Lewis acids reactions

Entry Solvent Lewis acid Reaction time Reaction temp. Yield (%) Ratio 193 194... 

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. 

Partially fluorinated vinyl ethers of fluoroolefins are quite susceptible to the action of Lewis acids. Reaction usually proceeds with ionization of the allylic C-F bond and results in formation of C=0 group and elimination of alkyl halide. Indeed, 3-chloro-2-methoxyhexafluoro-2-butene 82 reacts with A1C13 with formation of trichlorovinyl ketone 83, and cyclic alkoxyfluoroalkenes demonstrate similar behavior in reaction with aluminum or tin(IV) halides [170] ... 

As already mentioned, Meerwein-Ponndorf-Verley reductions are intrinsically selective, since only carbonyl group can coordinate with the Lewis acid reaction center, while C=C double bonds remain unactivated. In contrast, the selectivity... 

Neutral organoaluminum(m) compounds have broad applicability in organic synthesis as extremely strong Lewis-acidic reaction promoters. However, the exploration of increased reactivity in aluminum is a key subject for expanding their utility, especially in polymer synthesis and in the activation of relatively stable substrates... 

It is believed that complex formation between olefins and Lewis acids (reaction b) is quite common. Complexing of propene and aluminum organic compounds can be demonstrated in simple test tube experiments... 

An interesting situation arises when aromatic olefins are used. Since the ring moiety (particularly when methyl substituted rings are used) of the monomer is also nucleophilic as is the double bond, it will compete either for the Lewis acid (reaction b) or for the cocatalyst (reaction c). Reaction outcome is impossible to foretell and will depend on a balance of specific factors (i.e., steric, electrostatic, etc.). 

Halogen molecules are not strong electrophiles and, fluorine excepted, do not react with benzene. However, in the presence of a Lewis acid, reaction occurs readily. The role of the catalyst is to accept a lone pair of electrons from the halogen molecule, which then becomes electron deficient at one of the halogen atoms. The actual electrophile is probably the complex formed from the halogen and the catalyst, rather than a halonium ion, e.g. Cr or Br. Bromination of benzene serves as a good example of halogenation (Scheme 2.4). 

Dicationic tj -arene Ru half-sandwich complexes have also undergone development as Ru Lewis acids. Reaction of the readily available [Ru( -arene)Cl2]2 with bidentate ligands affords [Ru( -arene)(L-L )Cl][Cl], and halide removal with a silver salt then yields the corresponding dicationic Lewis acid. With chiral dissymmetric... 

The authors apphed this new concept to chemoselective functionalization of carbonyls rather than acetals [194], which is usually quite difficult to achieve because of the high reactivity of the acetal counterparts with Lewis acids. Reaction of a mixture of 1 equiv. each of acetophenone and its dimethyl acetal with ketene silyl acetal 191 under the influence of bidentate aluminum Lewis acid 188 in CH2CI2 at -78 °C for 3 h afforded aldol products 195 exclusively (88 % yield). It is worth noting that employment of dibutyltin bis(triflate) (DBTT) (10 mol%) as catalyst [195], which is quite useful for activation of aldehyde carbonyls rather than acetals, gave unsatisfactory results, producing the y3-methoxy ester preferentially (Sch. 147). 

Asymmetric cyanation of aldehydes is important in organic synthesis. Mukaiyama and Minowa have developed a new chiral Lewis acid catalyst which is readily prepared from l,l -dimethylstannocene, triflic acid, and (+)-cinchonine [49]. In the presence of this Lewis acid reaction of TMSCN with aldehydes proceed smoothly at -78 °C in dichloromethane to give the corresponding cyanohydrin trimethylsilyl ether in high yield with good to excellent ee. In this reaction the products are isolated as trimethylsilyl ethers and the reaction proceeds smoothly in the presence of 30 mol % tin(II) Lewis acid (Eq. 31). The catalyst, Sn(II) monoalkoxymonotriflate, is assumed to be regenerated from the initially produced Sn(II) alkoxide and trimethylsilyl triflate. 

Several racemic a- and /3-oxygenated aldehydes were examined under chelation-con-trolled conditions in which MgBra OEt2 served as the chelating Lewis acid. Reaction of a c -y-OTBS allylic stannane with a-benzyloxybutyraldehyde was highly selective for the syn, syn adduct (Eq. 31). /3-Oxygenated butyraldehydes were somewhat less selective. In these additions, the anti, syn adducts predominated by 4 1 (Table 33). 

Entry Lewis acid ) Reaction conditions % Yield ) Ratio (RSiRR) )... 

Table 2.8 Studies of Lewis acids, reaction medium, and chiral ligand structure in isopropyl addition to 663. ...

Spectacular results were reported by Mukaiyama and coworkers in the field of chiral Lewis acids. Reaction of thiol ester derived silylketene acetals with aldehydes promoted by the combined use of a chiral diamine coordinated with tin(II) triflate and tributyltin fluoride gave excellent yields of aldol products in very high enantiomeric excess (Scheme 19). 

Despite the lack of examples of Friedel-Crafts acylations catalyzed by Lewis acids, - reactions of stannanes with acyl halides catalyzed by palladium species have found considerable use for the preparation of ketones. Since alkyl groups are only transferred slowly from tin, more rapid transfer to the acyl chloride is observed for alkynyl, alkenyl and allyl, as well as aryl and benzyl, groups. This leads to a versatile synthesis of ketones.Acylations of alkenylstannanes are both regio- and stereo-specific. 

Friedel-Crafts acylation usually involves the reaction of an acyl halide, a Lewis acid catalyst, and the aromatic reactant. Several species may function as the active electrophile, depending on the reactivity of the aromatic compound. For activated aromatics, the active electrophile can be a discrete positively charged acylium ion or a complex formed between the acyl halide and the Lewis acid catalyst. For benzene and less reactive aromatics, it is believed that the active electrophile is a protonated acylium ion or an acyiium ion complexed by a Lewis acid. Reactions using acylium salts are slow with toluene or benzene as the reactant and do not proceed with chlorobenzene. The addition of triflic acid accelerates the reactions with benzene and toluene and permits reaction with chlorobenzene. These results suggest that a protonation step must be involved. 

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