Regiospecific, Diastereo- and Enantioselective Allylboration with Methylenecyclobutylboranes: Experimental and...

New methylenecyclobutane-derived allylboranes, diisopropyl(2-methylenecyclobutyl)borane and chiral (+)-diisopinocampheyl(2-methylenecyclobutyl)borane undergo rapid equilibration via facile [1,3]-B sigmatropic shifts that interconvert the endocyclic and exocyclic isomers. Both endocyclic isomers react with the Soai aldehyde, 2-(3,3-dimethylbut-1-yn-1-yl)pyrimidine-5-carbaldehyde, affording a single diastereomer of the corresponding homoallylic alcohol. With the achiral reagent the reaction is regio- and diastereoselective, yielding exclusively one diastereomer of the racemic alcohol in 83% yield. With the chiral reagent, the reaction is enantioselective, providing optically active (RR)-alcohol in 87% yield and 89% ee, demonstrating the synthetic potential of the new chiral organoboron reagent. The regio- and diastereospecific allylboration with the achiral reagent and the regio-, diastereo- and enantioselective allylboration with the novel chiral reagent, as well as their reactions with pyrimidinic aldehyde yielding new alcohol, were analyzed computationally, revealing the origins of high selectivity in dynamic multicomponent reaction mixtures. The exocyclic isomers are unreactive due to the universally high barriers of allylboration reactions with their participation. Relative stabilities of all intermediates and transition states are regulated by the effectiveness of weak non-covalent interactions in the corresponding species. X-Ray crystallographic analysis of the racemate, consisting of (RR) and (SS) enantiomers, confirmed its constitution and relative configuration, whereas the absolute configuration of the chiral compound was assigned on the basis of the DFT calculation results. The chiral alcohol was tested as a catalyst in the Soai reaction, which afforded a racemic product.