Highly enantioselective protonation of the 3,4-dihydro-2-methylnaphthalen-1(2H)-one Li-enolate by TADDOLs

A series of nine TADDOLs (= alpha,alpha,alpha',alpha'-tetraaryl-1,3-dioxola ne-4,5-dimethanols) 1a -1i, have been tested as proton sources for the enan tioselective protonation of the Li-enolate of 2-methyl-1-tetralone (= 3,4-d ihydro2-methylnaphthalen-1(2H)-one). The enolate was generated directly fro m the ketone (with LiN(i-Pr)(2) (LDA)/ MeLi) or from the enol acetate (with 2 MeLi) or from the silyI enol ether (with MeLi) in CH2CL2 or Et2O as the solvent (Scheme).The Li-enolate (associated with LiBr/LDA, or LiBr alone) w as combined with 1.5 -3.0 equiv. of the TADDOL at -78 degrees by addition o f the latter or by inverse addition. 2-Methyl-1-tetralone of (S)configurati on is formed (less than or equal to 80% yield) with up to 99.5% selectivity if and only if (R,R)-TADDOLs (Id, e, g) with naphthalen-1-yl groups on the diarylmethanol unit are employed (Table). The reactions were carried out o n the 0.1- to 1.0-mM scale. The selectivity is subject to non-linear effect s (NLE) when an enantiomerically enriched TADDOL Id is used (Fig.1). The pe rformance of TADDOLs bearing naphthalen-1-yl groups is discussed in terms o f their peculiar structures (Fig. 2).