Lewis acid-base interactions nitrogen donor

The term halogen bonding (XB) [3-8] indicates any D X-Y interaction in which X is the electrophilic halogen (Lewis acid, XB donor), D is a donor of electron density (Lewis base, XB acceptor), and Y is carbon, nitrogen, halogen, etc. (Fig. 1). 

The first step of the mechanism is the coordination of BFI3 (Lewis acid) to the tertiary nitrogen atom (Lewis base) of the CBS catalyst from the -face. This coordination enhances the Lewis acidity of the endocyclic boron atom and activates the BH3 to become a strong hydride donor. The CBS catalyst-borane complex then binds to the ketone at the sterically more accessible lone pair (the lone pair closer to the smaller substituent) via the endocyclic boron atom. At this point the ketone and the coordinated borane in the vicinal position are cis to each other and the unfavorable steric interactions between the ketone and the CBS catalyst are minimal. The face-selective hydride transfer takes... 

Boron-nitrogen and boron-phosphorous compounds are classical textbook examples of donor-acceptor complexes. The chemical bonds of the Lewis-acid Lewis-base complexes are usually explained in terms of frontier orbital interactions and/or quasiclassical electrostatic attraction in the framework of the Hard and Soft Acid and Base (HSAB) model [73]. We were interested in seeing if the differences between the bond strengths of boron-nitrogen and boron-phosphorous complexes for different boranes, amines and phosphanes can be explained with the EDA method. 

Taking into account the fact that the solvation of ambident anions in the activated complex may differ considerably from that of the free anion, another explanation for the solvent effect on orientation, based on the concept of hard and soft acids and bases (HSAB) [275] (see also Section 3.3.2), seems preferable [366]. In ambident anions, the less electronegative and more polarizable donor atom is usually the softer base, whereas the more electronegative atom is a hard Lewis base. Thus, in enolate ions, the oxygen atom is hard and the carbon atom is soft, in the thiocyanate ion the nitrogen atom is hard and the sulfur atom is soft, etc. The mode of reaction can be predicted from the hardness or softness of the electrophile. In protic solvents, the two nucleophilic sites in the ambident anion must interact with two electrophiles, the protic solvent and the substrate RX, of which the protic solvent is a hard and RX a soft acid. Therefore, in protic solvents it is to be expected that the softer of the two nucleophilic atoms (C versus O, N versus O, S versus N) should react with the softer acid RX. 

Calorimetric (enthalpy), i.r. (OH frequency shift), and n.m.r. (hydrogen-bond chemical shift) data have been reported for the interaction of 1,1,1,3,3,3-hexafluoropropan-2-ol with a series of Lewis bases (nitrogen and oxygen donors and the soft donor diethyl sulphide), and details of studies on the 1 1 and 2 1 complexes formed between this highly acidic alcohol [pJira(H20) 9 3] and dimethyl sulphoxide, tetramethylurea, tetramethylene sulphone, and 1,4-dioxan have become available. Hydrogen bonding between water vapour (the acceptor) and l,l,l,3,3,3-hexafluoropropan-2-ol has been detected by i.r. spectroscopy. ... 

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