Enol sulfonates allylic anions

Sodium hydride removes a proton from the sulfone to give an anion you can represent as enolate. Displacement of mesylate gives an allyl silane converted into an allyl anion with fluor Addition to the ketone gives the 5/5 fused system, necessarily with cis stereochemistry. 

Allyl anions with electron-withdrawing and anion-stabilising functional groups form anions much more easily and then react (mostly) in the a-position. There are many examples of this sort of substituent including extended enolates (chapter 11), and allyl anions stabilised by phospho-nium 159, phosphonate 160, sulfone 161, and cyanide 162 functional groups. All of these prefer to react in the a-position (at least kinetically) and we shall explore some of these. 

Stable enolates such as diethyl malonate anions react with allyl sulfones (or acetates) in the presence of nickel complexes to give a mixture of the a- and /-product83. The regioselectivity is generally poor in the nickel-catalyzed reaction, but the molybdenum-catalyzed reaction is selective for alkylation at the more substituted allylic site, thereby creating a quaternary carbon center84. 

It appears that the ketone enolates add to the vinyl sulfone, followed by a condensation that leads to the thiane sulfone. The latter may be desulfonylated to provide olefins Similarly, enamino vinyl sulfones (345) can undergo a thermally allowed electrocyclic reaction between the termini of the enaminic double bond and the allyl sulfonyl portion in the intermediate anion (346) to afford a, ) -unsaturated thiene dioxides (348) as shown in equation 126 ... 

Treatment of 1,3-dicarbonyl compounds with DBP in a methoxide/methanol system affords 2-alkyl-4-[(phenylsulfonyl)methyl]furans, where reaction proceeds by Initial addition-elimination on the vinyl sulfone moiety. In contrast, silyl enol ethers in the presence of silver tetrafluoroborate resulted in products derived from Sn2 displacement at the allylic site.11 Anions derived from 1,3-dicarbonyls substituted at the C-2 position are found to induce a complete reversal in the mode of ring closure.12 The major products obtained are 3-[(phenylsulfonyl)methyl]-substituted cyclopentenones. The internal displacement reaction leading to the furan ring apparently encounters an unfavorable Ai -interaction in the transition state when a substituent group is present at the 2-position ol the dicarbonyl compound. This steric Interaction is not present in the transition state leading to the cyclopentenone ring. 

It is possible to enhance the rate of a Claisen rearrangement, especially in the enolate Claisen reaction. Denmark et al. showed that other carbanionic centers accelerate the Claisen rearrangement (as in Table 11.23).467 Generation of the anion of sulfone 631 (sec. 8.6.A) with various bases led to acceleration of the reaction relative to the thermal reaction of 631 and also influenced the syn/anti ratio (632/633). In general, a donor group at the allyl position accelerates the rate and the presence of an amino stabilizing group increases the rate even more. 

Alkylation of enolate anions is achieved readily with alkyl halides or other alkylating agents. Both primary and secondary alkyl, allyl or benzyl halides may be used successfully, but with tertiary halides poor yields of alkylated product often result because of competing elimination. It is sometimes advantageous to proceed by way of the toluene-p-sulfonate, methanesulfonate or trifluoromethane-sulfonate rather than a halide. The sulfonates are excellent alkylating agents and can usually be obtained from the alcohol in a pure condition more readily than... 

With secondary and tertiary allylic halides or sulfonates, reaction of an enolate anion may give mixtures of products formed by competing attack at the a- and y-positions (1.6). Addition of the enolate anion to a TT-allylpalladium complex provides an alternative method for allylation (see Section 1.2.4). 

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