Boranes from boron halides

The main chemical products produced from these minerals are (a) boron oxides, boric acid and borates, (b) esters of boric acid, (c) refractory boron compounds (borides, eu .), (d) boron halides, (e) boranes and carbaboranes and (f) organoboranes. The main industrial and domestic uses of boron compounds in Europe (USA in parentheses) are ... 

Because they serve as versatile starting materials for a variety of other boranes, the dihalo boranes, particularly the fluoro and chloro derivatives, are the most important members of this class of compounds. Their chemistry, along with other pentafluor-ophenyl boron halides, has been reviewed recently from a personal perspective by Chi vers,15 whose PhD thesis described early explorations into the chemistry of these compounds. Most of the known literature on these compounds deals with pentafluor-ophenyl compounds, but the chemistry is likely extendable to other compounds with different AiF groups. 

Some authors have described the generation of boronates and stannanes bound to a support starting from the corresponding aryl haHdes [372, 6]. Boranes, boronic esters, and stannanes can furthermore be readily obtained from vinyl halides or from alkenes or alkynes by means of hydroboration or hydrostannylation (see Section 4.2). Boronates and silanes or stannanes can act as carbanion equivalents. Thus, support-bound boronates can release aryl alkenyl groups upon transmetala-tion to Rh. The intermediately formed Rh species can act as nucleophiles, and react with aldehydes to give alcohols (618) or can perform Michael additions (621, 622) [437, 438, 439] (Scheme 129) (see also Sections 4.7.15 and 4.2). 

The most versatile route to Ge—boron-containing species is from Ge halides and boron hydride anions, products embracing both oxidation states of Ge, inorganic and mono-, di-, and triorganogermyl derivatives, as well as borane and carborane substrates (Table 1) (cf. 5.3.7.3.2 for the analogous Si-based reactions). 

Group III.—Boron. It has only recently been conclusively shown that the stoicheiometry of the boric acid-tartaric acid complex is 1 1. Kinetic results from T-jump experiments indicate attack by an alcohol-oxygen lone-pair at the electron-deficient boron. Other references to reaction mechanisms of three-co-ordinate boron compounds deal with hydrolysis and methanolysis of boron halides, with disproportionation of boranes, and with Sh2 radical reactions of alkylboranes. ... 

Boron has a great affinity for oxygen and occurs in nature only in boric acid or borates. Borates are composed from clusters of flat trigonal BO3 and tetrahedral BO4 groups. The structural chemistry of borates is as rich and complicated as those of silicates, borides, or boranes. Boron oxide is an essential part of borosilicate glasses such as Pyrex. Boron halides are volatile molecular compounds. They are Lewis acids and react violently with water. The subhalides consist of boron chains or clusters that have terminally bound halogen atoms. They are substitution derivatives of the lower boranes. 

Phosphorus pentahalide complexes contain phosphonium cations and boron tetrahalide anions, for example, PCI4 BCI4 (Chapter 2.6). Phosphonium borohydrides, R4P BjH., can be prepared from phosphonium halides and sodium borohydride. On hydrolysis they give phosphine boranes. Salts with phosphonium cations and boron-containing anions are formed in other reactions (6.379) and (6.531). 

The formation of boron-group IB bonds succeeds in two ways by transfer of a boryl group from metal-boron compounds to other metals, and by reaction of anionic boranes or carboranes with transition-metal halides. 

The synthesis of compounds containing boron-group IIB bonds also occurs by two pathways. The most convenient method is using anionic borane or carboranc species, as in the group-IB case, to react with Zn, Cd, or Hg halides. Moreover, reactions between organometallic compounds (Zn, Cd) with unchanged boranes arc also realized, so Zn and Cd decaboranes, e.g., MB H 2 n solvent (M = Zn, Cd), result from reactions of decaborane(14) with MRj . 

Just as for group 5, 6, and 7 ( -CsF MCU species, Fehlner has shown that BH3-THF or Li[BH4] react with group 8 and 9 cyclopentadienyl metal halides to result in metallaborane clusters, many of them having a metal boron ratio of 1 3 and 1 4, and much of the synthetic chemistry and reactivity shows close connections with the earlier transition metals. The main difference between the early and later transition metallaboranes that result is that the latter are generally electron precise cluster species, while as has been shown, the former often adopt condensed structures. Indeed, as has been pointed out by King, many of the later transition metallaborane clusters that result from these syntheses have structures closely related to binary boranes and, in some cases, metal carbonyl clusters such as H2Os6(CO)18.159... 

Reagents of choice for reduction of epoxides to alcohols are hydrides and complex hydrides. A general rule of regioselectivity is that the nucleophilic complex hydrides such as lithium aluminum hydride approach the oxide from the less hindered side [511, 653], thus giving more substituted alcohols. In contrast, hydrides of electrophilic nature such as alanes (prepared in situ from lithium aluminum hydride and aluminum halides) [653, 654, 655] or boranes, especially in the presence of boron trifluoride, open the ring in the opposite direction and give predominantly less substituted alcohols [656, 657,658]. As far as stereoselectivity is concerned, lithium aluminum hydride yields trans products [511] whereas electrophilic hydrides predominantly cis products... 

The interaction of platinum halides with boron hydrides has been used to couple boranes. In the presence of PtBr2 at 25 °C the compound 1,2 -(B5H8)2 can be formed from B5H9 (equation 91). 2... 

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