Borane chemistry

Localized Bonds. Because boron hydrides have more valence orbitals than valence electrons, they have often been called electron-deficient molecules. This electron deficiency is partiy responsible for the great interest surrounding borane chemistry and molecular stmcture. The stmcture of even the simplest boron hydride, diborane(6) [19287-45-7] 2 6 sufficientiy challenging that it was debated for years before finally being resolved (57) in favor of the hydrogen bridged stmcture shown. 

One such reported example is the synthesis of polypropylene-6-polymethyl-methacrylate (PP-6-PMMA) copolymers utilizing metallocene catalysis and the borane chemistry. In the initial step, PP with chain-end olefinic unsaturations was prepared using metallocene catalysts such as Et(Ind)2ZrCl2/MAO. The unsaturation sites were then hydroborated by 9-borabicyclo[3.3.1]nonane (9-BBN) to produce borane-terminated PP (43) (Fig. 30), which was selectively oxidized and interconverted to a... 

In constrast to the previous section, the literature in this area over the period of the review was dominated by metalloheteroborane (mainly metallocar-borane) chemistry, narrowly focussed on charge-compensated carbollide C2B9H10L derivatives, with relatively few examples of metalloboranes. In... 

Even though qualitative bonding descriptions of metal atom clusters up to six or seven atoms can be derived and in some cases correlated with structural detail, it is clear that most structures observed for higher clusters cannot be treated thus. Nor do the structures observed correlate with those observed for borane derivatives with the same number of vertices. Much of borane chemistry is dominated by the tendency to form structures derived from the icosahedron found in elemental boron. However, elemental transition metals possess either a close-packed or body-centered cubic arrangement. In this connection, one can find the vast majority of metal polyhedra in carbonyl cluster compounds within close-packed geometries, particularly hexagonal close-packing. 

An unusual type of deltahedral cluster found in organoindium chemistry but not in deltahedral borane chemistry is the eight-vertex In8[Si(CMe3)3]6 (see Chapter 2.3.4.2) its structure is based on a bicapped octahedron with idealized D34 sym-... 

Today the chemistry of diborane and the polyboranes is well understood [2] and much of it is textbook knowledge. Therefore, after a brief survey, emphasis will focus on the development of polyhedral borane chemistry within recent decades, and even restricting discussions to homopolyboranes only certain areas can be dealt with. This incorporates synthetic procedures, the chemistry of some polyboranes and particularly polyborane anions. Other chapters of this book are devoted to heteropolyboranes such as the carbaboranes (see Chapter 3.1), azaboranes and related heteropolyboranes (see Chapter 3.3) of the main group elements. In these areas enormous progress has been achieved within the last two decades. 

One might expect that a large number of isomers of polyboranes should exist as the number of boron atoms increases, similar to hydrocarbon chemistry. This, however, is not the case in borane chemistry. There are indeed only a few examples of proven isomerism, for instance for the B2oHi82 anions (see Section 5.8). However, this point has been addressed in many theoretical papers, and energy differences between various isomers of a polyborane are, in general, larger than in hydrocarbons. 

These last results combine the Wittig ylide chemistry with Brownes hydroboration reaction. We hope that a preparatively interesting ylide-borane chemistry will arise from this new "alloy". 

Nonetheless, several attempts to quantify Lewis acidity have been made and a few methods are useful in the context of perfluoroaryl borane chemistry. The Childs method9 involves measurement of the perturbation of the 1H NMR signal for the (3-proton... 

Although carbohydrates are cheap and readily available chiral compounds, their application in stereoselective synthesis was for a long time limited to ex-chiral-pool syntheses [3]. They have been considered too complex compared to other chiral auxiliaries, for example a-pinene in borane-chemistry [4] or BINAP-derivatives in reduction chemistry [5]. However, it has been shown during the past few years that carbohydrates can be successfully applied as stereodifferentiating tools in many different reaction types such as aldol- [6], hydrogenation- [7], carbonyl addition- [8], Michael- [9], Diels-Alder- [10], hetero-Diels-Alder [11], and rearrangement reactions [12]. 

Such an anti-Markovnikov hydration was impossible until H. C. Brown, of Purdue University, discovered that diborane (B2H6) adds to alkenes with anti-Markovnikov orientation to form alkylboranes, which can be oxidized to give anti-Markovnikov alcohols. This discovery led to the development of a large field of borane chemistry, for which Brown received the Nobel Prize in Chemistry in 1979. 

In the First Edition of Regular Polytopes, Coxeter stated, ... the chief reason for studying regular polyhedra is still the same as in the times of the Pythagoreans, namely, that their symmetrical shapes appeal to one s artistic sense [23], The success of modem molecular chemistry does not diminish the validity of this statement. On the contrary. There is no doubt that aesthetic appeal has much contributed to the rapid development of what could be termed polyhedral chemistry. One of the pioneers in the area of polyhedral borane chemistry, Earl Muetterties, movingly described his attraction to the chemistry of boron hydrides, comparing it to M. C. Escher s devotion to periodic drawings [24], Muetterties words are quoted here [25] ... 

First achievements. The founder of borane chemistry is rightfully held to be the German chemist A. Stock although the first boron and hydrogen compounds were obtained back in 1881. But, according to N. V. Sidgwick, All statements about the hydrides of boron earlier than 1912, when A. Stock began to work upon them, are untrue . 

The methyl camouflaged species described above demonstrate the amplification of aromatic polyhedral borane chemistry by the prudent application of organic chemistry and the emergence of a novel family of organoboranes. 

The chemistry described above exemplifies the great similarity between aromatic polyhedral borane chemistry and the aromatic branch of organic chemistry. Modular syntheses with carboranes and the discovery of a variety of camouflaged derivatives clearly reveals the tentacles of organic chemistry reaching into the polyhedral borane field. Thus, the conflux of boron and carbon chemistries is broadened. 

A question of interest in deltahedral borane chemistry is whether supraicosahedral boranes B H 2 (n > 13) can be prepared and whether the Frank-Kasper polyhedra (Figure 1) are suitable models for their structures. The supraicosahedral metallacarborane (C5H5)2Co2B10C2H,2 is known21 and has a structure based on the Frank-Kasper bicapped... 

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