SYNTHESES AND STRUCTURES OF UNSYMMETRICALLY 1,1-DISUBSTITUTED AND 1,2-DISUBSTITUTED ALKENE COMPLEXES OF THE CHIRAL RHENIUM LEWIS-ACID [(ETA-5-C5H5)RE(NO)(PPH3)]- ENANTIOFACE BINDING SELECTIVITIES AND CONFORMATIONAL EQUILIBRIA( )

Reactions of L(eta5-C5H5)Re(NO)(PPh3)(ClR)]+BF4-(R = CH2Cl (1), C6H5(2 )) and H2C=C(CH3)R (R = CH2CH3 (a), CH2CH2CH3 (b), C6H5 (c)) at room t emperature (RT) give the alkene complexes [(eta5-C5H5)Re(NO)(PPh3) (H2 C=C(CH3)R)]+BF4 (3a-c; 53-98%) as (35-50):(65-50) mixtures of RS,SR/RR ,SS diastereomers. At 95-degrees-C, equilibration to (68-70):(32-30) ( 3a,b) and 36-64 (3c) mixtures occurs. Reactions of 2 and cis-CH3HC=CHR (R = CH2CH3 (a), CH2CH3)2 (b)) at RT or 95-degrees-C give (eta5-C5H5) Re(NO)(PPh3)(CH3HC=CHR)]+BF4-((Z)-4a,b; 90-99%) as (51-79):(49-21) mix tures of RSR,SRS/RRS,SSR diastereomers. Two Re-(C-C) conformers of eac h (Z)-4a diastereomer are detected by NMR (-60-degrees-C) and intercon vert with 11.6-11.9 kcal/mol barriers. Reactions of 2 and trans-CH3HC= CHR (R = CH2CH3 (a), CH(CH3)2 (b), C6H5 (c)) at RT give (E)-4a-c (85-9 7%) as 59:41, >99:<1, and 59:41 mixtures of RSS,SRR/RRR,SSS diastereom ers. Similar reactions at 95-degrees-C give greater-than-or-equal-to 9 9:1 mixtures (89-96%). The Re-(C-C) conformers of the RRR,SSS diastere omer of (E)-4c readily interconvert (DELTAG(double dagger)(T(c)) = 10. 3-10.4 kcal/mol), but those of the RSS,SRR diastereomers of (E)-4a,c s how higher barriers (greater-than-or-equal-to 17.0 kcal/mol). Reaction s of 2 and CH3HC=C(CH3)2 give [(eta5-C5H5)Re(NO)(PPh3)(CH3HC=C(CH3(2)] +BF4-as mixtures of RS,SR/RR,SS diastereomers that slowly equilibrate at RT ((91-94):(9-6) after 64-336 h). The equilibrium diastereomer and conformer ratios are rationalized stereoelectronically and compared t o those of analogous H2C=CHR and cis/trans-RHC=CHR complexes, thus pro viding a comprehensive picture of alkene binding equilibria for the ch iral Lewis acid [(eta5-C5H5)Re(NO)(PPh3)]+.