Dichlorocarbenes

Apparatus Ketene generator (500 ml distillation flask) (see Ref. 20, p. 529) for. the preparation of tetramethylallene 1-1 round-bottomed, three-necked flask, provided with a mechanical stirrer, a thermometer and a vent for the addition of dichlorocarbene 500-ml flask (see Fig. 1) for the dechlorination. 

FIGURE 14 3 (a) The unshared electron pair occupies an sp hybridized orbital in dichlorocarbene There are no electrons in the unhybridized p orbital (b) An electrostatic potential map of dichlorocarbene shows negative charge is concentrated in the region of the unshared pair and positive charge above and below the carbon... 

Make molecular models of the product of addition of dichlorocarbene to... 

The reaction of dihalocarbenes with isoprene yields exclusively the 1,2- (or 3,4-) addition product, eg, dichlorocarbene CI2C and isoprene react to give l,l-dichloro-2-methyl-2-vinylcyclopropane (63). The evidence for the presence of any 1,4 or much 3,4 addition is inconclusive (64). The cycloaddition reaction of l,l-dichloro-2,2-difluoroethylene to isoprene yields 1,2- and 3,4-cycloaddition products in a ratio of 5.4 1 (65). The main product is l,l-dichloro-2,2-difluoro-3-isopropenylcyclobutane, and the side product is l,l-dichloro-2,2-difluoro-3-methyl-3-vinylcyclobutane. When the dichlorocarbene is generated from CHCl plus aqueous base with a tertiary amine as a phase-transfer catalyst, the addition has a high selectivity that increases (for a series of diolefins) with a decrease in activity (66) (see Catalysis, phase-TRANSFEr). For isoprene, both mono-(l,2-) and diadducts (1,2- and 3,4-) could be obtained in various ratios depending on which amine is used. 

Pyrroles may be ring-expanded to pyridines in reactions having a greater academic than practical interest. Treatment of pyrrole with chloroform and sodium ethoxide (in effect, with dichlorocarbene, CCl2) gives a low yield of 3-chloropyridine [626-60-8]. A much better yield (33%) is obtained if chloroform and pyrrole are heated together in the vapor phase at 550°C some 2-chloropyridine (17) is also formed (71). 

Benzal chloride can be manufactured in 70% yield by chlorination with 2.0—2.2 moles of chlorine per mole of toluene. The benzal chloride is purified by distillation. Benzal chloride is also formed by the reaction of dichlorocarbene ( CCl2) with benzene (49). 

The benzyne adducts prepared from A -methylpyrrole (and A -methylisoindole) are deaminated conveniently by dichlorocarbene generated under phase-transfer conditions (81JOC1025 to give a convenient route to substituted naphthalenes (134) (and anthracenes) (Scheme 49). 

Trimethylpyrazole (336) adds dichlorocarbene generated under basic conditions (CHCla-EtONa) to give 10% of 4-dichloromethyl-3,4,5-trimethylisopyrazole (337 Scheme 28) (bromine also transforms (336) into an isopyrazole (312) Section 4.04.2.1.4(v)). Treatment with sodium ethoxide results in ring expansion of (337) into an ethoxymethyl-pyridazine (338) (B-76MI40402). 

Electron deficient species can attack the unshared electron pairs of heteroatoms, to form ylides, such as in the reaction of thietane with bis(methoxycarbonyl)carbene. The S —C ylide rearranges to 2,2-bis(methoxycarbonyl)thiolane (Section 5.14.3.10.1). A"-Ethoxycar-bonylazepine, however, is attacked by dichlorocarbene at the C=C double bonds, with formation of the trans tris-homo compound (Section 5.16.3.7). 

Other non-oxidative procedures have also been used to deaminate aziridines. For example, aziridines react with carbenes to yield ylides which subsequently decompose to the alkene. Dichlorocarbene and ethoxycarbonylcarbene have served as the divalent carbon source. The former gives dichioroisocyanides, e.g. (281), as by-products (72TL3827) and the latter yields imines (72TL4659). This procedure has also been applied to aziridines unsubstituted on the nitrogen atom although the decomposition step, in this case, is not totally stereospecific (72TL3827). 

CIAMICIAN - DENNSTEDT Cyclopropanation Cyclopropanation of alkenes with dichlorocarbene derived Irom CHCI3 and sometimes subsequent rirtg enlargement of fused cyclopropanes 1 Na... 

Isonitnle synthesis from pnmary amines and dichlorocarbene or dibromocarbene... 

Contents Introduction and Principles. - The Reaction of Dichlorocarbene With Olefins. - Reactions of Dichlorocarbene With Non-Olefinic Substrates. -Dibromocarbene and Other Carbenes. - Synthesis of Ethers. - Synthesis of Esters. - Reactions of Cyanide Ion. - Reactions of Superoxide Ions. - Reactions of Other Nucleophiles. - Alkylation Reactions. - Oxidation Reactions. - Reduction Techniques. - Preparation and Reactions of Sulfur Containing Substrates. -Ylids. - Altered Reactivity. - Addendum Recent Developments in Phase Transfer Catalysis. 

The reaction of dichlorocarbenes with enol acetates has been studied by Stork and co-workers. ° This procedure was used to convert 3-acetoxy-5a-cholest-3-ene (31) into 4-chloro-A-homo-5a-cholest-4-en-3-one (33). 

Reaction of 1-ethoxycyclohexene (34) with dichlorocarbene gives 1-ethoxy-7,7-dichloronorcarane (35) in 87 % yield. Rearrangement of dichlorocyclo-propane (35) in hot quinoline results in loss of both chlorine atoms to give l-ethoxycyclohepta-l,3,5-triene (37) in 37% yield. Hydrolysis of enol ether (37) with a very small quantity of hydrochloric acid in methanol produces cyclohepta-3,5-dienone (38) in 91 % yield. ... 

Treatment of dichlorocyclopropane (35) first with pyridine to give chloro-diene (36) followed by potassium /-butoxide affords 1-ethoxycyclohepta-1,3,5-triene (37) in 55% overall yield from dichlorocarbene adduct (35). 

Dibenzoy loxy-7 a, 8a-methylen-5a,8a-androstan-6a-ol, 113 Dibromocarbene, 366, 367, 372 1,4-Dichloro-2-butyne, 68 Dichlorocarbene, 364 9a,l 1 -Dichloroandrosta-1,4-diene-3,17-dione, 150... 

When the alkyltrifluorosilane is pyrolyzed, a more complex mixture is ob tamed, but trichloromethyltrifluorosilane gives dichlorocarbene under similar conditions [JOS] (equation 73)... 

The reaction of enamines derived from cyclohexanone with dichlorocarbene to give the 1 1 adducts is now well established (137-139). The morpholine enamine (113) reacted with dichlorocarbene at —10 to —20° in tetrahydrofuran to give the stable crystalline adduct (201). Thermal decomposition followed by an aqueous work-up gave an a,)3-unsaturated ketone identified as 2-chloromethylene-cyclohexan-l-one (202) (139). 

A similar difference in behavior of the dichlorocarbene adducts of cyclohexene and cyclopentene has been noted previously 140). 

The reaction of iminium salts such as 66 with salts of trichloroacetic acid has been shown to yield amides such as 84 on hydrolysis 126). It was suggested that the reaction proceeds by addition of dichlorocarbene to give an aziridinium intermediate (85), which was opened by trichloroacetate followed by hydrolysis to give the observed products 126). The observed products from the reaction can be accounted for by formation of CCI3,... 

Addition of dichlorocarbene (88) to the enamine of cyclohexanone gives a relatively stable adduct 89 n = 4) 103-105). Hydrolysis of this adduct... 

The reactions of dichlorocarbene with morpholine and piperidine enamines derived from cyclopentanone and cyclohexanone have been reported to lead to ring expanded and a-chloromethylene ketone products (355,356). Similarly a-chloro-a, -unsaturated aldehydes were obtained from aldehyde derived enamines (357). Synthesis of aminocyclopropanes (353,359) could be realized by the addition of diphenyldiazomethane (360) and the methylene iodide-zinc reagent to enamines (367). 

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