Diimides mechanism

In the above-mentioned reactions with hydrazine and hydroxylamine, the actual reducing species is diimide (NH=NH), which is formed from N2H4 by the oxidizing agent and from NH2OH by the ethyl acetate.The rate of this reaction has been studied.Although both the syn and anti forms of diimide are produced, only the syn form reduces the double bond, at least in part by a cyclic mechanism ... 

Catalytic hydrogenation transfers the elements of molecular hydrogen through a series of complexes and intermediates. Diimide, HN=NH, an unstable hydrogen donor that can be generated in situ, finds specialized application in the reduction of carbon-carbon double bonds. Simple alkenes are reduced efficiently by diimide, but other easily reduced functional groups, such as nitro and cyano are unaffected. The mechanism of the reaction is pictured as a concerted transfer of hydrogen via a nonpolar cyclic TS. 

In agreement with this mechanism is the fact that the stereochemistry of addition is syn.44 The rate of reaction with diimide is influenced by torsional and angle strain in the alkene. More strained double bonds react at accelerated rates.45 For example, the more strained trans double bond is selectively reduced in Z,K-1,5-cyclodecadiene. 

Williams, A., and Ibrahim, I.A. (1981) A mechanism involving cyclic tautomers for the reaction with nucleophiles of the water-soluble peptide coupling reagent l-ethyl-3-(3-dimethyla-minopropyl) carbo-diimide (EDC)./. Am. Chem. Soc. 103, 7090-7095. 

The redox reactions of hydrazine toward main-group and transition metal oxidants have been reviewed (73). Different stoichiometries have been found, with N2 appearing as the N-containing oxidized product, sometimes accompanied by the formation of NH3 and/or HN3. The mechanisms have been analyzed in terms of the one- or two-electron nature of the oxidants, and imply both outer-and inner-sphere routes, depending on the oxidant. The very reactive, key intermediate, diazene (diimide), N2H2, has been proposed in most of these reactions. 

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. 

Figure 1 Proposed mechanism for the generation of the radical anion and radical cation of perylene diimide and self-annihilation of the two to yield the triplet excited state.

While such a device has yet to be constructed, Debreczeny and co-workers have synthesized and studied a linear D-A, -A2 triad suitable for implementation in such a device.11641 In this system, compound 6, a 4-aminonaphthalene monoimide (AN I) electron donor is excited selectively with 400 nm laser pulses. Electron transfer from the excited state of ANI to Ai, naphthalene-1,8 4,5-diimide (NI), occurs across a 2,5-dimethylphenyl bridge with x = 420 ps and a quantum yield of 0.95. The dynamics of charge separation and recombination in these systems have been well characterized.11651 Spontaneous charge shift to A2, pyromellitimide (PI), is thermodynamically uphill and does not occur. The mechanism for switching makes use of the large absorption cross-section of the NI- anion radical at 480 nm, (e = 28,300). A second laser pulse at 480 nm can selectively excite this chromophore and provide the necessary energy to move the electron from NI- to PI. These systems do not rely on electrochemical oxidation-reduction reactions at an electrode. Thus, switching occurs on a subpicosecond time scale. 

A proposed mechanism for the formation of the diimides is shown in Scheme... 

However, produced a neutral [2]catenane (Scheme 11.5) [lib] bearing a mechanically interlocked bis-NDI macrocycle in 53% yield. This yield is far more attractive for a bis-NDI macrocycle than the 10% yield originally obtained in macrocycles [30a]. This approach involved an oxidative coupling of two acetylenic naphthalene diimides in the presence of crown ether. The efficiency of the catenation can be attributed to the crown ether component acting as a permanent template for the forming cyclophane. 

Like diimide, diazenes are unstable at room temperature. They decompose while releasing elemental nitrogen. The decomposition mechanism is a radical chain reaction. 

Reaction between nitrile oxides and other types of diazo compounds has not led to the expected oxatriazolines. Phenyl benzoyl diimide and ethyl benzoylazocarbonate gave rise to some substituted hydrazines by an unknown mechanism. Azobenzene did not react with nitrile oxides and 4-chlorobenzenediazocyanide reacted at the nitrile group instead of at the N=N bond (72CB2841). 

The mechanism for DCC coupling is not as complicated as it may seem. The car-boxylate ion adds to the strongly electrophilic carbon of the diimide, giving an activated acyl derivative of the acid. This activated derivative reacts readily with the amine to give the amide. In the final step, DCU serves as an excellent leaving group. The cyclohexane rings are miniaturized for clarity. 

A cis addition mechanism is generally accepted for the reaction, because cis addition to an olefinic bond generally occurs with predominant attack at trans bonds, and the Simmons-Smith reagent attacks preferentially one of the trans olefinic bonds of trans,trans,cis-1,5,9-cyclodode-catriene and then the cis double bond of the monoadduct (378). The close correspondence in relative rates of olefins for the cyclopropane formation by the Simmons-Smith reaction with those for diimide reduction and peroxide epoxidation supports the concept 409). The latter two reactions are generally considered to proceed via cis addition. 

Similar reaction mechanisms involving a diimide intermediate (17) probably account for the conversion of the 5 a-hydroxy 6 ketone (16) into the A -steroid (18) xmder Wolff-Kishner conditions [14], and for the formation of choiest-2-ene (20) when 2a-bromocholestan-3-one (19) was treated with... 

---