Diazene, propargylic

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... 

Reductive Transposition of Allylic Alcohols. Similar to the synthesis of allenes from propargylic alcohols, the Mitsunobu displacement of allylic alcohols with IPNBSH followed by hydrolysis, diazene formation, and sigmatropic loss of dinitrogen provides reductively transposed alkenes. This methodology has proven effective for the reductive transposition of a variety of allylic alcohols (eq 2). The overall transformation provides the desired olefin with high selectivity in the formation of the irans-alkene (eq 3). ... 

A new approach to trimethylsilylallenes unsubstituted in the 1-position has been developed. Following conversion into the corresponding tosylhydrazone, an a,j8-acetylenic aldehyde or ketone is treated with eight molar equivalents of sodium cyanoborohydride the subsequent reaction involves a concerted [1,5]-sigmatropic rearrangement of the intermediate propargylic diazene (63), and consequently furnishes the silylallenes free of their acetylenic isomers (Scheme 103). ... 

The oxidation of propargylic hydrazine derivatives with DAD to the corresponding propargylic diazenes (with subsequent spontaneous loss of N2) forms the basis of a powerful, yet mild, allene synthesis (eq 2). ... 

---