Cyclic Diaryl λ3-Bromanes as a Precursor for Regiodivergent Alkynylation Reactions

Regiodivergent reactions are a fascinating tool to rapidly access molecular diversity while using identical coupling partners. We have developed a new approach for regiodivergent synthesis using the dual character of hypervalent bromines. In addition to the recently reported reactivity of hypervalent bromines as aryne precursors, the first transition metal-catalyzed reaction is reported. Accordingly, the development of these two complementary transformations allows for the alteration of regioselectivity to furnish both ortho- and meta-substituted alkynylation products. Mechanistic and computational studies show how these selectivities are controlled.

Regioselective trans-Hydrostannation of Boron-Capped Alkynes

Alkynyl‐B(aam) (aam=anthranilamidato) derivatives are readily available bench‐stable compounds that undergo remarkably selective reactions with Bu₃SnH in the presence of [Cp*RuCl]₄ as the catalyst. The addition follows a stereochemically unorthodox trans‐selective course; in terms of regioselectivity, the Bu₃Sn‐ unit is delivered with high fidelity to the C‐atom of the triple bond adjacent to the boracyclic head group (“alpha,trans‐addition”). This outcome is deemed to reflect a hydrogen bonding interaction between the protic −NH groups of the benzo‐1,3,2‐diazaborininone ring system and the polarized [Ru−Cl] bond in the loaded catalyst, which locks the substrate in place in a favorable orientation relative to the incoming reagent. The resulting isomerically (almost) pure gem‐dimetalated building blocks are amenable to numerous downstream functionalizations; most remarkable is the ability to subject the −B(aam) moiety to Suzuki‐Miyaura cross coupling without need for prior hydrolysis while keeping the adjacent Bu₃Sn‐ group intact. Alternatively, the tin residue can be engaged in selective tin/halogen exchange without touching the boron substituent; the fact that the two ‐NH entities of −B(aam) do not protonate organozinc reagents and hence do not interfere with Negishi reactions of the alkenyl halides thus formed is another virtue of this so far underutilized boracycle. Overall, the ruthenium catalyzed trans‐hydrostannation of alkynyl‐B(aam) derivatives opens a practical gateway to isomerically pure trisubstituted alkenes of many different substitution patterns by sequential functionalization of the 1‐alkenyl‐1,1‐heterobimetallic adducts primarily formed.

Rhodium(III)-Catalyzed C(sp2)-H Functionalization of Cyclobutenes. Access to Cyclobuta[c]pyridones and -pyridines

O-(ω-Alkynyl) hydroxamates derived from cyclobutenyl carboxylic acids were identified as viable substrates in intramolecular rhodium(III)-catalyzed heteroannulations, which led to diversely substituted cyclobuta[c]pyridones. Further functionalization of the resulting cyclobutapyridones enabled the synthesis of cyclobuta[c]pyridines and other nitrogen heterocycles after electrocyclic ring opening of the four-membered ring.