Methylaluminum derivatives

Microwave heating has also been employed for performing retro-Diels-Alder cycloaddition reactions, as exemplified in Scheme 6.94. In the context of preparing optically pure cross-conjugated cydopentadienones as precursors to arachidonic acid derivatives, Evans, Eddolls, and coworkers performed microwave-mediated Lewis acid-catalyzed retro-Diels-Alder reactions of suitable exo-cyclic enone building blocks [193, 194], The microwave-mediated transformations were performed in dichloromethane at 60-100 °C with 0.5 equivalents of methylaluminum dichloride as catalyst and 5 equivalents of maleic anhydride as cyclopentadiene trap. In most cases, the reaction was stopped after 30 min since continued irradiation eroded the product yields. The use of short bursts of microwave irradiation minimized doublebond isomerization. 

S)-(-)-CITRONELLOL from geraniol. An asymmetrically catalyzed Diels-Alder reaction is used to prepare (1 R)-1,3,4-TRIMETHYL-3-C YCLOHEXENE-1 -CARBOXALDEHYDE with an (acyloxy)borane complex derived from L-(+)-tartaric acid as the catalyst. A high-yield procedure for the rearrangement of epoxides to carbonyl compounds catalyzed by METHYLALUMINUM BIS(4-BROMO-2,6-DI-tert-BUTYLPHENOXIDE) is demonstrated with a preparation of DIPHENYL-ACETALDEHYDE from stilbene oxide. A palladium/copper catalyst system is used to prepare (Z)-2-BROMO-5-(TRIMETHYLSILYL)-2-PENTEN-4-YNOIC ACID ETHYL ESTER. The coupling of vinyl and aryl halides with acetylenes is a powerful carbon-carbon bond-forming reaction, particularly valuable for the construction of such enyne systems. 

We note that while tin reagents have often been employed for the organoboron halides/ the use of organostannanes as starting materials can also be applied to the synthesis of heavier group 13 derivatives. In the context of polyfunc-tional Lewis acid chemistry, this type of reaction has been employed for the preparation of ort/ o-phenylene aluminum derivatives. Thus, the reaction of 1,2-bis(trimethylstannyl)benzene 7 with dimethylaluminum chloride, methylaluminum dichloride or aluminum trichloride affords l,2-bis(dimethylaluminum)phenylene 37, l,2-bis(chloro(methyl)aluminum)phenylene 38 and 1,2-bis(dichloroalumi-num)phenylene 39, respectively (Scheme 16). Unfortunately, these compounds could not be crystallized and their identities have been inferred from NMR data only. In the case of 39, the aluminum derivative could not be separated from trimethyltin chloride with which it reportedly forms a polymeric ion pair consisting of trimethylstannyl cations and bis(trichloroaluminate) anions 40. 

In early studies of these reactions, the turnover efficiency was not always high, and stoichiometric amounts of the promoters were often necessary to obtain reasonable chemical yields (Scheme 105) (256). This problem was first solved by using chiral alkoxy Ti(IV) complexes and molecular sieves 4A for reaction between the structurally elaborated a,/3-unsaturated acid derivatives and 1,3-dienes (257). Use of alkylated benzenes as solvents might be helpiul. The A1 complex formed from tri-methylaluminum and a C2 chiral 1,2-bis-sulfonamide has proven to be an extremely efficient catalyst for this type of reaction (258). This cycloaddition is useful for preparing optically active prostaglandin intermediates. Cationic bis(oxazoline)-Fe(III) catalysts that form octahedral chelate complexes with dienophiles promote enantioselective reaction with cyclopentadiene (259). The Mg complexes are equally effective. 

Long (81) showed that the complex from biscyclopentadienyltitanium dichloride and methylaluminum chloride or a simply derived product from it, was an active ethylene polymerization catalyst. There have been a number of attempts to determine the exact nature of initiation in polyethylene. However, by any techniques available until now, it has not been possible to determine the actual ionic nature of the active catalyst which polymerizes ethylene. Karapinka and Carrick (82) studied the polymerization of ethylene with biscyclopentadienyltitanium dichloride and various alkylaluminum compounds. They found that the alkyl group exchanged so readily between the aluminum and titanium, that the location of the initiating site could not be determined. All that could be concluded was that an ethyl group initiated the polymerization more easily than the phenyl. 

Claisen rearrangement.1 Allyl vinyl ethers such as 3 undergo Claisen rearrangement reluctantly and in low yield when treated with methylaluminum bis(2,6-di-f-butylphenoxide), MAD but this dibromo derivative, 1, effects this rearrangement readily at -78° with high (Z)-selectivity. Evidently the bulky r-butyl groups control the stereoselectivity, for use of methylaluminum bis(2,6-diphenylphenox-... 

A decade later, Corey introduced an effective aluminum-diamine controller for Diels-Alder and aldol additions. The C2-symmetric stilbenediamine (stien) ligands are available in good yield from substituted benzils, which are in turn derived from benzoic acids, aryl aldehydes, or aryl bromides [48]. Formation of the active catalyst 3 is achieved by treatment of the bis(sulfonamide) with tri-methylaluminum recovery of the ligand was essentially quantitative. Acryloyl and crotonyl imides 4 are particularly effective dienophiles for this system, as shown in Scheme 4. 

Cyclization of an acyclic terpene derivative to a seven-membered ring has also been achieved. Thus the silyl enol ether 2 related to nerol is converted into karahanaenone (3) in 95% yield by methylaluminum bis(trifluoroacetate) in CHaCla (0 25°). 

Formaldehyde undergoes Me2AlCl induced reactions with terminal alkynes to give a 2 3 mixture of the ene adduct 10 and the (Z)-chloroalcohol 11 in 50-75% yield (See Figure 4).i3 These products can both be derived from an intermediate such as 4 or 5 and 6. Reaction of formaldehyde, methyl 6-heptynoate and the stronger Lewis acid methylaluminum sesquichloride gives... 

ABSTRACT. With the aid of selected examples an overview is given of the development trends in highly discriminative reactions using novel Lewis acid catalysts. 1) Bulky organoaluminum reagent, methylaluminum 3,3 -bis(triphenylsilyl)-l,r-bi-2-naphthoxide, has been successfully utilized for enantioselective activation of carbonyl moiety. 2) A novel reactivity of acyloxyborane derivatives and their usefulness in organic synthesis is demonstrated. As an example, asymmetric Diels-Alder reaction catalyzed by new acyloxyboranes derived from chiral acids is described. 

MAPH and ATPH have also been known as more bulky Al Lewis acids compared to methylaluminum bis(2,6-di-tert-butyl-4-alkylphenoxide) derivatives such as MAD and MAT. MAPH can discriminate between structurally similar aldehydes such as pentanal and cyclohexanecarboxaldehyde, thereby MAPH acts as a carbonyl protector of less hindered aldehydes. Therefore, butyllithium reacts preferentially with uncomplexed, more hindered aldehydes (Scheme 6.39) [48]. 

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