Boron halides complex hydrides

Many phosphine-borane complexes Y3P BZ3 have been characterized. They include compounds where Y = alkoxy, alkyl, amino, halide, and hydride groups, and Z = alkyl, halide, and hydride. The stabilities of these complexes vary widely depending on the Lewis acidity and basicity of the boron and phosphorus moieties, respectively. The relative stabilities of Lewis acid-base complexes with BH3 are R3P > R3N > RgAs > R3Sb, but with BF3 the order is R3N > R3P > RsAs > R3Sb. The stabilities of the boron halide complexes of phosphines follow the same order as the amine complexes BI3 > BBt3 > BCI3 > BF3. 

A special kind of oxidative addition occurs between phosphine cobalt hydride complexes and boron halides ... 

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... 

In general, the simple metal hydrides are less reactive toward boron halides than the complex aluminum hydrides. Thus LiH gives very little, if any, diborane on direct interaction with, for example, BF3 or BBr3 at temperatures as high as 180 C (279). [It has recently been claimed that at still higher temperatures... 

Reactions.—Cleavage of prenyl and allyl ethers has been discussed earlier in this Report. Some aliphatic ethers can be cleaved by thallium(iii) nitrate in acetic anhydride.New reagents for the demethylation of aryl methyl ethers include the stable, crystalline, and easily handled complexes of boron halides BBr3 and BCI3 with dimethyl sulphide,and the salt obtained from treatment of N-methylaniline with sodium hydride in HMPA. The latter system cleaves methyl aryl ethers more readily than benzyl aryl ethers. 

Unlike boron, the development of heavier Group 13 congener complexes with carbenes has been sluggish. Like boron, however, early examples of carbene complexes with Al, Ga, and In were simple Lewis adducts of the carbene with ER3 species (E = Al, Ga, or In R = alkyl, aryl, halide, or hydride). Recently, Aldridge explored the coordination chemistry of NHC-EHj adducts (104, E = Al or Ga) with [M(CO)4(COD)] (M = Cr, Mo), and was able to prepare the first examples of o-gallane complexes (105, Seheme 5.18). ... 

A review of the chemistry of Group III complexes shows that many of the concepts of Werner s coordination chemistry may be applied to these elements. Furthermore, the differences observed in the complexing behavior between ammonia and trimethylamine refiect fundamental differences in the amines. Ammonia tends to give ionic compounds, whereas trimethylamine tends to give non-ionic products. Trends observed with the metal halides can be extrapolated into hydrides of boron, aluminum, and gallium. An extension of Werner s concepts provides a significant... 

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