded the pentacyclic triene sulfones 67a and 67b in 45% and 38% yields, respectively (Equation 4). In contrast, 1,6-heptadiyne 66c afforded only the three-component cycloadduct 68 in 56% yield (Equation 5). 

The first use of /3-hydroxysu I lone-based tethers in an intramolecular Diels-Alder reaction provides a novel route to fused 1,4-oxathiepinone sulfone derivatives the endo-adduct is favored (Equation 7) <20050L3203>. 

Van de Coevering et al. [32] utilized ion-pairing interactions to tether transition metal complexes with sulfonated anionic tails to cationic dendrimeric supports (Fig. 8). In these assemblies, eight anionic organometallic Pd-... 

Due to the relatively slow addition of aryl radicals to substituted benzenes [7], inter-molecular Gomberg-Bachmann type arylations often require favorable reaction conditions in which the substrate arene is used as solvent to be efficient. To achieve good control and selectivity in the addition step, radical biphenyl synthesis can alternatively be conducted in an intramolecular fashion by taking advantage of a removable tether. In the first examples, sulfonamides, sulfonates [130], and azetidinones [131] were employed as linkers between the two aryl moieties. Recently, phosphinates [132] and siloxanes [133] have been used for this purpose (Scheme 25). 

X = CH2, 31h-k) and sulfones (Y = S02. X = CH2, 311,m) participated effectively in the oxidative coupling reaction. Dibenzyl ethers (Y = O, X = CH2, 31n-r) were also coupled in fair to good yields. Cleavage of the temporary tether subsequently delivers the acyclic biaryls 33a-r. 

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