Ni(NHC)]-Catalyzed Cycloaddition of Diynes and Tropone: Apparent Enone Cycloaddition Involving an 8π Insertion

Abstract

A Ni/N-heterocyclic carbene catalyst couples diynes to the C(α)–C(β) double bond of tropone, a type of reaction that is unprecedented for metal-catalyzed cycloadditions with aromatic tropone. Many different diynes were efficiently coupled to afford [5–6–7] fused tricyclic products, while [5–7–6] fused tricyclic compounds were obtained as minor byproducts in a few cases. The reaction has broad substrate scope and tolerates a wide range of functional groups, and excellent regioselectivity is found with unsymmetrical diynes. Theoretical calculations show that the apparent enone cycloaddition occurs through a distinctive 8π insertion of tropone. The initial intramolecular oxidative cyclization of diyne produces the nickelacyclopentadiene intermediate. This intermediate undergoes an 8π insertion of tropone, and subsequent reductive elimination generates the [5–6–7] fused tricyclic product. This initial product undergoes two competing isomerizations, leading to the observed [5–6–7] and [5–7–6] fused tricyclic products.

Computational Studies of Synthetically Relevant Homogeneous Organometallic Catalysis Involving Ni, Pd, Ir, and Rh: An Overview of Commonly Employed DFT Methods and Mechanistic Insights

“…341 Calculations at the CPCM (THF) M06/6-31+G(2d,p) (SDD)//B3LYP/6-31G(d) (SDD) level of theory suggest that the mechanism of the reaction consists of an unique 8π insertion of tropone to the metallacyclopentene intermediate which is shown to be 12.3 kcal/mol more favored than the traditional 2π insertion.…”

Advances in Nickel-Catalyzed Cycloaddition Reactions To Construct Carbocycles and Heterocycles

“…Recently, we addressed this long-standing challenge by developing a Ni/SIPr-catalyzed cycloaddition of diynes with a single double bond of a tropone to selectively form fused tricyclic frameworks in high yields (Scheme 31). 19 The cycloaddition of 2,7-diynes bearing alkyl groups on alkyne termini led to the formation of [5–6–7] fused major products ( 89 and 90 ) and [5–7–6] minor products ( 89 ′ and 90 ′) in excellent combined yields. Symmetrical aryl-substituted diynes and unsymmetrical aryl–alkyl substituted diynes also underwent smooth cycloaddition to selectively form the desired major products ( 91 – 98 ).…”

π‐Lewis‐Base‐Catalyzed Asymmetric Vinylogous Umpolung Reactions of Cyclopentadienones and Tropone

“…Since simple cyclopentadienone is not an isolatable intermediate,c ommercially available tropone 22,u sually utilized as a4p,6p or 8p acceptor, [24,31] was tested to support the possibility of forming reactive h 2 -Pd 0 complex. Pleasingly, tropone 22 was indeed apparently HOMO-raised via Pd 0coordination based on DFT calculations (Scheme 4a, À6.82 eV vs. À5.01 eV), and performed as a2 p-partner [32] in IED aza-DA-type cycloaddition reaction with 1-azadiene 23.…”