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
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Reactions of [( 5 -C 5 H 5 )Re(NO)(PPh 3 )(ClR)] + BF 4 - (R = CH 2 Cl (1), C 6 H 5 (2)) and H 2 C=C(CH 3 )R(R = CH 2 CH 3 (a), CH 2 CH 2 CH 3 (b), C 6 H 5 (c)) at room temperature (RT) give the alkene complexes [( 5 -C 5 H 5 )Re(NO)(PPh 3 ) (H 2 C=C(CH 3 )R)] + BF 4 - (3a-c; 5398%) as (3550):(6550) mixtures of RS,SR/RR,SS diastereomers. At 95 C, equilibration to (6870):(3230) (3a,b) and 3664 (3c) mixtures occurs. Reactions of 2 and cis-CH 3 HC=CHR (R = CH 2 CH 3 (a), CH(CH 3 ) 2 (b)) at RT or 95 C give (Z)-[( 5 -C 5 H 5 )Re(NO)(PPh 3 )(CH 3 HC=CHR)] + BF 4 - ((Z)-4a,b; 9099%) as (5179):(4921) mixtures of RSR,SRS/RRS,SSR diastereomers. Two [formula omitted] conformers of each (Z)-4a diastereomer are detected by NMR (-60 C) and interconvert with 11.611.9 kcal/mol barriers. Reactions of 2 and trans-CH 3 HC=CHR (R = CH 2 CH 3 (a), CH(CH 3 ) 2 (b), C 6 H 5 (c)) at RT give (E)-4ac (8597%) as 59:41, >99:<1, and 59:41 mixtures of RSS,SRR/RRR,SSS diastereomers. Similar reactions at 95 C give 99:1 mixtures (8996%). The [formula omitted] conformers of the RRR,SSS diastereomer of (E)-4c readily interconvert (G (T c ) = 10.310.4 kcal/mol), but those of the RSS,SRR diastereomers of (E)-4a,c show higher barriers (17.0 kcal/mol). Reactions of 2 and CH 3 HC=C(CH 3 ) 2 give [( 5 -C 5 H 5 )Re(NO)(PPh 3 )(CH 3 -HC=C(CH 3 ) 2 )] + BF 4 - as mixtures of RS,SR/RR,SS diastereomers that slowly equilibrate at RT ((9194):(96) after 64336 h). The equilibrium diastereomer and conformer ratios are rationalized stereoelectronically and compared to those of analogous H 2 C=CHR and cis/trans-RHC=CHR complexes, thus providing a comprehensive picture of alkene binding equilibria for the chiral Lewis acid [( 5 -C 5 H 5 )Re(NO)(PPh 3 )] + . 1994, American Chemical Society. All rights reserved.
