Bimolecular condensation

Electron-spin resonance (e.s.r.) spectra with characteristic hyperfine structure have been recorded during the initial stages of the Maillard reaction between various sugar and amino compounds. The products responsible for the spectra appear to be IV, Af -disubstituted pyrazine radical cations. The pyrazine derivatives are assumed to be formed by the bimolecular condensation of two- and three-carbon enaminol compo-... 

The reaction of dimethyl 3,6-di-/i< r/-butyl-l,4-dihydropyrrolo[3,2-3]pyrrole-l,4-dicarboxylate 58a with CSI gave anhydride 328 with a trace of the expected 329. The structute of 328 was confirmed by solvolysis giving 330 or 331. On heating 328 in toluene for 1 h, evolution of CO2 occurred to give a bimolecular condensation product 332 (Scheme 32) <1996H(43)2361>. 

In the area of reaction energetics. Baker, Muir, and Andzehn have compared six levels of theory for the enthalpies of forward activation and reaction for 12 organic reactions the unimolecular rearrangements vinyl alcohol -> acetaldehyde, cyclobutene -> s-trans butadiene, s-cis butadiene s-trans butadiene, and cyclopropyl radical allyl radical the unimolecular decompositions tetrazine -> 2HCN -F N2 and trifluoromethanol -> carbonyl difluoride -F HF the bimolecular condensation reactions butadiene -F ethylene -> cyclohexene (the Diels-Alder reaction), methyl radical -F ethylene -> propyl radical, and methyl radical -F formaldehyde -> ethoxyl radical and the bimolecular exchange reactions FO -F H2 FOH -F H, HO -F H2 H2O -F H, and H -F acetylene H2 -F HC2. Their results are summarized in Table 8.3 (Reaction Set 1). One feature noted by these authors is... 

Moreover, the PES helps to define the scope of each elementary reaction. Thus, for instance, a bimolecular condensation that involves the formation of two new bonds between the reacting species may either proceed in a concerted fashion, with only a single predicted TS structure, or it may proceed as a stepwise process with two TS structures the stepwise process is really two elementary reactions - first a condensation and then a unimolecular rearrangement. 

Laser irradiation of isatin (70) gave 71 as a result of bimolecular condensation (78TL3007). The diazine ring of 73 was formed during the cyclocondensation of anthranilic acids with the imidate esters derived from indolinone (72) (81AP271). 

Table 25 Activation Parameters for the Bimolecular Condensation of sal- with dien and the Copper(II) Template Condensation ...

Formation of novel free radical products at an early stage of the Maillard reaction was demonstrated by use of ESR spectrometry. Analyses of the hyperfine structures for various sugar-amino compound systems led to the conclusion that the radical products are N,N -disubstituted pyrazine cation radicals. These new pyrazine derivatives are assumed to be formed by bimolecular condensation of a two-carbon enaminol compound involving the amino reactant residue. The presence of such a two-carbon product in an early stage reaction mixture of sugar with amine was demonstrated by isolation and identification of glyoxal dialkylimine by use of TLC, GLC, NMR, MS and IR. 

Synthesis of Ethers by Alkoxymercuration-Demercuration 636 14-7 Industrial Synthesis Bimolecular Condensation of Alcohols 637 Summary Syntheses of Ethers (Review) 638 14-8 Cleavage of Ethers by HBr and HI 638... 

The least expensive method for synthesizing simple symmetrical ethers is the acid-catalyzed bimolecular condensation (joining of two molecules, often with loss of a small molecule like water), discussed in Section 11-10B. Unimolecular dehydration (to give an alkene) competes with bimolecular condensation. To form an ether, the alcohol must have an unhindered primary alkyl group, and the temperature must not be allowed to rise too high. If the alcohol is hindered or the temperature is too high, the delicate balance between substitution and elimination shifts in favor of elimination, and very little ether is formed. Bimolecular condensation is used in industry to make symmetrical ethers from primary alcohols. Because the condensation is so limited in its scope, it finds little use in the laboratory synthesis of ethers. 

If the conditions are carefully controlled, bimolecular condensation is a cheap synthesis of diethyl ether. In fact, this is the industrial method used to produce millions of gallons of diethyl ether each year. 

Explain why bimolecular condensation is a poor method for making unsymmetrical ethers such as ethyl methyl ether. 

Bimolecular condensation of alcohols industrial synthesis (Sections 11-10B and 14-7)... 

Deleuze-Masquefa et al. [15] showed that the microwave assisted bimolecular condensation of 2-imidazole carboxylic acid, followed by coupling with ortho-fluoroaniline and subsequent substitution on imidazole ring by Suzuki cross-coupling reaction gave the imidazo[l,2-a]quinoxaline (xiii) analogues in good yields. All the synthesized compounds showed high activities when evaluated for antitumor activities. 

The hydrolysis of the lactam represents the initiation reaction providing end groups at which lactam molecules can be added, reaction (117), as well as the amino acid which can enter into the polymer through bimolecular condensation (116). 

The polymerization can proceed both as a stepwise addition (117) and condensation (116). The contribution of the individual processes to the over-all lactam consumption depends on the nature of the monomer and on the reaction conditions. In the polymerization of caprolactam, the prevailing fraction of lactam enters into the polymer through the stepwise addition [1, 215, 231, 233], and only a few percent of monomer units are incorporated into the polymer through hydrolysis (115) and bimolecular condensation (116) [1, 236]. 

The acyl chloride represents an activated growth centre which reacts with the strongly nucleophilic amine group either in the monomolecular process (139), (140) or in the bimolecular condensation reaction... 

It may favour bimolecular condensation reactions which are not acid-catalyzed, but which are accelerated by the proper geometry of sites at which the molecules are adsorbed. This property likely plays an important role in the mechanism of phthalic anhydride formation from w-pentane (19). 

Water can result, for instance, from the formation of bisphenol A and of its derivatives. Indeed bisphenol A could result from the acid catalyzed condensation of two molecules of phenol with one molecule of acetone (reaction 6) resulting from the bimolecular condensation of acetic add. Through this scheme one water molecule per molecule of bisphenol A is formed. 

Thus many heterocycles of various sizes can be formed and the reductive cyclization of polyfunctional nitro compounds offers a useful synthetic tool for the synthesis of complex molecules. When the interacting functions are not located at positions conducive to ring formation, bimolecular condensation or even polymerization may occur. 

By homogeneous reactions, vapour phase sulfuric acid is formed. This vapour condenses in the air by bimolecular condensation (H20 molecules also take part in the phase transition). The rate of this process depends, among other things, on the concentration of H 20 and H 2S04 molecules as well as on the temperature (Kiang et al., 1973). In the presence of suitable aerosol particles the condensation is heterogeneous, that is vapour molecules condense on the surface of aerosol particles (Cox, 1974). 

In the polymerization of eCLM only a few percent of lactam is incorporated into the polymer through hydrolysis and bimolecular condensation. Most monomer enters the polymer through stepwise addition, i.e. the cationic chain growth 22) ... 

Figure 5-13 A thioester at the C-terrninus of one peptide serves as the electrophile for bimolecular condensation with the thiol side chain of an N-terminal cysteine residue. The S,N-acy intramolecular rearrangement forms the native amide bond.

The Clemmensen reduction of aldehydes and ketones to methyl or methylene groups takes place by heating with zinc and hydrochloric acid. A non-miscible solvent can be used and serves to keep the concentration in the aqueous phase low, and thus prevent bimolecular condensations at the metal surface. The choice of acid is confined to the hydrogen halides, which appear to be the only strong acids whose anions are not reduced with zinc amalgam. The Clemmensen reduction employs rather vigorous conditions and is not suitable for the reduction of polyfunctional molecules, such as 1,3- or 1,4-diketones, or of sensitive compounds. However, it is effective for simple compounds that are stable to acid (7.38). A modification under milder conditions uses zinc dust and HCl dissolved in diethyl ether (ethereal HCl). Other methods for converting C=0 to CH2 are described in Schemes 7.87 and 7.105. 

The desilylative nitration involves initial O-nitration. with phenyl ni trate readily rearranging (catalyzed by the add formed in the intact ring nitration) to nitrophenols. In case of pentafluorophenyl ( -trimethylsil>l ether, no rearrangement can occur but there is indication of bimolecular condensation. 

High Dilution Although the preparation of five- and six-mem-bered rings from appropriate acyclic precursors usually proceeds without difficulty, the synthesis of larger rings requires more elaborate techniques since bimolecular condensations tend to become more favorable than simple cyclization. To favor the unimolecular reaction at the expense of the bimolecular condensation, high dilution techniques are used, and the apparatus (Fig. 1-14) used for the Dieckmann cyclization of diethyl tetradecan-1,14-dicarboxylate is excellent for this purpose.f... 

Related bimolecular condensations of enamines derived from jS-diketones with an aldehyde have also been described. ... 

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