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(Borohydrido)(18-crown-6)potassium and (borohydrido)(dibenzo-18-crown-6)(tetrahydrofuran)potassium
Claude Villiers1, Pierre Thuéry, Michel Ephritikhine
1CEA/Saclay, DSM/DRECAM/SCM (CNRS URA 331), Bâtiment 125, 91191 Gif-sur-Yvette, France.
Abstract:
In the two compounds (borohydrido)(1,4,7,10,13,16-hexaoxacyclooctadecane-kappa(6)O)potassium, [K(BH(4))(C(12)H(24)O(6))], (I), and (borohydrido)(1,4,7,10,13,16-hexaoxa-2,3:11,12-dibenzocyclooctadeca-2,11-diene-kappa(6)O)(tetrahydrofuran)potassium, [K(BH(4))(C(4)H(8)O)(C(20)H(24)O(6))], (II), the K atom is bound to the six O atoms of the crown ether and to a tridentate borohydride group, with further coordination to a tetrahydrofuran molecule in (II). The alkali metal ion environment is thus distorted hexagonal-pyramidal in (I) and bipyramidal in (II).
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Hydroboration proceeds in a concerted fashion with the attack of borane on the π bond, giving a cyclic four-centered transition state. The –BH2 group is bonded to the less substituted carbon and –H to the more substituted carbon. The concerted nature requires the simultaneous addition of –H and –BH2 across the same face of the alkene giving syn stereochemistry.
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