Addition reactions catalytic hydrogenation

The last decade has witnessed the application of 1-halogenoacetylenes as crucial intermediates for the synthesis of increasingly complex structures, especially in natural product chemistry. In pheromone synthesis it is essential to create double-bond systems diastereoselec-tively, and a route often taken consists in the preparation of a suitable alkyne precursor which is then converted into the final olefin by various addition reactions (catalytic hydrogenation, metalation, etc.). For the construction of the alkyne precursor to the pheromone, 1-bromo-(94) and 1-iodoalkynes (95) have been particularly valuable since they can easily be subjected to metal-catalyzed coupling reactions [105]. For ocample, the unsaturated ester 163, which is a sex attractant of Lepidoptera (moths and butterflies), has been prepared by first converting the terminal acetylene 160 into its 1-iodo derivative 161. This is subsequently hydrogenated... 

First let us consider some reactions we have learned. Addition reactions (catalytic hydrogenation and Diels-Alder reactions, for example) do not create any by-products. The same can be said for isomerization reactions. Such reactions are said to be atom economical —all of the atoms in the reactants appear in the product. On the other hand, eiimination reactions and substitution reactions necessariiy produce by-products. This does not mean that they are bad, but if a synthesis can be devised that focuses on addition and isomerization reactions, iess attention wiii have to be devoted to disposing of, or deveioping uses for, by-products. 

Benzene can undergo addition reactions which successively saturate the three formal double bonds, e.g. up to 6 chlorine atoms can be added under radical reaction conditions whilst catalytic hydrogenation gives cyclohexane. 

We have already discussed one important chemical property of alkynes the acidity of acetylene and terminal alkynes In the remaining sections of this chapter several other reactions of alkynes will be explored Most of them will be similar to reactions of alkenes Like alkenes alkynes undergo addition reactions We 11 begin with a reaction familiar to us from our study of alkenes namely catalytic hydrogenation... 

Chemical Properties. Higher a-olefins are exceedingly reactive because their double bond provides the reactive site for catalytic activation as well as numerous radical and ionic reactions. These olefins also participate in additional reactions, such as oxidations, hydrogenation, double-bond isomerization, complex formation with transition-metal derivatives, polymerization, and copolymerization with other olefins in the presence of Ziegler-Natta, metallocene, and cationic catalysts. All olefins readily form peroxides by exposure to air. 

AH (A)-menthol is made by synthetic methods. One method involves the cyclization of (+)-citroneIlal (68). Using a mild acid catalyst, (+)-citroneIlal [2385-77-5] undergoes an ene-reaction to produce a mixture of isopulegols (142). Catalytic hydrogenation of the isopulegol mixture gives a mixture of menthol and its isomers. The (A)-menthol is obtained after efficient fractional distillation and the remaining isomers can be equilibrated, usually with sodium menthol ate or aluminum isopropoxide. An equilibrium mixture is obtained, comprised of 62 wt % (A)-menthol, 23 wt % (+)-neomenthol, 12 wt % (+)-isomenthol, and 3 wt % (+)-neoisomenthol. The equilibrium mixture can be distilled to recover additional (+)-mentbol. 

Catalytic hydrogenation of the nitrile function of cyanohydrins can give amines. As in the case of ordinary nitriles, catalytic reduction of cyanohydrins can yield a mixture of primary, secondary, and tertiary amines. Addition of acid or acetic anhydride to the reaction medium minimizes formation of secondary or tertiary amines through formation of the amine salt or acetamide derivative of the primary amine. 

In a sense the formation of t) -H2 complexes can be thought of as an intermediate stage in the oxidative addition of H2 to form two M-H bonds and, as such, the complexes might serve as a model for this process and for catalytic hydrogenation reactions by metal hydrides. Indeed, intermediate cases between and... 

As in the case of the steroids, introduction of additional nuclear substituents yields morphine analogs of increased potency. The more important of these are derived from one of the minor alkaloids that occur in opium. Thebaine (14), present in crude opium in about one-tenth the amount of morphine, exhibits a reactive internal diene system that is well known to undergo various addition reactions in a 1,4 manner (e.g., bromination). Thus, reaction with hydrogen peroxide in acid may be visualized to afford first the 14-hydroxy-6-hemiketal (15). Hydrolysis yields the isolated unsaturated ketone (16). Catalytic reduction... 

Alkenes are reduced by addition of H2 in the presence of a catalyst such as platinum or palladium to yield alkanes, a process called catalytic hydrogenation. Alkenes are also oxidized by reaction with a peroxyacid to give epoxides, which can be converted into lTans-l,2-diols by acid-catalyzed epoxide hydrolysis. The corresponding cis-l,2-diols can be made directly from alkenes by hydroxylation with 0s04. Alkenes can also be cleaved to produce carbonyl compounds by reaction with ozone, followed by reduction with zinc metal. 

Other products have been obtained from T8[CH = CH2]8 via additions to the double bonds (Table 16 and Figure 25). For example, TgEtg can be prepared by catalytic hydrogenation of T8[CH = CH2]8 giving a better yield than from the reaction of TgHs with ethylene (see Section V.B) (Table 16, entries 1-3) and... 

Amides are very weak nucleophiles, far too weak to attack alkyl halides, so they must first be converted to their conjugate bases. By this method, unsubstituted amides can be converted to N-substituted, or N-substituted to N,N-disubstituted, amides. Esters of sulfuric or sulfonic acids can also be substrates. Tertiary substrates give elimination. O-Alkylation is at times a side reaction. Both amides and sulfonamides have been alkylated under phase-transfer conditions. Lactams can be alkylated using similar procedures. Ethyl pyroglutamate (5-carboethoxy 2-pyrrolidinone) and related lactams were converted to N-alkyl derivatives via treatment with NaH (short contact time) followed by addition of the halide. 2-Pyrrolidinone derivatives can be alkylated using a similar procedure. Lactams can be reductively alkylated using aldehydes under catalytic hydrogenation... 

When an oxidation or a reduction could be considered in a previous chapter, this was done. For example, the catalytic hydrogenation of alkenes is a reduction, but it is also an addition to the C=C bond and was treated in Chapter 15. This chapter discusses only those reactions that do not fit into the nine categories of Chapters 10-18. An exception to this rule was made for reactions that involve elimination of hydrogen (19-1-19-6), which were not treated in Chapter 17 because the mechanisms generally differ from those in that chapter. 

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