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:

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Boron possesses a vacant $\mathrm{p}_z$ orbital, which can interact with the $\pi$-cloud of the benzene ring.

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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?

  • 3
    $\begingroup$ Related: chemistry.stackexchange.com/q/15973/4325 $\endgroup$
    – jerepierre
    Jun 30 '17 at 7:11
  • $\begingroup$ You're talking about for EAS? $\endgroup$
    – Zhe
    Aug 14 '17 at 14:48
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    $\begingroup$ OK. Got it. So what makes you think this is a +I effect? For example. how is having a borane on the ring any different from having a carbonyl attached to the ring? $\endgroup$
    – Zhe
    Aug 14 '17 at 18:06
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    $\begingroup$ Screwed up the first response I was trying to write... I think this is an interesting idea. You might even consider other substituents instead of methyl on the borane. $\endgroup$
    – Zhe
    Aug 15 '17 at 2:09
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    $\begingroup$ This is very theoretical, but what if you added a subtituent such as orthonitrate so that the formula is Na2O3NPh, basically a nitrogen attached to a phenyl with 3 oxygens single bonded to it. The meta directing would be done similar to the nitro group, due to recent evidence pointing towards the pentavalent nitrogen structure being a part of resonance, and then the carbon gains electron density because of the pentavalent structure being in resonance with the traditional structure (following octet rule). $\endgroup$
    – AS_1000
    Aug 17 '17 at 13:50

Note: This requires an extremely high energy scenario to occur.

This is another example that looks promising, it is essentially an ylide that has enough EWing power to meta direct while giving some density to the ring in another resonance structure. [Resonance]

The phosphonium ylide derivative (phenylphosphonic acid) may provide the same effect if the hypervalent nature of nitrogen is challenged, or if this substance is unobtainable under any circumstances.

  • $\begingroup$ Can you explain whether there's an +I effect or not? $\endgroup$ Aug 18 '17 at 6:11
  • $\begingroup$ Also, can you comment on dimethylphenylborane, the example that I suggested for an activating meta director? $\endgroup$ Aug 18 '17 at 6:53
  • $\begingroup$ Product distribution is around 1:1:1 for the boranes, so a +l effect is definitely there, there is probably going to be a +l effect for this group, just as a phsophonium ylide used in the Wittig reaction gives that effect. $\endgroup$
    – AS_1000
    Aug 18 '17 at 13:40
  • $\begingroup$ This is highly dubious and speculative. $\endgroup$
    – Mithoron
    Apr 17 '18 at 17:12

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