Reactions with hydroalumination adducts

The second side reaction with alkenes is the configurational change at the jp -hybridized carbon-aluminum bond in the hydroalumination adduct (equation 31). ... 

The asymmetric nickel-catalyzed hydroalumination of prochiral terminal alkenes using adducts of BujAl and chiral amines was reported in 1981 [74], Among the different amines investigated, (-)-N,N-dimethylmenthylamine (DMMA) gave the best enantioselectivities. For example, reaction of 2,3,3-trimethyl-l-butene (39) at room temperature with 0.33 equiv. of the DMMA/iBu3Al adduct in the presence of 0.6 mol% of Ni(mesal)2 gave, after oxidation of the intermediate organoaluminum compounds, 2,3,3-trimethyl-l-butanol 40 in 76% yield and 27% ee (Scheme 2-19). 

Hydroalumination. Titanocene dichloride is an effective catalyst for hydro-uluminution of alkenes and alkynes with his(dialkylamino)alancs5 and various complex aluminum hydrides. The adducts can be quenched with water or iodine. The reaction is satisfactory for terminal alkenes and internal alkynes, but is not clcun for internal alkenes and terminal alkynes. 

E)-Allylsilanes These silanes can be prepared regio- and stereoselectively by hydroalumination of (chloromethyl)dimethylsilyl-l-alkynes (1) to give the cis-adduct 2. Reaction of CH3Li (3 equiv.) with 2 results in (E)-allyltrimethylsilanes (3). 

In 1981 the first attempt for the asymmetric hydroalumination with a chiral alane reagent took place. The reaction of a chiral amine-trialkylaluminum complex, Al((-Bu)3-[(-)-DMMA] (172), with 1,1-dialkylethylenes (excess) catalyzed by Ni(Il)rMesal]2 (173) (2 mol%) without solvent, followed by exposure of the resulting adduct to the oxygen atmosphere gave the corresponding alcohols with low enantiopurity, for example, the reaction of 2-tm-butylpropene with 172 in the presence of 173 gave 144 with 27% ee (R) (Scheme 2.19) [98],... 

Hydroboration proceeds without a catalyst, but hydroboration with less active catecholborane (225) is accelerated by catalysts. Usually 1,2-addition to conjugated dienes takes place, but the Pd-catalysed reaction of catecholborane (225) gives the 1,4-adduct 226. This reaction is not catalysed by Rh complexes [98], Hydroalumination of conjugated dienes catalysed by Cp2TiCl2 affords the allylic aluminium compounds 227 by 1,4-addition. The Pd-catalysed hydrostannation of isoprene with HSnBu3 affords the (Z)-2-alkenylstannane 228 with high regio- and stereoselectivities [99],... 

Despite this gamut of competing reactions, reaction conditions can usually be found for preparing either the syn or the anti adduct of many terminal alkynes with high regio- and stereo-selectivity (Table 2). Failing this the 1-alkynylsilanes can be employed as masked forms of 1-alkynes. In hydroalumination reactions, such silanes have little or no tendency to undergo the side reactions (equations 19-24) to... 

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). 

Highly enantioselective hydroalumination was realized in 1995 in the reaction of 174 with 1-BU2AIH (DIBAL, 175) promoted by the catalyst generated in situ from (/f)-BINAP (58) (21 mol%) and Ni(COD)2 (14 mol%) at -78°C, which gave 176 with 97% ee in 97% yield through the ring-opening of the initially formed adduct 177 (Scheme 2.20) [99]. 

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