Heteroatom borane

Heteroboranes contain heteroelements classified as nonmetals. The heteroatoms known to form part of a borane polyhedron include C, N, Si, P, As, S, Se, Sb, and Te either alone or in combination. In principle, most heteroboranes could have a wide range of skeletal sizes. However, with the primary exception of the carbaboranes, extensive chemistry has emerged only for the thiaboranes and azaboranes, which have the greatest availabiLity and demonstrated scope of chemistry. 

The field of transition metal-catalyzed hydroboration has developed enormously over the last 20 years and is now one of the most powerful techniques for the transformation of C=C and C=C bonds.1-3 While hydroboration is possible in the absence of a metal catalyst, some of the more common borane reagents attached to heteroatom groups (e.g., catecholborane or HBcat, (1)) react only very slowly at room temperature (Scheme 1) addition of a metal catalyst M] accelerates the reaction. In addition, the ability to manipulate [M] through the judicious choice of ligands (both achiral and chiral) allows the regio-, chemo-, and enantioselectivity to be directed. 

Heteroatom Cluster Compounds Incorporating Polyhedral Boranes as Ligands... 

Hydroboration, the addition of a boron-hydrogen bond across an unsaturated moiety, was first discovered by H. C. Brown in 1956. Usually, the reaction does not require a catalyst, and the borane reagent, most commonly diborane (B2H6) or a borane adduct (BH3-THF), reacts rapidly at room temperature to afford, after oxidation, the /AMarkovnikov alkene hydration product. However, when the boron of the hydroborating agent is bonded to heteroatoms which lower the electron deficiency, as is the case in catecholborane (1,3,2-benzodioxaborole) 1 (Scheme 1), elevated temperatures are needed for hydroboration to occur.4 5... 

Interestingly, homolytic substitution at boron does not proceed with carbon centered radicals [8]. However, many different types of heteroatom centered radicals, for example alkoxyl radicals, react efficiently with the organoboranes (Scheme 2). This difference in reactivity is caused by the Lewis base character of the heteroatom centered radicals. Indeed, the first step of the homolytic substitution is the formation of a Lewis acid-Lewis base complex between the borane and the radical. This complex can then undergo a -fragmentation leading to the alkyl radical. This process is of particular interest for the development of radical chain reactions. 

Building on and extending earlier studies, Schleyer, Najafian, et al. [57] employed computed geometric, energetic, and magnetic properties to quantify the aromaticity of the closo boranes B H 2 (6 < n < 12), and their isoelectronic counterparts, the CB iH and C2B 2H carboranes, and the NB H azaboranes [34]. All possible heteroatom placements were considered. The most stable iso-... 

In comparison with the enormous number of carboranes the family of the heavier group 14 heteroborane clusters is yet very small. Most of the heteropoly-borane skeletons with Si, Ge, Sn or Pb atoms belong to the group of larger sized closo- and nido-clusters and the chemistry of these heteroboranes is essentially concentrated on the heteroatoms. 

Borane Clusters with Croup 15 and Croup 16 Heteroatoms Survey of Compounds and Structures... 

Heteroboranes are those in which one or more non-boron atoms replace a BH vertex, together with groups that may be attached to these heteroatoms. Boranes that contain CH vertices constitute the vast family of carbaboranes. The possibility for carbon to participate in electron-deficient frameworks contradicted the former prejudice of the always electron-precise carbon as the well-behaved brother of naughty boron. So far, most elements have been introduced as heteroatoms into borane frameworks, with the exception of the halogens and the noble gases. 

In the following section, we give a survey of the known types of heteroboranes in question and discuss the structures in terms of the Wade-Williams rules. Readers, who are interested in the synthesis of heteroatom clusters, in their skeletal transformations, or in reactions at the ligand sphere, are referred to the cited literature. conjuncto- Boranes with heteroatoms in the skeleton are not considered in this brief discussion. 

In 1971, a note 164) was published favoring the hypothesis that the carboranes, boranes, their isoelectronic anions, Lewis base adducts, and heteroatom-substituted analogs should be viewed as constructed about the vertices of either the most spherical series of triangular-faceted polyhedra (deltahedra) found to be characteristic of the dicarba-cZoao-carboranes (Fig. 1) or, with one lone exception, fragments of the series of deltahedra produced by the successive removal of the highest coordinated vertices that sequentially define the nido and arachno classes. This position was in conflict with the then prevalent shibboleth that all nido and arachno compounds [except B5H9 (I-N5)] had or would prove to have icosahedral fragment structures. 

Chiral Dialkylboranes. Several allylic boranes have been developed as chiral auxiliary reagents (Fig. 5). The introduction of terpene-based reagents such as 12 and 64-68 has been pioneered by H.C. Brown, and the most popular class remains the bis(isopinocampheyl) derivatives (structures 12, 64-66). A wide variety of substituted analogs have been reported, including the popular crotylboranes but also a number of other reagents bearing heteroatom-... 

Arachno Clusters (2 n + 6 Systems). In comparison to the number of known closo and nido boranes and heteroboranes, there are rdatively fewer arachno species. Pardy because of the lack of a large number of structures on which to base empirical rules, arachno structures appear to be less predictable than their closo and nido counterparts. For example, there are two isomeric forms of B9H15, one with the arachno [19465-30-6] framework shown in Figure 2 (33), the other with a framework more reminiscent of that shown for the nine-atom nido classification (34). Structures of arachno molecules involve the presence of even more extra hydrogens or other electron-donating heteroatoms than nido molecules. Typical examples are given in Table 1. 

Fused Rings and Polymers. Borazine analogues of naphthalene and substituted phenalenes are known the latter compound is formed by reaction of distannylamine and a large excess of tris(methylthio)borane (137). A general method for the preparation of the polycyclic borazines using fused carbon—heteroatom rings, where X = O, NR and n = 2,3, has been given (137). 

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