Selective Iron-Mediated C- and O-Addition of Phenolic Nucleophiles to a Cyclohexadiene Scaffold Using Renewable Precursors

Abstract

Renewable phenols have been investigated as nucleophiles for the addition to a cationic cyclohexadienyl iron carbonyl scaffold. Benign conditions compatible with solvents such as ethanol and water were developed, and for the first time, selective C- or O-addition could be achieved. In addition, a novel atom-economic approach to forming the C-addition products directly from the neutral precursor complex in a single step using a catalytic acid is described. The formed C-addition product could then be selectively demetalated to form one of two different product classes, a functionalized arene or a cyclohexadiene.

Azulene Functionalization by Iron-Mediated Addition to a Cyclohexadiene Scaffold

“…In our previous studies involving phenolic nucleophiles, we performed the reaction in ethanol in the absence of base at an ambient temperature. 25 Similar conditions here afforded the desired addition product 3, albeit in a modest yield (Table 1, entry 1). Using water as the solvent afforded a heterogeneous reaction mixture where both guaiazulene and iron complex 2 were insoluble.…”

“…As the electrophile, we selected iron complex 2, which we have used in earlier studies. 25 Optimization of the reaction conditions for this addition was then performed (Table 1).…”

“…In our previous research on phenolic nucleophiles, we developed a shorter route for the coupling reaction starting from neutral iron cyclohexadiene complexes. 25 This method foregoes the formation and isolation of the cationic iron carbonyl complex by forming the electrophile in situ from a neutral precursor using a catalytic amount of acid. We applied this strategy here and found that guaiazulene could be alkylated directly from the neutral iron complex 32 (precursor to cationic complex 2) in the presence of tetrafluoroboric acid (Scheme 6).…”

“…24 The scope was recently increased by us to include the selective C-or O-addition of phenolic nucleophiles. 25 Although azulenes, with a nucleophilicity similar to that of indoles, 10d can be expected to be efficient nucleophiles, no previous report of their use in this context has been reported. Herein, we present a convenient iron-mediated electrophilic C−H functionalization of azulenes (Scheme 1).…”

Rules and mechanism for the oxidation of lignin-based aromatic aldehyde under alkaline wet oxygen

“…With the shortage of petroleum resources, the preparation of fuels and chemicals using renewable lignocellulosic biomass resources has become a hot research topic (Liao et al 2018;Duas et al 2019). The conversion of lignin into high value-added products is in line with the concept of biorefining, which calls for the utilization of the three major lignocellulosic components (cellulose, hemicelluloses, and lignin) efficiently (Ragauskas et al 2014;Anderson et al 2016;Beckham et al 2016;Rinaldi et al 2016).…”