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i) Li/naphthalene Ö) Br-(CH2)8OTHP
59% one pot, 3 steps
Table 4.3 gives some examples of the dehydrative alkylation-annulation of bis-sulfones.
TABLE 4.3 Synthesis of sulfones by alkylation-annulation
Reactants Product Yield (%)
, S02Ph .—-v .SOzPh
4.3 C=C BOND-FORMING REACTIONS
4.3.1 Use of sulfides and dithioacetals
Boekelheide and his collaborators  have described a two-step sequence for transforming sulfide linkages to carbon-carbon double bonds — Stevens rearrangement of sulfur ylides and Hofmann elimination — which they found particularly useful for the synthesis of cyclophane derivatives, such as the [2.2]metaparacyclophane-l,9-diene shown. The Ramberg-Backlund rearrangement (see Section 4.3.2) was unsatisfactory for such highly strained molecules.
I) methylation Ø base
As mentioned in Section 18.104.22.168, dithioacetals can be used in coupling reactions with Grignard reagents in sequences leading to the formation of carbon-carbon single and double bonds. The metal-catalysed reactions depicted here exemplify their uses for olefination reactions, which opened the way to many useful synthetic applications .
A “sulfur tandem ancillary effect” was involved to explain the facile cleavage of both carbon-sulfur bonds in those reactions. The use of cyclopropylmagncsium bromide as the Grignard reagent in the nickel-catalysed reaction with dithioacetals was rewarding. It led to a novel synthesis of butadienes through a ring-opening process and 0-hydride elimination of the organonickel species intermediary formed .
The “desulfurdimerization” reaction of dithioacetals proceeds via a thioketone intermediate.
Indeed, metal carbonyls were also reported to react with thioketones to yield dimeric olelins [31Ó, 409].
4.3.2 Sulfone-based eliminations
Sulfones have been shown to be versatile intermediates for the introduction of carbon-carbon double bonds . One of the best known routes to do so is the Ramberg-Backlund reaction of an a-halosulfone with an appropriate base.
R*R2CH-S02-CXR3R4 „ r1r2 CX /c^R4
±2. R'RY/CRSR4 — rVc=crW so2
This reaction was discovered as early as 1940  and has since received considerable attention. Many reviews deal with its synthetic aspects (see , ,  and , and references therein for discussion of its mechanism and utility).
Recently, intermediary episulfones have been isolated from low-temperature Ramberg-Backlund reactions of a-iodosulfones [414—416], opening the way to new perspectives for synthetic applications via these episulfones, although a-iodosulfones are not the most convenient a-halosulfones to prepare and handle.
In the example given here, the episulfone, prepared at -78°C from the iodosulfone, is converted into the alkene under more forcing conditions.
Preparation of an episulfone
A stirred solution of 2-(2'-iodo-2'-methanesulfonyl)ethyl-2-methyl-5.5-dimethyl-l,3-dioxane (76mg, 0.21 mmol) in dry THF (8ml) at -78°C
under nitrogen was heated with freshly sublimed potassium t-butoxide (28 mg, 1.2 eq) in THF (2 ml). After 5 min at -78°C, the reaction was quenched by the addition of saturated aqueous ammonium chloride solution. The mixture was diluted with water and given a standard dichloromethane work-up. Purification by column chromatography on silica (dichloromethane as the eluent) gave the episulfone as a viscous oil (40mg, 83%), R.; = 0.16 in dichloromethane.
From  with permission.
Analogous chemistry has been reported with a-chlorosulfoxides, though much less studied. Initial •y-dehydrochlorination yields thiirane 5-oxides, but from these intermediates two alternative pathways can occur leading to sulfenates and/or alkenes. The method was shown to be efficient for the synthesis of tetrasubstituted alkenes .
Another sulfone-based elimination of large applicability was proposed by Julia [418, 419]. Condensation of a metalled phenyl alkyl sulfone with a carbonyl compound, functionalization of the alkoxide and reductive elimination leads to an olefin.