1,2-Dialkyl-substituted cyclopropenes

The base peaks in the 70 eV electron impact mass spectra of 3,3-dimethylcyclopro-pene (223) , 3-substituted 1,2-diphenylcyclopropenes (224), 3-thiocyanato-1,2,3-triphenylcyclopropene (227) and tri-t-butylcyclopropenyl azide (229) just to mention a few examples, are often found to correspond to the loss of the (3)-substituent, thereby generating the substituted cyclopropenylium ions 215, 225, 226, 228 and 230, respectively (Scheme 33). It should be mentioned, however, that there exists at least one report describing the complete absence of cyclopropenyhum-type ions in 70 eV mass spectra of 1,2-dialkyl-substituted cyclopropenes. The reported spectra indicate that allylic cleavage seems to be operative, although no further experimental results are known which would lend support to such a suggestion. 

Another method used to prepare dialkyl-substituted diazomethanes involves the photolysis of 2-alkoxy-2,5-dihydro-1,3,4-oxadiazoles (209), which can be prepared by the oxidative cyclization of A(-acetyUiydrazones. The diazoalkanes are trapped in situ by cycloaddition with dimethyl acetylenedicarboxylate (54) (Scheme 8.49). The resulting pyrazoles 210 are converted into cyclopropenes 211 by continued irradiation. 

Problems with subsequent reaction of a 1-halocyclopropene may be avoided by reduction of the dihalocyclopropane to a monohalocyclopropane prior to dehydrohalogenation. Thus 3,3-dimethylcyclopropene may be obtained in multi-gram quantities by treatment of l-bromo-3,3-dimethylcyclopropane with potassium t-butoxide in DMSO at —78 °C19). Dehydrohalogenation of a series of related 3,3-dialkyl substituted monochloro- or monobromo-cyclopropanes leads to moderate yields of cyclopropenes (11, R = alkyl, alkenyl, aryl, CN, Cl, Rl = H, Ph, t-Bu, R2 = H)20) Indeed, dehydrobromination of (12) leads to either mono- or di-cyclopropenes 2l>. Reaction of a dihalocyclopropane with an alkyl lithium at low temperature followed by carboxylation of the derived 1-lithio-l-halocyclopropane provides a convenient source of 1-halocyclopropane carboxylates dehydrohalogenation leads to cyclo-... 

The reaction of the 2-substituted cyclopropenes (R = Et, Ph) with organocuprates favored, after hydrolysis, the 2,2- over 2,3-dialkyl adducts in a 29 1 to > 99 1 ratio [134] (Scheme 7-115). 

Cyclopropen-1-yl sodium derivatives are also readily prepared. Thus reaction of cyclopropene with one equivalent of sodium amide in liquid ammonia leads to 1-sodiocyclopropene which is alkylated by haloalkanes 77,78 reacts with ketones to produce tertiary alcohols and opens epoxides to produce 2-cyclopropenyl-ethanols in moderate to good yields79). Moreover, on reaction with two equivalents of base followed by haloalkane, 1,2-dialkylated species are obtained sequential reactions can also be used to produce unsymmetrically substituted cyclopropenes78). Reaction with a deficiency of sodium amide can also cause addition of the cyclopro-penyl anion to unreacted cyclopropene, leading to products derived from the 2-cyclo-propylcydopropen-l-yl anion and to 1,2-dicyclopropylcyclopropene 77). 

---