Dimethyl isomer, isomerization solvent

Figure 4. Isomerization of solvent to dimethyl isomer (-------), model predicted...

N-Substituted 3-aminocrotonic esters (67) tend to adopt a planar or near-planar structure. As a consequence of their push-pull nature these molecules show an increased facility for Z-67-is-67 isomerization around the C=C double bond as well as restricted rotation around the C—N and C—COOR2 single bonds. The IR and XH-NMR spectra of simple 3-(alkylamino)crotonic esters have shown that these substances exist either in the liquid state or in solution as equilibrium mixtures of the Z and E configurations, respectively. The position of the equilibrium is solvent-dependent, and the energy difference between the isomers varies from ca 7.3 kJ mol-1 in non-polar solvents to ca 0.8 kJ mol"1 in dimethyl sulphoxide, the intramolecularly bonded Z-form 68 or 69 being the most stable111-113. 

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] ... 

The product of the Michael addition of 5-aminouracil to dimethyl acetylenedicarboxylate in methanol is the fumarate resulting from the attack of the amino substituent whereas, in aprotic solvents such as dimethyl sulfoxide, the initial adduct is the maleate, which isomerizes within 1 hour to give predominantly the thermodynamically more stable Z-isomer. Upon heating a solution of the fumarate 1 in Dowtherm A at reflux temperature, cyclization occurs to give methyl 2,4,8-trioxo-l,2,3,4,5,8-hexahydropyrido[3,2-r/]pyrimidine-6-carboxylate... 

The relationship of the nitrous acid and nitrosoamide methods of deamination is more tenuous in water since only one example of the i nitrosoamide decomposition in aqueous media has been reported. The decomposition of A -nitroso-A -n-propylbenzamide in dimethyl-formamide-water (60/40 by vol) yielded a propyl benzoate fraction containing 9% of the isopropyl isomer and a propanol fraction con- taining 33% of isopropyl alcohoP . The nitrous acid deamination of n-propylamine under the same conditions yielded an alcohol fraction containing 31% of isopropyl alcohoP. The similarity in the isomer contents of the alcohol portions suggests that common intermediates were involved. The less extensive isomerization of the benzoates may be a reflection of the shorter lifetime of the propyl cations trapped by negatively charged coimter ions compared to those trapped by neutral solvent molecules on the other hand, the role of diazopropane must be determined before definitive conclusions are possible. 

Irradiations of alkenes are generally conducted with a low-pressure mercury lamp (185 nm), an ArF excimer laser (193 nm), or a zinc resonance lamp (214 nm). Highly substituted alkenes can also be irradiated with a medium-pressure lamp and quartz optics (>200 nm). Because of the presence of two or more close-lying singlet excited states, irradiation of alkenes usually results in several competing photoprocesses. Thus, for example, irradiation of 2,3-dimethyl-2-butene (1) in hydrocarbon or ether solvent results in rearrangement to the carbene-derived products 4 and 5 and the double bond isomer 9. E,Z-Isomerization also occurs but is not apparent in this case because of the symmetry of alkene 1. Similar behavior is exhibited in the gas phase. In alcoholic or aqueous media, the nucleophilic trapping products 8 are formed in competition with the carbene-derived products 4 and 5. ... 

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