Dicarba-closo-carboranes

But the discovery of the carboranes in the early 1960s revealed that bonding possibilities other than simple o-- or 7r-bonds between B and C centers were necessary to understand the structure of such compounds as 1,5-dicarba-closo-pentaborane(5) [6 in Eq. (1)], which is obtained in low yield in an electrical discharge.6 Ordinary valence conventions cannot account for the bonding of boron to five other atoms, and hence the concept of electron-deficient bonding must be invoked for boron. Although carbon seems to adhere to normal tetravalence, again it should be remembered... 

The discovery of polyhedral boranes and polyhedral heteroboranes, which contain at least one atom other than in the cage, initiated a new era in boron chemistry.1-4 Most commonly, of the three commercially available isomeric dicarba-closo-dodecaborane carboranes(l,2-, 1,7-, and 1,12-), the 1,2-isomer 1 has been used for functionalization and connection to organic molecules. The highly delocalized three-dimensional cage bonding that characterizes these carboranes provides extensive thermal and kinetic stabilization as well as photochemical stability in the ultraviolet and visible regions. The unusual icosahedral geometry of these species provides precise directional control of all exopolyhedral bonds. 

Figure 53 The 1,2-, 1,7-, and 1,12-dicarba-closo-dodecaboranes, better known by the more popular names o- (85), m- (86), and / -carborane (87), respectively. (Adapted from ref. 119.)...

Precise placement of metal complexing sites within the infrastructure of a cascade molecule is of importance from a variety of perspectives. In the construction of the above noted Micellane family (cf. Sect. 3.1), we reported the construction of dendrimers with four alkyne moieties at sites equidistant from each other in the interior (17, Fig. 8) [60]. These were treated with decaborane (B10H14) to afford 1,2-dicarba-closo-dodecaboranes (o-carboranes) [71]. Rendering boron clusters soluble in water is of interest because of their use in cancer treatment by Boron Neutron Cancer Therapy. First and second generation water-soluble dendrimers containing four and twelve precisely located boron cluster sites, respectively, were synthesized (e.g., 18). These water soluble dendrimers and their precursors were characterized by 13C-, and nB-NMR spectroscopy (Fig. 8). 

Reduction of the pentaalkyl-1,5-dicarba-closo-pentaboranes(5) 44 with elemental K opens the cluster and subsequent reaction with I2 enforces oxidative fusion of two C2B3 units to give a C4B6 cage (Scheme 3.2-36). The peralkylated hexaboraadamantane derivatives 70 (R = Me, Et X-ray structure analysis for R = Me [89]) rearrange irreversibly into the carboranes 71 with a wido-C4B6 framework [89] (X-ray structural analysis for R = Et [90]). The structure of the nido-C Bf, cluster is fluxional in solution [91]. 

Telluran [1,2-Dicarba-closo-dodecaboran( 12)-l -yl]-trichlor-E12b, 306 (O-Carboran + TeCl4)... 

The incorporation of a large number of boron atoms into a tumor for boron neutron capture therapy (BNCT) has been accorded considerable interest over the last few years [67]. It is known that if boron-containing cells are exposed to a neutron flux, they are destroyed without causing irreparable harm to adjacent healthy tissue. In the context of SERMs, for example, Endo et al. have prepared complex 18 [ 1 - (hydroxymethyl) -12- (4-hydroxyphenyl) -1,12-dicarba-closo-dodeca-borane] in which the hydrophobic carborane cage mimics the C and D rings of estradiol (Scheme 3.7) [68]. 

C4Hi4BioBr2, 1,2-Bis(bromomethyl)carborane, 29, 531 C4H14B10CI2, 5,12-Dichloro-1,7-dimethyl-1,7-dicarba-closo-dodecaborane ( 1 2), 39B, 468 C4H15B9, 6,9-Dimethylcarborane, 31B, 291... 

C4H15B1oClHg, 1-Methylmercury-2-chloromethyl-o-carborane, 44B, 573 C4H16AIB9, 3-Ethyl-3-alumina-1,2-dicarba-closo-dodecaborane(12),... 

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