Cyclopropane 1,1-dihalo

Cyclohexanone pyridylhydrazones, cyc-lization of, 91, 92, 94 Cyclopropanes, 1,1-dihalo-, formation of, 61... 

The same disconnection is also effective for cyclopropanes but the reagent for the earbene synthon is a diazocompound RCHN2 or a dihalo compound treated with a metal e.g. 

Treatment of geminal dihalocyclopropyl compounds with a strong base such as butyl lithium has been for several years the most versatile method for cumulenes. The dihalo compounds are easily obtained by addition of dihalocarbenes to double--bond systems If the dihalocyclopropanes are reacted at low temperatures with alkyllithium, a cyclopropane carbenoid is formed, which in general decomposes above -40 to -50°C to afford the cumulene. Although at present a number of alternative methods are available , the above-mentioned synthesis is the only suitable one for cyclic cumulenes [e.g. 1,2-cyclononadiene and 1,2,3-cyclodecatriene] and substituted non-cyclic cumulenes [e.g. (CH3)2C=C=C=C(CH3)2]. 

Cyclopropane, 1,1 -dibromo-2,2-diplienyl-[Benzene, l,l -(2,2-dibromocyclo-propylidene)bis-], 32 Cyclopropanecarboxyltc acid, 70 Cydopropanes, gem-dihalo, 32 CYCLOUNDECANONE, 107 Cycloundecanone, 2-hydroxy-, 110 Cycloundecene, 1-carboxy- [1-Cyclo-undecene-1-carboxylic acid], 111 Cycloundecene, 1-methoxy-, 111 1-Cycloundecene-l-carboxyhc acid, methyl ester, 108... 

Various alkyl- and aryl-substituted [3]radialenes could be prepared from 1,1-dihaloal-kenes using organometallic pathways. Hexamethyl-[3]radialene (25), the first [3]radialene to be synthesized, was obtained in a very low yield by treatment of l,l-dibromo-2-methyl-1-propene (22) with butyllithium8,9. The lithium carbenoid 23 and the butatriene 24 are likely intermediates of this transformation (Scheme 2), the former being the source of an unsaturated carbene moiety which is transferred onto the latter. However, the outer double bonds of 24 are more readily cyclopropanated than the central one. 

It has been shown that bis(chloromethyl)zinc in 1,2-dichloroethane (Denmark s modification) is particularly effective for the cyclopropanation of iodo-substituted allylic alcohols (equation 30) . The cyclopropanation of dihalo-substituted alkenes is a lot more difficult due to the significant decrease in the nucleophilicity of the alkene, thus resulting in much lower yields of the desired products (equation. In this case, significant amounts of 0-methylation were also observed. 

At elevated temperatures, methylenecyclopropane and its derivatives undergo a rearrangement which maintains the methylenecyclopropane skeleton. Obviously, for the parent system this process is degenerate [216, 217-222]. The 2,2-dihalo-methylene-cyclopropanes behave analogously, providing either mixtures of the... 

It is abundantly clear from the preceding discussion that dihalocyclopropanes are versatile intermediates in organic synthesis. Although a wealth of chemistry has already been uncovered, prospects remain bright for interesting developments in the future. Areas such as the application of dihalocyclopropanes in heterocyclic synthesis via carbene insertion into C—H bonds adjacent to heteroatoms, reactions of dihalocyclopropanes with organometallics and the synthetic applications of metallated derivatives deserve further exploration. The chemistry of difluoro-, diiodo- and mixed dihalo-cyclopropanes can be expected to attract some attention. Finally, other heteroatom-substituted cyclopropanes derived ftom dihalocyclopropanes will also invoke further investigation. 

The thermal decomposition of phenyl(trihalomethyl)-mercury compounds, C6H5.Hg.CX3, in the presence of olefins yields the dihalo-cyclopropane virtually quantitatively. A typical example is expressed in equation (15). The initial rate of disappearance of the organometallic... 

The intramolecular Wurtz-type coupling of dihaloorganic compounds with use of metallic zinc is a classical synthetic route to cyclic compounds. For example, cyclopropane derivatives can be prepared from 1,3-dihalo-propanes (29, 189a, 248, 451), and cyclobutane derivatives from 1,4-dihalobutanes (71). These reactions presumably proceed via the intermediate formation of organozinc compounds. The reaction of diethylzinc with esters of a,a -dibrominated aliphatic dicarboxylic acids leads to the... 

Cyclopropanation. Rhodium(II) pivalate is superior to copper catalysts for the cyclopropanation of l,l-dihalo-4-methylpenta-l,3-dienes (1) with ethyl diazoacetate. The yield is higher than that obtained with copper catalysts (48%), and the cis/trans ratio is considerably higher. Similar contrathermodynamic product ratios were observed with other substrates. 

Finally, dihalo cyclopropanes can be converted to dialkyl cyclopropanes by reaction with organocuprates (Section 26.1). For example, cyclohexene can be converted to a bicyclic product having four new C-C bonds by the following two-step sequence cyclopropanation with dibromocarbene ( CBr2) and reaction with lithium dimethylcuprate, LiCu(CH3)2. 

Cyclopropanation of alkenes carrying electron-withdrawing groups with dihalo-carbenes does not take place under the normal reaction conditions unless the dihalocar-bene is generated from trihalomethylphenylmercury compounds. By this procedure acrylonitrile was converted into l-cyano-2, 2-dichlorocyclopropane in 78% yield and other acrylic derivatives are transformed into dichloromethyl cyclopropane carboxylates (186) (equation 46). Treatment of electron-deficient olefins with dibromomethane in the presence of Ni(0) complex/Zinc/Lewis acid seems to be very effective for cyclopropanation. ... 

The S pathway for the preparation of electrophilic cyclopropanes is not well documented. The synthesis of 2-vinylcyclopropanecarboxylates (432) by reaction of 1,4-dihalo-2-butenes (431) with active methylene functions involves a Sj 2 mechan-ism so, 281 jjj analogous fashion, the reaction of 3,5-dibromocyclopentene (433) with diethyl sodiomalonate affords 6,6-diethoxycarbonylbicyclo[3.1.0]hex-2-ene (434)... 

Displacement reactions on 1,1-disubstituted cyclopropanes have been used to prepare other cyclopropanone equivalents. The most readily available 1,1-disubstituted cyclopropanes are geminal dihalo derivatives prepared by the addition of dihalocarbenes to olefins. Unfortunately, these materials do not undergo direct displacement easily and therefore do not provide a general route to other cyclopropanone derivatives. Solvolysis usually leads to ring-opened products, although dibromocyclopropanes with a barrier to... 

Those results being obtained, we wanted to examine reductions of gem dihalo geno cyclopropane with the goal of comparing NaH in HMPA, NaH-f-AmONa in THF and complex reducing agents. Moreover we hoped to get some more informa tions about our new reagents. 

A convenient procedure for the preparation of geminally disubstituted cyclopropanes is the successive alkylation and cyclization of active methylene compounds with 1,2-dihaloal-kanes. - This reaction can be termed a substitution initiated ring-closure reaction (SIRC), in analogy with the Michael initiated ring-closure reaction discussed in Section 1.1.3.7. 

There are only two examples for the conversion of allyl halides to nonheteroatom-substituted vinylcyclopropanes. Firstly, vinyl- and (2-chlorovinyl)cyclopropanes can be prepared in yields of 18-77% by oxidative addition of a 3,3-dihalo-l-alkene onto a low-valent metal, e.g. Cu(0), thus forming a carbenoid which either reacts directly or after formation of the carbene with an alkene (see Houben-Weyl, Vol. E19b, pp673-674). ... 

A copper-containing oxocarbenoid is assumed to be involved in the formation of both the cyclopropanes 9 and the 4,5-dihydrofurans 10. It is therefore not surprising that the formal cyclotrimers of oxocarbenes, triacylcyclopropanes 12, were obtained in the absence of suitable reagents that could trap the oxocarbenoid. Cyclopropanes 12 are likely to be formed via the formal oxocarbene dimers 11, which in some cases are also found among the reaction products. In addition to the copper-mediated debromination of a,a-dibromo ketones, triacylcyclopropanes can also be obtained from a,a-dihalo ketones by metal-induced a-elimination in other cases, e.g. from a,a-dibromo ketones with Ni(0), Fe(0), or Co(0) complexes,", and from a,a-dichloro ketones with a zinc-copper couple. 

Decomposition of dibromochloromethyl-, more seldom bromochloroiodomethyl(phen-yl)mercury or dibromochloromethyl(cyclohexyl)mercury in the presence of an alkene, afforded 1-bromo-l-chlorocyclopropanes in high yields uncontaminated by other 1,1-dihalo-cyclopropanes 1 (Houben-Weyl, Vol.E19b, pp 1595-1597). 

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