Kinetics and Mechanism of Addition of Tertiary Phosphines and Phosphites to the Dicarbonyl(η5-cyclopentadienyl)-(η-ethene)iron Cation

Article abstract of DOI:10.1039/DT9950002021

Kinetic studies of the addition of a range of tertiary phosphine and phosphite nucleophiles PR3 to the cation (1+) 1 (cp = η⁵-C5H5) revealed the general rate law.Rate = k₁3>.The second-order rate constants k₁ decrease markedly down the order P(C6H4OMe-2)3 > P(n-Bu)3 > P(C6H4OMe-4)3 > P(C6H4Me-4)3 > P(C6H4Me-4)Ph2 > PPh3 > P(C2H4CN-2)Ph2 > P(C2H4CN-2)3 > P(C6H4Cl-4)3 > P(O(n-Bu))3.This reactivity order parallels that of decreasing electron availability at the phosphorus centre, as shown quantitatively by the good correlation between log k₁ and the Tolman Σ_χ values.An excellent fit to the Hammett and Broensted equations is also observed for reaction (i) with the nucleophiles P(C6H4X-4)3. (1+) + PR3 --> (1+) The moderate Broensted slope α of 0.46 establishes the importance of phosphine basicity in determining nucleophilicity towards the ethene ligand in cation 1.These results, together with the large negative entropy of activation with PPh3 (ΔS(excit.)₁ = -103 J K^-1mol^-1 ), are interpreted in terms of direct addition (k₁) of the phosphorus nucleophiles to the ethene ligand in 1 and suggest a transition state in which there is build-up of positive positive charge on the phosphorus centre and considerable phosphorus-carbon bond formation.

Full text of DOI:10.1039/DT9950002021

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