Boronic angle strain

Figure 11.8 Computational results of the [3.2.0] system 22, its complex 24, and transition state 25. (a) Tricyclic boranes at the fourth through sixth homologated stages contain inverted pyramidalized boron centers arising to reduce transannular interactions and angle strain, (b) Complex formation with the ylide 1 is endothermic, (c) Transannular strain energy is partially relieved at the transition state.

The effect of angle strain on the hybridization of boron is illustrated by compounds ranging in structure from (22) through (23). The smallest member of the series, (22), is a tight dimer under all conditions measured, but (23) has a stable, planar BN3 arrangement. ... 

Normally, a number of different structures can be drawn for a specific boron hydride formula, and some selection is necessary. The most favored structures incorporate some symmetry and do not involve bond angles with excessive strain. Often, a number of equivalent but distinct structures that are satisfactory can be drawn for a particular boron hydride. In these cases the positions of the atoms are the same but the disposition of the electrons, that is, the details of the bonding, are different. For example, the structure of B5H9 given above can be oriented in four different directions. 

Figure 8 The compressed O-B-0 bond angle of the diphenylboronic acid - D-fructose complex" is found to be between the bond angles expected for sp and sp geometry, sp Geometry provides the closest match to the bound complex, meaning that formation of the tetrahedral boronate dial complex reduces ring strain and lowers the energy of the bound species. This results in a shift of the dynamic equilibrium between the neutral boronic acid dial complex 19 atui the boronate anion diol complex 20, causing the observed increase in the value of the acidity constant, Kf.

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