Table 1. Measured ³¹P-NMR chemical shifts for the monoadducts Et₃PO–Si(cat^X)₂ in CD₂Cl₂ in reference to free Et₃PO; σ_m Hammett parameters for the catechol substituents X ^a .

Si(catX)2–OPEt3	δ 31P-NMR [ppm]	Δ [ppm]	σ m of X27
Si(cattBu)2	81.6	31.1	–0.100
Si(catH)2	83.2	32.7	0.000
Si(catF)2	86.6	36.1	0.337
Si(catCl)2	87.2	36.7	0.373
Si(catBr)2	87.3	36.8	0.391

^aBlind sample of Et₃PO in CD₂Cl₂ (162 MHz): δ³¹P-NMR = 50.5 ppm.