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 stabilises the thermodynamically disfavoured endo transition state and product of a reversible Diels–Alder reaction; at 30°C the endo adduct is formed rapidly and almost exclusively. Linear porphyrin dimers containing ethyne or butadiyne links show related stereochemical preferences to the corresponding cyclic trimers; substantial rate accelerations are observed despite rotational freedom around the linkers. A qualitative correlation is observed between rate acceleration (i.e., transition state binding) and product binding, and the results are rationalised in terms of host geometry and flexibility.