Methylenecyclopropanes from cyclopropenes

FSg.. 1. Synthons derived from cyclopropenes and methylenecyclopropanes via transition metal catalysis (dipolar structures not considered)... 

Similarly, partially fluorinated and perfluorinated methylenecyclopropanes [57, 52], cyclopropenes [55, 84, 55], cyclobutenes [75, 56], and bicychc alkenes [57, 55, 59, 90] apparently denve dienophilic reactivity from relief of their ground-state strain during reaction Thus 2,2-difluoromethylenecyclopropane and perfluoromethylenecyclopropane undergo exclusive [244] cycloadditions [57, 52] (equations 72 and 73), whereas (difluoromethylene)cyclopropane undergoes only [24-2] cycloadditions [57]... 

The photochemical nitrogen extrusion from 3H-pyrazoles provides a major access to cyclopropenes. It has been shown that the 3H-pyrazoles are transformed into diazoalkenes in the first step of the decomposition, and the resultant diazoalkenes give the cyclopropenes by loss of nitrogen". Benzocyclopropenes have been prepared in a similar manner" The thermal or photochemical decomposition of 4-alkylidene-l-pyrazolines produces methylenecyclopropanes". The products obtained especially in the triplet sensitized photolysis are frequently derived by a methylenecyclopropane rearrangement. 

The preparation of cyclopropene has been achieved in the reaction of allyl chloride with sodium amide 1-Alkylcyclopropenes have analogously been synthesized from 2- and 3-alkylallyl chlorides but the products are liable to isomerize under the strongly basic conditions to alkylidenecyclopropanes and 1-alkenylcyclopropanes. The use of potassium amide gives the rearranged products in a higher extent than that with sodium amide (equation 24). Methylenecyclopropane has been prepared by the successive treatments... 

The elimination of hydrogen chloride from an allylic chloride provides one of the most effective routes to simple alkylcyclopropenes, and to cyclopropene itself The choice of base can be critical, particularly when the product cyclopropene has a 2-alkyl substituent and may undergo a base-induced rearrangement to a methylenecyclopropane, e.g. compare formation of methyl-enecyclopropane and 3-methylcyclopropene. ... 

Diazopropane is a potential source of ge w-dimethyl groups. It undergoes 1,3-dipolar addition to acetylenes and allenes/ and the adducts can be photolyzed to give cyclopropenes or methylenecyclopropanes/ respectively. In certain cases the adducts from a-substituted acetylenes give good yields of allenes and conjugated dienes on photolysis. ... 

The modest yields may be ascribed to a number of side reactions. Allylic halides are very prone to nucleophilic attack, which in this case will give rise to the formation of an allylic amine. One could also imagine a nucleophilic attack of the intermediary carbenoid on the allylic halide, or metallation of the cyclopropene by alkali amide and subsequent coupling of the cyclopropenylide with the allylic halide. The relatively high temperatures in the preparations of the cyclopropenes will certainly favour oligomerization, which may be a merely thermal one or an ionic process (similar to that observed by Dutch investigators [16] in the case of the parent hydrocarbon). Such reactions afford products which are considerably less volatile than the cyclopropenes and therefore will remain in the reaction mixture. One obvious subsequent reaction is the irreversible base-induced isomerization to methylenecyclopropane, which cannot be separated from the desired product [79] ... 

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