Solvent-induced reversibility

Arseniyadis, S. Valleix, A. Wagner, A. Mioskowski, C. Kinetic resolution of amines A highly enantioselective and chemoselective acetylating agent with a unique solvent-induced reversal of stereoselectivity. Angew. Chem. 

Convincing evidence was found that the majority of acyclic aldo-nitrones exist in the Z-form, by investigating the ASIS-effect (aromatic solvent induced shift effect) (399). However, in some cases, specified by structural factors and solvent, the presence of both isomers has been revealed. Thus, in C -acyl-nitrones the existence of Z -and -isomers was detected. Their ratio appears to be heavily dependant on the solvent polar solvents stabilize Z-isomers and nonpolar, E-isomers (399). A similar situation was observed in a- methoxy-A-tert-butylnitrones. In acetone, the more polar Z-isomer was observed, whereas in chloroform, the less polar E-isomer prevailed. The isomer assignments were made on the basis of the Nuclear Overhauser Effect (NOE) (398). /Z-Isomerization of acylnitrones can occur upon treatment with Lewis acids, such as, MgBr2 (397). Another reason for isomerization is free rotation with respect to the C-N bond in adduct (218) resulting from the reversible addition of MeOH to the C=N bond (Scheme 2.74). The increase of the electron acceptor character of the substituent contributes to the process (135). 

In most experimental studies, the environmental conditions of a given polypeptide sample are varied by changing either the temperature of the system or the composition of a solvent mixture consisting of a helix-supporting solvent and a helix-breaking solvent. A curve of helical fraction fN versus temperature at fixed solvent composition is called the thermally induced or thermal transition curve, while a curve of fN versus solvent composition at fixed temperature is called the solvent-induced transition curve. The former is classified into two types normal and inverse (or reverse), depending on whether fN decreases or increases with the rise in temperature. 

We found that cooling solutions of the anils of the 1 2- and 2 1-aldehydes in non-polar solvents induces a very marked, reversible,... 

In solution when iodine is excited to the bound B excited state, dissociation and recombination processes occur. The dissociation is the result of solvent-induced curve crossing to the dissociative a state, the recombination a result of momentum reversals arising from collisions with the surrounding solvent molecules. Eigenstates of the B state will decay in a continuous manner, whereas wavepackets—if the curve-crossing probability is less than unity—decay in a stepwise manner, giving rise to successive pulses of product. The B and a curves cross near the center of the B state, whereas the B state wavepacket is initially created near the left turning point thus there... 

Experimental results of an unprecedented haloform-type reaction in which 4-alkyl-4-hydroxy-3,3-difluoromethyl trifluoromethyl ketones undergo base-promoted selective cleavage of the CO-CF3 bond, yielding 3-hydroxy-2,2-difluoro acids and fluoro-form, have been rationalized using DFT (B3LYP) calculations.159 The solvent-induced effects on the two pathways, introduced within the SCRF formalism through PCM calculations, do not reverse the predicted preference of the CO-CF3 over the CO-CF2 bond cleavage in the gas phase. 

The structure of salmon calcitonin (sCT) has been characterized in a variety of co-solvent conditions (Motta et al., 1989 Meadows et ah, 1991 Meyer et ah, 1991 Motta et al., 1991 Arvinte and Drake, 1993). Salmon calcitonin in 90% TFE or 90% MeOH both formed a-helical structures with an N-terminal loop (Meadows et al., 1991 Meyer et al., 1991 Motta et al., 1991). Conversely, sCT in 90% DMSO exhibited an intermolecular (3-sheet structure with two (3-turns (Motta et al., 1989). These results indicate that sCT can adopt very different, reversible conformations in a variety of co-solvents. Similar conformational results for sCT (pH 5.0) were observed by FTIR in a series of stability studies at 37°C (Stevenson and Tan, 2000). Helical structures were observed in both 70% EtOH and 70% propylene glycol. A (3-sheet structure was observed in 70% DMSO however, a very loose a-helix was observed in 100% DMSO, probably caused by the aprotic nature of the solvent. In these studies, the conformation was correlated to the onset of gelation, where the co-solvents inducing an a-helical structure resulted in a decrease in gelation (Stevenson and Tan, 2000). 

Figure 1. Schematic representation of the influence of various factors on the miscibility and phase separation. The shaded areas represent the two-phase regions. The arrows show the paths for phase separation, while the reverse directions would be the paths for miscibility. From left-to-right Temperature-induced phase separation solvent-induced phase separation reaction (i.e. polymerization) -induced phase separation (the two-phase regions are entered with increasing degree of polymerization (DP)) and pressure-induced phase separation. < ) = polymer concentration, S2 and SI solvent and nonsolvent PI andP2 = polymer 1 and polymer 2.

Related chemistry has been seen with Tp PtPhH2. Protonation of one arm of the Tp ligand induces reversible reductive coupling of phenyl and hydride to give an t7 -C6H6 complex (Equation (13)). This species was found to be in rapid equilibrium with the Pt(iv) phenyl dihydride complex. Addition of 8(0565)3 to the methyl dihydride induces loss of methane and activation of benzene solvent. ... 

Numerous endeavors have been implemented in fermentation process development in order to alleviate the solvent-induced inhibitory effects and thus improve the fermentation productivity. Usually, the selective removal of fermentation products was integrated simultaneously into the fermentation process in order to maintain a low solvent concentration in the fermentation broth. Many online butanol removal techniques have been reported with various advantages and efficiencies, including liquid-liquid extraction, perstraction, gas stripping, pervaporation, adsorption, reverse osmosis, etc. 

J. Peng, Y. Xnan, H.F. Wang, Y.M. Yang, B.Y. Li, Y.C. Han, Solvent-induced microphase separation in diblock copolymer thin films with reversibly switchable morphology, Journal of Chemical Physics 120 (2004) 11163-11170. 

Guo C, Liu HZ, Chen JY (2000) A fourier transform infrared study on water-induced reverse micelle formation of block copoly(oxyethylene-oxypropylene-oxyethylene) in OTganic solvent. Colloids Surf A 175 193-202... 

Martinez A, Guy L, Dutasta J-P (2010) Reversible, solvent-induced chiraMty switch in atrane structirre control of the unidirectional motion of the molecular propeUer. J Am Chem Soc 132 16733-16736... 

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