Boron compounds metallated ring system

A new synthesis of dibenzoboroles by reductive cyclization of arylboron dibromides opens up access to several derivatives of this ring system (Scheme 6) <1996JA7981>. MonoaryIboron dibromides 61 and 62 were prepared by reaction of the appropriate aryl lithium compound 60 with boron tribromide in hexane. Reductive cyclization of arylboron dibromide 61 with an excess of lithium metal in diethyl ether gave bislithium dibenzoborole complex 63. At the time of writing, compound 63 was the first dibenzoborole dianion to be structurally characterized by X-ray... 

Early applications of chiral Lewis acid catalyzed stereoselective Diels Alder reactions used either boron- or aluminum-derived systems in carbocyclic ring formation18 1Q, or studied the effect of chiral shift reagents, such as Eu(hfc)3, in hetero-Diels-Alder cycloadditions of carbonyl compounds to dienes20 23,77, 78. The latter type of transition metal catalyzed addition is classified as heterocarboration and is described in Section 1.5.8.4. 

The heterocyclic compounds occupy an important place amongst boron-containing ligands [8], It is well known that borazines, having a six-member ring, form ri6(7i)-complexes, for example 554 [953], It was recently shown that cr(B)-metal-coordinated compounds of type 555 may also be obtained on the basis of this ligand system [954a] ... 

The empty 2/j,-orbital on boron interacting with the ji-electrons of the butadienylidene system in borole (5) is proved by the pronounced bathochromic shift in the visible spectrum compared with that of its pyridine adduct. This Ti-electron delocalization in boroles is destabilizing and produces a 471-electron antiaromatic system. The antiaromaticity is further evidenced by the rapidity of reactions that remove the 2p ,-orbital on boron from conjugation. These reactions include Lewis complexation, transition metal complexation, or borole ring opening reactions such as Diels-Alder addition, oxidation, and protodeboration. This removal of the antiaromaticity from borole results in transforming the compound to a typical, low reacting, sterically hindered arylborane. 

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