Aluminium ate complexes

Besides the well-known Grignard compounds, other magnesium, aluminium, lithium, and copper-lithium organometaliic compounds have been used as reagents — most recently, ate complexes of aluminium have been introduced. 

Vinyl aluminium compounds also carry out conjugate additions and illustrate well the difference in reactivity between vinyl alanes and the related ate complexes. If the enone can adopt the s-cis conformation, as in 175, a cyclic mechanism is possible in which aluminium both acts as a Lewis acid and delivers the vinyl nucleophile. Notice that the Zs-vinyl alane leads to the -y,8-unsaturated ketone 178 as this is again an SE2 reaction at the vinyl carbon.32... 

If the enone cannot adopt the s-cis conformation, it is necessary to convert the vinyl alane into an ate complex 179 before conjugate addition. Conjugate addition now occurs to s-trans enones, such as cyclic enones, to give the Zs-yS-unsaturated ketone 180 after aqueous work-up. Notice that this time the most stable anion is transferred - the choice is between Me, t-Bu, or vinyl. Aluminium is not a transition metal and simply releases the most stable anion as there must be some negative charge on the group being transferred. 

Hydroalumination of the alkyne in 135 gave the Z-vinyl aluminium 138. Treatment with MeLi gave the ate complex that reacted with the epoxide 136 to give 139. As the epoxide is opened, the oxyanion produced cyclises onto the Cbz group on nitrogen. This product was entirely Z and there was only a trace (<7%) of one other diastereoisomer. 

An aluminum-lithiiun catalyst, (R)-ALB, prepared from (R)-BINOL, and lithium aluminium hydride promoted the addition of malonate to 23 giving (R)-44 in 99% ee. X-ray analysis of the ALB catalyst showed an aluminum ate complex structure with li coordination to the oxygen atom. The asymmetric tandem Michael-aldol reaction of 46 was conducted with this catalyst giving a single isomer 47 containing three asymmetric centers. The aluminum enolate under-... 

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