Boronic trigonal geometry

The stereoselectivity of any particular reaction depends on the details of the structure of the transition state. The structures of several enone-Lewis acid complexes have been determined by X-ray crystallography.11 The site of complexation is the carbonyl oxygen, which maintains a trigonal geometry, but with somewhat expanded angles (130-140°). The Lewis acid is normally anti to the larger carbonyl substituent. Boron trifluoride... 

Boron trifluoride has a plane trigonal shape a 2p orbital on each fluorine atom overlaps with a boron sp2 hybrid. In general, we can expect that all molecules in which a central atom uses three equivalent sp2 hybrid orbitals will exhibit plane trigonal geometry, since this represents the most symmetrical, and hence equivalent , arrangement of the three bonds. 

To further the mechanistic understanding, computational studies have been conducted by Bock and co-workers, who analysed a range of possible transition states in the formation of the B-O bond. In these studies the reaction was studied on the basis that a molecule of water was eliminated in each step, trigonal geometry was therefore restored to boron in each step. The binding of methanol, 1,2-ethanediol and o-glucose at boron were examined. Where the reactions were simulated in vacuo or in acetonitrile the activation barriers were significant. On the other hand, if water, ammonia or NaOH were used in... 

Stereoelectronic factors are also important in determining the stmcture and reactivity of complexes. Complexes of catbonyl groups with trivalent boron and aluminum compounds tend to adopt a geometry consistent with directional interaction with one of the oxygen lone pairs. Thus the C—O—M bond angle tends to be in the trigonal (120-140°) range, and the boron or aluminum is usually close to die carbonyl plane. ... 

Two electron pairs are as far apart as possible when they are directed at 180° to one another. This gives BeF2 a linear structure. The three electron pairs around the boron atom in BF3 are directed toward the comers of an equilateral triangle the bond angles are 120°. We describe this geometry as trigonal planar. 

The formation of dimeric products is unique for the case of boron, because analogous complexes with other elements are all monomeric [95]. This can be attributed to the small covalent radius of the boron atom and its tetrahedral geometry in four-coordinate boron complexes. Molecular modeling shows that bipyramidal-trigonal and octahedral coordination geometries are more favorable for the formation of monomeric complexes with these ligands. 

Trimethylboron is an example of one type of Lewis acid. This molecule has trigonal planar geometry in which the boron atom is s hybridized with a vacant 2 p orbital perpendicular to the plane of the molecule (Figure 21-11. Recall from Chapter 9 that atoms tend to use all their valence s and p orbitals to form covalent bonds. Second-row elements such as boron and nitrogen are most stable when surrounded by eight valence electrons divided among covalent bonds and lone pairs. The boron atom in B (CH ) can use its vacant 2 p orbital to form a fourth covalent bond to a new partner, provided that the new partner supplies both electrons. Trimethyl boron is a Lewis acid because it forms an additional bond by accepting a pair of electrons from some other chemical species. 

The simplest type of Lewis acid-base reaction is the combination of a Lewis acid and a Lewis base to form a compound called an adduct. The reaction of ammonia and trimethyl boron is an example. A new bond forms between boron and nitrogen, with both electrons supplied by the lone pair of ammonia (see Figure 21-21. Forming an adduct with ammonia allows boron to use all of its valence orbitals to form covalent bonds. As this occurs, the geometry about the boron atom changes from trigonal planar to tetrahedral, and the hybrid description of the boron valence orbitals changes from s p lo s p ... 

Reaction of 2 equiv of H0Si(0 Bu)3 with B(0 Bu)3 as a neat mixture at 80 °C results in the formation of Bu0B[0Si(0 Bu)3]2, which can be readily isolated in crystalline form (79%) [64], X-ray crystallography revealed that BuOB[OSi(O Bu)3]2 is monomeric in the soUd state, with a trigonal planar coordination geometry about boron (Fig. 5). Similarly, B[0Si(0 Bu)3]3 can be formed by reaction of B(0 Bu)3 with 3 equiv of H0Si(0 Bu)3 in toluene [64]. The related triphenylsiloxide compound B(OSiPh3)3 had been previously reported [109]. 

In the molecule of 4-methylene-3-borahomoadamantane derivative 79, the structure of which was determined by X-ray analysis, the six carbon atoms of the triene system, the two boron and two silicon atoms all lie in one plane within experimental error (mean deviation 1.4 pm). The boron atoms deviate from the trigonal-planar geometry, since the sum of bond angles around the atoms is only 355.8° instead of 360°, as usually encountered in triorganoboranes. Considerable distortions of the bond angles at the terminal C-C double bond occurs in the vicinity of the boron atoms B-C(4)-C(ll) 130.60(19)° and B-C(4)-C(5) 107.38(17)° <2002CEJ1537>. 

Derivatives 80 and 81 have the same stereochemistry, indicating the deviation of the boron from trigonal-planar geometry into a pyramidal one (Table 5). 

The geometry of the BH3 molecule is trigonal planar. The net vectorial force applied on the boron atom by the three polar bonds is zero due to the symmetrical shape, so the molecule is nonpolsur. 

Hybridization. A satisfactory description of covalent bonding should also be able to account for molecular geometry, that is, for the mutual directions of bonds. Let us take for an example boron trifluoride, which is a trigonal planar molecule. Boron uses three orbitals to form three completely equivalent bonds to fluorine atoms. 

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