1 -halo-1 - cyclopropane alkene

The synthesis of halo-substituted cyclopropanes using zinc carbenoids can be accomplished using three different approaches by the cyclopropanation of a halo-substituted alkene, by the cyclopropanation using a halo-substituted zinc carbenoid, or by the cyclopropanation using, gem-dizinc carbenoids followed by trapping the cyclopropylzinc with an electrophilic halide source. 

Carbenes and substituted carbenes add to double bonds to give cyclopropane derivatives ([1 -f 2]-cycloaddition). Many derivatives of carbene (e.g., PhCH, ROCH) ° and Me2C=C, and C(CN)2, have been added to double bonds, but the reaction is most often performed with CH2 itself, with halo and dihalocarbenes, " and with carbalkoxycarbenes (generated from diazoacetic esters). Alkylcarbenes (HCR) have been added to alkenes, but more often these rearrange to give alkenes (p. 252). The carbene can be generated in any of the ways normally used (p. 249). However, most reactions in which a cyclopropane is formed by treatment of an alkene with a carbene precursor do not actually involve free carbene... 

Support-bound alkylating agents have been used to N-alkylate pyridines and dihydropyridines (Entries 7 and 8, Table 15.21). Similarly, resin-bound pyridines can be N-alkylated by treatment with a-halo ketones (DMF, 45 °C, 1 h [267]) or other alkylating agents [246]. Polystyrene-bound l-[(alkoxycarbonyl)methyl]pyridinium salts can be prepared by N-alkylating pyridine with immobilized haloacetates (Entry 8, Table 15.21). These pyridinium salts react with acceptor-substituted alkenes to yield cyclopropanes (Section 5.1.3.6). Pyridinium salts have also been prepared by reaction of resin-bound primary amines with /V-(2,4-dinitrophenyl)pyridinium salts [268,269]. 

The addition of a-deprotonated alkyl halides to alkenes or carbonyl compounds can, because of the good leaving-group properties of halides, also lead to formation of cyclopropanes [292] or epoxides [187, 304, 306, 310], respectively. Because of the inherent instability of a-halo organometallic compounds, these intermediates should be handled carefully and on a small scale only. The ketone produced by the last reaction in Scheme 5.34 is probably formed by Oppenauer oxidation of the intermediate alcohol by the excess benzaldehyde [310],... 

All of these carbenes are reactive intermediates that must be generated from the appropriate precursors in the presence of the alkene (or arene) which is to be cyclopropanated. The following methods of carbene-transfer reactions to C-C double bonds will be discussed path a. from a-halo-a-metal (or alkylmetal) compounds by a-elimination path b. from iodine or sulfur ylides by thermal, photochemical or transition metal catalyzed decomposition ... 

Formation of cyclopropane-containing oligocyclic systems such as nortricyclenes from nor-bornadienes also proceed via intramolecular insertion of homoallylmetal complexes. Palladated nortricyclene 19 can be converted with lithium aluminum hydride, chlorine or bromine to give 3-exo-methoxynortricyclene (20) and the en(/o-3-halo-exo-5-methoxy analogs 21 and 22, respectively. Intramolecular alkene insertion into the Pd — C bond is assumed to form the nortricyclene skeleton. ... 

This method can be extended to applications with halo-substituted allyl dichlorides. Chloro(2,2-dichlorovinyl)carbenes 6 have been formed by deprotonation with lithium tetramethylpiperid-ide, but yields were improved when sodium hexamethyldisilazanide was used (see Houben-Weyl, Vol. E19b, Table 97). 1,2,3,3-Tetrachloroprop-l-ene can also easily be deprotonated, but in this case subsequent decomposition formed two isomeric carbenes, as observed in the product ratio after cyclopropanation.2 However, treatment of 1,1,2,3,3-pentachloropropene (11) with a base gave only the [2 + 2] dimer 14 of tetrachloroallene 13, generated by jS-elimination of lithium chloride.2 Other pentahalopropenes with a fluorine in the 3-position (e.g. 15) reacted with x-elimination, forming the corresponding cyclopropanes 17 in the presence of alkenes (see Houben-Weyl, Vol. E 19b, Table 97). 

The issue of 1,2- versus 1,4-addition, as shown in Scheme 22, in the phase transfer catalyzed (benzyl-triethylammonium chloride TEBA) version of this cyclopropanation reaction, was addressed in a more recent publication. Simple a-halo esters do not react with acrolein, methyl vinyl ketone or 2-chloroac-rylonitrile under the conditions examined. However several electrophilic alkenes do react with diethyl bromomalonate to give good yields of substituted cyclopropanes (equation 38 Table 13). The only side... 

Cyclopropanes. Cyclopropanes are fcH-med by condrasation of a-halo-carbanions with electrophilic alkenes under phase-transfer conditions (equation... 

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