Stereospecific Oxidative Demetallation of Highly Functionalized CpCo(1,3-Diene) Complexes: An Experimental and Computational Study

Kudos to Professor K. Peter C. Vollhardt for his 25 years of innovative contributions to SYNLETT

Abstract

Three diastereomers of a highly functionalized tetrasubstituted cyclopentadienylcobalt–diene complex, (Cp)Co[η⁴-CH(CO₂Et)=C(SO₂Ph)C(SiMe₃)=CH(CO₂Et)] (4-ZE, 4-EZ, 4-ZZ; Cp = C₅H₅), undergo oxidative demetallation by ceric ammonium ­nitrate (CAN) to give the corresponding metal-free dienes, CH(CO₂Et)=C(SO₂Ph)C(SiMe₃)=CH(CO₂Et) (5-ZE, 5-EZ, 5-ZZ), with retention of configuration. The oxidation of 4-ZE by iodine occurs with a different stereoselectivity than that observed with CAN, to provide the fourth diastereomer 5-EE in high yield. B97D/Def2-TZVPP(CH₂Cl₂) calculations predict the diene free-energy ordering as: 5-EE < 5-ZE < 5-EZ < 5-ZZ.

Key words

cobalt - diene - oxidation - computation - demetallation

• References and Notes

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• 16 Representative Oxidation Procedure Diene complex ZZ-(η⁵-C₅H₅)Co[η⁴-CH(CO₂Et)=C(SO₂Ph)C(TMS)= CH(CO₂Et)] (4-ZZ; 0.120 g, 0.224 mmol) was dissolved in a 1:1 mixture (20 mL) of MeCN–pentane, and CAN (0.369g, 0.673 mmol) was added at –78 °C. The reaction was stirred at 4 °C for 18 h. Removal of volatiles, extraction of the residue with benzene, and chromatography (silica gel, 10% EtOAc–hexanes) gave 5-ZZ (76 mg, 83%) as a yellow oil. For representative spectroscopic data see ref. 18.
• 17 See Supporting Information for characterization data.
• 18 The experimental (CDCl₃) and calculated (CH₂Cl₂) vinyl hydrogen chemical shifts and assignments are as follows. For 5-ZZ: δ = 5.98 (H2; calcd 6.14), 5.80 (H5, calcd 5.01); for 5-EZ: δ = 6.77 (H2; calcd 6.88), 5.49 (H5, calcd 4.75); for 5-ZE: δ = 6.21 (H2; calcd 6.20), 6.02 (H5, calcd 6.51); for 5-EE: δ = 6.77 (H2; calcd 7.10), 6.11 (H5, calcd 6.22).
• 19 See Supporting Information for all computational details.