Aluminum reagent

See page 379, Section 13 page 382, Section 20 page 386, Section 21 and page 590, Section 3. 

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Diketene (635) is converted into 3-phenyl-3-butenoic acid (636) by the reaction of phenylzinc, magnesium, and aluminum reagents via C—O bond clea-vage[495j. 

Several highly enantioselective Diels-Alder reactions are known for which the di-enophile does not fit any of the above classes. Corey and coworkers applied the chiral aluminum reagent 36 with a C2-symmetric stilbenediamine moiety (videsu-pra) to the Diels-Alder reaction of maleimides as dienophiles [54] (Scheme 1.68). In most asymmetric Diels-Alder reactions the reactants are usually relatively simple dienes such as cyclopentadiene or monosubstituted butadienes, and unsym-... 

Thus one of the transferred hydrogens conies from the aluminum reagent, and the other one from the solvent. In addition to the mechanism via a six-membered cyclic transition state, a radical mechanism is discussed for certain substrates. ... 

The initial formation of an ate complex by attack of the nucleophile on the aluminum reagent, followed by reaction with the ketone, is unlikely since treatment of the ketone with a mixture of MAD and the organometallic reagent gave results comparable to those obtained with the organometallic reagent in the absence of MAD. 

As observed with cyclohexanones, the diastereoselectivity of the addition reaction of trimeth-ylaluminum to 2-methylcyclopentanone depends on the stoichiometry of the reactants. Thus, addition of one equivalent of trimcthylaluminum proceeds via preferential tram attack whereas, due to the "compression effect , addition of an excess of the reagent leads to the formation of the equatorial alcohol via predominant attack from the cis side (Table 3)6. In contrast to the addition reactions with trimethylaluniinum, no reversal of the diastereoselectivity upon change of reagent stoichiometry was observed in the addition of triphenylaluminum to 2-methylcyclopentanone6. Even with an excess of the aluminum reagent trans attack predominates. However, the diastereoselectivity is lower than with the use of an equimolar amount of the reactants. 

In fact, the highest anti-Cram selectivity reported to date (96% de) was observed with the MAT-mediated addition of methylmagnesium bromide to 2-(l-cyclohexenyl)propanal3 i 36. The stereochemical outcome of this addition reaction can be explained as follows on treatment of the carbonyl compound with the large aluminum reagent, the sterically least hindered complex 9 is formed. Subsequent addition of the nucleophile from the side opposite to the bulky aluminum reagent produces the anti-Cram diastereomer preferentially. 

Prior to the addition of the Grignard reagent the aldehyde was precomplexed with an aluminum reagent which was prepared in situ by treatment of 2,4,6-trimethylphenol with (dichloro)ethylaluminum. 

The oxazaborolidines are easily prepared by heating ephedrine with borane dimethyl sulfide or the appropriate boronate ester. The aluminum reagent C is obtained by mixing ephedrine and trimethylaluminum. Borolidinc A is superior to its methyl derivative B and to the aluminum analog C. The diastereomeric borolidine obtained from borane and (S,S)-pseu-doephedrine failed to show any cnantioselectivity25. A variety of aromatic aldehydes can be enantioselectively alkylated in the presence of A, however, with heptanal the enantioselectivity is poor25. 

This procedure, which is based on the work of Ishii and co-workers, affords a mild and general method for converting a wide variety of esters to primary, secondary, and tertiary amides (Table 1). While the preparation of the tertiary amide, N,N-dimethylcyclohexanecarboxamide, described here is carried out in benzene, aluminum amides derived from ammonia and a variety of primary amines have been prepared by reaction with trimethylaluminum in dichloromethane and utilized for aminolysis in this solvent. Although 1 equivalent of the dimethylaluminum amides from amines was generally sufficient for high conversion within 5-48 hours, best results were obtained when 2 equivalents of the aluminum reagent from ammonia was used. Diethyl-aluminum amides can also effect aminolysis, but with considerably slower rates. 

Computational studies of both hydroalumination and carboalumination have indicated a four-center TS for the addition.220 The aluminum reagents, however, have more nucleophilic character than do boranes. Whereas the TS for hydroboration is primarily electrophilic and resembles that for attack of CH3+ on a double bond, the... 

Effective catalysts have recently been developed for the addition of trialkyl-aluminum reagents to alkenes (carboalumination). 6 -(Pentamethylcyclopentadienyl) zirconium dimethylide activated by fra-(pentafluorophenyl)boron promotes the addition of trimethylaluminum to terminal alkenes.221... 

This selectivity is attributed to the steric bulk of the aluminum reagent favoring the migration of the larger alkyl group. The same selectivity pattern is observed with unbranched substituents. 

The alkyl methacrylate monomers were available from various sources. Isobutyl methacrylate (IBMA) (Rohm and Haas) and t-butyl methacrylate (TBMA) (Rohm Tech) may be purified first by distillation from CaH, followed by distillation from trialkyl aluminum reagents as described in detail earlier (20,21). In particular, t-butyl methacrylate (b.pt. 150°C) was successfully purified by distillation, from triethyl aluminum containing small amounts of diisobutyl aluminum hydride. The trialkyl aluminum and dialkyl aluminum hydride reagents were obtained from the Ethyl Corporation as 25 weight percent solutions in hexane. The initiator, -butyllithium, was obtained from the Lithco Division of FMC, and analyzed by the Gilman "double titration" (22). 

Haubenstock, H., Asymmetric Reductions with Chiral Complex Aluminum Hydrides and Tricoordinate Aluminum Reagents, 14, 231. 

Treatment of alkylidene-bridged zirconium—aluminum species with HMPA activates the C—A1 bond of the alkylidene unit, making it susceptible to electrophilic attack [146]. Ligand-based activation of the C—A1 bond can also be used to convert alkylidene-bridged zirconium-aluminum reagents to other bimetallic species. Thus, treatment of 3 with HMPA followed by addition of a weakly electrophilic metal salt can give rise to a new heterome-tallic species. Slow addition of a solution of R3SnCl in toluene to a solution of 3 and 1... 

Dichloroaluminum phenoxide, CUAIOQH, (1). This aluminum reagent is prepared by reaction of CH,A1C12 with phenol in CH,C12. 

Selective reduction of ketones.1 This reagent can be used to effect selective reduction of the more hindered of two ketones by DIBAH or dibromoalane. Thus treatment of a 1 1 mixture of two ketones with 1-2 equiv. of 1 results in preferential complexation of the less hindered ketone with 1 reduction of this mixture of free and complexed ketones results in preferential reduction of the free, originally more hindered, ketone. An electronic effect of substituents on a phenyl group can also play a role in the complexation. This method is not effective for discrimination between aldehydes and ketones, because MAD-complexes are easily reduced by hydrides. MAD can also serve as a protecting group for the more reactive carbonyl group of a diketone. The selectivity can be enhanced by use of a more bulky aluminum reagent such as methylaluminum bis(2-f-butyl-6-( 1,1-diethylpropyl)-4-methylphenoxide). 

Complexation of enones with MAD can be used to effect conjugate additions with organolithium reagents. This unusual 1,4-selectivity is observed mainly with cyclic systems and requires 2 equiv. of the aluminum reagent as well as of the nucleophile for full effect. 

Ni(sacsac)P(nBu3)Cl (sacsac = pentane-2,4-dithionate) was activated by AlEt2Cl to form a catalytically active species for the oligomerization of ethene and propene. This study is noteworthy in that it uses in situ UV-VIS spectroscopy to monitor the course of the polymerization. In this reaction, the aluminum reagent serves both to activate the transition metal and to scavenge any moisture present. 

One of the more important recent developments in organometallic aluminum chemistry has been the formation and isolation of low-coordinate compounds, and, in particular, cations. These were first prepared in reactions of various aluminum reagents with crown ethers to form the inclusion compounds known as liquid clathrates. 71,72 Most of the evidence supports the presence of ion pairs as the basis of the solvent inclusion effect. Indeed, the compound [AlMe2-18-crown-6]+[AlMe2Cl2] was isolated from one such system (the cation is shown in Figure 6(a)).73 This was the first time the Me2Al+ unit had been structurally characterized. 

Maruoka, K. Synthetic Utility of Bulky Aluminum Reagents as Lewis Acid Receptors. In Lewis Acid Reagents-, Yamamoto, H., Ed. Oxford University Press Oxford, 1999 pp 5-29. 

In the presence of an aluminum reagent, 2,3-butadienyltrimethylsilane can also accept the intramolecular electrophilic attack of the ketone-aluminum complex to afford bicyclic products via intermediate 60 [31]. The structures of the products depend on the aluminum reagent used [31]. 

ASYMMETRIC REDUCTIONS WITH CHIRAL ALUMINUM REAGENTS... 

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