1,1 -Dibromocyclopropanes, 1,2-metallate

Metal carbonyls in a low oxidation state are able to induce the carbonylative transformation. Reductive carbonylation of the gem-dibromocyclopropanes 150 is realized by treatment with nickeltetracarbonyl in an alkanol to give the alkyl cyclopropanecarboxylates 151 with reduction of another bromide [79], Amines, phenol, and imidazole can also be used instead of alcohols. (Scheme 53)... 

Doering and LaFlamme [10b] were the first to report that sodium and magnesium metal are capable of converting substituted gem-dibromocyclo-propanes to allenes in varying yield. However, it was found that sodium reacts best in the form of a high surface dispersion on alumina. At a later date, Moore and Ward [11a] and then Skattebol [12] reported that methyllithium or n-butyl-lithium reacts with gem-dibromocyclopropanes to give allenes in high yield. The related dichloro compounds were found to be inert to methyllithium but reacted slowly with -butyllithium. Several examples of the preparation of allenes from gem-dibromocyclopropanes are shown in Table I. 

Besides by these epoxidations, oxaspiropentanes have been prepared through the nucleophilic addition of 1-lithio- 1-bromocyclopropanes to ketones at low temperature. Thus for example, the dibromocyclopropane 96 prepared by addition of dibromo-carbene to cyclohexene 52) underwent metalation with butyllithium to give the lithio-bromocyclopropane 97 which was converted into the oxaspiropentane 98 upon simple addition to cyclohexanone, Eq. (28) 53,54). 

Despite the toxicity of volatile metal carbonyls, particularly Ni(CO)4, several useful transformations have been developed employing these reagents. Monocarbonylation of gem-dibromocyclopropanes may be accomplished with Ni(CO)4 in the presence of alcohols, amines or (less successfully) thiols, to afford cyclopropane carboxylic esters, amides or thioesters, respectively (equation 202)400. Silylamine or silylsulfide reagents may take the place of amines or thiols401. The intermediacy of a nickel enolate in the carbonylations is... 

As outlined in Scheme 5, tertiary cyclopropanol salts are also subject to the oxyanion-accelerated VCP rearrangement. In this sequence the requisite vinylcyclopropanol salts are conveniently prepared from readily available 1,1-dibromocyclopropanes by sequential halogen-metal exchanges, followed first by alkylation, and then by oxygenation. Products resulting from alternative 1,5-hydrogen shift (retro-ene) pathways were not detected in these reactions. 

Research trends of the last few years highlight applications to more involved systems either from the substrate/product side or from the catalyst side. Furthermore, a deeper insight into underlying mechanism is intended. Thus, reductive carbonylation of dibromocyclopropanes was performed in toluene/5 M KOH with syngas (CO/H2, 3 1) at elevated temperature (90 °C) using a mixture of CoCl2, KCN, and Ni(CN)2 for the metal catalyst and PEG-400 as PT catalyst which was much more efficient than a quaternary ammonium catalyst [81]. l,l-Dibromo-2-phenylcydopropane furnished a 72% yield of 2-phenylcydopropanecarboxylic add (1 1 cis/trans mixture). 

The debromination of gem-dibromocyclopropanes by dimsyl anion (73) involves removal of a soft halogen by the carbon base. The formation of allenes from the reaction of g cm-dibromocyclopropanes with alkyllithiums (74) must proceed at its early stages by a similar mechanism. Halogen-metal exchange of a e/n-dibromocyclopropanecarboxylic acid (75) is 2.5 times faster than proton abstraction from the acid This is a remarkable demonstration of the HSAB axiom. 

Treatment of an olefin with bromoform and an alkoxide to yield the 1,1-dibromocyclopropane which reacts with an active metal to produce an allene ... 

The transfonnations of gm-dihalocyclopropanes are synthetically useful because the cyclopropanes are readily prepared by the addition of dihalocarbene to olefins. In most of dehalogenation reactions to monohalocyclopropanes, the reagents are limited to metallic reductants such as organotin hydride, lithium aluminum hydride, sodium borohydride, Grignard reagent, and zinc-copper couple [1-9]. A versatile method for the reduction of gm-dibromocyclopropanes 3 with an organic reductant is achieved by use of diethyl phosphonate (commercially named diethyl phosphite) and triethylamine to give the monobromocyclopropanes 4 (Scheme 2.2) [10]. 

Reductive transformations are also achieved by using other low-valent metals, which permits the functionalization of gm-dihalocyclopropanes. For example, use of Ni(CO)4 as a reductant leads to reductive cabonylation of gm-dibromocyclopropanes via nickel enolates [46-50]. 

Scheme 1-4. Stereocontrolled debromination of gem-dibromoalkenes and ge/w-dibromocyclopropanes by halogen/metal permutation.

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