Cyclohexene with dibromocarbene

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. 

Dramatically differing effects of phase-transfer catalysts on the cyclopropanation of cw,trans,trans-cyclododecatriene (61) and a series of dienes have been reported. Addition of dichlorocarbene to (61) results in tris-cyclopropanation when cetyltri-methylammonium bromide (i) is employed, whereas with benzyl-P-hydroxyethyl-dimethylammonium ion (ii) as catalyst only monocyclopropanation (of the more strained bond) is observed (Scheme 7). From the extensive study it may be concluded that, for dichlorocarbene addition, the P-hydroxyethyl catalyst restricts potential polycyclopropanation to monocyclopropanation at the most highly substituted (or strained) double bond. With dibromocarbene a different situation results. Catalyst (i) does not effect the addition of dibromocarbene to styrene, cyclohexene, or allyl bromide while catalyst (ii), with the P-hydroxyethyl function, effects dibromocyclo-propanation, in yields of up to 80 %. 

Dibromocarbene has been generated by treatment of diethyl dibromomalonate with sodium methoxide, as shown by isolation of dibromonorcarane after trapping with cyclohexene." This dibromocarbene probably arises from a bromophilic attack from a dibromoacetyl carbanion followed by decarboxylation and loss of bromide. 

Alumina-potassium hydroxide can be used for the generation of dichlorocarbene or dibromocarbene from chloroform or bromoform. respectively.3 A 3 1 molar mixture of cyclohexene and the haloform is adsorbed on basic alumina and kept at room temperature for 4 days and then eluted with ether. The 7,7-dihalonorcarane was obtained in 12-15% yield (calculated on the basis of the base in the alumina). 

This organometallic reagent is prepared in 85-95% yield by the reaction of mercuric chloride and sodium tribromoacetate in the molar proportion 1 2.5-3.0 in monoglyme. The reagent is a convenient source of dibromocarbene. Thus it reacts with cyclohexene in refluxing benzene (N2) to give dibromonorcarane in 54% yield.1... 

N.m.r. chemical shifts of a number of monoterpenoids of this class are included in a study of bicyclo[4,l,0]heptanes. A further paper in a series on enthalpies of combustion and formation concerns the 3,4-epoxycaranes. (+)-Car-2-ene and (—)-car-2-ene of high optical purity have been synthesized from the respective (-)- and (+)-rran5-caran-2-ones, via the analogous and uncited tosylhydrazone/methyl-lithium route used by Cocker et al. for synthesizing (—)-cw-car-4-ene (Vol. 1, p. 47), thus providing access (Vol. 2, p. 29, ref. 108) from the readily available (+)- and (-)-dihydrocarvones. Cocker et al. have synthesized norcaran-3-one via dibromocarbene addition to 4,4-dimethoxy-cyclohexene and treatment with lithium dimethylcuprate) from which car-2-ene, car-3-ene, and car-3(10)-ene were prepared. ... 

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