Dihalocarbene intermediate

HFA reacts with trihalogenomethylphenylmercury compounds to form oxiranes 191, probably via dihalocarbene intermediates (248). 

Dihalocarbenes, in two-phase catalytic systems, 35 393-398, 408-414 Dihydronaphthalenes, intermediates of naphthalene hydrogenation, 18 32, 33 Diiodocyclodextrin, 32 437-438 Diisopropyl fluorophosphate, as inhibitor, 20 388... 

At the HF/6-31G level, ketenyl carbenes (1) were calculated to be intermediates in the decarbonylation of 1,2-bisketenes (2) to form cyclopropenones." At the MP2/6-31G and B3LYP levels, however, decarbonylation was predicted to form the cyclopropenones directly. The anfi-ketenyl carbenes were found to be 2.2-5.4kcalmoP higher in energy than the syn isomers (1). The mechanism of reaction of [l.l.ljpropellane with singlet dihalocarbene has been reported. ... 

Aromatization of dihalocarbene adducts to 1,4-cyclohexadiene or synthetic equivalents is the method of choice for the synthesis of the parent benzocyclo-propene (1). ° The mechanism of the aromatization step of the intermediate 7,7-dihalogenobicyclo[4.1.0]hept-2-ene (51) has been shown by labeling experiments with 51 depleted of C at Cl, to proceed via a series of elimination and double bond migration steps via cyclopropene- and alkylidenecyclopropane intermediates 52 to 54 with preservation of the original carbon skeleton. The synthesis of the benzannelated homologue, l//-cyclopropa[b]naphthalene (42), by the same route confirms these findings. Some skeletal rearrangement has, however, been observed in an isolated case. ... 

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]. 

Carbenes and substituted carbenes add to double bonds to give cyclopropane derivatives (1 + 2 cycloaddition).1008 Many derivatives of carbene, e.g., PhCH, ROCH,1009 Me2C=C, C(CN)2, have been added to double bonds, but the reaction is most often performed with CH2 itself, with halo and dihalocarbenes,1010 and with carbalkoxycarbenes1011 (generated from diazoacetic esters). Alkylcarbenes HCR have been added to olefins,1012 but more often these rearrange to give olefins (p. 201). The carbene can be generated in any of the ways normally used (p. 198). However, most reactions in which a cyclopropane is formed by treatment of an olefin with a carbene precursor do not actually involve free carbene intermediates. In some cases it is certain that free carbenes are not involved, and in other cases there is doubt. Because of this, the term carbene transfer is often used to cover all reactions in which a double bond is converted to a cyclopropane, whether a carbene or a carbenoid (p. 199) is actually involved. 

The addition of dihalocarbenes to sila- and germa-cyclopent-3-enes provides the intermediates for a convenient ring expansion to cyclohexadienes. The bicyclic derivatives (108) vary in thermal stability through the sequences Si[Pg.612]

Like enamines, dihalocarbenes add smoothly to enol ethers and in many cases it is possible to isolate the dihalocyclopropyl intermediates which are valuable synthons for chloroenones (cf. Section 4.7.3.7.1). The earliest example of the addition of a dihalocarbene to an enol ether was provided by Parham,6,79 who studied the addition of dichlorocaibene to dihydropyran (equation 22). An example which illustrates the synthetic potential of the process is the conversion of the cyclohexanone enol ether (6) to the dichlorocy-clopropane (7 equation 23).80 The latter served as a useful intermediate in a stereospecific synthesis of Prelog-Djerassi lactonic acid. 

The production of buta-1,3-dienes (37) by reaction of 1,2-diarylcyclopropenes with dihalocarbenes is thought to involve electrophilic attack of the carbene to give a dipolar intermediate (38).51 The addition of carbene to CO and H2C=0 has been studied by MNDO calculations.52 Photolysis of diethyl diazomalonate in a CO matrix at low temperature gave rise to ketenes by immediate trapping of the postulated carbene (39).53 The major products of reaction between dichlorocarbene and cyclone were CO and the gem-dichloro species (40).54 The predominance of this pathway over formation of the dichlorooxirane or the cyclopropane is attributed to the aromatic nature of the carbonyl ylide and its twist geometry. 

Under this heading a few borderline cases between direct and indirect electrochemical reactions can be included. Dihalocarbenes are probably intermediates in the cathodic reduction of certain polyhalogenated compounds172) as an example, cathodic reduction of carbon tetrachloride at a mercury cathode in acetonitrile-tetrabutylammonium bromide in the presence of tetramethylethylene gave a low yield of l,l-dichloro-2,2,3,3-tetramethylcyclopropane ... 

Dihalocarbenes, CX2, are especially useful reactive intermediates since they are readily prepared from trihalomethanes (CHX3) by reaction with a strong base. For example, treatment of chloroform, CHCI3, with KOC(CH3)3 forms dichlorocarbene, CCl2. 

Botta, M., De Angelis, F., Gambacorta, A. The reaction between 2,3-dialkylindoles and dihalocarbenes. Additional evidence for the interconversion of the reaction intermediates. Gazz. Chim. Ital. 1983, 113, 129-132. 

A comparison of olefin reactivities toward CeHjHgCBrCt in benzene at 80° with the reactivities of the same olefins toward sodium trichloroacetate in 1,2-dimethoxy-ethane at 80° established near identity of the relative reactivities toward both reagents, a result which favors the interpretation that both reactions involve free dichlorocarbene as an intermediate. Of practical significance is the fact that yields are consistently higher by the mercurial route. Thus the latter route proved effective as applied to olefins of low reactivity toward dihalocarbenes generated by other procedures. Examples are formulated ... 

The addition of dihalocarbenes to alkynes under many conditions results in hydrolysis of an intermediate 3,3-dihalocyclopropene to produce a cyclopropenonc. Such reactions are described in Section 5.A.1.1.1. Related reactions in which a halocyclopropene may be isolated are shown in Table 5. 

The process in which a dihalocarbene is formed from a trihalomethane corresponds to an elimination in which a proton and a halide are lost from the same carbon. It is an a-elimimtion proceeding via the organometallic intermediate [iCXg]. ... 

The Ciamician—Dennstedt reaction involves the addition of intermediate dihalocarbene, generated from haloforms (CHX3, X = Cl, Br, or I) and a strong base, to a pyrrole (330) to form an unstable dihalogenocyclopropane, which rearranges to a 3-halogenopyridine such as 331. The reaction was expanded for indoles to provide 3-chloroquinolines. ... 

Dihalocarbenes, synthetically useful intermediates, are normally generated by the action of a base on chloroform. 

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