Radialene ketones

Another [5]radialene ketone, the tetraallene 102a, has been prepared in 74% yield by treating the [5]cumulene 98a with stoichiometric amounts of Ni(CO)2(PPh3)267 the sterically very shielded ketone 102b is produced analogously in 32% yield from the corresponding terminally bridged [5]cumulene 98b (equation 11). In both cases the dimers of the [5]cumulenes, the peralkylated [4]radialenes are formed as well (see Section II.B, Scheme 11). 

The structure of 91, a new bisaikylidenesilirane (sila[3]radialene) system, was confirmed by X-ray structure analysis (Figure 3c). Because of the high kinetic stability of 91 due to steric protection, aldehydes such as benzaldehyde as well as ketones did not react. However, [2 + 4] cycloaddition chemistry of 91 with 4-methyl-l,2,4-triazoline-3,5-dione resulted in the formation of a new ring-fused silirene (97) (equation 26). 

A first preparation of cumulene (buta-2,3,4-triene)-carboxylic acids (50) features condensations between the anion (49) and ketones or a-keto-esters, the latter electrophiles leading to cumulene diacids (Scheme 8)Thermolysis of these acids can be used to prepare [4]radialenes. 

---