Dibromocarbene addition

A mixture of 0.20 mol of the adduct from cyclooctene and dibromocarbene (note 1) and 250 ml of dry diethyl ether was cooled to -65°C. A solution of 0.23 mol of ethyllithium (note 2) in 200 ml of diethyl ether (see Chapter II, Exp. 1) was added in 15 min with cooling between -60 and -50°C. The reaction was very exothermic (note 3). After the addition the cooling bath was removed and the temperature was allowed to rise to about -10°C and the reaction mixture was poured into 200 ml of ice-water. The aqueous layer was extracted twice with diethyl ether. After drying, the solvent was removed in a water-pump vacuum and the remaining liquid was distilled through a 40-cm Vigreux column. 1,2-Cyclononadiene, b.p. 62°C/22 mmHg, 1.5059, was obtained in 86 yield. 

The syn stereochemistry of dibromocarbene cycloaddition was demonstrated in experiments using as- and trans-2 butene Give the structure of the product obtained from addition of dibromocarbene to each alkene... 

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. 

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. 

Potassium /-butoxide is prepared by dissolving potassium metal in t-butanol followed by removal of the excess of -butanol by distillation under reduced pressure. The resultant cake is powdered and used directly in the dibromocarbene additions. 

Birch s procedure for tropone synthesis appears to be widely applicable to 2,3- or 2,5-dihydroanisole derivatives which are readily obtained by reduction of appropriate aromatic methyl ethers by alcoholic metal-ammonia solutions. " Additional functional groups reactive to dibromocarbene or sensitive to base such as double bonds, ketones and esters would need to be protected or introduced subsequent to the expansion steps. 

The cyclopropane derivative 1, formed by the addition of dibromocarbene to 4-ethoxy-l-tosyl-1,2-dihydroquinoline, in refluxing pyridine undergoes ring expansion and aromatization to the 1/f-l-benzazepine 2.168... 

The, g 7M-dibroniodihydrocyclopropa[r]isoqiiinolines 8 (R = H, Cl), prepared by addition of dibromocarbene to the corresponding 2-acctyl-l-phcnyl-l,2-dihydroisoquinolines, in the presence of silvcr(I) trifluoioacetate, undergo rearrangement to the 5/f-2-bcnzazepines 9, albeit in poor yields.3... 

Similarly, the ratio of dibromocarbene addition to CBr. substitution in the reaction of propyl bromide with tribromomethyl can be varied between 92 1 and 1 91, depending on whether a small and accessible quat or a bulky anion-activating quat is used (Dehmlow and... 

Direct attachment of a functional group to the carbon atoms of the cyclopropane ring is fairly difficult. gem-Dibromocyclopropanes are readily available through the addition of dibromocarbene to olefinic compounds. Their synthetic versatility is reviewed in a previous volume of this series [77]. Substitution of the bromide with the aid of nucleophilic organometallics to form a new carbon-carbon bond has been investigated [78]. 

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 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]. 

The cycloadditions of 1-substituted 1,2-cyclohexadienes and among them their dimerization are of interest because of the position selectivity. Does the reaction occur at the substituted or the unsubstituted ethylene subunit For that question to be answered, 1-methyl- (74), 1-phenyl- (75), 1-cyclopropyl- (76), l-(3-phenylpropyl)-(77) and l-trimethylsilyl-l,2-cyclohexadiene (79) were generated from the corresponding 1-substituted 6,6-dibromobicyclo[3.1.0]hexanes with methyllithium. Several of these dibromides are thermolabile, which particularly applies to the phenyl (93) [76] and the cydopropyl derivative [70], In those cases, it is advisable or necessary to prepare the dibromide in situ, that is, the dibromocarbene is liberated from tetrabro-momethane with methyllithium at -60 °C in the presence of the respective cyclopen-tene. Without workup, from the thus formed 6,6-dibromobicyclo[3.1.0]hexane, the 1,2-cyclohexadiene is then generated by addition of methyllithium at -30°C. 

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,... 

Alternative procedures for the generation of dichlorocarbene and dibromocarbene under phase-transfer catalysed conditions are also available. Where the reactive substrate is labile under basic conditions, the thermal decomposition of solid sodium trichloroacetate or bromoacetate under neutral conditions in an organic solvent is a valuable procedure [10-12], The decarboxylation is aided by the addition of a quaternary ammonium salt, which not only promotes dissolution of the trihaloacetate anion in the organic solvent, but also stabilizes the trihalomethyl anion. Under optimum reaction conditions, only a catalytic amount of the quaternary ammonium salt is required, as a large amount of the catalyst causes the rapid generation of the dichlorocarbene with resultant side reactions. 

The addition of dichloro- [114] and dibromocarbenes generated from Ph-HgBrg [63] or bromoform [33, 34, 115] gave the corresponding 7,7-dihalotri-spiro[2.0.2.1]heptanes 120 in 86-95% yields (Scheme 24). 

The additions of difluorocarbene onto the bicyclopropylidene derivative 121 (Scheme 24) [116a], of dibromocarbene onto oligospirocyclopropanated bicy-clopropylidenes 56 [33,65] (66% yield) and 62 [117] (94% yield), of dichloro-carbene onto 56 [33] (90% yield), 28a [118] (38% yield) and 121 (99% yield) 1116a] as well as of chloro(phenylthio)carbene (90%yield) [33,1191,of bromo-... 

Reaction of norbornane trithiolane (24) with both dichloro- and dibromocarbene afforded the trithiocarbonate (94), which upon further reaction with the dihalocarbene yielded the corresponding dithiocarbonate <90JOC1146>. The mechanism of formation of (94) from (24) is postulated to proceed by addition of the carbene to S-2 of the trithiolane, followed by a ring-expansion reduction sequence as outlined in Scheme 22. 

Acylation of limonene at the disubstituted double bond is favoured by a factor of 2.3 over reaction at the trisubstituted double bond using acetyl hexachloroan-timonate. Mixed alkylcuprate alkylation of tricarbonylcyclohexadienyliron salts has been used to synthesize the a-phellandrene tricarbonyliron complex. Dichlorocarbene addition to limonene in the presence of 1,4-diazabicy-clo[2,2,2]octane is almost 100% stereoselective at the trisubstituted double bond (no yield given) (cf. Vol. 6, p. 31) in contrast to dibromocarbene addition to carvone (Vol. 7, p. 34), dichlorocarbene addition to the carveols is not regio-specific. ... 

The ketone (48) can be obtained by Dieckmann cyclization, but this reaction failed for other isomers of (48) (72JCS(Pl)885). Friedel-Crafts cyclization has been used to obtain the N-tosyl ketone (49) (72JCS(Pl)2l3). Ring expansion reactions based on dibromocarbene additions have also been used to prepare benzo derivatives (72JCS(Pl)889). 

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... 

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