Thermally induced diastereoisomerization of (cyclobutadiene)cobalt complexes as a probe for the reversibility of their formation from complexed alkynes

Article abstract of DOI:10.1021/om00086a007

The synthesis and separation of a series of racemic diastereomers 2-5 of formula Co[η⁴-C(SiMe₃)C-(SiMe ₃)CHCCHMeR](η-C₅H₅) by cocyclization of Co(CO)₂(η-C₅H₅), Me₃SiC₂SiMe₃ (btmse), and HC₂CHMeR are described. Gas- and solution-phase pyrolyses for both diastereomers of Co[η₄-C-(SiMe₃)C(SiMe ₃)CHCCHMePh](η-C₅H₅) (5) reveal extensive mutual interconversion. A crossover experiment demonstrates the unimolecular character of this reaction and studies on enantiomerically enriched Co[η⁴-C(SiMe₃)C(SiMe ₃)CHCCHMeOH](η-C₅H₅) (2) show that the cobalt unit is the site of diastereoisomerization rather than the alternative carbon chiral center. During gas-phase pyrolysis of 5 small amounts of the geometrical isomer Co[η⁴-C(SiMe₃)CHC(SiMe ₃)CCHMePh](η-C₅H₅) (12) are formed. Similarly, gas-phase pyrolysis of 12 results in a small extent of positional isomerization to both diastereomers of 5. Heating one diastereomer of 26, i.e., Co[η⁴-C(SiMe₃)C(SiEt ₃)CHCCHMePh](η-C₅H₅) (26a), results in one diastereomer of 27, Co[η⁴-C(SiEt₃)C(SiMe ₃)CHCCHMePh](η-C₅H₅) (27b), but none of 26b. An experiment utilizing the 1,2-¹³C-labeled cyclobutadiene 26a results only in 1,3-¹³C-labeled 27b. The results are best interpreted by assuming that the cyclobutadiene rings open directly to bis(alkyne)cobalt species without the intermediacy of cobaltacyclopentadiene intermediates. Alternative rationales are discussed.