REACTIONS OF DIAZO-COMPOUNDS AND CHIRAL RHENIUM ALKYLIDENE COMPLEXES OF THE FORMULA [(ETA(5)-C5H5)RE(NO)(PPH(3))(=CHR)]+PF6- - A VERSATILE AND HIGHLY STEREOSELECTIVE ROUTE TO ALKENE COMPLEXES
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abstract
The methylidene complex [(5C5H5)Re(NO)(PPh3)(CH2)]+ PF6 or the ReCD2 analog react with diazo compounds N2CHR [R H, Si(CH3)3, COPh, CO2C2H5] in CH2Cl2 at 80C to give alkene complexes [(5C5H5)Re(NO)(PPh3)(2H2CCHR)]+ PF6 or D2CCHR analogs. The two ReC geometric isomers of benzylidene complex [(5C5H5)Re(NO)(PPh3)(CHPh)]+ PF6 and CH2N2 react to give opposite configurational diastereomers of styrene complex [(5C5H5)Re(NO)(PPh3)(2H2CCHPh)]+ PF6. Stereochemical features of these reactions are analyzed in detail, and are interpreted in terms of models involving (a) attack of the diazo compound upon the ReC face opposite to the bulky PPh3 ligand, (b) an antiperiplanar disposition of the ReC and CN2 bonds in the transition state, utilizing the diazo carbon face that minimizes interactions of the substitutent (R) and cyclopentadienyl ligand, and (c) N2 loss from the resulting intermediate [(5C5H5)Re(NO)(PPh3)(CHRCHRN2)]+ PF6 via a conformer with antiperiplanar PReCHRC and ReCHRCHRN linkages, with anchimeric assistance of the rhenium fragment d orbital HOMO from the backside. Copyright 1995 WILEYVCH Verlag GmbH & Co. KGaA, Weinheim
