Carbenes dihalo

As a final example in this section, a contribution by Grubbs et al. is discussed. The chloride-free ruthenium hydride complex [RuH2(H2)2(PCy3)2] (37) is believed to react, in the presence of alkenes, to form an unidentified ruthenium(O) species which undergoes oxidative additions with dihalo compounds, e.g., 38, to give the corresponding ruthenium carbene complex 9 (Eq. 4) [20]. 

The catalytic conditions (aqueous concentrated sodium hydroxide and tetraalkylammonium catalyst) are very useful in generating dihalo-carbenes from the corresponding haloforms. Dichlorocarbene thus generated reacts with alkenes to give high yields of dichlorocyclopropane derivatives,16 even in cases where other methods have failed,17 and with some hydrocarbons to yield dicholromethyl derivatives.18 Similar conditions are suited for the formation and reactions of dibromocar-benc,19 bromofluoro- and chlorofluorocarbene,20 and chlorothiophenoxy carbene,21 as well as the Michael addition of trichloromethyl carbanion to unsaturated nitriles, esters, and sulfones.22... 

Addition of halogens and pseudohalogens to the cyclopropylthiocarbene chromium complexes 122 affords the 1,4-dihalo-1-phenylthio-l-alkenes 123 stereoselectively [65]. Electrophilic halogen is likely to activate the carbene complexes, followed by the homo-Michael addition of halide anion. (Scheme 44)... 

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. 

In isolated examples, reactions of specific amides and thioamides with dihalo-carbenes can take unusual pathways. Thus, for example, using procedure 7.1.1, A,A-dialkylamides are converted into a-chloromethylene derivatives of the amides [48]. The initial step in which the carbene attacks the carbonyl oxygen atom is the same as for the dehydration of the A-alkyl amides, but subsequent steps, for which there is evidence from 2H/ H labelling experiments, lead to the formation of an enamine and further reaction with the carbene (Scheme 7.34). 

Preparation of spiro[2.3]hexanes is possible by the addition of methylene or a methylene equivalent to an exocyclic C —C double bond. Generally, the Simmons Smith reaction is used (see Section l.A.1.2.1.), but formation of carbenes from nitroso compounds can also be used.73-75,333 The 1,1-dihalo derivative can be prepared by phase-transfer-catalyzed reactions.48 For example, reaction of a mixture of cis- and trans-1,2-dimethyl-3-methylenecyclobutane with dibromo-carbene gave a mixture of cis- and lrans-1,1-dibromo-4,5-dimcthylspiro[2.3]hexanc (2).48... 

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

A. De Renzi, and E. O. Fischer, l,l-Dihalo-2-phenylvinyl Methyl Ethers from [Phenyl-(methoxy)carbene]pentacarbonylchromium(0) and Phenyl(trihalomethyl)mercurials, Inorg. Chim. Acta 8, 185-189 (1974). 

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. ... 

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). ... 

The cyclopropanation —aromatization of 9-substituted phenanthrenes, a sequence applied successfully to 1,1-dihalo-1//-cyelopropa[fl]naphthalenes, would be expected to lead to 1,1-dihalocyclopropa[/]phenanthrenes. Although carbene addition to 9-substituted phenanthrenes is often difficult, dichlorcarbene reacted with 9-phenylselanylphenanthrene in 35% yield. Oxidation of the selenide to the selenoxide, followed by selenoxide elimination gave products... 

A halogen atom, especially bromine, gives sufficient stabilization to an attached car-banion to allow preparation by metal halogen exchange from 1,1-dihalo compounds [39, 40]. These a-halocarbanions decompose rapidly via a carbene intermediate, since the halide is a good leaving group. The preparatively useful reactions involve in situ reactions of the transient carbanions in the presence of aldehydes or ketones [41]. 

Allenes can be synthesized simply and in good yield by treating 1,1-dihalo-cyclopropanes with methyl- or butyl-lithium in diethyl ether at —10° to — 30° 148 these dihalides are obtained by addition of dichloro- or dibromo-carbene to an olefinic double bond 149... 

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