Hydrogenation with Diimide

There are alternative ways to add hydrogen to a multiple bond besides the catalytic methods described in the previous sections. The most useful of these are homogeneous reactions utilizing diimide, HN=NH, and diborane, B2H6. 

The behavior and reactivity of diimide can be understood best by considering the thermochemistry of hydrogenation of nitrogen  

The first step is strongly endothermic and is the main hurdle to overcome in the hydrogenation of nitrogen to ammonia. Conversely, the reverse reaction, which is the dehydrogenation of diimide, is strongly exothermic. Therefore we may expect that diimide will have a pronounced tendency to revert to molecular nitrogen. This is in fact so and, at normal temperatures, diimide exists only as a transient intermediate that cannot be isolated. It is extremely reactive and readily transfers hydrogen to carbon-carbon multiple bonds  

In practice, diimide is generated as it is needed in the presence of the compound to be hydrogenated. There are various ways to do this, but one of the 

Hydrazine actually has been used as a hydrogenating agent for over sixty years, but it was not until the 1960 s that the diimide intermediate in such reactions was recognized. 

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Scheme 7.39 Ring-opening metathesis polymerization of various alicyclic monomers using K2lrCl6- Following polymerization, the polymer was hydrogenated with diimide produced in situ.

It was envisioned that hydrindanone 83 and cyclopentene 85 could be used as intermediates in the synthesis of e f-retigeranic acid A (1) and e f-retigeranic acid B (2), respectively. To prepare the building block 90, cyclopentene 85 was reduced with diimide (93 %) in order to prevent isomerization and subsequently deprotected with PPTS to yield hydrindanone 90 (quant.), which could provide access to <77/-retigeranic acid B (2) (Scheme 10.7). Hydrindanone 83 was reduced via an enol triflate and then subjected to Pd-catalyzed reduction to provide cyclopentene 91 (87 % from 83). Upon hydrogenation of 91 with Pd/C and cleavage of the acetal with iodine, protected hydrindanone 92 (95 % from 91) was obtained. The deprotection of 92 provided ent-60, whose enantiomer was used in previous syntheses of retigeranic acid A (1) by Corey [14] and Hudlicky [46, 47]. 

The superfluous carbonyl oxygen atom was removed by carbonyl reduction to provide the alcohol 171, subsequent Chugaev elimination (via 172 to 173) and double bond hydrogenation with in situ generated diimide (Scheme 27) [94]. The isopropenyl double bond was finally re-established by reductive cleavage of the a-bromo ether unit in 173 to afford the fully functionalized enantiomerically pure A-ring building block (162). 

Pd and Ni catalysts with the structural effects on reductions with diimide (diazene) (ref. 6) and the equilibrium constants for the association of substituted ethylenes with a Ni(0) complex (ref. 7). These particular reactions were chosen because of our perception of their relation to the mechanisms of catalytic hydrogenation, and the insightful analysis of the relationship between structure and reactivity provided by the authors of these studies. 

Hydrogenation with the use of H-donating compounds 9,10-dihydroanthracene (DHA), diimide N2H2, hydroboration. 

Stereochemical studies on the reduction of C==C and by diimide have shown that the transfer of hydrogens from diimide occurs in a completely syn manner. The reduction of (4) and (5) with dideute-riodiimide, generated by the deuterolysis of dipotassium azodiformate, resulted in the formation of the meso- and ( )-reduction products (6) and (7) in at least 97% stereospecificity (the lower limit of detectability with IR spectral analysis). The reduction of diphenylacetylene (8) produces only c/ s-stilbene (9) as an intermediate reduction product. It was considered that the reduction of multiple bonds by diimide occurred as a synchronous transport of a pair of hydrogens to a single face of the rr-system via a transition state represented as (10). ... 

In the reduction of alkylidenecyclohexanes the approach to the faces of the double bond are similarly sterically hindered, and roughly equal amounts of products derived from attack at the two faces of the double bond should be formed. This is the case with (26a). Replacement of the vinyl hydrogens with methyl groups should not greatly affect the approach of diimide to either face of the double bond. However, such substitution results in increases favoring formation of the trans isomer (28). This trend has been interpreted in terms of subtle changes in the conformations of the alkenes, which affect the ease of approach of diimide to the two faces of the double bonds. 

LiAlELj. LiAlELj does not usually reduce azo compounds (indeed these are the products from LiAlH4 reduction of nitro compounds, 19-80), but these can be reduced to hydrazo compounds by catalytic hydrogenation or with diimide (see I5-II). Diazonium salts are reduced to hydrazines by sodium sulfite. This reaction probably has a nucleophilic mechanism... 

Formation of l-cyclopropyl-2-methylhydrazine (7) occurred when cw-l-methyldiazenyl-2-phenylcyclopropane was reacted with diimide generated by thermolysis of the anthracene-diimide adduct The reaction has not been utilized on a preparative scale. Furthermore, hydrogenation of 1-azidocyclopropanecarboxylic acid over palladium gave 1-aminocyclo-propanecarboxylic acid in 69% yield. 

Hydrogenation has also been carried out successfully with diimide, ... 

Singlet oxygen adds readily to tetrahydrobenzopyrans (166) derived from the photoisomerization of j6-ionone <73ABC224i> and its derivatives, for example (167) <88JMC713>, to afford the corresponding trioxanes (168) and (169) (Equation (21)). The saturated derivatives are obtainable by reduction with diimide in the case of (165) and (169 and by hydrogenation over Pt for (168). 

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