PENTADIENYL METAL PHOSPHINE CHEMISTRY .20. REGIOSELECTIVE AND DIASTEREOSELECTIVE ADDITION OF ALKYL ANIONS TO ELECTRON-RICH (ETA-5-PENTADIENYL)IRON COMPLEXES

Treatment of (η5-pentadienyl)(η3-pentadienyl)Fe(PEt3) (1) with Ag+PF6- and PEt3 or with HPEt3+PF6 produces (η5-pentadienyl)Fe(PEt3)2+PF6- (2), a 16e- paramagnetic complex. Compound 2 crystallizes in the monoclinic space group C2/c (no. 15) with a = 20.556 (6) Å, b = 7.894 (2) Å, c= 15.000 (4) Å, β = 103.80 (2)°, V = 2363.9 (11) Å3, and Z = 4. The two phosphine ligands reside in inequivalent environments, one under the open “mouth” of the η5-pentadienyl ligand and the other under the pentadienyl “backbone”. The electronic origins of compound 2's ligand orientation and paramagnetism have been probed by performing Fenske-Hall molecular orbital calculations on a model complex. Treatment of 2 with 1 equiv of carbon monoxide or ferf-butylisonitrile (L) leads to the production of (η5-pentadienyl)Fe(PEt3)2(L)+PF6- (L = CO, 3a; L = CNCMe3, 3b). These complexes adopt unsymmetrical three-legged piano stool structures, wherein one PEt3 ligand and L reside under the pentadienyl backbone, while the other PEt3 ligand resides under the pentadienyl mouth. Complexes 3a,b possess stereogenic centers at iron by virtue of the fact that the two PEt3 ligands are inequivalent. Treatment of 3a or 3b with methyllithium at-78 °C results in exo nucleophilic attack at C2 of the pentadienyl ligand, producing (2-methyll, 3,4,5-η-pentenediyl)Fe(PEt3)2(L) (L = CO, 4a; L = CNCMe3, 4b). The analogous reactions involving ierf-butyllithium produce (2-tert-butyl-1,3,4,5-η-pentenediyl)Fe(PEt3)2(L) (L = CO, 5a; L = CNCMe3, 5b). In each case, attack occurs with >85% diastereoselectivity on the “L-side” of the pentadienyl ligand, i.e., at the internal pentadienyl carbon which resides above ligand L and opposite the bulky PEt3 ligand. Compound 5b crystallizes in the triclinic space group PI (no. 2) with a = 9.433 (2) Å, b = 9.849 (2) Å, c = 18.015 (4) Å, α = 100.98 (2)°, β = 93.51 (2)°, γ = 111.44 (2)°, V= 1513.6 (6) Å3, and Z = 2. The solid-state structure shows the attacked carbon, C2, to be displaced 0.59 Å out of the C1/C3/C4/C5 plane. Bond distances within the pentenediyl ligand are consistent with its σ, π-allyl character. © 1990, American Chemical Society. All rights reserved.