Radialenes from 1,1-dihaloalkenes

It appears that neither the lithium carbenoid pathway nor the cyclopropanation of buta-trienes are general routes to [3]radialenes. More successful is the cyclotrimerization of 1,1-dihaloalkenes via copper or nickel carbenoids, provided the substituents at the other end of the C=C double bond are not too small. Thus, tris(fluoren-9-ylidene)cyclopropane 27 was formed besides butatriene 28 from the (l-bromo-l-alkenyl)cuprate 26 generated in situ from (9-dibromomethylene)fluorene (Scheme 3)10. The cuprate complexes formed... 

The most recent strategy to prepare [3]radialenes is the treatment of 1,1-dihaloalkenes with activated nickel. Thus, the aryl-substituted [3]radialenes (Z,E,E)-30 and (E,E,E) 30, 27 and 32 were obtained together with the corresponding butatrienes (29, 28, 31) from the 1,1-dibromo- or 1,1-dichloroalkenes with the help of nickel activated by ultrasound (Scheme 4)11. It is worth mentioning that the mixed-substituted radialene 33 was produced, when the nickel carbenoid derived from 9-(dichloromethylene)xanthene was generated in the presence of butatriene 2811. 

Various alkyl- and/or aryl-substituted [3]radialenes couldbe prepared from 1,1-dihaloalkenes via organometallic intermediates. These transformations, hov rever, often give the radialene in low yield due to competing reaction pathways. The first radialene, hexamethyl[3]radialene (35), was obtained from dibromoalkene 32 and -butyl lithium in very low yield, the lithium carbenoid 33 and the [3]cumulene 34 being the likely reaction intermediates (Scheme 4.8) [32]. Preferential cyclo-propanation of the outer double bonds of the [3]cumulene by the carbenoid (or carbene) is a major factor for the complexity of the product mixture. 

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