On the mechanisms of degenerate halogen exchange in phosphorus(iii) halides

Abstract

Diazaphospholidine heterocycles of phosphorus(III) containing halogens or trifluoromethylsulfonate (triflate) display dynamic behaviour due to exchange of the X (Cl, OTf) groups. Full line-shape ¹³C{¹H}-NMR simulations on (3R,8R)-1-chloro-2,9-bis(4-tert-butylbenzyl)-2,9-diaza-1-phosphabicyclo[4.0.3]nonane (4a) using the gNMR software package reveal proportionality between the rate of [P–Cl] exchange and the square of the concentration of the phosphorus compound, supportive of a second-order, bimolecular process. However, in the presence of an additional chloride ion source, a second, more rapid, process which is both first order in phosphorus compound and added chloride ion becomes competitive, consistent with a mechanism of the S_N2(P) type. This tends to support the existence of parallel pathways which operate under slightly different conditions. Added nitrogen bases such as N-methylmorpholine do not affect the rate of exchange. Variable temperature full line-shape ¹³C{¹H}-NMR analyses on both 4a and an analogue, (3R,8R)-1-chloro-2,9-bis(2,6-difluorobenzyl)-2,9-diaza-1-phosphabicyclo[4.0.3]nonane (4c), reveal isothermal exchange rates at 206 K of 22.2 and 8.0 mol L⁻¹ s⁻¹ (0.36 M), respectively, suggesting that the exchange is slower for 4c. Moreover, activation parameters (ΔG^‡, ΔH^‡ and ΔS^‡ of 50.6(9), 20.8(9) kJ mol⁻¹ and −100(3) J K⁻¹ mol⁻¹, respectively, for 4a at 0.36 M and 55.8(1), 15.6(1) kJ mol⁻¹ and −135(5) J K⁻¹ mol⁻¹, respectively, for 4c at 0.36 M) are comparable and are more consistent, especially in terms of entropy change, with a bimolecular mechanism of exchange in the absence of added chloride ion. We argue on the basis of precedence that a bimolecular mechanism proceeding via a stabilised phosphenium intermediate is most probable, since it is well known that phosphorus(III) is capable of stabilising phosphenium centers. Finally, single-crystal X-ray diffraction analysis of (3R,8R)-1-chloro-2,9-bis(4-cyanobenzyl)-2,9-diaza-1-phosphabicyclo[4.0.3]nonane (4b) reveals a fused ring system with a three-coordinate phosphorus atom in a distorted trigonal pyramidal geometry; the sum of the angles around P(1) is ca. 295°. The P–Cl, P–N and C(1)–C(2) bond lengths compare well with literature values for similar compounds; these range between 2.0–2.3, 1.6–1.7 and 1.5–1.56 Å, respectively.