REGIOSELECTIVITY AND DIASTEREOSELECTIVITY IN THE REARRANGEMENT OF CATIONIC IRON(II) ETA-1-1-(1-METHYLCYCLOALKYL)METHYLIDENES

A series of (CO)(η5-C5H5)(L)Fe-η1-{[–(CH2)n–]C(CH3)CH(OCH3)| complexes [7a-d (n = 2–5, L = CO) and 8a-d or 9a-d (n = 2–5, L = P(OMe)3)], when protonated in dichloromethane at -80 to -60 °C, produce (CO)(η5-C5H5)(L)Fe-η1-{[–(CH2)n–]C(CH3)CH)}+ [10a–d+ (n = 2–5, L = CO) or 11a–d+ (n = 2–5, L = P(OMe)3)] whose rearrangement to iron(II) η2-olefin complex(es) has been monitored at reduced temperature by NMR. The putative Fp [l0a–d (n = 2–5, L = CO)] and Fp’ [11b (n = 3, L = P(OMe)3)] alkylidenes themselves are too reactive to detect at -80 °C, but the P(OMe)3-ligated (Fp’) alkylidenes 11a,c,d (n = 2, 4, 5), being less reactive, can be observed by13C,1H, and/or31P NMR at -80 °C. The resulting Fp’(n2-olefin) complexes in all cases are appreciably less stable thermally than their Fp counterparts. The thermally unstable η2-olefin rearrangement products are identified by reduced temperature NMR of the reaction mixtures and by decomplexation with excess tetrabutylammonium iodide followed by capillary GLPC of the resulting olefin(s) 18a-d and 19d. These iron(II) η1-alkylidene rearrangements are highly regioselective: ring enlargement is exclusive in the Fp (l0a–c) and Fp’ (11a–c) cases while methyl migration is prepoderant in the Fp and Fp’ cyclohexyls [10d and 11d (n = 5), respectively]. The rearrangements of the chiral Fp’ alkylidenes 11a-d are apparently also diastereoselective, producing substantially more of one diastereomeric η2-olefin complex than of its diastereotopic “epimer”. The less stable of the two diastereomeric Fp’ η2-l-methylcyclobutenes [14a or 15a (n = 2) (and possibly of the others as well)] isomerizes to the more stable under the reaction conditions, thus preventing an assessment of the true kinetic diastereoselectivity of this (and presumably of all these) Fp’ alkylidene rearrangment(s). The relative rates and regiospecificities of these η1-alkylidene to η2-olefin rearrangements apparently reflect the strain-energy differences between the starting Fp or Fp’ alkylidene and the rearranged β-Fp or Fp’ carbocation as estimated by molecular mechanics (PCMODEL) with ametallic cations as models for the starting alkylidenes and rearranged olefin complexes. © 1990, American Chemical Society. All rights reserved.