Hydroalumination reaction rates

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]. 

In 1954, Ziegler and coworkers [11,12] reported that traces of nickel salts dramatically alter the course of the growth reaction of ethylene with trialkylalanes, the Aufbau process. Instead of the low molecular weight polyethylene which was expected, the only product was butene. This observation culminated in Ziegler s discovery of transition metals that were highly effective in polymerizing ethylene, an accomplishment for which he later shared the Nobel Prize. It also opened the door to transition metal catalyzed hydroalumination reactions. In 1968, Eisch and Foxton showed that addition of nickel(II) salts increased the rate of the hydroalumination of alkynes by approximately 100-fold [13]. The active catalyst was believed to be a nickel(O) species. 

With MAIIL,H4-fl reagents, knowing the kinetic order of the reaction would prove most helpful in discerning whether the A1—H addition occurs initially in an anti manner, or whether a syn addition is followed by an isomerization to the anti adduct. A direct anti hydroalumination would likely require the synergistic action of 2 mol of MAIR3H in the rate-determining step and thus would be second-order in complex hydride (equation 37). 

Since hydroalumination by neutral aluminum hydrides is an electrophilic attack on a C= or C=C linkage, the reaction can be accelerated by Lewis acids such as aluminum halides, and be retarded by Lewis bases like R3N, R2O or even unsaturated R3AI cf. equation 38). Such reagents also exert an effect on the syn or anti character of the A1—H adduct. Evidence suggests that Lewis acids or bases principally affect the rate of isomerization of the initial syn adduct into the generally more stable anti adduct Lewis bases retard such isomerizations, while Lewis acids promote them. The presence of ethers or tertiary amines stabilize the syn adducts of alkynyl-silanes and -germanes (47) and permit such adducts to be formed in >95% geometrical purity (Scheme 12). ... 

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-... 

Deuterolysis gives the trans-deuterio-olefin, and small amounts of 1-deuterio-hex-l-yne (metallation) and 1,1-dideuterio-n-hexane (bis-hydroalumination). The predictable stereochemistry, the available starting materials and the known reactivity pattern of Al—C= bonds encourages synthetic application of this reaction - " . The R group at the C=C bond influences reaction (m) secondary or tertiary alkyl groups increase the rate more acidic alk-l-ynes (e.g. R = Ph, olefin) increases the amount of metallation. ... 

In donor solvents (EtjO, THF, RjN) the rate of hydroalumination is slower than with hydrocarbons. Nickel compounds catalyze the reaction, but isomerization may take place more readily. The regiospecificity of cis-hydroalumination of 1,2-disubstituted acetylenes with i-BujAlH is inferior to dialkylboranes (see 5.3.2.5.1) ... 

Comparative kinetic measurements for the hydroalumination have shown that with, for example, diethylalane and excess of a 1-alkene which is unbranched at carbon atom 2, reaction is half complete in 15 minutes at 65°C (291). In contrast to this, the time for 50% dehydroalumination with tripropylalane is 6 hours at 120°C (307) and 20 minutes at 160°C (242). Since hydroalumination proceeds only slowly at room temperature it is advantageous to work at about 50°C for preparative purposes. The rate of addition of the A1—H group to the C=C double bond decreases in the sequence... 

Oxabicyclic alkenes were hydroaluminated with /BU2AIH using Ni(COD)2 or its combination with phosphines as a catalyst. The regio- and enantioselectivity of the reactions depend on the ligand environment of the catalyst central atom, the catalyst concentration and component ratio, the nature of the solvent, and the rate of introduction of /BU2AIH into the reaction mixture [32-35] (Scheme 7). 

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