A New Access to Chiral Phospholanes

The hydrogenation of 5‐oxo‐5 H ‐5λ 5 ‐dibenzophosphol‐5‐ol gives, almost quantitatively, only one geometric isomer of 5‐oxododecahydro‐5λ 5 ‐dibenzophosphol‐5‐ol ( 2 ). An X‐ray structure analysis confirmed the formation of a meso isomer with a 4aβ,5aβ,9aβ,9bβ configuration ( 2a ). Another possible way to get 2a is the hydrogenation of (4aβ,5aβ)‐5‐oxo‐2,3,4,4a,5,5a,6,7,8,9‐decahydro‐1 H ‐5λ 5 ‐dibenzophosphol‐5‐ol ( 11 ), which is easily available starting from cyclohexanone. Highly selective epimerization has been achieved using LDA as base at elevated temperature. Resolution of 2e was achieved by salt formation with ( R )‐(+)‐ and ( S )‐(–)‐ N ‐benzyl‐α‐methylbenzylamine to give (+)‐ and (–)‐(4aα,5aβ,9aβ,9bβ)‐5‐oxododecahydro‐5λ 5 ‐dibenzophosphol‐5‐ol. The geometry of 2e was confirmed by X‐ray structure analysis. Reduction of (+)‐ or (–)‐ 2e gave diastereomeric phospholanes (4aα,5 R / S ,5aβ,9aβ,9bβ)‐dodecahydrodibenzophosphole ( 1a ), which were oxidized to the corresponding secondary phosphane oxides (SPOs) (4aα,5 R / S ,5aβ,9aβ,9bβ)‐dodecahydrodibenzophosphole 5‐oxide ( 1b ). These phosphanes and phosphane oxides were used as ligands and preligands, respectively, in the Rh‐catalyzed enantioselective hydrogenation of benchmark substrates, where up to 79 % ee have been achieved. B3LYP density functional calculations reveal that among the three possible epimers at the 4a C‐ and 5a C‐positions the chiral isomer (4aα,5aβ,9aβ,9bβ)‐5‐oxododecahydro‐5λ 5 ‐dibenzophosphol‐5‐ol ( 2e ) is 3.52 kcal mol –1 more stable than the related meso isomer 2a in Gibbs free energy. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)