Alkenes Aluminates

Alkali metall-trialkyl-hydrido-aluminate reagieren dagegen hauptsachlich unter Mono-hydroaluminierung zu Alken-(l)-yl-aluminiumhydriden12 (weiteres s. ds. Handb., Bd. XIII/4, S. 159ff.). 

Scheme 6.12. Enantioselective carbo-aluminations of unactivated alkenes are promoted by neutral (37) and cationic (38 + B(C6F5)3) chiral Zr complexes.

The conductimetric, kinetic, and radiochemical experiments lead to the conclusion that alkenes, but not strainless alkanes, are aluminated by A1X2+. This and the balance of the number of C-Al bonds in our polyisobutylenes support Proposition 2 that initiation is by reaction (iii). 

Another common trapping method is an intramolecular aldol reaction of the initially formed anion, as shown in equation (91) and Schemes 53 and 54.% In the first case, an aldol-like trapping of the iminium salt produced (411 equation 91 ).96b The initial heteronucleophile in the other two cases is ultimately lost from the product by oxidation and elimination, so that the overall process is C—C bond formation at the a-center of an enone. Thus, treatment of the formyl enone (412 Scheme 53) with an aluminum thiolate afforded in 60% yield the trapped product (413) which could be oxidized and eliminated to give (414).96c Addition of the corresponding aluminate species to the ketoacrylate (415 Scheme 54) produced only one diastereomer of the aldol product (416) which was converted into the alkene (417) in excellent yield.96 1... 

Hydroalumination. CpaTiClj is a particularly efficient catalyst for hydro-alumination of alkenes with these alanes (equation II). The intermediate adducts can also be trapped with carbonyl compounds, oxygen, or carbon dioxide, but yields are not impressive. cp,na,... 

The stereocontrolled synthesis of alkenes and the use of vinyl- and alkyl-aluminum and -aluminate reagents as nucleophiles has been described above. In addition, the aluminum reagents are also useful in diene synthesis, in reactions with enones and epoxides, and in cyclizations. 

Preparation.—Alcohols can be obtained from terminal alkenes by hydro-alumination with lithium aluminium hydride in the presence of a titanium complex, followed by oxidative cleavage of the adduct (Scheme 1). The sequence provides anti-Markovnikov alcohol. 

Alkenes can be converted into primary alkyl acetates via titanium-catalysed hydroalumination (Scheme 2) followed by lead tetra-acetate oxidation of the dialkyldihydroaluminate addition products (2). Only two equivalents of alkene are used per aluminium atom as it seems that only two alkyl groups from the aluminates can participate in the oxidation. In a closely related study a titanium-boron complex has been found to promote catalytic hydroboration of alkenes (also Scheme 2) cw-addition predominates for non-terminal alkenes. The adducts (3) can be oxidized to alcohols, and it may be seen that both sequences provide anti-Markovnikov products. 

In a related sequence, a new method for conversion of the tetra-alkyl-aluminate adducts (63), from titanium-catalysed hydroalumination of alkenes, into chlorides and bromides involves treatment with the Cu halides (Scheme... 

Keywords Alkenes/alkynes Aluminacyclopentane Alumination Dienes Organoaluminum compounds... 

Use of Aluminate Reagents Derived from 1-Alkenes. The nucleophilic substitution was achieved using lithium tetraalkylaluminates produced by the Ti-catalyzed hydroalumination reaction of 1-alkenes with UAIH4 (5). The whole transformation is illustrated in Scheme 7, and the results are summarized in Table 6. Because the hydroalumination is site-selective, optically active phenyl-substituted or olefinic trifluoro alcohols are now readily available. 

Scheme 7, Alkylation of Hemiacetal Tosylates with Aluminates Derived from 1-Alkenes...

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