(CROSS-CONJUGATED DIENYL)TRICARBONYLIRON CATIONS .2. 4-METHYL DERIVATIVES

Tricarbonyl(cross-conjugated dienyl)iron cations have been generated from precursor alcohols in strong acid solutions and studied by NMR spectroscopy in the temperature range 0 to -50°C. The 4-anti-methyl cation (11) coordinates reversibly, but slowly, with fluorosulfonate ion to give the adduct 12. Broadening of1H and13C NMR signals in the region -40 to -9°C demonstrates rotation about the C2-C3bond of 11. The observations exclude 12 as an intermediate in the rotation process. Relief of steric strain associated with the 4-anti-methyl enhances the driving force for FS03coordination and lowers the barrier for C2-C3rotation. The 4-syn-methyl cation (15) does not give a fluorosulfonate adduct and fails to exhibit NMR signal broadening caused by C2-C3rotation at temperatures below 0°C. Neither cation coordinates with carbon monoxide. Lack of evidence of coordinative unsaturation and the substantial barrier to C2-C3rotation rule out the η3-allyl type structure predicted by the simplest (one interaction) frontier orbital model. © 1979, American Chemical Society. All rights reserved.