Other Lewis acids

Hoffmann found that 2,2-dialkylated 8-oxabicyclo[3.2.1]oct-6-en-3-ones such as 222 efficiently open in the presence of Lewis acid and an amine base, Eq. 134 [190]. The mechanism is apparently an enolization of the ketone followed by opening of the ether bridge. The reagent combination that was most successful was a 1 1 complex of ZrCl4 and piperidine. Substrates which are not 2,2-di-substituted give tropones. 

3 equiv ZrCI4, 3 equiv piperidine CH2CI2, -30°C, 1.5 h 79% yield 

It follows that the corresponding enol ethers can be ring-opened by treatment with Lewis acid [190]. Simpkins subjected the enantiomerically enriched silyl enol ether 224 (obtained by deprotonation using a homochiral lithium amide) to titanium tetrachloride [121]. Alkene 224 was obtained in 88% ee at -95°C, and the ring opened product is expected to be of comparable enantiomeric purity, Eq. 135. 

The acid catalyzed ring opening of l,4-dimethyl-2,3-dicarbomethoxy-7-oxa-bicyclo[2.2.1]hepta-2,5-diene yielded the aromatized product, Eq. 136. However, in the presence of [Rh(CO)2Cl]2, methanol acts as a nucleophile and gives the cyclohexadienol. The reaction was shown to be both regio and stereoselective, Eq. 137 [191]. 

Pyridine readily forms stable coordination compounds. Thus, boron, aluminum and gallium trihalides react at 0°C in an inert solvent to give 1 1 adducts (cf. 85). Steric factors are important, and a-substituents decrease the ease of reaction. This is illustrated by the heats of reaction of pyridine, 2-methylpyridine and 2,6-dimethylpyridine with boron trifluoride which are 101.3, 94.1 and 73.2 kJ mol-1, respectively. The marked decrease in exothermicity here should be contrasted with the small steric requirement of the proton as shown by the pA., values of substituted pyridines (see Section 3.2.1.3.4). 

Alkyl-substituted pyridines have been complexed with a wide variety of other boron Lewis acids, BH3, B(OH)3, and BX3, where X is a selection of alkyl, aryl, hydroxy, alkoxy, and aryloxy groups 87JCS(P2)77l). 

Sulfur trioxide gives the expected adduct, a sulfonating agent (see also Section 3.2.1.4.5) similarly A-nitropyridinium tetrafluoroborate is formed with N02+ BF4.  

Van der Waals complexes of C2v symmetry between 1,2,4,5-tetrazine and a number of light gases (He, Ar, H2) have been observed and characterized by laser spectroscopic studies of free supersonic jet expansion of the tetrazine in the carrier gas (84CHEC-(3)53l). In these complexes, one equivalent of noble gas sits on top of the aromatic TT-system of the heterocycle. 1,2,4,5-Tetrazine, its 3-methyl, and 3,6-dimethyl derivative as well as aminotetrazine have all been used as heterocycles with noble gases, water, HC1, benzene, and acetylene, playing the role of the second partner. 

Cocatalytic features have been reported for the conjoint action of gallium chloride and hydrogen chloride in the addition of the latter to variousolefins No detailed study of the mechanism of these reactions was however carried out and the interpretation of the results is somewhat complkated by the fact that in both studies toluene was used as solvent, and it is known that hydrc en chloride gives a complex with this aromatic hydrocarbon. 

Sal nikov et investigated the polymerisation of butyl vinyl ether by ferric chloride in the presence of hydrogen chloride. The products had a high degree of polymerisation, in contrast with the black resinous liquids produced when hydrogen chloride alone was added to the monomer. R rettably, these experiments were carried out in the presence of butanol, which makes any discussion abcut the details of the initiation mechanism impossible. 

Variations to the above scheme can of course be envisaged, particularly in terms of the intervention of HX as the assisting entity for the intermediate concerted complex (as in some AdgS mechanisms proposed for the addition of Br(Ansted acids to olefins), and of the formation of homoconjugated Lewis acid anions of the type Mt2X2 +i. 

Water is the most frequently invoked cocatalyst in Fiiedel-Crafts reactions and in particular in cationic polymerisations promoted by Lewis acids. This is not only due to the almost inevitable presence of traces of water as an impurity in most re nts and solvents, but also to its higji efficiency compared to many other cocatalysts. Thus, when a polymerisation system employing a Lewis acid as catalyst is examined, it is Mways indispensable to check and assess the possible role of adventitious moisture as the main cocatalyst. 

As we have already mentioned in the preceding sections, water is often a more powerful cocatalyst than hydrogen halides. This observation clearly excludes the possibility that the general cause of cocatalysis by water originates from the hydrolysis of the Lewis acid, 

This is one of the fields that has developed considerably since CHEC-I 84CHEC-I(5)167 . The compounds containing metalloids on the nitrogen are described in the order of the periodic group of the element. 

Yalpani has prepared other boron derivatives of azoles compounds (120) resulted from the reaction of pyrazole with triorganylboroxins 88CB1553 , while compounds (121) and (122) are formed when crowded 3,5-di-r-butylpyrazoles-4-substituted (R = H, Me, Et) react with BEtj 

Dialkylindium derivatives of pyrazole, 3(5)-methylpyrazole and 3,5-dimethylpyrazole, pz-lnMcj or pz-InEt2 have been prepared they are dimeric compounds with structures similar to (124) 91JOM(420)1 . 

Begtrup 90ACS1050 has developed a general procedure for silylation of azoles including 1-(trimethylsilyl)pyrazole. 

Sodium tris(pyrazol-l-yl)germanate (125) has been prepared and characterized by x-ray crys- 

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In order to be able to provide answers to these questions, a Diels-Alder reaction is required that is subject to Lewis-acid catalysis in aqueous media. Finding such a reaction was not an easy task. Fortunately the literature on other Lewis-acid catalysed organic reactions in water was helpful to some extent... 

These studies at the same time aroused my interest in the mechanistic aspects of the reaetions, including the complexes of RCOF and RF with BF3 (and eventually with other Lewis acid fluorides) as well as the complexes they formed with aromatics. 1 isolated for the first time at low temperatures arenium tetrafluoroborates (the elusive (T-complexes of aromatic substitutions), although I had no means to pursue their structural study. Thus my long fascination with the chemistry of car-bocationic complexes began. 

Nitronium tetrafluoroborate was first prepared by adding a mixture of anhydrous hydrofluoric acid and boron trifluoride to a solution of dinitrogen pentoxide in nitromethane. Nitric acid can be used in place of dinitrogen pentoxide, and by replacing boron trifluoride by other Lewis-acid fluorides Olah and his co-workers prepared an extensive series of stable nitronium salts. ... 

Friedel-Crafts (Lewis) acids have been shown to be much more effective in the initiation of cationic polymerization when in the presence of a cocatalyst such as water, alkyl haUdes, and protic acids. Virtually all feedstocks used in the synthesis of hydrocarbon resins contain at least traces of water, which serves as a cocatalyst. The accepted mechanism for the activation of boron trifluoride in the presence of water is shown in equation 1 (10). Other Lewis acids are activated by similar mechanisms. In a more general sense, water may be replaced by any appropriate electron-donating species (eg, ether, alcohol, alkyl haUde) to generate a cationic intermediate and a Lewis acid complex counterion. 

Alkylation involving formaldehyde in the presence of hydrogen chloride is known as chloromethylation (eq. 3). The reagent may be a mixture of formalin and hydrochloric acid, paraformaldehyde and hydrochloric acid, a chloromethyl ether, or a formal. Zinc chloride is commonly employed as a catalyst, although many other Lewis acids can be used. Chloromethylation of sahcyhc acids yields primarily the 5-substituted product 5-chlotomethylsahcyhc acid [10192-87-7] (4). 

Methylarsine, trifluoromethylarsine, and bis(trifluoromethyl)arsine [371-74-4] C2HAsF, are gases at room temperature all other primary and secondary arsines are liquids or solids. These compounds are extremely sensitive to oxygen, and ia some cases are spontaneously inflammable ia air (45). They readily undergo addition reactions with alkenes (51), alkynes (52), aldehydes (qv) (53), ketones (qv) (54), isocyanates (55), and a2o compounds (56). They also react with diborane (43) and a variety of other Lewis acids. Alkyl haUdes react with primary and secondary arsiaes to yield quaternary arsenic compounds (57). 

Aromatic compounds may be chlorinated with chlorine in the presence of a catalyst such as iron, ferric chloride, or other Lewis acids. The halogenation reaction involves electrophilic displacement of the aromatic hydrogen by halogen. Introduction of a second chlorine atom into the monochloro aromatic stmcture leads to ortho and para substitution. The presence of a Lewis acid favors polarization of the chlorine molecule, thereby increasing its electrophilic character. Because the polarization does not lead to complete ionization, the reaction should be represented as shown in equation 26. 

Aminating agents 3.14 Other Lewis acids Electrophilic Attack at Carbon... 

In the preceding parts of Section 4.04.2.1.3 the electrophilic attack on pyrazolic nitrogen with the concomitant formation of different classes of N—R bond has been examined N—H (iv, v), N—metal (vi), N—C(sp ) (vii, viii, xi), N—C(sp ) (be, x, xi), N—SO2R (x), N—halogen (xii), N—O (xiii) and N—-N (xiv). In this last part the reaction with other Lewis acids leading to the formation of pyrazole N—metalloid bonds will be discussed, and the study of their reactivity will be dealt with in Section 4.04.2.3.lO(viii). 

The most important reaction with Lewis acids such as boron trifluoride etherate is polymerization (Scheme 30) (72MI50601). Other Lewis acids have been used SnCL, Bu 2A1C1, Bu sAl, Et2Zn, SO3, PFs, TiCU, AICI3, Pd(II) and Pt(II) salts. Trialkylaluminum, dialkylzinc and other alkyl metal initiators may partially hydrolyze to catalyze the polymerization by an anionic mechanism rather than the cationic one illustrated in Scheme 30. Cyclic dimers and trimers are often products of cationic polymerization reactions, and desulfurization of the monomer may occur. Polymerization of optically active thiiranes yields optically active polymers (75MI50600). 

Technical grade zinc cyanide was used as supplied by Matheson, Coleman and Bell. Other Lewis acids, notably aluminum chloride, zinc bromide, and zinc iodide may be used as catalysts for the reaction. 

The Friedel-Crafts reaction is a very important method for introducing alkyl substituents on an aromatic ring. It involves generation of a carbocation or related electrophilic species. The most common method of generating these electrophiles involves reaction between an alkyl halide and a Lewis acid. The usual Friedel-Crafts catalyst for preparative work is AICI3, but other Lewis acids such as SbFj, TiC, SnCl4, and BF3 can also promote reaction. Alternative routes to alkylating ecies include protonation of alcohols and alkenes. 

Alternative high-yield syntheses of these various boranes via hydride-ion abstraction from borane anions by BBra and other Lewis acids have recently been devised l (see p. 162). 

Metal halides like zinc chloride are used as Lewis-acid catalysts. Other Lewis-acids or protic acids, as well as transition metals, have found application also. The major function of the catalyst seems to be the acceleration of the second step—the formation of the new carbon-carbon bond. 

Other Lewis-acidic alkoxides might also be employed however aluminum isopropoxide has the advantage to be sufficiently soluble in organic solvents, and acetone as oxidation product can be easily removed for its low boiling point. Recently lanthan isopropoxide has been used with success, and showed good catalytic activity. 

Azaaromatics can be perfluoroalkylated selectively in the 2-position using RLi-BF3 in a Ziegler-Zeisser-type reaction. The BF3 was essential as a promoter and other Lewis acids were found to be less effective. 

In the Brpnsted picture, the acid is a proton donor, but in the Lewis picture the proton itself is the acid since it has a vacant orbital. A Brpnsted acid becomes, in the Lewis picture, the compound that gives up the actual acid. The advantage of Lewis theory is that it correlates the behavior of many more processes. For example, AICI3 and BF3 are Lewis acids because they have only 6 electrons in the outer shell and have room for 8. Both SnCU and SO3 have eight, but their central elements, not being in the first row of the periodic table, have room for 10 or 12. Other Lewis acids are simple cations, like Ag. The simple reaction A + B- A—B is not very common in organic chemistry, but the scope of the Lewis picture is much larger because reactions of the types... 

Aldehydes can be converted to acylals by treatment with an anhydride in the presence of BF3, other Lewis acids, proton acids, or PCI3. The reaction cannot normally be applied to ketones, though an exception has been reported when the reagent is trichloroacetic anhydride, which gives acylals with ketones without a catalyst. ... 

Hepatite Virus NS3/4A having the pyrrolidine-5,5-trans-lactam skeleton [83], starting from (R)- and (S)-methionine, respectively. The key step is the addition of the proper silyl ketene acetal to an iminium ion, e.g., that generated by treatment of the intermediate 177 with boron trifluoride, which provided the adduct 178 with better diastereoselectivity than other Lewis acids. Inhibitors of hepatitis C virus NS3/4A were efficiently prepared by a similar route from (S)-methionine [83]. The addition of indole to a chiral (z-amino iminium ion was a completely diastereoselective step in a reported synthesis of tilivalline, a natural molecule which displays strong cytotoxicity towards mouse leukemia L 1210 [84]. 

The aluminum trihalides are particularly important Lewis acids in the chemical industry. They promote or catalyze a large variety of reactions. One of the most important applications is the Friedel-Crafts reaction, in which two molecules combine, forming a new C—C bond. For example, aluminum chloride or some other Lewis acid catalyzes the reaction between an acid chloride and benzene to form acetophenone ... 

Quite a number of other Lewis acids can catalyze the Mukaiyama aldol reaction, including Bu2Sn(03SCF3)2,51 Bu3SnC104,52 Sn(03SCF3)2,53 Zn(03SCF3)2,54 and... 

Trialkylsilyl cations may play a key role in other Lewis acid-catalyzed reactions.59 For example, trimethylsilyl triflate can be formed by intermolecular transfer of the silyl group. When this occurs, the trimethylsilyl triflate can initiate a catalytic cycle that does not directly involve the Lewis acid. 

The stereoselectivity of aldol addition is also affected by chelation.81 a- and P-Alkoxy aldehydes can react through chelated structures with Li+ and other Lewis acids that can accommodate two donor groups. 

Chelation alone, however, is not sufficient to induce high enantioselectivity since other Lewis acids capable of chelation, such as SnCl4 and TiCl4, give lower enantioselectivity. 

Various other Lewis acids have been explored as catalysts, and the combination InCl3-(CH3)3SiCl has been found to be effective.88 The catalysis requires both components and is attributed to assistance from O-silylation of the carbonyl compound. 

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