Dienes dibromocarbene addition

The reaction of dibromocarbene with cyclooctatetraene affords mono-, di-, tri- and tetraad-ducts, the latter two in yields below 1% (Houben-Weyl, Vol. E19b, p 1614). For recent examples of dibromocarbene additions to polycyclic unconjugated dienes, see ref 77. 

Hydrolysis in buffered solution of the bromomethylenecyclopropanes (275) leads almost exclusively to allenic tertiary alcohols (276). This constitutes a homologation sequence for allenes since the monobromo-compounds are accessible by tributyltin hydride reduction of the dibromo-analogues which are themselves obtained by dibromocarbene addition to allenes. Cyclonona-1,2-diene (277) having high optical purity has been prepared by... 

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. 

An ingenious approach to the synthesis of steroids incorporating a tropone A ring has been developed by Birch and co-workers. Addition of dibromocarbene to 3-methoxyestra-2,5(10)-dien-17-one 17-ethylene ketal (42) gives a monodibromocarbene adduct formulated as (43) accompanied by a minor amount of a bisadduct. This confirms earlier observations that electrophilic halocarbenes add mainly to 2,3- or 2,5-dihydroanisoles at the double bond bearing the methoxyl group. 

The next higher homologs of 28-30 have all been prepared without difficulty as shown in Scheme 5.37. Single and double addition of dibromocarbene to 1,5-hexa-diene (biallyl) (240) leads to the adducts 241 and 242, respectively, which on methyl-lithium treatment are debrominated/rearranged to 32 and 34 in the usual way [43, 96, 97]. 

Addition of Dibromocarbene to 3-Methoxy estra-3,5( 0)-dien-17-one ll-Ketal (46). Resublimed potassium r-butoxide (1.36 g) is suspended in a solution of... 

The mechanism of the cycloaddition of singlet dibromocarbene to formaldehyde was studied using DFT at the B3LYP/6-31G level of theory 47 The energy barrier is estimated as 13.7 kJ mol-1. Reaction paths for the addition of dichlorocarbene to 1,2-disubstituted cyclopropenes were studied using the same level of theory as above.48 The addition gives 1,3-dienes or bicyclobutanes and was predicted to be concerted following an asymmetric approach. 

Dibromocarbene undergoes addition to the more substituted (more nucleophilic) double bond of unconjugated di- and polyenes di- and polyadducts may also be obtained, particularly using the bromoform/base/phase-transfer catalyst method. For the preparation of monoadducts, using an excess of diene possessing equivalent double bonds, bromoform and base/phase-transfer catalyst or potassium /er/-butoxide are recommended. The examples 1, 2 and 4 are typical. 

In the case of compounds with a fixed c -l,3-diene system, 1,4-addition may compete with 1,2-addition of dibromocarbene, e.g. formation of 6 and 1. ... 

Addition of dibromocarbene to bicyclo[3.2.1]octa-2,6-diene, under the basic conditions used to generate the carbene, yielded 3,4-dibromobicyclo[3.3.l]nona-2,7-diene (27), which was de-hydrobrominated on treatment with potassium ter/-butoxide in dimethyl sulfoxide, to give 3-bromotricyclo[3.3.1.0 ]nona-3,6-diene (28). ... 

Silver nitrate-catalysed rearrangements in methanol of dibromocyclopropyl propellanes have been examined. For example, (80 R = Br) gave a mixture of the corresponding methyl ketal (80 R = OMe) and bicyclo[5,4,0]undec-l(7)-en-2-one, together with minor products. One step in a reported synthesis of 1,7-methano-[12]annulene involved addition of dibromocarbene to tricyclo[4,4,l,0 ]undeca-3,8-diene (81) followed by silver acetate-catalysed ring-opening of the resultant bis-dibromocarbene adduct to give a mixture of isomeric acetates. ... 

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. 

The bis-addition of dibromocarbene to the ( , )-dienes (58 R = Bu or Ph) results in the exclusive formation of meso-bicyclopropyl (60 R = Bu or Ph). Since the same monocarbene adduct (59) must result from addition to either double bond, the observed stereospecificity can only be due to the second addition being controlled by the conformational preference of (59), as shown in Scheme 6. Indeed, with (58 R = Me), where conformational constraints are diminished, a 7 3 mixture of meso-to d/-bis-addition products was obtained. 

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 %. 

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