Hydrogenolysis epoxide

Thus, hydrogen peroxide has found application in the implementation of the well-known organic reactions in the gas phase—dehydrogenation, epoxidation, hydrogenolysis, nitrile formation from /V-alkylbcnzcne, nitrogen fixation, etc. 

Epoxide opening with nucleophiles occurs at the less substituted carbon atom of the oxlrane ting. Cataiytic hydrogenolysis yields the more substituted alcohol. The scheme below contains also an example for trons-dibromination of a C—C double bond followed by dehy-drobromination with strong base for overall conversion into a conjugated diene. The bicycKc tetraene then isomerizes spontaneously to the aromatic l,6-oxido[l0]annulene (E. Vogel, 1964). 

As a further application of the reaction, the conversion of an endocyclic double bond to an c.xo-methylene is possible[382]. The epoxidation of an cWo-alkene followed by diethylaluminum amide-mediated isomerization affords the allylic alcohol 583 with an exo double bond[383]. The hydroxy group is eliminated selectively by Pd-catalyzed hydrogenolysis after converting it into allylic formate, yielding the c.ro-methylene compound 584. The conversion of carvone (585) into l,3-disiloxy-4-methylenecyclohexane (586) is an example[382]. 

The Pd-catalyzed hydrogenolysis of vinyloxiranes with formate affords homoallyl alcohols, rather than allylic alcohols regioselectively. The reaction is stereospecific and proceeds by inversion of the stereochemistry of the C—O bond[394,395]. The stereochemistry of the products is controlled by the geometry of the alkene group in vinyloxiranes. The stereoselective formation of stereoisomers of the syn hydroxy group in 630 and the ami in 632 from the ( )-epoxide 629 and the (Z)-epoxide 631 respectively is an example. 

Acetylenic epoxides are reduced readily to the olehnic epoxide, provided the resulting epoxide is not allylic (27). In the latter case, one might surmise that hydrogenolysis could best be avoided by use of rhodium in a neutral nonpolar solvent (81) or a Lindlar catalyst (13). Reduction of l,2-epoxydec-4-yne over Lindlar catalyst gave (Z)-l,2-epoxydec-4-ene in 95% yield (69). Hydrogenation ceased spontaneously. 

The C2-symmetric epoxide 23 (Scheme 7) reacts smoothly with carbon nucleophiles. For example, treatment of 23 with lithium dimethylcuprate proceeds with inversion of configuration, resulting in the formation of alcohol 28. An important consequence of the C2 symmetry of 23 is that the attack of the organometallic reagent upon either one of the two epoxide carbons produces the same product. After simultaneous hydrogenolysis of the two benzyl ethers in 28, protection of the 1,2-diol as an acetonide ring can be easily achieved by the use of 2,2-dimethoxypropane and camphor-sulfonic acid (CSA). It is necessary to briefly expose the crude product from the latter reaction to methanol and CSA so that the mixed acyclic ketal can be cleaved (see 29—>30). Oxidation of alcohol 30 with pyridinium chlorochromate (PCC) provides alde-... 

Asymmetric hydrogenolysis of epoxides has received relatively little attention despite the utility such processes might hold for the preparation of chiral secondary alcohol products. Chan et al. showed that epoxysuccinate disodium salt was reduced by use of a rhodium norbornadiene catalyst in methanol/water at room temperature to give the corresponding secondary alcohol in 62% ee (Scheme 7.31) [58]. Reduction with D2 afforded a labeled product consistent with direct epoxide C-O bond cleavage and no isomerization to the ketone or enol before reduction. 

When hydrogenolysis of vinylepoxides is used sequentially, it allows for the controlled formation of 1,3-polyols. In the synthesis of the C11-C23 fragment 92 of preswinholide A, hydrogenolysis of ( ) olefin 93 gave the syn isomer 94 (Scheme 9.37) [159]. Methylation, reduction, epoxidation, oxidation, and olefmation of this material then gave vinylepoxide 95, which was subjected to hydrogenolysis to afford 96 in excellent yield. Repetition of this sequence ultimately afforded the desired derivative 94. 

From the chemical point of view, in diasteieoselective syntheses, several kinds of reactions like hydrogenation [273,277-286], hydrogenolysis [287-293], isomerization [294], and epoxidation [295-300] are involved. Hydrogenation is the most important application of heterogeneous catalysis because of its potential to produce a wide variety of chiral functional groups. 

Since group 4 derived species are of particular interest as catalysts for olefin polymerization and epoxidation reactions, the thermal stability of surface metal-alkyl species, as weU as their reactivity towards water, alcohols and water, deserve some attention. On the other hand, mono(siloxy) metaUiydrocarbyl species can be converted into bis- or tris(siloxy)metal hydrides by reaction with hydrogen [16, 41, 46-48]. Such species are less susceptible to leaching and can be used as pre-catalysts for the hydrogenolysis of C-C bonds, alkane metathesis and, eventually, for epoxidation and other reactions. 

Solutions of low-valence titanium chloride (titanium dichloride) are prepared in situ by reduction of solutions of titanium trichloride in tetrahydrofuran or 1,2-dimethoxyethane with lithium aluminum hydride [204, 205], with lithium or potassium [206], with magnesium [207, 208] or with a zinc-copper couple [209,210]. Such solutions effect hydrogenolysis of halogens [208], deoxygenation of epoxides [204] and reduction of aldehydes and ketones to alkenes [205,... 

In bromohydrins, hydrogenation over Raney nickel may lead to epoxides rather than to alcohols since regular Raney nickel contains enough alkali to cause dehydrobromination. Pure hydrogenolysis of bromine can be achieved over Raney nickel which has been washed free of alkah by acetic acid [631]. 

Open-chain aliphatic ethers are completely resistant to hydrogenolysis. Cyclic ethers (for epoxides, see p. 83) may undergo reductive cleavage under strenuous conditions. The tetrahydrofuran ring was cleaved in vigorous hydrogenations over Raney nickel [420] and copper chromite [420] to give, ultimately, alcohols. 

Hydrogenolysis of epoxides to alcohols by catalytic hydrogenation over platinum requires acid catalysis. 1-Methylcyclohexene oxide was reduced to a mixture of cis- and /ranj-2-methylcyclohexanol [652]. Steroidal epoxides usually gave axial alcohols stereospecifically 4,5-epoxycoprostan-3a-ol afforded cholestan-3a,4/J-diol [652 ]. 

Unsaturated epoxides are reduced preferentially at the double bonds by catalytic hydrogenation. The rate of hydrogenolysis of the epoxides is much lower than that of the addition of hydrogen across the carbon-carbon double bond. In a, -unsaturated epoxides borane attacks the conjugated double bond at -carbon in a cis direction with respect to the epoxide ring and gives allylic alcohols [660], Similar complex reduction of epoxides occurs in a-keto epoxides (p. 126). 

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