Conjugated dienes from cyclopropanes

Only a few results are available concerning competitive cyclopropanation of non-conjugated dienes. The case of 1,4-hexadiene72 (mixture of Z and E isomers) illustrates the reactivity difference between a monosubstituted and a 1,2-disubstituted double bond, whereas in limonene (24)47, a 1,1-disubstituted and a trisubstituted double bond compete for the carbenoid derived from ethyl diazoacetate. In both cases, the less substituted double bond reacts preferentially (Scheme 8). 

Conjugated dienes, styrenes and electron-rich alkenes are cyclopropanated with ethyl diazoacetate using a triarylamminium salt of appropriate oxidation potential as a cata-lyst/initiator (equation 96)185. These reactions are initiated by electron transfer from the unsaturated substrate to the amminium ion and the double additions of the diazo esters to the conjugated dienes are effectively suppressed. Cyclopropanes geminally bearing two... 

CO)2Fe (THF) BFT A transition state model for the syn stereoselective cyclo-propanations of alkenes with diazoacetic ester by Rh-porphyrin catalysts has been proposed. Alkenes , conjugated dienes and enol ethers are stereoselectively cyclopropanated with Rh(II) -stabilized 1- (alkoxycarbonyl)vinyl carbenoids derived from the diazo precursors and Rh2(OAc)4 (equation 95). The Cu(acac)2-catalyzed reactions of Me3SiCH2COCHN2 with alkenes provide the expected adducts in good yields ". ... 

So far the systems under consideration for the Diels-Alder reaction with MA were conjugated dienes or systems that could give rise to diene intermediates. The latter could be generated, as we saw, by isomerization, tautomeriz-ation, pyrolysis, or even as the result of DA reaction when in some cases the primary products are unstable. In addition, there are some cases where a double bond in the diene may be replaced by a cyclopropane ring. An example of this is the vinylcyclopropane derivative reported by Sarel and Breuer. They found a-cyclopropyl-styrene 184 to react with MA. Two main products were obtained. Product 185, obtained in a 40% yield, resulted from participation of the vinylcyclopropane unit in an apparent 1,5 addition process. The other product, 186, results from participation of the styrene unit in the fashion discussed earlier. 

Bis(camphorquinone-a-dioximato)cobalt(II) (10) has been developed as a catalyst for enan-tioselective cyclopropanation reactions. It allows selective carbene transfer from diazoacetic esters to terminal C-C double bonds which are in conjugation with vinyl, aryl, alkoxycarbonyl or cyano groups, but not to alkyl-substituted alkenes, cycloalkenes, 1,3-dienes and al-lenes. The unusual chemoselectivity and some other experimental observations make the two mechanistic pathways proposed vide supra) questionable for these special carbene-transfer reactions. In contrast, the cobalt(II) complex 11 allows not only the cyclopropanation of styrene but also of oct-l-ene, a nonactivated alkene (ethyl diazoacetate, 35 °C, 3mol% of catalyst yield 50-60%). ... 

Cobalt is another metal which has been successfully used in asymmetric cyclopropanation. A chirally modified catalytic system for selective cyclopropanation of phenyl-, vinyl- or alkoxy-carbonyl-conjugated terminal double bonds with diazoacetates is formed from cobalt(ll) chloride and (+)-a-camphorquinonc dioxime27,69 71 and similar systems 09. Best optical yields are achieved with styrene and the bulky 2,2-dimethylpropvl diazoacetate which gives 2,2-dimethylpropyl /ra .v-2-phenyl-l-cyclopropanecarboxylate in 88% ee and the as-isomer in 81%ee7n. No cyclopropanation occurs with alkyl-substituted or cyclic alkenes, cyclic or sterically hindered acyclic 1.3-dienes, vinyl ethers and phenylethyne. 

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