Modified Organoaluminums

Organoaluminum compounds are highly oxygenophilic, and hence are capable of forming long-lived monomeric 1 1 complexes with carbonyl substrates. For example, the reaction of benzophenone with McsAl in 1 1 molar ratio gives a yellow, long-lived monomeric 1 1 species which decomposes unimolecularly to dimethylaluminum 1,1-diphenylethoxide after some minutes at 80 °C or many hours at 25 °C [124]. 

Entry Alkylation agent Chemical yield (%) Axiakequatorial ratio 

This approach has been quite useful in the stereoselective alkylation of steroidal ketones. Reaction of 3-cholestanone (128) with MeLi gave predominantly 3/8-methyl-cholestan-3a-ol (129, axial alcohol), whereas amphiphilic alkylation of the ketone with MAD-MeLi or MAT-MeLi afforded 3a-methylcholestan-3/3-ol (130, equatorial alcohol) exclusively (Sch. 89) [125b]. 

In the course of the synthesis of ( )-3a-acetoxy-15/3-hydroxy-7,16-secotrinervita-7,11-diene, a defense substance isolated from the soldier termite, the need to introduce the methyl group from the hindered a side was achieved for the first time when the amphiphilic alkylation system MAD-MeLi was used (Sch. 90) [126]. 

In the alkylation of a-chiral aldehydes with no ability to chelate with organometal-lic compounds such as Grignard reagents, erythro alcohols are usually obtained preferentially according to the Cram s rule [127], and high Cram selectivity can be achieved with alkyltitanium reagents developed by Reetz [128]. In contrast, application of amphiphilic alkylation to a-chiral aldehydes enables one to achieve the hitherto difficult anti-Cram selectivity, affording threo alcohols selectively as shown in Sch. 91 [125]. 

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Maruoka K, Murase N, Ooi T, Yamamoto H (1991) A new cyclization of olefinic epoxides by modified organoaluminum reagents via epoxide rearrangement and subsequent intramolecular ene reaction. Synlett 857-858... 

This reaction has been further extended to the asymmetrization of the symmetric acetal, and to different modified organoaluminum reagents including i-BuaAl and bulky organoaluminum amides (Sch. 57) [85],... 

Polymers with LM in side groups can also be obtained by polymerization of other monomers. Thus, poly(ethylene oxide) containing LM (Table 1 LMg) is formed by copolymerization of ethylene oxide and 9-anthryaldehyde in the presence of a modified organoaluminum catalyst ... 

The modified organoaluminum reagent, BINAL, can be used as a chiral Lewis acid catalyst in the asymmetric hetero-Diels-Alder reaction [41]. Reaction of various aldehydes with activated dienes under the influence of catalytic BINAL (5-10 mol%) at -20 °C gave, after exposure of the resulting hetero-Diels-Alder adducts to tri-fluroacetic acid, predominantly c/v-dihydropyrone in high yield with excellent enan-tioselectivity. 

It is not the subject of this chapter to cover all domains of the catalytic properties of nanoparticles modified by organometallics. In particular, we will not cover the work of Bormeman on metallic particles have been modified by organoaluminum compounds. 

Scheme76 Modified allyl insertion mechanism of the neodymium-based 1,3-diene polymerization taking into account organoaluminum-mediated chain transfer [148]...

In practice, a general impression of the throwing power of an electrolyte can be obtained by observing deposition in a Hull [144], a Haring-Blum [75], or another suitable cell. To compare the throwing power of different organoaluminum electrolyte systems, a modified Haring-Blum cell was developed to allow work under inert atmosphere [130]. 

Another unprecedented stereochemical control in a Claisen rearrangement was reported recently by Yamamoto using organoaluminum reagents (Scheme 59). Sometimes (e.g. with R = Cu2CH==CH— CsHi i) complete reversal of double bond geometry could be achieved simply by modifying the bulky aluminum catalyst. 

N.S. Mani and co-workers utilized the organoaluminum promoted modified Beckmann rearrangement during their efficient synthetic route to chiral 4-alkyl-1,2,3,4-tetrahydroquinoline. (4R)-4-Ethyl-1,2,3,4-tetrahydroquinoline was obtained by rearrangement of the ketoxime sulfonate of (3R)-3-ethylindan-1-one. The resulting six-membered lactam product was reduced to the corresponding cyclic secondary amine with diisobutylaluminum hydride. 

The authors have designed modified bis(organoaluminum) reagent 49 for the efficient simultaneous coordination toward carbonyls (type E2), and successfully elucidated its reactivity and selectivity in the typical synthetic transformations [58J. 

Alkyl aluminum compounds are known to be radical initiators. Kabalka and coworkers reported fhe first unequivocal example of a free-radical process in which the classical organoaluminum reagent PrsAl was used in fhe presence of O2 or under UV irradiation [203]. In modern studies, modified alkylaluminum reagents enabled more practical improvement in fhe control of diastereo- and enantioselectivity in radical initiation and reactions. 

The catalyst systems may roughly be divided into two groups (1) Ziegler-Natta type catalysts composed of a Ni(II) salt or complex activated by a Lewis acid (frequently an organoaluminum compound) or a Bronsted acid and often modified by a P-donor [PR3 or P(0R)3] and (2) one-component systems, most of them composed of a Ni(II) chelate complex with a Ni—C bond that are active without a Lewis acid or reducing agent as cocatalyst. 

Asymmetric modifications of hydrovinylation are one of the earliest examples of successful asymmetric transition metal catalysis. After optimization of various dimerization and codimerization reactions using phosphane modified nickel catalysts, the first examples of asymmetric olefin codimerization were reported with n-allylnickel halides activated by organoaluminum chloride and modified by chiral phosphanes7. Thus, codimerization of 2-butene with propene using n-allylnickel chloride/A]X, (X = Cl, Br) in the presence of tris(myrtanyl)phosphane gives low yields of (—)-( )-4-methy 1-2-hexene (I) with 3% ee7,7 . 

Hydrated iron group metal salts supported on porous carriers after drying acid calcination modified with organoaluminum compounds are used as catalysts in Huels-UCP- Hexall and Octol processes for propylene oligomerization and propylene-butene co-oligomerization [622-624]. 

Carlini C, Marchionna M, Galletti AMR, Sbrana G (2001) Olefin oligomerization by novel catalysts prepared by oxidative addition of carboxylic acids to nickel(O) precursors and modified by phosphine ancillary ligands and organoaluminum compounds. J Mol Catal A 169 79-88... 

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