Alkenes geometry affecting

Charette and coworkers investigated the stereoselectivity of gem-zinc carbenoids in the reaction with allylic alcohols 100 and 101 (Scheme 1.52). Configuration at the allylic stereogenic center and alkene geometry affected the stereoselectivity of cyclopropanation [87]. 

Substitution has no significant effect on the geometry of the n complex and RuHCl(PH3)2(CH2=CH2) and RuHCl(PH3)2(H2C=CH(OMe)) have very similar shape. Of interest the OMe group cannot reach the second empty coordination site (trans to H) of Ru. Remarkably, the Ru-alkene bond dissociation energy is also not affected by the presence of OCH3 (less than 3 kcal.mol 1 difference in binding dissociation energy). The methyl vinyl ether... 

Identify which protonation reaction (alkene A —> protonated alkene A, alkene B —>protonated alkene B) is more favorable. The energy of proton is given at right. Compare geometries of the two alkenes. Which is more strained Why How is this likely to affect the proton affinity Compare electrostatic potential maps for the two alkenes. Is the n bond in one more susceptable to protonation than that in the other Compare maps for the two protonated forms. Is the charge in one more delocalized than that in the other Suggest an explanation to account for both the reactivity difference and the structural changes. 

The effect of substituents on the stereoselectivity of the intramolecular photocycloadditions of alkenes to cyclohexenones was systematically examined by Becker and coworkers84 who obtained high stereofacial selectivity in compounds 283a-c. However, small changes in the position, geometry or steric effect of the substituents have dramatically affected the selectivity, indicating the complexity in predicting the stereoselectivity in such system (Scheme 61). 

In contrast to the [4+2]-additions of butadiene to ethene or acetylene (Figures 15.8 and 15.9), the two HOMO/LUMO interactions stabilize the transition state of the [2+2]- addition of ketenes to alkenes to a very different extent. Equation 15.2 reveals that the larger part of the stabilization is due to the LUMOketene/HOMOethene interaction. This circumstance greatly affects the geometry of the transition state. If there were only this one frontier orbital interaction in the transition state, the carbonyl carbon of the ketene would occupy a position in the transition state that would be perpendicular above the midpoint of the ethene double bond. The Newman projection of the transition state (Figure 15.11) shows that this is almost the case but... 

The geometry of alkene coordination appears to affect the reactivity toward nucleophiles. 7-Methylenenorbomene reacts with PdCl2 to give the product from Pd-Cl addition across the exo-methylene double bond (equation 37). In the intermediate PdCl2-diene complex, which could not be isolated, the 7-exo double bond would be coordinated so as to lie in the ring plane, while the other aUcenic group would lie perpendicular to the palladium square plane, as is normal. From the structure of the product, trans addition of Pd and Cl occurs. 

In contrast with other electrophilic additions, the peracid epoxidation is syn-stereospecific. With sterically strongly hindered alkenes the reaction takes place on the less sterically hindered side. In other cases, the stereochemistry of the reaction is affected by polar effects or the geometry of the transition state. Important conclusions regarding the mechanism of the reaction can be drawn from the steric pathways in the synthesis of the oxiranes. This has been dealt with comprehensively by Berti, who reviewed the topic up to 1971, with special emphasis on the peracid oxidation. A noteworthy account of the topic of peracid epoxidation is given in a review by Rebek. ... 

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