Fedorov et al. assign the $^1$H-NMR ($\ce{CDCl3}$) chemical shifts of the $\ce{CH2}$ group in 2-(bromomethyl)phenylboronic acid
to three singlets at $4.75$, $5.09$ and $5.15~\ce{ppm}$ in a ratio of $3:3:14$.
Can someone explain their reasoning?
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asked Jul 8, 2015 at 23:27
pH13 - Yet another PhilipppH13 - Yet another Philipp
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$\begingroup$ Could you post the entire 1H-NMR analysis? The article is behind a paywall for me, and perhaps there is something in the rest of the data that is helpful. $\endgroup$– Ben NorrisCommented Jul 9, 2015 at 0:06
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$\begingroup$ Regarding long's answer - do you still want this information? $\endgroup$– pH13 - Yet another PhilippCommented Jul 9, 2015 at 0:27
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$\begingroup$ Nope. I was going to try to partially blame the potential NMR-activity of $\ce{^{11}B}$, which is a spin $\frac{3}{2}$, but I forgot about the tendency of boronic acids to oligomerize. $\endgroup$– Ben NorrisCommented Jul 9, 2015 at 1:12
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The paper specifically addresses this issue and ascribes it to the tendency for boronic acids to exist as solvent and concentration dependent oligomers:
The dependence of the intensity ratio for these signals and the chemical shifts in the 1H NMR spectra on the concentration of the samples under study as well as on the nature of the solvents used indicates that anhydrides of boronic acids with different degrees of oligomerization can exist along with monomeric forms of these derivatives (ArB(OH)2) in solutions of substituted arylboronic acids. The conclusions about the existence of oligomeric forms of boronic acids and the absence of byproducts in the compounds isolated are supported by the results published earlier.
This type of equilibrium can be expressed like:
Higher order oligomers are also possible.
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$\begingroup$ Thank you for pointing this out. I tend to overlook the "real" contents when searching for experimental data. $\endgroup$ Commented Jul 9, 2015 at 0:46