Allenes Carrying Other Unsaturated Substituents

A large number of allenes are known in which the unsaturated substituents possess a different structure than in the cases discussed so far. Rather than trying to be comprehensive, a number of representative cases will be discussed. 

Beginning with vinylallene (2) as a prototype, four double bond extended hydrocarbons can be constructed 3, 4, 22 and 23. They have all been prepared and characterized or generated as reactive intermediates. 

The reaction sequence to the latter hydrocarbons is the most flexible one and starts from the allenic alcohols 212, which are first converted to the l,3-hexadien-5-ynes 213 by an elimination reaction the allene group is then generated by a pro-pargylic rearrangement initiated by the addition of a Grignard reagent. 

Many hydrocarbon derivatives of 22 have since been prepared [81, 82] they are mostly of interest because of their thermal cyclization reactions (see below) [8, 83, 84], although their behavior in cycloaddition reactions could also lead to new preparative insights [79]. 

Coupling of excess (Z)-l,2-dichloroethene (217) with propargyl alcohol first led to the enyne 218, which, when subjected to a second Pd-catalyzed coupling step with trimethylsilylacetylene, provided the mixed diacetylene 219. With all carbon atoms assembled, the allene function was generated by first producing the (unprotected) hydrazine derivative 220, which on treatment with either diethyl azodicarboxylate (DEAD) or 4-methyl-l,2,4-triazoline-3,5-dione (MTAD) under anaerobic conditions at 0 °C yielded the hydrocarbon 27. According to mechanistic studies, the latter process leads first to a mixture of ( )- and (Z)-diazenes. Sigmatropic elimination of 

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