Tetrahydrofuran complexes Grignard reagents

The addition of vinylmagnesium bromide to methyl (S)-3-benzyloxy-4-oxobutanoate (5) in tetrahydrofuran proceeded with a slight preference for the nonchelation-controlled reaction product (40 60)5°. A reversal of the diastereoselectivity (80 20) could be observed when the Grignard reagent, as a solution in tetrahydrofuran, was added to a dichloromethane solution of the aldehyde which had been precomplexed with one equivalent of magnesium bromide. The almost exclusive formation of the chelation-controlled reaction product 6 was achieved when tetrahydrofuran was completely substituted by dichloromethane the presence of tetrahydrofuran interferes with the formation of the chelate complex, which is a prerequisite for high chelation-controlled diastereoselection. 

Feringa reports that alkyl heterozincates (131), made from zinc chloride-TMEDA complex, potassium f-butoxide and alkyl Grignard reagents in ether (or tetrahydrofuran), are highly effective in 1,4-additions to ot,P-enones (>85%) while the aryl heterozincates are less efficient (<50%).106... 

Probably the best-known pentanuclear organogold complex is the homoleptic complex [Au(p-mes)]5, whose synthesis by reaction of [AuCl(CO)] with the appropriate Grignard reagent in tetrahydrofuran solution [Eq. (50)] was first reported in 1983 by Floriani.143 An alternative synthetic procedure that takes place with precipitation of insoluble byproducts and uses a mesitylgold(I) derivative as a starting material [Eq. (51)] has been described more recently.31... 

Tris(N-methylanilino)borane has previously been prepared by the reaction of boron trifluoride-ether complexes with three equivalents each of N-methylaniline and a suitable Grignard reagent,1,2 by the reaction of (N-methylanilino)potassium with boron trifluoride-ether complexes,2 and by aminolysis of boron trichloride by N-methylaniline.3 The present general procedure describes a convenient preparation of tris(N-methylanilino)-borane by the reaction of (N-methylanilino)lithium and boron trifluoride-diethyl ether in tetrahydrofuran-hexane as solvent. 

Spescha et al. [4] used the copper complex 6, which was obtained from a thioglucofuranose derivative, as catalyst for 1,4-additions of Grignard reagents to 3, and observed enantioselectivities of up to 60 % ee. The dihydrooxazolylthiophenolato copper complex 7 was employed by Pfaltz et al. 5] for the enantioselective catalysis of Michael additions to cyclic enones the best results were obtained with tetrahydrofuran as solvent and HMPA as additive. There was a pronounced dependence of the stereoselectivity on the ring size of the substrate 16-37 % ee for 2-cyclopente-none, 60-72 % ee for 3, and 83-87 % ee for 2-cycloheptenone. Alexakis et al. [6] used the heterocycle 8, which is readily accessible from... 

Ruthenium, the homologue of iron in this group, was also shown to form complexes quite early. Ruthenocene, Ru( 5H5)2, is obtained by treatment of the acetylacetonate of tervalent ruthenium with five times the theoretical quantity of the Grignard reagent (206), or, better, by the action of cyclopentadienyl sodium on ruthenium trichloride in tetrahydrofuran (47). It forms pale yellow scales which sublime at 120° and melt at 200°. Its properties are closely similar to those of ferrocene it is soluble in organic solvents, and in the absence of air is not attacked by bases or by sulfuric or hydrochloric acid. Oxidation converts it into the pale yellow [Ru( 5H6)2] + ion. 

Table 2 lists the results found for the Grignard reagents investigated. In diethyl ether solution, the Grignard reagents preferentially add to C-1 [2], whereas in tetrahydrofuran (THF) and other solvents C-4 addition dominates [3], It was also reported that the addition of complexing agents such as iV,iV,iV, /V -tetramethylethylenediamine (TMEDA) or hexa-methylphosphoramide (HMPA) in diethyl ether resulted in the reversal of selectivity. 

Transition-metal-catalyzed stereoselective reductions of vinylic sulfones with Grignard reagents are achieved with excess n-BuMgCl in tetrahydrofuran at room temperature.196,48 Better yields and selectivities are obtained with palladium catalysts [Pd(acac)2] than with nickel complexes such as [Ni(acac)2], especially if external ligands such as DABCO, triethylamine (TEA), or (n-Bu)2P are used.196,48 Nickel catalysts are used more often for reducing sulfonyl-1,3-dienes than palladium catalysts. When using this method, it is very important to remove the catalyst before isolation of the products in order to avoid isomerization of the... 

The most common solvents used in the preparation of Grignard reagents are ethers, typically diethyl ether or tetrahydrofuran (THI-). Ethereal solvents offer the advantage of stabilizing the Grignard reagent. The stabilization of phenylmag-iiesiuni bromide by two diethyl ether molecules was proven by the crystallization of the complex ] 12[. Due to the low Hash point of diethyl ether. 1 Ub is frequently the solvent of choice oil an industrial scale... 

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