Olefins, activated acrolein

Scheme 21.1 Heck arylation of acrolein and acrolein diethylacetal. was the most important parameter among all those evaluated (i.e. KCl, solvent...) that affect the selectivity (Scheme 21.1, Route 3). However, moderate activity and selectivity were achieved when using the 9-bromoanthracene whatever the olefin (8). This was attributed to the large steric hindrance of this substrate.

The P-alkoxy elimination pathway is important during the incorporation of oxygen-containing monomers. Therefore, it is often necessary to provide distance between the olefin and the polar group, or to prevent chain walking close to the group that can be eliminated by the placement of a quaternary carbon spacer [87], The incorporation of acrolein dimethyl acetal is accompanied by reduced activity and full catalyst... 

The investigation of the mechanism of olefin oxidation over oxide catalysts has paralleled catalyst development work, but with somewhat less success. Despite extensive efforts in this area which have been recently reviewed by several authors (9-13), there continues to be a good deal of uncertainty concerning the structure of the reactive intermediates, the nature of the active sites, and the relationship of catalyst structure with catalytic activity and selectivity. Some of this uncertainty is due to the fact that comparisons between various studies are frequently difficult to make because of the use of ill-defined catalysts or different catalytic systems, different reaction conditions, or different reactor designs. Thus, rather than reviewing the broader area of selective oxidation of hydrocarbons, this review will attempt to focus on a single aspect of selective hydrocarbon oxidation, the selective oxidation of propylene to acrolein, with the following questions in mind ... 

Grubbs first generation metathesis catalyst 145 was found to be an active catalyst for the Kharasch addition, provided its metathesis activity for the chosen olefin is low [206]. Snapper and coworkers found at the same time that the Grubbs I olefin metathesis catalyst is efficient for the Kharasch addition of less activated halides, such as CHC13, 1,1,1-trichloroethane, or ethyl trichloroacetate, to olefins like styrenes, acrylates, or acrolein [207, 208]. 

The Baylis-Hillman reaction has become a very powerful carbon-carbon bond forming reaction in the past 20 years. A typical reaction involves an activated olefin (i.e., an acrylate) and an aldehyde in the presence of a tertiary amine such as diazobicyclo-[2.2.2]octane (DABCO) to form an a-meihylhydroxyacrylale. A host of activated olefins have been utilized including acrylates, acroleins, a, 3-unsaturated ketones, vinylsulfones, vinylphosphonates, vinyl nitriles, etc. The Baylis-Hillman has been successfully applied inter- and intramolecularly. In addition, there are numerous examples of asymmetric Baylis-Hilhnan reactions. Reviews (a) Ciganek, E. Org. React. 1997, 51, 201-478. (b) Basavaiah, D. Rao, P. D. Hyma, R. S. Tetrahedron 1996, 52, 8001-8062. (c) Fort, Y. Berthe, M. C. Caubere, P. Tetrahedron 1992, 48, 6371-6384. 

In allylic oxidation, an olefin (usually propylene) is activated by the abstraction of a hydrogen a to the double bond to produce an allylic intermediate in the rate-determining step (Scheme 1). This intermediate can be intercepted by catalyst lattice oxygen to form acrolein or acrylic acid, lattice oxygen in the presence of ammonia to form acrylonitrile, HX to form an allyl-substituted olefin, or it can dimerize to form 1,5-hexadiene. If an olefin containing a jS-hydrogen is used, loss of H from the allylic intermediate occurs faster than O insertion, to form a diene with the same number of carbons. For example, butadiene is fonned from butene. 

The data in Table 1 summarize catalytic activities for epoxidation of a variety of olefins over an unpromoted 5%Ag/Al203 catalyst. These data illustrate the preferential reactivity at the allylic position relative to addition of oxygen across the C=C bond. While the selectivity to ethylene oxide is typical for an unpromoted catalyst, the selectivities to propylene oxide and butylene oxides are non-existent for propylene, 1-butene, and 2-butene, respectively. In addition to small amounts of the selective allylic oxidation products (acrolein in the case of propylene and butadiene in the case of 1-butene), the only products are those of combustion. However, the results for butadiene reveal it is possible to epoxidize this non-allylic olefin at moderate selectivity and activity. What is not obvious from Table 1 is the short-lived nature of this activity. After 2-3 hours of reaction time, activity and selectivity typically decreased to approximately <1% conversion of C4H6 and approximately 50-75% selectivity to epoxybutene. A typical chromatogram of the activity of an... 

Moiseev and coworkers showed [10,13] that giant palladium clusters with an idealized formula Pd56iL5o(OAc)igo (L = phenanthroline or bipyridine) are highly active catalysts for allylic oxidation of olefins. The catalytically active solution was prepared by reduction of Pd(OAc)2, e. g. with H2, in the presence of the ligand, L, followed by oxidation with O2. The giant palladium cluster catalyzed the oxidation of propylene to allyl acetate under mild conditions. Even in 10% aqueous acetic acid, allyl acetate selectivity was 95-98 % [10]. Oxidation catalyzed by Pd-561 in water afforded a mixture of allylic alcohol (14%), acrolein (2%), and acrylic acid (60%), and only 5% acetone [10]. 

Addition at activated olefinic groups (e.g. /3,y-unsatu-rated carbonyls) are quite varied. A typical substrate is acrolein (CH — CH — CHO). Quinones (ortho md para)... 

Many acids appear as minor by-products when the higher olefins are oxidized. Selective oxidations are rare. Ethylene can be oxidized rather weU to acetic acid. For example, Gurdjian 155) reported 74% selectivity to acetic acid at 39 % conversion of ethylene over phosphomolybdic acid on silica at 290°. Propylene or acrolein can be oxidized to acrylic acid with quite good selectivity, as is shown in a number of patents. Generally molybdenum catalysts are used, moderated with P, B, Bi, Te, or As. With a highly active catalyst composed of vanadium and molybdenum oxides, propylene is oxidized mainly to acetic acid, according to Aliev and co-workers 165). 

Acrolein rarely features as the activated olefins in the MBH reaction with aldehydes, clearly because of its propensity to form oligomers or polymers under the basic catalysts employed. However, this hurdle has been surmounted by selection of appropriate reaction conditions. The DABCO-catalyzed addition of acetaldehyde and propionaldehyde with acrolein proceeded in good yields under a low catalyst concentration, perhaps to minimize the polymerization of acrolein. It is also the exclusive path in the attempted addition of 2-pyridinecarboxaldehyde" and a-diketones to acrolein. In addition, successful MBH reactions of acrolein with aldehydes have been reported xmder high pressure, which affected a dramatic acceleration in the rate of MBH reaction." More reactive electrophiles, including halo ketones, fluoro-carbonyls and activated imines,all reacted very rapidly with acrolein. 

The MBH reactions of non-enolizable a-diketones precursors (Figure 1.4) with the activated olefins, e.g. acrolein, methyl acrylate and acrylonitrile, have been investigated systematically. The reaction of 3,3,5,5-tetramethyl-cyclopentane-l,2-dione (162) with acrolein and acrylonitrile, but not methyl acrylate, afforded the mono-a-hydroxyalkylation products in high yields. Other nonenolizable a-diketones, such as camphorquinone (159), homo-adamantane-2,3-dione (160) and bicyclo[ 3.3.2]decane-9,10-dione (161) reacted only with acrylonitrile, probably due to the hindered nature of the a-dicarbonyl compounds and the difference in steric demand between nitrile and ester. 

Because a different stereochemistry for the aza-MBH reaction involving different Michael acceptors was observed, in a continuation of our work, we reinvestigated systematically the reaction of iV-sulfonated imines with various activated olefins, including ethyl vinyl ketone (EVK), acrolein, phenyl acrylate and a-naphthyl acrylate. An interesting inversion of absolute configuration between the adducts derived from MVK or EVK and those from acrolein, methyl acrylate, phenyl acrylate or a-naphthyl acrylate was observed, indicating that the substitution patterns of the olefin may alter or even invert this trend.Similar to the addition to HFIPA, the (3-ICD-mediated addition of methyl, phenyl and naphthyl acrylates 149 to V-sulfonyl imines afforded adducts 150 with an (5) configuration, which is opposite to that observed with aldehydes (Scheme 2.71). ... 

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