Allylaluminum reagents

In contrast to the large number of applications of allylboron reagents in organic synthesis (Section 1.3.3.3.3.), the analogous allylaluminum reagents have received only scant attention. The few applications that have appeared are reviewed in this section. 

Alkoxy-substituted allylaluminum reagents diethyl[(Z)-3-methoxy-2-propenyl]- and -[(Z)-3-(l-methoxy-l-methylethyl)-2-propenyl]aluminum have been prepared by treatment of the corresponding alkoxyallyllithiums with diethylaluminum chloride in tetrahydrofuran at — 78 =C4. These reagents provide the syn-diastereomer with 9-11 1 selectivity in reactions with aldehydes at — 78 °C. The reaction of diethyir(Z)-3-methoxy-2-propenyl]a]uminum and acetophenone provided the iy -diastereomer with 4 1 selectivity. 

Diastereoselective homoaldol reaction. The allylaluminum reagent (E)-2, obtained by deprotonation of (E)-l followed by transmetallation, undergoes a highly diastereoselective reaction with an aldehyde (or a ketone) to give E)-anti-3. In contrast, (Z)-2 reacts to give (E)-JV -3. ... 

Allylaluminum reagents, prepared in situ from triscrotylborane and triethylaluminum, showed a comparatively higher reactivity (-10 to 20 C) and regioselectivity on additions to 1-butene, 1-hexene and styrene yielding, after methanolysis, alkenes (14) (entries 5-7, 50-55%). Both zinc- and aluminum-ene reactions suffer from a lack of diastereochemical control. 

Table 2 Metallo-ene Reactions of Dicrotylzinc and In Situ Prepared Allylaluminum Reagents widi Alkenes...

The reaction of aldehydes with another titanium complex generated from E-crotyl iV-diisopropyl carbamate, s-BuLi, sparteine 2.5 and Ti(0 -Pr)4 gives anti homoallylic alcohols with a good enantioselectivity [109, 1162, 1163] (Figure 6.29). Addition of allylaluminum reagents to aromatic aldehydes in the presence of Sn(OTf>2 and a chiral amine also gives useful results [842],... 

There is one report of a chiral reagent based on allylaluminum chemistry1 2 3 4 5 10. Bis(2-methyl-propyl)-2-propenylaluminum is treated with tin(II) triflate and chiral diamine ligand 4 to give a reagent, presumably a chiral allyltin species, that reacts with aldehydes at — 78 "C. Good enantioselectivity (80 -84% ee) is obtained with aromatic aldehydes, but with aliphatic aldehydes the selectivity is somewhat lower (53-64%)10. 

An allylaluminum species, which was generated by the reaction of allyl Grignard reagent with diisobutylaluminum chloride, was found to react with terminal alkynes in... 

Allyllithium gives even higher yields of B-allyl-9-BBN than allylaluminum sesqui-bromide. However, the stoichiometry of the reaction must be controlled to avoid the formation of a diallyl ate complex, and the isolation of product is not simple. Alkylations of B-methoxy-9-BBN with the RMgX reagents meet with limited success. Only the methyl group is quantitatively transferred. Lower yields are obtained with other groups , e.g., n-Bu (42%), Ph (27%). 

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