Aluminum complexes acid derivatives

The enantioselective inverse electron-demand 1,3-dipolar cycloaddition reactions of nitrones with alkenes described so far were catalyzed by metal complexes that favor a monodentate coordination of the nitrone, such as boron and aluminum complexes. However, the glyoxylate-derived nitrone 36 favors a bidentate coordination to the catalyst. This nitrone is a very interesting substrate, since the products that are obtained from the reaction with alkenes are masked a-amino acids. One of the characteristics of nitrones such as 36, having an ester moiety in the a position, is the swift E/Z equilibrium at room temperature (Scheme 6.28). In the crystalline form nitrone 36 exists as the pure Z isomer, however, in solution nitrone 36 have been shown to exists as a mixture of the E and Z isomers. This equilibrium could however be shifted to the Z isomer in the presence of a Lewis acid [74]. 

Sulfonamide derivatives of a-amino acids and the similar bissulfonamide derivatives of diamines can be used to prepare reactive Lewis acid complexes. Corey20 reported the Lewis acid (R,R)- or (5,5 )-complex 69, which can be employed at 10 mol% level to catalyze the Diels-Alder reaction of cyclopentadiene and imide. Reactions catalyzed by this complex give an endo.exo ratio of over 50 1, as well as a high ee (91%) at —78°C, and this can be further improved to 95% by carrying out the reaction at 90°C.20 The related aluminum complex 69b shows very similar reactivity at —78°C, with generally higher ee values, typically over 95%, for the reaction of cyclopentadiene derivatives with imide.20,21... 

Both CO2 activation and enolate formation are combined in the preparation of malonic acid derivatives. The reaction of CO2 with methacrylic esters or methacry-lonitrile and under visible light irradiation produced the corresponding aluminum porphyrin malonate complex. When diethylzinc was added to this system, Al(TPP)Et could be regenerated by axial ligand exchange reactions, and the malonic acid derivatives were formed catalytically with respect to the aluminum porphyrins in a one-pot photosynthetic route (Scheme 1). The first step in this... 

However, it could be shown that in principle simple imines can be used successfully as well for asymmetric aminoalkylation reactions catalyzed by chiral Lewis acids. The asymmetric allylation of simple imines 16 with allyltributylstannane (Scheme 6) catalyzed by the / -pinene derivative 17 [32a], for example furnished comparatively good results (for related asymmetric catalytic allylations of simple imines, see [32b-d]). Moreover, it was demonstrated on the basis of several Strecker-type syntheses [33-35] that catalysts such as the chiral aluminum complex 18 (Scheme 6) [33a—b] are also well suited for enantioselective aminoalkylations with simple imines. The mechanism indicated in Scheme 6 shows that the... 

Aluminum chloride is used in the petroleum industries and various aspects of organic chemistry technology. For example, aluminum chloride is a catalyst in the alkylation of paraffins and aromatic hydrocarbons by olefins and also in the formation of complex ketones, aldehydes, and carboxylic acid derivatives. 

Straight-chain alkyl carboxylic acids derived from petroleum that also have a terminal cyclohexyl or cyclopentyl group are known as naphthenic acids. They form complexes, presumably polymeric, with many transition metals, and these compounds are freely soluble in petroleum. Copper naphthenates are used as fungicides, aluminum naphthenate was used as a gelling agent in napalm, and cobalt naphthenates are used in paints. 

Asymmetric Reductions. Asymmetric reductions of prochiral ketones to optically active secondary alcohols have been extensively studied. The most common method involves the use of chiral unidentate or bidentate ligands in conjunction with Lithium Aluminum Hydride. However, an (5)-aspartic acid derived tridentate ligand has been shown to be very effective in certain cases, presumably due to the rigidity of aluminum complex (4) (eq 5-7). ... 

Diels-Alder Reactions. Reaction of the bis(triflamide) (2) with Diisobutylaluminum Hydride or Trimethylaluminum affords chiral Lewis acids that catalyze Diels-Alder reactions of acryloyl or crotonoyl derivatives with cyclopentadienes (eq 4). The aluminum complex must be crystallized before use to remove traces of trimethylaluminum. High diastereo- and enantioselectivities are achieved with as little as 0.1 equiv of the Lewis acid, and the chiral sulfonamide is recoverable. 

It has been noted that although the linear gemnetry is consistently predicted for cationic Lewis acid carbonyl ctxnplexes in ab initio calculations, extrapolation of these results to neutral Lewis acid c< n-plexes may not be justified. Semiempirical MNDO calculations predicted the bent conformation as the lowest energy structure for neutral Lewis acidic derivatives of beryllium, boron and aluminum cmn-plexed with rranr-2,3-dimethylcyclopropanone, whereas linear structures were predicted for the cationic complexes of beryllium and aluminum (Table 1). ... 

Few examples have been reported demonstrating enantioselective cyclization methodology. One known example, however, is similar to the diastereoselective cyclization of 175, which uses a menthol-derived chiral auxiliary and a bulky aluminum Lewis acid (see Eq. (13.55)). The enantioselective variant simply utilizes an achiral template 188 in conjunction with a bulky chiral binol-derived aluminum Lewis acid 189 (Eq. (13.59)) [75]. Once again the steric bulk of the chiral aluminum Lewis acid complex favors the s-trans rotamer of the acceptor olefin. Facial selectivity of the radical addition can then be controlled by the chiral Lewis acid. The highest selectivity (48% ee) was achieved with 4 equivalents of chiral Lewis acid, providing a yield of 63%. 

Asymmetric ene Reaction In 1988 Yamamoto and coworkers provided the first indication that asymmetry in ene-reactions could be induced by catalytic amounts of chiral Lewis acids in the presence of 4-A molecular sieves (Scheme 6.64) [88]. They described the first example of asymmetric ene-reaction between prochiral, halogenated aldehydes and alkenes catalyzed by chiral binaphthol-derived aluminum complexes. The hindered 3,3-silyl substituents in the chiral catalyst are essential to achieve good enantioselectivity and high yield. In fact, the use of a catalyst derived from MesAl and 3,3 -biphenylbinaphthol led to the racemic product in a low yield. 

Chiral aluminum complexes, derived from N-sulfonyl-a-amino acids and diethylaluminum chloride, catalyze the cycloaddition of methacrolein (1) to cyclopentadiene (2) albeit with low enantioselectivities. Much better results are achieved with boron complexes (vide infra)6. 

Evans, D.A., Janey, J.M., Magomedov, N. and Tedrow, J.S. (2001) Chiral salen-aluminum complexes as catalysts for enantioselective aldol reactions of aldehydes and 5-alkoxyoxazoles an efficient approach to the asymmetric synthesis of syn and anti P-hydroxy-a-amino acid derivatives. Angewandte Chemie - International Edition, 40, 1884—1888. 

As early works on asymmetric ring-opening reactions utilizing chiral aluminum Lewis acids, in 1998, Yamamoto and coworkers reported the chiral chloroaluminate mediated asymmetric desymmetrization of meso-cyclohexene oxide giving rise to trans-cyclohexane chlorohydrin with modest enantioselectivity (Scheme 6.75) [95]. For example, the use of chiral aluminate complexes (100) derived from Et2AlCl,... 

Enantioselective Michael addition catalyzed by chiral aluminum Lewis acid is one of the most important methods to obtain enantiomerically pure compounds. As an early work in this fleld, in 1986, Shibasaki and coworkers reported catalytic enantioselective Michael addition of malonates to cyclic enones catalyzed by Li-Al bimetallic catalyst (72) (ALB) derived by premixing LiAlH4 and 2 equivalent of (R)-BINOL in THF (Scheme 6.86) [106, 107]. The structure of (R)-ALB was confirmed by X-ray crystallographic analysis of ALB-cyclohexenone complex. One notable advantage of ALB catalyst is that it works nicely in the tandem Michael-aldol sequence. 

Kobayashi and coworkers reported catalytic asymmetric Simmons-Smith type reaction of allylic alcohols (Scheme 6.98). In this reaction, Lewis acid (R,R)-(112) prepared by premixing of (1R,2R)-cyclohexane bis-sulfonamide and i-Bu2AlH was found to realize good enantioselectivity. Since, in the similar reaction catalyzed by chiral Zn complex derived from (1R,2R)-cyclohexane bis-sulfonamide and Et2Zn instead of chiral aluminum complex, the same enantioselectivity was observed, chiral Zn carbenoid species formed from (R,R)-(112) and Et2Zn via Al-Zn transmetallation was proposed as an active species [117]. 

In 1990, Yamamoto and Maruoka reported the first example of chiral aluminum Lewis acid-catalyzed asymmetric Claisen rearrangement (Scheme 6.162) [192]. Since chiral Lewis acids (2a) and (2b) derived from 3,3 -substituted BINOL derivative (1) exists as a mononuclear complex, high catalytic activity was realized. In this reaction, simple allyl vinyl ethers gave only poor results, although silylated substrates resulted in good enantioselectivity. 

At the surface of the raised bog, the pore waters are derived primarily from recharging precipitation, and are high in dissolved organic acids from the oxidation of plant material. pH is low (<5), and dissolved aluminum is high, whereas dissolved silicon and alkaline earth cation concentrations are low. Examination of silt grains from the peat show that aluminosilicate minerals are chemically weathered but quartz is not. In this type of water, aluminum is mobilized as organic-aluminum complexes at acidic pH, and... 

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