Aspects of selective oxidation and ammoxidation mechanisms over bismuth molybdate catalysts. 3. Substituent effects in the selective oxidation of allylbenzenes

Abstract

3,3-Dimethylbicyclo[2.2.2]octa-5,7-dien-2-one (1). Electrolysis of the Diels-Adler adduct of 6,6-dimethylcyclo-hexadienone13 and maleic anhydride by using the procedure reported by Luibrand and Fujinari12 resulted in the bicyclic ketone 1: UV max (95% C2H2OH) 212 nm (e 3700), 230 (sh, 832), 297 (40). Deuterium-labeled ketone 412 was prepared by exchange of hydrogens of the adduct by using deuterium oxide and sodium deuterioxide heated to 150 °C for 24 h in a stainless-steel bomb calorimeter, prior to the electrolysis. The deuterium analysis17 of the resulting dicarboxylic acid is as follows: caled for exchange of two hydrogens, 16.67% excess deuterium; found, 13.17%, corresponding to 79% incorporation of two deuterium atoms. of 3,3-Dimethylbicyclo[2.2.2]octa-5,7dien-2-one (1). A solution of 0.63 mg of ketone 1 in 5 mL of ether in a quartz tube was irradiated with a 450-W, Hanovia, medium-pressure lamp fitted with a Vycor filter. GLC (FFAP) analysis after 75 min revealed that most of the starting material had reacted, and only peaks at very short retention times were present. Direct IrradiationNone of the tricyclic ketone 6 formed.Photorearrangement of 3,3-Dimethylbicyclo[2.2.2]octa-5,7-dien-2-one (1). A solution of 0.49 g (3.3 mmol) of 3,3-dimethylbicyclo[2.2.2]octadien-2-one in 36 mL of acetone was placed in a quartz tube and degassed by bubbling nitrogen through the solution for 1 h. The tube was irradiated at room temperature with a 450-W, Hanovia, medium-pressure lamp fitted with a Vycor filter. Aliquots were analyzed by GLC (FFAP or di-n-decyl phthalate) and indicated the formation of a number of peaks with short retention times (one of which corresponded to the retention time of benzene), a primary photoproduct (6), and an unidentified product which formed upon continued irradiation of 6. GLC retention times (di-n-decyl phthalate, 0.64 cm X 3 m, 120 °C, He flow rate 60 mL/min) were as follows: starting ketone 1, 35 min; unidentified secondary photoproduct, 33 min; tricyclic ketone 6, 59 min. (A control experiment in which pure 6 was irradiated under the same conditions confirmed that the unidentified product was formed from 6.) After 16 h of irradiation, the solution was concentrated, and the products were isolated by preparative GLC (FFAP, 0.96 cm X 6 m, 150 °C, He flow rate 250 mL/min). Some starting ketone 1 (6.3 mg, retention time 43 min) was recovered, in addition to 48 mg (10%) of 4,4-dimethyltricyclo[3.3.0.02,8]oct-6-en-3-one (6, retention time 106 min) as a colorless liquid: IR (neat) 3030 (w), 2940 (m), 2905 (m), 1620 (s, carbonyl), 1590 (w), 1465 (m), 1376 (m), 1359 (m), 1342 (m), 1299 (m), 1087 (m), 893 (m), 747 (m) cm"1; NMR, see text and structure 6 for results; mass spectrum, m/e 148, 133, 70, 42; UV max (95% C2H6OH) 202 nm (e 7300), 233 (sh, t 1500). Anal. Caled for C10H12O: C, 81.04; , 8.16.Found: C, 80.82; H, 7.94. Purification of deuterated 6 from irradiation of 4 was carried out using a di-n-decyl phthalate GLC column. NMR results are shown in Table I.

A DFT Investigation of the Mechanism of Propene Ammoxidation over α-Bismuth Molybdate

“…3 that the transition-state geometry is very similar to the final-state geometry, with the molybdenum-oxygen bond highly elongated, the oxygen-hydrogen bond almost completely formed, the carbon-hydrogen bond already completely broken and the two carbon-carbon bonds of almost equal length. Consistent with the DFT-calculated final state 2, experimental studies have shown that acrolein produced from 1-13 C-propene 19 contains a 50/50 mixture of 13 C label in both terminal C positions of the product, 12 demonstrating that a free allyl species is produced is produced as an intermediate that can…”

The mechanism and kinetics of propene ammoxidation over α-bismuth molybdate

“…Further experiments with isotopically labeled carbon demonstrate that this first step generates a physisorbed, symmetric allyl [17,18], most likely with radical character [19]. 18 O 2 labeling studies have shown that the O in the product acrolein comes from the catalyst surface via a Marsvan Krevelen mechanism [20,21].…”

Identifying the Unique Properties of α-Bi2Mo3O12 for the Activation of Propene

“…6 , 7 , 8 , 9 Both experimental and theoretical studies have identified this step to be the abstraction of a hydrogen atom from the terminal methyl group to produce a physisorbed allyl radical. 10,11,12,13,14 Isotopic labeling studies have shown that the oxygen atoms involved in propene oxidation come from the catalyst surface and not from adsorbed O2, which only serves to reoxidize the catalyst. 15,16, The elementary steps by which allyl radicals are converted to acrolein or acrylonitrile have been the subject of a number of studies.…”

“…The best single-phase catalyst for propene oxidation and ammoxidation is considered to be alpha bismuth molybdate, α-Bi 2 Mo 3 O 12 , though some researchers have suggested that other bismuth molybdate phases, such as β-Bi 2 Mo 2 O 9 and γ-Bi 2 MoO 6 , exhibit similar activities. − Mechanistic studies of propene oxidation and ammoxidation have therefore focused on the three primary bismuth molybdate phases, with particular emphasis on the α-Bi 2 Mo 3 O 12 phase. The activation energy and partial pressure dependencies for both reactions are the same, leading to the conclusion that the rate-limiting step is the same. − Both experimental and theoretical studies have identified this step to be the abstraction of a hydrogen atom from the terminal methyl group to produce a physisorbed allyl radical. − Isotopic labeling studies have shown that the oxygen atoms involved in propene oxidation come from the catalyst surface and not from adsorbed O 2 , which only serves to reoxidize the catalyst. , The elementary steps by which allyl radicals are converted to acrolein or acrylonitrile have been the subject of a number of studies. ,− …”

“…Considerable effort has been devoted to identifying the characteristics of a mixed metal oxide that make it active for propene oxidation and ammoxidation. While much is known about the structure and electronic properties of bismuth molybdates, the reason why Bi 2 Mo 3 O 12 is particularly active remains a subject of discussion. ,− The crystal structure of Bi 2 Mo 3 O 12 is related to that of the tetragonal mineral scheelite (CaWO 4 ) but with a monoclinic distortion due to the vacancies that result from the replacement of Ca 2+ with Bi 3+ (Bi 2/3 φ 1/3 MoO 4 , where φ is a vacancy) . The flexibility of the scheelite structure enables the synthesis of many single-phase, solid-solution tri- and tetra-metal oxides, and has therefore formed the basis for much of this work .…”