Isopropylzinc alkoxides

The mechanism of the asymmetric autocatalysis with amplification of has been examined experimentally by us171 and other groups172. It is basically understood that the aggregation of the isopropylzinc alkoxide of 5-pyrimidyl alkanol is involved in the reaction. Kinetic analysis of the reaction shows that the reaction is second order in the isopropylzinc alkoxide of 5-pyrimidyl alkanol171. 

Chiral isopropylzinc alkoxide 50 of pyrimidyl alkanol formed in situ from the chiral pyrimidyl alkanol and i-Pr2Zn is considered to be an initial actual asymmetric autocatalyst, which automultiplies itself by catalyzing the addition of i-Pr2Zn to aldehyde 48 (Scheme 9.25). 

Chiral pyrimidyl alkanol reacts with i-Pr2Zn to form chiral isopropylzinc alkoxide 57, which serves as the true asymmetric autocatalyst to multiply itself with the same configuration in the addition reaction of i-Pr2Zn to pyrimidine-5-carbaldehyde 55 (Scheme 9.29). 

The first experimental invention of spontaneous asymmetric synthesis was achieved only a little more than a decade ago in an organic reaction system by Soai and coworkers [9-15]. The Soai reaction (Scheme 1) comprises the addition of diisopropylzinc to prochiral pyrimidine carbaldehydes yielding isopropylzinc alkoxides that, after hydrolysis, are usually converted into stable chiral pyrimidyl alkanols. 

We have given kinetic insight into a number of experimental features of the Soai reaction. It was shown that chiral amplification and mirror-symmetry breaking are driven by a reaction network that contains enantioselective autocatalysis and mutual inhibition as the essential ingredients. In this sense, the Soai reaction moves the early concepts of Frank forward into experimental reality. Taking into account the formation of isopropylzinc alkoxide dimers, an evaluation of the parameter space in which amplification and symmetrybreaking are observed indicates that the heterochiral dimers display a higher thermodynamic stability and have to be formed faster than the homochiral ones. The necessity of such sensitive interplay may explain why such reactions systems are so scarce. 

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