Anti-coplanar orientation

The relative reactivities of two diastereomeric vicinal dibromide derivatives of cholanic acid also provided evidence for an anti-coplanar orientation of the two carbon-bromine bonds in iodine-promoted dehalogenation. In the lla,12)3-dibromo compound 20, each of the two bromine atoms is held in an axial conformation by the rigid steroid skeleton. On the other hand, each of the two bromine atoms is held in an equatorial conformation in the llj8,12a-dibromo isomer 21. As a result, the conformation of the Br—C—C—Br grouping is anti in 20 but is gauche in 21. When treated with sodium iodide in acetone, 20 underwent debromination readily, while 21 was unreactive under the same conditions. 

Only the deuterium atom can assume the anti coplanar orientation necessary for an E2 reaction to occur. 

Anti (coplanar) orientation A Newman projection of added bromine atoms of the product... 

It has generally been assumed that phosphorous oxychloride-pyridine dehydrations, the elimination of sulfonates, and other base catalyzed eliminations (see below) proceed by an E2 mechanism (see e.g. ref. 214, 215, 216). Concerted base catalyzed eliminations in acyclic systems follow the Saytzelf orientation rule i.e., proceed toward the most substituted carbon), as do eliminations (see ref 214). However, the best geometrical arrangement of the four centers involved in 2 eliminations is anti-coplanar and in the cyclohexane system only the tran -diaxial situation provides this. 

Nearly all cyclohexanes are most stable in chair conformations. In the chair, all the carbon-carbon bonds are staggered, and any two adjacent carbon atoms have axial bonds in an anti-coplanar conformation, ideally oriented for the E2 reaction. (As drawn in the following figure, the axial bonds are vertical.) On any two adjacent carbon atoms, one has its axial bond pointing up and the other has its axial bond pointing down. These two bonds are trans to each other, and we refer to their geometry as trans-diaxial. 

Models show that the H on C-3 cannot be anti-coplanar with the Cl on C-2. Thus, this E2 elimination must occur with a yn-coplanar orientation the D must be removed as the Cl leaves. 

In E2, the two groups to be eliminated must be coplanar. In conformationally mobile systems like acyclic molecules, or in cyclohexanes, anti-coplanar is the preferred orientation where the H and leaving group are 180° apart. In rigid systems like norbomanes, however, SYN-coplanar (angle 0°) is the only possible orientation and E2 will occur, although at a slower rate than anti-coplanar. 

Cornforth proposed a different explanation for the diastereoselective addition of Grignard reagents to a-chloro aldehydes and ketones. The underlying premise of this model is that electrostatic effects such as dipole-dipole interactions favor a reactant conformation in which the C=0 group and the C —Cl bonds are oriented anti-coplanar. The preferred path for approach of the nucleophile could then be predicted on the basis of the sizes of the other substituents on the a carbon (Figure 9.57). 

The dehydrohalogenation of vinyl halides in the s5mthesis of alk3mes shows steric preferences analogous to those of alkyl halides. The reaction of (Z)-)3-bromostyrene with hydroxide ion in isopropyl alcohol at 43°C was 2.1 X 10 faster than the reaction of the ( ) isomer. The results were interpreted in terms of differing mechanisms for the eliminations of the two compounds. As shown in equation 10.28, the (Z) isomer can undergo concerted elimination of hydrogen and bromine because they are in the proper orientation for anti-coplanar elimination. 

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