Lewis acids oxophilic

LP-DE has a weaker catalytic activity than BF3-Et20, AICI3 and TiCU because the Lewis acidity of the lithium cation is moderated by complex-ing with diethyl ether and perchlorate anion [37], but it becomes a highly oxophilic Lewis acid when concentrated solutions are used [38]. The concentration of LP-DE is therefore sometimes essential for the success of the reaction. 

This simple example may illustrate that in general the reaction of an organic halide salt [cation]X with an excess of a Lewis-acid MXy can result in new catalytic materials even if other Lewis-acids are applied than AICI3. In contrast, the use of other Lewis-acids to form the ionic liquid of type [cation][MXy+i] + excess MXy (the excess of MXy may be dissolved in the neutral ionic liquid or may form acidic anionic species such as e.g. [M2X2y+i]-) gives access to new combinations of properties (e.g. a liquid, less oxophilic, Lewis-acidic catalyst with defined solubility and acidity properties). In Table 2 other examples of ionic liquids are presented which are formed by the reaction of an organic halide salt with different Lewis-acids. All these systems should be in principle useful acidic catalysts for synthetic organic chemistry even if not all displayed examples have been already discribed in the literature for this application. 

Synthesis of thiazoline 201 was accomplished from fully protected cysteine 202 using PhsPO (3.0equiv) and Tf20 (l.Sequiv) to generate an oxophilic Lewis acid catalyst 203 in solution. The reaction gave 201 in 98% yield and >99.5% ee <2003AGE83>. The proposed mechanism is shown in Scheme 79. 

Allylation reactions can be designed to effect high stereoselectivity in the case of chiral /3-alkoxy aldehydes, in which the ether oxygen provides for effective coordination with a Lewis acid. Multi-valent, oxophilic Lewis acids serve to pre-organize the aldehyde substrate in a six-membered chelation complex. As in the examples of a-chelation control, an open transition state is deployed with synchnal or antiperiplanar orientations based upon the consideration of steric interactions with placement of the small (hydrogen) vinyl substituent of the allylic stannane over the preformed metallocycle. Several examples are illustrated in Scheme 5.2.20. i... 

The results in Table 16.3 show that the hard oxophilic Lewis acids afford the carboxamide, but that the soft zinc halides cannot be used. An interesting result is... 

They are base metals, and their most common oxidation state is -LIII. The corresponding metal ions Ln + are hard and oxophilic Lewis acids. It is therefore not surprising that chelate complexes of... 

While trying to use allylic alcohols as nucleophiles in Au-catalyzed reactions, electrophilic reactivity was unexpectedly observed and the potential for a variety of mechanistic pathways responsible for this reactivity was intriguing. Au-complexes are typically reported as soft, carbophiUc 7t-acids, but for the observed reaction, it seemed more reasonable that a cationic mechanism whereby the Au-complex functioned as a more traditional oxophilic Lewis acid ° was operative. This piqued our interest, and we decided to change the goals of the project to see where it would take us. Fortunately, pursuing these Au-catalyzed dehydrative transformations developed from a single observation into a research program (vide infra). 

These authors also developed the synthesis of three-fold axially chiral tri-podal ligands, since virtually no studies have been reported on the combination of Cs-symmetry and axial chirality (Scheme 5.51). The first step was the reaction between a chiral bromide with liquid ammonia giving the corresponding primary amine, which was further alkylated in situ with two additional equivalents of the chiral bromide. A final deprotection with BCI3 furnished the required tripodal ligand, the biarylic portions of which formed a cavity capable of incorporating oxophilic Lewis acidic elements. 

As Oxophilic Lewis Acid. In search for Lewis acids of mild acidity and devoid of nucleophilic anion, Yu and coworkers found that PhsPAuOTf offered the best results in the glycosidation of 1,2-anhydrosugars, increasing yields by >20% compared to the conventional protocol based on superstoichiometric anhydrous ZnCl2. Complex 1 was less efficient (eq 18). 

The epoxidation of olefins was discovered by a Russian chemist, N. Prileschajew, in 1909 using peracids RCO3H, and this reaction has been used for a long time. Per-acids often are dangerous (explosive), however, and must therefore imperatively be avoided. Transition-metal catalysis is now used together with hydroperoxides ROOH (most often R = t-Bu, in short TBHP) whose first example was discovered by Hawkins in 1950 with [V2OS] as catalyst. The catalysts most often are d° complexes with an oxophilic Lewis acid able to bind an oxygen atom of hydroperoxide. 

As one would expect, the employment of an oxophilic Lewis acid leads to cation formation at the higher substituted carbon atom of the epoxide, while attack with a strong nucleophile takes place at the less substituted one. 

It was quite clear at that time that a very fast equihbrium must be involved. To shift this in the desired direction an oxophilic Lewis acid (e.g., TiCl4), as expected, served very well and additionally stabilized the amide group obtained in the process (see 219) [82]. 

Since one has to look for oxophilic Lewis acids in selective epoxide splitting, aluminum compounds can be expected to be of quite some value in this field. 

If R in 103 is assumed to be a simple unbranched hydrocarbon chain and one operates with a mild oxophilic Lewis acid, both epoxide bonds will be polarized to a certain extent, but we will be far away from the cation status. 

Bearing in mind that most of the epoxide transformations are catalyzed by oxophilic Lewis acids, we will also have to consider rearrangements that may occur... 

The brutal classical methods of treatment with strong oxophilic Lewis acids or boiling in hydriodic acid will, in most cases, be absolutely unacceptable, although the principle idea is still at work in some modern regioselective ring cleavages. 

Carbonyls. The title compound is widely used as an oxophilic Lewis acid, efficiently increasing the reactivity of carbonyl groups toward the additions of many nucleophiles. a-Dicarbonyls are particularly effective substrates for this kind of activation. A catalytic amount of In(OTf)3 combined with a multidentate chiral ligand allows nucleophiles, such as allylstannanes (eq 23), electron-rich... 

Zhang and coworkers reported two divergent redoxisomerization pathways for yne-enones 151 to afford ring-fused tetrahydroquinolines and tetrahy-droazepines. Product selectivity depends on whether a carbophilic or oxophilic Lewis acid was used. Ring-fused tetrahydroazepines 152 could be obtained in moderate to excellent yields via the carbophihc gold(I)-catalyzed domino reaction under the mild conditions (Scheme 12.66) [70]. 

The gold(III)-catalysed rearrangement of tetrasubstituted 1-alkynyloxiranes shows that the gold complexes act as oxophilic Lewis acids rather than r-acids (Scheme 132). ... 

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