I have heard about activating ortho-para directors. For example: methoxy group, hydroxy group, alkyl group, et cetera.
I have heard about deactivating ortho-para directors. For example: halogen substituents
I have heard about deactivating meta directors. For example: nitro, trifluromethyl group, carboxylic acid group, et cetera.
So, naturally, I have embarked on a quest to find Activating Meta directors.
I have found one potential candidate, which is dimethylphenylborane:
Boron possesses a vacant $\mathrm{p}_z$ orbital, which can interact with the $\pi$-cloud of the benzene ring.
Considering the fact that boron is treated as a metalloid, it will definitely be less electronegative than carbon, so the dimethylborane group should offer a good +I effect, which could possibly outweigh the -M effects.
Can anyone shed some light on this issue? Are there any other examples of activating meta directors?