Boron-carbon hydride types

CARBORANE. A cry stalline compound composed of boron, carbon, and hydrogen. It can be synthesized in various ways, chiefly by the reaction of a borane (penta-or deca-) with acetylene, either at high temperature in the gas phase or in the presence of a Lewis base. Alkylated derivatives have been prepared. Carborancs have different structural and chemical characteristics and should not be conTused with hydrocarbon derivatives or boron hydrides. The predominant structures arc the cage type, the nest type, and the web type, these terms being descriptive of the arrangement of atoms in the crystals. Active research on cargorane chemistry has been conducted under sponsorship of the U.S. Office of Naval Research, http //www.onr.navy.mil/... 

The element before carbon in Period 2, boron, has one electron less than carbon, and forms many covalent compounds of type BX3 where X is a monovalent atom or group. In these, the boron uses three sp hybrid orbitals to form three trigonal planar bonds, like carbon in ethene, but the unhybridised 2p orbital is vacant, i.e. it contains no electrons. In the nitrogen atom (one more electron than carbon) one orbital must contain two electrons—the lone pair hence sp hybridisation will give four tetrahedral orbitals, one containing this lone pair. Oxygen similarly hybridised will have two orbitals occupied by lone pairs, and fluorine, three. Hence the hydrides of the elements from carbon to fluorine have the structures... 

Hydroboration is a reaction m which a boron hydride a compound of the type R2BH adds to a carbon-carbon bond A carbon-hydrogen bond and a carbon-boron bond result... 

In the second type of mixed hydride, hypercarbon atoms feature alongside boron atoms in the polyhedral molecular skeleton. Most of these mixed hydrides contain more boron atoms than carbon atoms, and their formulae and structures can be understood simply by isoelectronic replacement of B by C, B by C+, or BH units by C atoms in the parent borane. They are therefore known as carbaboranes, though the shorter name carboranes, coined soon after their discovery, "- is that most often used for this important category of compound and will be used here. Their now well-documented chemistry, " particularly structural and bonding aspects, is the concern of the present chapter. 

The second-row diatomic hydrides AH, A = Li to F, with data given in Table 2, exhibit all three types of surfaces. This is made clear in Figure 2, which displays the arms of the interatomic surfaces for the hydrides relative to a fixed position of the proton. The change in the direction of charge transfer, initially from A to H up to boron and from H to A beyond carbon (which has close to zero charge transfer) is evident in the advance of the surface towards the proton - the advance of the bond critical point - the... 

The addition of metal and metalloid hydrides to carbon-carbon double bonds is not a new reaction, having been observed from time to time with silanes of the type R3SiH under free-radical conditions (4%, 85) and with boron hydrides (68). The versatility of such hydride-olefin interactions, nevertheless, first became evident with the recent researches of Ziegler with lithium and aluminum alkyls (139). The observation that attempted distillation of ethyllithium led to decomposition into lithium hydride, ethylene, and higher olefins prompted the following formulation of the reaction course (see 18) ... 

Another interesting example is the use of ILs in Suzuki cross-coupling reactions (carbon-carbon bond formation between an sp3 carbon or non-P-hydride containing electrophile and a boronic acid derivative Miyaura Suzuki, 1995). Cyphos IL 101 has been recently reported as an extremely successful medium for such types of reaction (see Figure 7, McNulty et al, 2002). The palladium catalyst (Pd2(dba)3CHCl3) was dissolved in the IL, forming a stable solution in absence of oxygen, and hence could also be effectively recycled after solvent extraction of reaction products. The Suzuki reactions in Cyphos IL 101 were efficient even under moderate conditions (50-70°C), whereas the use of imidazolium-based ILs in contrast requires elevated temperatures (up to 110°C) (Mathews et al., 2000). 

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