Hydrogenation regio-selective

Hydrogen bromide is unique among the hydrogen halides m that it can add to alkenes either by electrophilic or free radical addition Under photochemical conditions or m the presence of peroxides free radical addition is observed and HBr adds to the double bond with a regio selectivity opposite to that of Markovmkov s rule... 

The pronounced acidity of the bridgehead hydrogen atoms in 4 (R = H) facilitates the regio-selective introduction of electrophiles. Rearrangements of 4 (R = H, Me, CHO, C02Me) catalyzed by dicarbonyldichlororhodium(I) lead to 4-substituted 1-benzothiepins 5,10 except in the case of R = Me where a mixture (1 1.3) of 3- and 4-methyl-l-benzothiepin is obtained (total yield 98 %). In the case of the dimethyl-substituted derivative 8 (R1 = R2 = Me), however, the rhodium(I)-catalyzed isomerization reaction leads to the thiophene isomer. 

The nucleophiles that are used for synthetic purposes include water, alcohols, carboxylate ions, hydroperoxides, amines, and nitriles. After the addition step is complete, the mercury is usually reductively removed by sodium borohydride, the net result being the addition of hydrogen and the nucleophile to the alkene. The regio-selectivity is excellent and is in the same sense as is observed for proton-initiated additions.17... 

Both uncatalyzed and catalyzed [4+2]-cycloaddition reactions of furans with the allenic esters have been reported (Table 12.6) [93]. The allene adds from the less hindered C1-C2 Jt-face. The unfavorable steric interaction between the a-hydrogen atom of the furan and the methyl group at C4 of the allene is responsible for this selectivity. The more reactive 2-methylfuran adds to the allenic ester also in a regio-selective manner. The C2 carbon atom of 2-methylfuran was exclusively attached to the Cl carbon atom of the allenic ester, providing a mixture of endo- and exoadducts. 

A fundamental topic in hydroformylation research is the control of regio-selectivity and the suppression of side-reactions like the hydrogenation reac-... 

It has been shown experimentally that attack by strong nucleophiles also occurs regio-selectively at this C atom, stereo selectively from the face opposite to the metal [287]. Since the alkyl group a bonded to the metal is very carbanion-like, it is susceptible to protonation by acids, yielding an alkane. The overall reaction provides the mechanism for homogeneous hydrogenation of alkenes. It may be extended to hydrogenation of C=N and C=0 pi bonds. 

The ease of reduction is 1-alkynes > disubstituted alkynes > 1-alkenes, indicating that selective semihydrogenation of triple bonds can be achieved in molecules containing double bonds. For example, nonconjugated enynes can be hydrogenated regio-and stereospecifically in high yields. 

The reaction may proceed as homo- or cross-dehydrodimerization [105] and takes place with a wide range of substituted substrates such as higher alcohols, ethers, silanes, and partially fluorinated alcohols and ethers, but also with ketones, carboxylic acids, esters, amides, and amines [106]. Besides the formation of 1,2-diols from saturated alcohols, unsaturated substrates are also dimerized under hydrogen to form l,n-diols other than the 1,2-isomers [107]. The regio-selectivity of the diols is controlled by the formation of the most stable radical, which then dimerizes. 

The hydroboration of alkenes is a very important way to convert alkenes in other organic groups.1 For example, the hydroboration of an alkene followed by the oxidation with hydrogen peroxide and sodium hydroxide produces an alcohol1 in a highly stereo- and regio-selective reaction. 

Case 11. (See also Case 5 above.) The formation of Thiele s ester (223)184 is a remarkable example of several kinds of selectivity, all of which can be explained by frontier orbital theory. The particular pair of cyclopentadienes which do actually react together (221 and 222) are not the only ones present. As a result of very rapid 1,5-sigmatropic hydrogen shifts (see p. 99), all three isomeric cyclopentadiene carboxylic esters are present, and any combination of these is in principle possible. As each pair can combine in several different ways there are, in fact, 72 possible products. We shall take up other aspects of this selectivity later, in their proper place (p. 167), but for now we may note that the regio-selectivity shown is a vinylogous version of case 11. 

The local softness has been applied with much success in interpreting and predicting the regio-selectivities of different types of organic reactions including radical additions , nucleophilic additions " , pericyclic [2 + i]330-333 2 + 2] " and [3 + 2] " " additions, hydrogen shifts and internal rotations. . ... 

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