In many cases, 13C signal of the carbon which linked with boric acid directly often can't appear in C-NMR. Could someone can tell me the reason? If can share the article about this problem will be better. Thanks very much.
Check these out:
http://www.chemistry.sdsu.edu/research/BNMR/
J. Chem. Soc., Perkin Trans. 2, 1991, 321-327
Inorganica Chimica Acta 363 (2010) 4031–4037
Hope you get to it!
This is a nice reference about 11B NMR spectroscopy and about the usual chemical shifts there. However, He Yuhun asked about 13C signals coupled (directly) by boron. B has two quadrupolar nuclei, 10B (~ 20%) with a spin of 3, and for NMR the more favorable 11B(~80 %) with a spin of 3/2. Even the latter causes splitting of 13C signals into a quintet-like signal. Thus your 13C signal is expected to give a broad and not so nicely resolved peak, which is difficult to detect especially on a guaternary 13C connected directly to a B.
http://chem.ch.huji.ac.il/nmr/techniques/1d/row2/b.html
https://www.chem.wisc.edu/areas/reich/nmr/07-multi-04-quadrupolar.htm
The answers provided by Imre and Riaz are good. I would also add that if you need to see that carbon, I have had luck using HMBC to see all 13C signals in aryl boronates (of course, this requires that there is a proton bound to one of the neighboring carbon atoms...)
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