Allenes diimide

The reduction of dienes by diimide depends on the nature of the substitution of the diene. Several studies of relative reactivity have been carried out and they indicated that an increasing degree of alkyl substitution on the double bond results in decreasing reactivity82. In the case of allenes, the reduction of the less substituted allenic double bonds and the formation of the thermodynamically less stable cis olefin can be explained by the steric control of the approach of the diimide (equation 23)83. 

Simple and complex hydrazides, with the general formula RCONHNHCOR, were readily dehydrogenated by DIB to the corresponding azo compounds, some of which were used as dienophiles for in situ Diels-Alder reactions [65,66]. Hydrazine itself in the form of its hydrate was similarly converted into diimide, NH — NH, which served for some in situ hydrogenations [67], Further reactivity accompanied by solvent participation was observed in some cyclic derivatives of hydrazine, i.e. pyrazolones. At low temperature these underwent ffagmentive loss of dinitrogen to give either methyl alkynoates or allenic esters [68] ... 

Like the double bond, the carbon-carbon triple bond is susceptible to many of the common addition reactions. In some cases, such as reduction, hydroboration and acid-catalyzed hydration, it is even more reactive. A very efficient method for the protection of the triple bond is found in the alkynedicobalt hexacarbonyl complexes (.e.g. 117 and 118), readily formed by the reaction of the respective alkyne with dicobalt octacarbonyl. In eneynes this complexation is specific for the triple bond. The remaining alkenes can be reduced with diimide or borane as is illustrated for the ethynylation product (116) of 5-dehydro androsterone in Scheme 107. Alkynic alkenes and alcohols complexed in this way show an increased structural stability. This has been used for the construction of a variety of substituted alkynic compounds uncontaminated by allenic isomers (Scheme 107) and in syntheses of insect pheromones. From the protecting cobalt clusters, the parent alkynes can easily be regenerated by treatment with iron(III) nitrate, ammonium cerium nitrate or trimethylamine A -oxide. ° ... 

Reduction of allenes. Cyclic and acyclic allenes are reduced stereospecifically by diimide (generated from hydrazine, H2Oa, CuS04) to cis-alkenes.6... 

The methods for generating acyl ketenes (Scheme 7-V) and their subsequent in situ participation in [4 + 2] cycloadditions with a wide range of hetero- or olefinic and acetylenic dienophiles (Scheme 7-VI), including acyl ketenes,185 186,197 carbonyl compounds, 86-188 nitriles,1874,189,191 isocyanates and isothiocyanates,1864,190,191 ketenes,191 imines,1864,1874,191,192 carbo-diimides,l87c 190,191,193 ynamines,194 ketene acetals,1864,195 enol ethers,1864,191,196 and V-sulfinylamines197 have been extensively reviewed.5,9,12 Two reports have detailed the 4-n- participation of allenic ketones in [4 + 2] cycloaddition reactions [Eq. (51)].198,199... 

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