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(η 5 –C 5 H 5 )(CH 2 = CHR)(NO) (PPh 3 )] + BF 4 − )

Reactions of monosubstituted alkenes RCH = CH 2 and [Re(η 5 –C 5 H 5 )(CH 2 Cl 2 ) (NO)(PPh 3 )] + BF   4 −give complexes ([Re(η 5 –C 5 H 5 ))(CH 2 CHR)(NO) (PPh 3 )] + BF   4 −( 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 = PhCH 2 ), 70:30; 1d (R = Ph), 75:25; 1e (R = i‐Pr), 64:36; 1f (R = t ‐Bu), 84:16; 1g (R = Me 3 Si), 69:31; Scheme 2 ). These differ in the CC enantioface bound to the chiral Re fragment. In most cases, the analogous reactions of RCHCH 2 and [Re(η 5 –C 5 H 5 ) (C 6 H 5 Cl)(NO)(PPh 3 )] + BF   4 +give comparable results. When 1a – e , g are heated in PhCl at 95–100°, 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 ). Thus, thermodynamic enantioface‐binding selectivities are much higher than kinetic binding selectivities. This phenomenon is analyzed in detail. A crystal structure of ( RS , SR )‐ 1e (monoclinic, P 2 1 / c , a = 10.256(1) Å. b = 17.191(1) Å, c = 16.191(1) Å, β = 101.04(1)°, Z = 4) shows that the Re–C(1)–C(2) plane (see Fig.2 ) is nearly coincident with the Re–P bond (angle 15°), and that the i‐Pr group is ‘ syn ’ to the nitrosyl ligand.