Carboranes and Organoboranes

Boranes in Organic Chemistry, 11, 1 Carboranes and Organoboranes, 3, 263 Catalyses by Cobalt Carbonyls, 6, 119... 

This review will restrict itself to boron-carbon multiple bonding in carbon-rich systems, as encountered in organic chemistry, and leave the clusters of carboranes rich in boron to the proper purview of the inorganic chemist. Insofar as such three-dimensional clusters are considered at all in these review, interest will focus on the carbon-rich carboranes and the effect of ring size and substituents, both on boron and carbon, in determining the point of equilibrium between the cyclic organoborane and the isomeric carborane cluster. A typical significant example would be the potential interconversion of the l,4-dibora-2,5-cyclohexadiene system (7) and the 2,3,4,5-tetracarbahexaborane(6) system (8) as a function of substituents R (Eq. 2). 

ORGANOBORANE. A compound composed of an unsaturated organic group and a borane obtained by the hydroboration reaction. Such compounds are useful catalytic reagents in organic syntheses of some complexity, e,g, cis- or (/awf-olefins, optically pure alcohols, alkanes, and ketones. Prostaglandins and insect pheromones have been synthesized by tlus means. A particularly versatile example is triphenylboron B(CgHsty. See also Borane Carborane and Hydroboration. 

As discussed in the introduction (Section 3.2.1), derivatives of the diboracyclo-propane 1C are non-classical organoboranes having 8 SE, and according to the 2n+2 SE rule may be classified as the simplest doso-carboranes of the series CH(BH) H ( = 2). Compounds IB and 1C have been computed [5] to be 17.5 and 47.6 kcal mol-1 lower in energy, respectively, than the classical diboracyclopropane 1A. They are 2e aromatics and possess planar-tetracoordinate centers in IB this unusual geometry is found at the carbon and one boron atom, in 1C at both boron atoms (Scheme 3.2-2). 

Bernd Wrackmeyer received his Diploma in Chemistry from the University of Munich in 1971 and his Ph.D. from the same University with Heinrich Noth in 1973. He performed postdoctoral studies at John Cass College of Science and Technology in London with William McFarlane in 1974 and moved back to the University of Munich in 1975, where he finished his habilitation in 1979. He became a Heisenberg-Fellow in 1983 and moved to the University of Bayreuth in 1986. His work focuses on the application of multinuclear magnetic resonance spectroscopy (NMR), on the chemistry of organoboranes, carboranes, Group 14 metal chemistry and on metal amides. 

This article covers only a part of the chemistry of boron. Boron-carbon compounds are covered in other articles in this volume see Boron Organoboranes Boron Metallacarbaboranes, and Boron Polyhedral Carboranes). The main subject of the latter two articles, and the separate one on Boron Hydrides is the extensive chemistry of the multicenter bonded boron-hydride systems. This area has been a major focus of boron research for the past 60 years. There is some direct overlap between the two articles Borides Solid-state Chemistry and Borates Solid-state Chemistry, and this more general one covering the inorganic chemistry of boron. Boron-Nitrogen Compounds are also covered separately. These articles should be consulted for more detailed discussions of the structure, bonding, and properties of borides, solid-state borates, and boron-nitrogen compounds. 

Cyclic organoboranes containing mnltiple boron atoms within the ring stracture have been extensively stndied as precursors to (multidecker) transition-metal complexes and to nonclassical organoboranes (see Boron Metallacarbabo-ranes and Boron Polyhedral Carboranes). Earlier efforts have been thoroughly reviewed. Recently, a series of triboracyclopentane derivatives were reported by Bemdt and Schleyer (Scheme 6). " Reduction of these boracy-cles gave the nonclassical bishomoaromatic anions (28) and homoaromatic dianions (29). Related trishomoaromatic dianions obtained via reduction of 1,3,5-triboracyclohexane derivatives with lithium were studied by Siebert and Schleyer. i 5> ... 

The field of borane chemistry continues to expand as evidenced by the extent of published literature in the field during the past years borane lectures [1] included transition metal-promoted coupling of boranes and carboranes [2], the kinetics and mechanisms of the thermolysis and photolysis of binary boranes [3], organoboranes for synthesis [4], and a description of a career in organoborane chemistry [5]. 

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