Hydrogenation apparent anti addition

The amount of each product obtained depends on the catalyst and the nature of R and R, but the linear form generally tends to predominate. The unsaturated vinylsilane, RCH=CHSiR3, is also a product. Although minor in most cases, conditions can be found in which it predominates. The Chalk-Harrod mechanism cannot explain the formation of this dehydrogenative sU-ation product, but the alternate mechanism of Fig. 9.76 in which the alkene inserts into the M—Si bond first does explain it because 3 elimination of the intermediate alkyl leads directly to the vinylsilane. As in hydrogenation, syn addition is generally observed. Apparent anti addition is due to isomerization of the intermediate metal vinyl, as we saw in Eqs. 7.21 and 7.22, a reaction in which initial insertion of alkyne into the M—Si bond must predominate (>99%). Co2(CO)g also catalyzes a number of other reactions of silanes, as shown in Fig. 9.8. 

Hydrogenation of the strained hexamethylbicyclo[2.2.0]hexa-2,5-diene (29) is highly stereospecific over the platinum metals at 25 °C, 1 atm. Addition of hydrogen from the less-hindered exo direction yields the bicyclohexene (30) but further addition is very slow (Scheme 4). Raising the pressure (150 atm) results in the syn, exo addition of a second mol of H2 over Ru and Rh to form (31 >98%). The resistance to addition can be attributed to the increase in nonbonded interactions among the four endo methyl groups which accompanies the formation of the alkyl intermediate. These interactions are avoided in the dissociative mechanism of isomerization to the e ro-methylene intermediate, apparently the path taken on Pt and Ni at 50 C, where both the syn, exo-(3l) and the anti addition product (32) are formed. 

The addition reaction of allylsilane to acetaldehyde with BF3 as the Lewis acid has been modeled computationally.95 The lowest-energy TSs found, which are shown in Figure 9.2, were of the synclinal type, with dihedral angles near 60°. Although the structures are acyclic, there is an apparent electrostatic attraction between the fluorine and the silicon that imparts some cyclic character to the TS. Both anti and syn structures were of comparable energy for the model. However, steric effects that arise by replacement of hydrogen on silicon with methyl are likely to favor the anti TS. 

Now, just the same sort of rationalization can be applied to the radical addition, in that the more favourable secondary radical is predominantly produced. This, in turn, leads to addition of HBr in what is the anti-Markovnikov orientation. The apparent difference is because the electrophile in the ionic mechanism is a proton, and bromide then quenches the resultant cation. In the radical reaction, the attacking species is a bromine atom, and a hydrogen atom is then used to quench the radical. This is effectively a reverse sequence for the addition process but, nevertheless, the stability of the intermediate carbocation or radical is the defining feature. The terminologies Markovnikov or anti-Markovnikov orientation may be confusing and difficult to remember consider the mechanism and it all makes sense. 

Abnormal olefin arylation reactions which are of interest mechanistically and preparatively occur with some allylically substituted compounds. The ailylic esters and ethers appear normal and produce cinnamyl derivatives exclusively while ailylic alcohols and chlorides are abnormal. Ailylic alcohols and "arylpalladium acetates form 3-arylaldehydes from primary ailylic alcohols and 3-arylketones from secondary alcohols 3°). The mechanism of reaction apparently involves anti-Markovnikov addition of the palladium compound to the double bond followed by elimination of the hydrogen atom on the hydroxyl-bearing carbon rather than the benzylic hydrogen. This again would be elimination of the more electronegative hydrogen atom. 

The mixed Tishchenko reaction involves the reaction of the aldol prodnct 113 from one aldehyde with another aldehyde having no a-hydrogens to yield an ester The products were proposed to be formed through an aldol step (equation 33), followed by addition of another aldehyde (equation 34) and an intramolecular hydride transfer (equation 35). However, several aspects of this mechanism need to be clarified. As part of the continuing mechanistic studies carried out by Streitwieser and coworkers on reactions of alkali enolates ", it was found that the aldol-Tishchenko reaction between certain lithium eno-lates and benzaldehyde proceeded cleanly in thf at room temperature". Reaction of the lithium enolate of isobutyrophenone (Liibp) with 1 equiv of benzaldehyde in thf at — 65 °C affords a convenient route to the normal aldol product 113 (R = R" = Ph, R = Me). At room temperature, however, the only product observed after acid workup was the diol-monoester 116, apparently derived from the corresponding lithium ester alcoholate (115, R = R" = Ph, R = Me), which was quantitatively transformed into 116 after quenching. As found in other systems", only the anti diol-monoester diastereomer was formed. 

Since the keto esters A and C readily isomerize into one another in an alkaline medium [224, 236], it must be concluded that the keto ester C is the epimer of the keto ester A at C3, i.e., it has the 8Q, 9o -configuration [123]. However, an explanation of the stereochemistry of the formation of the estrone isomers (127) and (130) from the keto esters (119) and (123) presents serious difficulties. Johnson [83] assumed that the unsaturated diester with mp 95-97°C obtained from (119) was not a geometrical isomer of the higher melting (mp 113-115°C) diester (121) but a product of the isomerization of the double bond from the to the position (120). The addition to it of hydrogen from the p -direction leads to compounds of the trans-anti-cis series (127) at the same time, the formation from (120) of isomers of the type of (130) remains difficult to explain. The synthesis of the keto ester C (123) may also, apparently, take place through (120). 

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