Synthetic and mechanistic studies of cyclopropenes
1,2-Dehalogenation of 1,1,2,2-tetrahalocyclopropanes (A, y= Cl, ' X= OMe, Ar) by reaction with one equiv. of methyl-lithium at, 0-20 OC leads' to 1,2-dichlorocyclopropenes (B, X= OMe, Ar); the pentachloride (A, X=Y= Cl) also -react by 1,2-dechlorination to give (B, X= Cl), whereas, the bromide (A, X= Cl, Y= Br) apparently undergoes a 1,3-dehalogenation on reaction with two mol. equiv. of methyl lithium at -70 0C, leading eventually to the cyclobutene (C). The cyclopropenes (13, X= Cl, OMe, Ar) undergo ring-opening at 0-20 OC and in principle produce two stereoisomeric carbenes (D) and (E). Reaction of the cyclopropene (ß, X= Ph) with methyl methacrylate leads predominantly to cyclopropane (F), the structure of which was established by an X-ray crystallographic study. This isomer is apparently derived by strapping of the - carbene (E, X= Ph) rather than (D, X= Ph) Moreover, the ester and alkene substituents are obtained with a cis-stereochemistry. A minor product derived by addition of (D, X= Ph) is also thought to have cis-stereochemistry of ester and vinyl groups. The products derived from the corresponding cyclopropenes (B, X= Cl, OMe) reflect an even more selective trapping of the carbenes (E, X= Cl, OMe). Studies of the relative rates of cyclopropanation' of a standard series of alkyl-substituted alkenes by the carbene derived from (B, X= OMe) showed that the reactivities of this carbene do not give a good "linear correlation with those of (: CC'2); however, they do give a linear correlation when compared to (G). The relative rates of formation of (H) when (I, Y=H, Me, OMe, CF3) were allowed to ring open in the presence of an excess of 2,3-dimethylbut-2-ene gave a good linear correlation with the aI constant for the substituent. Dehydrochlorination of the derived cyclopropanes provides easy access to two series of allylidene cyclopropanes, e. g., (J) and (K). 1.2-Dehalogenation of 1.1.2-trihalocyclopropanes by reaction with one mol. equiv. of methyl lithium leads to I -halocyclopropenes. The latter reacts with second equiv. of the reagent under more vigorous conditions by a lithium halogen exchange, and the resulting lithiocyclopropenes could be trapped by (R)-(+)- or (S)-(-)-methyloxirane to give the alcohols (L) and (M), which are converted to the optically active methylenefuran (N) and dihydropyran (0) respectively by reaction with bromine or with acid.