Alkenes with dibromocarbene

Prepared by the reaction of the corresponding alkene with dibromocarbene generated from CHBr3... 

A similar method was used to generate dibromocarbene using a combination of sonication and PTC for the reaction of alkene with dibromocarbene formed from bromoform and solid sodium hydroxide in situ [42], Compared with the preparation of dichlorocarbene, however [39], the amount of PTC required had to be increased 10-fold. The combination of the effect of sonication and PTC was significant in that the presence of PTC allowed a shortening of the reaction period to 20-30 min and an increase in yield to 96% compared with 3 h and 40-50% using sonication alone. 

This is the only case of nonstereospecific cyclopropanation of electrophilic alkenes with dibromocarbene, generated from tribromomethyl(phenyl)mercury diethyl ( )-but-2-enedioate, (Z)- and ( )-but-2-enenitrile, (Z)- and ( )-but-2-enoate vide supra) and, of course, electron-rich alkenes undergo addition of dibromocarbene in a stereospecific manner. ... 

The reaction of dibromocarbene with haloalkenes (vinyl halides, alkenes with a halogen more distant from the double bond) leads to the formation of 1,1-dibromocyclopropanes in poor to good yields. However, reactions with allyl halides, particularly substituted allyl bromides, require comment. These alkenes furnish, apart from 1,1-dibromocyclopropanes 1, the alkylation products of tribromomethyl anion 2 and, occasionally, the products of their further transformations (dibromocarbene adducts 3, products of elimination of hydrogen bromide 4 etc.) if they react with bromoform under phase-transfer catalysis conditions (Houben-Weyl, Vol.E19b, pi620). 

Enol ethers, alkenes with a more remote oxygen atom, ketene acetals or esters of unsaturated alcohols undergo addition of dibromocarbene to give the corresponding 1,1-dibromocyclo-propanes, often in high yield. Some of these products are of limited stability, which creates problems with their isolation and purification. 

M kosza et al. have confirmed the strong catalytic effect of trialkylamines in the generation of dichlorocarbene and also of dibromocarbene. Actually, di-bromocarbene generated by catalysis with trialkylamines adds to 1-alkenes, a reaction that is not observed with dibromocarbene generated with tetraalkyl-ammonium halide catalysts. The authors have presented indirect evidence that dichlorocarbene reacts with a trialkylamine to form a basic salt that abstracts a proton from chloroform ... 

Carbenes generated the more traditional way react similarly. In Trost s synthesis of (-)-anatoxin,297 the alkene unit in 366 reacted with dibromocarbene, generated from bromoform and base, to give an 85% yield of 367 and 368 as a 3.5 1 mixture of exo and endo isomers. 

Entries 1 and 2 in Scheme 2.9 are typical of concerted syn addition to alkene double bonds. On treatment with peroxyacetic acid, the Z-alkene affords the cis-oxirane, whereas the -alkene affords only the iraws-oxirane. Similarly, addition of dibromocarbene to Z-2-butene yields exclusively l,l-dibromo-cw-2,3-dimethylcyclopropane, whereas only 1,1-dibromo-/ra 5-2,3-dimethylcyclopropane is formed from -2-butene. There are also numerous stereospecific anti additions. Entiy 3 shows the anti stereochemistry typical of bromination of simple alkenes. 

Because of the yield of only 16% in the synthesis of 239, the overall yields of cycloadducts with reference to indene according to Scheme 6.54 are rather low, however. A substantial improvement was achieved by the development of a one-pot procedure, which starts from indene and takes advantage of its dibromocarbene adduct (254) (Scheme 6.55). This was prepared at -60 °C with tetrabromomethane and MeLi as source for the carbene and remained unchanged in solution up to temperatures around 0 °C [92]. If an activated alkene and MeLi were added sequentially to such a solution at -30 C, cydoadducts of 221 were isolated in a number of cases in relatively good yields. In Scheme 6.55, this procedure is illustrated by the example of 1,3-cyclopentadiene, which furnished the [4 + 2]-cydoadducts 255 and 256, both as a mixture with endo exo= 2 1, in the ratio of 8 1 in 23% yield with reference to indene [67]. Analogously, the products from 221 and styrene, 1,3-butadiene [92] and 2,3-dimethylbutadiene [66], namely the compounds 240, 241, 246-249 and 250-253, were obtained in yields of 40, 24 and 25%, respectively, by means of the one-pot procedure from indene. 

Dibromocarbene undergoes addition to alkenes in a stereospecific manner. The sole case of nonstereospecific dibromocyclopropanation using bromoform/base/phase-transfer catalyst concerns ( )-cyclooctene, and is explained by isomerization of this cycloalkene caused by reversible addition of tribromomethyl or ethoxide anion the latter is formed from the ethanol present in bromoform (see also ref 2 and Houben-Weyl, Vol. El 9b, p 1617 for stereomutation in the reactions of dibromocarbene, generated from organomercury reagents, with low-active alkenes, see Section 1.2.1.4.3.1.5.1. and Vol. E19b, pp 1615 1616). 

Determination of the optimal conditions for the reaction of dibromocarbene with alkenes is more difficult than for the corresponding reaction of dichlorocarbene. This is due to the high reactivity of dibromocarbene which enters into other competitive reactions, particularly hydrolysis under the conditions of phase-transfer catalysis and, in the case of alkenes of low reactivity, its precursor bromoform forms products of free radical reactions if the reaction system is not protected from air and oxygen and from light. These processes have been studied and are described in detail in Houben-Weyl, Vol.E19b, pp 1609-1612. 

The preparation of mono- and diadducts 5 and 6, respectively, of dibromocarbene with cyclo-hexa-1,4-diene is described. The synthesis of the diadduct encounters difficulties in its isolation, due to the formation of an emulsion and polymeric products. Such problems occur occasionally if an alkene is allowed to react with bromoform under phase-transfer catalysis conditions. 

A similar phenomenon has been observed in the reaction of dibromocarbene with strongly electrophilic alkenes (Section 1.2.1.4.3.1.6.1.). 

Electrophilic alkenes react with bromoform using base/phase-transfer catalyst to give either Michael adducts of the tribromomethyl anion or adducts of dibromocarbene. The 1,1-di-bromocyclopropanes result via direct addition of carbene to the alkene or via the cyclization of the anion of the Michael adduct. Such discrimination between mechanisms is not always made in the literature, therefore all syntheses of 1,1-dibromocyclopropanes, irrespective of the way in which they are formed, are described below (see Houben-Weyl, Vol. El 9b, pp 1618-1620). 

Access to cyclopropanone thioacetals was provided by dibromocarbene addition to an alkene followed by sequential replacement of the bromine atoms with methylsulfanyl groups. Bi-cyclo[n.l. 0]alkanone thioacetals prepared in this way rearranged in the ring-enlargement mode when treated with formic or trifluoroacetic acid (Table 2). 

The reactivity of the singlet and triplet states should be different, as proposed by SkelL The triplet state is a diradical, and would be expected to exhibit a selectivity in its reactions toward olefins similar to that of other species having unpaired electrons. The singlet state, with its unfilled p-orbital, should be electrophilic and exhibit reactivity toward olefins similar to that of other electrophilic species. In general, carbenes add to alkenes to give cyclopropane derivatives. Dibromocarbene is typical in this respect. This reaction could involve the singlet dibromocarbene in a concerted addition, or the triplet state in a stepwise process ... 

Convenient new routes to tricyclo[4,1,0,0 ]hept-3-ene and its derivatives have been disclosed. Acetone-sensitized irradiation of bicyclo[3,2,0]hept-6-en-2-one affords the ketone (624), whose enol phosphate is reduced by lithium-ammonia to give the parent alkene (625). " A second route is also described, starting from the 7,7-di-bromonorcarane derivatives (626) and (627). Reaction of (626) with methyl-lithium in ether at 0°C afforded a 3 1 mixture of the tricycloheptenes (628) and (629) similar reaction of (627) gave (630 40%), but reactions of the parent dibromide were unsuccessful. Catalytic Ag ion causes the rearrangement of (628) and (629) to, respectively, 3-methyl- and 1-methyl-cycloheptatriene. Initial Ag" attack at the least hindered edge bond is implicated. Attempted preparation of the tetrahedrane dimer (631) by the addition of dibromocarbene to homobenzvalene followed by treatment of the adduct so obtained with excess methyl-lithium in ether at 0°C afforded instead 5-ethynyl-cyclohexa-1,3-diene. 

Bromoform gives dibromocarbene, which reacts stereospedfically with the alkene to give a dibromocyclopropane. 

Reaction of Dichloro- or Dibromocarbene with an Alkene (Section 15.3B) The... 

The olefinic selectivity of dibromocarbene generated from bromoform has been reinvestigated and, by employing 3-ethyl-3-heptoxide as the base (to suppress alkene formation), the results reveal minimal kinetically effective carbenoid involvement. This same group of workers have shown that MeSCCl is a free carbene while PhCF is in fact a carbenoid when generated by oxygenated base. A careful study of the relative reactivity of the vinylcarbene (48) by addition to a series of para-substituted styrenes has revealed a Hammett correlation with p = —0.75 treatment of the vinyl triflate with potassium t-butoxide results in the free carbene. ... 

Dibromocarbene 302 was generated by the reductive treatment of CBr4. For example, the treatment of CBr4 with iron and copper resulted in two single-electron reductions of CBr4 to give dibromocarbene, which underwent cycloaddition to alkenes to afford cyclopropane 303 (Scheme 1.148) [214]. 

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