Organometallic compounds in organic synthesis. Part 12. Methods of determination of the stereochemistry of tricarbonylcyclohexa-1,3-dieneiron complexes

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Abstract
The steric configuration (α- or β-) of a substituent attached to an sp3 carbon atom in tricarbonylcyclohexadieneiron complexes can be determined by 1H n.m.r. spectroscopy if both epimers are available. With only one epimer present, the most useful method involves examination of the vicinal coupling constants of hydrogens attached to sp3 carbon atoms. The β-H atoms have a vicinal coupling constant of 10–12 Hz and the corresponding α-H atoms have a value of ca. 8 Hz. The CH2 splitting patterns are indicative of stereochemistry when flanked by two vicinal protons. In some cases simplification of the spectrum by use of a paramagnetic shift reagent is necessary for evaluation. Usually δ(β-substituent) > δ(α-substituent). The CO2Me substituents show δ(β-CO2Me) > 3.63 > δ(α-CO2Me) and often, but not invariably, δ(β-H) > δ(α-H). Computer simulation confirms the expected pattern. Aromatic solvent induced shifts are not useful; 13C n.m.r. have only a limited usefulness and require both isomers; mass spectrometric fragmentation is intepretable in steric terms, but is not useful for definition. 1H N.m.r. spectra of the isomeric cations (45), (46), and the alkyl analogue (47) show indications of utility in connection with 6-H, but more examples are needed. The vicinal coupling of CH2 seems to be at present the only reliable method for the assignment of configuration in neutral diene complexes.

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