Hydroalumination regioselectivity

A variety of other addition reactions occurring regioselectively are also known. These include hydrocyanation, hydroalumination, hydrosilylation, and hydrozirconation. 

The regiochemistry of Al-H addition to unsymmetrically substituted alkynes can be significantly altered by the presence of a catalyst. This was first shown by Eisch and Foxton in the nickel-catalyzed hydroalumination of several disubstituted acetylenes [26, 32]. For example, the product of the uncatalyzed reaction of 1-phenyl-propyne (75) with BujAlH was exclusively ds-[3-methylstyrene (76). Quenching the intermediate organoaluminum compounds with DjO revealed a regioselectivity of 82 18. In the nickel-catalyzed reaction, cis-P-methylstyrene was also the major product (66%), but it was accompanied by 22% of n-propylbenzene (78) and 6% of (E,E)-2,3-dimethyl-l,4-diphenyl-l,3-butadiene (77). The selectivity of Al-H addition was again studied by deuterolytic workup a ratio of 76a 76b = 56 44 was found in this case. Hydroalumination of other unsymmetrical alkynes also showed a decrease in the regioselectivity in the presence of a nickel catalyst (Scheme 2-22). 

The mechanism of the second reaction is a trans hydroalumination helped by coordination of the alane to the triple bond and external nucleophilic attack. The regioselectivity of the hydroalumination is again determined by silicon the electrophilic alane attacks the alkyne on the carbon bearing the silyl group (the ipso carbon). 

Finally, the high locoselectivity or chemoselectivity of hydroalumination toward substrates having both C==C and C C bonds is exemplified by the reaction of diisobutylaluminum hydride with 1-octen-7-yne (7). The only organoaluminum product detected (8) was that of a regioselective syn stereospecific addition to the bond (equation 3). 

As part of the same comprehensive study Ziegler and Gellert also showed that at 180-200 °C complex (10) did catalyze the oligomerization of ethylene into higher ot-alkenes. Subsequently, the unstable AIH3 was found to add even more readily to ethylene and other a-alkenes (11) regioselectively to provide quantitative yields of the tri-n-alkylaluminum (12b). This reaction is thus the prototype for the hydroalumination of unsaturated hydrocarbons (equation 5). 

The experimental observations favoring this proposal are the following (i) the regioselectivities for nickel-catalyzed hydroaluminations differ significantly from those obtained from the uncatalyzed process e.g. equation 11) (ii) certain hindered alkenes do not undergo uncatalyzed hydroalumination under conditions where the nickel-catalyzed reaction is essentially complete (e.g. equation 39) (iii) nickel(O) complexes have been shown to insert into A1—H bonds to yield R2A1-N1—H intermediates (equation 40) and (iv) such Al-Ni—H intermediates react with A1—H bonds, with the rate depending upon sub-... 

Once the hydroalumination adducts of C=C and C C bonds have been cleanly formed, protolysis or oxidation can produce useful derivatives. With H2O or O2, generally all available C—Al bonds are destroyed. Protolysis or deuteriolysis of such adducts constitutes an overall reduction or deuteriating reduction of the original C—C unsaturation (equations 13, 39 and 41 Schemes 6-8). ° Any stereoselectivity or regioselectivity of the original hydroalumination is preserved. 

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. 

The hydroalumination of alk-l-enes is regioselective, placing Al at the terminal carbon (95%) ... 

Hydrotitanation of 1-silyl- and I-stannyl-1-alkynes leads to P-silyl (or p-stannyOalkenyltitanates, thus showing opposite regioselectivity to other related processes (hydroboration, hydroalumination, hydromagnesiation, hydrotitanation, hydrozincation, hydrozirconation).Well-defined alkenes are obtained on further reaction of the alkenylti-tanates, for example, with allylic carbonates an 8 2 displacement occurs to afford 1,4-di-... 

Hydroalumination Certain bridged oxabicycles are cleaved by i-BUjAlH. The regioselectivity can be different when Nifcod) and PhjP are present. 

The hydroalumination reactions, as hydroborations, are stereospecifically cis. An advantage, compared to hydroboration, is the ease of cleavage of the Al-C bond that does not require a peroxide. The regioselectivity, however, is not as good as that of the hydroboration reactions, which clearly is a major disadvantage. 

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