Carbene, dichloro-, reactions with

An important synthetic application of this reaction is in dehalogenation of dichloro- and dibromocyclopropanes. The dihalocyclopropanes are accessible via carbene addition reactions (see Section 10.2.3). Reductive dehalogenation can also be used to introduce deuterium at a specific site. The mechanism of the reaction involves electron transfer to form a radical anion, which then fragments with loss of a halide ion. The resulting radical is reduced to a carbanion by a second electron transfer and subsequently protonated. 

Cycloocta-2,5-diene yields the mono adduct with dimethylvinylidene carbene ( 19%) [156]. With dichloro- and dibromocarbene, the syn- and cmh-bis-adducts are obtained in a ratio which favours the syn-isomer [55, 104] whereas, with bromochlorocarbene, the mono-adduct is reported to be the major product (55%) with only 9% of the bis-adduct [142]. In contrast, cyclo-octatetraene is converted into the mono-, syn-1,2 5,6-bis-, tris-, and tetra-adduct with dichlorocarbene depending on the reaction conditions [4, 17, 25, 55] and the 1,2 5,6-bis-adduct with... 

In reactions which have some analogy with the interaction of dichloro-carbene/trichloromethyl anions with ketones, 2-dichloromethyloxazolines yield chloro-oxiranes and a-chlorocarbonyl compounds (Scheme 7.18). The formation of the oxiranes is favoured with aldehydes and lower homologue ketones, whereas cyclic ketones and aryl ketones are converted preferentially into the a-chloro carbonyl derivatives [18]. 

The major product isolated from the reaction of secondary amines with dichloro-carbene the reaction under phase-transfer conditions is the A-formylamine [12-14], The isolated yields are considerably higher (Table 7.14) than those recorded for classical procedures and are not inhibited by steric effects [15]. Diphenylamine is converted into its A-formyl derivative in low yield by conventional procedures and, although application of the phase-transfer catalysed procedure increases the yields, the E- and Z-l,2-bis(diphenylamino)-l,2-dichloroethenes are also obtained as byproducts of the reaction [13],... 

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. 

Nielsen et al. have introduced a monoether linked bis-carbene [209] modelled on an amino linked bis-carbene ligand that acts as a C,N,C pincer ligand in a corresponding palladium(II) complex [156]. Synthesis of the ether linked bis-carbene is facile and involves the reaction of the l-co-dichloro-diethylether with 2 equiv. of methylimidazole. Subsequent reaction with silver oxide and carbene transfer to suitable transition metal precursor complexes affords the corresponding complexes (see Figure 3.73). 

Phenylmercury derivatives are the best carbene-transfer reagents in this category. 1 -Bromo-1,2,2,2-tetrafluoroethyl(phenyl)mercury (1) reacted with alkenes at elevated temperature to give 1-fluoro-l-trifluoromethylcyclopropanes 3 and 4 (X = F).3 Similarly, thermolysis of 1-bromo-l-chloro-2,2,2-trifluoromethyl(phenyl)mercury (2) in the presence of excess alkene yielded 1-chloro-l-trifluoromethylcyclopropanes 3 and 4 (X = Cl) and occasionally a small amount of 1 -bromo-1 -trifluoromethylcyclopropane 5.3,4 1,1 -Dichloro-2,2,2-trifluoroethyl(phenyl)mer-cury is thermally too stable to be useful for the carbene-transfer reaction.5 The carbene pre-... 

An interesting study reports carbene additions to the androstane enol ether (139). Addition of dibromo- and dichloro-carbene gave the D-homo-a-halo-genoenones (140a) and (140b), respectively. The Simmons-Smith reaction on (139) afforded the l6a,17a-cyclopropyl steroid (141). Whereas treatment of (141) with acid provided the D-homo-derivative (142), reaction with iodine, followed by... 

Polyisoprenes and polybutadienes can also be modified by reactions with carbenes. Dichloro-carbene adds to natural rubber dissolved in chloiofoim in a phase transfer reaction with aqueous NaOH and a phase transfer reagent. Solid sodium hydroxide can be used without a phase transfer reagent. There is no evidence of cis-trans isomerization and the distribution of the substituents is random. ... 

The cheletropic reactions of dihalogenocarbenes with norbomadienes have been the subject of further investigations. The main reactions involved are 1,2-addition, which can occur from the endo and exo directions, and homo-1,4-addition. For reaction with norbomadiene itself it has been found that the exo endo selectivity ratio increases som hat in the series difluoro-, fluorochloro-, dichloro-, and dibromo-carbene (respectively 1.2,1.8,4.0, and 2.0), whereas the selectivity ratio for the 1,2 1,4... 

A similar reaction sequence accounts for the formation of 1,l-dichloro-2,2,3,3-tetramethylcyclopropane in 15% yield from the reaction of pinacol with dichloro-carbene. Likewise, reaction of m 5o-dihydrobenzoin with dichlorocarbene gave a 15% yield of cw-stilbene [10] (Eq. 3.7). 

The only known reaction of a furan with a dihalocarbene is that recently reported between benzofuran and dichlorocarbene in hexane at 0°. The initial adduct (7) could not be isolated but on hydrolysis gave the ring-expanded product 8, possibly via 9, in 15% yield. Benzothiophene was recovered in 92% yield under the same conditions. 2,5-Dihydrofuran reacted with dichloro- and dibromo-carbene to give the products of allylic insertion, 2-dihalogenomethyl-2,5-dihydrofuran, as well as the normal addition products. ... 

Cyclopropylchlorocarbene [20] has been generated by UV photolysis (A = 335 nm) of cyclopropylchlorodiazirine [21] frozen in a nitrogen matrix at 12 K (Ho et al., 1989). IR and UV spectra of [20] have been recorded. The reaction of [20] with HCl resulted in the formation of (dichloromethyl)-cyclopropane [22], and annealing of the matrix gave (dicyclopropyl)dichloro-ethene [23]. Subsequent irradiation (A = 450 nm) of the carbene [20] led to its isomerization to 1-chlorocyclobutene [24], which was partialy destroyed to give ethene and chloroacetylene. Ab initio calculations predict the existence of two carbene conformers, but attempts to distinguish them in IR or UV spectra were unsuccessful. 

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