Hydroalumination scope

In contrast to the related organoboranes, which are mostly used in the addition to non-polar carbon-carbon multiple bonds, aluminum hydrides have found their widest use in organic synthesis in the addition reaction to polar carbon-carbon and carbon-heteroatom multiple bonds including carbonyl, nitrile and imino groups as well as their a,(J-unsaturated analogs. Although these reduction reactions are also sometimes referred as hydroalumination reactions in the Hterature, they are outside the scope of this review. 

Early attempts by Asinger to enlarge the scope of hydroalumination by the use of transition metal catalysts included the conversion of mixtures of isomeric linear alkenes into linear alcohols by hydroalumination with BU3AI or BU2AIH at temperatures as high as 110°C and subsequent oxidation of the formed organoaluminum compounds [12]. Simple transition metal salts were used as catalysts, including tita-nium(IV) and zirconium(IV) chlorides and oxochlorides. The role of the transition metal in these reactions is likely limited to the isomerization of internal alkenes to terminal ones since no catalyst is required for the hydroalumination of a terminal alkene under these reaction conditions. 

In addition to the enhanced rate of hydroalumination reactions in the presence of metal catalysts, tuning of the metal catalyst by the choice of appropriate ligands offers the possibility to influence the regio- and stereochemical outcome of the overall reaction. In particular, the use of chiral ligands has the potential to control the absolute stereochemistry of newly formed stereogenic centers. While asymmetric versions of other hydrometaUation reactions, in particular hydroboration and hydrosi-lylation, are already weU established in organic synthesis, the scope and synthetic utiHty of enantioselective hydroalumination reactions are only just emerging [72]. 

Negishi reported the hydrogen transfer hydroalumination of alkenes with (/-Bu AKTIBA) and catalytic amounts of palladium and other late transition metal complexes.125 Although uncatalyzed hydroaluminations of alkenes with di-and trialkylalanes at elevated temperatures have long been known, their scope and limitations as well as their synthetic utility have not been extensively explored. 

The availability of the Schwartz s reagent Cp2Zr(H)Cl (see Schwartz s Reagent) and its versatile behavior explain its extensive use in orgaiuc chemistry. The scope of hydrozirconation (see Hydrozirconation) with respect to both substrate structure and chemoselectivity lies somewhere between those of hydroboration (see Hydroboration) and hydroalumination. 

Table 1 Scope of Suitable Alkenic Substrates for Hydroalumination with Dialkylaluminum Hydrides"- ...

Table 2 Scope of Suitable Alkynic Substrates for Hydroalumination with R2AIH or M AIR3H Reagents...

Hydroalumination. The treatment of alkynes with diisobutylaluminum hydride in hydrocarbon solvents results in a aT-addition of the Al-H bond to the triple bond to produce stereodefined alkenylalanes. The hydroalumination of alkynes is more limited in scope than the corresponding hydroboration reaction of alkynes (see Chapter 5) with regard to accommodation of functional groups and regioselectivity. Whereas hydroalumination of 1-alkynes is highly regioselective, placing the aluminum at the terminal position of the triple bond, unsymmetrically substituted alkynes produce mixtures of isomeric alkenylalanes. 

Metal catalyzed hydrometalations, specifically hydroboration and hydrosi-lylation, are particularly attractive due to the potential for enantiocontrol by the use of chiral ligands coordinated to the metal center. While the first metal catalyzed hydroalumination was described more than 40 years ago, the first synthetically useful enantioselective hydroalumination was described in 1995 and the scope and synthetic utility of this reaction are only just emerging. 

Despite the low intrinsic reactivity of alkylboranes, Pd-catalyzed all lation with alkylboron compounds holds considerable promise as a unique and chemoselective alkylation method. It is unique in part because hydioboration of alkenes is by far the most general and dependable method for stoichiometrically converting alkenes into organometals and in part because alkylboron compounds thus generated can satisfactorily participate in Pd-catalyzed alkylation under basic conditions. In this connection, it should be noted that the current scopes of hydroalumination and hydiozirconation of alkenes are much more limited than... 

As a follow-up of the aforementioned studies, Hoveyda and co-workers then addressed one of the limitations in scope their method still had. Indeed, more sterically demanding Si-containing aryl vinyl aluminiums were incompatible with the reaction conditions. Hence, they turned their attention to alternative methods for the preparation of organoaluminiums compatible with the asymmetric allylic substitution of allylic phosphates for the formation of quaternary stereogenic centres. Firstly, they studied the enantioselective substitution of alkyl vinyl aluminiums obtained from the hydroalumination with DIBAL-H of alkyl-substimted alkynes (Scheme 35) [65]. When the proper catalyst is chosen (Lll as chiral ligand), this... 

Cp2ZrH(Cl) = ([Zr]-H) reacts with unsaturated C—C bonds to give isolable addition products (in opposition to boranes and alanes, organo-Zr are rather stable in dry air) that can be hydrolyzed with diluted acidic solutions. The scope of hydrozirconation with respect to chemoselectivity and substrate structure lies between those of hydroalumination and hydroboration. The facile isomerization of secondary alkylzirconium derivatives into primary alkyl derivatives occurs very readily (even below room temperature, note the difference with A1 and B ). 

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