Symmetry and Geometry Considerations of Atom Transfer: Deoxygenation of (silox)3WNO and R3PO (R = Me, Ph, tBu) by (silox)3M (M = V, NbL (L = PMe3, 4-Picoline), Ta; silox = tBu3SiO)

Deoxygenations of (silox)3WNO (12) and R3PO (R = Me, Ph, tBu) by M(silox)3 (1-M; M = V, NbL (L = PMe3, 4-picoline), Ta; silox = tBu3SiO) reflect the consequences of electronic effects enforced by a limiting steric environment. 1-Ta rapidly deoxygenated R3PO (23 °C; R = Me ((delta)G°rxn(calcd) = -47 kcal/mol), Ph) but not tBu3PO (85°, >2 days), and cyclometalation competed with deoxygenation of 12 to (silox)3WN (11) and (silox)3TaO (3-Ta; (delta)G° rxn(calcd) = -100 kcal/mol). 1-V deoxygenated 12 slowly and formed stable adducts (silox)3V-OPR3 (3-OPR3) with OPR3. 1-Nb(4-picoline) (S = 0) and 1NbPMe3 (S = 1) deoxygenated R3PO (23 °C; R = Me ((delta)G°rxn(calcd from 1-Nb) = -47 kcal/mol), Ph) rapidly and 12 slowly ((delta)G°rxn(calcd) = -100 kcal/mol), and failed to deoxygenate tBu3PO. Access to a triplet state is critical for substrate (EO) binding, and the S (right arrow) T barrier of ~17 kcal/mol (calcd) hinders deoxygenations by 1-Ta, while 1-V (S = 1) and 1-Nb (S (right arrow) T barrier ~ 2 kcal/mol) are competent. Once binding occurs, significant mixing with an 1A1 excited state derived from population of a (sigma)*-orbital is needed to ensure a low-energy intersystem crossing of the 3A2 (reactant) and 1A1 (product) states. Correlation of a reactant (sigma)*-orbital with a product (sigma)-orbital is required, and the greater the degree of bending in the (silox)3M-O-E angle, the more mixing energetically lowers the intersystem crossing point. The inability of substrates EO = 12 and tBu3PO to attain a bent M-O-E due to sterics explains their slow or negligible deoxygenations. Syntheses of relevant compounds and ramifications of the results are discussed. X-ray structural details are provided for 3-OPMe3 (V-O-P = 157.61(9)), 3-OPtBu3 (V-O-P = 180°), 1-NbPMe3, and (silox)3ClWO (9).