Dichlorocarbene addition

Evidence for the generation of these reactive intermediates was obtained from a study of the effect of added cydohexene on the sonication of chloroform. The presence of free radicals in the system was confirmed by the appearance of chlorocydohexane as a product and by the increased rate of decomposition of CHCI3 in the presence of cyclohexene. The increased decomposition rate is a consequence of the presence, in the cavitation bubble, of the alkene which mops up the Cl radical as it is formed and prevents the regeneration of chloroform - i. e. the kinetic steps in Scheme 3.4 are driven from left to right. Carbene intermediates are implicated by the formation of tricyclic compounds such as (1) via dichlorocarbene addition to cydohexene. 

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. ... 

Although the gem-dihalocyclopropanes are fairly stable compounds, they can participate — as has been shown in the above sections — in quite a number of chemical transformations. Several reactions between dihalocarbenes and alkenes have been described in which no dihalocyclopropane formation could be observed that these intermediates might have been produced was only inferred from the type of products finally isolated. A typical process of this type is the e/ufo-addition of dihalocarbenes to norbomene and norbomadiene as discussed above. Comparable rearrangements have been observed, when dichlorocarbene additions either lead to aromatic products or when they cycloadd to rather inert aromatic systems. In the latter case a ring-enlargement takes place. A reaction related to the concerted opening of two cyclopropane rings in a bicyclopropyl system as discussed above takes place when dichlorocarbene is added to spiro[2.4]hepta-4,6-diene [227]. 

N,N-Dichlorobenzenesulfonamide, in. N-chlorination of amines, 41, 84 Dichlorocarbene, addition to dihydropy-ran, 41, 76... 

The ring expansion of furans by dichlorocarbene addition gives unstable 2,3-dichlorochromenes, which are trapped by nucleophilic displacement reactions (Scheme 13).232 233... 

Hi) By formation of seven- from six-membered rings The expansion of a di- or a tetra-hydropyran ring fused to a three-membered ring has been used as a synthetic approach to oxepins. Thus the synthesis of oxepin (193) was attempted by thermal dehydrochlorination of a tetrahydropyran (equation 57) obtained from a dichlorocarbene addition reaction (65CI(L)184). Unfortunately the equilibrium appeared to favor the keto tautomer to the apparent exclusion of the enolic oxepin form (193). 

The 7,7-dihalobicycloheptene route appears easily adaptable to the synthesis of 6n 5 atom cyclopropa[c]heteroarenes 61 as the three-membered ring is suitably located and the requisite precursors—the dichlorocarbene addition products of, for example, 2,5-dihydrofuran—are easily accessible. Unfortunately, treatment of halides 59 with r-BuOK fails to provide any evidence for sought after 61. Dehydrochlorination does occur but the strained 1,3-bridged cyclopropenes 60 ring expand to carbene or add a nucleophile faster than rearrangement and loss of a second molar equivalent of HCl. The precise outcome of these reactions is very dependent upon the nature of the ring system as the detailed studies show . ... 

Dichlorocarbene addition to aikenes. Dehmiow and LisseP have examined the reaction variables in the generation of dichlorocarbene by PTC. Optimal conditions include use of 4 molar excess each of CHCI3 and 50% aqueous NaOH, 1 mole % of catalyst, and efficient stirring. The reaction should be conducted initially at 0-5°, then at 20° for 1-2 hours, and finally at 50° for 2-4 hours. Most quaternary ammonium salts are suitable as catalysts the anions should be chloride or hydrogen sulfate. From the point of cost/efficiency, the most useful are benzyltriethylammonium chloride, tetra-n-butylammonium chloride, Aliquat 336, and tri-n-propylamine. The reaction rate is strongly dependent on the nucleophilicity of the alkene. 

Dichlorocarbene addition to azodicarboxylate esters, Phenyl(bromodichloro-mcthyl)mercury reacts with diethyl azodicarboxylate to give phenylmercuric bromide (98% yield) and a product (87% yield) which was found not to be the expected di-aziridine( I), but to have the structure (2). The same product is obtained by decarboxylation of CCIjCOONa in the presence of diethyl azodicarboxylate in refluxing 1,2-dimethoxyethane (69% yield). 

Since dibromocarbene is more susceptible to hydrolysis than dichlorocarbene, dibro-mocarbene addition by the PTC method often gives the expected adducts in lower yields than those of the dichlorocarbene addition. Use of excess tribromomethane , execution of the reaction at ambient or lower temperature and addition of a small amount of alcohol have been recommended to improve the yields. Diiodocarbene has also been generated by PTC procedures and has been added to alkenes . However, the CHl3/r-BuOK method appears to be superior . It has also been pointed out that the Cl2 adducts are sometimes rather unstable when they are prepared by the PTC method, probably due to contamination of some sensitive byproducts . 

It was found that alkylidenemalononitriles 41 undergo 1,1-dichlorocyclopropane (223) formation by reaction with trichloroacetic acid. This reaction is presumed not to involve a dichlorocarbene addition, but instead a trichloromethylated carbanionic intermediate (equation 64) Substitution of one of the cyano groups by another EWG such as an ester... 

The relative rates of dichlorocarbene addition to cyclopropylalkenes showed some significant differences (Table 23) that were attributed to the combined influence of steric and electronic factors It was concluded that unique effects due to the cyclopropyl groups did not occur in this concerted cycloaddition. ... 

Reaction of piperidone 145 with dichlorocarbene under phase-transfer catalysis conditions affords a mixture of 146 and 147 in 85-90% yields (80JOC1513, 80TL119) with the ratio determined by the catalyst (80JOC1513). The reaction presumably occurs via dichlorocarbene addition onto the carbonyl group of 145 followed by rearrangement with ring contraction. 

The regioselectivity of dichlorocarbene addition to 1-alkyl-1,3-dienes is sometimes only slightly marked. Furthermore, the addition of dichlorocarbene to the double bond more distant from the aryl group in 1-aryl-1,3-dienes and particularly 1-arylpolyenes, assures energetically suitable Tiaryr vinyi Conjugation in the products. The examples of 3 and 4," 5 and 6, and 8 support these conclusions. 

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