Methyl-frans-chrysanthemate

Sulfur ylides, like the phosphorus ylides of Chapter 21, are useful intermediates in organic synthesis, as shown in Problem 21.79. Methyl frans-chrysanthemate, an intermediate in the synthesis of the insecticide pyrethrin I (the chapter-opening molecule), can be prepared from diene A and a sutfur ylide. Draw a stepwise mechanism for this reaction. 

Methyl 5-methyl-fra7i5-2,4-hexadienoate added at - 70° to a soln. containing di-phenylsulfonium isopropylide (prepn. s. original) with exclusion of air and moisture throughout, allowed to warm to -20° during 3 hrs., and kept 12 hrs. at this temp. methyl frans-chrysanthemate. Y 12.b%, F. e. s. E. J. Corey and M. Jautelat, Am. Soc. 89, 3912 (1967). 

When phosphane-free nickel complexes, such as bis(cycloocta-l,5-diene)nickel(0) or te-tracarbonylnickel, are employed in the codimerization reaction of acrylic esters, the codimer arising from [2 + 1] addition to the electron-deficient double bond is the main product. The exo-isomer is the only product in these cyclopropanation reactions. This is opposite to the carbene and carbenoid addition reactions to alkenes catalyzed by copper complexes (see previous section) where the thermodynamically less favored e/iob-isomers are formed. This finding indicates that the reaction proceeds via organonickel intermediates rather than carbenoids or carbenes. The introduction of alkyl substituents in the /1-position of the electron-deficient alkenes favors isomerization and/or homo-cyclodimerization of the cyclopropenes. Thus, with methyl crotonate and 3,3-diphenylcyclopropene only 16% of the corresponding ethenylcyc-lopropane was obtained. Methyl 3,3-dimethylacrylate does not react at all with 3,3-dimethyl-cyclopropene, so that the methylester of frans-chrysanthemic acid cannot be prepared in this way. This reactivity pattern can be rationalized in terms of a different tendency of the alkenes to coordinate to nickel(O). This tendency decreases in the order un-, mono- < di- tri- < tet-... 

A striking example for the preferred formation of the thermodynamically less stable cyclopropane is furnished by the homoallylie halides 37, which are cyclopro-panated with high c/s-selectivity in the presence of copper chelate 3891 The cyclopropane can easily be converted into cw-permethric acid. In contrast, the direct synthesis of permethric esters by cyclopropanation of l,l-dichloro-4-methyl-l,3-pentadiene using the same catalyst produces the frans-permethric ester (trans-39) preferentially in a similar fashion, mainly trans-chrysanthemic ester (trans-40) was obtained when starting with 2,5-dimethyl-2,4-hexadiene 92). 

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