THERMODYNAMIC ENANTIOFACE-BINDING SELECTIVITIES OF MONOSUBSTITUTED ALKENES TO A HIGHLY DISCRIMINATING CHIRAL TRANSITION-METAL LEWIS ACID - EQUILIBRATION OF DIASTEREOISOMERIC (CYCLOPENTADIENYL)(ALKENE)(NITROSYL)(TRIPHENYLPHOSPHINE)RHENIUM COMPLEXES ([RE(ETA-5-C5H5)(CH2=CHR)(NO)(PPH3)]+BF4-)

Reactions of monosubstituted alkenes RCH = CH2 and [Re(5C5H5)(CH2Cl2) (NO)(PPh3)]+BF4 give complexes ([Re(5C5H5))(CH2CHR)(NO) (PPh3)]+BF4 (1ag) in 6399% yields as mixtures of (RS,SR) and (RR,SS)diastereoisomers (1a (R = Me), 66:34; 1b (R = Pr), 63:37; 1c (R = PhCH2), 70:30; 1d (R = Ph), 75:25; 1e (R = iPr), 64:36; 1f (R = tBu), 84:16; 1g (R = Me3Si), 69:31; Scheme 2). These differ in the CC enantioface bound to the chiral Re fragment. In most cases, the analogous reactions of RCHCH2 and [Re(5C5H5) (C6H5Cl)(NO)(PPh3)]+ BF4+ give comparable results. When 1ae, g are heated in PhCl at 95100, equilibration to 96:4, 97:3, 97:3, 90:10, > 99:< 1, and > 99:< 1 (RS,SR)/(RR,SS) mixtures occurs (7999% recoveries; Tables 1 and 2). Thus, thermodynamic enantiofacebinding selectivities are much higher than kinetic binding selectivities. This phenomenon is analyzed in detail. A crystal structure of (RS,SR)1e (monoclinic, P21/c, a = 10.256(1) . b = 17.191(1) , c = 16.191(1) , = 101.04(1), Z = 4) shows that the ReC(1)C(2) plane (see Fig.2) is nearly coincident with the ReP bond (angle 15), and that the iPr group is syn to the nitrosyl ligand. Copyright 1992 Verlag GmbH & Co. KGaA, Weinheim

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