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Abstract
The purpose of this project has been to study the bromination reaction of a very old, classical cis-trans pair of isomeric acids: maleic and fumaric acid, maleic and fumaric methyl esters, and maleic and fumaric anions. The study was carried out in a variety of solvents whose dielectric constants range from 2.2 to 78.5.
CO(,2)H CO(,2)H CO(,2)Me
H-C H-C H-C
H-C C-H H-C
CO(,2)H CO(,2)H CO(,2)Me
-+ -+
CO(,2)Me CO(,2)NR(,4) CO(,2)NR(,4)
H-C H-C H-C
C-H H-C C-H
CO(,2)Me CO(,2)H CO(,2)H
Purification procedures for the purchased solvents and reagents are outlined in the experimental section. The cost of dimethylfumarate was prohibitive when compared to the cost of the starting materials; therefore three methods of preparation of dimethylfumarate are compared. Five new quaternary ammonium salts of maleic and fumaric acid were prepared and their physical properties listed. One salt (ditetraalkylammonium maleate) could not be prepared by the various methods attempted.
The bromination reactions were all carried out in red (low actinic) flasks that were wrapped with aluminum foil in a darkened hood at room temperature. They were all consistently stirred for seven days before work up. The dibromides (d,l and meso) were analyzed by proton NMR and verified with authentic samples. There were mechanistic and isomerization checks made by brominating maleic anhydride and running reactions with only the salt or acid and without bromine present.
Bromine added with the highest percent (91-100) stereospecific
trans-addition in non-polar solvents to dimethyl maleate, dimethyl-
fumarate, tetraethylammonium hydrogen maleate, and tetraethyl-
ammonium hydrogen fumarate. When the above compounds were
reacted with two equivalents of bromine and one equivalent of
tetraethylammonium bromide the stereospecific trans-addition
dropped to 69-100 percent. With maleic acid the reverse was true;
the addition of bromide salt raised the stereospecific trans-addition
from 95-100 precent to all 100 percent. In water the stereospecific
trans-addition was lowered to 36-83 percent. Also as the cation
size of the quaternary ammonium salt increased the percentage
of stereospecific trans-addition decreased in both polar and
non-polar solvents. These results are explained using a
cyclobromonium ion, lopsided cyclo bromonium ion, and the possibility of rotation of the lopsided cyclobromonium ion to explain "cis" addition products. The cis-isomer was always the most reactive compared to the trans-isomer under the some conditions.
