In contrast to diethyl fumarate (DEF), diethyl maleate (DEM) did not homopolymerize with 2,2′-azobisisobutyronitrile (AIBN), but was found to undergo radical polymerization in the presence of amines such as morpholine (Mor). From the results of radical polymerization of DEF with AIBN and isomerization of DEM with Mor, the radical polymerization of DEM in the presence of both AIBN and Mor is considered to proceed via a new monomer-isomerization radical polymerization mechanism, i.e., DEM is isomerized first to DEF which is then homopolymerized. The most efficient isomerization catalyst was Mor among the amines examined. The apparent activation energies for the monomer-isomerization radical polymerization of DEM was calculated to be 91.9 kJ mol−1 and that for this isomerization was also obtained as 24.7 kJ mol−1. Moreover, Mor also acted as a retarder of radical polymerization of DEF. The composition relation of the copolymer to the feed monomer mixture in the copolymerizations of DEM with styrene, isobutyl vinyl ether, and acrylonitrile in the presence of Mor was found to be similar to that of the respective copolymerizations of DEF.

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Polymers from 1,2-Disubstituted Ethylenic Monomers VI. Monomer-isomerization Radical Polymerization of Diethyl Maleate

Me2NCH:CHCH:CH2 is allowed to react with Me acrylate, acrylonitrile, maleo- and fumaronitrile, di-Me maleate and fumarate, and with di-Me dicyanomaleate and dicyanofumarate. In all cases except the last two mixts. (endo/exo) of cycloadducts are formed. Structures are detd. by a combination of one- and two-dimensional NMR spectroscopy. The E/Z isomers maleo- and fumaronitrile add stereospecifically to the diene. Cycloaddn. of di-Me maleate and fumarate leads to the same mixt. of isomers in almost the same ratio. The reaction of di-Me dicyanomaleate and di-Me dicyanofurmarate yields a single cycloadduct which seems to be the thermodynamically most stable one. The results are interpreted in such a manner that a concerted cycloaddn. takes place with maleo- and fumaronitrile, while stepwise reactions, presumably proceeding via zwitterions, are obsd. with di-Me maleate, fumarate, dicyanomaleate and dicyanofumarate. With increasing capability of the dienophile to stabilize a neg. charge the mechanism changes from a concerted to a stepwise reaction.

trans‐1‐(Dimethylamino)‐1,3‐butadiene – Concerted and Stepwise (4 + 2) Cycloadditions

Generation of NH-azomethine imine intermediates through the 1,2-hydrogen shift of hydrazones and their intermolecular cycloaddition reaction with olefinic dipolarophiles

Density functional theory (DFT) calculations at B3LYP/6-31 G (d,p) and B3LYP/6-311 + G(d,p) levels for the substituted pyridine-catalyzed isomerization of monomethyl maleate revealed that isomerization proceeds via four steps, with the rate-limiting step being proton transfer from the substituted pyridinium ion to the C=C double bond in INT1. In addition, it was found that the isomerization rate (maleate to fumarate) is solvent dependent. Polar solvents, such as water, tend to accelerate the isomerization rate, whereas apolar solvents, such as chloroform, act to slow down the reaction. A linear correlation was obtained between the isomerization activation energy and the dielectric constant of the solvent. Furthermore, linearity was achieved when the activation energy was plotted against the pKa value of the catalyst. Substituted-pyridine derivatives with high pKa values were able to catalyze isomerization more efficiently than those with low pKa values. The calculated relative rates for prodrugs 1–6 were: 1 (406.7), 2 (7.6 × 106), 3 (1.0), 4 (20.7), 5 (13.5) and 6 (2.2 × 103). This result indicates that isomerizations of prodrugs 1 and 3–5 are expected to be slow and that of prodrugs 2 and 6 are expected to be relatively fast. Hence, prodrugs 2 and 3–5 have the potential to be utilized as prodrugs for the slow release of monomethylfumarate in the treatment of psoriasis and multiple sclerosis.

Stanley Brown Graham

Papers

XXX.—The cis-trans ethenoid transformation