Lewis catalyst

Acid Halides (Lewis Acids). AH metal haUde-type Lewis catalysts, generally known as Friedel-Crafts catalysts, have an electron-deficient central metal atom capable of electron acceptance from the basic reagents. The most frequendy used are aluminum chloride and bromide, followed by... 

In the chlorination of 2,4-dichlorophenol it has been found that traces of amine (23), onium salts (24), or triphenylphosphine oxide (25) are excellent catalysts to further chlorination by chlorine ia the ortho position with respect to the hydroxyl function. During chlorination (80°C, without solvent) these catalysts cause traces of 2,4,5-trichlorophenol ( 500 1000 ppm) to be transformed iato tetrachlorophenol. Thus these techniques leave no 2,4,5-trichlorophenol ia the final product, yielding a 2,4,6-trichlorophenol of outstanding quaUty. The possibiUty of chlorination usiag SO2CI2 ia the presence of Lewis catalysts has been discussed (26), but no mention is made of 2,4,5-trichlorophenol formation or content. 

Catalytic asymmetric synthesis with participation and formation of heterocycles (including asymmetric phase transfer reactions and asymmetric reactions with chiral Lewis catalysts) 93MI1. 

The mechanism of the acylation reaction of PS with acetic anhydride in the presence of Lewis catalysts can be considered as follows ... 

Aziridines 180 (Scheme 3.65) react with acetonitrile and BF3 Et20, a Lewis catalyst, to give imidazolines 181 in 65-95% yield [98], Under the same reaction conditions, however, the C-3 phenyl-substituted aziridine 182 (Scheme 3.66) afforded oxazoline 183 in 59% yield [97]. 

The extent of the stereoselectivity depends on the protection of the nitrogen and also on the catalyzing Lewis acid. However, the monoprolected a-amino aldehydes 1 (R3 = H) show good to excellent chelation-controlled syn preference, independent of the Lewis catalyst employed6fi S9. 

Chlorine molecules are cleaved at high temperatures by photoinduced radical formation. By this means, a gas/liquid reaction can be performed in the side chain of alkyl aromatics quite selectively. The electrophilic ring substitution, instead, is favored using Lewis catalysts in polar solvents at low temperature. 

Typically, the reaction mechanism proceeds as follows [6], By photoreaction, two chlorine radicals are formed. These radicals react with the alkyl aromatic to yield a corresponding benzyl radical. This radical, in turn, breaks off the chlorine moiety to yield a new chlorine radical and is substituted by the other chlorine, giving the final product. Too many chlorine radicals lead to recombination or undesired secondary reactions. Furthermore, metallic impurities in micro reactors can act as Lewis catalysts, promoting ring substitution. Friedel-Crafts catalyst such as FeClj may induce the formation of resin-Uke products. 

Kobayashi has found that scandium triflate, Sc(OTf)3,36 and lanthanide triflate, Ln(OTf)3, are stable and can be used as Lewis catalysts under aqueous conditions. Many other Lewis acids have also been reported to catalyze Diels-Alder reactions in aqueous media. For example, Engberts reported37 that the cyclization reaction in Eq. 12.7 in an aqueous solution containing 0.010 M Cu(N03)2 is 250,000 times faster than that in acetonitrile and about 1,000 times faster than that in water alone. Other salts, such as Co2+, Ni2+, and Zn2+, also catalyze the reaction, but not as effectively as Cu2+. However, water has no effect on the endo-exo selectivity for the Lewis-acid catalyzed reaction. 

The ring expansion of 2-f-butyloxetanes to tetrahydrofurans in the presence of a Lewis catalyst has been reported. Treatment of the oxetane (341) with boron trifluoride etherate in diethyl ether leads to the ring expanded product (342). It is suggested that an intermediate carbocation is formed (Scheme 92) (79CC382). 

Heyns et al. [53] started with di-O-acetyl-L-rhamnal (28) which after reaction with sodium azide in the presence of boron trifluoride etherate as Lewis catalyst gave again the four products 79-82 (R=Ac). Subsequent quenching with NIS and the aglycone led to ristosamine and acosamine glycosides 86 and 87. 

RCHO RCOCHiCOiCiCs.1 This reaction can be effected with 1 in the presence of various Lewis catalysts, but SnCl2, GeCl2, and BF3 are the most efficient, permitting reactions at -25°. Yields are generally 50-90%. 

In addition to the strong Lewis catalysts already mentiotud. trialkylalutninum and trialkoxyaluTninuni csialysts, " i - and... 

Thus, the formation of symmetrical (caused by perpendicular arrangement of the naphthalene and cyclohexadienone ring systems) cyclohexadienone 215 proceeds not as a [4 + 2]-cycloaddition of two carbocation species but rather as a two-step electrophilic substitution, the first stage of which is intermolecular and the second intramolecular. The reasons for the different direction of the cyclodimerisation reactions of carbocation 211 in protic media and in the presence of Lewis catalysts have been discussed elsewhere29,207. 

Silylallylation of alkynes. Hafnium(IV) chloride is a suitable Lewis catalyst to... 

The pioneering work in this field was performed by F. Swarts starting from 1892. Treatment of different haloalkanes with HF in the presence of Lewis acids such as SbFs, SbFs, AgF, HgF2, and AIF3 yielded mixtures of partially and fully fluori-nated alkanes, depending on the exact reaction conditions (Scheme 2.14). Stoichiometric amounts of the Lewis catalysts themselves can also serve as the fluoride source [41]. 

Lanthanoid salts with non-coordinating anions carrying long perfluoroalkyl chains, Yb[C(SO2CgFi7)3[3 (19) and Sc[C(SO2CgFi7)3[3 (20), have been successfully used as Lewis catalysts for O-acylations, Friedel-Crafts, Diels-Alder, and Mu-kayama aldol reactions in fluorous biphasic media [21] (Scheme 3.6). In these reactions the fluorous medium avoids deactivation of the Lewis acid by solvent coordination. The catalyst can also be recycled and reused. 

A number of studies on the polymerization of acetal derivatives of sugars by means of Lewis catalysts,166,167 or 6-O-alkene derivatives,168 have been published. 

C13H22O2, Mr 210.32, bpo iy kpa 120-122 °C, df 0.9941, rag0 1.4915, is a fragrance substance that does not occur in nature. It is a colorless, viscous liquid with a sweet odor reminiscent of lily of the valley. The aldehyde can be prepared by a Diels -Alder reaction of myrcenol and acrolein in the presence of a Lewis catalyst (e.g., zinc chloride) [117] ... 

Catalyzed and uncatalyzed homo-Diels-Alder reactions behave mechanistically in the same way as Diels-Alder reactions. However, in the presence of a Lewis catalyst two competing processes, the homo-Diels-Alder reaction and the [2 + 2] cycloaddition, take place. The smaller absolute value of AV observed for the [2 + 2] cycloaddition is good evidence of a stepwise process. In this case, the size of the pressure effect depends on the possibility of electrostriction due to the polarity of the transition state [14],... 

Esterification with alcohols using the Lewis catalyst BF3 is a well-established procedure. Organic acids are derivatized with BF3/butanol to their corresponding butylesters and then extracted into hexane. For dicarboxylic acids derivatizations, excess butanol was removed to avoid interference with the analytes.Butylesters are preferred over methylesters since these are more easily separated from the solvent peak due to their higher boiling points. Dicarboxylic acids as well as a-ketoacids were quantified with this procedure in rainwater samples. 

The feasabihty of using metal triflates as Lewis catalysts for different aromatic acetylations in non-chloroaluminate ILs such as [BMIM][BF4] has also been demonstrated. The faster rates were obtained with copper(II) triflate for the benzoylation of anisole but triflates are restricted to activated substrates [13]. 

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