1,1-Dichlorocyclopropanes

Synthesis From Other Ring Systems. These syntheses are further classified based on the number of atoms in the starting ring. Ring expansion of dichlorocyclopropane carbaldimine (53), where R = H and R = ryl, on pyrolysis gives 2-arylpyridines. Thermal rearrangement to substituted pyridines occurs in the presence of tungsten(VI) oxide. In most instances the nonchlorinated product is the primary product obtained (63). 

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). 

If dichlorocarbene is generated in the presence of an alkene, addition to the double bond occurs and a dichlorocyclopropane is formed. As the reaction of dichlorocarbene with ds-2-pentene demonstrates, the addition is stereospecific, meaning that only a single stereoisomer is formed as product. Starting from a cis alkene, for instance, only cis-disubstituted cyclopropane is produced starting from a trans alkene, only trans-disubstituted cyclopropane is produced. 

The catalytic conditions (aqueous concentrated sodium hydroxide and tetraalkylammonium catalyst) are very useful in generating dihalo-carbenes from the corresponding haloforms. Dichlorocarbene thus generated reacts with alkenes to give high yields of dichlorocyclopropane derivatives,16 even in cases where other methods have failed,17 and with some hydrocarbons to yield dicholromethyl derivatives.18 Similar conditions are suited for the formation and reactions of dibromocar-benc,19 bromofluoro- and chlorofluorocarbene,20 and chlorothiophenoxy carbene,21 as well as the Michael addition of trichloromethyl carbanion to unsaturated nitriles, esters, and sulfones.22... 

Both phase transfer and crown ether catalysis have been used to promote a-elimination reactions of chloroform and other haloalkanes.153 The carbene can be trapped by alkenes to form dichlorocyclopropanes. 

The addition of dichlorocarbene, generated from chloroform, to alkenes gives dichlorocyclopropanes. The procedures based on lithiated halogen compounds have been less generally used in synthesis. Section D of Scheme 10.9 gives a few examples of addition reactions of carbenes generated by a-elimination. 

Macdonald and Dolan have reported a new route to the tropinone system (88). 2-Cyclohexenone (160) is transformed to 2-[(trimethylsilyl)oxyl]-l,3-cyclohexadiene (161), and then by dichlorocyclopropanation to 162 and by acid-catalyzed hydrolysis to 2-chloro-2,6-cycloheptadienone (163). Addition... 

The 1-bromo-l-chlorocyclopropanes 191 are reduced to the corresponding chlorocyclopropanes 192 selectively although em-dichlorocyclopropanes do not undergo reduction with dialkyl phosphonate and triethylamine [97]. (Scheme 72)... 

Furthermore, the addition of dichlorocarbene to ene-ynes proved to be remarkably sensitive to substituent effects. Trans-1,4-diphenyl butenyne gave only the cyclopropenone 17 via hydrolysis of dichlorocyclopropene 16, however, 2-methyl-pentene-l-yne-3 favored the formation of the dichlorocyclopropane 18 with only traces of products resulting from addition to the triple bond ... 

Allylic silanes react with dichlorocarbenes, generated from dechlorination of carbon tetrachloride with low valent titanium species, to furnish dichlorocyclopropanes, which in turn get desilylated with CsF in DMF to generate 3-chloro-l,3-butadienes (equation 30)63. 

The electron diffraction studies of 1, 1-dichlorocyclopropane has given the following values for bond distances and bond angles. 

Compared with the classical procedures, which employ chloroform and dry potassium /ert-butoxide, Makosza s method is several magnitudes superior, in spite of the normally recognized requirements that the dichlorocarbene should be produced under totally anhydrous conditions. Several early reports of the reactions of dichlorocarbene, generated by Makosza s procedure, led to suggestions that the activity of the carbene was considerably greater than that of the classically produced carbenes. This assumption was based on the overall higher yields of dichlorocyclopropanes derived from the reaction with alkenes, and upon the observation that weakly activated alkenes reacted with Makosza carbenes, but not with the classically produced carbenes. A consideration of the mechanism of formation of the carbenes under phase-transfer catalytic conditions exposes the fallacies in the assumptions. 

Alkynes tend to be much less reactive than alkenes. For example, 1,2-diphenylethyne produces only 23% of the dichlorocyclopropene from its reaction with dichlorocarbene, compared with 96% of the dichlorocyclopropane obtained from rrans-stilbene under analogous conditions [4]. Conjugated eneynes react preferentially at the C=C bond with dihalocarbenes [18-20, 22, 38] and with dimethylvinylidene carbene [158],... 

Evaporation of the dried (MgS04) extracts gives the dichlorocyclopropane, which is then heated in aqueous HBr (47%) at 100°C for 9 h. The mixture is diluted with H20 and extracted with CH2C12. Evaporation of the extracts produces the cyclopent-3-enone. 

With the exception of the parent compounds, where the Michael adducts are isolated, acrylic esters [see, e.g. 6,7,31,105,111 ] and nitriles [6,7], and vinyl ketones [26, 113, 115] generally yield the cyclopropanes (Table 7.6) under the standard Makosza conditions with chloroform. Mesityl oxide produces a trichlorocyclopropy-lpropyne in low yield (10%) [7]. When there is no substituent, other than the electron-withdrawing group at the a-position of the alkene, further reaction occurs with the trichloromethyl anion to produce spiro systems (35-48%) (Scheme 7.12) [7, 31]. Under analogous conditions, similar spiro systems are formed with a,p-unsaturated steroidal ketones [39]. Generally, bromoform produces cyclo adducts with all alkenes. Vinyl sulphones are converted into the dichlorocyclopropane derivatives either directly or via the base-catalysed cyclization of intermediate trichloromethyl deriva-... 

Dichlorocarbene reacts with oxiranes to produce dichlorocyclopropanes [18] via an initial deoxygenation reaction (Scheme 7.22). 

Substituted 4,4,6-trimethyl-1,4-dihydropyrimidine-2(3//)-thiones behave as enamines [49] to yield dichlorocyclopropanes (Scheme 7.35). Further reaction... 

Methylisoquinol-l-one behaves as an enamine with dichlorocarbene to produce the dichlorocyclopropane derivative (83%). The corresponding reaction with dibromocarbene produces a thermally labile compound, which is assumed to have an analogous structure. Rearrangement of the dichloro compound under basic conditions leads to the isoindole derivative (96%), whereas controlled thermolysis... 

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