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. 2012 Jul 18;134(28):11350-3.
doi: 10.1021/ja303443m. Epub 2012 Jul 3.

Outer-sphere direction in iridium C-H borylation

Philipp C Roosen  1 , Venkata A Kallepalli, Buddhadeb Chattopadhyay, Daniel A Singleton, Robert E Maleczka Jr, Milton R Smith 3rd
Affiliations

Outer-sphere direction in iridium C-H borylation

Philipp C Roosen et al. J Am Chem Soc. .

Abstract

The NHBoc group affords ortho selective C-H borylations in arenes and alkenes. Experimental and computational studies support an outer sphere mechanism where the N-H proton hydrogen bonds to a boryl ligand oxygen. The regioselectivities are unique and complement those of directed ortho metalations.

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Figures

Figure 1
Figure 1
Calculated energies of intermediates (5) vs. the total Normal Population Analysis (NPA) charge on the C–H activated heterocycles in 5. Computational data are excerpted from Ref 7b.
Figure 2
Figure 2
Lowest-energy M06/SDD(Ir)/6-31+G**(C H O N B) transition state (a) and intermediate (b) for C–H activation of pyrrole at the 2-position by model complex Ir(bipy)(Beg)3 (eg = ethyleneglycolate). The NH–O distances and distortions for Ir–C–C and Ir–C–N angles indicate N–H–O hydrogen bonding.
Figure 3
Figure 3
Plot of the ortho/meta product ratios using 4,4′-di(R1)-2,2′-dipyridyl catalysts vs. pKas of 4-R1-pyridium ions.
Figure 4
Figure 4
Lowest energy transition state for C–H activation of PhNHCO2Me by model complex 15. The o:m:p ratios predicted by theory and those found for borylation catalyzed by 2 are given.
Scheme 1
Scheme 1
Strategies for ortho-Directed C–H Borylation
Scheme 2
Scheme 2
Boc Substitution and Borylation Regioselectivity
Scheme 3
Scheme 3
Experiments to Probe Mechanism
Scheme 4
Scheme 4
C–H borylation and DoM regioselectivities.
See this image and copyright information in PMC

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    1. Although “outer-sphere” commonly refers to eletron transfer, it has been used to describe mechanisms for hydrogen transfer and nucleophilic addition to π-allyl Pd compounds: Samec JSM, Backvall JE, Andersson PG, Brandt P. Chem. Soc. Rev. 2006;35:237. Trost BM, Xu JY, Schmidt TJ. Am. Chem. Soc. 2009;131:18343.

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