BC13

Another definition of acids and bases is due to G. N. Lewis (1938). From the experimental point of view Lewis regarded all substances which exhibit typical acid-base properties (neutralisation, replacement, effect on indicators, catalysis), irrespective of their chemical nature and mode of action, as acids or bases. He related the properties of acids to the acceptance of electron pairs, and bases as donors of electron pairs, to form covalent bonds regardless of whether protons are involved. On the experimental side Lewis definition brings together a wide range of qualitative phenomena, e.g. solutions of BF3, BC13,... 

In contrast to this, very moisture-sensitive black OsCl5, prepared by chlorinating OsF6 using BC13 as the chlorinating agent, has the dimeric... 

Boron trichloride, a colorless, reactive gas of BC13 molecules, behaves chemically like BF3. However, the trichloride of aluminum, which is in the same group as boron, forms dimers, linked pairs of molecules. Aluminum chloride is a volatile white solid that vaporizes at 180°C to a gas of Al2Cl6 molecules. These molecules survive in the gas up to about 200°C and only then fall apart into A1C13 molecules. The Al,CI6 molecule exists because a Cl atom in one AlCI, molecule uses one of its lone pairs to form a coordinate covalent bond to the Al atom in a neighboring AICI molecule (33). This arrangement can occur in aluminum chloride hut not boron trichloride because the atomic radius of Al is bigger than that of B. 

The B atom has an incomplete octet in all its trihalides. The compounds consist of trigonal planar molecules with an empty 2p-orbital perpendicular to the molecular plane. The empty orbital allows the molecules to act as Lewis acids, which accounts for the catalytic action of BF3 and BC13. 

In 1978, Sugasawa et al., at Shionogi Pharmaceutical Co. reported ortho-selective Friedel-Craft acylation with free anilines with nitrile derivatives [4]. Sugasawa reported that the reaction requires two different Lewis acids (BC13 and A1C13) and does not proceed when N,N-dialkyl anilines are used. He proposed that boron bridging between nitriles and anilines led to exclusive ortho-acylation but a conclusive mechanism was not elucidated. The report did not offer any reason why two different Lewis acids were required and why the reaction did not progress with N,N-dialkyl anilines. Therefore, we initiated mechanistic studies. 

This reaction generates lmole of HAICI4, which protonates anilines. Since protonated anilines could not coordinate with BC13, the reaction shuts down. 

Scheme 18. Reagents a, BC13, CH2C12 b, 5-chloro-l-phenyl-l//-tetrazole, benzene c, H2/Pd-C, AcOH d, CH2N2, MeOH.

Der zwischen A1H3 und BC13 beobachtete Austausch von Hydrid gegen Chlorid findet sich auch zwischen A1H3 und Aluminiumhalogeniden C196,193,230-232) ... 

Umsetzung von BC13 mit monomerem Al(NPr2)3 rasch in Analogic zu (177) ab (147). 

Dies ist insofem uberraschend, als BC13 von LiAlH4 nach (234) zu Diboran hydriert wird, d.h. daB auch (A1H3)X in der Lage sein miiBte, das nach (233) anfallende Bortrichlorid zu hydrieren. 

Das Bortrisazid laBt sich aus BC13 und LiN3 nicht darstellen. In Benzol tritt keine Reaktion ein, und in Methylenchlorid bleibt die Azidierungs-reaktion auf der Stufe des triraeren Bordichloridazids stehen, das auf diesem Wege zum erstenmal in 20-proz. Ausbeute erhalten wurde (10) ... 

Figure 8.3 Contour maps of the electron density distribution of LiCI, BeCL, BC13, and CC14.

Figure 8.8 Contour maps of the Laplacian L for LiCI, BeCl2, BC13 and CO4.

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