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)(3)WNO (12) and R3PO (R = Me, Ph, Bu-t) by M(Silox)(3) (1-M; M = V, NbL (L = PMe3, 4-picoline), Ta; silox = (BU3SiO)-B-t) reflect the consequences of electronic effects enforced by a limiting steric environment. 1-Ta rapidly deoxygenated R3PO (23 degreesC; R = Me (DeltaGdegrees(rxn)(calcd) = -47 kcal/mol), Ph) but not (Bu3PO)-Bu-t (85degrees, >2 days), and cyclometalation competed with deoxygenation of 12 to (silox)(3)WN (11) and (silox)(3)TaO (3-Ta; DeltaGdegrees(rxn)(calcd) = -100 kcal/mol). 1-V deoxygenated 12 slowly and formed stable adducts (Silox)(3)V-OPR3 (3-OPR3) with OPR3. 1-Nb(4-picoline) (S = 0) and 1-NbPMe3 (S = 1) deoxygenated R3PO (23 degreesC; R = Me (DeltaGdegrees(rxn)(calcd from 1-Nb) = -47 kcal/mol), Ph) rapidly and 12 slowly (DeltaGdegrees(rxn)(calcd) = -100 kcal/mol), and failed to deoxygenate (Bu3PO)-Bu-t. Access to a triplet state is critical for substrate (EO) binding, and the S --> T barrier of similar to17 kcal/mol (calcd) hinders deoxygenations by 1-Ta, while 1-V (S = 1) and 1-Nb (S --> T barrier similar to 2 kcal/mol) are competent. Once binding occurs, significant mixing with an (1)A(1) excited state derived from population of a sigma*-orbital is needed to ensure a low-energy intersystem crossing of the (3)A(2) (reactant) and (1)A(1) (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)(3)M-O-E angle, the more mixing energetically lowers the intersystem crossing point. The inability of substrates EO = 12 and (Bu3PO)-Bu-t to attain a bent angleM-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 (angleV-O-P = 157.61(9)degrees), 3-(OPBu3)-Bu-t (angleV-O-P = 180degrees), 1-NbPMe3, and (Silox)(3)CIWO (9).