Alkenyl aluminium

Carboalumination of alkynes is possible in the presence of the zirconium catalyst Cp2ZrCl2. The method is particularly attractive for methylalumination using Me3 A1 and can tolerate a wide variety of functional groups. Terminal alkynes react with very good regioselectivity ( 95 5) and excellent stereoselectivity (>98%) in favour of the alkenyl aluminium species, such as 58 (2.69). Mixtures of regioisomers often result when using internal alkynes with two different substituents. 

Alkenyl-aluminium or -zinc species cross-couple with allylic halides or derivatives of allylic alcohols (c.g. acetates or phosphates) under the influence of Pd(PPh3)4 to give 1,4-dienes. In favourable examples coupling occurs with complete retention of stereo- and regio-chemistry of both alkenyl and allyl substrates, and it is free from formation of homocoupled products (Scheme 73). However, partial allylic inversion was observed in reactions involving E-crotyl... 

OAlMea > 0P(0R)2 > OSiRs alkenyl aluminiums, prepared for example by the carboalumination of acetylenes," participate in this reaction with retention of stereochemistry to give 1,4-dienes." Aryl phosphates are found to have a reactivity similar to that of aryl chlorides in their reaction with organoaluminium reagents, and have been used to prepare alkyl, alkenyl, and aryl benzenes in a nickel-catalysed reaction." ... 

Phosphoramidite ligands are also suitable for the challenging enantioselective addition of alkenyl aluminium reagents to p-substituted cychc enones [36]. Although the introduction of alkenyl groups to enones has been largely the domain of Rh catalysis [40, 41], the remarkable advances in copper-catalysed ECA have now made this transformation possible. 

Alkenyl aluminium reagents can be easily generated by hydroalumination of the corresponding alkynes with DIBAL-H, under Zweifel conditions (Scheme 9)... 

Scheme 9 Copper-phosphoramidite-catalysed ECA of alkenyl aluminium reagents (prepared via hydroalumination) to p-substituted cyclohexenones by Alexakis [36]...

Alkenyl aluminium reagents can be also prepared by halogen/lithium exchange from the corresponding iodoalkene, followed by transmetalation with Et2AlCl. In this way, the formation of A1 acetylides is avoided and lower catalysts loading for the ECA are allowed (Scheme 10) [32, 44]. 

It is worth mentioning that no transfer of the isobutyl group from the alkenyl aluminium reagents is observed in any case the preferential transfer of the vinylic group occurred exclusively. 

Scheme 17 Preparation of alkenyl aluminium reagents from bromoafkenes by Alexakis [49, 50]...

The supernatant solution of the generated alkenyl aluminium reagent 13 can be used directly in the ECA to p-substituted cyclohex-2-enones (Scheme 19), providing good yields and enantioselectivities with only 10 mol% of the Cu-... 

General observations and limitations on the copper-phosphinamine-catalysed CA of alkenyl aluminium reagents include ... 

The use of alkenyl aluminium reagents as nucleophiles for the copper—NHC-catalysed ECA has also been described through a tandem hydroalumination-CA process. To prevent the formatimi of aluminium acetyUdes during the hydro-aluminatimi reactimi, which could perturb the chiral complex, Hoveyda and co-workers opt for the use of sUyl-protected alkynes as starting material, which imdergo clean cis-hydroalumination with DlBAL-H in coordinating solvents (Scheme 27) [64]. 

The addition of alkenyl aluminium nucleophiles to linear p,p-disubstituted enones to give all-carbon quaternary stereogenic centres can be also achieved with consistently high yields and enantioselectivities (up to 99% ee) by copper-NHC-catalysed ECA, using very low catalyst loadings at room temperature (Scheme 30) [66],... 

Scheme 30 Multicomponent N1-, Zr- and Cu-catalysed strategy for ECA of alkenyl aluminium reagents to p,p-disubstituted linear enones by Hoveyda [66]...

Many of the palladium- and nickel-catalysed reactions of alkenyl-aluminium or -zirconium compounds which fail or give low yields of cross-coupled products can be promoted to give the desired product in high yield by the addition of catalytic amounts of zinc chloride. " Trisubstituted olefins (40), for example, can be synthesized in good yield (ca. 70%) from the metal complex (38) in the presence of a palladium or nickel phosphine complex and zinc chloride. This reaction is particularly attactive for the one-pot synthesis of natural products containing... 

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