Bases, used in elimination reactions

The most common bases used in elimination reactions are negatively charged oxygen compounds such as OH and its alkyl derivatives, OR, called alkoxides, listed in Table 8.1. Potassium tert-butoxide, OC(CH3)3, a bulky nonnucleophilic base, is especially useful (Section 7.8B). 

The majority of preparative methods which have been used for obtaining cyclopropane derivatives involve carbene addition to an olefmic bond, if acetylenes are used in the reaction, cyclopropenes are obtained. Heteroatom-substituted or vinyl cydopropanes come from alkenyl bromides or enol acetates (A. de Meijere, 1979 E. J. Corey, 1975 B E. Wenkert, 1970 A). The carbenes needed for cyclopropane syntheses can be obtained in situ by a-elimination of hydrogen halides with strong bases (R. Kdstcr, 1971 E.J. Corey, 1975 B), by copper catalyzed decomposition of diazo compounds (E. Wenkert, 1970 A S.D. Burke, 1979 N.J. Turro, 1966), or by reductive elimination of iodine from gem-diiodides (J. Nishimura, 1969 D. Wen-disch, 1971 J.M. Denis, 1972 H.E. Simmons, 1973 C. Girard, 1974),... 

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],... 

L is the hydrogen or deuterium atom that is not transferred in the elimination reaction and T is tritium that is present in tracer quantities. These substrates were chosen so that the reactions would have transition states ranging from very ElcB-like for [22], to central or intermediate for [23], to El-like for [24]. For practical reasons, the base/solvent system could not be kept constant as was originally intended. EtO /EtOH was used in the reaction with substrates [22] and [23] whereas Bu,0"/But0H was used with substrate [24]. Although the secondary tritium KIE (when L = H) for the reaction of [22] was... 

The reaction proceeds well with unhindered secondary amines as both nucleophiles and bases. The yield of allylic amine formed depends upon how easily palladium hydride elimination occurs from the intermediate. In cases such as the phenylation of 2,4-pentadienoic acid, elimination is very facile and no allylic amines are formed with secondary amine nucleophiles, while phenylation of isoprene in the presence of piperidine gives 29% phenylated diene and 69% phenylated allylic amine (equation 30).84 Arylation occurs at the least-substituted and least-hindered terminal diene carbon and the amine attacks the least-hindered terminal ir-allyl carbon. If one of the terminal ir-allyl carbons is substituted with two methyl groups, however, then amine substitution takes place at this carbon. The reasons for this unexpected result are not clear but perhaps the intermediate reacts in a a- rather than a ir-form and the tertiary center is more accessible to the nucleophile. Primary amines have been used in this reaction also, but yields are only low to moderate.85 A cyclic version occurs with o-iodoaniline and isoprene.85... 

It is noteworthy that the use of DMSO in this manner contrasts with its established usage in elimination reactions. DMSO has normally been used because of its ability to promote eliminations by weakly basic anions (Parker, 1971), its effectiveness in enhancing the basicity of alkoxides (Cram et al., 1961), and also because it allows variation of base without the usual variation of solvent (Dolman and Stewart, 1967). Full details of such studies and their importance in the development of an understanding of elimination mechanisms are given in a recent text (Saunders and Cockerill, 1973).8... 

Two problems had to be solved for these reactions to be made usefiil. First, reductive elimination to form C N and bonds was not a well-known reaction with classical ligands such as PPh3. Second, jS-hydride elimination is very facile for primary and secondary heteroatom substrates. As with other cross-coupling reactions, the use of hindered, basic phosphines turned out to be cmcial. Amination reactions tend to give better yields, since reductive elimination is faster for more basic groups. For example, the base used in catalytic aminations is Na-O-t-Bu, but the product is the aryl amine. 

Alkali metal oiganometallics, when used as bases in elimination reactions, often afford many by-products however, with triphenylmethylpotassium good results were obtained. On the other hand, some weak bases, such as alkali halides, sometimes in the presence of the corresponding carbonates, in DMF or HMPA have been used successfully for eliminations. In some cases, dehydrohalogenation has... 

Figure S. Free energy relationship (activation barrier versus ApKa) comprising experimental data on acetone deprotonation by various bases ([19], squares) and on base catalyzed phosphate elimination reactions [10] of DHAP (circles) and LGAP (triangles), using the estimated pKa values of DHAP and LGAP. The latter pA, was related to the former by the observed equilibrium constant of 1/22 between LGAP and DHAP in solution [10]. All values have been corrected for the number of equivalent protons.

TTF can also be prepared by the elimination of a proton and of triphenylphosphine in the final step. Thus, 1,3-dithiolylium salts (402) react with 2-triphenylphosphino-l,3-dithioles to afford intermediates which give TTF (403) upon treatment with base at low temperatures (Scheme 76). This method also permits the selective preparation of unsymmetrical TTF <78JOC369,82TLl8l3,83Mi 312-02). The phosphorus ylides used in these reactions are easily accessible by the reaction at — 78°C between -butyllithium and the corresponding phosphonium salt which is obtained by the reaction of triphenylphosphane with 1,3-dithiolylium salts <78JOC369>. 

A(, -Dimethyl(methylene)iminium salts have been the most widely used class of preformed iminium salts, mainly due to their applications in the synthesis of a,p-unsaturated carbonyl compounds, normally accomplished by subjecting the /V, -dimethyl Mannich base to quatemarization followed by base-induced elimination. Table 3 outlines various counterion forms of /V, -dimethyl(methylene)iminium salts that have been used in Mannich reactions as well as their synthetic precursors. The crystalline iodide (30), known also as Eschenmoser s salt , has seen the most widespread use and is prepared by thermal fragmentation of (iodomethyl)trimethylammonium iodide or, more conveniently, by a variant of the... 

Several other handles and resins allow cleavage by base-catalyzed (B-elimination reactions [39-42]. The 2-(2-nitrophenyl)e liyl handle 10 has been used for the synthesis of protected peptides and nucleopeptides [43,44]. Cleavage can be accomplished in high yield on treatment with 0.1 m 1,8-diazabicyclo[5.4.0] undec-7-ene (DBU) in dioxane, or 20% piperidine in DMF, for 2 h at room temperature. 

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