Stepwise additions

Historically, the discovery of one effective herbicide has led quickly to the preparation and screening of a family of imitative chemicals (3). Herbicide developers have traditionally used combinations of experience, art-based approaches, and intuitive appHcations of classical stmcture—activity relationships to imitate, increase, or make more selective the activity of the parent compound. This trial-and-error process depends on the costs and availabiUties of appropriate starting materials, ease of synthesis of usually inactive intermediates, and alterations of parent compound chemical properties by stepwise addition of substituents that have been effective in the development of other pesticides, eg, halogens or substituted amino groups. The reason a particular imitative compound works is seldom understood, and other pesticidal appHcations are not readily predictable. Novices in this traditional, quite random, process requite several years of training and experience in order to function productively. 

Novolak Resins. In a conventional novolak process, molten phenol is placed into the reactor, foHowed by a precise amount of acid catalyst. The formaldehyde solution is added at a temperature near 90°C and a formaldehyde-to-phenol molar ratio of 0.75 1 to 0.85 1. For safety reasons, slow continuous or stepwise addition of formaldehyde is preferred over adding the entire charge at once. Reaction enthalpy has been reported to be above 80 kj /mol (19 kcal/mol) (29,30). The heat of reaction is removed by refluxing the water combined with the formaldehyde or by using a small amount of a volatile solvent such as toluene. Toluene and xylene are used for azeotropic distillation. FoHowing decantation, the toluene or xylene is returned to the reactor. 

Another subclass of substituted amides that is of great commercial value is the ethoxylated amides. They can be synthesized from alkanolamides by chain extending with ethylene or propylene oxide or by ethoxylation directly from the primary amide (46—48). It was originally beheved that the stepwise addition of ethylene oxide (EO) would produce the monoethano1 amide and then the diethanolamide when sufficient ethylene oxide was added (49), but it has been discovered that only one hydrogen of the amide is substituted with ethylene oxide (50—53). As is typical of most ethylene oxide adducts, a wide distribution of polyethylene oxide chain length is seen as more EO is added. A catalyst is necessary to add ethylene oxide or propylene oxide to a primary or an ethoxylated amide or to ethoxylate a diethoxy alkanolamide synthesized from diethanolamine (54). 

Higher ethyl chloride efficiency is claimed for a process utilising a hydrocarbon diluent coupled with stepwise addition of sodium hydroxide (80). Product work-up includes distillation to remove residual unreacted ethyl chloride, added diluent, methanol, and diethyl ether neutralization of excess sodium hydroxide washing in water to remove salts drying and grinding. 

In stepwise additions, ketenes are usually the nucleophilic component, so that such additions can be catalyzed by Lewis acids, such as the additions of trimethylsilylketenes to aldehydes, catalyzed by BF3 (Scheme 14) (79JOC733). However, the roles can be reversed, such as in the addition of chlorocyanoketene to benzaldehyde (79JA5435). 

Polymerization methods [I], [II], and [III] (Fig. 1) indicate, respectively, the dropwise addition of VAc and initiator the dropwise addition of VAc and the stepwise addition of initiator the batch method, in which all ingredients of water, VAc, PVA, and initiator were put into the reaction vessel before starting polymerization. In method [I], when the temperature of the PVA solution in the flask attained 70°C, dropwise additions of 20 g of an aqueous solution containing initiator and 250 g of VAc were started. In method [II], the process was similar to method [I], except the initiator was added stepwise. When the temperature of the contents in the flask was raised to 70°C, 24 g of an aqueous solution containing half the prescribed amount of initiator was first added. 

The oxygen level in primary tantalum powder can be also increased by adjustment of the reduction process parameters [594] or by controlled stepwise additions of sodium to the reactor [595]. 

The synthesis of aziridines through reactions between nitrenes or nitrenoids and alkenes involves the simultaneous (though often asynchronous vide supra) formation of two new C-N bonds. The most obvious other alternative synthetic analysis would be simultaneous formation of one C-N bond and one C-C bond (Scheme 4.26). Thus, reactions between carbenes or carbene equivalents and imines comprise an increasingly useful method for aziridination. In addition to carbenes and carbenoids, ylides have also been used to effect aziridinations of imines in all classes of this reaction type the mechanism frequently involves a stepwise, addition-elimination process, rather than a synchronous bond-forming event. 

Difficulties due to side reactions (cyclization) and a broad molecular weight distribution accompanying the polycondensation of active esters led to the application of methods wherein the polymers are built up stepwise. In 1968, Sakakibara et al.31) introduced the solid-phase technique using Merrifield s resin. By stepwise addition of tert-pentyloxycar-bonyl tripeptides, they have synthesized (Pro-Pro-Gly)n with n = 5, 10, 15 and 20. 

Although the nucleophilic addition of secondary amines to thiirene dioxides can be interpreted as following the same mechanistic pathway, the reaction was found to be second order in amine119 (which is typical for the addition of amines to olefins in appropriate solvents13 2 133), and the addition is syn. As a result, mechanisms with a cyclic-concerted addition across the carbon-carbon bond, or a stepwise addition involving two molecules of amine per one molecule of thiirene dioxide, have been proposed. 

If processing occurs down to the core heptasaccharide (ManjiGIcNAclj), complex chains are synthesized by the addition of GIcNAc, removal of two Man, and the stepwise addition of individual sugars in reactions catalyzed by specific transferases (eg, GIcNAc, Gal, NeuAc transferases) that employ appropriate nucleotide sugars. 

Stepwise addition of the aniline moieties in analogy to Scheme 7 (Route A) allows the preparation of asymmetrically substituted derivatives.55,56 The use of pyridine-3,4-dicarboxylic acid anhydride similarly results in the formation of mixtures of 5- and 6-azaphthalides.57 Quinoline-2,3-dicarbo-xylic anhydride has also been converted into the corresponding azaphthal-ides in a similar manner.58 Pyrazine-2,3-dicarboxylic acid anhydride yields... 

A typical result obtained during the reduction by H2 at 350°C of NO stored up to saturation at the same temperature over the Pt-Ba/ y-Al203 (1/20/100 w/w) catalyst is presented in Figure 6.10. Upon the stepwise addition of 2000 ppm of H2 at t = 0 s the stored NO was reduced to N2. Indeed H2 was completely consumed while the N2 outlet concentration increased immediately to the level of 360 ppm and then it kept almost constant. Accordingly, at the beginning the reaction is very fast and selective to nitrogen, and limited by the concentration of H2. Small amounts of NO were also observed, immediately after the H2 stepwise addition. 

The reduction of NO also produced water, which however did not desorb immediately, showing a delay of about 50 s due to adsorption onto the catalyst and most likely onto Ba sites to form Ba(OH)2. The stepwise addition of hydrogen to the reactor was accompanied by a small increase of the catalyst temperature (3-5°C), due to the occurrence of the exothermic reduction, so that the run was actually performed in the absence of significant thermal effects. The following main reactions were thus likely involved in the reduction of stored NO by H2 ... 

The compounds [Au3Ira(MeN=COMe)3] ( = 2, 4, 6) obtained by stepwise addition of iodine to the cyclic tetranuclear gold(I) complexes [Au3(MeN=COMe)3] have been revisited. The structure of [Au3I6(MeN=COMe)3] (338) consists of columns of the molecular units united by weak iodine- iodine interactions which range in length from 3.636(2) A to 3.716(2) A. The solid-state molecular packing of the di-iodo and tetra-iodo complexes reveal no unusual features.1971... 

Methylene insertion into C—H bonds is believed to be concerted for the singlet species and stepwise for the triplet.<164,156) The C—H insertion of methylene into the 14C-labeled isobutylene shown below results in 92% unrearranged isopentenes and 8% rearranged isopentene [Eq. (11.22)]. Assuming that an additional 8% of the unrearranged isopentene arises from the stepwise addition, it is clear that 84% of the insertion products result from insertion by singlet methylene and 16% by triplet methylene ... 

FIGURE 8.10 H2S consumption in rat aorta smooth muscle cells (RASMCs). Accumulated data from several experiments showing RASMC H2S consumption rates (filled circles and squares) as a function of H2S concentration, compared to H2S oxidation rates in solution without cells (open circles and squares). Heat-inactivated RASMC H2S consumption rates (open plus symbols) were equivalent to background rates without cells. Inset Representative PHSS traces showing stepwise additions of Na2S stock, at arrows, in the presence (thin line) and absence (thick line) of RASMCs (after [41]). 

Parallel to the NMR study we tried to determine the number of water molecules that coordinate to Li+ by DFT calculations by adding stepwise additional water molecules. In the gas phase the addition of a further water molecule is in all cases exothermic up to [Li(H20)6]+ and the resulting structures are also local minima (see Fig. 1). 

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