Lewis acidic organoboron compounds

Of particular sigiuficance in the chemistry of organoboranes is their behavior as Lewis acids, which is a result of the empty / -orbital of tricoordinate boron. Much recent effort has been devoted to the synthesis and development of highly Lewis-acidic organoboron compounds through introduction of fluorinated organic substituents. Boron can reach the... 

AbRfi (or AIR3) react readily with ligands to form adducts, LAIR3. They are stronger Lewis acids than are organoboron compounds, BR3, and can be considered as hard (or class a)... 

We note that while tin reagents have often been employed for the organoboron halides/ the use of organostannanes as starting materials can also be applied to the synthesis of heavier group 13 derivatives. In the context of polyfunc-tional Lewis acid chemistry, this type of reaction has been employed for the preparation of ort/ o-phenylene aluminum derivatives. Thus, the reaction of 1,2-bis(trimethylstannyl)benzene 7 with dimethylaluminum chloride, methylaluminum dichloride or aluminum trichloride affords l,2-bis(dimethylaluminum)phenylene 37, l,2-bis(chloro(methyl)aluminum)phenylene 38 and 1,2-bis(dichloroalumi-num)phenylene 39, respectively (Scheme 16). Unfortunately, these compounds could not be crystallized and their identities have been inferred from NMR data only. In the case of 39, the aluminum derivative could not be separated from trimethyltin chloride with which it reportedly forms a polymeric ion pair consisting of trimethylstannyl cations and bis(trichloroaluminate) anions 40. 

Lack of space prevents descriptions of a number of additional types of boron compounds such as the numerous interstitial metallic borides, the subhalides (of type B2CI4), the organoboron compounds (such as B(CH3)3), and the boron-nitrogen addition compounds, many of which have been prepared by H. C. Brown to study the effects of structures on the stability of Lewis acid-base adducts. ... 

The mechanistic aspects of this Lewis acid promoted reaction have been examined by low temperature NMR studies, and reaction of the lithium alkynides with the Lewis acid activated epoxides is indicated. The order of the addition of the reagents does not affect the product yields provided that the reaction is carried out at -78 C addition of BF3-OEt2 to a mixture of an epoxide and an alkynide or addition of an epoxide to a mixture of an alkynide and BF3-OEt2 are both possible. The transmetalation between RLi and BF3 which produces unreactive organoboron compounds is shown to be very slow at this temperature. - ... 

Sugimoto, H., Aida, T., and Inoue, S., 1993, Organoboron Compounds as Lewis Acid Accelerators for the Aluminum Porphyrin-Mediated Living Anionic Polymerization of Methyl Methaciylate. Macromolecules, 26 4751... 

In addition to organoaluminum compounds, organoboron compounds can also be used as Lewis-acid accelerators for the polymerization of 21 initiated with la, where triphe-nylboron and tris(perfluorophenyl)boron are effective accelerators (See Table 6). Tributylboron (Bu B) bearing no particular steric hindrance neither terminates nor accelerates the polymerization of 21, indicating a low transmetalation activity and a rather poor Lewis acidity of Bu B. This is in sharp contrast with the polymerization upon addition of trimethylaluminum (Me Al) as a monomer activator where the chain growth is terminated by the transfer of the enolate moiety from 32 to Me Al, unless the temperature is lowered to —40°C or below. 

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