Alkenes reduction with diimide

Table 1 Relative Reactivities of Substituted Alkenes Toward Reduction with Diimide ...

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... 

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. 

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

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. ° ... 

TABLE 2. Alkanes from the reduction of alkenes with diimide... 

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. 

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. 

Alder Ene Reactions. Like the Diels-Alder reaction, Alder ene reactions usually take place only when the enophile has a Z-substituent, the regiochemistry is that expected from the interaction of the HOMO of the ene and the LUMO of the enophile, and Lewis acids increase the rate. All these points can be seen in the reaction of T-pinene 6.449 with the moderately activated enophile methyl acrylate, which takes place at room temperature in the presence of aluminium chloride,928 but which would not have taken place easily without the Lewis acid. The lowering of the LUMO energy of the methyl acrylate accounts for the increase in the rate of reaction. Similarly, the rates of diimide reductions of alkenes show some correlation with ionisation potential and hence orbital properties.929... 

Reduction of alkenes. Siegel et al have discussed the stereochemistry of diimide reduction of alkenes as compared with catalytic hydrogenation. The same laboratory investigated the reduction of conjugated dienes to enes no evidence for 1,4-reduction was obtained. 

In agreement with this mechanism is the fact that the addition has been demonstrated to be syn for several typical alkenes. The rate of diimide reductions has been shown to be affected by torsional and angle strain in the alkene/ More strained double bonds react at accelerated rates. For example, the more strained trans double bond is selectively reduced in cis, trans-l,5-cyclodecadiene. ... 

A well-known example of group transfer reaction with p = 0, q = 2 is represented by the concerted reduction of alkenes and alkynes with the reactive intermediate diimide stereoselectively in the cis fashion (Scheme 6.2). The driving force of the reaction is the formation of the stable nitrogen molecule. 

Again, as seen with alkenes, both borane (B2H6) (Equation 6.22) and diimide (H-N=N-H) (Scheme 6.15) can be used to reduce alkynes. Indeed, the reaction of internal alkynes with borane is apparently more facile than that with the alkene that results from consummation of the reduction. Further, as would be expected, suprafacial addition of hydrogen and boron obtains and Z- (or cis-) alkene is the only product. The use of deuterated boron compounds (e.g., the hindered 9- H-9-borabicyclo[3.3.1]nonane [9- H-9-BBN]), commercially viable since it contains only one deuterium ( H), followed by use of deuterated acetic acid (CH3C02, i.e., reductive workup) produces Z- (or cw-)-dideuteroalkene of high stereospecificity and in high yield (Scheme 6.65). 

Problem 623. The reduction of alkenes can be accomplished in a number of ways. The methods include using hydrogen (H2) with (a) Adam s Catalyst and (b) Wilkinson s Catalyst as well as with (c) diimide and (d) borane. Using 1,2-dimethylcyclopentene as a substrate, compare and contrast these methods. 

Exocyclic bis-silylated olefins have been constructed through the Pd(OAc)2-catalyzed reaction of alkynes with a tethered disi-lanyl group. The reactions are carried out in the presence of a tert-alkyl isocyanide, although the precise role of this ligand is unclear. Diimide reduction of the disilylated alkene so-formed followed by Fleming-Tamao-type oxidation of the two C-Si bonds in the saturated product then affords 1,2,4-triols in a stereoselective manner (eq 83).l ... 

Reduction of tosylhydrazones of a,/3-unsaturated ketones gives alkenes with the double bond being located between the former carbonyl and a-carbon atoms. This reaction is believed to proceed via an initial conjugate reduction followed by decomposition of the resulting vinylhydrazine to a vinyl diimide. 

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