1H NMR Shift of Single Molecular Water and Its Ligand Interactions in Organic Solvents

Abstract

Loose H-bonded water named as “free water” or “confined water” plays an important role in “on water catalysis”, or on properties of lignocellulose-based composites, as well as protein-ligand binding (lock and key inhibition) and protein mediated multivalent weak cooperative interactions like carbohydrate-protein complexes in biological systems. To quantify the number of banded hydrogen bonds for single molecular water is challenging. In order to identify the “free water” via 1H NMR, a theoretical value of chemical shift of single molecular water with zero bonding is rationally set here. Water ligand interactions with common organic solvents were investigated systematically. The role of water ligand interactions in “on water” catalysis is elucidated.

The Mechanism to Enhance the Nucleophilicity of Water in Hydroxylation of Alkenes, Epoxides, and Allylic Carbonates

“…Based on the above hypothesis (Figure 1), the most efficient strategy to break the hydrogen bonding networks of water is to use a suitable organic solvent in the presence of little water, 11 because water would be in its single molecular water form when it is an impurity in organic solvents 11,12 . The second strategy is to create a confined space where the real active species could accumulate; it could be either via the design of the heterogeneous porous structure, or by the use of catalysts that could be able to confine/encapsulate/localize water via hydrogen bonding.…”

“…Very recently, with set chemical shift of the single molecular water, the water ligand interactions in various organic solvents were also investigated and the water organic complexes were proposed correlated to the reported chemical shifts of the water impurities. 11 This newly created knowledge would make the manipulation of the nucleophilicity of water through the use of a suitable organic solvent possible. Hence, a hypothesis (Figure 1) that loose H-bonded water is a good nucleophile was proposed; generally, the bigger upfield change in comparison with that of single molecular water, the better of the nucleophilicity of the water in the catalytic system, if not considering the steric effect of the water organic complex.…”

“…Combining the new knowledge 10,11 , the water data parts from the recent two dozen successful water addition cases (reported mainly in non-asymmetric hydroxylation of alkenes, enantioselective hydroxylation of epoxides (Scheme 1B) and allylic carbonates) were interpreted and discussed carefully in order to gain insights on the mechanism to enhance the nucleophilicity of water at the molecular level. These literatures were not cherry-picked, but were selected randomly (only if the organic transformations were performed at or below 70℃, to avoid a possible water vapor participation pathway).…”

“…According to the water organic solvent complexes correlated with the 1 H NMR chemical shifts of the water hydrogens, the higher electron density (the lower δ value of the water impurity peak) of the protons indicated the more electronegative oxygen of the water inside the complex. 11 Being more electronegative usually means being more nucleophilic if the organic solvent molecule is not so bulky in size. In fact, most of the hydroxylation reactions via water addition in organic solvents were carried out in the median polar organic solvents like acetonitrile (CH3CN) [13][14][15] , acetone [16][17][18] , nitromethane (CH3NO2) [19][20][21] , and dichloromethane (DCM) 22 ; two of them were performed in the relatively polar solvent DMF (N, N -dimethylformamide) 23,24 , and one was done in the non-polar solvent toluene 25 .…”

“…In fact, most of the hydroxylation reactions via water addition in organic solvents were carried out in the median polar organic solvents like acetonitrile (CH3CN) [13][14][15] , acetone [16][17][18] , nitromethane (CH3NO2) [19][20][21] , and dichloromethane (DCM) 22 ; two of them were performed in the relatively polar solvent DMF (N, N -dimethylformamide) 23,24 , and one was done in the non-polar solvent toluene 25 . Chemical shifts of water impurities ranged from 0.43 to 2.84 (δ, ppm) in different deuterated organic solvents 11,26,27 (0.43 in toluene-d6, 1.52 in CD2Cl2, 1.90 in DMF-d7, 28 2.10 in CD3NO2, 29 2.13 in CD3CN, 2.84 in (CD3)2CO) at ambient conditions 30 . The strong correlation between the chemical shift of water impurities in organic solvents and the reactivity/selectivity in hydroxylation reactions supports the proposed hypothesis in general.…”