Sung Kie-Moon, Holm R H
Department of Chemistry and Chemical Biology, Harvard University, Cambridge, Massachusetts 02138, USA.
J Am Chem Soc. 2002 Apr 24;124(16):4312-20. doi: 10.1021/ja012735p.
The recent development of structural and functional analogues of the DMSO reductase family of isoenzymes allows mechanistic examination of the minimal oxygen atom transfer paradigm M(IV) + QO M(VI) O + Q with the biological metals M = Mo and W. Systematic variation of the electronic environment at the WIV center of desoxo bis(dithiolene) complexes is enabled by introduction of para-substituted phenyl groups in the equatorial (eq) dithiolene ligand and the axial (ax) phenolate ligand. The compounds [W(CO)2(S2C2(C6H4-p-X)2)2] (54-60%) have been prepared by ligand transfer from [Ni(S2C2(C6H4-p-X)2)2] to [W(CO)3(MeCN)3]. A series of 25 complexes [W(IV)(OC6H4-p-X')(S2C2(C6H4-p-X)2)2]1- ([X4,X'], X = Br, F, H, Me, OMe; X' = CN, Br, H, Me, NH2; 41-53%) has been obtained by ligand substitution of five dicarbonyl complexes with five phenolate ligands. Linear free energy relationships between E1/2 and Hammett constant p for the electron-transfer series [Ni(S2C2(C6H4-p-X)2)2]0,1-,2- and [W(CO)2(S2C2(C6H4-p-X)2)2]0,1-,2- demonstrate a substituent influence on electron density distribution at the metal center. The reactions [WIV(OC6H4-p-X')(S2C2(C6H4-p-X)2)2]1- + (CH2)4SO [W(VI)O(OC6H4-p-X')(S2C2(C6H4-p-X)2)2]1- + (CH2)4S with constant substrate are second order with large negative activation entropies indicative of an associative transition state. Rate constants at 298 K adhere to the Hammett equations log(k([X4,X']/k[X4,H]) = rho(ax)sigma(p) and log(k[X4,X']/k([H4,X']) = 4rho(eq)sigma(p). Electron-withdrawing groups (EWG) and electron-donating groups (EDG) have opposite effects on the rate such that k(EWG) > k(EDG). The effects of X' on reactivity are found to be approximately 5 times greater than that of X (rho(ax) = 2.1, rho(eq) = 0.44) in the Hammett equation. Using these and other findings, a stepwise oxo transfer reaction pathway is proposed in which an early transition state, of primary W(IV)-O(substrate) bond-making character, is rate-limiting. This is followed by a six-coordinate substrate complex and a second transition state proposed to involve atom and electron transfer leading to the development of the W(VI)=O group. This work is the most detailed mechanistic investigation of oxo transfer mediated by a biological metal.
二甲基亚砜还原酶同工酶家族结构和功能类似物的最新进展，使得利用生物金属M = Mo和W，对最小氧原子转移范式M(IV) + QO M(VI) O + Q进行机理研究成为可能。通过在赤道（eq）二硫烯配体和轴向（ax）酚盐配体中引入对位取代苯基，可实现脱氧双（二硫烯）配合物中WIV中心电子环境的系统变化。化合物[W(CO)2(S2C2(C6H4-p-X)2)2]（54 - 60%）是通过配体从[Ni(S2C2(C6H4-p-X)2)2]转移至[W(CO)3(MeCN)3]制备而成。通过用五个酚盐配体对五个二羰基配合物进行配体取代，得到了一系列25种配合物[W(IV)(OC6H4-p-X')(S2C2(C6H4-p-X)2)2]1-（[X4,X']，X = Br、F、H、Me、OMe；X' = CN、Br、H、Me、NH2；41 - 53%）。电子转移系列[Ni(S2C2(C6H4-p-X)2)2]0,1-,2-和[W(CO)2(S2C2(C6H4-p-X)2)2]0,1-,2-的E1/2与哈米特常数p之间的线性自由能关系表明，取代基对金属中心的电子密度分布有影响。反应[WIV(OC6H4-p-X')(S2C2(C6H4-p-X)2)2]1- + (CH2)4SO [W(VI)O(OC6H4-p-X')(S2C2(C6H4-p-X)2)2]1- + (CH2)4S在底物恒定的情况下为二级反应，具有很大的负活化熵，表明是缔合过渡态。298 K时的速率常数符合哈米特方程log(k([X4,X']/k[X4,H]) = rho(ax)sigma(p)和log(k[X4,X']/k([H4,X']) = 4rho(eq)sigma(p)。吸电子基团（EWG）和供电子基团（EDG）对反应速率有相反的影响，使得k(EWG) > k(EDG)。在哈米特方程中，发现X'对反应活性的影响约为X的5倍（rho(ax) = 2.1，rho(eq) = 0.44）。利用这些及其他发现，提出了一个逐步的氧转移反应途径，其中具有主要W(IV)-O（底物）键形成特征的早期过渡态是限速步骤。随后是一个六配位底物配合物和第二个过渡态，该过渡态被认为涉及原子和电子转移，导致W(VI)=O基团的形成。这项工作是对生物金属介导的氧转移进行的最详细的机理研究。
