Bromoform-Diethylzinc

An unprecedented cyclopropanation reaction was observed during the reaction of ketene alkylsilyl acetals (191) with bromoform-diethylzinc. When monosubstituted acetals were used, cyclopropanecarboxylic esters (195) were formed by a novel C-H insertion. When disubstituted ketene acetals were used, byproducts such as a,)5-ethylenic esters (197) were also formed presumably via 196 (equation 49). This reaction provides a convenient method for the preparation of the bicyclo[3.1.0] hexane system and can be advantageously compared to the copper-catalysed intramolecular cyclization of unsaturated a-diazoketones . 

Alternatively, dibromoiodomethane may be used instead of bromoform. An experimental procedure for the synthesis of 9-bromobicyclo[6.1.0]nonane by reaction of bromoform, diethylzinc and cyclooctene is described in Houben-Weyl, Vol. El 9 b, pp 1605-1606. Some products prepared by this method are collected in Table 2. 

Table 2. Bromocyclopropanes from Bromoform, Diethylzinc and Alkenes in the Presence...

Reaction of diethylzinc with bromoform produced dibromomethyl(ethyl)zinc which, in the presence of an alkene, gave bromocyclopropanes oxygen greatly accelerated this reaction. ... 

A useful sabinene (767) synthesis has been described by Rousseau and Slougui. The keto ester 768 (R = O) was prepared conventionally from ethyl 4-methyl-3-oxopentanoate and converted to the methylene compound 768 (R = CH2) by a Wittig reaction. The trimethylsilylketene acetal 769 was then treated with bromoform and diethylzinc in pentane, yielding 70% of the ester 765 (R =... 

Monobromocydopropanes (cf. 1,253 2,134 4,153). The carbenoid reagent derived from diethylzinc and bromoform in the presence of oxygen converts olefins into monobromocyclopropanes in 70-85% yield. Little reaction occurs in the absence of O2. iyw-Isomers are formed selectively. In the reaction with c/s-butene-2 only traces of the anti-isomer were detected by glc. 

CYCLOPROPANATION Copper-lsonitrile complexes. Cupric chloride. Diethylzinc-Bromoform-Oxygen. Palladium acetate. Titanium(IV) chloride-Lithium aluminum hydride. 

In terms of the synthesis of substituted cyclopropane derivatives using either halo- or alkyl-substituted reagents, endo exo selectivity of the product is often poor. For example, cyclohexene (39) was converted into an approximately 2 1 ratio of 40 and 41 on exposure to bromoform and diethylzinc. ... 

Treatment of aryl aldehydes with zinc amalgam and boron trifluoride has been shown to lead to carbenoid (94) or its equivalent, which proves to be an effective cyclopropanating agent (Scheme 14). The generation of carbenoid (95) from bromoform by treatment with diethylzinc has resulted in a new and highly efficient... 

Oxygen accelerates the generation of the bromo-carbenoid reagent of zinc from diethylzinc and bromoform. 

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