The stereochemistry of boron

The stereochemistry of boron is simple in many of its compounds with the halogens (but note BgClg), oxygen, nitrogen, and phosphorus, three coplanar or four tetrahedral bonds being formed. The more complex stereochemistry of the element in electron-deficient systems (elementary B, some borides, and the boranes) is dealt with separately. The planar arrangement of three bonds from a B atom has been demonstrated in many molecules BX3 and BR3 (Table 24.1), in cyclic molecules of the types noted in the previous section, in many oxy-ions (see later section), and in crystals such as the graphite-like form of BN (p. 847) and AlBj (p. 842). 

The tetrahedral arrangement of four bonds from B was established by the methods of classical stereochemistry. A molecule such as that of borosalicylic acid (1) would possess a plane of symmetry if the boron bonds were coplanar. The optical activity of this molecule was demonstrated by its resolution into optical antimers using strychnine as the active base. This really only eliminates a strictly 

BOF (B-F, 1-43 A, B-0, 1-435 A) in salts such as BaBOF3.( The B(OH) ion and many examples of tetrahedral BO4 coordination groups are mentioned under boron-oxygen compounds later in this chapter. 

Simple tetrahedral molecules include H3B. N(CH3)3, borine carbonyl, H3B.C0, (a), the numerous adducts of BF3, and the remarkable molecule B4F6. Pp3, (b). This compound is formed as a very reactive colourless solid when the high-temperature species BF is condensed on to a cold surface with PF3. 

This molecule is of special interest as containing one B atom bonded tetrahedrally to 1 P (B-P, 1-83 A) and to 3 B (B-B, 1-68 A), these B atoms forming three coplanar bonds (B-F, 1-31 A, P-F, 1-51 A). 

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Both the nature of solvent and the temperature influenced the stereochemistry of boron enolates, with the solvent effect being greater than that of the temperature. Aliphatic and alicyclic hydrocarbon solvent favored the formation of yy/z-aldols at — 78 °C, whereas aromatic and chlorinated aliphatic solvents favored the formation of anti-dXdoh at 0-25 °C (Equation (179)).682 683... 

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