Tetraphenyl cumulenes

The reduction of cinnamylidene fluorene by aluminum amalgam in moist ether to 1-fluorenyl-3-phenyl-l-propene (I), a 1,4-addition, exemplifies the use of Al(Hg) rather than Na(Hg) for the reduction of alkali-sensitive compounds in the presence of Na(Hg), I might be tautomerized to 3-phenyl-1-propylidene fluorene [26]. Another example is the reduction of a cumulene, l,l,6,6-tetraphenyl-l,2,3,4,5-hexapentaene, to 1,1,6,6-tetraphe-nyl-l,2,4,5-hexatetraene by Al(Hg) [27]. 

Partial hydrogenation of cumulenes affords m-polyenes selectively in the presence of the Lindlar catalyst (a palladium-calcium catalyst deactivated by lead acetate see p. 19) hydrogenation ceases almost entirely after a rapid absorption of (n — l)/2 moles of hydrogen (n = number of double bonds). According to Kuhn and Fischer,139 tetraphenylbutatriene absorbs one equivalent of hydrogen, yielding l9l9494-tetraphenyl-l93-butadiene ... 

Correspondingly, electronic equivalencies based on similar C nmr resonances may be established for higher cumulenes, such as the tetraphenyl derivatives 225 and 226 (121). 

If one looks at the uv spectra of tetraphenylallene (94) and tetraphenyl-pentatetraene (227) on the one side and tetraphenylbutatriene (225) and te-traphenylhexapentaene (226) (131) on the other side (Fig. 23), one can see lucidly that within the corresponding class of cumulenes the uv spectra are qualitatively very similar. The same feature is observed for cumulenic hydrocarbons 241-244(132). 

The [4] cumulenes or pentatetraenes are not well investigated. However, a few of their dimerization and cycloaddition reactions are reported. Some pentatetraenes, such as tetraphenyl-, bis-cyclohexylidene- and tetraferrocenylpentatetraene are stable at room temperature. 

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