Modified reaction

Anilines react with ct-haloacetophenones to give 2-arylindoles. In a typical procedure an W-phenacylaniline is heated with a tw o-fold excess of the aniline hydrobromide to 200-250°C[1]. The mechanism of the reaction was the subject of considerable investigation in the 1940s[2]. A crucial aspect of the reaction seems to be the formation of an imine of the acetophenone which can isomerize to an aldimine intermediate. This intermediate apparently undergoes cyclization more rapidly (path bl -> b2) than its precursor (Scheme 7.3). Only with very reactive rings, e.g, 3,5-dimethoxyaniline, has the alternative cydiz-ation (path al a2) to a 3-arylindole been observed and then only under modified reaction conditions[3],... 

The reaction is of wide scope (R = alkyl, aryl) however the substrate molecule should not contain other functional groups that can react with diazomethane. With unsaturated acyl halides the yield can be poor, but may be improved by modified reaction conditions. ... 

The Hantsch pyridine synthesis provides the final step in the preparation of all dihydrop-yridines. This reaction consists in essence in the condensation of an aromatic aldehyde with an excess of an acetoacetate ester and ammonia. Tlie need to produce unsymmetrically subsrituted dihydropyridines led to the development of modifications on the synthesis. (The chirality in unsymmetrical compounds leads to marked enhancement in potency.) Methyl acetoacetate foniis an aldol product (30) with aldehyde 29 conjugate addition of ethyl acetoacetate would complete assembly of the carbon skeleton. Ammonia would provide the heterocyclic atom. Thus, application of this modified reaction affords the mixed diester felodipine 31 [8]. 

Referring to Fig. 9, the effect of the shear is to catalyze the reaction, presumably through suppression of the interfacial barrier by stretching the flow. The latter is believed to reduce the diffusion path, promoting the reaction rate, and hence the rate of increase in the viscosity. A similar effect is produced with temperature as a parameter, which also augments the reaction rate. The modified reaction rate constant in case of any external stimulus or perturbation acting on the system may be computed from the scalar K, where ... 

The story of the ozone hole illustrates how important it is to learn the molecular details of chemical reactions. Some chemists use information about how reactions occur to design and synthesize useful new compounds. Others explore how to modify reaction conditions to minimize the cost of producing industrial chemicals. This chapter explores how chemical reactions occur at the molecular level. We show how to describe a reaction from the molecular perspective, introduce the basic principles that govern these processes, and describe some experimental methods used to study chemical reactions. 

In acetic acid media in the presence of 0.5 M HCIO4, a modified reaction... 

Scheme 2.20 gives some examples of the application of the Julia olefination in synthesis. Entry 1 demonstrates the reductive elimination conditions. This reaction gave a good E.Z ratio under the conditions shown. Entry 2 is an example of the use of the modified reaction that gave a good E.Z ratio in the synthesis of vinyl chlorides. Entry 3 uses the tetrazole version of the reaction in the synthesis of a long-chain ester. Entries 4 to 7 illustrate the use of modified conditions for the synthesis of polyfunctional molecules. 

Several modified reaction conditions have been developed. One involves addition of silver salts, which activate the halide toward displacement.134 Use of sodium bicarbonate or sodium carbonate in the presence of a phase transfer catalyst permits Reactions involving especially mild conditions to be used for many systems.135 Tetraalkylammonium salts... 

Modified Reaction Time (t) (h kg/kmol) Partial Conversion (y) ... 

Table IV lists the mechanical stress-strain data for a series of hybrid TEOS-PTMO materials containing different levels of Ti-isop in the starting reaction mixture. These materials, with the exception of the first one, were all made using a modified reaction scheme (see experimental section) in order to incorporate the titanium into the network. The starting reaction mixtures in all cases contained 50% by weight of the glass precursors (TEOS and Ti-isop) and 50% by weight of PTMO(2000) (endcapped with triethoxysilane). One set of samples without titanium was made in order to compare the effects of the reaction scheme on the observed mechanical properties.

The formation of protonated H+(H20)n species can affect the acidity of the non-solvated protonic sites. Therefore, as the acid strength of the protonic sites in zeolites plays a key role in the hydrocarbon transformation reactions, driving the rate of the hydrocarbon protonation [4-6], the presence of water vapor among the reactants can modify reaction rates of the individual reactions involving in the hydrocarbon transformations. 

A prototype of a microwave reaction vessel that takes advantage of bottom filtration techniques was presented by Erdelyi and Gogoll in a more recent publication. Therein, the authors described the use of a modified reaction vessel (Fig. 7.2) for the Emrys instruments (see Section 3.5.1) with a polypropylene frit, suitable for the filtration /cleavage steps in their microwave-mediated solid-phase Sonogashira coupling (see Scheme 7.19) [21]. 

A modified reaction mechanism to the one suggested by Hurd and Mori is proposed for the preparation of some thieno[2,3- 7][l,2,3]thiadiazole derivatives in order to explain the formation of a chlorinated by-product <1998J(P1)853>. 

The oxaspirocyclization was applied to the synthesis of theaspirone and vitispirane (equations 26 and 27)59. Under slightly modified reaction conditions where water is employed as the major solvent, palladium-catalyzed 1,4-oxidation of 64 afforded 65. Alcohol 65 was oxidized to theaspirone, which was obtained as a 1 1 isomeric mixture of cis and trans isomers. When the analogous reaction was performed at a lower pH by the use of trifluoroacetic acid, vitispirane was formed in high yield, again as a 1 1 isomeric mixture of stereoisomers. 

A slightly different catalytic system was reported by Lau and collaborators [14], These authors used Ru(t 5 T l-C5H4-(CH2)3NMe2)(dppm) (dppm = bis(diphenylphosphino)methane) as a catalyst and proposed a modified reaction mechanism where H2 undergoes heterolytic activation between the Ru center and the NMe2 group in the ligand. 

A number of modified reaction conditions have been developed. One involves addition of silver salts, which activate the halide toward displacement.94 Use of sodium bicarbonate or sodium carbonate in the presence of a phase-transfer catalyst permits especially mild conditions to be used for many systems.95 Tetraalkylammonium salts often accelerate reaction.96 Solid-phase catalysts in which the palladium is complexed by polymer-bound phosphine groups have also been developed.97 Aryl chlorides are not very reactive under normal Heck reaction conditions, but reaction can be achieved by inclusion of triphenylphosphonium salts with Pd(OAc)2 or PdCl2 as the catalyst.98... 

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