Structural Contributions to Autocatalysis and Asymmetric Amplification in the Soai Reaction

Diisopropylzinc alkylation of pyrimidine aldehydes – the Soai reaction, with its astonishing attribute of amplifying asymmetric autocatalysis, occupies a unique position in organic chemistry and stands as an eminent challenge for mechanistic elucidation. A new paradigm of ‘mixed catalyst substrate’ experiments with pyrimidine and pyridine systems allows a disconnection of catalysis from autocatalysis, providing insights into the role played by reactant and alkoxide structure. The alkynyl substituent favorably tunes catalyst solubility, aggregation and conformation while modulating substrate reactivity and selectivity. The alkyl groups and the heteroaromatic core play further complementary roles in catalyst aggregation and substrate binding. In the study of these structure activity relationships, novel pyridine substrates demonstrating amplifying autocatalysis were identified. Comparison of three autocatalytic systems representing a continuum of nitrogen Lewis basicity strength suggests how the strength of N-Zn binding events is a predominant contributor towards the rate of autocatalytic progression.