Carbenes addition, alkenes carbenoid species

Two types of addition reactions involving carbenoid species have been used to synthesize cyclopropanes. The first approach, the reaction of a vinyl carbene species, obtained by a metal-catalyzed ring-opening reaction of cyclopropenes, with alkenes is discussed in Section 1.1.6.3.2., as it is, from a topological point of view, a codimerization of eyclopropene and an alkene. In this case the ring carbons of the cyclopropane are not identical to those of the eyclopropene substrate. 

Reaction of bromofluorophenylmethane (for a preparation see ref 118) and potassium tert-butoxide with an alkene afforded l-fluoro-l-phenylcyclopropanes (Houben-Weyl, Vol.4/3, p233 Vol. E19b, p980). To identify the reacting species, e.g. carbene or carbenoid, the reaction was carried out without and with an equimolar amount of 18-crown-6. ° Alternatively, chlorofluorophenylmethane, in place of the bromo derivative, can be used. Reactions with bromofluorophenylmethane were performed at 25 °C (sealed tube), whilst those with chlorofluorophenylmethane at 60-80 °C. Addition of fluoro(phenyl)carbene to alkenes is at least 98% stereospecific e.g. formation of . ... 

Carbenoid species, such as the Simmons-Smith reagent (Eq. 10.55), undergo facile additions to alkenes to create cyclopropanes. The reactions are stereospecific, making carbenoids synthetically useful versions of carbenes. Carbenoid compounds do not normally perform insertion reactions. 

The cyclopropanation reaction of simple alkenes appears to proceed via stereospecific 5yn-addition of a Zn-carbenoid (carbene-like species) to the double bond without the involvement of a free carbene. 

In a large number of carbene and carbenoid addition reactions to alkenes the thermodynamically less favored syn-isomers are formed 63). The finding that in the above cyclopropanation reaction the anti-isomer is the only product strongly indicates that the intermediates are organonickel species rather than carbenes or carbenoids. Introduction of alkyl groups in the 3-position of the electron-deficient alkene hampers the codimerization and favors isomerization and/or cyclodimerization of the cyclopropenes. Thus, with methyl crotylate and 3,3-diphenylcyclopropene only 16% of the corresponding vinylcyclopropane derivative has been obtained. 2,2-Dimethyl acrylate does not react at all with 3,3-dimethylcyclopropene to afford frons-chrysanthemic add methyl ester. This is in accordance with chemical expectations 69) since in most cases the tendency of alkenes to coordinate to Ni(0) decreases in the order un-, mono-< di- < tri- < tetrasubstituted olefines. 

Although the Simmons-Smith reaction has found considerable use in organic synthesis, it is not readily applicable to the formation of highly substituted cyclopropanes, since 1,1 -diiodoalkanes (other than diiodomethane) are not readily available. Substituted zinc carbenoids can be prepared from aryl or a,p-unsaturated aldehydes (or ketones) with zinc metal, and these species can be trapped with an alkene to give substituted cyclopropanes.The addition of chromium carbenes (see Section 1.2.2) to alkenes can be used to effect cyclopropanation to give substituted cyclopropanes. Thus, addition of excess 1-hexene to the chromium carbene 113 gave the cyclopropane 114 as a mixture of diastereomers, with the isomer 114 predominating (4.92). ... 

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