MECHANISM OF ISOMERIZATION OF ALKYLIDENE COMPLEXES [(ETA-5-C5H5)RE(NO)(PPH3)(=CRCHR'R'')]+X- TO ALKENE COMPLEXES [(ETA-5-C5H5)RE(NO)(PPH3)(RHC=CR'R'')]+X- - AN ORGANOMETALLIC WAGNER-MEERWEIN-TYPE REARRANGEMENT
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Propylidene complex [(5-C6H5)Re(NO)(PPh3)(=CHCH2CH3)]+PF6 (1c+PF6) cleanly rearranges to propene complex [(8-C6H5)Re(NO)(PPh3)(H2C=CHCH3)]+PF6 (CeD5Cl, 6586 C.; H* = 27 1 kcal/mol; S* = 3 3 eu). Experiments with deuterated 1c+PF6 show a modest primary kinetic deuterium isotope effect (k(=CHCH2CH3)/k(=CHCD2CH3) = 1.951.40) but a considerable inverse secondary kinetic deuterium isotope effect (k(Re==CH)/k(Re==CD) = 0.500.58). A crossover experiment shows intramolecular hydride migration and the absence of PPh3 ligand dissociation. Optically active 1c+PF6 rearranges with retention of configuration at rhenium, but the hydride shift is not stereospecific at the migration terminus. However, the rearrangement of isobutylidene complex [(6-C6H5)Re(NO)(PPh3)(=CHCH(CH3)2)]+SO3F to isobutylene complex [(5-C8H5)Re(NO)(PPh3)(H2C=C(CH3)2)]+SO3F is highly stereoselective at the migration origin and terminus, and much faster than that of 1c+PF6 (CD2Cl2, 325 C.; H* = 21 1 kcal/mol; S* = 3 3 eu). Rearrangements of analogous pentylidene and cyclopentylidene complexes are also studied. An orbital symmetry analysis of these reactions is given. 1991, American Chemical Society. All rights reserved.
