Selenium-substituted cyclopropanes

Despite the high toxicity of selenium compounds there has been a lot of activity in this field during the last decade. Not surprisingly, selenium-substituted cyclopropane derivatives have also been recommended for synthetic purposes. An example is disclosed in equation 119 one should remember, however, that most transformations can be achieved by other and less poisonous reagents. 

The chloroform with base/phase-transfer catalyst or potassium rcrt-butoxide methods have been used for the preparation of selenium-substituted cyclopropanes 1-3. ... 

Heteroatom Analogs of Methylenemalonate Esters. Phosphorus-substituted cyclopropanes can be prepared by the [2 + 1]-cycloaddition of 1 with 2-phosphonoacrylates (eq 13). Sulfone-substituted cyclopropanes were also prepared (eq 14).Secondary orbital interactions between the selenium and carbonyl... 

Cyclopropanations are known for several other carbanionic intermediates of the general type (7), in which the substituent G is ultimately lost as an anionic leaving group in the last step of the ring-forming pathway (see Scheme 3 above). The substituent G is most often a functional group based upon sulfur, selenium or nitrogen. Halide-substituted derivatives probably react via the a-elimination pathway in most cases (see Section 4.6.3.1), but in some reactions with electron deficient alkenes as substrates, the normal order of steps may be altered (e.g. Table 10, ref. 162). 

Krief has applied selenium chemistry to some anionic cascade cyclisations.128 For example, 293 can be cyclised in two successive 5-exo reactions to give a mixture of the stereoisomers of 297. If 294 is warmed to 0 °C, an alternative sort of tandem process occurs after cyclisation onto the alkene, the organolithium 295 undergoes an intramolecular displacement, stereoselectively generating the 5,3-fused system of 296. Similar intramolecular cyclopropanations (of a-bromo organolithiums) have been described by Hoffmann151 but are probably mediated by carbenes rather than a sequential cyclisation-substitution sequence. 

The reaction of selenium ylides stabilized by a carbonyl function with a,jS-unsaturated ketones afforded cyclopropanes substituted with two ketone functions, e. g. formation of 19 ° (see also refs 161 and 162). 

An equilibrium has been invoked to account for the unusual chemistry of tris(phenylthio)methyl-lithium (74). Compound (74) in Scheme 8 may be made in several ways, and it undergoes cycloaddition with hetero-substituted olefins to give cyclopropanes. A selenium analogue of (74) has also been reported which gives a cyclopropane in 63% yield with l,l-bis(phenylthio)-ethylene. ... 

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