78736-27-3Relevant articles and documents
Thermally induced diastereoisomerization of (cyclobutadiene)cobalt complexes as a probe for the reversibility of their formation from complexed alkynes
Ville, Guy A.,Vollhardt, K. Peter C.,Winter, Mark J.
, p. 1177 - 1187 (2008/10/08)
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.
