REVERSING THE STEREOCHEMISTRY OF A DIELS-ALDER REACTION - USE OF METALLOPORPHYRIN OLIGOMERS TO CONTROL TRANSITION-STATE STABILITY

A cyclic Zn-porphyrin trimer with ethyne and butadiyne links stabilise s the thermodynamically disfavoured endo transition state and product of a reversible Diels-Alder reaction; at 30 degrees C the endo adduct is formed rapidly and almost exclusively. Linear porphyrin dimers cont aining ethyne or butadiyne links show related stereochemical preferenc es to the corresponding cyclic trimers; substantial rate accelerations are observed despite rotational freedom around the linkers. A qualita tive correlation is observed between rate acceleration (i.e., transiti on state binding) and product binding, and the results are rationalise d in terms of host geometry and flexibility.