Elimination reactions leaving group effect

On page 284, the differing order of leaving-group effects for elimination-addition reactions promoted by organolithium compounds as opposed to those promoted by NaNH2 is described. Explain in more detail how the nature of the... 

TABLE 5.6. Leaving Group Effects on Orientation in Elimination Reactions ... 

Brown, H. C. Wheeler, O. H. Steric Effects in Elimination Reactions. IX. The Effect of the Steric Requirements of the Leaving Group on the Direction of Bimolecular Elimination in 2-Pentyl Derivatives J. Am. Chem. Soc. 1956, 78, 2199-2202. Also see Bartsch, R. A. Bunnett, J. F. Orientation of Olefin-Forming Elimination in Reactions of 2-Substituted Hexanes with Potassium frrf-Butoxide-fiprf-Butyl Alcohol and Sodium Methoxide-Methanol /. Aw. Chem. Soc. 1969, 91, 1376-1382. Provide the products expected from the following olefin-forming reactions. (CJH-7)... 

For E2 eliminations in 2-phenylethyl systems with several different leaving groups, both the primary isotope effect and Hammett p values for the reactions are known. Deduce from these data the relationship between the location on the E2 transition state spectrum and the nature of the leaving group i.e., deduce which system has the most El-like transition state and which has the most Elcb-like. Explain your reasoning. 

The scope of heteroaryne or elimination-addition type of substitution in aromatic azines seems likely to be limited by its requirement for a relatively unactivated leaving group, for an adjacent ionizable substituent or hydrogen atom, and for a very strong base. However, reaction via the heteroaryne mechanism may occur more frequently than is presently appreciated. For example, it has been recently shown that in the reaction of 4-chloropyridine with lithium piperidide, at least a small amount of aryne substitution accompanies direct displacement. The ratio of 4- to 3-substitution was 996 4 and, therefore, there was 0.8% or more pyridyne participation. Heteroarynes are undoubtedly subject to orientation and steric effects which frequently lead to the overwhelming predominance of... 

The net effect of the addition/elimination sequence is a substitution of the nucleophile for the -Y group originally bonded to the acyl carbon. Thus, the overall reaction is superficially similar to the kind of nucleophilic substitution that occurs during an Sn2 reaction (Section 11.3), but the mechanisms of the two reactions are completely different. An SN2 reaction occurs in a single step by backside displacement of the Leaving group a nucleophilic acyl substitution takes place in two steps and involves a tetrahedral intermediate. 

An approach to isobacteriochlorins1 ln-e makes use of Pd(II) or metal-free bilatrienes 1 as starting materials. Cyclization of the corresponding bilatriene derivatives is induced by base in the presence of palladium(II) or zinc(II) which exercise a template effect. Zinc can be readily removed from the cyclized macrotetracycles by acid whereas palladium forms very stable complexes which cannot be demetalated. Prior to the cyclization reaction, an enamine is formed by elimination of hydrogen cyanide from the 1-position. The nucleophilic enamine then attacks the electrophilic 19-position with loss of the leaving group present at the terminal pyrrole ring. 

Effect on Reactivity. The leaving groups in elimination reactions are similar to those in nucleophilic substitution. The E2 eliminations have been performed with the following groups NRj, PR, SRj, OHR", SO2R, OSO2R,... 

The most comprehensive examination of the Rokita kinetic procedure from a synthetic standpoint was carried out by Barrero and coworkers.6 They examined the effects of various leaving groups, solvents, nucleophiles, and their equivalents on subsequent [4 + 2] cycloadditions. Avast excess of the intended nucleophile (50-100 equiv) must be employed, because the fluoride triggered (3-elimination proves nearly instantaneous at room temperature resulting in a high concentration of a species that is prone to undergo dimerization and other undesired side reactions that are irreversible at these low temperatures (Fig. 4.7). Use of fewer equivalents of the intended nucleophile led to a rapid drop off in yield. For example, 5-10 equivalents of ethoxy vinyl ether (EVE) affords only a 5-10% yield of the desired benzopyran... 

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