Are phenylboranes susceptible to ipso or meta electrophilic aromatic substitution, i.e. what's the regioselectivity?
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2 Answers
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A SciFinder search turns up very little on this type of transformation.
Qui and coworkers run halogenations of phenyl boronate esters using N-halosuccinimide using gold(III) catalysis. The regioselectivity was essentially 1:1:1 for ortho:meta:para. When the reaction was carried out with other substituents as well, the other substituent's directing group preference dominated the regioselectivity. Gold(III)-Catalyzed Halogenation of Aromatic Boronates with N-Halosuccinimides
There is one other reaction in SciFinder that is relevant, however I don't have access to the full text (and it's in Russian, so I wouldn't be able to read it anyway). The substrate is a benzene ring substituted with a borane cluster. The reported regioselectivity is 80:20 for meta:para.
Electrophilic substitution in the benzene ring of 9-phenyl-o- and -m-carborane. Zakharkin, L. I. et al Zhurnal Organicheskoi Khimii, 23(8), 1691-5; 1987.
answered Sep 19, 2014 at 21:45
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1$\begingroup$ I think that it's somewhat strange that this isn't examined more thoroughly. I am also unsure whether a boronate ester can be compared to a borane. The carborane looks more appropriate for consideration (excluding the steric effect contribution which doesn't allow ortho). The meta-direction would be in agreement with the resonance withdrawal. Seems like there is no evidence on ipso substitution. $\endgroup$– EJCSep 20, 2014 at 10:47
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$\begingroup$ I don't think there's been a real need to investigate that type of reaction. Boron relatively rarely appears in the target compound of a synthesis. Even then, it is probably introduced late in the synthesis. $\endgroup$ Sep 20, 2014 at 13:45
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$\begingroup$ One other note, SciFinder reports that phenyl borane is typically prepared through the organo mercury compound, something many people probably don't want to do. As a reactant, phenyl borane has been oxidized to the phenol, used in hydroboration, and to make boron complexes. $\endgroup$ Sep 20, 2014 at 13:49
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1$\begingroup$ Practically speaking yes, but theoretically speaking I think that it would be interesting to examine this topic, as boranyl is the only group that gives electrons by inductive effect and withdraws them by conjugation. $\endgroup$– EJCSep 20, 2014 at 21:03
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9-(N-pyridine)-11-I-7,8-dicarba-nido-undecaborate are susceptible to ipso substitution and have substitution I-atom on Cl-atom. Read
Rudakov D. A., Potkin V. I. Chlorination of 9-dimethylsulfonio-7,8-dicarba-nido-undecaborane and 9-(N-pyridine-)-11-iodo-7,8-dicarba-nido-undecaborane. pp. 69--72
Summary:
Chlorination of 9-dimethylsulfonio-7,8-dicarba-nido-undecaborane and 11-iodo-9-(N-pyridine)-7,8-dicarba-nido-unde-caborane with chlorine in dichloromethane produces 6,11-dichloro-9-dimethylsulfonio-7,8-dicarba-nido-undecaborane and 11-chloro-6-iodo-9-(N-pyridine)-7,8-dicarba-nido-undecaborane in 20% and 44% yields, respectively.
