Solvent and Temperature Effects

During our efforts in profiling (comparative investigations) of several commercial available metathesis catalysts bearing NHC ligands in different types of metathesis reactions remarkable solvent effects were observed [4], Interestingly, the efficiency of most transformations studied frequently depended more on solvent and temperature effects rather than on the nature of Ru precursor and NHC ligands. 

The solvent and temperature effects for the Michael addition of amidoxime 7 to DMAD were probed because the reaction itself occurs without any other catalysts. As shown in Table 6.2, the reaction gave a high ratio of 8E in strongly aprotic polar solvents such as DMF and DMSO (entry 1 and 2). 8E was also found as the major product in MeCN (entry 3), dichloromethane (entry 4), and xylenes (entry 5). To our delight, the desired 8Z was obtained as the major component in methanol (entry 6). The stereoselectivity of 8Z versus 8E was better at low temperature (entry 7). A similar result was observed when the reaction was run in THF or dichlo-roethane in the presence of a catalytic amount of DABCO (entries 9 and 10). 

Solvent and temperature effects have been especially studied in simple compounds—white phosphorus,(1969 180) PH3, (1967>43> PFS, (1968,51) phosphorus acid, phosphines, phosphonium compounds etc. (1965> 39 1967,169 1968,155 1969,53) phosphorus chemical shifts or line-widths,... 

Solvent and temperature effects on the diastereoselective addition of n-BuLi to 2-phenylpropanal have been reassessed95 and the stereochemistry of addition of organolithium reagents to carbohydrate enones has been studied.96 No compelling evidence has been found to support the suggestion that addition of n-BuLi to benzoic acid might compete with formation of the lithium salt and thereby provide an alternative to the sequential route to PhCOBu".97... 

The solvent and temperature effect on the 71-route cyclization of endo-bicyclo[3.3.1]-non-6-ene-3-carboxylic acid (24) has been studied.40... 

Molecular dynamics simulations have been used to predict solvent and temperature effects in the nucleophilic addition of a-chiral carbonyl compounds.253 Prediction of diastereoselectivity break temperatures (i.e. inversion points) has been achieved with fair accuracy by comparison with experimental data on n-BuLi addition. Dramatic differences are seen for additions to 2-phenylpropanol in pentane solvent, compared with octane. 

The question concerning the nature of allyllithium in solution is of great interest (2). The interpretation of experimental studies is complicated because the bonding situation is considerably influenced by solvent and temperature effects (14,15). Possible structures for allyllithium species in solution are aggregates (A), contact ion pairs (B and C), solvent separated ion pairs (D) and free ions (E) (bases coordinated to the lithium atoms have been omitted for clarity). 

In this approach the R-amine moiety in in was considered likely, in view of the work of Yamada and Koga5 and later Kametani et al.,6 to provide some inductive control in the sodium borohydride reduction of IV. Moreover, the R-amine moiety is a necessary component of dilevalol. Desired inductive control was quickly demonstrated by Gold et al. (internal communication, October 25,1979). However, a broad study of process conditions, particularly of solvent and temperature effects, only gave, at best, a ratio of RR to SR of... 

Solvent and temperature effects on the 15N shieldings are important, as expected. 

For the purpose of estimating conformational parameters of silaiylene carboorganocyclosiloxanes of the structure XV, two approaches were used [117] computerized mathematical simulation using the Monte-Carlo method and experimental estimation of the flexibility parameters in solution under natural conditions. In the first case, mathematical simulation has determined the skeletal flexibility of the molecule in the absence of substituting agents at atoms forming the backbone. In the second case, all fragments of the chain, possible interactions with the solvent and temperature effect have been taken into account in the flexibility estimation. 

Work on indole, tryptophan, etc. continues because of their relevance to the complex field of protein photophysics. Creed has produced reviews of the photophysics and photochemistry of near-u.v.-absorbing amino-acids, viz. tryptophan and its simple derivatives, tyrosine and its simple derivatives, and cysteine and its simple derivatives. The nature of the fluorescent state of methylated indole derivatives has been examined in detail by Meech et al. Another investigation on indole derivatives deals particularly with solvent and temperature effects. Fluorescence quenching of indole by dimethylfor-mamide has also been examined in detail. Fluorescence excitation spectra of indoles and van der Waals complexes by supersonic jets give microscopic solvent shifts of electronic origin and prominent vibrational excitation of L(, states. Conventional flash photolysis of 1-methylindole in water shows R, e p, and a triplet state to be formed. " Changes in the steady-state fluores-... 

Vrentas JS and Duda JL. Solvent and temperature effects on diffusion in pol3mier solvent systems. J. Appl. Polym. Sci. 1977 21 1715-1728. 

Table 3 Solvent and Temperature Effect on the Stereochemistry of Cyclopropyl Carbanion...

With this technique for measuring protonic activity at low temperatures, we were able to titrate proteins in mixed solvents at subzero temperatures. Such investigations using the potentiometric method, which allow us to study the solvent and temperature effects on the net... 

Discussion. Changes in eluent composition as well as temperature variations determine new separation patterns. This could lead, if generalized, ta unwanted separation processes or yet, when correctly understood, to new separation methods. However, the problems raised by these solvent and temperature effects are far from simple and deserve some discussion. 

Since this review was completed, a number of interesting developments have been obtained in this laboratory and elsewhere in various protein fractionation techniques at subzero temperatures. Let us mention a work on isoelectric focusing and electrophoresis by M. Per-rella, A. Heyda, A. Mosca and L. Rossi-Bemardi (Anal. Biochem., in press) a work by C. Le Peuch and C. Balny on the sequential elution of proteins bound by hydrophobic interaction chromatography [FEBS Lett. 87,232(1978)] a work by K. Andersson, Y. Benyamin, P. Douzou, and C. Balny about organic solvents and temperature effects on desorption from immunoadsorbents (J. Immunol. Methods, in press, 1978). 

F. Minisci, F. Recupero, A. Cecchetto, C. Gambarotti, C. Punta, R. Paganelli, G. F. Pedulli, F. Fontana, Solvent and temperature effects in the free radical aerobic oxidation of alkyl and acyl aromatics catalysed by transition metal slats and N-hydroxyphatUmide New processes for the sysnthesis of p-hydroxybenzoic acid, diphenols, and dienes liquid for liquid crystals and cross-linked polymers, Org. Proc. Res. Devel. 8 (2004) 163. 

Quantitative studies of the solvent and temperature effects coupled to molecular mechanics studies give access to a wide variety of parameters [44], The monitoring of CTI using fluorine as an internal probe (i.e. 4-fluorophenylalanine) is also particularly attractive due to the high sensitivity of 18F [54],... 

Andersson PO, Bachilo SM, Chen R-L and Gillbro T (1995) Solvent and temperature effects on dual fluorescence in a series of carotenes. Energy gap dependence of the internal conversion rate. J Phys Chem 99 16199-16209... 

Other applications of laser flash photolysis have included (a) a study of the bimolecular rate constant for the reaction between singlet oxygen and several lipid-soluble substances,(b) an investigation of quenching, solvent, and temperature effects on the photolysis of indoles, (c) the time dependence of the quenching of aromatic hydrocarbons by tetramethylpiperidine TV-oxide, and d) the kinetics of the geminate recombination of aromatic free radicals. Finally, flash photolysis has been utilized in order to examine the feasibility of a tunable IF laser (479—498 nm) and an ICl laser (430 nm). ... 

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