Subject olefin hydrogenation

As a special case, the formation of hemiacetals 2 (lactolization) during the hydroformylation of hydroxy-functionalized olefins, such as allyl or homoallyl alcohols, has to be mentioned (1, Y= O, Scheme 5.70). With these substrates, the reaction occurs in an intramolecular manner. In the presence of an external alcohol, the cyclic hemiacetal can further react to give a nonsymmetric cyclic acetal 3. Hemiacetals can be subjected to hydrogenation to afford diols 4. Under reducing conditions and in the presence of amines, amino alcohols 5 are formed both are valuable building blocks in fine chemistry. Alternatively, oxidation gives lactones 6 [5]. By dehydration of hemiacetals, cychc vinyl ethers 7 are formed. The same transformation with allylamines (Y=NR) gives cyclic hemiaminals, A/ ,0-acetals, lactames, or vinyl amines. 

In the reaction of 88 with /(-phenethyl bromide, l-phenethyl-3-phenylpropyl methyl sulfoxide and bis-3-phenylpropyl sulfoxide, besides 3-phenylpropyl methyl sulfoxide are obtained118. Sulfoxides, bearing a /1-hydrogen to the sulfmyl function, give olefins upon thermolysis. Utilizing this reaction, Trost and Bridges120 alkylated benzyl phenyl sulfoxide, 3,4-methylenedioxybenzyl phenyl sulfoxide, phenylthiomethyl phenyl sulfoxide, phenylsulfinylmethyl phenyl sulfoxide and cyanomethyl phenyl sulfoxide with alkyl, allyl and benzyl halides and subjected these sulfoxides to thermolysis, obtaining olefins in one-pot processes. 

As depicted in Scheme 21, subjection of 86 with 20 mol% of freshly prepared 2 after 4 h at 22 °C indeed afforded 87 in 92% yield after silica gel chromatography (>98% Z). Stereocontrolled hydrogenation of the trisubstituted olefin (72% yield) and removal of the acetate and trifluroacetate groups, effected by subjection of the hydrogenated adduct with hydrazine in MeOH, delivered Sch 38516 (1) in 96% yield to complete the total synthesis. 

How is the course of halogen substitution in the benzene nucleus to be explained It is not at all probable that direct replacement of hydrogen occurs, such as we must assume in the formation of benzyl chloride and in the reaction between methane and chlorine, since the hydrogen attached to the doubly bound carbon atom of olefines exhibits no special reactivity. However, various facts which will be considered later (p. 164) indicate that benzene reacts with halogen in fundamentally the same way as does ethylene. The behaviour of ethylene towards bromine is the subject of the next preparation. 

We shall not continue any further into the labyrinth of autoxidation, but shall merely point out that the complexities we have described are multiplied manyfold when one considers the situations that will arise in oxidation of an olefin that reacts by a combination of the addition-polymerization and the abstraction routes, or when the temperature is high enough to homolyze the peroxide products and the reaction is thus producing its own initiator, or when there are several nonequivalent hydrogens in the substrate. Furthermore, the products will themselves be subject to oxidation. Clearly the possibilities are almost without limit. 

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