C/s-Alkenes

In most cases, a multistep synthesis is not a simple string of reaction steps. You need to look ahead (and behind). For example, you may have a perfectly good reaction that forms a c/s-alkene, but that s the wrong answer because two steps later, you need a trans-alkene. 

M- Alkylaziridines can be stereospecifically deaminated to alkenes by reaction with m-chloroperbenzoic acid (70AG(E)374>. The reaction and work-up are carried out in the dark to avoid isomerization of the c/s-alkene, and the mechanism is thought to involve an initial oxidation to an amine oxide followed by a concerted elimination. Aziridine oxides have been generated by treating aziridines with ozone at low temperatures (71JA4082). Two... 

S-methyl dithiocarbonate (76S413). Stereoselective isomerization of 1,2-disubstituted alkenes may be achieved by a sequence such as the following rra/ts-alkene -> bromo-hydrin -> /3-hydroxythiocyanate -> cis-thiirane — c/s-alkene (75TL2709). 

Preparation of cis-alkenes Lindlar s catalyst, which is also known as poisoned catalyst, consists of barium sulphate, palladium and quinoline, and is used in selective and partial hydrogenation of alkynes to produce c/s-alkenes. Hydrogen atoms are delivered simultaneously to the same side of the alkyne, resulting in syn addition (cw-alkenes). Thus, the syn addition of alkyne follows same procedure as the catalytic hydrogenation of alkyne. 

The absorption of symmetrical disubstituted trans-alkenes or tetrasubstituted alkenes may be extremely weak or absent. The c/s-alkenes, which lack the symmetry of the trans structure, absorb more strongly than frans-alkenes. Internal double bonds generally absorb more weakly than terminal double bonds because of pseudosymmetry. 

Lindlar catalyst (Section 9.9) A catalyst for the hydrogenation of alkynes to c/.s-alkenes. It is composed of palladium, which has been poisoned with lead(II) acetate and quinoline, supported on calcium carbonate. 

As an illustration of the stereospecificity of eliminations, the meso compound 4 gives the c/s-alkene 5, whereas the d,l isomers 6 give the trans-alkene 7 with ethoxide. Both reactions clearly proceed by antarafacial elimination ... 

Requires a transition metal catalyst, Pt, Pd, Ni, etc. Addition is supra-facial from least hindered side. Rearrangements can occur. Alkynes are reduced to c/ s-alkenes over Lindlar catalyst, Pd-Pb (Section 11-2). 

The important feature is that no exceptional, contrived, or unprecedented chemistry has to be invoked to rationalize the stereoselective formation of the cw-alkene preferential rapid hydronation of the metal (or binding of an alkyne to a preformed hydrido species) naturally results in the c/s-alkene. 

Reduction of an alkyne with hydrogen on a metal catalyst gives the corresponding alkane. By selectively poisoning the catalyst it is possible to reduce an alkyne to an alkene. Once again, the reaction is stereoselective, adding both hydrogen atoms from the same side of the C—C bond to form the c/s-alkene. 

The diol can be prepared from syn hydroxylation of (Z)-2-butene. The c/s-alkene can be prepared by hydrogenation of 2-butyne, and 2-butyne can be prepared by alkylation of propyne. The retrosynthetic analysis is ... 

Hydrogenation using the Lindlar catalyst resulted in the formation of the c/s-alkene (see Section 11.12). 

Using retrosynthetic analysis, we recognize that the c/.v-epoxide can be prepared from the c/s-alkene. The m-alkene can be prepared by catalytic hydrogenation of an alkyne. Finally, substituted alkynes can be prepared by nucleophilic substitution reactions using acetylide ion nucleophiles (see Section 10.8). On the basis of this analysis, the synthesis reported in the literature was accomplished as shown in Figure 23.3. 

The human eye uses a c/s-alkene, 11-c/s-retinal, to detect light, and a cis-trans isomerism reaction is at the heart of the chemical mechanism by Which we see. The light-sensitive pigment in the cells of the retina is an imine, formed by reaction of ll-ci ... 

The c/s-alkene dienophile gives stereospecific addition—in each product the CC Me is cis to the alkyl chain (and therefore trans to the H atom). But we get about a 50 50 mixture of endo and exo products. This does not seem to be because there is anything wrong with the transition state for endo addition, which leads in this case to cis-fused rings. 

Alkynes show the same reaction, and again the product obtained is the trans isomer. After a suitable elimination from the metal the alkene obtained is the product of the trans-addition. Earlier we have seen that insertion into a metal-hydride bond and subsequent hydrogenolysis of the M-C bond will afford the c/s-alkene product. Thus, with the borohydride methodology and the hydrogenation route, both isomers can be prepared selectively. 

Simple 1,3-dienes are reduced mainly to monoenes. Some alkynes are reduced satisfactorily to c(s-alkenes. 

The chemical shifts of H-3 and H-4 in coumarin (19) are similar to those in pyran-2-one (Figure 4). The values are closely related to the shifts of the a and /3 protons of o-coumaric acid (56), implying that the heteroring has little or no aromatic character (62PiA(A)(56)7l). The signal for H-3 appears at higher field than that from H-4 and distinction between 3-and 4-substituted isomers is usually possible. Coupling between the alkenic protons is typical of a c/s-alkene (73,4 = 9.8 Hz) and the pair of doublets is a characteristic feature of the spectra of coumarins. 

Highly stereo- and chemo-selective hydrogenation of internal alkynes to c/ s-alkenes has been attained with [(arene)Cr(CO)3] complexes. For example, reduction of 7-tetradecyne (78) gives (79) in quantitative yield. 

The reduction of a carbon-carbon multiple bond by the use of a dissolving metal was first accomplished by Campbell and Eby in 1941. The reduction of disubstituted alkynes to c/ s-alkenes by catalytic hydrogenation, for example by the use of Raney nickel, provided an excellent method for the preparation of isomerically pure c -alkenes. At the time, however, there were no practical synthetic methods for the preparation of pure trani-alkenes. All of the previously existing procedures for the formation of an alkene resulted in the formation of mixtures of the cis- and trans-alkenes, which were extremely difficult to separate with the techniques existing at that time (basically fractional distillation) into the pure components. Campbell and Eby discovered that dialkylacetylenes could be reduced to pure frani-alkenes with sodium in liquid ammonia in good yields and in remarkable states of isomeric purity. Since that time several metal/solvent systems have been found useful for the reduction of C=C and C C bonds in alkenes and alkynes, including lithium/alkylamine, ° calcium/alkylamine, so-dium/HMPA in the absence or presence of a proton donor,activated zinc in the presence of a proton donor (an alcohol), and ytterbium in liquid ammonia. Although most of these reductions involve the reduction of an alkyne to an alkene, several very synthetically useful reactions involve the reduction of a,3-unsaturated ketones to saturated ketones. ... 

With dialkylacetylenes, the products of hydrolysis and oxidation are c/s-alkenes and ketones, respectively. 

The facial selectivity required for an asymmetric epoxidation can be achieved with manganese complexes to provide sufficient induction for synthetic utility (Scheme 17) [120-126], This manganese (III) salen complex 8 can also use bleach as the oxidant rather than an iodosylarene [127,128], The best selectivities are seen with c/s-alkenes. 

TABLE 5.15 Catalytic asymmetric epoxidation of various c/s-alkenes using TPPP and catalyst 36 ... 

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