Kail Brian W, Pérez Lisa M, Zarić Snezana D, Millar Andrew J, Young Charles G, Hall Michael B, Basu Partha
Department of Chemistry and Biochemistry, Duquesne University, Mellon Hall, Pittsburgh, PA 15282, USA.
Chemistry. 2006 Sep 25;12(28):7501-9. doi: 10.1002/chem.200600269.
The oxygen-atom-transfer (OAT) reactivity of [LiPrMoO2(OPh)] (1, LiPr=hydrotris(3-isopropylpyrazol-1-yl)borate) with the tertiary phosphines PEt3 and PPh2Me in acetonitrile was investigated. The first step, [LiPrMoO2(OPh)]+PR3-->[LiPrMoO(OPh)(OPR3)], follows a second-order rate law with an associative transition state (PEt3, DeltaH not equal=48.4 (+/-1.9) kJ mol-1, DeltaS not equal=-149.2 (+/-6.4) J mol-1 K-1, DeltaG not equal=92.9 kJ mol-1; PPh2Me, DeltaH not equal=73.4 (+/-3.7) kJ mol-1, DeltaS not equal=-71.9 (+/-2.3) J mol-1 K-1, DeltaG not equal=94.8 kJ mol-1). With PMe3 as a model substrate, the geometry and the free energy of the transition state (TS) for the formation of the phosphine oxide-coordinated intermediate were calculated. The latter, 95 kJ mol-1, is in good agreement with the experimental values. An unexpectedly large O-P-C angle calculated for the TS suggests that there is significant O-nucleophilic attack on the P--C sigma* in addition to the expected nucleophilic attack of the P on the Mo==O pi*. The second step of the reaction, that is, the exchange of the coordinated phosphine oxide with acetonitrile, [LiPrMoO(OPh)(OPR3)]+MeCN-->[LiPrMoO(OPh)(MeCN)]+OPR3, follows a first-order rate law in MeCN. A dissociative interchange (Id) mechanism, with activation parameters of DeltaH not equal=93.5 (+/-0.9) kJ mol-1, DeltaS not equal=18.2 (+/-3.3) J mol-1 K-1, DeltaG not equal=88.1 kJ mol-1 and DeltaH not equal=97.9 (+/-3.4) kJ mol-1, DeltaS not equal=47.3 (+/-11.8) J mol-1 K-1, DeltaG not equal=83.8 kJ mol-1, for [LiPrMoO(OPh)(OPEt3)] (2 a) and [LiPrMoO(OPh)(OPPh2Me)] (2 b), respectively, is consistent with the experimental data. Although gas-phase calculations indicate that the Mo--OPMe3 bond is stronger than the Mo--NCMe bond, solvation provides the driving force for the release of the phosphine oxide and formation of [LiPrMoO(OPh)(MeCN)] (3).
研究了[LiPrMoO2(OPh)]（1，LiPr = 氢化三（3 - 异丙基吡唑 - 1 - 基）硼酸酯）与叔膦PEt3和PPh2Me在乙腈中的氧原子转移（OAT）反应活性。第一步，[LiPrMoO2(OPh)] + PR3 → [LiPrMoO(OPh)(OPR3)]，遵循二级速率定律，具有缔合过渡态（PEt3，ΔH≠ = 48.4（±1.9）kJ mol-1，ΔS≠ = -149.2（±6.4）J mol-1 K-1，ΔG≠ = 92.9 kJ mol-1；PPh2Me，ΔH≠ = 73.4（±3.7）kJ mol-1，ΔS≠ = -71.9（±2.3）J mol-1 K-1，ΔG≠ = 94.8 kJ mol-1）。以PMe3作为模型底物，计算了形成氧化膦配位中间体的过渡态（TS）的几何结构和自由能。后者为95 kJ mol-1，与实验值吻合良好。为TS计算出的意外大的O - P - C角表明，除了预期的P对Mo == O π的亲核进攻外，还存在对P - C σ的显著O亲核进攻。反应的第二步，即配位的氧化膦与乙腈的交换，[LiPrMoO(OPh)(OPR3)] + MeCN → [LiPrMoO(OPh)(MeCN)] + OPR3，在乙腈中遵循一级速率定律。对于[LiPrMoO(OPh)(OPEt3)]（2a）和[LiPrMoO(OPh)(OPPh2Me)]（2b），分别具有解离交换（Id）机制，活化参数为ΔH≠ = 93.5（±0.9）kJ mol-1，ΔS≠ = 18.2（±3.3）J mol-1 K-1，ΔG≠ = 88.1 kJ mol-1和ΔH≠ = 97.9（±3.4）kJ mol-1，ΔS≠ = 47.3（±11.8）J mol-1 K-1，ΔG≠ = 83.8 kJ mol-1，这与实验数据一致。尽管气相计算表明Mo - OPMe3键比Mo - NCMe键更强，但溶剂化提供了释放氧化膦并形成[LiPrMoO(OPh)(MeCN)]（3）的驱动力。
