Sterically Congested LLX-Type PNSi Tridentate Ligands for Iridium-Catalyzed C(sp³)-H Borylation

Area of Science:

• Organometallic Chemistry
• Catalysis
• Synthetic Chemistry

Background:

• Direct C(sp3)-H functionalization is crucial for organic synthesis.
• Developing selective catalysts for C(sp3)-H activation remains challenging, especially in the absence of directing groups.
• Iridium catalysts are widely explored for C-H activation reactions.

Purpose of the Study:

• To develop novel iridium catalysts for efficient and selective C(sp3)-H borylation of alkanes and alkyl groups.
• To investigate the chemoselectivity of these new catalysts in comparison to existing systems.
• To explore the application of these catalysts in the presence of C(sp2)-H bonds.

Main Methods:

• In situ preparation of iridium catalysts using [Ir(OMe)(cod)]2 and novel LLX-type PNSi tridentate ligands.
• Synthesis of PNSi ligands from commercially available 8-bromo-2-methylquinoline.
• Evaluation of catalyst performance in C(sp3)-H borylation reactions of various substrates, including alkanes and alkylbenzenes.

Main Results:

• The developed Ir/PNSi catalysts demonstrated high efficiency for C(sp3)-H borylation of alkanes lacking directing groups.
• These catalysts exhibited superior chemoselectivity over standard Ir/Me4phen and previously reported Ir/NNSi catalysts.
• Selective borylation of sterically less hindered C(sp3)-H bonds was achieved in substrates like 2-methylheptane and dibutyldimethylsilane.
• C(sp3)-H selective borylation in the presence of C(sp2)-H bonds was successfully accomplished using 4-substituted alkylbenzenes.

Conclusions:

• The novel Ir/PNSi catalysts are effective for the direct C(sp3)-H borylation of unactivated alkanes.
• The mer-tridentate coordination of PNSi ligands creates a sterically congested iridium center, leading to enhanced chemoselectivity.
• These findings offer a promising new avenue for selective C-H functionalization in organic synthesis.