Oxidative Coupling with Zr(IV) Supported by a Noninnocent Anthracene-Based Ligand: Application to the Catalytic Cotrimerization of Alkynes and Nitriles to Pyrimidines

Abstract

We report the synthesis and reactivity of Zr complexes supported by a 9,10-anthracenediyl-linked bisphenoxide ligand, L. ZrIVLBn2 (1) undergoes facile photolytic reduction with concomitant formation of bibenzyl and ZrIVL­(THF)3 (2), which displays a two-electron reduced anthracene moiety. Leveraging ligand-stored reducing equivalents, 2 promotes the oxidative coupling of internal and terminal alkynes to isolable zirconacyclopentadiene complexes, demonstrating the reversible utilization of anthracene as a redox reservoir. With diphenylacetylene under CO, cyclopentadienone is formed stoichiometrically. 2 is competent for the catalytic formation of pyrimidines from alkynes and nitriles. Mechanistic studies suggest that selectivity for pyrimidine originates from preferred formation of an azazirconacyclopentadiene intermediate, which reacts preferentially with nitriles over alkynes.

Multicomponent syntheses of 5- and 6-membered aromatic heterocycles using group 4–8 transition metal catalysts

“…31 ). 120 When terminal alkynes were used as substrates, the major product was the 2,4,6-trisubstituted pyrimidine, similar to the selectivity observed in NbCl 5 -catalyzed reactions ( Fig. 29 ).…”

“…119 An analogous zirconacyclopentadiene intermediate was also recently isolated and shown to yield pyridines upon reaction with nitrile (vide infra). 120 3.1.3 Formal [3 + 2 + 1] strategies. While the majority of pyridine syntheses rely on variations of formal [2 + 2 + 2] cycloadditions, there are several Ti-catalyzed formal [3 + 2 + 1] methods that capitalize on multistep, one-pot hydroamination/ functionalization sequences, similar to those seen in the various hydroamination-derived routes to 5-membered heterocycles.…”

“… 119 An analogous zirconacyclopentadiene intermediate was also recently isolated and shown to yield pyridines upon reaction with nitrile ( vide infra ). 120 …”

Transition‐Metal‐Mediated/Catalyzed Synthesis of Pyridines, Pyrimidines, and Triazines by [2+2+2] Cycloaddition Reactions

“…More recently, Agapie and co-workers reported synthesis and use of a Zr-bisphenoxide ligand complex to catalyze pyrimidine synthesis (Scheme 68). [61] The redox-active complex Zr IV À Anth red 168 was derived by photolytic reduction. This cycloaddition proceeds in the presence of 5 mol% of the Zr IV À Anth red complex and leads to pyrimidines 169-174.…”

“…More recently, Agapie and co‐workers reported synthesis and use of a Zr–bisphenoxide ligand complex to catalyze pyrimidine synthesis (Scheme ) . The redox‐active complex Zr IV −Anth red 168 was derived by photolytic reduction.…”