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

Reactions of monosubstituted alkenes RCH = CH2 and [Re(eta(5)-C5H5)(CH 2Cl2)(NO)(PPh3)]+BF4- give complexes ([Re(eta(5)-C5H5)(CH2 = CHR)(NO)( PPh3)]+BF4- (1a-g) in 63-99% yields as mixtures of (RS,SR)- and (RR,SS )-diastereoisomers (1a (R = Me), 66:34; 1b (R = Pr), 63:37; 1c (R = Ph CH2), 70:30; 1d (R = Ph), 75:25; 1e (R = i-Pr), 64:36; 1f (R = t-Bu), 84:16; 1g (R = Me3Si), 69:31; Scheme 2). These differ in the C = C ena ntioface bound to the chiral Re fragment. In most cases, the analogous reactions of RCH = CH2 and [Re(eta(5)-C5H5)(C6H5Cl)(NO)(PPh3)]+BF4+ g ive comparable results. When 1a-e, g are heated in PhCl at 95-100-degr ees, equilibration to 96:4, 97:3, 97:3, 90:10, > 99:< 1, and > 99:< 1 (RS,SR)/(RR,SS) mixtures occurs (79-99% recoveries; Tables 1 and 2). T hus, thermodynamic enantioface-binding selectivities are much higher t han kinetic binding selectivities. This phenomenon is analyzed in deta il. A crystal structure of (RS,SR)-1e (monoclinic, P2(1)/c, a = 10.256 (1) angstrom, b = 17.191(1) angstrom, c = 16.191(1) angstrom, beta = 1 01.04(1)-degrees, Z = 4) shows that the Re-C(1)-C(2) plane (see Fig. 2 ) is nearly coincident with the Re-P bond (angle 15-degrees), and that the i-Pr group is 'syn' to the nitrosyl ligand.