Ketones dialkylidene

Five-membered ring compounds are obtained from conjugated bisallenes under a variety of conditions. Whereas oxidation with m-chloroperbenzoic acid (MCPBA) provides ketones of type 323 [137], carbonylation by iron pentacarbonyl in THF at 50 °C leads to the symmetrical dialkylidene cyclopentenones 326 [45]. With phosphi-nidene complexes such as 325, biallenyls react in a stepwise process that eventually leads to adducts such as 327 [138], mimicking the behavior of singlet carbenes [139]. 

In their reactions with aldehydes in the presence of alkali cyclic ketones such as cyclopentanone and cyclohexanone generally give only the dialkylidene or diarylidene compounds, and not the monosubstituted derivatives, since the two methylene groups vicinal to the carbonyl group are equally activated. Monosubstituted cyclanones can be obtained in this way only when about a six-fold excess of the ketone and special precautions are used.949 On the other hand, Birkofer and his co-workers showed monoalkylidenecycloalkanones to be easily obtained starting from enamines 950 yields are between 30% and 90% ... 

Conjugate addition of allylboronic acid pinacol ester to nons)mimet-ric dialkylidene ketones with an in situ generated phosphonite nickel catalyst. 

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