STUDY OF THE STEREOCHEMISTRY AT IRON OF ELECTROPHILIC CLEAVAGE OF THE IRON-CARBON SIGMA-BOND IN METHYL(ETA-5-1-METHYL-3-PHENYLCYCLOPENTADIENYL)-CARBONYL(TRIPHENYLPHOSPHINE)IRON(II)

The stereochemistry at iron of electrophilic cleavage, with insertion or elimination, of the Fe-CH3 σ bond in the diastereomers (used as enantiomeric pairs) of the “pseudotetrahedral” (η5-1-CH3-3-C6H5C5H3)Fe(CO)[P(C6H5)3]CH3 (2) was investigated by 1H N MR spectroscopy and X-ray crystallography. The insertion of SO2 in 2 to yield the corresponding S-sulfinate, (η5-1-CH3-3-C6H5C5H3)Fe(CO)[P(C6H5)3]S(O)2CH3 (3), is essentially stereoselective (>95% stereospecificity) in CH2Cl2 solution, and highly (79%) stereospecific in neat SO2. Eliminative cleavage of 2 by a deficiency of each of HI, I2, and Hgl2 to afford the iodo complex, (η5-1-CH3-3-C6H5C5H3)Fe(CO)[P(C6H5)3]I (4), proceeds with variable stereospecificity (0-67%). The stereospecificity of this cleavage by a given electrophile is unequal for the two diastereomers of 2 (0-38 and 3867%), and higher with I2 (38-67%) than with HI and Hgl2 (0-47%). Unreacted 2 undergoes epimerization during the reaction to an extent (20-42%) which appears to be independent of the diastereomer employed. To elucidate the stereochemistry of these eliminative cleavage reactions crystal structure determinations were carried out of one diastereomer each of (η51-CH3-3-C6H5C5H3)Fe(CO)[P(C6H5)3]C(O)CH3 (1) and 4. The structural results, taken in conjunction with the known stereochemistry of the photochemical decarbonylation of acyl(η5-cyclopentadieny)iron complexes, show that the cleavage of 2 by the iodine-containing electrophiles (EI) proceeds with net retention of configuration at iron. A mechanism is proposed which involves attack of the electrophile at the iron center to generate an ionic intermediate, (η5-1-CH3-3-C6H5C5H3)Fe(CO)-[P(C6H5)3](CH3)E+I- (5), containing a square pyramidal cation. Reductive elimination of CH3E followed by coordination of iodide or, alternatively, nucleophilic attack of iodide at the ligated CH3 (when E = I) lead to product 4. These individual steps are discussed in the context of the observed net retention of configuration at iron and the previously reported stereochemical results at α carbon of such cleavage processes. A possible fluxional behavior of 5 and its relationship to partial epimerization of unreacted 2 are also considered. Crystallographic details follow. Crystals of the iodide complex (4a) are monoclinic, space group P21/n, with a = 9.773 (7) Å, b = 15.635 (8) Å, c = 17.235 (8) Å, β = 92.43 (4)°, V= 2631 (6) Å3, Z = 4. The structure, refined to an R value of 0.078 for 2541 reflections, consists of a racemic mixture of molecules having RS and SR configurations. The acetyl complex (1b) crystallizes as a benzene solvate in the triclinic space group P1, with a = 15.359 (4) Å,b = 9.072 (3) Å, c = 14.112(4) Å, α = 86.22 (3)°,β= 119.64 (2)°, γ= 109.38 (2)°, V= 1601 (2) Å3, Z = 2. Final R factor = 0.081 for 2370 reflections. Crystals of the acetyl complex are also racemic, and contain equal numbers of each member of the enantiomeric pair (RR/SS). © 1979, American Chemical Society. All rights reserved.