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. 2016 Nov 2;55(45):14085-14089.
doi: 10.1002/anie.201605548. Epub 2016 Oct 6.

Visible Light Activation of Boronic Esters Enables Efficient Photoredox C(sp2 )-C(sp3 ) Cross-Couplings in Flow

Fabio Lima  1 , Mikhail A Kabeshov  1 , Duc N Tran  1 , Claudio Battilocchio  1 , Joerg Sedelmeier  2 , Gottfried Sedelmeier  2 , Berthold Schenkel  2 , Steven V Ley  3
Affiliations

Visible Light Activation of Boronic Esters Enables Efficient Photoredox C(sp2 )-C(sp3 ) Cross-Couplings in Flow

Fabio Lima et al. Angew Chem Int Ed Engl. .

Abstract

We report herein a new method for the photoredox activation of boronic esters. Using these reagents, an efficient and high-throughput continuous flow process was developed to perform a dual iridium- and nickel-catalyzed C(sp2 )-C(sp3 ) coupling by circumventing solubility issues associated with potassium trifluoroborate salts. Formation of an adduct with a pyridine-derived Lewis base was found to be essential for the photoredox activation of the boronic esters. Based on these results we were able to develop a further simplified visible light mediated C(sp2 )-C(sp3 ) coupling method using boronic esters and cyano heteroarenes under flow conditions.

Keywords: boronic esters; cross-coupling; flow chemistry; photoredox catalysis; synthetic methods.

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Figures

Figure 1
Figure 1
Use of boronic pinacol esters for photoredox C(sp3) radical generation.
Figure 2
Figure 2
Scope of the dual Ir/Ni‐catalyzed benzyl boronic ester arylation in flow (0.5 mmol scale, 0.1 m in acetone). Isolated yields, a slug containing all the premixed reaction mixture was eluted through the photo‐reactor at 100 μL min−1, collected, filtered through a plug of Celite and concentrated before being purified by flash column chromatography. [a] Isolated yield reported by Molander for the product.4 [b] Isolated as the phenol after oxidation of the aryl boronic pinacol ester with H2O2/NaOH.
Figure 3
Figure 3
Scope of photoredox benzyl and allyl boronic esters arylation with cyanoarenes in flow (for benzyl: 0.25 mmol scale, 0.25 m in acetone). Isolated yields, a slug containing all the premixed reaction mixture was eluted through the reactor at 100 μL min−1, collected and concentrated before being purified on flash column chromatography. [a] Allylation experiments were performed on a 1.0 mmol scale (0.5 m in acetone), using 2.5 equivalents of the allylboronic acid pinacol ester and 0.5 mol % of cat(1) at 200 μL min−1 (τ=50 min).
Figure 4
Figure 4
Proposed mechanistic description for the photoredox net neutral coupling of cyanoarenes with organo boronic esters.
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References

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