Pt^II‐Catalyzed Hydroxylation of Terminal Aliphatic C(sp³)−H Bonds with Molecular Oxygen

Abstract

The practical application of Shilov‐type Pt catalysis to the selective hydroxylation of terminal aliphatic C−H bonds remains a formidable challenge, due to difficulties in replacing PtIV with a more economically viable oxidant, particularly O2. We report the potential of employing FeCl2 as a suitable redox mediator to overcome the kinetic hurdles related to the direct use of O2 in the Pt reoxidation. For the selective conversion of butyric acid to γ‐hydroxybutyric acid (GHB), a significantly enhanced catalyst activity and stability (turnover numbers (TON)>30) were achieved under 20 bar O2 in comparison to current state‐of‐the‐art systems (TON<10). In this regard, essential reaction parameters affecting the overall activity were identified, along with specific additives to attain catalyst stability at longer reaction times. Notably, deactivation by reduction to Pt0 was prevented by the addition of monodentate pyridine derivatives, such as 2‐fluoropyridine, but also by introducing varying partial pressures of N2 in the gaseous atmosphere. Finally, stability tests revealed the involvement of PtII and FeCl2 in catalyzing the non‐selective overoxidation of GHB. Accordingly, in situ esterification with boric acid proved to be a suitable strategy to maintain enhanced selectivities at much higher conversions (TON>60). Altogether, a useful catalytic system for the selective hydroxylation of primary aliphatic C−H bonds with O2 is presented.

Direct C(sp3)–H Activation of Carboxylic Acids

“…The authors demonstrated that both reactivity and selectivity could be improved either by the addition of 2-pyridone as a ligand or by the addition of boric acid to stop the oxidation process at the stage of GHB. 14 Lee and Chang used a related catalytic system in 2006 to achieve the lactonization of ortho-methyl-substituted benzoic acid derivatives (Scheme 3, a). 15 For example, using 2,4,6-trimethylbenzoic acid (8a) as the substrate the authors isolated the corresponding lactone 9a in 56% yield.…”

The Emergence of Palladium‐Catalyzed C(sp3)−H Functionalization of Free Carboxylic Acids

“…To be mentioned, van Gemmeren's short review on direct C(sp 3 )−H activation of carboxylic acids provided a brief overview of different C−H functionalizations [14] . Few examples for C(sp 3 )−H lactonization [15] and hydroxylation [16] of carboxylic acids were reported under platinum catalysis. They are not included in this report.…”

Regioselective C–H hydroxylation of n-alkanes using Shilov-type Pt catalysis in perfluorinated micro-emulsions

“…alkyl ammonium salts, as well as lower carboxylic, sulfonic and phosphonic acids. 21,22 Hence, Shilovinspired Pt catalysis has hitherto not been applied for the selective oxidation of non-polar aliphatic compounds. While we demonstrated the potential of implementing acetic acid as a solvent in converting n-heptane to 1-heptanol, 23 the use of an aqueous solution would be highly desirable from an ecological perspective.…”