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Here we report a new 6,6′-bipyridine based ligand framework for the iridium-catalysed regioselective ortho borylation of diverse classes of arenes containing different functional groups. Moreover the developed method is highly selective for the directed C(sp (Altus and Love, 2021) [3]-H borylation of the 2-pyridyl amines as well as benzyl and homobenzyl amides directed by pyridine group and amide directed borylation of N-adjacent C–H bond of amides.
]. Despite these difficulties, so many efficient methods have been discovered to functionalise these inert C–H bonds from proximal to remote C–H bonds [
In this context, among various metal-catalysed C–H borylation reactions, the employment of iridium-metal-catalysed C–H borylation is the major contributing factor. Moreover, while directed ortho metalation (DoM) strategy is a widely accepted synthetic approach, many C–H borylation catalysts have been discovered, which have enabled to activate and functionalise arene's C–H bonds [
] came up with a powerful strategy by employing solid-supported monodentate phosphine ligand for the ortho borylation under iridium-catalysed heterogeneous conditions. Notably, by the passing of time, this ligand showed huge utility for the ortho borylation [
]. Subsequently, electron-deficient monodentate phosphine and arsine ligand was introduced by the Ishiyama and Miyaura for the ortho borylation of esters [
] introduced an elegant concept using hemilabile ligand for the selective ortho borylation of hydrazones and 2-phenyl pyridines. These are the seminal reports where iridium-catalysed regioselective borylation occurred through the formation of Iridium tris(boryl) complex [
] for the selective ortho borylation of specific substrates by the modification of the bidentate bipyridine ligand using noncovalent interactions. On the other hand in 2014, Smith and coworkers reported an approach [
] for the ortho borylation of diverse classes of substrates using P/N–Si ligand and the reaction underwent through the formation of iridium bis(boryl) complex. After that, Li et al. [
] developed a method for the ortho and directed aliphatic borylation of diverse classes of substrates by using simplified N–C monoanionic bidentate ligand (PYT ligand). In these above-mentioned reports where heterobidentate [
] were used for the ortho borylation of diverse classes of substrates, there was no report for the bipyridine-based systems for the ortho borylation of diverse classes of arenes irrespective of their electronic nature.
Herein, we report a simple bidentate bipyridine ligand having phenyl groups at 6,6′- position of bipyridine ligand, which showed high regioselectivity towards ortho as well as aliphatic borylations (Fig. 1).
To get the optimised reaction conditions, we first tested the borylation with ethyl benzoate (1a) as a model substrate (Fig. 2). Initial investigation with the conventional bidentate ligands (L1-L3) with [Ir(cod)OMe]2 showed that there was no ortho selective borylation, rather resulted mixture of meta and para borylated products. Next, we tested various ring-substituted bpy derived ligands and found that while the 6-substituted ligand L5 gave 50% ortho selectivity but the 6-methyl substituted ligand L4 gave no ortho borylation under the same reaction conditions. Based on this result, we have designed and synthesized 6,6′-diphenyl substituted bpy ligand (L6) and employed in the borylation reaction. Interestingly, we found this developed ligand showed very high ortho selective borylation (96%) with 76% conversion. With increasing the amount of B2pin2 to 1.5 equiv., the conversion was increased to 94%. Then to see any effect of other substitution at the 6-position, L7-L9 ligands were tested and resulted lower ortho selectivity. Also, it was observed that 5,6′-disubstituted ligand L10 gave very poor ortho selectivity. Moreover, testing some monodentate ligands (L11-L13) in the borylation also found to be nonselective. Notably, to see the effect of the hemilabile ligands, we performed borylation using L14 [
] which also gave nonselective borylation. Frome the above screening it is concluded that the optimised ligand is L6.
Fig. 2Ligand screening. Reactions are in 0.5 mmol scale. In parenthesis isolated yield is reported. Conversions and ratios are determined from the GC/MS analysis. a1.5 equiv. B2pin2 was used (with 1.0 equiv. B2pin2 76% conversion (o/others = 96/4)). See SI for details.
Next ortho borylation of a series of aromatic esters were performed under the developed reaction conditions (Fig. 3). It was observed that arenes containing substitution at ortho positions (1b-1h) were well tolerated. The developed conditions were also compatible for arenes containing meta (1i-1k), para (1l-1r) substituents and as well as disubstituted arenes(1s-1w) with good isolated yield and selectivity. Ester derived from bioactive borneol (1x) gave 86% ortho borylation. The developed reaction condition was applicable for the ortho borylation of methyl benzoate substrate (1y).
Fig. 3Substrates scope for aromatic esters. Reactions are in 0.5 mmol scale. Isolated yields are reported. Ratios are determined from the GC/MS analysis. a1.0 equiv. B2pin2 was used. b1.2 equiv. B2pin2 was used. See SI for details.
To this end, we have found that the developed conditions exhibited excellent outcomes for the regioselective ortho borylation of a range arenes containing different functional groups (Fig. 4). For example, the developed ligand (L6) was compatible for the ortho borylation of N, N disubstituted amide (3a), acetophenone (3b), benzophenone (3c), thioanisole [
] (3d), benzyl ether (3e), imine (3f), carbamate (3g), N, N-disubstituted benzylamine (3h), 2-phenoxypyridine (3i), 2-anilinopyridine (3j) and 8-aryl quinoline (3k) and phenyl acetate (3k). The developed ligand system was also examined for the aliphatic borylations. For example, the aliphatic borylation of N-substituted 2-pyridylamines (3m-3o), where directed aliphatic borylation occurred by the coordination of the pyridyl nitrogen to the iridium center with a decent number of isolated yields (Fig. 4). This developed method also equally effective for the amide-directed sp [
] of C–H bond adjacent to the nitrogen atom of N,N-dimethyl benzyl amide and homologous benzyl amides.
Fig. 4Substrate scopes for other classes. Reactions are in 0.5 mmol scale. Ratios are determined by GC/MS analysis. aNMR conversions are reported. See SI for details.
The practical utility of the developed methodology was showcased by the transformation of the in-situ generated boronate ester to various functional groups. Thus, ortho borylated product (2a) was converted into ethyl 2-hydroxybenzoate (5, 86%), Pd-catalysed arylation (1h, 82%), Cu-catalysed bromination (6, 87%) and chlorination (1c, 85%) could also be performed. By treatment with AgNO3 and D2O, deuteration (7, 90%) was also be performed (Fig. 5, see SI for details).
Fig. 5In-situ synthetic transformations. See SI for details.
To understand the mechanism of the ortho selectivity of benzoate ester with 6,6′-diphenyl bipyridine ligand (L6), we looked into some ligand screening results including 2-phenyl pyridine (L11) with other ligand systems (Fig. 6). We observed that while the ligand (L11) resulted in 62% ortho borylated product, the ligand 2-methyl pyridine (L12) gave almost same amount of ortho borylation (66%), which indicated a monodentate behaviour of the L11 ligand. Moreover, we noticed that 6-substituted ligands L5 and L16 also gave almost 42–50% ortho selectivity, which is same result with the L7 and L9 ligands. These results indicated that L6 ligand may act as a hemilabile ligand (one N-coordination acts as labile due to the sterically crowded phenyl substitution at the nitrogen binding site) during the C–H activation process (oxidative addition). From these results, we proposed a mechanism where L6 acts as a hemilabile ligand (Fig. 6). Also, there may be other possible mechanism where N, C bidentate coordination may play role in the ortho selectivity, which is matter of further study.
In summary, we have developed a new ligand framework for the ortho borylation of a diverse class of arenes containing different functional groups along with aliphatic borylation. The developed method showed excellent functional group tolerance and site selectivity. We believe that the developed method will find wide application for the C–H functionalisation reactions.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This work was supported by SERB-STAR AWARD grant (STR/2019/000045). SD, SG and JD thanks CSIR for JRF and MMH and JC thanks UGC for SRF. MEH and SD equally contributed to this work.
Appendix A. Supplementary data
The following is the Supplementary data to this article:
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