Polar solvents stepwise

In non-polar solvents, the reaction with piperidine is best represented by a two-term kinetic form indicating a mixed 2nd- and 3rd-order reaction. Also, base catalysis by tri-ri-butylamine was observed. This kinetic pattern is strongly reminiscent of the results obtained with nitro-activated benzenes.Another interesting result is that stepwise replacement of chlorine atoms by amino groups results in marked... 

Polar, water-soluble analogs of these molecules (23 and 24) have also been studied and have been shown to form helical columns in a variety of polar solvents such as methanol, butanol, acetonitrile, and water, by virtue of solvophobic interactions between the large aromatic cores.82,83 A stepwise association process is observed when alcohol solutions of 24 are cooled. The... 

Fulleride anions are often more soluble, especially in more polar solvents, than the parent fullerenes. For example, in bulk electrolysis experiments with tetra-n-butylammonium perchlorate (TBACIO4) as supporting electrolyte, carried out in acetonitrile where Cjq is completely insoluble, fairly concentrated, dark red-brown solutions of 50 can be obtained [81]. Upon reoxidation, a quantitative deposition of a neutral Cjq film on the surface of a gold/quartz crystal working electrode takes place. This Cjq film can be stepwise reductively doped with TBA, leading to (Cjo )... 

Allenylidenecomplexes 46 also react with thecarbon-carbon triple bond ofynamines to yield similar mononuclear cydobutenyhdene derivatives 48, although mixtures with the corresponding alkenyl-aminoallenylidene spedes 49 are formed (Scheme 2.20) [ 1 Oc]. The former isomer results from the addition of the C=C bond of y namines across the Co,=Cp unsaturation, while the latter is provided bythe formal [2 + 2] cycloaddition between C=C and Cp=Cy bonds and subsequent cycloreversion. In both processes, stepwise cyclization initiated by the addition of the nucleophilic R C=CNEt2 carbon at the Co, or Cy position, respectively, is proposed. Relative proportions of 49 with respect to 48 increase with the electron-releasing capacity of the para-substituents of the diarylallenylidene skeleton (NMe2 > OMe > Me > H). In contrast, the formation of 48 is favored when the reaction is carried out in low polarity solvents. 

An important point, however, is that although the configurations of the reactants are preserved in the products (i.e. the additions are stereoselective), some cycloadditions, including those of ketenes to imines, occur more rapidly in polar rather than in non-polar solvents (Scheme 8.9). For such examples it may well be that the addition proceeds in a stepwise manner (non-concerted), and the collapse of a dipolar intermediate is so quick that the stereochemistry of the reacting species is not compromised. 

The transition states for the stepwise (fej, Fig. 2.3) and concerted (fecon) reactions of (4-MeO,X)-3-Y lie at distinct well-separated positions on the More O Ferrall diagram and show different sensitivities to changes in solvent polarity, meta substituents X at the aromatic ring, and the leaving group Y. For example, in 50 50 (v/v) water/trifluoroethanol (4-MeO,H)-3-Cl reacts with azide ion exclusively by a stepwise mechanism through the primary carbocation intermediate (4-MeO,H)-3" with a selectivity for reaction with azide ion and solvent of feaz/ s = 25 However, two-thirds of the azide ion substitution product obtained from the reaction of (4-MeO,H)-3-Cl in the less polar solvent 80 20 acetone/water forms by concerted bimolecular substitution and only one-third forms by trapping of the carbocation intermediate (4-MeO,H)-3 with a selectivity of k z/h = 8 The preferred... 

The rates of Diels-Alder reactions are little affected by the polarity of the solvent. If a zwitterionic intermediate were involved, the intermediate would be more polar than either of the starting materials, and polar solvents would solvate it more thoroughly. Typically, a large change of solvent dipole moment, from 2.3 to 39, causes an increase in rate by a factor of only 10. In contrast, stepwise ionic cycloadditions take place with increases in rate of several orders of magnitude in polar solvents. This single piece of evidence rules out stepwise ionic pathways for most Diels-Alder reactions, and the only stepwise mechanism left is that involving a diradical. 

The preference for the 1,2,3-thiadiazoline structure in more polar solvents is explained by the higher dipole moment (ca 5 D as compared to ca 2 D) of the transition structure452. Extensive ab initio calculations on the reaction of diazomethane with thioformaldehyde concluded that both regioisomers should be formed via concerted pathways452, but some semiempirical methods (AMI, MNDO-PM3) suggest that the reaction takes place in a stepwise manner. 

The parent 2-vinylindole and 2-(2-methylvinyl)indole also reacted with the carbodienophiles methyl-(E)-3-benzoylacrylate, l-penten-3-one, and methyl acrylate these reactions proceeded through a Diels-Alder step to produce the corresponding carbazoles (90JOC5368). The unsymmetrical dienophiles reacted regioselectively in accordance with the predictions of the FMO concept. In none of these reactions was it possible to detect either a betaine intermediate originating from a stepwise process or a Michael-type adduct. The stereochemistry of the cycloadducts was not changed when the reactions were carried out in the polar solvent... 

It may be expected that other, highly structured solvents with a tri-dimensional network of strong hydrogen bonds, would also permit micelle formation by surfactants, but little evidence of such occurrences has been reported. On the other hand, surfactants in non-polar solvents, aliphatic or aromatic hydrocarbons and halocarbons tend to form so-called inverted micelles, but these aggregate in a stepwise manner rather than all at once to a definite average size. In these inverted micelles, formed, e.g., by long-chain alkylammonium salts or dinonyl-naphthalene sulfonates, the hydrophilic heads are oriented towards the interior, the alkyl chains, tails, towards the exterior of the micelles (Shinoda 1978). Water and hydrophilic solutes may be solubilized in these inverted micelles in nonpolar solvents, such as hydrocarbons. 

Both concerted and stepwise mechanisms have been proposed for these reactions although the experimental facts can be readily accounted for by the formation of a 1,4-dipolar intermediate (27 Scheme 27). The reaction rates are markedly increased in polar solvents. The cycloadditions of isocyanates to alkenes are always regiospecific, the reaction taking place so as to form the most stable 1,4-dipole. 

Superior coumarin compounds separation with use of the described flat sandwich chambers was achieved with gradient chromatography on silica gel and stepwise variation of polar modifier concentration in mobile phase, as less polar solvents (hexane, cyclohexane, toluene, or dichloromethane) and polar modifiers (acetonitrile, diisopropyl ether, ethyl acetate) are used. 

However, Hamilton and Comai described a procedure which allows good recovery and separation of the neutral lipids from FFA on a silica SPE matrix [5]. Separation of the different phospholipids can be also achieved on an aminopropyl column. These polar compounds are tightly bound to the matrix, so they are retained while eluting neutral lipids and FFAs. Then, the different phospholipids can be washed from the column by changing the pH and the ionic strength of the solvent. Stepwise elution of phospholipids was obtained (PC, PE, PS, and PI) by increasing, progressively, the polarity of the solvent and its pH [7]. 

Most prominent and well investigated is the group of donor-substituted cyclopropanes that readily react with electron-deficient dienophiles. Rate increase in polar solvents, the negative value of the partial molar volume and trapping reactions with methanol indicate a stepwise reaction via dipolar intermediates 2 for singly alkoxy-substituted cyclopropanes 1. 

Cyclic enones, such as substituted cyclohex-2-enones or cyclohexa-2,5-diones, also undergo sigmatropic photorearrangement to form bicyclo[3.1.0]hexanones (lumiketones) or bicyclo[3.1.0]hex-3-en-2-ones, respectively, for which both concerted and stepwise (biradical) reaction mechanisms have been proposed.640,641,770 For example, a [l,2]-shift concurrently with the ring contraction (termed the type A reaction) is observed upon irradiation of the methylphenyl derivative 159 in polar solvents, whereas phenyl migration (termed the type B reaction) predominates in nonpolar solvents (Scheme 6.70).771,772 The reactions are believed to proceed via both the n,n and n,Tt triplet ketone states. In the presence of alkenes, cyclic enones may readily undergo a competitive photocycloaddition reaction (Section 6.1.5). 

A beautiful illustration of a delicate balance between a stepwise and a concerted reaction has been found in the reactions of 1,1-dimethylbutadiene 6.133.716 This diene rarely adopts the s-cis conformation necessary for the Diels-Alder reaction with tetracyanoethylene giving the cyclohexene 6.136. However, it can react in the more abundant s-trans conformation in a stepwise manner, leading to a moderately well stabilised zwitterion 6.134. The intermediate allyl cation is configurationally stable, and a ring cannot form to C-l, because that would give a trans double bond between C-2 and C-3 in the cyclohexene 6.137. Instead a cyclobutane 6.135 is formed. All this is revealed by the solvent effect. In the polar solvent acetonitrile the stepwise ionic pathway is favoured, and the major product (9 1) is the cyclobutane 6.135. In the nonpolar solvent hexane, the major product (4 1) is the cyclohexene 6.136 with the Diels-Alder reaction favoured. 

For the separation of substances having small Rf values, it is advisable to run the TLC plate more than once, drying the layer before it is developed again. If the solvents used for subsequent runs are the same as in the first run it is called multiple or repeated development, but when the solvents are different in the first and later runs, the technique is called stepwise development. These techniques are specially useful for mixtures which contain polar as well as non-polar substances. Polar solvent systems are usually employed for the first run and a non-polar solvent system for the later runs. 

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