The First Detection of an Exclusive 1,2-Silatropic Shift in the Enantiopure Silylcyclopentadiene [C5H5(SiMe2(1R)-endo-(+)-OC10H17)]

• Published in: Organometallics Vol. 18; no. 22; pp. 4646 - 4653
• Main Authors: Grimmond, Brian J, Corey, Joyce Y
• Format: Journal Article
• Language: English
• Published: WASHINGTON American Chemical Society 25.10.1999; Amer Chemical Soc
• Subjects: Chemistry; Chemistry, Inorganic & Nuclear; Chemistry, Organic; Physical Sciences; Science & Technology; ZIRCONIUM; CRYSTAL-STRUCTURE; NMR-SPECTROSCOPY; OLEFIN POLYMERIZATION; METAL; LIGAND; DYNAMIC BEHAVIOR; METALLOCENE COMPLEXES; GROUP-4 METALLOCENES; EXCHANGE-REACTIONS
• ISSN: 0276-7333; 1520-6041
• DOI: 10.1021/om990504+

The enantiopure silylcyclopentadiene 1 was obtained as a mixture of regioisomers where the dominant isomer 5-(1) had the silyl substituent located on the allylic or 5-position of the cyclopentadiene ring. The full characterization of 1 by 1H, 13C{1H}, 13C{1H} DEPT, 29Si{1H} DEPT, 1H−1H NOESY, 1H−1H COSY, 1H−13C HMQC, 1H−29Si HMBC, and 1H-difference NOE spectroscopy was accompanied by trapping of 5-(1) as well-characterized Diels−Alder adducts with DMAD and TCNE. Above 250 K, compound 5-(1) undergoes a degenerate [1,5]-sigmatropic silyl rearrangement which exchanges the five sites of the cyclopentadiene ring. The rate of silyl migration around the CpH ring increased with temperature, leading to coalescence of the allylic and vinylic CpH 1H NMR resonances above 410 K in biphenyl-d 10. When the fluxional process was monitored by VT 1H and VT 13C{1H} experiments in toluene-d 8, line shape analysis of the allylic region for 5-(1) afforded the activation energy E a = 17.4 kcal mol-1 for the sigmatropic rearrangement. The mechanism for the rearrangement was independent of solvent polarity and was found to occur exclusively by a 1,2-Si shift at 250 K in toluene-d 8 by 1H−1H EXSY in combination with a resonance-selective 1H-NOE difference experiment at 200 K.