Simple Lewis-base catalysts

R = aryl, heteroaryl, alkyl, alkenyl R = H, aryl, alkyl 

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As shown above, it was not so easy to optimize the Michael addition reactions of l-crotonoyl-3,5-dimethylpyrazole in the presence of the l ,J -DBFOX/ Ph-Ni(C104)2 3H20 catalyst because a simple tendency of influence to enantio-selectivity is lacking. Therefore, we changed the acceptor to 3-crotonoyl-2-oxazolidi-none in the reactions of malononitrile in dichloromethane in the presence of the nickel(II) aqua complex (10 mol%) (Scheme 7.49). For the Michael additions using the oxazolidinone acceptor, dichloromethane was better solvent than THF and the enantioselectivities were rather independent upon the reaction temperatures and Lewis base catalysts. Chemical yields were also satisfactory. 

Two patterns are possible in the activation mechanism by simple chiral Lewis base catalysts. One is through the activation of nucleophiles such as aUyltrichlorosilanes or ketene trichlorosilyl acetals via hypervalent silicate formation using organic Lewis bases such as chiral phosphoramides or A-oxides. " In this case, catalysts are pure organic compounds (see Chapter 11). The other is through the activation of nucleophiles by anionic Lewis base conjugated to metals. In this case, transmetal-lation is the key for the nucleophile activation. This type of asymmetric catalysis is the main focus of this section. 

Strecker-type reaction of TMS cyanide with chiral sulfinimines gives diastereoselective cyanations at the imine carbon, at —78 °C in DMF, using simple metal-free Lewis base catalysts such as tetraalkylammonium carboxylates.73... 

Mukaiyama developed a variety of catalytic carbon-carbon bond-forming reactions with the use of trimethylsilyl nucleophiles in the presence of simple lithium Lewis-base catalysts, such as lithium acetate, lithium pyrro-lidone, lithium succinimide, lithium benzamide, lithium diphenylamide, and lithium benzyloxide (Scheme 2.1). For example, the Mukaiyama aldol... 

Ishihara developed a highly efficient Mukaiyama aldol reaction between ketones and trimethylsilyl enolates catalysed by sodium phenoxide-phosphine oxides (46) as simple homogeneous Lewis-base catalysts (0.5-10 mol%) (Scheme 2.29). For a variety of aromatic ketones and aldimines, aldol and Mannich-type products with an a-quaternary carbon centre were obtained in good to excellent yields. Remarkably, a retro-aldol reaction was not observed. On a scale of up to 100 mmol, benzophenone and trimethylsilyl enolate gave the product in 97% yield (34.8 g) using 0.5 mol% of catalyst. 

Scheme 2.28 Mukaiyama aldol reaction and Mukaiyama-Michael reaction with the use of simple sodium(i) Lewis-base catalysts.

A syn-selective asymmetiic nih o-aldol reaction has been reported for structurally simple aldehydes using a new catalyst generated from 6,6-bis[(tiiethylsilyl)ethynyl]BINOL (g in Scheme 3.18). The syn selectivity in the nitro-aldol reaction can be explained by steric hindrance in the bicyclic transition state as can be seen in Newman projection. In the favored h ansition state, the catalyst acts as a Lewis acid and as a Lewis base at different sites. In conbast, the nonchelation-controlled transition state affords anti product with lower ee. This stereoselective nitro-aldol reaction has been applied to simple synthesis of t/ireo-dihydrosphingosine by the reduction of the nitro-aldol product with H2 and Pd-C (Eq. 3.79). 

The formation of covalent substrate-catalyst adducts might occur, e.g., by single-step Lewis-acid-Lewis-base interaction or by multi-step reactions such as the formation of enamines from aldehydes and secondary amines. The catalysis of aldol reactions by formation of the donor enamine is a striking example of common mechanisms in enzymatic catalysis and organocatalysis - in class-I aldolases lysine provides the catalytically active amine group whereas typical organocatalysts for this purpose are secondary amines, the most simple being proline (Scheme 2.2). 

Diastereoselective Mannich-type reactions between ketene silyl acetals and chiral sulfinimines using simple metal-free Lewis bases such as tetraalkylammonium car-boxylates have been reported. The sulfinimine can even be generated in situ (from aldehyde and a chiral sulfonamide), using cesium carbonate, followed by addition of ketene silyl acetal at -78 °C, and as little as 1 mol% of catalyst.32... 

At the end of considerations of the role of alkylaluminoxanes as activators for metallocenes as Ziegler-Natta olefin polymerisation catalysts, it should be noted that the analogy between methylaluminoxane and simple acidic moieties does not appear to hold. For example, highly Lewis acidic perfluorinated boranes, such as Bf CgFs, show enhanced activity as activators when compared with [Al(Me)0]x, but the stability of the resulting zirconocene-based catalyst is drastically lower than that of the catalyst formed with [Al(Me)0]x [98]. 

Simple cyclooctatetraenediyls [M(C8H8)] (M = Eu, Yb) can be made by reaction of cyclooctatetraenewith solutions ofthe metal in liquid ammonia. Another route to [M(CsH8)] (Sm, Yb) is the reaction of the metals with cyclooctatetraene using iodine as catalyst. These compounds are not monomers, but the Lewis base adduct [M(C8H8)(py)3] has a piano-stool structure. 

A simple example illustrates the setup An engineer needs to add 3 gal of an organic liquid to a 10-gal reactor over 90 min (0.0333 gpm). The organic is saturated with hydrochloric acid and contains a significant amount of a partially dissolved Lewis-acid catalyst [chemistry buffs will remember that, in 1923, G. N. Lewis introduced the useful concept that any acceptor of an electron pair is an acid, and an electron donor is a base]. 

With the simple triorganotin hydrides, reaction 15-31 occurs only in the presence of a Lewis base such as an amine, or of a transition metal catalyst. The amine-catalysed reaction has been used for preparing catenary and cyclic oligostannanes (e.g. equations 15-33 and 15-34 and Section 18.2.1.3).92... 

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