Veige Adam S, Slaughter LeGrande M, Lobkovsky Emil B, Wolczanski Peter T, Matsunaga Nikita, Decker Stephen A, Cundari Thomas R
Department of Chemistry & Chemical Biology, Cornell University, Baker Laboratory, Ithaca, New York 14853, USA.
Inorg Chem. 2003 Oct 6;42(20):6204-24. doi: 10.1021/ic0300114.
Deoxygenations of (silox)(3)WNO (12) and R(3)PO (R = Me, Ph, (t)Bu) by M(silox)(3) (1-M; M = V, NbL (L = PMe(3), 4-picoline), Ta; silox = (t)Bu(3)SiO) reflect the consequences of electronic effects enforced by a limiting steric environment. 1-Ta rapidly deoxygenated R(3)PO (23 degrees C; R = Me (DeltaG degrees (rxn)(calcd) = -47 kcal/mol), Ph) but not (t)Bu(3)PO (85 degrees, >2 days), and cyclometalation competed with deoxygenation of 12 to (silox)(3)WN (11) and (silox)(3)TaO (3-Ta; DeltaG degrees (rxn)(calcd) = -100 kcal/mol). 1-V deoxygenated 12 slowly and formed stable adducts (silox)(3)V-OPR(3) (3-OPR(3)) with OPR(3). 1-Nb(4-picoline) (S = 0) and 1-NbPMe(3) (S = 1) deoxygenated R(3)PO (23 degrees C; R = Me (DeltaG degrees (rxn)(calcd from 1-Nb) = -47 kcal/mol), Ph) rapidly and 12 slowly (DeltaG degrees (rxn)(calcd) = -100 kcal/mol), and failed to deoxygenate (t)Bu(3)PO. Access to a triplet state is critical for substrate (EO) binding, and the S --> T barrier of approximately 17 kcal/mol (calcd) hinders deoxygenations by 1-Ta, while 1-V (S = 1) and 1-Nb (S --> T barrier approximately 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 (t)Bu(3)PO to attain a bent 90 degree angle 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-OPMe(3) (90 degree angle V-O-P = 157.61(9) degrees), 3-OP(t)Bu(3) ( 90 degree angle V-O-P = 180 degrees ), 1-NbPMe(3), and (silox)(3)ClWO (9).
M(silox)₃（1-M；M = V、NbL（L = PMe₃、4-甲基吡啶）、Ta；silox = (t)Bu₃SiO）对(silox)₃WNO（12）和R₃PO（R = Me、Ph、(t)Bu）的脱氧反应反映了在有限空间环境中电子效应的结果。1-Ta能迅速使R₃PO脱氧（23℃；R = Me（计算得到的ΔG°(rxn) = -47 kcal/mol）、Ph），但不能使(t)Bu₃PO脱氧（85℃，>2天），并且环金属化与12脱氧生成(silox)₃WN（11）和(silox)₃TaO（3-Ta；计算得到的ΔG°(rxn) = -100 kcal/mol）相互竞争。1-V缓慢地使12脱氧，并与OPR₃形成稳定的加合物(silox)₃V-OPR₃（3-OPR₃）。1-Nb(4-甲基吡啶)（S = 0）和1-NbPMe₃（S = 1）能迅速使R₃PO脱氧（23℃；R = Me（从1-Nb计算得到的ΔG°(rxn) = -47 kcal/mol）、Ph），缓慢地使12脱氧（计算得到的ΔG°(rxn) = -100 kcal/mol），并且不能使(t)Bu₃PO脱氧。进入三重态对于底物（EO）结合至关重要，大约17 kcal/mol（计算值）的S→T能垒阻碍了1-Ta的脱氧反应，而1-V（S = 1）和1-Nb（S→T能垒约为2 kcal/mol）则可以进行。一旦发生结合，就需要与由σ轨道占据产生的¹A₁激发态进行显著混合，以确保(³A₂)（反应物）和¹A₁（产物）态之间的低能量系间窜越。需要反应物σ轨道与产物σ轨道相关，并且(silox)₃M-O-E角的弯曲程度越大，混合在能量上就越能降低系间窜越点。由于空间位阻，底物EO = 12和(t)Bu₃PO无法达到90°的弯曲M-O-E角，这解释了它们缓慢或可忽略不计的脱氧反应。讨论了相关化合物的合成及结果的影响。提供了3-OPMe₃（V-O-P的90°角 = 157.61(9)°）、3-OP(t)Bu₃（V-O-P的90°角 = 180°）、1-NbPMe₃和(silox)₃ClWO（9）的X射线结构细节。
