Raman and SERS Spectroscopy of N-Methyliminodiacetic Acid (MIDA)-Protected Boronate Esters

摘要

N-methyliminodiacetic acid (MIDA)-protected boronate esters represent a new class of reagents that offer great promise in iterative Suzuki-Miyaura cross-coupling reactions. The Suzuki-Miyaura reaction between an organohalide and boronic acid is a powerful tool in generalizing the method for C-C bond formation in the synthesis of complex molecules. However, boronic acid usually proves problematic in this process because of the reactivity of its sp~2 center. Martin Burke and co-workers have developed a solution to this problem through complexation of boronic acid with a trivalent ligand, N-methyliminodiacetic acid.' This complexation rehybridizes the boron center to spa through use of a B-N dative bond, dramatically increasing the stability of the boronic acid moiety. Compared to earlier reagents, the MIDA esters are easily handled and are bench-top stable under air indefinitely, making them very attractive to many synthetic chemists. Once the desired synthesis has occurred, the boron can be de-protected easily using mild aqueous basic conditions. Here, we study the spectroscopic properties of the key structural component common to this new molecular class – the B-N dative bond. This is accomplished by comparing Raman and surface-enhanced Raman (SERS) spectra of methylboronic acid MIDA ester and 1-naphthylboronic acid MIDA ester (structures shown in Figure 1) to the results of high level electronic structure calculations.