Silyl borane anions

Silyl enol ethers react with aldehydes in the presence of chiral boranes or other additives " to give aldols with good asymmetric induction (see the Mukaiyama aldol reaction in 16-35). Chiral boron enolates have been used. Since both new stereogenic centers are formed enantioselectively, this kind of process is called double asymmetric synthesis Where both the enolate derivative and substrate were achiral, carrying out the reaction in the presence of an optically active boron compound ° or a diamine coordinated with a tin compound ° gives the aldol product with excellent enantioselectivity for one stereoisomer. Formation of the magnesium enolate anion of a chiral amide, adds to aldehydes to give the alcohol enantioselectively. 

These reactivities encouraged the investigation of reduction into NH3 and spectacular results were obtained with triphosphine-borane Fe complexes. Various V-bound Fe complexes 65 relevant to the catalytic reduction of into NH were prepared. Most significant are the protonation of the [FeJ-NH complex into [Fe]-NHj and the reductive displacement of NH by N, which are the toal steps proposed to account for the fixation/reduction of at Fe (Scheme 25). This stoichiometric reaction sequence was adapted into a catalytic transformation. Treatment of the anionic complex 65d with excess and KCj at -78°C results in NHj production. Under these conditions, up to 7 NH3 molecules were generated per Fe complex and more than 40% of the furnished protons were delivered to Nj. The borane moiety plays a major role in this process, being capable of accommodating the various [FelN H, )] species involved in the catalytic cycle due to its versatile coordination properties. This is further supported by the inability of the related triphosphine-silyl complex to promote the reduction of Nj under similar conditions. 

Tris(pentafluorophenyl)borane (35) is a powerful Lewis acid with a hard boron center. It interacts strongly with hard Lewis basic centers and abstracts hard anion such as methyl anion and hydride to initiate selective reactions that are otherwise difficult to achieve. The B(CgF5)3-promoted early transition-metal-catalyzed olefin polymerizations are typical reactions [16]. The B(C6F5)3-Ph3SiH system is useful for hydro-silylation of the carbonyl group, in which the boron of 35 activates the Si—H bond rather than the carbonyl group (see 40 in Scheme 3.39) and reduces even the ester carbonyl group effectively [17]. A number... 

Reactions. - The reactions of diorganoiodophosphines with varying quantities of iodine have been studied by NMR techniques, which indicate the formation of a dialkyldi-iodophosphonium iodide as the initial product, followed by polyhalide anion formation in the presence of excess iodine. A modified McCormack synthesis of 3-silylated phosphol-3-enes (168) from phenyl-dichlorophosphine and 2-silylated 1,3-butadienes has been described. The reaction is carried out in the presence of a small amount of copper stearate in acetic anhydride at 50 °C, these conditions preventing the complete desilylation observed under standard conditions. The consecutive reaction of lithium phenylacetylide with triphenylborane and diphenylchlorophosphine results in the formation of the intramolecularly coordinated phosphino-borane (169). Treatment of the alkoxyvinyldichlorophosphines (170) with Grignard reagents... 

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