Against the flow? What factors dictate the relative merits of microflow reactors versus batch‐reaction flasks for homogeneous catalytic reactions? The optimal reaction protocol must be decided on a case‐by‐case basis. Flask reactors equipped with in situ detection devices provide a concise and information‐rich means of obtaining the intrinsic kinetic information required to make this decision.

Observations of an intriguing inverse temperature dependence on reaction rate and a profound induction period in the Soai autocatalytic reaction are reported along with detailed kinetic and NMR investigations of the product alkoxide at low temperatures, leading to the suggestion that the active catalyst is derived in situ from the tetrameric ground state.

The amplifying asymmetric autocatalysis of alkylzinc addition to aldehydes discovered by Soai and co-workers in 1995 remains the only effective example of this genre, [1,2] despite widespread continuing interest in asymmetric amplification, primarily in the context of studies probing the origin of biological homochirality.[3] The autocatalytic process is highly circumscribed, being effective only within a narrowly defined regime: only 2-substituted pyrimidin-5-als such as 1 function as reactant [4] and only di-isopropylzinc 2 functions as reagent. The active catalyst is thought to be a higher-order alkoxide species derived from alkoxide 3 and is normally generated in situ from the alcohol 4. Significant advances in the period since the initial discovery include absolute asymmetric synthesis, [5,6] amplification from extremely low to high levels of enantiomer excess, [7] and initiation by diverse chiral entities or by isotopically chiral promoters. [8,9] Kinetic and spectroscopic studies have provided much information about the nature of the autocatalytic process, and kinetic and computational modeling have contributed to our understanding. To date, however, neither experimental observations of intermediate species, nor evidence of the nature of the catalytic entity, have been reported. Hence the molecular mechanisms by which amplifying autocatalysis occur are not yet properly understood.We report the direct observation of active species during catalytic turnover in the Soai autocatalytic Zn alkylation by 1 H NMR spectroscopy. This work confirms the surprising dependence on reaction temperature reported previously and reveals the first temporal observation of an intermediate species present during the reaction, which is transient at 0 8C but stable below À20 8C. The species is observed from the onset of the accelerating phase of autocatalytic turnover. Our earlier kinetic work indicated an equilibrating reaction product, formed without selectivity between homo-and heterochiral dimers, for which only the homochiral form was catalytically active.[10] Further studies demonstrated that the reaction was first-order in catalyst, close to second-order in aldehyde, but zero-order in the zinc alkyl reagent.