Diastereoselectivity diazoalkane cycloaddition reactions

Due to their tendency to undergo side reactions and the lack of stereospecificity, free methylene or alkylcarbenes, as generated from diazoalkanes by photolysis or thermal nitrogen extrusion, are of minor synthetic importance for [2 4- 1] cycloadditions. However, transition metal catalysis, most commonly with copper or palladium compounds, offers a convenient solution to this problem (Vol. E19b. p 278)s. Probably the most active catalyst is copper(I) trifluoromcthanesulfonate9. The simple diastereoselectivity of these reactions is often negligible, as demonstrated by the copper(I) chloride or palladium(II) bis(benzonitrilo)dichloride promoted cyclopropanation of phenylethene with diazoethane10. 

The oldest known type of diastereoselectivity is the addition to dipolarophiles that form chiral centers at the reacting atoms. In the context of Buchner s pioneering work on cycloaddition of diazoalkanes (see Sects. 1.1 and 6.2), Buchner investigated the reactions of methyl diazoacetate with ethyl (E)-3-phenylprop-2-enoate (ethyl cinnamate) and of ethyl diazoacetate with methyl ( )-3-phenylprop-2-enoate (Scheme 6-31) with his coworkers Dessauer (1893) and von der Heide (1902). They found two isomeric 4,5-dihydro-3//-pyrazoles 6.69 and 6.70 which, on dehydrogenation, gave the same prototropic mixture of ethyl methyl 4-phenylpyrazole-3,5-dicarboxylates (6-32). The ratio found was 80 20. Ihis surprising result was rationalized 78 years after Buchner s discovery ... 

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