Erica Henry-Basch

The ate complexes of aluminium: Reactivity and stereoselectivity with respect to epoxides and carbonyl compounds. Catalytic activation by salts of transition metals

Abstract When used in non-coordinating solvents (hydrocarbons) NaAlEt4 and LiAlnBu4 are good alkylation agents for epoxides. The presence of catalytic quantities of transition-metal salts, particularly NiCI2 or NiBr2, greatly accelerate the reactions, making them possible within a reasonable time in the case of disubstituted epoxides such as cyclohexene oxide, 2-3 epoxybutane, 1 phenyl-2,3-epoxybutane. In the case of aliphatic epoxides, dialkylmagnesium, NaAlEt4 and LiAlnBu4lead mainly to alkylation of the least substituted carbon of the epoxide ring; while in the case of epoxides with a C-O bond in the benzyl position, it is this carbon that is alkylated. The reaction always proceeds by total inversion of the configuration of the carbon in the epoxide ring, namely the site of the alkylation. NaAlEt4 is also a good agent for alkylating carbonyl compounds when used in solvents of low basicity such as diethylether, or in totally non-coordinating solvents such as the hydrocarbons. The yields of the alcohol are greatly improved by using catalytic quantities of NiCl2. The behaviour of NaAlEt4 with 2-phenylpropanol is quite remarkable: in diethylether NaAl-Et4 gives predominantly the pair of enantiomers predicted by Crams rule, and with greater stereoselectivity than if EtMgBr was used, while in pentane the reaction is no longer stereoselective. Finally, with a cyclic ketone, 4-t-butylcyclohexanone, NaAlEt4 in diethylether and in hexane in the presence of NiCI2 gives predominantly the equatorial alcohol resulting from an axial attack, which is generally not favoured at all.

Asymmetric alkylation of carbonyl compounds with lithium or sodium tetraalkylaluminates modified by chiral aminoalcohols

Asymmetric alkylation of benzaldehyde and acetophenone by modified aluminium “ate” complexes are reported. LiAlMe4, NaAlEt4, LiAln-Bu4, NaAln-Bu4 modified by either (−)-N-methylphedrin or (+)cinchonine or (−)quinine were used. Using hydrocarbon solvents and sometimes under nickel catalysis the treatment of carbonyl compounds by modified “ate· complexes produced chiral alcohols with both chemical and optical good yields. NaAlEt4 modified by (−)-N-methylephedrin reacted with benzaldehyde to give S(−)1-phenyl-1 propanol in 20% enantiomeric excess. The same reagent reacted with acetophenone to give S(−)2-phenyl-2 butanol in 33% e.e. NaAln-Bu4 modified by (−)-N-methylephedrin reacted with benzaldehyde to give S(−)1-phenyl-1-pentanol in 27% e.e. or with acetophenone to give (−)2-phenyl-2-hexanol in 44% e.e. Since a wide range of new asymmetric alkylating reagents has been obtained from aluminium “ate” complexes and chiral compounds, it can be assumed that the method described could be useful to synthetise chiral alcohols with high optical yield.