Sulfoxides, allenic

Following studies on the rearrangement of allylic arenesulfinates, Braverman and coworkers have investigated a number of natural extensions of this unique transformation, including the predictable [2,3] sigma tropic rearrangements of allylic sulfenates to sulfoxides and of propargylic sulfenates and sulfinates to allenic sulfoxides and sulfones respectively. The last reaction is described below, while the other two are described in Chapter 14. 

C. [2,3]-Sigmatropic Rearrangements of Propargylic Sulfenates to Allenic Sulfoxides... 

Arai Y., Koizumi T. Synthesis and Asymmetric Diels-Alder Reactions of Chiral. Alpha.,.Beta.-Unsaturated Sulfoxides Bearing a 2-Exo-Hydroxy-lO-Bornyl Group As an Efficient Ligand on the Sulfur Center Rev. Heteroat. Chem. 1992 6 202-217 Keywords allenic sulfoxide, a-sulfinylmaleate, a-sulfinylmaleimide, asymmetric synthesis, chiral unsaturated sulfoxides... 

Cutting and Parsons described the transformation of acetylenic alcohols 314 into allenyl phenyl thioethers 316 by a two-step procedure (Scheme 8.85) [174], Deprotonation of alkynes 314 with n-butyllithium is followed by addition of phenylsulfenyl chloride, forming sulfenyloxy intermediates which subsequently rearrange to allenic sulfoxides 315. Treatment of allenes 315 with methyllithium results in loss of the sulfoxide moiety to form allenyl sulfides 316 in reasonable yields. 

Direct hydrohalogenation of 1,2-allenic sulfoxides with MX in HOAc failed. A1X3 was found to be a good X- donor to afford 2-haloallylic sulfoxides in 56-89% yields. In this reaction, an Al3+ species activates the reactivity of the allene moiety by its interaction with the sulfmyl group [87, 98]. 

The sulfenate-sulfoxide and sulfinate-sulfone rearrangements are very reliable and proceed with complete syn stereoselectivity17, ls. The allenic sulfoxides can be used for the synthesis of chiral alkylallenes with retention of configuration (see Section 1.1.3.). The relative configuration at sulfur in the allenic sulfoxides is not important for further synthetic purposes and racemization at sulfur is often observed without affecting the allenic axial dissymmetry. 

Reaction with allene sulfoxides.1 Allenic sulfoxides are desulfurized by reaction with methyllithium. An example is the transformation shown in equation (I). The propynylcarbinol 1 is treated with benzenesulfenyl chloride to form the allene 2,... 

Ethynylation of the ketone 1 and treatment of the propargyl alcohol thus obtained with phenylsulfenyl chloride in the presence of triethylamine gave the expected allene sulfoxide. Treatment of this sulfoxide with aluminium trichloride in benzene gave the sulfide 2 in 15% yield. 

Scheme 12 [2,3]Sigmatropic rearrangement of propargylic trichloromethanesulfinates to allenic sulfoxides...

Synthetic Utility of the Propargylic Sulfenate-to-Allene Sulfoxide Rearrangement... 

Scheme 13 [2,3]Sigmatropic rearrangements of nitroaryl propargyl sulfenates to allene sulfoxides and their cycloaddition reactions...

A somewhat related [2,3]sigmatropic rearrangement of 6-allenic sulfoxides to vinyl sulfenates has been reported by Padwa and coworkers [65]. 

Scheme 23 /S-Allenic sulfoxides in the one-pot synthesis of functionalized 4-oxo-2-alkenoate esters...

Allenic sulfoxides can be made from propargylic sulfenates by a 2,3-sigmatropic shift. In the example reported,the product is not stable at room temperature and undergoes a 1,5-sigmatropic hydrogen shift (Scheme 56). These allenic sulfoxides can react as Michael acceptors (Scheme 57). ... 

The synthetically most useful oxygen-sulfur transpositions (equation 16) are the allyl sulfenate-allyl sulfoxide and the propargyl sulfenate-allene sulfoxide rearrangements, the driving force of both being the formation of the strong S=0 bond at the cost of the weak O—S bond (Scheme 12). ... 

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