Parallel achiral reagent

Several papers have appeared where principles relevant to a two-reagent (PKR) scenario were mentioned with reference to the Brooks example of Scheme 6.36 [28d, 61). One of these reports demonstrated an advantage in efficiency when the slow-reacting enantiomer of a racemic pair in simple KR was consumed by a parallel reaction using an achiral reagent [28dj. The other publication discussed a mathematical model for the specific case where one of the competing parallel... 

Results of representative reactions of substituted allylboronates and achiral aldehydes are summarized in Table 1. It is noteworthy that in the majority of cases the reaction diastereoselectivity closely parallels the isomeric purity of the reagents, thus underscoring the requirement that the allylboronate syndesis be highly stereoselective. Dimethyl crotylboronates (68) and (69) are more reactive than (1) and (2), as indicated by the fact that the reactions of (68) and (69) are complete within a few hours at -78 °C while those of (1) and (2) include an overnight period at room temperature. Reagents (73)-(79) are even less reactive than (1) and (2), their reactions requiring several days at room temperature to reach completion. A detailed study of the temperature dependence of diastereoselectivity, however, has not been reported to date. 

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