Hydrogenation, catalytic alkenes

The formation of the linear carbon-carbon skeletons of lipids and carotenes usually depends on the reaction of the electropositive carbonyl carbon atoms of aldehydes or ketones with carbanions. Most popular is the Wittig reaction between a phosphorus ylene as obtained from alkylbromides and an aldehyde to form alkenes. Catalytic hydrogenation then converts alkenes to alkanes. The reaction has been adjusted to all kinds of functional groups, which can be connected to both educts, namely aldehydes and bromides (Scheme 1.3.1). 

Alkynes can be reduced to either a cis or trans alkene. Catalytic hydrogenation of an alkyne using a poisoned catalyst (H2, Lindlar catalyst) results in the syn addition of one equivalent of H2 to give a cis alkene product. Dissolving metal reduction (Li, NH3) of an alkyne produces the corresponding trans alkene. This strategy is well suited for synthesizing monosubstituted alkenes and disubstituted alkenes with a specific stereochemistry. 

The reaction of furans with ethyl 2,2-difluoro-l-diethylamino-carboxyacrylate generates fluorinated 7-oxanorbomenes [163]. These cycloadducts undergo alkene catalytic hydrogenation (or other reactions) into the corresponding 7-oxabicyclo[2.2.1]heptanes. Examples have been presented with the synthesis of norcantharidin, 5, with the synthesis of the 2-epimer of rac-3, 6 -epoxyauraptene, 32 (Scheme 3), and with the synthesis of thromboxane mimetics (Scheme 8). In 1929, Diels and Alder first reported the reaction of furan with maleic anhydride that produces at room temperature the exo-adduct 132 [164]. In 1962, Anet found that at low temperature, the reaction produces first the ew fo-adduct 133 in agreement with the endo Alder rule [165]. At 25 °C and in MeCN, the exo-adduct 132 is more stable by 1.9 kcal/mol compared with 133 [166] (Scheme 20). 

Catalytic hydrogenation is mostly used to convert C—C triple bonds into C C double bonds and alkenes into alkanes or to replace allylic or benzylic hetero atoms by hydrogen (H. Kropf, 1980). Simple theory postulates cis- or syn-addition of hydrogen to the C—C triple or double bond with heterogeneous (R. L. Augustine, 1965, 1968, 1976 P. N. Rylander, 1979) and homogeneous (A. J. Birch, 1976) catalysts. Sulfur functions can be removed with reducing metals, e. g. with Raney nickel (G. R. Pettit, 1962 A). Heteroaromatic systems may be reduced with the aid of ruthenium on carbon. 

The solvent used m catalytic hydrogenation is chosen for its ability to dissolve the alkene and is typically ethanol hexane or acetic acid The metal catalysts are insoluble m these solvents (or indeed m any solvent) Two phases the solution and the metal are present and the reaction takes place at the interface between them Reactions involving a substance m one phase with a different substance m a second phase are called het erogeneous reactions... 

On being heated with a solution of sodium ethoxide in ethanol compound A (CyHisBr) yielded a mixture of two alkenes B and C each having the molecular formula C7H14 Catalytic hydrogenation of the major isomer B or the minor isomer C gave only 3 ethylpentane Suggest structures for compounds A B and C consistent with these observations... 

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... 

A useful alternative to catalytic partial hydrogenation for converting alkynes to alkenes IS reduction by a Group I metal (lithium sodium or potassium) m liquid ammonia The unique feature of metal-ammonia reduction is that it converts alkynes to trans alkenes whereas catalytic hydrogenation yields cis alkenes Thus from the same alkyne one can prepare either a cis or a trans alkene by choosing the appropriate reaction conditions... 

The stereochemistry of metal-ammonia reduction of alkynes differs from that of catalytic hydrogenation because the mechanisms of the two reactions are different The mechanism of hydrogenation of alkynes is similar to that of catalytic hydrogenation of alkenes (Sections 6 1-6 3) A mechanism for metal-ammonia reduction of alkynes is outlined m Figure 9 4... 

Catalytic hydrogenation (Sections 6.1-6.3) Alkenes react with hydrogen in the presence of a platinum, palladium, rhodium, or nickel catalyst to form the corresponding alkane. 

Alcohol A (CioHi O) is converted to a mixture of alkenes B and C on being heated with potassium hydrogen sulfate (KHSO4). Catalytic hydrogenation of B and C yields the same product. Assuming that dehydration of alcohol A proceeds without rearrangement, deduce the structures of alcohol A and alkene C. 

Until the second half of the twentieth century, the structure of a substance—a newly discovered natural product, for example—was determined using information obtained from chemical reactions. This information included the identification of functional groups by chemical tests, along with the results of experiments in which the substance was broken down into smaller, more readily identifiable fragments. Typical of this approach is the demonstration of the presence of a double bond in an alkene by catalytic hydrogenation and subsequent determination of its location by ozonolysis. After-considering all the available chemical evidence, the chemist proposed a candidate structure (or structures) consistent with the observations. Proof of structure was provided either by converting the substance to some already known compound or by an independent synthesis. 

Catalytic hydrogenation of alkynes on a metal surface provides cis alkenes (see Chapter 7, Problem 13), while treatment with sodium in liquid ammonia nearly always leads to trans alkenes, e.g., hydrogenation of 2-butyne. 

Problem 7.13 I What product would you obtain from catalytic hydrogenation of the following alkenes ... 

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. 

The product i n this case is a cis-disubstituted alkene, so the fi rst question is, " What is an immediate precursor of a cis-disubstituted alkene " We know that an alkene can be prepared from an alkyne by reduction and that the right choice of experimental conditions will allow us to prepare either a trans-disubstituted alkene (using lithium in liquid ammonia) ora cis-disubstituted alkene (using catalytic hydrogenation over the Lindlar catalyst). Thus, reduction of 2-hexyne by catalytic hydrogenation using the Lindlar catalyst should yield cis-2-hexene. 

Alkynes can be reduced to yield alkenes and alkanes. Complete reduction of the triple bond over a palladium hydrogenation catalyst yields an alkane partial reduction by catalytic hydrogenation over a Lindlar catalyst yields a cis alkene. Reduction of (he alkyne with lithium in ammonia yields a trans alkene. 

Figure 1.26 Abbreviated mechanism for the catalytic hydrogenation of a terminal alkene using...

Catalytic hydrogenation of triple bonds and the reaction with DIBAL-H usually give the eis alkene (15-11). Most of the other methods of triple-bond reduction lead to the more thermodynamically stable trans alkene. However, this is not the case with the method involving hydrolysis of boranes or with the reductions with activated zinc, hydrazine, or NH2OSO3H, which also give the cis products. 

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