Allylic dibromocarbene addition

Dibromocarbene has been successfully generated under phase transfer conditions and added to a variety of olefmic substrates. These include isolated double bond systems, styrenes, conjugated dienes, allenes, cyclopropanated olefins, vinyl ethers, allylic halides, and enynes. It is interesting to note that with the latter class of compounds, dibromocarbene addition to double bonds appears to be favored over addition to triple bonds. The simple addition of dibromocarbene according to equation 4.1 to a number of substrates is recorded in Table 4.1. 

The grem-dibromocyclopropanes 152 bearing a hydroxyalkyl group, prepared by the addition of dibromocarbene to allylic or homoallylic alcohols, undergo an intramolecular reductive carbonylation to the bicyclic lactones 153. bicyclic lactone derived from prenyl alcohol is an important precursor for the synthesis of ris-chrysanthemic acid. (Scheme 54)... 

The cocatalytic effects of pinacol in the phase transfer catalysis (PTC) of dihalocarbene additions to alkenes were noted by Dehmlow and co-workers who showed that pinacol accelerates the PTC deprotonation of substrates up to pKa 27.7 Dehmlow also studied the effects of various crown ethers as phase transfer catalysts in the addition of dibromocarbene to allylic bromides.8 In Dehmlow s study, elevated temperature (40°C) and dibenzo-18-crown-6 did not give the highest ratio of addition/substitution to allyl bromide. However, the submitters use of pinacol,... 

Unsaturated compounds (especially cyclic ones), which possess allylic C-H bond(s), are prone to insertion of dibromocarbene, in addition to cycloaddition to the double bond. Due to the similar physical properties of these products, their separation is troublesome, e.g. formation of 4 and 5. ... 

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 %. 

Addition of dibromocarbene to strained carbon-carbon double bonds can lead to rearranged products. These products result from ionization of the initial 1,1-dibromo-cyclopropane adduct with ring opening to yield an allylic carbonium ion/bromide anion pair. Alkyl group shifts followed by ion pair collapse can then lead to rearranged products (see Sect. 2.6). Three examples of this phenomenon are illustrated as equations 4.2—4.4 [9, 12, 13]. For related examples, see equations 2.17—2.21. 

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