LiBr complex

To a solution of hexamethyldisilane (2.5 mmol) in HMPA (CAUTION— CANCER SUSPECT AGENT) (3 ml) at 0-5 °C was added methyl lithium (2.5 mmol, 1.5 m MeLi.LiBr complex in ether) dropwise. After being stirred for 3 min, the red solution was treated with Cul (2.5 mmol) in Me2S (1 ml), the resulting black reaction mixture was stirred for 3 min. and 2,3-dibromo-propene (1 mmol) was added rapidly via a syringe. The reaction mixture was allowed to warm to room temperature, and was stirred for 1.5 h. It was then poured into pentane (25 ml) and saturated ammonium chloride solution (25 ml, buffered to pH 8 by the addition of ammonium hydroxide), and the mixture was stirred vigorously for 1 h. The aqueous phase was re-extracted with pentane, and the combined organic extracts were dried. Removal of... 

To a solution of hexamethyldisilane (2.5 mmol) in HMPA (CAUTION— CANCER SUSPECT AGENT) (3 ml) at 0-5 °C was added methyl lithium (2.5 mmol, 1.5 m MeLi.LiBr complex in ether) dropwise. After being stirred for 3 min, the red solution was treated with Cul (2.5 mmol) in Me2S (1 ml), and the resulting black reaction mixture was stirred for 3 min. Ether (6 ml)... 

The reaction of MeLi in Et20 with alkyl- and aryl-substituted branched tetrasilanes proceeds by cleavage of one Si—Si bond, thus affording various trisilanes, which are lithiated at the central silicon atom (equation 26)62. The outcome of these reactions apparently is insensitive as to whether halide-free MeLi or the MeLi-LiBr complex is used62. The decrease in the reaction rate in the order R = Tip > Mes > Ph has been suggested by Fink and coworkers to reflect the relief of steric strain on converting the tetrasilane into the metalated silane626. 

The greater size mismatch between Li+ and Br makes aggregation less pronoimced in LiBr complexes. Simple ring dimers, for example, (LiBr TMEDA)2, are common. The complex LiBr-PMDETA is dimeric in the solid state with 5-coordinate lithium, but in benzene solution this rather asymmetric dimer is in equilibrium with a monomeric form. An unusual dimeric LiBr complex which may be considered as two monomers of LiBr connected only by bridging HMPA ligands is also known. This complex, [LiBr-(HMPA)i.5)2 (12), was synthesized by the in situ ammonium salt route . 

A LiBr complex with another, rather weaker, monodentate pyridine donor in the same ligand salt ratio results in a more aggregated structure, [(2,6-Me2CsH3N)6Li4Br4], and is a product of dissolution in excess donor. A 4-runged step ladder arrangement results because the internal lithiums are solvated giving a pseudotetrahedral 4-coordination at each... 

It has been demonstrated that LiBr complexes with urea (Geschwind, 1960) and also forms a crystalline complex with W-methylacetamide in which each Li+ is surrounded by four carbonyl oxygen atoms and two oxygen atoms of water, while each Br is surrounded by four NH groups and hydrogen atoms from two water molecules (Bello and Bello, 1961,... 

Diastereoselective conjugate addition of the MeLi LiBr complex to 127 and the subsequent 2-step conversion gave 128. The Vasella-type reductive ring-opening of 128 with Zn and the subsequent 3-step conversion gave the C1-C8 segment 120. 

Substituting lithium 2-thienylcyanocuprate for the CuCN/LiBr complex and omitting boron trifluoride etherate from the reaction of the organozinc halide species with 2-cyclohexenone indeed gave a reasonable yield (76% GC) of the... 

DFT and CCSD(T)//DFT computations have been used to model the structure, coordination state, and ring opening of 1-bromo-l-lithiocyclopropane as a model for cyclopropylcarbenoid chemistry. The initial disrotatory stereochemistry becomes conrotatory on route to the allene-LiBr complex. Pre-dissociation of the carbenoid to cyclopropylidene and LiBr was not supported by computations. DFT computations predict modestly exergonic dimerization of the carbenoid, with or without solvation, and the dimer appears to be the most likely reactive species in solution. 

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