Mechanism of the Phospha-Wittig–Horner Reaction**

General. All reactions were performed under argon using Schlenk techniques. Diethyl ether and THF were freshly distilled from sodium/benzophenone prior to use. 1 H, 13 C and 31 P spectra were recorded on a 400 MHz and 300 MHz spectrometers. Chemical shifts (ppm) were reported and referenced to the internal signal of residual protic solvent. High resolution mass spectral analyses (HRMS) were performed on high resolution and FTMS+pNSI mass spectrometer (OrbitrapXL). Bruker SMART APEX diffractometer equiped with an APEXII CCD detector, a graphite monochromator and a 3-circles goniometer. The crystal-to-detector distance was 5.0 cm, and the data collection was carried out in 512 x 512 pixel mode. The initial unit cell parameters were determined by a least-squares fit of the angular setting of strong reflections, collected by a 10.0 degrees scan in 33 frames over three different parts of the reciprocal space (99 frames total). Cell refinement and data reduction were performed with SAINT V7.68A (Bruker AXS). Absorption correction was done by multi-scan methods using SADABS96 (Sheldrick).