Water-Soluble Phosphines. 6.¹ Tailor-Made Syntheses of Chiral Secondary and Tertiary Phosphines with Sulfonated Aromatic Substituents: Structural and Quantum Chemical Studies

Chiral water-soluble secondary phosphines (2-6) were obtained by nucleophilic phosphination of FC₆H₄-4-SO₃K (1a), FC₆H₃-2,4-(SO₃K)₂ (1b), and FC₆H₄-2-SO₃K (1c) with RPH₂ (R = Ph, 2,4,6-Me₃C₆H₂, 2,4,6-iPr₃C₆H₂) in the superbasic medium DMSO/KOH by employing steric control of substitution at phosphorus by bulky substituents R and sulfonic groups in the ortho position of the aromatic ring systems in 1b or 1c. The secondary phosphines may be deprotonated in DMSO/KOH to give phosphido anions which on reaction with alkyl halides (PhCH₂Cl, Br(CH₂)₃Br, and C₁₂H₂₅Br) yield mono- or bidentate tertiary phosphines (7-10). Ligands of this type are alternatively accessible by nucleophilic arylation of secondary phosphines, e.g. Ph(Me)PH or Ph(H)P(CH₂)₃P(H)Ph with 1a or 1b, respectively. The crystal structure of the starting material 1b·H₂O (space group P2₁/m) has been determined. In the solid state of 1b·H₂O the individual molecules are interconnected by ionic interactions between the potassium cations and the SO₃^- anions. The C-F bond (C(1)-F 1.347(4) Å) is shorter than that in C₆H₅F (1.356(4) Å). The unit cell of 7a·0.5H₂O (space group P1̄), the first structurally characterized chiral phosphine with a sulfonated phenyl substituent, contains the two enantiomers. Due to the asymmetrical substitution at phosphorus the PC₃ skeletons are significantly distorted (P(1)-C(1,11,31) 1.864(10), 1.825(8), 1.841(7) Å). The electronic structure of sulfonated fluorobenzenes FC₆H_5-n(SO₃M)_n (M = K, NH₄, n = 1-3) is discussed on the basis of quantum chemical calculations. In particular, the reactivity difference toward nucleophilic phosphination within the series is rationalized in terms of steric factors and of the -I effect of the sulfonic groups.