Bases combined with Lewis acids

A second pivotal contribution from the Grigg group asserted the positive influence of the use of amine bases in combination with Lewis acids to catalyze the thermal... 

On the other hand using LiLn(Allyl)4dioxane in combination with Lewis acids like AlBr3 or base like TMEDA or THF, where Ln = Ce, Nd, Sm, Gd gives yields in the range of 70-94 percent. When a Lewis base is involved, the vinyl content of the product is about 80 percent. 

Subsequently, Ko et al. investigated the MBH reaction of benzaldehyde with methyl acrylate in various bmim-based ion liquids and found that [bmimJIPFe] was the best, resulting in the maximum rate enhancement. A moderate acceleration in the reaction rates was observed if the above ionic liquid was used in combination with Lewis acid or H-bond donors. Notably, the rate-enhancing effects of the ionic liquid and Lewis acid or H-bond donors were not additive. A two-fold rate increase was achieved in mixtures of [bmim][PF6], La(OTf)3 and 2,2, 2-nitrilotris(ethanol) compared to the same reaction in MeCN. ... 

Bases in combination with Lewis acids are powerful in promoting the HWE reaction with many substrates that are incompatible with strong bases. In 1984, Masamune and Roush et al. reported mild conditions using lithium chloride and DBU (or i-Pr2NEt) [172], and later Rathke et al. extended this Lewis acid/base system to lithium or magnesium halides with triethylamine [173]. Recently, Helquist et al. have employed Zn(OTf)2, TMEDA, and DBU to promote the HWE reaction of aldehydes with diethylphosphonoacetic acid to give a,p-unsaturated carboxylic acids with excellent selectivity [174]. 

Moreover, these experiments reveal some unique properties of the chlorostan-nate ionic liquids. In contrast to other known ionic liquids, the chlorostannate system combine a certain Lewis acidity with high compatibility to functional groups. The first resulted, in the hydroformylation of 1-octene, in the activation of (PPli3)2PtCl2 by a Lewis acid-base reaction with the acidic ionic liquid medium. The high compatibility to functional groups was demonstrated by the catalytic reaction in the presence of CO and hydroformylation products. 

Not only cyanide but also an isocyanide behaves as a nucleophile to attack a carbonyl compound or an imine that is prepared in situ from an carbonyl compound. " In these reactions, an isocyanide is a synthetic equivalent to an aminocarbonyl anion. Asymmetric version of this reaction appeared in 2003. Using a combination of Lewis acid SiCU and a Lewis base chiral bisphosphora-mide, the corresponding a-hydroxyamide is obtained in 96% yield with >98% ee (Scheme 4.23). 

Based on the precedented reduction of 2//-dihydropyrones,86 the combination of Lewis acid and hydride source exemplified by Et3SiH/BF3 seemed ideally suited to our needs (Scheme 9). While (+)-artemisinin 1 could not be reduced directly to 10-deoxoartemisinin 108 with Et3SiH/BF3, dihydroartemisinin (175, R = H) was smoothly converted at low temperature to desired tetrahydropyran 108 in 96% yield. Further, this method was insensitive to scale being readily accomplished on the gram or milligram level. It was also found that small scale reductions could be more conveniently conducted utilizing diisobutylaluminum hydride in place of sodium borohydride. As applied to the problematical case, it was found that lactone 125 could be reduced to lactol and thence 115 as outlined in Scheme 3 in excellent yield. Furthermore, the yield for the conversion of lactone 45 into 9-butyl-10-deoxoartemisinin 112 could be similarly improved from 58 to 90%. 

The flow of electrons from a Lewis base to a Lewis acid is often indicated with a curved arrow. The arrow starts on a pair of nonbonding electrons on the Lewis base and points toward the Lewis acid with which it reacts. Because adding a pair of electrons to one point on a molecule often displaces electrons in the molecule, combinations of curved arrows are often used to describe even simple chemical reactions. Consider the following example, in which a pair of electrons on an NH2 ion are donated to the H+ ion formed when the electrons in one of the CH bonds in acetylene are given to the carbon atom instead of being shared by the C and H atoms in this bond. 

Metal ions such as Cu2+ are Lewis acids that combine with Lewis bases such as H20 to form complex ions, e.g., CuCOH ". The number of ligands, the coordination number, varies from one metal ion to another the most common coordination numbers are 4 and 6, but 2, 3, 5, 7, and 8 are also known. In the presence of another Lewis base such as NH3, the H20 ligands can be replaced in stepwise equilibria ... 

Complex 9.12 and its analogues, with other rare earth and alkali metals, have the unique combination of properties of both a Brpnsted base and a Lewis acid. The Li-O bond (Brpnsted base) abstracts a proton from nitromethane. At the same time coordination of the carbonyl oxygen atom to La3+ (Lewis acid) activates the carbonyl group. The formulations for 9.12 and its analogues were first established by a sophisticated mass-spectrometric technique called laser-desorption/ionization time-of-flight (LDI-TOF) mass spectrometry. The molecular structures of several such complexes have subsequently been established by single-crystal X-ray studies. 

Sinou and co-workers [73] studied the influence of different surfactants on the palladium-catalyzed asymmetric alkylation of l,3-diphenyl-2-propenyl acetate with dimethyl malonate in presence of potassium carbonate as base and non-water-soluble chiral ligands. Best results in activity and enatioselectivity (> 90% ee) were observed with 2,2 -bis(diphenylphosphino)-l,l -binaphthyl (BINAP) as ligand and cetyltrimethylammonium hydrogen sulfate as surfactant in aqueous medium. Water-stable Lewis acids as catalysts for aldol reactions were developed by Kobayashi and co-workers [74]. An acceleration of the reaction was indicated in presence of SDS as anionic surfactants. An additional promotion could be observed by combination of Lewis acid and surfactant (LASCs = Lewis acid-surfactant-combined catalysts) as shown in Eq. (3). Surfactant the anion of dodecanesulfonic acid. 

As compounds of coordinatively unsaturated trivalent aluminum, the organoaluminum compounds are Lewis acids and will combine with Lewis bases to form molecular compounds or complex anions. 

The HSAB theory of Pearson has been one of the key organizing concepts in the study of nucleophiles. This theory is applied and examined in Chapters 15 and 16. In Chapter 15, Fuji applies the HSAB principles to design nucleophilic reagents for cleaving C-X bonds. Fuji notes that all bonds are made of a combination of Lewis acid and Lewis base and have hard-soft dissymmetry for the typical C-X bond, the carbon is a soft acid and the X is a hard base. Thus, in accord with the HSAB principles, a soft base (the nucleophile) and a hard acid are required to cleave this bond selectively. Applying these ideas, Fuji then shows the utility of several soft base-hard acid reagents for cleaving various C-X bonds in complex molecules. 

The formation of ionic liquids by the reaction of halide salts with Lewis acids (most notably AICI3) dominated the early years of this area of chemistry. The great breakthrough came in 1951 with the report by Hurley and Weir on the formation of a salt that was liquid at room temperature based on the combination of 1-butylpyridinium with AICI3 in the relative molar proportions 1 2 (X = 0.66) [26]. ... 

A Lewis acid is an electron pair acceptor and a Lewis base is an electron pair donator. A complex is formed when a Lewis acid and a Lewis base share a pair of elecfrons and form a bond. When an organic compound is used as a ligand in a complex formation with a metal it acts as a Lewis acid and the combination in many cases produces real acids. This means that the metal Lewis bases compete with the acidic "loose protons" for the sharing electron and will be replaced by the proton at pH values that are low compared to the pKa of the ligand in question. The metals of the d block, on the otiier hand, are known for forming insoluble hydroxides or oxides at neutrality or high pH. So a complex is often stable only inside Ihe pH interval between ligand deprotonation and metal hydroxide precipitation. 

Isobutylene polymerisation, in the presence of metals halides (Lewis acids) combined with Bronsted acids (exemplified by a catalyst based on MeX and H2O), proceeds according to the following scheme ... 

Flagrant examples which violate the HSAB principle are known. Most of these can be attributed to the inherent strength of the acid-base complex which overrides the match in softness. That the extremely soft Lewis base combines with the very hard acid H exothermically to give the stable hydrogen molecule is a case in point. 

Other chiral zinc based Lewis acid, such as zinc(II) complex with pybox, showed good stability in aqueous media and gave syn-adducts in moderate to excellent catalytic activity and enantioselectivity for asymmetric Mukaiyama aldol reactions (113,114). A simple combination of Lewis acidic zinc salt (Zn(OTf)2) and organocatalyst is also shown to be effective catalysts for the direct aldol reaction of acetone and aldehydes in the presence of water (115). 

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