Content area
Full Text
In these experiments, which were used in the problemsolving mode (1), the stereoselectivity of the Diels-Alder cycloaddition of N-phenylmaleimide to furan is deduced by the characteristic splitting patterns in the proton-NMR spectra. The relationship of coupling constants to dihedral angle, as described by the Karplus equation, is illustrated. The data can also be used to demonstrate the concept of kinetic versus thermodynamic control.
Two recent papers in this Journal reported laboratory experiments illustrating the stereochemistry of the DielsAlder reaction (2, 3). Pickering (2) described the reaction of maleic anhydride with cyclohexadiene, a-phellandrene, and furan. Cyclohexadiene and a-phellandrene gave the endo stereoisomer, whereas furan gave the exo stereoisomer. These experiments were used in the problem-solving mode and students were asked to decide, by comparing their data with that in the literature, which stereoisomer was formed. In another experiment, Harrison (3) used NMR to study the Diels-Alder reaction between norbornadiene and phencyclone (reaction 1). The stereochemistry of this reaction was established from the fact that the methylene protons lie in the shielding cone of the aromatic system.
The majority of general organic chemistry texts present the Diels-Alder reaction as yielding endo products. In most cases the exo product is the thermodynamically more stable, but the endo adduct forms much more rapidly, and kinetic control is observed (4). The exceptional exo stereochemistry of the furan-maleic anhydride adduct was first demonstrated by Woodward and Baer (5) using classical methods and later confirmed by X-ray crystallography (6). More recently Lee and Herndon ( 7) demonstrated that the endo isomer forms more rapidly in a reversible reaction, resulting in the ultimate dominance of the thermodynamic (exo) product.
In the norbornene (bicyclo[2.2.1]heptene) system, 1 (which results from Diels-Alder additions to cyclopentadiene), endo and exo stereochemistry can be deduced experimentally from differences in the coupling constants of the bridgehead protons on C-1 and C-4 (Hb) to the exo (Hx) or endo (Hn) protons on C-5 and C-6 (see below) (8).The geometry of the 7oxabicyclo[2.2.1]heptene system is very similar to that of the norbornenes; consequently, the corresponding coupling constants are very similar.
We felt that an ideal experiment would be one in which both endo and exo products are formed, with each product giving an NMR spectrum that students could use to assign stereochemistry....