Other Substrates

Other Substrates. The popularity of tungsten as a substrate for field emission studies reflects the fact that it is sometimes difficult to achieve and establish the cleanliness of other substrate surfaces. There is, however, an increasing interest in the more easily 

Gadzuk in Structure and Chemistry of Solid Surfaces , ed. G. Somorjai, Wiley, New York, 1969. 

The structure of zinc oxide ensures that the dipolar layer which constitutes the (0001) surface is of opposite polarity to that of the (OOOT) surface. Marien has investigated the effect of this on field emission from field evaporated zinc oxide tips, and by measuring the work function of both orientations has shown that they have different electron affinity lAoooi = 3.35 0.15 eV, i/ oooT = 5.05 0.15 eV. 

Current fluctuations from clean (310), (211), and (100) tungsten surfaces have been studied by Swanson and attributed to the earliest stages in surface self-diffusion in which surface atoms diffuse on the terraces between plane edges. The observed threshold temperatures for noise on (310), (211), and (100) surfaces which are respectively 300 K, 600 K, and 1000 K, serve as a salutary reminder of the ease with which substrate atoms may become mobile and hence involved in adsorbate behaviour. 

Recent advances include alkyl iodides as substrates that can be activated by metal complexation. Also Jt-allyl anions , when co-ordinated to palladium, are activated toward attack by nucleophiles. This is very similar to the activation of co-ordinated alkenes and it shows the very high electrophilicity of palladium. The valence state of palladium, and/or the charge on palladium, and therefore also the ligands attached to it are very important  

There is obviously a relation with the classic activation of molecules by Lewis acids, but here we have confined ourselves to the activation of soft substrates by soft acids. Examples of hard acid activated reactions include Diels-Alder additions, nitrile solvolysis, ester solvolysis, ester formation, Oppenauer reactions etc (see Lewis acid catalysed reactions, 2.11). 

While alcohol oxidations have been the most common metal promoted reactions involving molecular oxygen, a number of other metal catalyzed oxidations of potential synthetic interest have been reported. Supported palladium catalysts are comparable to many soluble palladium catalysts in promoting the selective oxidations of alkenes and aromatics. 2-Butene was oxidized primarily to crotonic acid over Pd/C in water but methyl vinyl ketone and crotonaldehyde were also formed in significant amounts. When this oxidation was run in acetic acid the allyl acetates were the major products, particularly when a Pd/Al203 catalyst 

Ni0-Fe203 catalysts were used for the vapor phase oxidation of benzoic acid to phenol. This reaction took place in a stream of air at 400°C. A catalyst having a Ni Fe atomic ratio of one gave phenol in 88% selectivity at 100% conversion. 0  

Supported platinum catalysts have been used to promote the oxidation of alkenes in water at 180°C and two atmospheres of oxygen to give the 1,2 diols. Oxidation of ethylene gave a mixture of the diol and acetic acid in about a 2 1 ratio. Oxidation of propene gave the diol as the predominant product with some acetone also produced. When a small amount of carbon monoxide was added to 

Treating alkenes with air in the presence of iso-butraldehyde and a clay catalyst that was impregnated with nickel acetonylacetate gave good yields of the corresponding epoxides (Eqn. 21.39). The reaction of oxygen with the aldehyde gave the hydroperoxide which, in turn, was used to convert the alkene to the epoxide over this clayniac catalyst. 

Toluene was oxidized to a mixture of benzyl acetate (31) and benzylidene diacetate (32) on reaction with oxygen in the presence of a silica supported Pd-Sn catalyst.A reaction run in HOAc/KOAc at 70°C under an atmosphere of oxygen gave a near 3 1 ratio of the monoacetate to the diacetate at 98% conversion (Eqn. 21.40). The two products are formed in parallel reactions as the benzylacetate does not react further under these reaction conditions. Substituted diphenylmethanes were oxidized to the diphenyl ketones by refluxing them in air in a DMF solution containing a copper powder catalyst.  

Although Knowles catalyst has been known for 30 years, there are still applications that are coming to light. 

The phosphorus analogs of enamide esters, the dimethyl phosphonates can be reduced by Knowles catalyst to provide the corresponding a-amido phosphonates (Fig. 8) [25]. 

The reduction of itaconic acid derivatives was found to be selective and was used to access compounds under investigation as HIV protease inhibitors (Fig. 9) [26-28], 

Knowles had shown that a-keto esters could be reduced to a-hydroxy esters if the vinyl acetate was employed (Fig. 10) [9]. If the R-group is aryl, the enol ether must have the Z-geometry for reduction to occur [19]. When the substituent R is an alkyl group this requirement is relaxed. Unfortunately, turnover numbers are not high [29]. 

The enol acetates of a-keto acids can be accessed by a variation of the azlactone synthesis. They can also be reduced by Knowles catalyst, but ee values are 87-88% and the substrate to catalysis ratio needed for reduction does not allow this to be an economic approach to a-hydroxy acids although the reaction times are reduced compared with the corresponding carboxylate esters [9, 29]. 

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By changing the enzyme and mediator, the amperometric sensor in Figure 11.39 is easily extended to the analysis of other substrates. Other bioselective materials may be incorporated into amperometric sensors. For example, a CO2 sensor has been developed using an amperometric O2 sensor with a two-layer membrane, one of which contains an immobilized preparation of autotrophic bacteria. As CO2 diffuses through the membranes, it is converted to O2 by the bacteria, increasing the concentration of O2 at the Pt cathode. 

Polyacrylamide, whether charged or not, can be detected by reactions of the amide group (67,68) however, a number of substances can interfere with the determination. If the molecular weight is high enough, flocculation of a standard slurry of clay or other substrate is a sensitive method for detecting low levels of polyacrylamide (69). Once polymers are adsorbed on a surface, many of these methods caimot be used. One exception is the use of a labeled polymer. 

Ethylene vinyl acetate copolymer (EVA) forms a soft, tacky film with good water-vapor barrier but very poor gas-barrier properties. It is widely used as a low temperature initiation and broad-range, heat-sealing medium. The film also serves for lamination to other substrates for heat-sealing purposes. 

At high neutralization levels with alkaH metal ions, many ionomers spontaneously form coUoidal suspensions in water when stirred vigorously at 100—150°C under pressure. Depending on soHds content and acid level, the dispersions range in viscosity from water-like to paste-like. These provide convenient methods for applying thin coatings of ionomers to paper and other substrates. 

Isocyanates are derivatives of isocyanic acid, HN=C=0, ia which alkyl or aryl groups, as weU as a host of other substrates, are direcdy linked to the NCO moiety via the nitrogen atom. StmcturaHy, isocyanates (imides of carbonic acid) are isomeric to cyanates, ROCMSI (nitriles of carbonic acid), and nitrile oxides, RCMSI—>0 (derivatives of carboxyUc acid). 

A review covers the preparation and properties of both MABS and MBS polymers (75). Literature is available on the grafting of methacrylates onto a wide variety of other substrates (76,77). Typical examples include the grafting of methyl methacrylate onto mbbers by a variety of methods chemical (78,79), photochemical (80), radiation (80,81), and mastication (82). Methyl methacrylate has been grafted onto such substrates as cellulose (83), poly(vinyl alcohol) (84), polyester fibers (85), polyethylene (86), poly(styrene) (87), poly(vinyl chloride) (88), and other alkyl methacrylates (89). 

Microwave or radio frequencies above 1 MHz that are appHed to a gas under low pressure produce high energy electrons, which can interact with organic substrates in the vapor and soHd state to produce a wide variety of reactive intermediate species cations, anions, excited states, radicals, and ion radicals. These intermediates can combine or react with other substrates to form cross-linked polymer surfaces and cross-linked coatings or films (22,23,29). 

Low viscosity cellulose acetate is used in lacquers and protective coatings for paper, metal, glass, and other substrates and as an adhesive for cellulose photographic film because of its quick bonding rate and excellent bond peel strength (135) (see Coatings). Heat-sensitive adhesives for textiles have also been prepared from cellulose acetate (136). Extmded cellulose acetate film makes an excellent base for transparent pressure-sensitive tape (137) (see Adhesives). 

Specifications and Standards Test Methods. Hydroxyethylcellulose is included in the Hst of materials that are in compHance with requirements of the U.S. EDA for use in adhesives and in resinous and polymeric coatings employed on the food-contact surfaces of metal, paper, or paperboard articles, and other substrates intended for use in food packaging as specified in CER 21. HEC made dispersible by cross-linking with glyoxal is cleared only as an adhesive and as a component of paper and paperboard in contact with food. It has not been cleared as a direct food additive. 

The contact ends of printed circuit boards are copper. Alloys of nickel and iron are used as substrates in hermetic connectors in which glass (qv) is the dielectric material. Terminals are fabricated from brass or copper from nickel, for high temperature appHcations from aluminum, when aluminum conductors are used and from steel when high strength is required. Because steel has poor corrosion resistance, it is always plated using a protective metal, such as tin (see Tin and tin alloys). Other substrates can be unplated when high contact normal forces, usually more than 5 N, are available to mechanically dismpt insulating oxide films on the surfaces and thereby assure metaUic contact (see Corrosion and corrosion control). 

Adsorption and Desorption Adsorbents may be used to recover solutes from supercritical fluid extracts for example, activated carbon and polymeric sorbents may be used to recover caffeine from CO9. This approach may be used to improve the selectivity of a supercritical fluid extraction process. SCF extraction may be used to regenerate adsorbents such as activated carbon and to remove contaminants from soil. In many cases the chemisorption is sufficiently strong that regeneration with CO9 is limited, even if the pure solute is quite soluble in CO9. In some cases a cosolvent can be added to the SCF to displace the sorbate from the sorbent. Another approach is to use water at elevated or even supercritical temperatures to facilitate desorption. Many of the principles for desorption are also relevant to extraction of substances from other substrates such as natural products and polymers. 

Microorganisms exhibit nutritional preferences. The enzymes for common substrates such as glucose are usually constitutive, as are the enzymes for common or essential metabohc pathways. Furthermore, the synthesis of enzymes for attack on less common substrates such as lactose is repressed by the presence of appreciable amounts of common substrates or metabolites. This is logical for cells to consei ve their resources for enzyme synthesis as long as their usual substrates are readily available. If presented with mixed substrates, those that are in the main metabolic pathways are consumed first, while the other substrates are consumed later after the common substrates are depleted. This results in diauxic behavior. A diauxic growth cui ve exhibits an intermediate growth plateau while the enzymes needed for the uncommon substrates are synthesized (see Fig. 24-2). There may also be preferences for the less common substrates such that a mixture shows a sequence of each being exhausted before the start of metabolism of the next. 

Structural strength. Combined with phenolic resins or epoxies, high shear strength can be obtained in joints produced with aluminium and other substrates. 

Lap shear samples performed according to ASTM-D1002 or D3163. Oily metal was coated with hexadecane. All other substrates were bonded unprepared. 

Extension of this reaction to other substrates, however, revealed that it is more complex, and that side products are formed depending on (1) the nature of the substrate, (2) the reaction conditions, e.g. temperature and solvent,and (3) the method of work-up." Thus, in addition to the desired substitution products, primary and secondary hydroxy steroids generally yield esters and ethers and undergo simple dehydration as well as dehydration accompanied by rearrangement. 

In the last decade two-dimensional (2D) layers at surfaces have become an interesting field of research [13-27]. Many experimental studies of molecular adsorption have been done on metals [28-40], graphite [41-46], and other substrates [47-58]. The adsorbate particles experience intermolecular forces as well as forces due to the surface. The structure of the adsorbate is determined by the interplay of these forces as well as by the coverage (density of the adsorbate) and the temperature and pressure of the system. In consequence a variety of superstructures on the surfaces have been found experimentally [47-58], a typical example being the a/3 x a/3- structure of adsorbates on a graphite structure (see Fig. 1). 

A parameterization of many different surface potentials, ranging from (100) surfaces of FCC crystals to graphite surfaces, has been given by Steele [146-148]. Since most of the systems discussed below are adsorbed layers on graphite surfaces, we consider the graphite substrate in detail. The interaction potential between an adsorbate particle at the position r = (x,y, z) and all other substrate particles consists of two contributions,... 

The reaction involves only one substrate, or if the reaction is multisubstrate, the concentration of only one substrate is varied while the concentration of all other substrates is held constant. 

The transaldolase functions primarily to make a useful glycolytic substrate from the sedoheptulose-7-phosphate produced by the first transketolase reaction. This reaction (Figure 23.35) is quite similar to the aldolase reaction of glycolysis, involving formation of a Schiff base intermediate between the sedohep-tulose-7-phosphate and an active-site lysine residue (Figure 23.36). Elimination of the erythrose-4-phosphate product leaves an enamine of dihydroxyacetone, which remains stable at the active site (without imine hydrolysis) until the other substrate comes into position. Attack of the enamine carbanion at the carbonyl carbon of glyceraldehyde-3-phosphate is followed by hydrolysis of the Schiff base (imine) to yield the product fructose-6-phosphate. 

Danishefsky et al. were probably the first to observe that lanthanide complexes can catalyze the cycloaddition reaction of aldehydes with activated dienes [24]. The reaction of benzaldehyde la with activated conjugated dienes such as 2d was found to be catalyzed by Eu(hfc)3 16 giving up to 58% ee (Scheme 4.16). The ee of the cycloaddition products for other substrates was in the range 20-40% with 1 mol% loading of 16. Catalyst 16 has also been used for diastereoselective cycloaddition reactions using chiral 0-menthoxy-activated dienes derived from (-)-menthol, giving up to 84% de [24b,c] it has also been used for the synthesis of optically pure saccharides. 

Other substrates were tested the results are summarized in Table 5.2. Vinyl ethers (2b-2d) also worked well to afford the corresponding tetrahydroquinoline derivatives (3a-3e) in good to high yields with good to excellent diastereo- and en-antioselectivity (entries 1-10). Use of 10 mol% of the chiral catalyst also gave the adducts in high yields and selectivity (entries 2 and 6). As for additives, 2,6-di-t-bu-... 

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