Alkenes diimides

Dia ene deductions. Olefins, acetylenes, and azo-compounds are reduced by hydrazine in the presence of an oxidizing agent. Stereochemical studies of alkene and alkyne reductions suggest that hydrazine is partially oxidized to the transient diazene [3618-05-1] (diimide, diimine) (9) and that the cis-isomer of diazene is the actual hydrogenating agent, acting by a concerted attack on the unsaturated bond ... 

Certain N-substituted aziridines are particularly labile towards deamination. N-Aminoaziridines (271) decompose with high stereospecificity to alkenes and diimide between 20 and 60 °C in good yield (70HCA1479). 

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. 

Several 1,2,4-trioxolanes are known which contain a double bond linked in some way to the ring, with interest focusing on furan endoperoxides. The alkene moiety of 2,5-dimethylfuran endoperoxide (41) can be selectively functionalized in the presence of the 1,2,4-trioxolane ring. Reaction with diimide gives the saturated ozonide <81TL3509> and a single addition product was obtained with p-nitrophenylazide, although the stereochemistry of the reaction was not determined. 

Alkyl-5-alkyliminothiatriazolines (50) decompose slowly around 40-60 "C and rapidly at 125°C with formation of sulfur, nitrogen, and carbodiimide (51) (Equation (4)). However, the carbo-diimides (51) formed react with undecomposed thiatriazoline (50), which in part explains the low yields of isolable carbodiimide. It has not been possible by trapping experiments to decide whether the decomposition involves an intermediate as addition of electron-rich alkenes or heterocumulenes induces immediate nitrogen evolution in a bimolecular reaction (see Section 4.19.5.2) <78JCS(P1)1440>. 

The relative reactivity of alkenes toward reduction by diimide depends on the degree of substitution. Increasing alkyl substitution results in decreasing reactivity, and strained alkenes exhibit higher reactivity than nonstrained compounds 187... 

Figure 5.1. Reduction of Wang resin bound alkenes and alkynes with diimide (HN=NH) [6].

STRUCTURE-REACTIVITY IN THE HYDROGENATION OF ALKENES. COMPARISONS WITH REDUCTIONS BY DIIMIDE AND THE FORMATION OF A Ni(O) COMPLEX... 

The correlation between the apparent association constants, K. which are derived from the competitive rates on Pt and reductions by diimide indicates that structural changes in the alkene generally have parallel effects on these reactions, Fig. 2. Because the diimide reduction is essentially free of steric effects, this effect is liable to account for some of the differences which are observed in extended groups of compounds. The small range of individual reactivities on Pt, which are zero order in alkene, can be understood in that the variation in structure which increases the driving force towards... 

We already have discussed a few addition reactions that appear to occur in a concerted manner. These include the addition of diimide, ozone, and boron hydrides to alkenes (Sections 11-5, 11-7A, and 11-6B). Concerted reactions that have cyclic transition states often are called pericyclic reactions. Other examples will be considered in later chapters. 

The other well-known type of group transfer reaction is represented by the concerted syn delivery of two hydrogen atoms from the reactive intermediate diimide 1.25 to an alkene or alkyne, driven by the formation of the stable molecule nitrogen. 

As regards the protecting effect, the complex is stable to Lewis acids. Also, no addition of BH3 occurs. As Co2(CO)6 can not coordinate to alkene bonds, selective protection of the triple bond in enyne 137 is possible, and hydroboration or diimide reduction of the double bond can be carried out without attacking the protected alkyne bond to give 138 and 139 [32], Although diphenylacetylene cannot be subjected to smooth Friedcl Crafts reaction on benzene rings, facile /7-acylation of the protected diphenylacetylene 140 can be carried out to give 141 [33], The deprotection can be effected easily by oxidation of coordinated low-valent Co to Co(III), which has no ability to coordinate to alkynes, with CAN, Fe(III) salts, amine /V-oxidc or iodine. 

It seemed prudent that the same ethers be examined in the absence of potentially labile functionality, thus removal of unsaturation in 262 and 263 was considered. Hydrogenation of 259 over Pd/C or Pt was unsuccessful in either case reduction of the peroxide group was problematical. Hydrogenation over Wilkinson s catalyst gave a new product, but with the unsaturation retained. While selective alkene hydrogenation can sometimes be achieved in the presence of a peroxide bond, the double bond of 259 was apparently too hindered in this case. Diimide, on the other hand, worked reasonably well for this reduction. Thus, treatment of 259 in dichlo-romethane solution with potassium azodicarboxylate followed by addition of acetic acid led, after several days, to roughly 60% conversion of 259 to the saturated version, 264. Now, ether formation as before provided the saturated methyl and benzyl ethers 265 and 266, respectively, in good yields. 

Garbisch et al. (1965) examined the reactivity of a wide variety of alkenes with diimide. The observed range in reactivity of 38000 was ascribed chiefly to terms such as Fbend, -Etors. > and a-alkyl substitution. Unfortunately, those reactivity differences pertaining to SS generally turned out to be small and could not be generated by this approach the reduction of 1 -methyl-4-t-butylcyclohexene at 80° gave transjcis = 2-3. 

A further difference to electrophilic alkenes shown by DAD and other diimides is that tertiary amines, such as A-cyclohexylpyrrolidine (167), are dehydrogenated to the enamine. Further reaction then occurs to give the product of 2,6-disubstitution (168)352. Diacyl diimides, such as dibenzoyldiimide (DBD), are even more remarkable. The action of DBD on 167 at room temperature results in dehydrogenation of the pyrrolidine ring and reaction with a further two equivalents of DBD to give 169353 (Scheme 178). 

A non-catalytic procedure for the syn-addition of hydrogen to alkenes makes use of the unstable compound diimide (N2H2). This strongly exothermic reaction is favoured by the elimination of nitrogen gas. The diimide reagent must be freshly generated in the reaction... 

Reduction of alkenes and alkynes with diimides is also an example of an ene reaction. 

Commercially available 30% hydrogen peroxide solution can oxidize alkenes readily in the presence of a carbo-diimide promoter <1996SL649, 1998JOC2564, 1998JOC1730> (Equation 67). A method to epoxidize propene using aqueous hydrogen peroxide and a reaction controlled phase-transfer catalysts was developed <20040PD131>. 

---