Pyrolysis of amine oxides

Pyrolysis of Amine Oxides The Cope Elimination Reaction ... 

This type of reaction is termed an a -(3 elimination and it is isoelec-tronically similar to the pyrolysis of amine oxides 30>. However, the occurrence of an a -(3 elimination was subsequently disproven in this particular reaction because the trimethylamine which forms in the decomposition of s-2-phenyl-2-deuterocyclohexyltrimethylammonium hydroxide contains no deuterium T>. Therefore, a cis E-2 mechanism was proposed to account for the experimental observations 36>. 

A mild method for introducing double bonds is by pyrolysis of amine oxides (the Cope-Mamlock-Wolffenstein reaction92,93) it occurs at low temperatures (85-115°) and gives high yields and ds-elimination.94,95 When heated at 120° the amine oxide from optically pure Ar,Ar-dimethyl-(3-phenylbutyl)amine gives optically pure 3-phenyl-l-butene 96... 

Some cycloeliminations use a ring of five atoms instead of six but still involve six electrons. This is no longer a retro ene reaction but it is still a retro group transfer and is allowed in the all-suprafacial mode. Some common examples of such reactions are pyrolysis of amine oxides, sulphoxides, and selenoxides. All these reactions are syn stereospecific. 

The pyrolysis of amine oxides is called Cope elimination and typically takes place at 120 °C (Scheme 6.21). The reaction is a syn periplanar elimination in which six electrons move in a five-membered ring according to a concerted, thermally induced mechanism to yield an alkene and a hydroxylamine. 

Pyrolysis of the oxide of tertiary amine 1 yields olefin 2 and hydroxylamine 3. This olefination method is known as the Cope elimination reaction.1-3... 

The sulphurdi-imines are hydrolysed in acidic media to sulphur dioxide and two equivalents of amine. Oxidation with ozone afforded a nitrocompound and sulphinylamine. Pyrolysis of the diphenyl derivative (100 R = Ph) gave azobenzene, but pyrolysis of the di-t-butyl derivative (100 R = t-butyl) resulted in an intramolecular elimination to isobutylene, hydrogen sulphide, and ammonia. The bis-tosyl imine (100 R = toluene-p-sulphonyl) reacted with diamines (1,2----------1,6-) in an... 

In the pyrolysis of pure amine oxides, temperature has a significant effect on the ratio of products obtained (22). The principal reaction during thermal decomposition of /V,/V-dimetby11 amyl amine oxide [1643-20-5] at 80—100°C is deoxygenation to /V,/V-dimetby11 amyl amine [112-18-5] (lauryl = dodecyl). 

However, when the temperature is increased to 120°C, the principal reaction is the elimination to olefin. The thermal decomposition of dimethyl dodecyl amine oxide at 125°C in a sealed system, as opposed to a vacuum used by Cope and others, produces 2-methyl-5-decyhsoxa2ohdine, dimethyl dodecyl amine, and olefin (23). The amine oxide oxidi2es XW-diaLkylhydroxylainine to the nitrone during the pyrolysis and is reduced to a tertiary amine in the process. 

Ammonia is used in the fibers and plastic industry as the source of nitrogen for the production of caprolactam, the monomer for nylon 6. Oxidation of propylene with ammonia gives acrylonitrile (qv), used for the manufacture of acryHc fibers, resins, and elastomers. Hexamethylenetetramine (HMTA), produced from ammonia and formaldehyde, is used in the manufacture of phenoHc thermosetting resins (see Phenolic resins). Toluene 2,4-cHisocyanate (TDI), employed in the production of polyurethane foam, indirectly consumes ammonia because nitric acid is a raw material in the TDI manufacturing process (see Amines Isocyanates). Urea, which is produced from ammonia, is used in the manufacture of urea—formaldehyde synthetic resins (see Amino resins). Melamine is produced by polymerization of dicyanodiamine and high pressure, high temperature pyrolysis of urea, both in the presence of ammonia (see Cyanamides). 

Isoprene (2-methyl-1,3-butadiene) can be telomerized in diethylamine with / -butyUithium as the catalyst to a mixture of A/,N-diethylneryl- and geranylamines. Oxidation of the amines with hydrogen peroxide gives the amine oxides, which, by the Meisenheimer rearrangement and subsequent pyrolysis, produce linalool in an overall yield of about 70% (127—129). 

For the regioselectivity similar rules as for the ester pyrolysis do apply. With simple, alkylsubstituted amine oxides a statistical mixture of regioisomeric olefins is obtained. On the other hand with cycloalkyl amine oxides the regioselectivity is determined by the ability to pass through a planar, five-membered transition state. This has been demonstrated for the elimination reaction of menthyl dimethylamine oxide 10 and neomenthyl dimethylamine oxide 11 ... 

Amine oxide pyrolysis occurs at temperatures of 100°-150°C. The reaction can proceed at room temperature in DMSO.323 If more than one type of (3-hydrogen can attain the eclipsed conformation of the cyclic TS, a mixture of alkenes is formed. The product ratio parallels the relative stability of the competing TSs. Usually more of the /f-alkene is formed because of the larger steric interactions present in the TS leading to the Z-alkene, but the selectivity is generally not high. 

The primary products obtained from 2-butanol are of mechanistic. significance and may be compared with other eliminations in the sec-butyl system 87). The direction of elimination does not follow the Hofmann rule 88) nor is it governed by statistical factors. The latter would predict 60% 1-butene and 40% 2-butene. The greater amount of 2-alkene and especially the unusual predominance of the cis-olefin over the trans isomer rules out a concerted cis elimination, in which steric factors invariably hinder the formation of cis-olefin. For example, the following ratios oicisjtrans 2-butene are obtained on pyrolysis of 2-butyl compounds acetate, 0.53 89, 90) xanthate, 0.45 (S7) and amine oxide, 0.57 86) whereas dehydration of 2-butanol over the alkali-free alumina (P) gave a cisjtrans ratio of 4.3 (Fig. 3). 

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