Other 1,2-Elimination Reactions

A number of 1,3-elimination processes other than those described above have been observed. Treatment of diketone 282 with hydroxide initially formed 283, but the ring opened to give the enolate anion (284). Proton transfer gave 285 and hydrolysis led to keto-acid 286. 34 xhis particular fragmentation relied on an equilibrium that favored the resonance stabilized enolate anion, and a proton transfer to give the keto-carboxylate. 

Elimination of carbon dioxide (CO2) has been observed with p-halo carboxylates. Fragmentation of the carboxylate anion of 3-halo acrylic acids (see 288) can lead to alkenes, as in the conversion of cinnamic acid 

The formic ° and acetic acid decompositions are probably not unimolecular, intramolecular eliminations as reported. An. 4-factor lower than 10 sec (Table 38, HCOOH- H2 + CO2) is quite impossible. In addition, decomposition of formic acid dimer, (HCOOH)2 - 2H2O + CO, is equally suspect. The authors 

Chain decomposition of formaldehyde would be fast, and give carbon monoxide and hydrogen as products. Although absence of any appreciable inhibition by isobutene and propene was reported, this does not exclude free radical chains since formic acid and formaldehyde are both excellent free radical scavengers, (i.e., i)(H-COOH) = 88 kcal.mole Z)(0=CH-H) = 85 kcal.mole ). Chains can also be propagated readily by allylic radicals at the reaction temperatures (709-805 °K). 

All the radicals in the system are good chain carriers, and propagation processes should be fast. It should also be noted that extensive wall conditioning was required before the rate data were reproducible. This is a characteristic of long chain processes. 

Oxalic acid decomposes to formic acid and carbon monoxide, viz. 

This is not energetically reasonable since the enthalpy change for such a process is probably 65 kcal.mole . Lapidus et subsequently studied deuterium and isotope effects in the oxalic acid decomposition. The deuterium effect (kiilko) was found to vary from 1.3 (400 °K) to 0.87 (435 °K), a reverse isotope effect. A similar reversal was found in the study, viz. 

In the cases where the fragments A and B form a stable, strongly bonded compound A-B of reasonable volatility, the reaction of E-A and M-B species can lead to elimination of A-B and the formation of an E-M bond. A notable example is the reaction of E-C bonds with metal hydrides, which can lead to the thermodynamically favored elimination of R-H while 

CuCl + PHPh2 + PhP(SiMe3)2 + Ph2PSiMe3 Cu]8(PHPh2)3(PPh2)]o(PPh)4 

CuCl + PhP(SiMe3)2 + PMe3 = Cui4(PMe3)8Cl2(PPh) 5 (35) 

CuCl+ P(SiMe3)3 + PEt3 = Cu% P(SiMe2)6 (PEt3)3P3o (36) 

NiCl2(PPh3)2 + PhAs(SiMe3)2 = Ni9(PPh3)sCl3(As)6 + Ni9(PPh3)eCl2(As) 5 (41) 

FIGURE 10.41 Chromium(VI) oxidation of axial and equatorial alcohols. 

Your first thought on looking at these problems might be EEK I have not seen anything like this before. However, as we have said earlier, once you understand the mechanistic principles, you CAN tackle and understand new types of problem. In both cases, once you work out which atoms/groups have been eliminated, the arrow pushing is relatively easy. 

FIGURE 10.42 Oxidation of primary and secondary alcohols using chromium(VI). 

Alkynes can be formed by elimination reactions, which may be E2 or ElcB in character. Very strong bases are generally required. 

Benzyne may be formed by elimination of HBr from bromobenzene using sodamide as a base. This contains a very weak triple bond, and the molecule is highly reactive, especially toward addition processes. 

---

J Olefin Syntheses by Dehydrogenation and Other Elimination Reactions 137... 

Over most catalysts, with the notable exception of thoria [130], the thermodynamically more stable olefins are formed (Saytzeff rule) as primary products when two elimination directions are possible. This is in agreement with the results concerning other elimination reactions, both in the liquid phase (cf. refs. 64 and 65) and over solid catalysts. The striking difference in the action of thoria has been explained on the basis of a different mechanism [68],... 

Much less information is available on the deamination and related reactions over solid catalysts than on some other elimination reactions but it suffices for comprehension of the general features. 

Effects of structure on reactivity have been studied several times. The sulphides are more stable than the thiols [248,250], In both series of thiols and of sulphides, the reactivity increases with the inductive effect of the alkyl group [248,251,252], in accordance with other elimination reactions. A linear relation between the logarithm of the rate coefficient and the enthalpy change on carbonium ion formation from the corresponding alkanes has been observed [248]. As Fig. 9 shows, linear correlation of the same rate data by means of the Taft equation is also possible. 

By analogy with other elimination reactions and on the basis of observed structure effects on the rate, the El and E2-like mechanisms may also be accepted for the dehydrosulphidation. Sugioka and Aomura [248, 249,253] have proposed mechanisms which correspond to the above designations. Their results on ethanethiol decomposition over a series of... 

By elimination reactions of selenides or selenoxides Diphenyl diselenide, 125 Hydrogen peroxide, 145 Other elimination reactions Chloromethyldiphenylsilane, 74 m-Chloroperbenzoic acid, 76 Hydrogen peroxide, 145 Iodomethyltrimethylsilane, 315 Lithium 4,4 -di-/-butylbiphenylide, 162... 

This chapter is concerned with a group of thermally induced elimination reactions widely used for the introduction of carbon-carbon double Irands into complex molecules. These reactions form a discrete group of elimination reactions in that they proceed with syn stereochemistry via concerted cyclic transition states. Related syn elimination processes are believed to be involved in other elimination reactions, e.g. alcohol dehydration using the Burgess reagent, but are not discussed here. One of the advantages of the syn elimination reactions discussed in this chapter is that they do not require the use of... 

Other elimination reactions reported include the formation of the l(10),5-diene from a 5,6-dibromo-19-nor-steroid, and also from a 6j -methoxyoestr-5(10)-... 

These reaction products have also been observed in laboratory experiments with sohd acid catalysts [37-44], However, since a coherent mass balance has not been provided, other reaction products as well as other elimination reactions (e.g. adsorption) may well be possible. Recently, other possible reaction products such as isobutene have been identified [38],... 

Other elimination reactions have been reported for various drug substances, as shown in Scheme 43. Trimelamol eliminates its hydroxymethyl groups and forms formaldehyde.171 Levothyroxine eliminates iodine. 172 ADD-17014, a derivative of triazoline, eliminates nitrogen and forms a derivative of aziridine.173 Ditiocarb eliminates carbon disulfide.174... 

The intramolecular elimination just illustrated is under kinetic control, rather than thermodynamic control, and the transition state leading to removal of a P-proton from the less substituted P-carbon is lower in energy, so reaction occurs faster. This is the fiindamental difference between E2 reactions and intramolecular elimination reactions. The transformation of amine 42 to alkene 43 is called Hofmann elimination, named after August Hofmann (Germany 1818-1892). Other elimination reactions proceed by an intramolecular elimination mechanism, but Hofmann ehmination is the only one that will be discussed. 

The Hofmann elimination has also been used in the synthesis of alkenes not readily available by other elimination reactions. A recent example is the synthesis of trimethylenecyclopropane (equation 6)... 

The mechanism of the Hofmann elimination has received considerable attention in the last two decades and a number of reviews on the mechanism of this and other elimination reactions have appeared . Two facets of the reaction have been studied in particular, the degree of concertedness of the reaction and the orientation of the elimination. 

---