Asymmetric reduction and Meerwein‐Ponndorf‐Verley reaction of prochiral aromatic ketones in the presence of optically pure 1‐aryl‐2,2‐dimethylpropane‐1,3‐diols

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Abstract
In THF solution, NaBH4 in the presence of (S)‐(‐)‐1‐(2‐chlorophenyl)‐2,2‐dimethylpropane‐1,3‐diol together with 2‐chlorobenzoic acid reduced propiophenone to 1‐ phenylpropan‐1‐ol in up to 40% enantiomeric excess (ee). Only 21% ee was obtained without an added acid. (R)‐(‐)‐butane‐1,3‐diol and (R)‐( + )‐1,1′‐binaphthalene‐2,2′‐diol did not form clear solutions with NaBH4 in THF and only gave a low ee. Of the organic acid additives investigated, (S)‐(–)‐4‐(2‐chlorophenyl)‐5,5‐dimethyl‐2‐hydroxy‐1,3,2‐dioxaphosphorinane 2‐oxide gave the highest ee (46%). However, with AlCl3/NaBH4/(S)‐(–)‐1‐(2‐chlorophenyl)‐2,2‐dimethylpropane‐1,3‐diol in THF the best result was obtained (ee 55%).Asymmetric Meerwein‐Ponndorf‐Verley (MPV) reductions of acetophenone by isopropanol in the presence of these chiral diols as auxiliary have also been carried out. For the case of aluminium triisopropoxide, (R)‐(–)‐1‐phenyl‐2,2‐dimethylpropane‐1,3‐diol and acetophenone(1/1/1), an ee of up to 20% for 1‐phenylethanol was obtained whereas with (R)‐(+)‐1,1′‐binaphthalene‐2,2′‐diol an ee of 32% was obtained. Lanthanide triisopropoxides, Ln(OiPr)3, have also been used. Of the various lanthanides (Ln = La, Y, Er, Tb, and Eu) the Er triisopropoxide with chiral (R)‐(–)‐1‐phenyl‐2,2‐dimethyl‐propane‐1,3‐diol gave the best result; 46% ee for 1‐phenylethanol. Lanthanide chloride diisopropoxides, LnCl(OiPr)2, were also used as catalysts; although these have high catalytic activities they unfortunately give little chiral recognition.

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