Lewis acid common examples

Allylsilylation is an addition reaction of allylorganosilanes to carbon-carbon multiple bonds of unsaturated hydrocarbons in the presence of Lewis acids. Common examples of unsaturated hydrocarbons for allylsilylations include al-kenes, cycloalkenes, allyltriorganosilanes, " 5-(trimethylsilyl)-l-pent-... 

Additions of allenic tributylstannanes to aldehydes, like those of their allylic counterparts, require Lewis-acid promoters [83]. The favored promoter is BF3-OEt2 (Table 50). Promotion can also be effected by MgBr2, although less effectively. The use of other common Lewis acids, for example TiCU or AICI3, is complicated by competing exchange reactions. This type will be covered in a later section. 

Lanthanide Lewis acids catalyze many of the reactions catalyzed by other Lewis acids, for example, the Mukaiyama-aldol reaction [14], Diels-Alder reactions [15], epoxide opening by TMSCN and thiols [14,10], and the cyanosilylation of aldehydes and ketones [17]. For most of these reactions, however, lanthanide Lewis acids have no advantages over other Lewis acids. The enantioselective hetero Diels-Alder reactions reported by Danishefsky et al. exploited one of the characteristic properties of lanthanides—mild Lewis acidity. This mildness enables the use of substrates unstable to common Lewis acids, for example Danishefsky s diene. It was recently reported by Shull and Koreeda that Eu(fod)3 catalyzed the allylic 1,3-transposition of methoxyace-tates (Table 7) [18]. This rearrangement did not proceed with acetates or benzoates, and seemed selective to a-alkoxyacetates. This suggested that the methoxy group could act as an additional coordination site for the Eu catalyst, and that this stabilized the complex of the Eu catalyst and the ester. The reaction proceeded even when the substrate contained an alkynyl group (entry 7), or when proximal alkenyl carbons of the allylic acetate were fully substituted (entries 10, 11 and 13). In these cases, the Pd(II) catalyzed allylic 1,3-transposition of allylic acetates was not efficient. 

Ikeda et al. reported that a combined system of bismuth triflate [Bi(OTf)3] and BF3-Et20 in dichloromethane is an efficient promoter for the glycosylation of sialyl acetates among various Lewis acids, for example, BiCl3, Sc(OTf)2, Yb(OTf)3, Zn(OTf)2, and TMSCl, and their combinations, which showed the first example of utilization of Bi(OTf)3 in the glycosylation of common sialic acid derivatives (O Scheme 17) [22]. 

Displacement of activated chlorine atoms also proceeds with certain types of organic compounds, but only in the presence of Lewis acid catalysts. Particular examples include epoxides, polyhydric alcohols, trialkylphosphites (12), and P-aminocrotonates (13). These additives are commonly used in conjunction with metallic stabilizers to provide complete, high performance, commercial stabilizer packages. 

Phosphites. Tertiary phosphites are also commonly used and are particularly effective ia most mixed metal stabilizers at a use level of 0.25—1.0 phr. They can take part ia a number of different reactions duting PVC processing they can react with HCl, displace activated chlorine atoms on the polymer, provide antioxidant functionaHty, and coordinate with the metals to alter the Lewis acidity of the chloride salts. Typical examples of phosphites are triphenyl phosphite [101 -02-0], diphenyl decyl phosphite [3287-06-7], tridecyl phosphite [2929-86-4], and polyphosphites made by reaction of PCl with polyols and capping alcohols. The phosphites are often included in commercial stabilizer packages. 

Covalent fluondes of group 3 and group 5 elements (boron, tin, phosphorus, antimony, etc ) are widely used m organic synthesis as strong Lewis acids Boron trifluoride etherate is one of the most common reagents used to catalyze many organic reactions. A representative example is its recent application as a catalyst in the cycloadditions of 2-aza-l,3-dienes with different dienophiles [14] Boron trifluoride etherate and other fluonnated Lewis acids are effective activators of the... 

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... 

The types of ionic liquids shown in Figure 5.4 have been most extensively studied, especially ones based on chloroaluminate. Whilst these chloroaluminate materials also display useful Lewis acid properties they are highly air and moisture sensitive, which renders them relatively commercially unattractive. Newer ionic liquids containing C104 and NOa anions, for example, which are less air and moisture sensitive, are now being more widely studied, but these are less catalytically active. Other than lack of vapour pressure and catalytic properties there are several other features common to most ionic liquids that make them attractive reaction solvents. These include ... 

Examples of the use of heterodienophiles under the action of microwave irradiation are not common. Soufiaoui [84] and Garrigues [37] used carbonyl compounds as die-nophiles. The first example employed solvent-free conditions the second is an example of the use of graphite as a susceptor. Cycloaddition of a carbonyl compound provided a 5,6-dihydro-2H-pyran derivative. These types of reaction proceed poorly with aliphatic and aromatic aldehydes and ketones unless highly reactive dienes and/or Lewis acid catalysts are used. Reaction of 2,3-dimethyl-l,3-butadiene (31) with ethyl glyoxylate (112) occurred in 75% yield in 20 min under the action of microwave irradiation. When conventional heating is used it is necessary to heat the mixture at 150 °C for 4 h in a sealed tube to obtain a satisfactory yield (Scheme 9.33). 

The two most common series of chromium halides have the formulas CrX2 and CrX3 (where X = F, Cl, Br, or I). However, CrF6 is also known. Compounds having the formula CrX3 are Lewis acids, and they also form many coordination compounds. For example, CrX3 reacts with liquid ammonia to yield... 

Like so many other reactions, the ene reaction has been given new life by metal catalysis. The use of metals ranges from common Lewis acids, which simply lower the barrier of activation of the hetero-ene reactions to transition metal catalysts which are directly involved in the bond-breaking and -forming events, rendering reactions formal ene processes. This review is meant to serve as a guide to the vast amount of data that have accumulated in this area over the past decade (1994-2004). If a particular subject has been reviewed recently, the citation is provided and only work done since the time of that review is included here. Finally, the examples included within are meant to capture the essence of the field, the scope, limitations, and synthetic utility therefore, this review is not exhaustive. 

As a specific example of the vinylallenes, 294 demonstrates that not only can common dienophiles such as maleic anhydride (MA) be added (to furnish adduct 295 with a rich functionality) [122], but also carbonyl compounds such as propanal to afford 296 when the reaction is carried out in the presence of a Lewis acid catalyst [27]. 

In earlier days it was fairly common to suggest that sulfenium ions, RS+, were involved as intermediates in a number of these substitutions, particularly those in which sulfenyl halides RSX reacted with very weak nucleophiles, or those where electrophilic catalysis of the substitution was observed (Parker and Kharasch, 1959). However, it has since become evident (Owsley and Helmkamp, 1967 Helmkamp et al., 1968 Capozzi et al., 1975) that sulfenium ions are almost impossible to generate as intermediates. For example, Capozzi et al., (1975) showed that although treatment of a sulfenyl chloride RSC1 with the powerful Lewis acid antimony pentafluoride led to the complete conversion of the sulfenyl chloride to a cation, what was formed was, not the sulfenium ion RS+, but rather the cation [59] in reaction (172). These results, and others... 

Ionic polymerization may also occur with cationic initiations such as protonic acids like HF and H2SO4 or Lewis acids like BF3, AICI3, and SnC. The polymerization of isobutylene is a common example, shown in Fig. 14.5. Note that the two inductively donating methyl groups stabilize the carbocation intermediate. Chain termination, if it does occur, usually proceeds by loss of a proton to form a terminal double bond. This regenerates the catalyst. 

Stronger Lewis acids such as SnCLi, TiCLt, and CH3AICI2 yield fast but uncontrolled polymerization with broad PDI. LCP of vinyl ethers can be achieved if the other components and reaction parameters are appropriately adjusted by various combinations of lower reaction temperature, added nucleophile, added common salt, and solvent prolarity. For example, polymerization of isobutyl vinyl ether using HC1 as the initiator (or one can use the preformed adduct of monomer and HC1) with SnCLt or TiCLj in CH2CI2 is non-LCP... 

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