Alkenes 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. 

The photocycloaddition chemistry of oc,P-unsaturated 8-lactones is similar to the chemistry of y-lactones. Complications arise as with cyclohexenones because anti-addition to the a,P-unsaturated double bond can occur, particularly in the intermolecular addition mode. Even if one product prevails, intermolecular [2 + 2]-photocycloaddition reactions are often sluggish. Despite the fact that alkene 92, for example, was employed in a twofold excess relative to dihydropyranone 91, the reaction delivered only 32% of the desired product 93 (43% based on recovered starting material Scheme 6.33). The relative product configuration, which was established by X-ray crystallography, came as a surprise because the lactone apparently... 

Thus the observed orientation in both kinds of HBr addition (Markovnikov electrophilic and anti-Markovnikov free radical) is caused by formation of the secondary intermediate. In the electrophilic case, it forms because it is more stable than the primary in the free-radical case because it is sterically preferred. The stability order of the free-radical intermediates is also usually in the same direction 3°>2°>1° (p. 241), but this factor is apparently less important than the steric factor. Internal alkenes with no groups present to stabilize the radical usually give an approximately 1 1 mixture. 

Apparently similar reactions do not, however, follow these orbital symmetry rules. According to Neureiter (1966), addition of sulfur dioxide to alkenes or its subsequent elimination appears to be syn-stereospecific instead of anti, as predicted. According to Freeman and... 

Scheme 11.5 gives some examples of these radical addition reactions. Entries 1 to 3 show anti-Markovnikov addition of HBr. The reaction in Entry 1 was carried out by passing HBr gas into the alkene, using benzoyl peroxide as the initiator, apparently near room temperature. Entry 2 is an example of anti-Markovnikov addition to... 

This process is synthetically important because a sacrificial alkene such as 1-decene or 1-hexadecene can be added to bias the overall reaction toward a specific target alkene. Mechanistically, thermal isomerization apparently involves a series of reversible elimination reactions of borane followed by re-addition in an anti-Markovnikov manner.2 If the sacrificial alkene has a significantly higher boiling point, then the desired alkene can he distilled from the reaction medium.22a xhe sacrificial alkene must not boil < 160°C, however, which is the temperature required for borane isomerization. This process makes it possible to isomerize the double bond of an alkene to a less substituted isomer, a process that Brown termed contrathermodynamic isomerization.29,30 methylenecyclohexane (41) was prepared from 1-methylcyclohexene by... 

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