Lewis acids, acceleration

The presence of one or more heteroatoms in either the 4jt or In component allows the synthesis of six-memebered heterocycles via the Diels-Alder process. These strategies have been successfully developed (218) leading to a diverse array of natural products. Similar to the carbo-Diels-Alder reaction, many of these processes are amenable to Lewis acid acceleration, and several have been developed using chiral catalysts. 

With the aid of density functional theory, the ZnCl2 acceleration of the Simmons-Smith reaction of ethylene and allyl alcohol has been investigated. A pathway involving direct Lewis acid acceleration of the leaving halogen atom (327) was found to be a more facile process than the more popular pathway involving 1,2-chlorine migration (328). 

Tris(pentafluorophenyl)boron [(CgF5)3B], a triarylboron bearing electron-withdrawing perfluorinated phenyl rings, was found to be much more powerful than (05115)36 as a Lewis acid accelerator for the present polymerization. When (0565)36 was added at room temperature to the polymerization system at an equimolar ratio of (0565)36 to the growing species 2, the polymerization took place rapidly with considerable heat evolution, attaining 100% monomer conversion within only about 10 min [6ig. 13 ( )]. This polymerization was estimated to be 150-times faster than that in the absence of (0565)36, and 12.5-times faster than that with (05115)36 as a Lewis acid under similar conditions. 

Organometallic crown ethers have also been synthesized.96-98 Recently, a crown-cation group was shown to interact with an appended transition metal acyl ligand (29)." Complexes of this type have potential applications in Lewis acid-accelerated alkyl migration to coordinated carbonyls. 

Lewis acids accelerate the addition of zinc organometallics to carbonyl derivatives. Titanium and zirconium(iv) salts are especially efficient catalysts. Oguni and co-workers have shown in pioneering work that various... 

Diazo compounds react with aldehydes and ketones to give homologated carbonyl compounds and epoxides. Lewis acids accelerate these processes, and in some cases direct the reactions to single products. frara-Epoxides result from aldehydes and ethyldiazoacetates in the presence of MeReOs (although this can occur via a metal carbene pathway rather than a Lewis acid mechanism) [139], whereas... 

Though even higher oligomers of 9 could undoubtedly be prepared by further repetition of this procedure, a somewhat more streamlined, convergent synthesis proved more practical, even for the preparation of pentayne 11 (Fig. 9-4). Noteworthy here is the utility of acetyl chloride for the mild conversion of a methyl ether to an alkyl chloride (and methyl acetate), presumably via an S l process that is facilitated by the adjacent triple bond Lewis acids accelerate the reaction but are not essential. 

Sugimoto, H., Aida, T., and Inoue, S., 1993, Organoboron Compounds as Lewis Acid Accelerators for the Aluminum Porphyrin-Mediated Living Anionic Polymerization of Methyl Methaciylate. Macromolecules, 26 4751... 

In addition to organoaluminum compounds, organoboron compounds can also be used as Lewis-acid accelerators for the polymerization of 21 initiated with la, where triphe-nylboron and tris(perfluorophenyl)boron are effective accelerators (See Table 6). Tributylboron (Bu B) bearing no particular steric hindrance neither terminates nor accelerates the polymerization of 21, indicating a low transmetalation activity and a rather poor Lewis acidity of Bu B. This is in sharp contrast with the polymerization upon addition of trimethylaluminum (Me Al) as a monomer activator where the chain growth is terminated by the transfer of the enolate moiety from 32 to Me Al, unless the temperature is lowered to —40°C or below. 

Several studies show that Lewis acids accelerate the insertion of CO into alkyl groups. This effect has been shown to result from two different reactivities of Lewis acids. In one case, this acceleration is likely related to stronger coordination of Lewis acids to the oxygen of the acyl product than to tlie oxygen of the CO ligand. In a second case, the acceleration is due to an affect on ion pairing. 

Platinum group metal catalyzed Lewis acid Accelerates the rate... 

Sugimoto, H. Kuroki, M. Watanabe, T. Kawamura, C. Aida, T. Inoue, S. High-speed living anionic polymerization of methacrylic esters with aluminum porphyrin initiators. Organoaluminum compounds as Lewis acid accelerators. Macromolecules 1993, 26, 3403-3410. 

Scheme 2-141. Lewis-acid accelerated addition of alkylzincs to carbonyl groups.

Coordination of carbonyl groups by Lewis acids accelerates simple carbonyl transformations, conjugate additions to unsaturated carbonyl compounds, and Diels-Alder reactions, by polarizing the carbonyl group. 

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