Diastereoselective arbuzov dealkylation and evidence for phosphorus-carbon bond activation of prochiral cobalt(III) phosphonite complexes

Article abstract of DOI:10.1021/om00032a029

(S_c)-(η⁵-Cp)CoI₂(PPh ₂NHC*H(Me)Ph) reacts with 1 equiv of dimethyl tert-butyl- or ethylphosphonite in methylene chloride solution to give diastereomeric P-chiral phosphinate Arbuzov products (R,S_Co;R_P;S_C)-(η ⁵-Cp)CoI(PPh₂NHC*H(Me)Ph)(P(0)(OMe)(t-Bu)) (3a) and (R,S_Co;R,S_P;S_C)-(η ⁵-Cp)CoI(PPh₂NHC*H(Me)Ph)(P(O)(OMe)(Et)) (3b), respectively. Apparent phosphorus-carbon bond cleavage also occurs to give (R,S_Co;S_C)-(η⁵-Cp)CoI(PPh ₂NHC*H(Me)Ph)(P(O)(OMe)₂) (4), which was isolated in low yield after aerial workup. Two diastereomers of the tert-butyl series 3a and one diastereomer of the four isolated for the ethyl series 3b were characterized by single-crystal X-ray diffraction. (S_Co;R_P;S_C)-3a-1 crystallizes in the space group P2₁2₁2₁ with a = 9.815(4) A?, b = 14.314(6) A?, c = 21.120(9) A?, V = 2967(2) A?³, Z = 4, and R_F = 2.83% for 5958 reflections (F > 4.0σ(F)) at 198 K. (A_Co;S_P;S_C)-3a-2 crystallizes in the space group P2₁ with a = 13.073(5) A?, b = 8.731(2) A?, c = 13.537(5) A?, β = 106.49(3)°, V = 1481.5(9) A?³, Z = 2, and R_F = 3.19% for 4846 reflections (F > 4.0σ(F)) at 198 K. (R_Co;S_P;S_C)-3b-2 crystallizes in the space group P2₁2₁2₁ with a = 8.7004(7) A?, b = 12.0769(18) A?, c = 27.490(3) A?, V = 2888.5(6) A?³, Z = 4, and R_F = 4.4% for 3118 reflections (F > 2.5σ(F)) at 297 K. The absolute configurations of all diastereomers were assigned on the basis of crystallographic results, circular dichroism, and chemical cycles involving Co epimerization. P=O?H-N intramolecular hydrogen bonding establishes a chaise longue conformation with pseudoaxial η⁵-Cp and pseudoequatorial iodide in the solid state. Proton nuclear Overhauser difference (NOED) spectra show that the solid-state conformation is retained in solution. Optical yields increased with increasing steric demands of the prochiral phosphonite substituent. A model based on 1,3-diaxial steric interactions in the transition state leading to dealkylation is proposed to account for the observed Co→P chiral induction.