Deoxygenations of (silox)₃WNO and R₃PO by (silox)₃M (M = V, Ta) and (silox)₃NbL (silox = ^tBu₃SiO):  Consequences of Electronic Effects

Abstract

Oxygen atom transfers involving terminal metal-oxo functionalities are central to many biological transformations, 1 prominent in applications to organic synthesis, 2-4 and of increasing importance in inorganic systems as synthetic tools, 5-7 objectives in biomimicry, 1,8,9 and targets of fundamental studies. [5][6][7][8][9][10][11][12][13] As a synthetic route to (silox) 3 WN (4, silox ) t Bu 3 SiO), the deoxygenation of (silox) 3 WNO (2) by (silox) 3 Ta (1-Ta) was attempted without success, despite ample precedent in cleavages of epoxides, 10 N 2 O, NO, 11 CO 2 , and CO. 12 A comparison study involving sources of M(silox) 3 (1-M; M ) V, Nb, Ta) revealed that features of deoxygenations of 2 and R 3 PO (R ) Me, Ph, t Bu) are the consequences of electronic effects enforced by a limiting steric environment. ) converted 2 to the nitride. (silox) 3 Nb(η 2 -N,C-4-picoline) (1-Nb-(4-pic), S ) 0) 11 and (silox) 3 NbPMe 3 (1-NbPMe 3 , S ) 1) 14 deoxygenated 2 and formed 4 and (silox) 3 NbO (3-Nb) swiftly at first, then more slowly as the released 4-picoline and PMe 3 inhibited the reactions, respectively. With a 4-picoline scavenger (1-Ta) present in the former, swift cyclometalation to (silox) 2 -HNbOSi t Bu 2 CMe 2 CH 2 (5-Nb, 23°C, <5 min) 14 competed with deoxygenation; 5-Nb then slowly deoxygenated 2, presumably via reversible formation of 1-Nb. The thermodynamics of deoxygenation 15 were investigated by high-level quantum calculations, 16 with (HO) 3 M serving as the model of respective tris-silox centers in 1-M and 3-M. In each case the reaction was extremely exoergic (25°C: M ) V, ∆G°r xn ) -66 kcal/mol; M ) Nb, Ta, -100 kcal/mol). With favorable thermodynamics, the uncompetitive (1-Ta) and relatively slow (1-V, 1-Nb) deoxygenations are puzzling.Since (silox) 3 V (1-V, S ) 1) binds various L (L ) THF, py, etc.), while (silox) 3 Ta (1-Ta, S ) 0) does not, 17 the singlet and triplet states of 1-M were examined via quantum calculations. 16 Figure 1 reveals that 1-V is a triplet at the optimized geometries for, and the TfS barrier is 17 kcal/mol, assuming a facile intersystem crossing. 1-Ta is a singlet at the optimized S ) 0 and S ) 1 geometries and its intersystem crossing barrier is 17 kcal/mol. 1-Nb is a singlet, but the conversion barrier to a triplet of nearly the same energy is 2 kcal/mol. If the approach of (silox) 3 WNO (2) to the 1-M center is linear because of intermolecular silox/silox interactions, then a 4e-repulsion will result in the case of 1-Ta, but successful docking to an S ) 1 intermediate (silox) 3 MONW(silox) 3 (1-M-2) will occur for M ) V, Nb. The additional S-T barrier forced on 1-Ta allows unimolecular cyclometalation to compete with the bimolecular deoxygenation of 2. Table 1 lists the results of R 3 PO deoxygenations by (1-V, Ta) and 1-NbL (L ) 4-pic, PMe 3 ), which are predicted by quantum calculations to be exothermic for V (-15 kcal/mol) and Nb or Ta (-45 kcal/mol) with Me 3 PO. Curiously, 1-Ta and 1-NbL both deoxygenated Me 3 PO and Ph 3 PO, but failed with t Bu 3 PO; 1-Ta cyclomet...

Improved Oxidative C–C Bond Formation Reactivity of High-Valent Pd Complexes Supported by a Pseudo-Tridentate Ligand

“…The EPR spectra of 1 + and 2 + (in 3:1 PrCN/MeCN frozen glass at 77 K) reveals that the two axial amino donors interact to a different extent with the Pd III center, in contrast to the previously reported Pd III complexes supported by the symmetric R N4 ligands ( Figure 4). 3,4,12 For complex 1 + the Pd III center strongly couples to the N atom of the N-Me group (Az(N1) = 35.5 G) and weakly couples to the N atom of the N-Ts group (Az(N2) = 10.0 G) in the gz direction.…”

“…This combination of hybrid functional and basis sets have been previously shown to work well for reproducing experimental parameters of Pd complexes. 11,12 The ground state wave function was investigated by analyzing the frontier MOs, and the atomic contributions to…”

“…However, recently an increasing number of high-valent Pd IV and/or Pd III active intermediates have been reported in several catalytic and stoichiometric aerobic transformations. [9][10][11][12][13][14][15][16][17] We have recently reported several Pd II Me2 complexes supported by tetradentate ligands N,N'-di-alkyl-butyl -2,11-diaza [3.3] (2,6)pyridinophane ( R N4, R = t Bu, i Pr, Me) that are capable of undergoing facile oxidation by O2 or peroxides to produce Pd III and Pd IV intermediates, which undergo elimination of ethane under ambient conditions (Scheme 1). [18][19] Similarly, we have reported the aerobic oxidation of (Me3tcan)Pd II Me2 (Me3tacn = N,N′,N″-trimethyl-1,4,7triazacyclononane) to produce a stable high-valent Pd IV complex [(Me3tacn)Pd IV Me3] + (Scheme 2), which can undergo reductive elimination of ethane at elevated temperatures.…”

Oxygen Activation Chemistry of Pacman and Hangman Porphyrin Architectures Based on Xanthene and Dibenzofuran Spacers

“…A general depiction of this cycle appears in Fig. 25 is believed to proceed under stereoelectronic control, which is preferred for the side-on attack of substrate on the electrophilic MÀ À À À O subunit (166)(167)(168)(169)(170)(171). To this end, the oxidative photocatalysis builds on the chemistry of unbridged PFe III À ÀOÀ ÀFe III P constructs (172)(173)(174)(175)(176)(177) with the enhancement stemming from the fact that the Pacman assembled from a single rigid pillar sterically directs substrate to the electronically preferred side-on approach to the metal-oxo bond.…”

Recent advances in metal-mediated nitrogen oxyanion reduction using reductively borylated and silylated N-heterocycles

“…The synthesis of a metal nitride from deoxygenation is rare, with several examples reported using low-valent vanadium to remove the oxygen. [58][59][60] Our DFT calculations show that deoxygenation of Cr(NO)(N(SiMe 3 ) 2 ) 3 to the known Cr(N) (N(SiMe 3 ) 2 ) 3 , as well as deoxygenation of proposed (PNNH)Mn (CO)(NO) to the proposed nitride (PNNH)Mn(N)(CO) are both quite exergonic. 46,61 Metal nitrides display a wide variety of reactivity depending on the electronic character of the metal nitride fragment, 62 and developing new synthetic methodologies to access metal nitrides from deoxygenation of nitrate and nitrite ligands could lead to transient late metal nitrides that can undergo subsequent functionalization to create new N-C bonds (Fig.…”