Donor strength

Donor strengths, taken from ref. 207b, based upon the solvent effect on the symmetric stretching frequency of the soft Lewis acid HgBr2. Gutmann s donor number taken from ref 207b, based upon AHr for the process of coordination of an isolated solvent molecule to the moderately hard SbCL molecule in dichioroethane. ° Bulk donor number calculated as described in ref 209 from the solvent effect on the adsorption spectrum of VO(acac)2. Taken from ref 58, based on the NMR chemical shift of triethylphosphine oxide in the respective pure solvent. Taken from ref 61, based on the solvatochromic shift of a pyridinium-A-phenoxide betaine dye. 

Compare energies fox meta andpara-nitroanilinium ions (intermediates in nitration of aniline). Are these differentiated to a lesser or greater extent than the intermediates in toluene nitration Examine electrostatic potential maps. What do these suggest about the relative electron-donor strengths of methyl and amino groups ... 

The data indicate that the formation of cyclic intermediates creates a stabilization of the cationic chain ends (AH° < 0 and AH s < 0), also expressed by a decrease of both the acceptor strength (Ae(LUMO) > 0) and the donor strength (Ae(HOMO) < 0) of the cations. The positive charge of the cationic centre is distinctly decreased (Aqc+ < 0) as a consequence of the interaction of this centre with the oxygen of the methoxy group. A partially covalent C + —O-bond is formed (pt Q(f) > 0.6 rc+ 0if) 146 pm). 

This shows that the activation energy for elimination can be reduced by either increasing the electron density on the a-carbon (by ligands of greater donor strength) or decreasing the electron density on the jS-carbon. 

Replacement of the phosphorous ligand with an NHC is a logical next step toward stabilising the D-type intermediate due to the o-donor strength of the NHC. Thus, choosing the correct NHC should allow for high selectivities without excess ligand. 

Since the beginning of the 1980s, two different approaches to quantify the H-bond contribution to properties at the 2D and 3D levels developed independently. The carefully parameterized methodology of HYBOT allows one to take into account the influence of substituents on H-bond acceptor and donor strengths. Modern procedures based on X-ray data of ligand-macro molecular complexes consider the... 

Calculated molecular descriptors including H-bond parameters were used for QSAR studies on different types of permeabiUty. For example, the new H-bond descriptor characterizing the total H-bond ability of a compound, was successfully appUed to model Caco-2 cell permeability of 17 drugs [30]. A similar study on human jejunal in vivo permeabiUty of 22 structurally diverse compounds is described in Ref. [62]. An exceUent one-parameter correlation of human red ceU basal permeabiUty (BP) was obtained using the H-bond donor strength [63] ... 

In addition to the variation in electronic configuration, the geometric details of the coordination sphere and the properties of iron-ligand bonds (different a- or 71-donor strength) also influence the isomer shift as observed for a series of compounds ... 

Summary of Facial Stereoselectivity in Aldol and Mukaiyama Reactions. The examples provided in this section show that there are several approaches to controlling the facial selectivity of aldol additions and related reactions. The E- or Z-configuration of the enolate and the open, cyclic, or chelated nature of the TS are the departure points for prediction and analysis of stereoselectivity. The Lewis acid catalyst and the donor strength of potentially chelating ligands affect the structure of the TS. Whereas dialkyl boron enolates and BF3 complexes are tetracoordinate, titanium and tin can be... 

Figure 5.3 illustrates the key features of the Fujita study. In relation to the reference phenol in frame (a), frames (b), and (c) illustrate the effect of H-bonding, and frames (d) and (e) illustrate steric hindrance. Given that the H-bond donor strength of (b) is greater than that of (c), since pKa (b) membrane partitioning, 8, increases in (b) and decreases in (c), relative to (a). Similarly, steric hindrance in (d) produces negative 8, compared to (e). 

Figure 5.3 The effect of hydrogen bonding and steric hindrance on the difference between liposome-water and octanol-water partition coefficients 8 increased H-bond donor strength and decreased steric hindrance favor membrane partitioning in the substituted phenols [381]. [Avdeef, A., Cun Topics Med. Chem., 1, 277-351 (2001). Reproduced with permission from Bentham Science Publishers, Ltd.]...

Fig. 20.3. H-bond donor groups found in the transmenbrane sequences of P-gp shown in order of abundance, and H-bond acceptor groups shown in order of H-bond donor strength.

Iodine molecules adsorbed onto the structural aromatic rings provide information on the electron-donor strength, which appears to be higher in phenylene-bridged PMO than that reported for benzene and comparable with that reported for p-xilene. Traces of I3" species were also detected. From a comparison between two phenylene-bridged PMOs with different degree of order in the walls, aromatic rings in the material with amorphous walls appear more available for interaction with iodine. 

Comments on the thermal nitration of enol silyl ethers with TNM. The strikingly similar color changes that accompany the photochemical and thermal nitration of various enol silyl ethers in Table 2 indicates that the preequilibrium [D, A] complex in equation (15) is common to both processes. Moreover, the formation of the same a-nitroketones from the thermal and photochemical nitrations suggests that intermediates leading to thermal nitration are similar to those derived from photochemical nitration. Accordingly, the differences in the qualitative rates of thermal nitrations are best reconciled on the basis of the donor strengths of various ESEs toward TNM as a weak oxidant in the rate-limiting dissociative thermal electron transfer (kET), as described in Scheme 4.40... 

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