The fluxional behavior of iron complexes of 1,2-cycloheptadiene: the role of the allyl cation

Structures and Dynamic Solution Behavior of Cationic, Two‐Coordinate Gold(I)–π‐Allene Complexes

“…However, Rosenblum and co‐workers confidently excluded the involvement of η 1 ‐allylic cation species in the intramolecular π‐face exchange of cationic iron cyclopentadienyl allene complexes [(η 5 ‐C 5 H 5 )Fe(CO) 2 (π‐allene)] + through employment of an optically active π‐cyclononadiene complex 30. Supporting the conclusions of Rosenblum and co‐workers, Oon and Jones identified two distinct fluxional processes for the chiral iron 1,2‐cycloheptadiene complex [(η 5 ‐C 5 H 5 )Fe(CO)(PPh 3 )(π‐1,2‐cycloheptadiene)] + BF 4 − 31. The lower energy process ( E a ≈17 kcal mol −1 ), which led to π‐face exchange without racemization, was proposed to occur through a chiral η 1 ‐allene species and the higher energy process ( E a ≈23 kcal mol −1 ), which led to racemization, was proposed to occur through an η 1 ‐allylic cation intermediate.…”

Kinetics and Mechanism of the Racemization of Aryl Allenes Catalyzed by Cationic Gold(I) Phosphine Complexes

“…Cramer31 and Willis32 have reported the rhodium‐catalyzed racemization of allenes, which Cramer proposed to occur via allene hydro‐metallation, isomerization through a rhodium‐allyl species, and subsequent elimination. In addition to these catalytic processes, planar η 1 ‐allylic cation intermediates have been invoked to account for the stereomutation of chiral iron,33 platinum,34 and silver35 allene complexes. Bergman reported the stoichiometric stereoinversion and racemization of 1,3‐disubstituted allenes by a zirconocene imido complex that was proposed to occur through stereospecific [2+2] cycloaddition followed by racemization of the resulting azazirconacyclobutane 36…”

“…To account for this behavior, we invoked the involvement of chiral, η 1 ‐allene intermediates or transition states III in which gold is bound 45° relative to the allene axes, presumably interacting with both p ‐orbitals of the central carbon atom (Scheme ). Similar chiral η 1 ‐allene species have been invoked to account for π‐face exchange in platinum44 and iron33, 45 π‐allene complexes and gold η 1 ‐allene species have been evaluated computationally 8. 9 Germane to the present study, analysis of the 4,5‐nonadiene complexes 1 a and 1 b revealed two discrete π‐face exchange pathways associated with two diastreomeric η 1 ‐allene intermediates/transition states that lay well below the transition state for allene stereomutation (Δ G ≠ =≥17.4 kcal mol −1 ):12 a lower energy process (Δ G ≠ =8.8 kcal mol −1 ) involving either the trans,trans ‐ III or cis,cis ‐ III that interconverts the allenyl protons of the more stable η 2 ‐allene stereoisomer ( cis ‐ 5 or trans ‐ 5 ) and a higher energy process (Δ G ≠ =9.7 kcal mol −1 ) involving cis,trans ‐ III that interconverts the η 2 ‐allene diastereomers cis ‐ 5 and trans ‐ 5 (Scheme ) 12.…”

“…[12] To account for this behavior, we invoked the involvement of chiral, h 1 -allene intermediates or transition states III in which gold is bound 458 relative to the allene axes, presumably interacting with both p-orbitals of the central carbon atom (Scheme 4). Similar chiral h 1allene species have been invoked to account for p-face exchange in platinum [44] and iron [33,45] p-allene complexes and gold h 1 -allene species have been evaluated computationally. [8,9] Germane to the present study, analysis of the 4,5-nonadiene complexes 1 a and 1 b revealed two discrete p-face exchange pathways associated with two diastreomeric h 1 -allene intermediates/transition states that lay well below the transition state for allene stereomutation (DG°= !…”

[2,2'-(1,3'-Bicyclohexenyl)]bis(triphenylphosphine)platinum(0) Tetrahydrofuran Solvate

“…For some years we have been interested in the preparation and characterization of transition metal complexes of strained cyclic allenes (Omrcen, Conti & Jones, 1991;Abboud, Lu & Jones, 1992;Lu, Jones & Winchester, 1993;Lu, Abboud & Jones, 1993). Prior to 1993, the smallest complexes that had been isolated were iron (Manganiello, Oon, Radcliffe & Jones, 1985;Oon, Koziol, Palenik & Jones, 1987;Oon & Jones, 1988) and platinum complexes of 1,2-cycloheptadiene (Visser & Ramakers, 1972) and 1,2,4,6-cycloheptatetraene. The former was prepared by generating the allene moiety from a previously complexed ligand and the latter by ligand displacement involving previously generated allenes.…”