Induced solvent

The very first report on the use of ionic liquids as soluble supports was presented by Fraga-Dubreuil and Bazureau in 2001 [102]. The efficacy of a microwave-induced solvent-free Knoevenagel condensation of a formyl group on the ionic liquid (IL) phase with malonate derivatives (E1CH2E2) catalyzed by 2 mol% of piperidine was studied (Scheme 7.89). The progress of the reaction could be easily monitored by 1H and 13C NMR spectroscopy, and the final products could be cleaved from the IL... 

Generally solvents chosen for NMR spectroscopy do not associate with the solute. However, solvents which are capable of both association and inducing differential chemical shifts in the solute are sometimes deliberately used to remove accidental chemical equivalence. The most useful solvents for the purpose of inducing solvent-shifts are aromatic solvents, in particular hexadeuterobenzene (CgDg), and the effect is called aromatic solvent induced shift (ASIS). The numerieal values of ASIS are usually of the order of 0.1 - 0.5 ppm and they vary with the moleeule studied depending mainly on the geometry of the complexation. 

Studies of the effect of a-helix inducing solvents such as trifluoroethanol (TFE) on the product distribution of oxidative folding of endothelin-1 (6) (0.01 mM peptide concentration in 80% TFE, pH 9.5) clearly revealed that induction of the a-helical conformation favors the onset of the cystine-stabilized a-helix structural motif with concomitant enhancement of isomer 1 (88 12 vs 75 25) (Table 1)J491 A similar enhancement of the correctly folded 6 by the addition of TFE has also been observed under other conditions. [5°1... 

A related view of the physical basis of the solvent coordinate is the well-known reaction field R, which is the field located on the solute dipole due to the dipole induced solvent polarization. For example, the reaction field for the equilibrated excited state dipole Rlq is given by... 

Hino, T., Anzai, T. and Kuramoto, N. (2006) Visible-light induced solvent-free photooxygenations of organic substrates by using [60]fullerene-linked silica gels as heterogeneous catalysts and as solid-phase reaction fields. Tetrahedron Letters, 47 (9), 1429-1432. 

The benzene-induced solvent shifts (relative to CC14 solution) of perhy-dropyrido[2,l-c][l,4]oxazin-3-one (36) and -4-one (34) and their monosub-stituted derivatives have been studied [720MR(4)283,72OMR(4)509].The structure of a tram-3,9a-H-3-[(aroylamino)methyl]perhydropyrido[2,l-c]-... 

Here m is the mass and v is the velocity (dx/dt) associated with the x coordinate. The remaining, dynamically induced solvent effects show up in F(t), the so-called fluctuating force, and 17 (t), the dynamical friction. Indeed, the last two terms in Equation (1) are the key — without them there would be no mechanism for a solvent to accept energy from a solute (29,30). 

Keywords Absolute configuration and conformation determinations Density functional theory Induced solvent chirality Transient VCD measurements Vibrational circular dichroism... 

Modeling Chiral Solute-Water H-Bonding Networks by Induced Solvent Chirality... 

Although ionic liquids do not boil it is useful to know the temperature range under which they can operate. For many ionic liquids, the onset of decomposition takes place at temperatures exceeding 250°C and often much higher/181 However, 1,3-dialkylimidazolium phosphates were found to slowly decompose below 200°C 19 and when the liquid is composed of less common cations or anions, the thermal stability may be considerably lower. Nevertheless, since the overwhelming majority of catalysed reactions take place at much lower temperatures, thermally induced solvent decomposition tends not to be a problem, although chemically induced decomposition can be a real concern For example, thermal decomposition of [C4Ciim][BF4] takes place a much lower temperature if nucleophiles are present/201... 

Other recent microscopic approaches are based on the Langevin dipoles solvent model or on the all-atom solvent model, using a standard force field with van der Waals and electrostatic terms as well as intramolecular terms, and molecular dynamics simulations of the fluctuation of the solvent and the solute, incorporating the potential from the permanent and induced solvent dipoles in the solute Hamiltonian in a self-consistent way (Luzhkov and Warshel, 1991). 

Furthermore, the higher aromatics content and the possible presence of oxygenates in motor gasoline can induce solvent characteristics that are unsuitable for seals, gaskets, fuel lines, and some fuel tank materials in aircraft. Motor gasoline may also contain additives that could be incompatible with certain in-service aviation turbine fuel (ASTM D-4054, ASTM D-4307). For example, alcohols or other oxygenates can increase the tendency for the fuel to hold water, either in solution or in suspension. 

McRae87 88 has derived an expression for the solvent-induced frequency shift, from the second order perturbation theory, taking into account all the types of interactions suggested by Bayliss and McRae86. On the basis of a simple electrostatic model, the frequency shift, Av, is related to the refractive index and the static dielectric constant of the solvent by an equation consisting of four terms. The first term in the equation represents the contribution from dispersive interactions which give rise to a general red shift the second term represents the contribution from the solute dipole-induced solvent dipole interactions the third term accounts for the solute dipole-solvent dipole interactions and the fourth term represents the contribution from the... 

The mechanism by which benzene solvent molecules are able to produce differential solvent shifts in the proton resonances of the same molecule has been widely interpreted in terms of a benzene-solute collision complex. The term collision complex is meant to imply a short-lived orientation of the benzene molecule(s) that has been brought about by dipole-induced dipole, dipole—quadrupole, or other weak chemical association. Any mechanism that is proposed for benzene-induced solvent shifts should take into account the nature, stoicheiometry, strength and time-averaged stereochemical environment of the interaction and we shall deal with each of these points in turn. 

It has been widely assumed that the stoicheiometry of the benzene-solute collision complex is 1 1, and consistent with this assumption is the observation that the solvent shift at various concentrations of benzene in carbon tetrachloride is approximately proportional to the mole fraction of benzene. Dilution studies such as these do not exclude the co-occurrence to a small extent of other solute-solvent combinations, such as 2 1 or 1 2. Phase-independent 1 1 associations at isolated polar sites can account for the additivity of benzene-induced solvent shifts. For example, addition of the solvent shifts (d = CDci3 <5cgH6) of the H-18 and H-19 protons of 5a-androstan-ll-one (12) and 5a-androstan-2-one (13) gives A values which are in... 

If this equilibrium has any real significance, then the benzene-induced solvent shifts should be temperature dependent. This has been shown to be true for aliphatic and aromatic solutes. On the basis of a 1 1 collision complex, Abraham has developed a method by which the heat and entropy of formation together with the equilibrium constant of the complex may be calculated. Briefly, if a fraction p of the solute is complexed at a temperature T, the equilibrium constant, X, for a dilute solution is given by—... 

Extensive studies have been carried out on the empirical use of benzene solvent-induced shifts in structural and stereochemical correlations. Saturated aliphatic ketones in particular have been the subject of many investigations, and an important generalization, the so-called carbonyl plane rule , has been proposed by Williams and Bhacca and independently by Connolly and McCrindle. This rule states that in the specific case of an isolated carbonyl group, as found in 5a-androstan-ll-one for example, the benzene-induced solvent shift (Ac He ) positive for protons... 

In any conformationally mobile a,j3-unsaturated ketone that exists as a mixture of s-cis and s-trans isomers at room temperature there will usually be one conformer that will be energetically more favourable. It is therefore to be expected that a decrease in temperature will increase the population of the favoured isomer, and this may be detectable in the benzene-induced solvent shifts of the substituents. Variable-temperature studies in toluene-c g solution of some model fixed s-m and s-trans a,) -unsaturated ketones have been carried out and the data for 3-methyl-cyclohexenone (33) are fairly typical (see Fig. 4 the chemical shift is given in c./sec. at 100 Me./sec. relative to internal TMS). 

In view of the frequency with which methoxy substituents occur in natural products, the use of benzene-induced solvent shifts as an aid in structural elucidation has obvious possibilities. A recent study of the benzene-induced shifts of the methoxy resonances of some flavones has resulted in the following observations—... 

A recent study of the benzene-induced solvent shifts of the 2-CH2 protons of some AT-formyl- and iV-thioacetylindolenes demonstrates admirably the elegance of the method in distinguishing between various conformational isomers [see 51-54 the solvent shifts A (= CDci3 C6H6 P-P na.) are shown in parentheses]. 

Wharton and Bair have demonstrated that in the cyclopropane derivatives of the type shown in (55), the methyl group trans to the ester group suffers a larger positive benzene-induced solvent shift than the methyl group cis to it. These studies have been extended by Seyden-Penne and co-workers and Boykin et to many... 

---