Boron heterocycles bonding

Another boron heterocycle with a formula apparently appropriate for a nido-pyramidal structure is the compound H4C4B2F2 (193) (Fig. 28). The fluorine substituents in this molecule, however, can 7r-bond to the boron atoms, thereby generating a quinonoid electronic structure. Elsewhere in boron cluster chemistry, the presence of halogen substituents appears to modify the skeletal electron requirements of the cluster. 

Molecular dimensions measured by X-ray diffraction show that several boron containing rings are close to planarity and that many bond lengths are intermediate between single and double bonds. However, no monocyclic unsaturated boron heterocycle has so far been subjected to this kind of investigation. 

B-Aminoborazines are of particular interest for fundamental studies. In these compounds, boron is bonded to three nitrogen atoms with two different types of environment. B-Aminoborazines are also useful precursors for the synthesis of thermally stable polymers. Quite a few polycondensates of aminoborazines and copolymerisates with organic difunctional molecules have been described 4>. Of major interest are difunctional borazines yielding linear polycondensates. The condensation of l,3,5-tris(2,6-dimethylphenyl)-2,4-dichloroborazine (cf. Section II.2.5) with aliphatic, aromatic, and heterocyclic diamines, as well as the preparation of the same linear polyborazines by transamination of 1,3,5-tris(2,6-dimethylphenyl)2,4-bis(diethyl-amino)borazine with diamines was studied 139). 

The chemical reactivity of several heterocyclic derivatives is closely associated with a reactivity one might expect for open-chain unsaturated derivatives. The boron heterocycles are sensitive to oxygen or hydrogen peroxide 111, 169) and so apparently is 1,1,2,5-tetraphenylsilacyclo-pentadiene 17, 246). These reactions probably lead to ring cleavage but the products have not been characterized. Vinyl-metal bonds are frequently cleaved by electrophilic reagents and the heterocycles are no exception. 

I. Bally and A. T. Balaban Stud. Cercet. Chim. 17. 431 (1969) Unsaturated organoboron heterocycles with boron-oxygen bonds (Rom.) 32 (164)... 

The relatively high stability of carbon-silicon/-boron/-tin bonds allows the metaT to be carried through many heterocyclic syntheses as an inert substitutent some examples are shown below. ... 

Compounds with Isolated Trigonal Boron Atoms and Covalent Boron-Nitrogen Bonding (Aminoboranes and B-N Heterocycles). Literature coverage from 1950 to the end of 1973. 

Organolithium compounds may be used to prepare boron heterocyclics containing B—C,yi bonds. Derivatives of dibenzodihydroborepins may be prepared from tributoxyborane and o,o -dilithiodibenzyl (40). 

Many aliphatic boron heterocyclics can be readily prepared by hydroboration of dienes or polyolefins (30, 45-49). Complete reaction of C=C bonds in polyolefins, such as butadiene, isoprene, 1,5-hexadiene, and cyclododecatrienes, yields colorless liquids which are distillable in many cases under reduced pressure 30, 45-49). 

The dimerization energy of (LXXXVIII) is low in comparison to other bis(borolanes), apparently because the boron is bonded to a secondary carbon. Five-membered boron heterocyclics with B—H bonds, whose boron is attached to an aromatic ring, react as readily as open-chain alkyl or aryl-diboranes at room temperature (68). The dimerization tendency of bora-indane (LXXXIX) (68), boratetralin (XC) (68), and 9-borafluorene (XCI)... 

The current year has seen the appearance of comprehensive data on known carbaboranes up to the end of 1976. - Other reviews have discussed Group IV carbaboranes, carbaborane crystal structures, wfo-monocarbaboranes, car-baborane nomenclature, metallocarbaboranes, and triple decker complexes with boron heterocycles as ligands. Theoretical studies have considered molecular orbitals utilized in bonding carbaboranes and their relationship to cage counting rules. ... 

Boron-nitrogen heterocycles [4,5-diethyl-2,2,3-trimethyl-l-(<9-tnfluoro-methyl)phenyl-2,5-dihydro-l(-)-],2,5-azasilaboroles and -azastannaboroles] are formed in good yields The products demonstrate atropoisomensm due to the hmdered rotation about the N-aryl bond [113] (equation 88)... 

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