E2 Elimination Reaction

Double dehydrohalogenation of gemmal dihalides (Section 9 7) An E2 elimination reaction of a gemmal dihalide yields an alkenyl halide If a strong enough base IS used sodium amide for example a second elimination step follows the first and the alkenyl halide IS converted to an alkyne... 

Is the electrophilic addition of hydrogen chloride to 2-methyl-propene the reverse of the El or the E2 elimination reaction of fe/t-butyl chloride ... 

O-isopropylidene derivative (57) must exist in pyridine solution in a conformation which favors anhydro-ring formation rather than elimination. Considerable degradation occurred when the 5-iodo derivative (63) was treated with silver fluoride in pyridine (36). The products, which were isolated in small yield, were identified as thymine and l-[2-(5-methylfuryl)]-thymine (65). This same compound (65) was formed in high yield when the 5 -mesylate 64 was treated with potassium tert-hx Xy -ate in dimethyl sulfoxide (16). The formation of 65 from 63 or 64 clearly involves the rearrangement of an intermediate 2, 4 -diene. In a different approach to the problem of introducing terminal unsaturation into pento-furanoid nucleosides, Robins and co-workers (32,37) have employed mild base catalyzed E2 elimination reactions. Thus, treatment of the 5 -tosylate (59) with potassium tert-butylate in tert-butyl alcohol afforded a high yield of the 4 -ene (60) (37). This reaction may proceed via the 2,5 ... 

Thomson KJW Click Organic Process to view an animation showing the mechanism of an E2 elimination reaction. 

Zaitsev s rule (Section 11.7) A rule stating that E2 elimination reactions normally yield the more highly substituted alkene as major product. 

Table 36 Secondary /3-tritium KIEs for the E2 elimination reactions of PhCHTCH2—X at 50°C.a...

Saunders and co-workers (Amin et al., 1990) used E2 elimination reactions in the p-substituted 2-phenylethyl system to test the new criteria for tunnelling suggested by the above calculations. The actual substrates and base/solvent systems they used were (2-phenylethyl-2-f)-trimethylammonium bromide, [19], with sodium ethoxide in ethanol, 2-phenylethyl-2-f bromide, [20], with potassium t-butoxide in t-butyl alcohol and 2-(p-chlorophenyl)ethyl-2-f tosylate, [21], with potassium t-butoxide in t-butyl alcohol. When equation (57) was applied to the experimental secondary (kB/ S) KIEs in Table 39, the calculated /th h KIEs were 1.106 0.033 and 1.092 0.026 for [19] and [21],... 

In a recent paper, Lin and Saunders (1994) reported experiments aimed at identifying the structural factors that promote tunnelling in E2 elimination reactions. In that investigation, the alkyl group of the doubly labelled 2-arylethyl substrates, [22]—[24], were varied while the leaving group was kept constant. 

Suggest an explanation based on orbital interactions for the observed stereochemistry for E2 elimination reactions, that is, the strong stereoelectronic preference that the C—H and C—X bonds be anti-coplanar. 

The enolate anions of thioacetaldehyde and thioacetone were generated in a flow tube by rapid (presumable) E2 elimination reactions of F with the appropriate sulphide (equations 36 and 37, respectively)181. 

J) In the most stable conformation of this compound, chlorine occupies an axial site, and so it is ideally situated to undergo an E2 elimination reaction by way of an anti arrangement in the transition state. 

At high temperatures, or in the presence of a strong base, secondary halides undergo E2 elimination reactions. 

Upon E2 elimination. (I R.2R)-1 -bromo-1,2-diphenylpropane produces only (Z)-1,2-diphenylpropene. This result demonstrates that the reaction occurs entirely from the anti-periplanar conformation shown. The enantiomer of this compound, (IS,2S)-l-bromo-l,2-diphenylpropane, also produces only the (Z)-alkene when it undergoes an E2 elimination reaction. (To simplify viewing, the phenyl groups are shown as single blue atoms in the ball-and-stick models.)... 

---