El reactions can be stereoselective

The kinetic isotope effect tells us that the C-H (or C-D) bond is being broken during the rate-determining step, and so the reaction must be an E2 elimination. It s evidence like this that allows us to piece together the mechanisms of organic reactions. 

How do kinetic Isotope effects come about Even in its lowest energy state a covalent bond never stops vibrating. If it did it would violate a fundamental physical principle, Heisenberg s uncertainty principle, which states that position and momentum cannot be known exactly at the same time a nonvibrating pair of atoms have precisely zero momentum and precisely fixed locations. The minimum vibrational energy a bond can have is called the zero point energy (Eq) - given by the expression E = 

For some eliminations only one product is possible. For others, there may be a choice of two (or more) alkcne products that differ either in the location or stereochemistry of the double bond. We shall now move on to discuss the factors that control the stereochemistry (geometry) and rcgio-chemistry (that is, where the double bond is) of the alkenes, starting with El reactions, only one alkene possible 

For steric reasons, E-alkenes (and transition states leading to E-alkenes) are usually lower in energy than 2-alkenes (and the transition states leading to them) because the substituents can get 

The E/Znomenclature was introduced in Chapter , and rtdyv) that you have read Chapter 16 we can be more precise with our definition. For disubstituted alkenes, Ecorrespondsto tran and Zcorresponds to cis. To assign Eor Zto tri-or tetrasubstituted alkenes, the groups at either end of the alkene are given an order of priority according to the same rules as those outlined for f and Sin Chapter 16. If the two higher priority groups are c/s, the alkene is Z if they are frans the alkene is Of course, molecules don t know these rules, and sometimes (as in the second example here) the Ealkene is less stable than the Z. 

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Chiral, unsymmetrical divinyl methanols can be prepared by reaction of CH3Cu(CN)Li BF3, prepared from CH3Li, with a monoprotected (E,E)-dienoate such as 1, derived from an L-tartrate. The reaction involves an SN2 reaction with the mesyloxy group. The reaction proceeds readily at —78° with high regio- and (El-stereoselectivity.5... 

The as-decalin is formed because the enone, though flattened, is already folded to some extent. A conformational drawing of either molecule shows that the top surface is better able to bind to the flat surface of the. catalyst. Each of these products shows interesting stereoselective reactions. The ketal can be converted into an alkene by Grignard addition and El elimination and then epoxidized. Everything happens from the outside as expected with the result that the methyl group is forced inside at the epoxidation stage. 

Stereoselective radical reactions. Gicsc el al have found that radicals can show high 1,2-slcreoinduction comparable to ionic reactions, and that the selectivity can be reversed when proceeding from cyclic to acyclic radicals. Thus the Bu-tSnH radical addition to the dioxanone 1 provides the only one isomer (2) with rran.r-selectivity. The same radical with a corresponding acyclic system (3) shows di-selectivity. 

An El reaction is regioselective. The major product is the most stable alkene, which is generally the most substituted alkene. An El reaction is stereoselective. The major product is the alkene with the bulkiest groups on opposite sides of the double bond. The carbocation formed in the first step can undergo both syn and anti elimination therefore, the two groups to be eliminated in a cyclic compound do not have to be trans or both in axial positions. Alkyl substitution increases the stability of a carbocation and decreases the stability of a carbanion. 

In principle, a planar carbonium ion should lose a beta proton from either side of the plane with equal facility and consequently there should be no stereoselectivity in El reactions. This simple interpretation can be distorted, however, in solvents which do not favour ionic dissociation, or if neighbouring group participation in the ionisation is afforded by a beta hydrogen or another group. 

In Chapter 17 you saw that El elimination reactions usually give mainly E alkenes (there s an example earlier in this chapter) because the transition state leading to an E double bond is lower in energy than that leading to a Z double bond. In other words, El reactions are stereoselective, and their stereoselectivity is kinetically controlled. E2 reactions are similar if there is a choice of protons that can be removed the E alkene is preferred, but a mixture is still formed. Again, this is kinetic control. 

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