Carbonyl compounds Lewis acid complexes

Enolates are undoubtedly the most versatile intermediates for C-C, C-N, C-O bond-forming reactions [36]. Continuous progress has been made not only in fundamental operations involving these anionic species but also during the synthesis of complex natural products. Compared with metal enolates with counter cations of, e.g., B, Si, Li, Na, K, Mg, Ti, Sn, Cu, etc., aluminum enolates have found fewer apphcations, probably because no particular advantages over the other metals have been perceptible. There are, however, still intriguing aspects of novel reactivity and selectivity in the formation and reaction of aluminum enolates. Specifically, very recent development have highhghted pre-formation of Lewis acid-carbonyl complexes by use of bulky aluminum compounds as precursors of aluminum enolates the behavior of these complexes is unprecedented. 

Conjugate addition to carbonyl compounds. Complexation to the bulky Lewis acid prevents attack at the carbonyl group, and the addition occurs at the o- or p-positions of aromatic aldehydes and ketones. With relatively bulky nucleophiles, such as s-BuLi and. BuLi, products from addition in the p-position prevail. 

The addition of cyanotrialkylsilanes to carbonyl compounds, and especially the development of catalysts for this transformation continues to attract considerable attention owing to the role of cyanohydrin trialkysilyl ethers and cyanohydrins as versatile intermediates in organic synthesis [149]. A variety of catalysts has been developed for such reactions including Lewis acids, transition metal complexes, 18-crown-6 complexes of alkali metals, tetracyanoethylene, Lewis bases, and alkali earth bases. Trimethylcyanation of ketones using Lewis bases does not seem to have been mentioned in the literature. 

Design of Lewis acids toward selective complexation mode of carbonyl group has opened new and efficient chemistry of carbonyl compounds including aldol reactions. 

The mechanism of the cycloaddition reaction of benzaldehyde 2a with Danishefsky s diene 3a catalyzed by aluminum complexes has been investigated theoretically using semi-empirical calculations [14]. It was found that the reaction proceeds as a step-wise cycloaddition reaction with the first step being a nucleophilic-like attack of Danishefsky s diene 2a on the coordinated carbonyl compound leading to an aldol-like intermediate which is stabilized by interaction of the cation with the oxygen atom of the Lewis acid. The next step is the ring-closure step, giving the cycloaddition product. 

Lewis acids, particularly the boron trifluroride diethyl ether complex, are used to promote the reaction between allyl(trialkyl)- and allyl(triaryl)stannanes and aldehydes and ketones52-54. The mechanism of these Lewis acid promoted reactions may involve coordination of the Lewis acid to the carbonyl compound so increasing its reactivity towards nucleophilic attack, or in situ transmetalation of the allyl(trialkyl)stannane by the Lewis acid to generate a more reactive allylmetal reagent. Which pathway operates in any particular case depends on the order of mixing of the reagents, the Lewis acid, temperature, solvent etc.55- 58. 

Lewis-acid-catalyzed cycloadditions of dienophiles, such as a,/l-unsaturated carbonyl compounds, with open-chain carbon-dienes, are generally highly ortho-para regioselective because the oxygen complexation increases the difference of LUMO coefficients of the alkene moiety. 

Several aluminum- and titanium-based compounds have been supported on silica and alumina [53]. Although silica and alumina themselves catalyze cycloaddition reactions, their catalytic activity is greatly increased when they complex a Lewis acid. Some of these catalysts are among the most active described to date for heterogeneous catalysis of the Diels-Alder reactions of carbonyl-containing dienophiles. The Si02-Et2AlCl catalyst is the most efficient and can be... 

Thermal [2h-2] cycloaddition reactions of carbonyl compounds were catalyzed by a Lewis acid. The catalyst forms complexes with the carbonyl compounds and enhances the electron-accepting power. The reaction shifts from the delocalization band to the pseudoexcitation band. Catalyzed [2h-2] cycloaddition reactions were observed with acetylenic compounds [28] and ketenes [29-31]. 

Si. rra(pentafluorophenyl)boron was found to be an efficient, air-stable, and water-tolerant Lewis-acid catalyst for the allylation reaction of allylsilanes with aldehydes.167 Sc(OTf)3-catalyzed allylations of hydrates of a-keto aldehydes, glyoxylates and activated aromatic aldehydes with allyltrimethylsilane in H2O-CH3CN were examined. a-Keto and a-ester homoallylic alcohols and aromatic homoallylic alcohols were obtained in good to excellent yields.168 Allylation reactions of carbonyl compounds such as aldehydes and reactive ketones using allyltrimethoxysilane in aqueous media proceeded smoothly in the presence of 5 mol% of a CdF2-terpyridine complex (Eq. 8.71).169... 

This reaction sequence of conjugate reduction followed by aldol reaction is known as the reductive aldol reaction. In certain instances, reductive elimination from the M-TM-enolate species may occur to furnish M-enolate, which itself may participate in the aldol reaction (Scheme 3). This detour may be described as the background path or stepwise path in one-pot. Indeed, it has been reported that certain cationic Rh complexes such as [Rh(COD)(DPPB)] (COD = 1,5-cyclooctadiene, DPPB = diphenylphosphinobutane) catalyze the aldol reactions of silyl enol ethers and carbonyl compounds by serving as Lewis acids [5-8]. 

Complexes 17-19 can be written in one valence structure as a, /3-unsaturated carbonyl compounds in which the carbonyl oxygen atom is coordinated to a BF2(OR) Lewis acid. The C=C double bonds of such organic systems are activated toward certain reactions, like Diels-Alder additions, and complexes 17-19 show similar chemistry. Complexes 17 and 18 undergo Diels-Alder additions with isoprene, 2,3-dimethyl-1,3-butadiene, tram-2-methyl-l,3-pentadiene, and cyclopentadiene to give Diels-Alder products 20-23 as shown in Scheme 1 for complex 17 (32). Compounds 20-23 are prepared in crude product yields of 75-98% and are isolated as analytically pure solids in yields of 16-66%. The X-ray structure of the isoprene product 20 has been determined and the ORTEP diagram (shown in Fig. 3) reveals the regiochemistry of the Diels-Alder addition. The C-14=C-15 double bond distance is 1.327(4) A, and the... 

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