More-substituted olefin

The triethylsilane/trifluoroacetic acid reagent system reduces alkenes to alkanes in poor to excellent yields depending largely on the ability of the alkene to form carbocations upon protonation. Under these conditions the more substituted olefins are reduced in better yields and styrene double bonds are reduced in high yields.127,202,207,221-228 The reduction of 1,2-dimethylcyclohexene with this reagent gives a mixture of cis- and trans- 1,2-dimethylcyclohexane.229 The formation of the trifluoroacetate esters is a side reaction.205,230... 

The Bamford-Stevens reaction and the Shapiro reaction share a similar mechanistic pathway. The former uses a base such as Na, NaOMe, LiH, NaH, NaNHa, heat, etc., whereas the latter employs bases such as alkyllithiums and Grignard reagents. As a result, the Bamford-Stevens reaction furnishes more-substituted olefins as the thermodynamic products, while the Shapiro reaction generally affords less-substituted olefins as the kinetic products. 

Acid-catalyzed alkyl group migration of alcohols to give more substituted olefins. 

Modification of base and solvent can also induce a change from Saytzeff to Hofmann orientation even in alkyl bromide eliminations . In one unimolecu-lar elimination , the least substituted olefin is formed predominantly - ", but in this case it is the thermodynamically more stable product by about 1.8 kcal.mole, due to eclipsing between the /-butyl and methyl substituents in the more substituted olefin , and consequently the Saytzeff pattern is not violated (115). 

E2 thermodynamic elimination gives the more substituted olefin as the major product because it is more stable. 

Oxidative rearrangements, via oxythallation, have been improved in yield and selectivity by the use of thallium(iii) nitrate supported on clay rather than in methanolic solution. Thus, cyclohexene gave an 85% yield of dimethoxymethyl-cyclopentane while 1-tetralone, which normally gives a complex mixture of products, gave a 1 1 mixture of methyl indane-l-carboxylate and 2-methoxytetralone. An efficient, large-scale procedure for the direct cis-dihydroxylation of olefins has been reported. The oxidant is t-butyl hydroperoxide and the catalyst osmium tetroxide, with the reaction conducted under alkaline conditions (E%N OH ), so facilitating a rapid turnover of catalyst via enhanced hydrolysis of the osmate esters. The method appears to be more advantageous for the more substituted olefins than the Hofmann and Miles procedure. 

Reactions of both t5ipes are generally restricted to mono- or disubstituted olefins. More highly substituted olefins coordinate only weakly to most metal centers. Displacement of the more-substituted olefins from the metal by the nucleophile is a major competing reaction, particularly if the nucleophile is also a good ligand for the metal (Equation 11.20, bottom),... 

Insertion of propylene with 1,2-regioselectivity is known as primary insertion or regioregular insertion because it is the most prevalent type of monomer enchainment for metallocenes. Some metallocenes do insert propylene in other, regioirregular ways (Scheme 1.4). The monomer may be inserted with 2,1-regioselectivity (Scheme 1.4a), known as secondary insertion, with the more substituted olefinic carbon bonded to the metal. Following 2,1-insertion, a unimolecular isomerization may take place to give a 3,1 -insertion (Scheme 1.4b). Alternatively, monomer insertion can... 

Isomerization reaction may occur by both direct and sensitized irradiation. The direct irradiation is difficult to achieve in the case of the simple olefins. However, with more substituted olefins the direct excitation can be more readily achieved. When the cis-trans- somQV z2ii on occurs, it usually produces more cw-form. The reason is that the trans-fovm absorbs more light at the excitation wavelength than does the cis-form and consequently the trans-isomer is converted more readily into the cz -form (Horspool, 1976). 

Desulfonylation of allylic sulfones with LiHBEts catalyzed by C Pdldppp) proceeds through attack at the less substituted side to give the more substituted olefins (Scheme 17). This method was applied successfully to the total syntheses of various natural products such as squalene (Scheme 18), coenzyme (Scheme 19), ambremt (Scheme 20),... 

Pyrolysis of xanthate esters known as Chugaev reaction requires temperatures of 150-250 °C. For example, the pyrolysis of xanthates 4 and 5 gives more substituted olefins (Saytzeff products) as major products. 

E2 thermodynamic elimination to give the more substituted olefin. 

---