Jyn-elimination

The thermal jyn-elimination reactions of X-oxides have assumed great importance in synthesis because of their mildness and regioselectivity. 

The preference for jyn-elimination leading to the fra/ii-olefins is not too difficult to understand in the alicyclic series. The use of models clearly indicates that the non-reacting alkyl residue exerts a severe limitation on the... 

Realising the significance and necessity of jyn"-elimination in Ej processes. Barton showed that this concept correctly predicted the direction of elimination of xanthates and esters of many natural products, based on 5- and 6-membered alicyclic rings and he subsequently used this method to predict unknown configurations of certain compounds. 

This marked preference for 1,4-jyn-elimination is not surprising as it involves two simultaneous l,2-am/-eliminations. On the other hand, l,4-ant/-elimination involves a 1,2-an -elimination coupled with a 1,2-iy -c/ma/-elimination and consequently involves less favourable overlap of the developing p orbitals in the transition state. 

The thermodynamics of the complexation of dicycIohexyl-18-crown-6 with HBr in chloroform has been investigated, and the high basicity of the ether and the low entropy of complexation were demonstrated. A number of mechanistic studies involving crown ethers have been carried out. They are found to accelerate the potassium alkoxide-DMSO isomerization of 2-methylbicyclo-[2,2,l]hepta-2,5-diene to 5-methylenebicyclo[2,2,l]hept-2-ene, to decelerate the addition of bromine to stilbene in chloroform in the presence of HBr, and to influence the relative amounts of cis- and /m/75-isomers arising from base-catalysed anti- and jyn-eliminations. Such effects may be generalized as arising largely from the polyether-assisted dissociation of ion pairs in these processes. 

In step 4, the elimination of HBr is reasonable because it gives an aromatic system and because jyn-E2 elimination is a common reaction. 

One of the limitations of the Warren s adaptation of Homer-Wittig olefina-tion, the failure of the (Z)-selective route when the alkene has a branched chain substituent, has now been overcome. Reduction of the p-ketophosphonates carrying a-branches, e.g. (112) and (113), with sodium borohydride and cerium chloride gives excellent a / -stereoselectivity and hence (Z)-alkene on base-induced elimination. Enantioselective synthesis of both jy -(115) and anti- ll) P-hydroxy-phosphine oxides has been achieved with up to 90% e.e. through two separate approaches. The jyn-isomer was obtained by reduction of the corresponding ketone (114), while the anti-isomer is the product of the reaction of the oxazolidine substituted aldehyde (116) with lithiated diphenylmethyl-phosphine oxide (Scheme 10). A new, highly stereoselective approach to trisubstituted alkenes has been reported. Cerium(III) chloride-promoted... 

On pyrolysis, sulphoxides with a (3-hydrogen undergo syn elimination to give alkenes (Scheme 6.24). The pyrolysis of sulphoxides takes place at about 80 °C. Like other concerted pericyclic reactions, this reaction is also highly stereoselective. For example, the anff-sulphoxide (1) on pyrolysis gives mainly frawi-methylstilbine (2), whereas the corresponding jyn-sulphoxide (3) gives predominantly cw-methylstilbine (4). 

Isomerization of the C=C bond may occur by a jyn-readdition of HPdX with the reverse regioselectivity, followed by a syn-P"-hydride elimination (Scheme 19.14b and d). 

When the alkene is an aUylic alcohol, a nonclassical Heck reaction takes place due to a more favored jyn-P -hydride elimination, leading to a ketone (Scheme 19.14e). The classical Heck reaction involving the jyn-p-hydride elimination occurs only in the presence of Ag+ that generates the cationic [ArPdLjS]. The coordination of the OH onto the Pd center (Scheme 19.14f) prevents the C—C bond rotation observed previously [3c, m]. 

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