Metallate Rearrangements, Hold the Metal

Forming new C-C bonds using organoborons is a staple disconnection that chemists across the world in both industry and academia use to stitch molecules together. You may think of the Suzuki cross-coupling, Zweifel olefinations (and their renaissance piloted in large part by Varinder Aggarwal), or even the Matteson homologation. Transition-metal catalysts have played key roles in many of the C-C bond formations we can do with organoborons. What struck me about this recent work from Chen and Huang is the marriage of two well-understood concepts — sulfonium ylide formation and metallate rearrangements at boron— into something greater than the sum of the parts: a borylative C-C coupling of allyl electrophiles.

The scope of the reaction accommodates a range of allylic electrophiles (typically allyl bromides) and notable shows selectivity for the halides over other common leaving groups used in transition metal-catalyzed alternatives ex. allylic esters. Also unlike the transition metal counterparts, aryl and sp2 halides remain untouched, allowing modular assembly of complex building blocks for further cross-coupling.

Critically, the authors demonstrate that potential trace palladium from preparation of the sulfide catalyst is not responsible as catalysts prepared in a Pd-free sequence were equipotent to catalysts prepared via Pd.

Overall, this is a really useful way to stitch molecules together and definitely makes me think harder about sulfur ylides.
Graphical summary and link to the paper