Dimethyl isomerisation

Two commercial disazo disperse dyes of relatively simple structure were selected for a recent study of photolytic mechanisms [180]. Both dyes were found to undergo photoisomerism in dimethyl phthalate solution and in films cast from a mixture of dye and cellulose acetate. Light-induced isomerisation did not occur in polyester film dyed with the two products, however. The prolonged irradiation of Cl Disperse Yellow 23 (3.161 X = Y = H) either in solution or in the polymer matrix yielded azobenzene and various monosubstituted azobenzenes. Under similar conditions the important derivative Orange 29 (3.161 X = N02, Y = OCH3) was degraded to a mixture of p-nitroaniline and partially reduced disubstituted azobenzenes. 

This discussion is certainly an over-simplification. Unfortunately there are no detailed experimental results for this reaction under strictly homogeneous conditions, but even with heterogeneous catalysts (e.g., AlCl3 and Ni [13]) only mixtures of branched paraffins, naphthenes and polyenes of low molecular weight are obtained. If isomerisation is slower than propagation, as indicated, e.g., by the experiments of Meier [5] on the polymerisation of 3,3-dimethyl butene-1, this would modify in detail but would not invalidate the above general conclusions. 

Above we have mentioned several heterogeneous applications such as the OCT process and SHOP. Neohexene (3,3-dimethyl-1-butene), an important intermediate in the synthesis of fine chemicals, is produced from the dimer of isobutene, which consists of a mixture of 2,4,4-trimethyl-2-pentene and 2,4,4-trimethyl- 1-pentene. Cross-metathesis of the former with ethene yields the desired product. The catalyst is a mixture of W03/Si02 for metathesis and MgO for isomerisation at 370 °C and 30 bar. The isobutene is recycled to the isobutene dimerisation unit [48],... 

As shown in the synthetic sequence, to induce isomerisation of the c -B/C junction to a trans configuration, a double bond between C(6) and C(7), that can transmit the electronic effects of carbonyl group at C(5) must be created ex profeso. Yhis relatively simple structural modification requires no less than five different synthetic operations i) bromination at C(6) ii) substitution of the bromine atom by an OH group iii) oxidation iv) 0-methylation with dimethyl sulphate, and... 

Amino-1 -p-chlorophenyl-4-dimethylamino-1,2-dihydro-2,2-dimethyl-s-triazine (CXXXVII) was obtained from acetone and 1-p-chlorophenyl-5,5-dimethylbiguanide under acid conditions, and was isomerised by alkali to 6-p-chloroanilino-4-dimethylamino-l,2-dihydro-2,2-dimethyl-s-triazine (CXXXVIII), which also arose directly under basic conditions (95). Piperidine was found useful in promoting an analogous condensation (534). 

Studies have recently become available concerning the kinetics and mechanism of the isomerisation, in dilute aqueous solution, of dihydrotriazines of this type (e.g. 4,6-diamino-l-(3,5-dichlorophenyl)-l,2-dihydro-2,2-dimethyl-l,3,5-triazine), and of their degradation to substituted biguanides 683). 

The influence of o-, wt-, and p-methoxy substituent is cli-.uly evident from isomerisation of correspondingly substituted oxides Eq. 449). Thus, jB -dimethyl-o-methoxyatyrene oxido and its p-methoxy isomer give only a kotona, whereas tho m-mothuvy isomc- ... 

For instance in the synthesis of dimethyl arsinic acid (cacodylic acid) trivalent arsenic is isomerised to the pentavalent state... 

Cis-1-alkyl(or aryl)-2,5-dimethyl-3-phospholenes were noted to isomerise to the trans-isomers in CDCI3, the isomerization being a result of small quantities of COClj in the solvent. Addition of an excess of phosgene to the cis- isomer gave the quaternary salt depicted in the following scheme, which quickly equilibrated to another salt (which could also be prepared from the reaction of the trans- isomer with an excess of phosgene [877] ... 

Because of the ready availability of very pure geraniol (37) and nerol (40) of defined configuration. Stork et al. have used them to prepare olefins of specific geometry. The first stage depends on the fact that the acetates can be cleaved by ozone specifically at the trisubstituted double bond, leading to the aldehyde acetates which are isolated as their dimethyl acetals (38) and (41). The corresponding chloroacetals (39) and (42) were more difficult to obtain without isomerisation of the double bond, but when the alcohol in ether-hexamethylphosphoramide (2 1) was treated with commercial methyl-lithium in ether, followed by p-... 

Ergot Alkaloids.—The hemiterpenoid unit of the ergot alkaloids is interesting in that, as expected, only one carbon atom is labelled by [2- C]mevalonic acid [marked by in formulae (14)—(17)] but the stereochemistry of this atom changes. Extensive studies by Floss etalf indicate that chanoclavine I (14) is converted into elymoclavine (17) via the aldehyde (15) and possibly via agroclavine (16) (however, see ref. 36a). This process involves two isomerisations about the isolated double bond. In dimethyl allyl pyrophosphate (5) the trans-methyl is labelled by... 

The three thioamidium salts differ in reactivity because of steric, electronic and polar substituent-effects, the thioaminals isomerised normally only in the tm-butyl case, lost the S-substituent by H-reduction in the phenyl derivative and yielded dimerisation of N,N-dimethyl amino and dicyanomethyl residue. The reduction of the C-phenyl derivative is suppressed when S-phenyl replaced S-methyl. Now the dipolar isomerisation occurs again (Scheme 12). 

Based on the process developed by Schrader (1948c), its production began in 1949. It is 2-3 times more sensitive to hydrolysis than parathion. In a manner similar to parathion, it undergoes thion-thiol isomerisation at high temperatures. However, isomerisation results in different products. While 0,0-diethyl-S-p-nitrophenyl phosphorothioate (25) is formed from parathion, the isomerisation of parathion-methyl under identical conditions yields 0,S-dimethyl-0-(p-nitro-phenyl) phosphorothioate (26) (McPherson and Johnson, 1956). 

Its isomerisation proceeds in a way similar to that of parathion-methyl (24), resulting in the 0,S-dimethyl isomer (S-methyl fenitrothion, 29). 

Dimethyl phosphorothiochloridate can easily be converted with ammonia to 0,0-dimethyl phosphorothioamidate which, in the presence of a methylating agent undergoes thion-> thiol isomerisation, and is converted to the 0,S-dimethyl... 

In heterogeneous catalytic conversions in the vapor phase good results have been obtained on Mo03-Si02 [7]. But we were unable to reproduce these results. This paper describes an investigation of the activity of Mo03-Si02 catalysts in vapor phase isomerisation of dimethyl ethynyl carbinol (DMEC), scope and limitation of that process. 

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