Regioselectivity olefin structures

Substituted TMMs also participate smoothly in imine cycloaddition to generate more structurally elaborate pyrrolidines. The regioselectivity of these reactions is similar to that of olefin addition, although subsequent isomerization of the initial adduct is often observed. For example, the cyano system produced the thermody-... 

The transition state was shown to have a four-centered nonplanar structure and the product showed a strong jS-agostic interaction.59 Molecular-mechanics (MM) calculations based on the structure of the transition state indicated that the regioselectivity is in good agreement with the steric energy of the transition state rather than the stability of the 7r-complex. The MM study also indicated that the substituents on the Cp rings determine the conformation of the polymer chain end, and the fixed polymer chain end conformation in turn determines the stereochemistry of olefin insertion at the transition state.59... 

The /Tamino alcohol structural unit is a key motif in many biologically important molecules. It is difficult to imagine a more efficient means of creating this functionality than by the direct addition of the two heteroatom substituents to an olefin, especially if this transformation could also be in regioselective and/ or enantioselective fashion. Although the osmium-mediated75 or palladium-mediated76 aminohydroxylation of alkenes has been studied for 20 years, several problems still remain to be overcome in order to develop this reaction into a catalytic asymmetric process. 

The proposed mechanism is illustrated in Figure 8-5.60a Oxidative addition of the phenyl triflate to the palladium(0)-BINAP species A gives phenylpalla-dium triflate B. Cleavage of the triflate and coordination of 2,3-dihydrofuran on B yields cationic phenyl palladium olefin species C. This species C bears a 16-electron square-planar structure that is ready for the subsequent enantio-selective olefin insertion to complete the catalytic cycle (via D, E, F, and G). The base and catalyst precursor have profound effects on the regioselectivity and enantioselectivity. 

Stmctural features of olefin cycloadducts (i.e., isoxazolines) were extended when regioselective 4-endo- and 3 -exo-deprotonations were discovered, with the broad range of ensuing reactions of these carbanions that are common with electrophiles (19-22, 26-28). Thus, sequences consisting of cycloaddition-modification-ring cleavage were utilized to build up complex structures (17, 20-23). 

The same Pt/SiC catalysts were poisoned by CS2 the hydrogenation of (+)-apopinene was used as an indicator reaction (ref. 14). The amount of CS2 necessary to eliminate the hydrogenation activity permits calculation of the fraction of metal sites active in olefin hydrogenation. This fraction correlates with the number of step sites. Similarly, a good correlation is found between this fraction and the rate of methyloxirane transformation (Table 1). These results reveal that the structure-sensitivity is caused by the variation in the number of active sites, and the steps appear to be the active sites for the regioselective hydrogenation of methyloxirane. 

Alkenes are relatively low oxidation level hydrocarbons. The most common way to prepare alkenes is to cany out the elimination of a small molecule from between vicinal carbon atoms. However, this is only a viable strategy if the regiochemistry of elimination can be controlled. That is, traditional dehydrohalo-genations or dehydrations often are regioselective but not regiospecific, so that mixtures of structurally isomeric olefins are formed. For example,... 

Intramolecular cycloaddition of fV-benzyl-substituted 3-O-allylhexose nitrones furnishes chiral oxepane derivatives. The regioselectivity of the cycloaddition depends on several factors such as (1) the structural nature of the nitrone, (2) substitution and stereochemistry at 3-C of the carbohydrate backbone, and (3) substitution at the terminus of the O-allyl moiety. A mixture of an oxepane and a pyran is formed in the intramolecular oxime olefin cycloaddition of a 3-O-allyl carbohydrate-derived oxime <2003T4623>. The highly stereoselective synthesis of oxepanes proceeds by intramolecular nitrone cycloaddition reactions on sugar-derived methallyl ethers <2003TA3899>. 

Tosyl chloride 361 can be applied similarly (entry 23) [404], Here the best cobalt complex was dependent on the structure of the starting olefin. For terminal alkenes 2 mol% of the (salen)Co complex 357a was preferred. Secondary alkyl chlorides were obtained with complete regioselectivity in 73-94% yield, while the catalyst derived from Co(BF4)2 and /V-salicylidene diphenylglycinate 353 proved to be better for the hydrochlorination of 1,1-disubstituted olefins (entry 22). Tertiary alkyl chlorides 362 were obtained in 67-96% yield. The reaction conditions are mild so that acid- and base-sensitive protecting groups are compatible. 

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