Disubstituted olefins, hydrogenation

Prior literature indicated that olefins substituted with chiral sulfoxides could indeed be reduced by hydride or hydrogen with modest stereoselectivity, as summarized in Scheme 5.10. Ogura et al. reported that borane reduction of the unsaturated sulfoxide 42 gave product 43 in 87 13 diastereomer ratio and D20 quench of the borane reduction mixture gave the product 43 deuterated at the a-position to the sulfoxide, consistent with the hydroboration mechanism [10a]. In another paper, Price et al. reported diastereoselective hydrogenation of gem-disubstituted olefin rac-44 to 45 with excellent diastereoselectivity using a rhodium catalyst [10b],... 

The configuration of the product in diastereoselective hydrogenation -whether 1,2-syn or 1,2-anti - is related to the substitution pattern of the starting alkene. The allyl alcohol with a 1,1-disubstituted olefin unit affords the antiproduct, while the syn-product is formed from the allyl alcohol with a trisubsti-tuted olefmic bond (Table 21.8, entries 6-9). The complementarity in diastereoselective hydrogenation of di- and tri-substituted olefins may be rationalized based on the conformation analysis of the intermediary complex (Scheme 21.1)... 

The enantioselective hydrogenation of terminal 1,1-disubstituted olefins presents a particular challenge. Enantioface differentiation in the compounds relies on the different interactions of the catalyst with the two substituents which, as in the case of 1 and 2 (Fig. 30.1), are sterically quite similar. 

Ligand 19 performs excellently with the wide variety of l,l -disubstituted olefins reported. Substrates 61a-m are efficiently reduced at 1 bar of hydrogen in high enantioselectivity with very little dependence on the bulk of the alkyl substituents. Strongly coordinating olefins such as 611 and 61m tyqrically perform poorly in iridium-catalyzed hydrogenations, but reduction with 19 clearly breaks this rule and the substrates are reduced in excellent selectivity and yield. 

A number of examples involving nitrile oxide cycloadditions to cyclic cis-disubstituted olefinic dipolarophiles was presented in the first edition of this treatise, notably to cyclobutene, cyclopentene, and to 2,5-dihydrofuran derivatives (15). The more recent examples discussed here also show, that the selectivity of the cycloaddition to 1,2-cis-disubstituted cyclobutenes depends on the type of substituent group present (Table 6.8 Scheme 6.41). The differences found can be explained in terms of the nonplanarity (i. e., pyramidalization) of the double bond in the transition state (15) and steric effects. In the cycloaddition to cis-3,4-diacetyl-(197) and cis-3,4-dichlorocyclobutene (198), the syn-pyramidalization of the carbon atoms of the double bond and the more facile anti deformability of the olefinic hydrogens have been invoked to rationalize the anti selectivity observed. 

A full account5 describes the enantioselective carbonyl-ene reaction of glyoxylate esters catalyzed by a binaphthol-derived chiral titanium complex that is potentially useful for the asymmetric synthesis of a-hydroxy esters of biological and synthetic importance.6 The present procedure is applicable to a variety of 1,1-disubstituted olefins to provide ene products in extremely high enantiomeric purity by the judicious choice of the dichloro or dibromo chiral catalyst (see Table). In certain glyoxylate-ene reactions involving removal of a methyl hydrogen, the dichloro catalyst... 

The hydroboration of an olefin involves a cis addition of a boron-hydrogen bond to an alkene linkage, and for unsymmetric olefins occurs in a counter-Markownikoff fashion. 1-Alkenes and simple 1,2-disubstituted olefins undergo rapid conversion to the corresponding trialkylborane, whereas addition of diborane to tri- and tetrasubstituted olefins may be conveniently terminated at the respective di- and monoalkylborane stage. 1-Alkenes yield trialkylboranes in which there is a preponderant (approximately 94%) addition of the boron atom to the terminal carbon.2,3... 

In contrast to the case of allylsilanes, anodic oxidation of disubstituted olefins provides in general four regioisomeric products because all the allylic carbon-hydrogen bonds can be cleaved. In the case of allylsilane, the cleavage of a C—Si bond takes place... 

Another relatively simple system for the epoxidation of tri- and c/r-disubstituted olefins is formamide-hydrogen peroxide in an aqueous medium. This reagent has the advantage of being pH-independent, which makes it attractive for biochemically mediated transformations. No reaction was observed in the case of /ran.v-disubstituted and terminal olefins. With bifunctional alkenes, the more reactive double bond is selectively epoxidized [95TL4015]. 

Diels-Alder reactious. This triazine reacts with unactivated disubstituted olefins to produce dichloropyridines (4). This reaction appears to involve a [1,5] hydrogen migration in the initial adduct (2) followed by loss of hydrogen chloride. 

Transfer hydrogenation l-methyl-l,4-cyclohexadiene, Pd/C, CaCO, EtOH, 100% yield.This method was compatible with a disubstituted olefin. Benzyl... 

Transfer hydrogenation Pd-C, l-methyl-l,4-cyclohexadiene, CaCOs, EtOH, 98% yield. A disubstituted olefin survives these conditions. ... 

High enantioselectivities in the hydrogenation of 2-phenyl-l-butene have been achieved using chiral samarium complexes such as 55 (96% ee at — 80°C, 64-80% cc at 25°C)129. The reaction was carried out in heptane at 1 bar of H2 using a substrate/catalyst ratio of 200 1, and quantitative conversion and high turnover frequencies were observed under these conditions. The same catalyst gave 72% ee in the deuteration of styrene with D2 at 25 "C. Substantial enantiomeric excesses in the hydrogenation of 1,1-disubstituted olefins have also been obtained with the chiral bis(cyclopentadienyl)titanium complex 5690. 

Arylation and vinylotion of olefins. Aryl and vinylic bromides and iodides react with mono- and disubstituted olefins in the presence of a tertiary amine and a catalyst composed of palladium acetate and 2 eq. of triphenylphospine to form new olefins in which the vinylic hydrogen of the original olefin has been replaced by the aryl or vinyl group of the halide. 

Shapiro reaction. (2, 418-419 6, 598-600). Several laboratories have used EDA instead of an alkyllithium for decomposition of tosylhydrazones of ketones to olefins. Trisubstituted alkenes can be prepared by this modification in moderate yields from tosylhydrazones that contain only tertiary a-hydrogens. This modification also favors formation of the (Z)-disubstituted olefin. ... 

Other Reductions. The (porphinato)irons could realize the reduction of alkenes and alkynes with NaBILj. Various unsaturated carbon-carbon bonds were saturated by meso-tetraphenylporphinatoiron chloride (TPPFe Cl) derivatives (up to 81% yield). Ruthenium(III) complexes also pair with NaBH in the reduction of unsaturated carbon-carbon bonds (as does cobalt boride). In the presence of a catalytic amount of Ru(PPh3)4H2 (0.5-1 mol %) and NaBHj, unsaturated carbon-carbon bonds in a wide variety of alkenes and alkynes were saturated in toluene at 100 Addition of water was required to provide a proton source. Similar systems with RUCI3 in aqueous solution reduce unsaturated bonds under milder conditions. Various unactivated mono- or disubstituted olefins and activated trisubstituted olefins were reduced with RUCI3 (10 mol %) and NaBH4 in THF-H2O at 0 °C to room temperature (eq 36). When the RuCl3-catalyzed reductions of olefins were carried out in aqueous amide solution, unactivated trisubstituted olefins were also hydrogenated. ... 

In general, radical hydrosilylation of alkenes cannot be conducted with tri-alkylsilanes, which is due to a rather strong Si—H bond in the latter. However, the hydrosilylation of carbon-carbon multiple bonds with modified silanes such as tris(trimethylsilyl)silane has been successfully used in radical hydrosilylation (16). The reversible addition of tris(trimethylsilyl)silyl [(TMSlsSi] radical to the C=C bonds is due to the ability of this radical to isomerize alkenes. The hydrosilylation of monosubstituted and gem-disubstituted olefins are efficient processes and have been shown to proceed with high regioselectivity for both electron-rich and electron-poor olefins (140). Walton and Studer presented the results of the radical hydrosilylation with silylated cyclohexadienes as radical initiators (141). The bisvinylic methylene group acts as the hydrogen donor in these reactions. H-transfer leads to a cyclohexadienyl radical (2) that subsequently rearranges to provide er -butyldimethylsilyl radical and arene (3) (see Scheme 20) (141). 

Fig. 7.1. Olefinic vibrations. The left-hand column illustrates the in-plane vibrations of the vinyl group. The right-hand column illustrates the out-of-plane vibrations (+ and —) of the vinyl group and for comparison the in-phase, out-of-plane hydrogen wagging vibrations of trans-, cis, and 1,1-disubstituted olefins and trisubstituted olefins. The approximate frequencies are given for hydrocarbon-substituted olefins.

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