Thermal syn eliminations

Another important family of elimination reactions has as its common mechanistic feature cyclic TSs in which an intramolecular hydrogen transfer accompanies elimination to form a new carbon-carbon double bond. Scheme 6.20 depicts examples of these reaction types. These are thermally activated unimolecular reactions that normally do not involve acidic or basic catalysts. There is, however, a wide variation in the temperature at which elimination proceeds at a convenient rate. The cyclic TS dictates that elimination occurs with syn stereochemistry. At least in a formal sense, all the reactions can proceed by a concerted mechanism. The reactions, as a group, are often referred to as thermal syn eliminations. 

Furthermore, as described by Mori and coworkers, the domino aldol/cyclization reaction of the 3-keto sulfoxide 2-422 with succindialdehyde (2-423) in the presence of piperidine at r.t. afforded the chromone 2-424 which, on heating to 140 °C, underwent a thermal syn-elimination of methanesulfenic acid to provide 2-426 in 22 % overall yield (Scheme 2.100) [227]. This approach was then used for the synthesis of the natural products coniochaetones A (2-425) and B (2-427) [228]. 

Ab initio and density functional calculations of the thermal syn elimination transition states for E reaction of organic amine oxide, sulfoxide, and phosphoxide have confirmed the expected planar geometry and known order of reactivity.71... 

Reaction of alcohol 4.12 with o-nitrophenyl selenocyanate (4.13) and tributylphosphine gives 4.14. Oxidation of 4.14 with hydrogen peroxide gives selenoxide 4.15. Aryl selenoxide 4.15 bearing a (3-hydrogen atom is unstable and undergoes thermal syn-elimination to give styrene (4.16) with the expulsion of a selenol 4.17 in a fashion similar to that of the Cope elimination (Scheme 4.13). 

Chugaev elimination Thermal syn elimination of xanthate esters to form alkenes. 82... 

The power of this methodology becomes fully apparent however on further synthetic manipulation of the geminally functionalized pyridyl sulfide adducts, with two general reactions being reductive removal of the sulfide by Raney nickel, nickel boride or tri-n-butyltin hydride, or controlled oxidation to the sulfoxide and subsequent thermal syn elimination (Scheme 31). 

A typical synthetic transformation using the Hofmann elimination also illustrates an alternative synthetic approach to this reaction. Reaction of iodomethane with the dimethylamino moiety in 215 leads to the trimethylammonium salt. This ammonium leaving group allows the usual Hofmann elimination sequence, with thermal syn-elimination giving the vinyl derivative, 216.203 204 j is variation is important since using an amine as a Hofmann precursor is often more convenient than relying on an Sn2 reaction of an amine and a halide to produce the requisite ammonium salt (sec. 2.1.A). 

C.vi. Selenoxide Pyrolysis.218 just as sulfides are oxidized to sulfoxides, selenides (R—Se—R) can be oxidized to selenoxides. It is reasonable to assume, therefore, that replacing the sulfoxide with a selenoxide will also lead to thermal syn-elimination to the less substituted alkene. The increased polarity of the Se-O bond of the selenoxide, relative to the S-0 bond of the sulfoxide, and the loss of the unstable R—Se—OH allows even lower temperatures for thermal syn-elimination (typically 0-25 °C). Elimination of PhSeOH from 245 is clearly analogous to the sulfoxide pyrolysis of 243, and the exocyclic-methylene derivative (246) rather than... 

The reaction of a-phenylthio lactones 13 with two equivalents of difluoroiodotoluene results in fluorination-oxidation to give a-fluoro sulfoxides 14, which then undergo thermal syn elimination to produce vinyl fluorides 15 (Scheme 3.6) [25]. 

The cyclic nature of the transition states in these reactions dictates that elimination will proceed with syn stereochemistry. A cyclic transition state involving only five or six atoms cannot accommodate an anti stereochemical relationship at the site of elimination. For this reason, this family of reactions is often referred to as thermal syn eliminations. 

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