REACTANTS AND REACTION CONDITIONS

In addition to the catalytic allylation of carbon nucleophiles, several other catalytic transformations of allylic compounds are known as illustrated. Sometimes these reactions are competitive with each other, and the chemo-selectivity depends on reactants and reaction conditions. 

Figure 7 illustrates the use of HPGPC to aid a resin chemist in developing an in-house isocyanate crosslinker for a powder coating system. Isocyanate crosslinker X-02 gave desired properties and is considered the standard. At the early stage of the development, resin X-03 was initially made. By changing the types of reactants, molar ratio of reactants and reaction conditions, resin X-36 was the next iteration in the resin synthesis process. Finally, X-36 was fine-tuned to produce X-38 which matched X-02 in both its chemical reaction properties and its MWD. 

Intramolecular alkylation of enolates can be used to synthesize bi- and tricyclic compounds. Identify all the bonds in the following compounds that could be formed by intramolecular enolate alkylation. Select the one that you think is most likely to succeed and suggest reasonable reactants and reaction conditions for cyclization. 

The following molecules have been synthesized by radical cyclization and tandem radical cyclizations. Identify the bond or bonds that could be formed by radical cyclizations and suggest an appropriate reactant and reaction conditions that would lead to the specified products. 

In 1978 we described a synthetic methodology as a repeating-step principle , which led us to the first cascade molecules , today known as dendritic molecules [17]. We recognized then that a synthetic pathway, which allows consecutive repetition, implies the advantage of likewise reactants and reaction conditions and is suited for the building of more or less structure perfect highly branched molecules, particularly of polyamines (Fig. 4). 

Crosslinking of polyurethanes proceeds in different ways depending on the stoichiometry and choice of reactants and reaction conditions. For example, an isocyanate-terminated trifunctional prepolymer is prepared by reaction of a polyol and... 

Density functional theory calculations (B3LYP/6-31G level) have provided an explanation for the stereodivergent outcome of the Staudinger reaction between acyl chlorides and imines to form 2-azetidinones (/3-lactams). When ketene is formed prior to cycloaddition, preferential or exclusive formation of ct5-j6-lactam (50) is predicted. If, however, the imine reacts directly with the acid chloride, the step that determines the stereochemical outcome is an intramolecular 5n2 displacement, and preferential or exclusive formation of trans isomer (51) is predicted. These predictions agree well with the experimental evidence regarding the stereochemical outcome for various reactants and reaction conditions. 

Another group of synthetically useful reductions employs a metal as the reducing agent. The organic substrate under these conditions accepts one or more electrons from the metal. The subsequent course of the reaction depends on the structure of the reactant and reaction conditions. Three broad classes of reactions can be recognized, and these will be discussed separately. These include reactions in which the overall change involves (a) net... 

REACTANTS AND REACTION CONDITIONS Inhibitors and Their Removal... 

A wide variety of reactants and reaction conditions can be used to prepare chromones via a Claisen condensation and the review by Ellis contains a comprehensive list of examples <77HC(31)496). 

Nucleophilic attack is faster at unsubstituted ring positions than at similarly activated but substituted ring positions.8-15-43 5 Since the addition is in most cases reversible, the opportunity exists for competing reactions. Indeed an extremely varied spectrum of reactivities is found in these systems, depending on reactants and reaction conditions. Examples are known of competing SNAr and cine substitution,46 NAr and tele substitution,47 SnAt and cine and tele substitution (equation l),5 SsAr and VNS,16 and SnAt and Sn(ANRORC).48... 

The reaction of 5-amino-l,2,4-thiadiazoles (16) with aldehydes produces a-amino alcohols (145), azomethines (146) or bis-amines (147) depending on the reactants and reaction conditions, as illustrated in Scheme 58. The nitration of 5-amino-3-methyl-l,2,4-thiadiazole (25) in a mixture of 98% sulfuric acid and 95% nitric acid at 0°C is reported to produce (148) in good yield (65AHC(5)119), but a similar reaction with 3-amino-l,2,4-thiadiazoles has not been described. Products of type (149) can be obtained, however, by the ring closure of Af-nitro-AT-thiocarbamylguanidines with alkaline hydrogen peroxide. 

It was shown in the laboratory of P.T. Kaye that the reactions of 2-hydroxybenzaldehydes and 2-hydroxy-1-naphthaldehydes with various activated aikenes proceeded with regioseiective cyclization under Baylis-Hillman conditions to afford the corresponding 3-substituted 2H-chromene derivatives in high yields. Previous attempts to prepare 2H-chromenes chemoselectively via the cyclization of 2-hydroxybenzaldehyde-derived Baylis-Hillman products had proven unsuccessful. Complex mixtures containing coumarin and chromene derivatives were obtained. Good results were observed after the careful and systematic study of the various reactants and reaction conditions. 

When substitution or elimination reactions are used in synthesis, care must be taken to choose reactants and reaction conditions that will maximize the yield of the desired product. In Section 10.4, you saw that nucleophilic substitution reactions of alkyl halides can lead to a wide variety of orgaiuc compounds. For example, ethers are synthesized by the reaction of an alkyl halide with an alkoxide ion. This reaction, discovered by Alexander Williamson in 1850, is still considered one of the best ways to synthesize an ether. 

Factors Affecting PEO Agglomeration. By using Equation 3, it is easily possible within a relatively short time to study the effects of the many factors which usually play a part in polymer-colloid reactions and which have to be considered in PEO agglomeration, too. These factors are tabulated in Table I according to reactants and reaction conditions. 

The second part deals with how polymers are prepared from monomers and the transformation of polymers into useful everyday articles. It starts with a discussion of the various polymer preparation methods with emphasis on reaction mechanisms and kinetics. The control of molecular weight through appropriate manipulation of the stoichiometry of reactants and reaction conditions is consistently emphasized. This section continues with a discussion of polymer reaction engineering. Emphasis is on the selection of the appropriate polymerization process and reactor to obtain optimal polymer properties. The section terminates with a discussion of polymer additives and reinforcements and the various unit operations in polymer processing. Here again, the primary focus is on how processing conditions affect the properties of the part produced. 

Ease of Use and User Friendliness. CAMEO was found to be easy to use. CAMEO S menu screens are well designed and easy to follow, and greatly facilitate use and operation of the program, particularly structure entry. To explore fully all potential reactions and the products of these reactions, however, the user must separately evaluate a given set of starting materials and reaction conditions under several if not all of the available reaction modules (i.e., Carbenoid, Radical, Heterocyclic, Basic/Nucleophilic, Acidic/Electrophilic, Electrophilic Aromatic, Oxidative/Reductive, and Pericyclic). Thus, the user can enter reactants and reaction conditions, and, depending upon which module is selected, CAMEO may predict different results. For example, CAMEO correctly predicted carbaryl as the product from the reaction of methyl isocyanate with 1-naphthol only if the Acidic/Electrophilic mechanistic module was selected no product was predicted when the Basic/Nucleophilic module was selected. 1-Naphthol is clearly the nucleophile in this reaction, and it seems that CAMEO should have recognized it as such. 

The following arrangements are possible, depending on the reactants and reaction conditions ... 

In complex reaction systems, several stable products are produced by more than one reaction, and some of the products are not desirable. Selectivity is a measure of the extent to which the catalyst accelerates the formation of desired product(s) and is usually a function of the degree of conversion of reactant and reaction conditions, particularly temperature. A number of different definitions of selectivity are used according to purpose. 

The reaction of 2-butanone with LiNMeg in THF at -100°C generates the enolate anion in 10 minutes. Identify the reaction conditions as kinetic or thermodynamic control and briefly discuss how each reactant and reaction condition influences this. 

However, the second mechanism alone may account for all the experimental observations if appropriate assumptions are made concerning the rates of the two steps in this sequence (757). The stereochemistry of the products in the triphenylphosphine-induced carbonylation is also strongly dependent on reactants and reaction conditions. Triphenylphosphine and -arsine gives largely initial products but some amines give exclusively cis products (75J). 

When substitution or elimination reactions are used in synthesis, care must be taken to choose reactants and reaction conditions that will maximize the yield of the desired product. Thus, Sn2 and E2 reactions are preferred. 

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