New Bond Formations Just Keep On Coming

Key Takeaways

• Copper-catalyzed sp3‑sp3 coupling retains stereochemistry at B(pin) carbon
• t‑BuLi‑activated B(pin) salts can be isolated and stored
• Method tolerates silyl ethers, acetals, pyridines, cyclopropyl groups
• Demonstrated in six-step synthesis of macrocycle spongidepsin
• Enables iterative assembly of diverse alkyl fragments for automation

Pulse Analysis

Iterative bond‑forming strategies have reshaped synthetic planning over the past decade. Starting with MIDA boronates, chemists demonstrated that boron‑protected intermediates could be deprotected and re‑coupled in a modular fashion, a concept later extended to TIDA boronates and carbenoid insertions. These advances hinted at a future where complex molecules could be assembled by a sequence of predictable, automated steps, reducing the need for bespoke route design and accelerating discovery pipelines.

The latest contribution builds on that foundation by exploiting the familiar pinacol boronate ester (B(pin)) at alkyl centers. Activation with t‑butyllithium generates a lithium‑boron species that can be isolated, stored, and later coupled with alkyl electrophiles bearing iodide or benzenesulfonate leaving groups. In the presence of CuI, triphenylphosphine, and an acetylide ligand, the reaction delivers direct sp³‑sp³ carbon‑carbon bonds while preserving stereochemistry—a rarity in this domain. Scope studies reveal compatibility with silyl ethers, acetals, tertiary amines, pyridines, and cyclopropyl groups, and electrophiles featuring alkenes, alkynes, esters, nitriles, and epoxides. Limitations include sensitivity to acidic protons and reduced yields with β‑branched partners, underscoring areas for further optimization.

For the pharmaceutical and materials sectors, the ability to stitch together diverse sp³ fragments with stereocontrol opens new avenues for rapid analog generation and macrocycle construction, as exemplified by the spongidepsin synthesis. By providing a potentially storable, modular building block, the protocol aligns with emerging robotic platforms that aim to execute multi‑step sequences without human intervention. Continued refinement could lower barriers to high‑throughput synthesis, accelerate lead optimization, and ultimately shift the skill set of chemists toward designing automated workflows rather than labor‑intensive stepwise routes.