1.3- Cyclohexadiene selective reduction

Selective reduction of the methyl ester to the corresponding aldehyde using DIBAL at low temperature and subsequent reductive amination with iodopiperonyl-ammonium chloride affords the tricarbonyliron-cyclohexadiene complex with the secondary alkylamine in the side chain. Iron(O)-mediated oxidative cydization... 

To study the effects of water and other solvents on titanocene(III)-mediated processes we used the transannular cychzation of epoxygerma-crolides as a model reaction [47]. Thus, we found that in anhydrous, non-halogenated solvents such as THF the reaction led selectively to decalins with an exocyclic double bond (Scheme 5). In an aqueous medium (THF/H2O), however, the characteristic lime green color of Cp2TiCl turned deep blue and the main product was a reduced decalin (Scheme 5). Under these conditions, water (either H2O or D2O) proved to be more effective than the toxic and expensive hydrogen-atom donor 1,4-cyclohexadiene for the reduction of tertiary radicals [47]. This is an unusual phenomenon in free-radical chemistry [48-50], subsequently exploited by us for the selective reduction of aromatic ketones as we shall see later [51,52]. 

The trianionic cobalt catalyst has been successfully employed in the hydrogenation of 1,3-butadiene in [bmim][BF4] [10], The product from this reaction is 1-butene which is formed with 100% selectivity. Unfortunately the catalyst undergoes a transformation to an inactive species during the course of the reaction and reuse is not possible. The cationic rhodium catalyst together with related derivatives have been used for numerous reductions, including the hydrogenation of 1,3-cyclohexadiene to cyclohexane in [bmim][SbF6] [11],... 

Selectivity. Benzene itself is transformed into 1,4-cyclohexadiene by employing sodium and ethanol in liquid ammonia207 [Eq. (11.54)] the Benkeser method affords further reduction to cyclohexene and cyclohexane208 [Eq. (11.55)] ... 

Benzene is reduced in 95% current yield to a mixture of 23% cyclohexadiene, 10% cyclohexene and 67% cyclohexane. HMPTA as a solvent additive seems to play a dual role. Firstly it is selectively adsorbed at the cathode surface, thereby preventing hydrogen evolution from the protic solvent. Thus it permits the attainment of a potential sufficiently cathodic for the generation of the solvated electron. It secondly stabilizes the solvated electron, thus suppressing its reaction with protic solvents (eq. (130) ). With decreasing HMPTA concentration in the electrolyte the current efficiency for reduction decreases and hydrogen evolution dominates. In pure ethanol the current efficiency is less than 0,4%. 

The reduction of benzene itself may then be achieved either in LiBr + HMPA solution (water as proton donor) [313] or in ethanol-HMPA (66.6 mol% ethanol) at an aluminum catghode [311] this gives a mixture of 22% cyclohexadiene, 10% cyclohexene, and 67% cyclohexane with an overall current efficiency of 95% [311]. The electrolysis must be carried out with a large excess of benzene. However, product distribution may depend on both the concentration of ethanol and the current density. As previously studied under chemical conditions (lithium metal added to ethanol in ammonia [314]), the product distribution would depend on the rates of reduction of benzene (/cb) and of cyclohexene (kc) by the reducing species (here the solvated electron), the nature of the solvent, and efficiency of the proton donor in the considered solvent. In ethylenediamine, the ratio /kc was found to be equal to 200, and this value explains why reduction stops when an isolated double bond has been formed. In contrast, in HMPA-ethanol mixtures, the ratio k[)/kc is only 1.4 this may explain the lack of selectivity and high yield of cyclohexane when a large excess of proton donor is used. 

PCoWu0395 (R)-(+)-limonene, 4-vinyl-l-cyclo hexene, 1 -methyl-1,4-cyclohexadiene Epoxides o2 MeCN or CH2C12 Aldehyde reductant quite selective 400... 

Stereoselective Birch reduction is possible and a number of examples have been reported, particularly for selective alkylation of the intermediate enolate anion. For example, reduction of the chiral benzamide 69 with potassium in ammonia, followed by alkylation with ethyl iodide gave essentially a single diastereomer of the cyclohexadiene 70, which was used in a synthesis of (-l-)-apovincamine (7.50). 

Thermal cycloadditions of chiral vinylsulfoxides B with cyclopentadiene, 1,3-cyclohexadiene and 1,3-butadiene have been examined in toluene at high temperature leading to the adducts in moderate selectivities. The electron-deficient sulfur residue activates the alkene unit and serves as a temporary substituent that can be easily removed by reduction or transformed into a variety of other functionalities. An increase in reactivity and excellent selectivities are observed in the presence of additional electron-withdrawing substituents at the alkene unit as shown for the examples and in the table below. At 20°C in dichloromethane the cycloadditions of C with cyclopentadiene proceed smoodily and highly stereoselective. In reactions of menthyl-3-(3-trifluoromethyl-2-pyridylsulfinyl)acrylate A with dienes, e.g. 2-methoxyfliran or cyclopentadiene, high diastereoselectivities (>98 2) were obtained, too. 

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