Imines solvent effects

Keywords imines derived from formylphosphonate undergo Diels-Alder reactions only in those cases which carry a strongly electron-withdrawing N-substituent. Lewis acidity, solvent effect, lithium perchlorate in diethyl ether... 

Aryl and acyl isothiocyanates were also found to react with alkyl azides, but yield thiatriazoline-5-imines only as elusive intermediates which are transformed in situ into a series of bisadducts after loss of nitrogen. Interestingly, L abbe et al. have found that picryl isothiocyanate is an exception to this and reacts smoothly with alkyl azides to yield 4-alkyl-5-picrylimino-l,2,3,4-thiatriazolines (177) which are stable below 100°C <90JHC1059>. Kinetic experiments indicate that the reaction between picryl isothiocyanate and alkyl azides most likely is concerted as no significant solvent effect was observed. 

Irradiation of triazolines with UV light of k > 240 nm, the region of maximum absorption, constitutes one of the principal methods for the synthesis of various substituted aziridines.79 "3 The photolysis reaction is essentially independent of substituent and solvent effects in some cases aziridines are accompanied by imines.79... 

Computational studies [89] on the model reactions shown in Scheme 22 at the RHF/6-31G, MP2/6-31G, B3LYP/6-31G in vacuo and including solvent effects via SCRF methods showed that both processes take place via stepwise mechanisms similar to those found for the reaction between ketenes and imines. In both cases the nucleophilic attack of the iminic moiety was the rate-limiting step, the electrocyclic ring closure of intermediates (93, 96) being much faster. However, the first... 

All of the molecules examined show low barriers (of torsional type) for isomer interconversion according to theoretical results. These barriers are lower than in unsubstituted double bond derivatives and in imines. For 3c and 3d, the values for the rotational barrier are similar to open-chain derivatives. The calculated values, including solvent effects, are close to the experimental values. 

Similarly small rate factors were obtained for 1,3-dipolar cycloadditions between diphenyl diazomethane and dimethyl fumarate [131], 2,4,6-trimethylbenzenecarbonitrile oxide and tetracyanoethene or acrylonitrile [811], phenyl azide and enamines [133], diazomethane and aromatic anils [134], azomethine imines and dimethyl acetylenedi-carboxylate [134a], diazo dimethyl malonate and diethylaminopropyne [544] or N-(l-cyclohexenyl)pyrrolidine [545], and A-methyl-C-phenylnitrone and thioketones [812]. Huisgen has written comprehensive reviews on solvent polarity and rates of 1,3-dipolar cycloaddition reactions [541, 542]. The observed small solvent effects can be easily explained by the fact that the concerted, but non-synchronous, bond formation in the activated complex may lead to the destruction or creation of partial charges, connected... 

Jain, D. C., Sapse, A. M., and Cowburn, D., Solvent effect on some imine-carboxyl complexes, J. Phys. Chem. 92, 6847-6849 (1988). 

THEOCHEM), in press. Electrostatic Solvent Effect on the Ketene—Imine Cycloaddition Reaction. 

TABLE 7. CHIRAL SOLVENT EFFECTS ON IMINE OXIDATION... 

Lopez, R., Suarez, D., Ruiz-Lopez, M. F., Gonzalez, J., Sordo, J. A., Sordo, T. L. Solvent effects on the stereoselectivity of ketene-imine cycloaddition reactions. J. Chem. Soc., Chem. Common. 1995,1677-1678. 

Akiyama et al. have reported that the cycloaddition of 102 to imines is effectively accelerated by 10 mol% of a Bronsted acid such as HBF4 in aqueous media, affording dihydro-4-pyridones in good to high yield (Scheme 10.109) [295]. This catalytic system is applicable to three-component synfhesis of dihydro-4-pyridones from aldehydes, anilines, and 102. The three-component coupling can be achieved efficiently in water without any organic solvent by using SDS as surfactant. 

The answer to this dilemma was provided by Huisgen et al. (1962 a, 1962 b) by the cycloaddition reaction of C-methyl-A-phenylsydnone (6.10) with styrene, which yields, via the bicyclic intermediate 6.11 and elimination of CO2, 4,5-dihydro-5-methyl-l,3-diphenyl-li/-pyrazole (6.12). As mentioned briefly in Section 6.2 (Table 6-1, footnote a), sydnones are cyclic azomethine imines. As Huisgen (1968) demonstrated later, sydnone and azomethine imine cycloadditions are kinetically very similar with respect to solvent effects and in the sequence of reactivity with a series of 11 dipolarophiles. 

Mechanistic investigations can also provide insight into unexpected side reactions. For example, in Ir(III)-catalyzed indole synthesis, Messerle and coworkers noted an unexpected side product, N-vinyhndole, when 2-(2-phenylethynyl)anihne was used as substrate with acetone as the solvent By probing solvent effects, deuterium-labeling experiments, and isolation of dormant species formed in stoichiometric experiments, two possible reaction pathways were considered. In collaboration with Eisenstein [303], these pathways were compared computationally, yielding a proposed reaction mechanism that proceeds via the initial formation of imine between the aniline and acetone solvent, followed by imine nucleophilic attack of coordinated alkyne for indole formation. 

The kinetics of the aza-Morita-Baylis-Hillman reaction have been studied for a range of imine substrates in various solvents, using triphenylphosphine as catalyst, and p-nitrophenol as a Brpnsted acid co-catalyst. The effects of varying the phosphineiphenol catalyst ratio on the rate indicate interdependence between them. This and the solvent effects support reversible protonation of zwitterionic intermediates within the mechanism. ip-NMR and quantum calculations also support such a route. 

Substituent and solvent effects on the Kabachnik-Fields reaction of the dialkyl hydrogenphosphite-cyclohexylamine-benzaldehyde system have been reported kinetic analysis shows evidence of a hydroxyphosphonate intermediate. Kinetics are also reported for several substituted benzaldehydes, probing for evidence of an alternative imine intermediate, and for salicylaldehyde, which does proceed via an imine (as the hydroxyphosphonate is unstable). When the dialkyl hydrogenphosphonate reactant is changed to a phosphonate, the rate and mechanism of the reaction are significantly altered. ... 

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