Lewis acids, reaction with

It should be kept in mind that Lewis acids reactions with DHA also led to the formation of dimers and/or of anhydroartemisinin (AHA) 23, often as a major product. To circumvent this problem, DHA was converted to the C12 acetate, which was displaced by ether formation. 

ANSWER (d) Hydroxide must be the Lewis base, but where is the empty orbital Where is the Lewis acid Recall the structure of the sp hybridized borane (p. 56). Boron is neutral, but there is an empty 2p orbital on boron, and borane is very definitely a Lewis acid. Reaction with the Lewis base hydroxide looks just like the reaction of the methyl cation with hydroxide (Rg. 2.59) except that the product is negatively charged. 

The transition-state structures were also found to be more asynchronous in the presence of BF3 Lewis acid compared with the uncatalyzed reactions. 

A similar study performed by Welton and co-workers studied the rate and selec-tivities of the Diels-Alder reaction between cyclopentadiene and methyl acrylate in a number of neutral ionic liquids [44]. It was found that endo. exo ratios decreased slightly as the reaction proceeded, and were dependent on reagent concentration and ionic liquid type. Subsequently, they went on to demonstrate that the ionic liquids controlled the endo. exo ratios through a hydrogen bond (Lewis acid) interaction with the electron-withdrawing group of the dienophile. 

The stereochemical outcome of the Mukaiyama reaction can be controlled by the type of Lewis acid used. With bidentate Lewis acids the aldol reaction led to the anti products through a Cram chelate control [366]. Alternatively, the use of a monoden-tate Lewis acid in this reaction led to the syn product through an open Felkin-Anh... 

We can now write equations for many reactions that involve a Lewis acid reacting with a Lewis base. The following are a few examples ... 

Flynn et al., also described the synthesis of the fused indoles [73]. The o-iodotrifluoroacetanilide 110 was coupled to aryl alkyne 111 under Sono-gashira conditions followed by subsequent reaction with aryl iodide, 107 with gaseous carbon dioxide produced the fused indole 158. Lewis acid dealkylation with aluminum trichloride produced the deprotected alcohol 159. 

Mixed-valence compounds. The ground state of monomeric species RM (M = B, Al, Ga, In), which have an electron lone pair at the metal center, is singlet and the singlet-triplet energy gap increases with increasing atomic number. Consequently, these compounds are able to act as Lewis bases. Reactions with Lewis-acidic... 

Intramolecular addition of trialkylboranes to imines and related compounds have been reported and the main results are part of review articles [94, 95]. Addition of ethyl radicals generated from Et3B to aldimines affords the desired addition product in fair to good yield but low diaster control (Scheme 40, Eq. 40a) [96]. Similar reactions with aldoxime ethers [97], aldehyde hydrazones [97], and N-sulfonylaldimines [98] are reported. Radical addition to ketimines has been recently reported (Eq. 40b) [99]. Addition of triethylborane to 2H-azirine-3-carboxylate derivatives is reported [100]. Very recently, Somfai has extended this reaction to the addition of different alkyl radicals generated from trialkylboranes to a chiral ester of 2ff-azirine-3-carboxylate under Lewis acid activation with CuCl (Eq. 40c) [101]. 

Compounds 9-16 were found to undergo an acidic reaction in aqueous solution.21 Possibly, these zwitterionic A5S/-silicates behave as Lewis acids reacting with the Lewis base OH- to generate protons (2 H20 OH- + H30+). This interpretation is supported by the observation that the related compounds 6 and 7 react with [HNMe3]F in aqueous solution to yield the ionic A6S/-silicates 25 and 26, respectively (Scheme 4 the Lewis bases F-and OH- are isoelectronic).24 The identity of these hexacoordinated silicon... 

The reaction of alkynyllithium compounds with alkoxycarbene tungsten complexes leads to anionic propargyl tungsten complexes (Figure 2.33 see also Figure 3.9). These intermediates are stable at low temperatures and react upon Lewis acid catalysis with aldehydes or A -sulfonyl imines to yield five-membered heterocycles [280]. Oxidative methoxycarbonylation [375] of the intermediate vinyl tungsten complex, followed by elimination of methanol leads to pyrroles or furanes (Figure 2.33 Entry 6, Table 2.22). 

Next to the above presented use of SiCl for the in situ preparation of a Lewis acid catalyst with a Lewis base for the aldol reaction, it is possible to apply this compound as a reagent in the ring opening of epoxides leading to chlorinated alcohols. Denmark [104] reported that the chiral phosphoramide 38 catalyzed the asymmetric ring opening reaction of meso-epoxides in the presence of tetrachlo-rosilane. Similar examples were provided by Hashimoto in 2002 [105], applying the A -oxide 39 as catalyst (Scheme 30). 

The metalloporphyrin-initiated polymerizations are accelerated by the presence of steri-cally hindered Lewis acids [Inoue, 2000 Sugimoto and Inoue, 1999]. The Lewis acid coordinates with the oxygen of monomer to weaken the C— O bond and facilitate nucleophilic attack. The Lewis acid must be sterically hindered to prevent its reaction with the propagating center attached to the prophyrin structure. Thus, aluminm ortho-substituted phenolates such as methylaluminum bis(2,6-di-/-butyl-4-methylphenolate) accelerate the polymerization by factors of 102-103 or higher. Less sterically hindered Lewis acids, including the aluminum phenolates without ortho substituents, are much less effective. 

Water, alcohols, ethers, or amines can cause inhibition of ionic polymerization. However, these substances can act in different ways according to their concentration. For example, in polymerizations initiated by Lewis acids (BF3 with isobutylene) or organometallic compounds (aluminum alkyls), water in small concentrations behaves as a cocatalyst, but in larger concentrations as an inhibitor (reaction with the initiator or with the ionic propagating species). 

Acids and Lewis acids react with quinoline at the basic nitrogen atom to form quinolinium salts, and there is a question over the nature of the substrate for electrophilic attack, i.e. is it quinoline or the quinolinium cation The answer is not a simple one and appears to depend upon the reagents and reaction conditions. Thus, whereas acetyl nitrate at 20 °C gives mainly 3-nitroquinoline (Scheme 3.2), fuming nitric acid in concentrated sulfuric acid containing sulfur trioxide at 15-20 °C yields a mixture of 5-nitroquinoline (35%) and 8-nitroquinoline (43%) (Scheme 3.3). In the case of acetyl nitrate, the reaction may proceed by the 1,4-addition of the reagent to quinoline, followed by electrophilic attack upon the 1,4-dihydro derivative. 

Nucleophilic ring opening of salicyl alcohol acetals was effected by borane-trimethylamine upon Lewis acid activation with AICI3. This reaction was particularly useful for the construction of fluorescently labeled compounds (Equation 29) <2006JOC3444>. 

A rather more complex tertracyclic indole based compound lowers blood pressure by selective blockade of a 1-adrenergic receptors. Reaction of the anion from indole (72-1) with butyrolactone (72-2) leads to the scission of the carbon-oxygen bond in the reagent and the formation of the alkylated product (72-3). The acid is then cyclized onto the adjacent 2 position to give the ketone (72-4) by treatment with a Lewis acid such as polyphosphoric acid. Reaction with bromine then leads to the brominated ketone (72-5). This is subjected to reductive alkylation with ethylene... 

Lewis acids are important catalysts for promoting organic reactions because they coordinate heteroatoms of functional groups. Lewis acids interact with carbonyl oxygen in its plane in either syn or anti fashion 248). Such perturbation of acceptor molecules lowers the LUMO level... 

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