Anti-periplanar arrangement

The relative instability of the cis-2-oxopyrido[l,2-u]pyrimidines (158 R1 = Ph) was explained by the steric conditions of the molecule the anti-periplanar arrangement of bonds a and "b" facilitates a concerted olefinforming elimination reaction leading to the amide (159 R1 = Ph). The reaction is also aided by the unfavorable interactions arising among the cis-4-phenyl groups and between the 4-phenyl group and the 6-H atom. 

E2 elimination proceeds via anti-periplanar arrangement of the OTs and the adjacent H. 

An anti-periplanar arrangement of C-Br and C-H is attainable with a vinylic bromide too, provided the Br and H are trans to one another. E2 elimination from the Z isomer of a vinyl bromide gives an alkyne rather faster than elimination from the E isomer, because in the E isomer the C-H and C-Br bonds are syn-periplanar. 

In the last chapter, we looked at some stereospecific eliminations to give double bonds, and you know that E2 elimination reactions occur best when there is an anti-periplanar arrangement between the proton and the leaving group. 

The more stable conformations of each of the two isomers are pictured above the larger tert-butyl group is always equatorial in the more stable conformation. The cis isomer reacts faster under E2 conditions because -Br and -H are in the anti periplanar arrangement that favors E2 elimination. 

The dihedral angle for the C-H and C-X bonds equals 0° for the syn periplanar arrangement and 180° for the anti periplanar arrangement. 

Stereochemistry Anti periplanar arrangement of H and X (8.8) Base Favored by strong bases (8.4B)... 

Draw the reactant with the -H and the leaving group in an anti periplanar arrangement. Then carry out the elimination while keeping all substituents in approximately their same positions, and see what alkene results. 

FIGURE 5.11 Conformations of cis- and trans-4-te/ t-butylcyclohexyl bromide and their relationship to the preference for an anti periplanar arrangement of proton and leaving group. 

Effects that arise because one spatial arrangement of electrons (or orbitals or bonds) is more stable than another are called stereoelectronic effects. There is a stereoelec-tronic preference for the anti periplanar arrangement of proton and leaving group in E2 reactions. 

Dehydrohalogenation of alkyl halides (Sections 5.14-5.16) Strong bases cause a proton and a halide to be lost from adjacent carbons of an alkyl halide to yield an alkene. Regioselectivity is in accord with the Zaitsev rule. The order of halide reactivity is I > Br > Cl > F. A concerted E2 reaction pathway is followed, carbocations are not involved, and rearrangements do not normally occur. An anti periplanar arrangement of the proton being removed and the halide being lost characterizes the transition state. 

The orange hydrogen cannot be lost by E2 as it is cis to the iodine and E2 reactions prefer a trans (anti-periplanar) arrangement. The green hydrogens are not lost because they are less acidic than the black hydrogens. In fact, this is not an E2 elimination at all. Because one of the black hydrogens can be lost in enolate formation, this is an ElcB elimination. 

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