Base-catalysed condensation of aryl aldehydes and valine-derived boroxazolidones
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Emäskatalysoitu aryylialdehydien ja valiinista johdettujen boroksatsolidonien kondensaatio
Imines are an important group of compounds for many chemical reactions in organic chemistry, mostly as electrophiles. In nature, imines are important for the transamination reaction. N-B bonds are interesting because they can be thought of as an analogy to a C-C bond. However, unlike a C-C bond, the N-B bond is polarized. Imines and N-B bond-containing compounds both have similar potentials as pharmaceuticals. Both of these groups can have antibacterial, antifungal and anticancer effects. The N-B bond focused on in this thesis is formed when an amino acid reacts with a boron compound to form a hetero ring structure known as a boroxazolidone. In this master’s thesis, the imine condensation between aldehydes and boroxazolidones, and the N-B bond were studied. Boroxazolidones with differing substituents at the boron were prepared from the corresponding triethylammonium tetra-arylborates (TEATABs) and (L)-valine. These were then reacted with different aryl aldehydes in the presence of a base as a catalyst to afford the corresponding imines. The general procedures for preparing the TEATABs, the boroxazolidones and imines had already been established in prior projects in the laboratory. Two Lewis bases, 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD) and sodium methoxide (NaOMe), were found to produce the most simple ketimine and aldimine respectively in good yields and without side products. However, the aldimine is very labile and reverts easily to its starting materials. When testing other aldehydes with TBD, it was found that the yields range from 63 to 0 %, in this general order depending on the aldehyde’s substituents: halogen-substituted > electron withdrawing but non-halogen-substituted > electron donating. 4-Methoxybenzaldehyde was an exception to this because it produced a decent yield. Five different boroxazolidones were prepared from the corresponding TEATABs. The effect of the boroxazolidones’ substituents at boron had a lesser effect on the imine yield than the aldehydes’ substituents. Difluoro- and methyl-substituted boroxazolidones produced good yields but the 4-methoxy-substituted boroxazolidone produced a much lower yield.