Bismuth aryloxide reactivity: kinetics of thermal decomposition and resulting organic oxidation products

Abstract

The Standard Oil of Ohio Company (SOHIO) process is an industrial method for the oxidation and ammoxidation of propene to produce acrolein and acrylonitrile. This process uses a bimetallic catalyst based on Bi2O3*MoO3. Bismuth has been shown to be necessary in the rate determining step as well as in later steps of the catalytic cycle. Our group was the first to describe a molecular bismuth model system with reactivity similar to the SOHIO catalysts using bismuth aryloxides as a model system. This represented the first experimental support that the fundamental step in SOHIO propene activation involves Bi(III)-O bond homolysis to form Bi(II). In an effort to obtain kinetic rates, we chose to study a phenoxy bismuth complex of intermediate steric bulk. We anticipated that 2-tert-butyl-6-isopropylphenol would give a corresponding bismuth aryloxide that would decompose at an intermediate rate.^A new synthesis of this phenol was developed.^In order to have authentic materials for comparative analysis in the decomposition of the Bi(III) aryloxide Bi(O-2-tBu-6-iPrC6H3)3, four potential decomposition products were synthesized; 2-tert-butyl-6-isopropylquinone, 2-tert-butyl-6-isopropylhydroquinone, 3,3'-di-tert-butyl-5,5'-diisopropyldiphenoquinone, and 3,3'-di-tert-butyl-4,4'-dihydroxy-5,5'-diisopropyldiphenyl. We observed during the isolation of 3,3'-di-tert-butyl-5,5'-diisopropyldiphenoquinone that only the E isomer crystallizes, and slowly equilibrates to a 51:49 E/Z mixture when in solution. We successfully obtained isomerization kinetics that favor a diradical isomerization mechanism. Computational studies in collaboration with the Borden research group at University of North Texas supported our experimental results but giving a smaller negative entropy value.^In an effort to obtain the Bi(OAr)3 product by reacting bismuth tert-butoxide with 2-tert-butyl-6-isopropylphenol, we obtained instead tBuOBi(OAr)2.^When this bismuth phenoxide was heated in C6D6, decomposition to the corresponding phenol and both E and Z isomers of 3,3'-di-tert-butyl-5,5'-diisopropyldiphenoquinone was observed by 1H NMR. Kinetic studies on this decomposition were initiated and will be discussed. The re-synthesis of a large number of bismuth aryloxides was also performed and their stabilities studied. Kinetic studies of the C-H activation of two bismuth complexes were also initiated. Both experimental and computational studies with the Janesko research group at TCU support a base catalyzed concerted pathway.