Hydrogen acceptor-donor centers

LigandScout pharmacophores solely consist of chemical features classified as layers 3 and 4 (Table 6.2). Visualization mainly distinguishes between point and vector features point features (layer 4) are defined as a center with a tolerance this group encompasses hydrophobic, positively ionizable and negatively ioniz-able areas, in addition to excluded volume spheres. Hydrogen donors, acceptors and donor-acceptor pairs belong to the vector features group. 

GRID), hydrogen bond donor-acceptor centers (OH or 01 type in GRID) and shape (H type in GRID)). There is always the opportunity for the user to customize the selection of the probes, especially in the case where a known interaction between the target and a given is of particular interest. 

This study supports rate-determining H-OH bond breaking, which constrasts with previous reports that identified vinylidene isomerization as the key step in catalytic alkyne activation. The results indicate an enzyme-like mechanism is operative involving cooperative substrate activation by a metal center and proximal hydrogen bond donor/acceptors. In the future we will apply these principles to the activation of additional species. 

The high stereopreference was rationalized by considering complex 388 in which an attractive n-n donor-acceptor interaction favors co-ordination of the dienophile to the face of the boron center which is cis to the 2-hydroxyphenyl substituent. Hydrogen bonding of the hydroxyl proton of the 2-hydroxyphenyl group to an oxygen of the adjacent B—O bond played an important role in the asymmetric induction. Protection of this hydroxy functionality with a benzyl group caused reversal of enantioselectivity in the cycloaddition of cyclopentadiene with methacrolein (model 389)244. 

In these three ligands the and atoms are suitable for covalent coordination with the metal center (see Scheme 3.5 for labeling). Although also possesses an electron pair, this will not be the preferred position for metal coordination because normally it is involved in the formation of donor-acceptor hydrogen bonds together with one or two hydrogen atoms of the amino groups. Very few examples of the coordination of an N atom with two vicinal NH2 functions have been reported... 

---