Olefin metathesis - a mechanistic study of high-valent Group VI catalysts
Olefin metathesis - a mechanistic study of high-valent Group VI catalysts
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烯烃复分解——高价VI族催化剂的机理研究
DOI：
10.1021/ja00366a013
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发表时间：
1982
期刊：
影响因子：
--
通讯作者：
W. Goddard
中科院分区：
文献类型：
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作者：
A. Rappé;W. Goddard
从头算理论机理研究结果表明，氧-亚烷基配合物是高价Mo、W、Re配合物的活性链状复分解催化剂，氧配体与催化过程密切相关。此外，我们认为在负载型钼酸盐和钨酸盐催化剂上形成了氧代亚烷基络合物，而二氧杂环前驱体可以提供一条方便的途径来形成用于烯烃歧化和氧化反应的稳定的表面催化剂。在这些反应中，旁观者氧基在稳定关键中间体方面起着中心作用，并且在金属氧化物(例如，Mn04-、Oso、Ru04、负载型过渡金属氧化物)的其他反应中可能是重要的。金属有机化学中最耐人寻味、研究最深入的催化反应之一是烯烃歧化反应。形式上，该反应涉及两个烯烃双键的同时裂解，然后形成交替的双键c=c+c=c2c=c(A)，其中M是合适的金属络合物。目前公认的机理涉及金属-亚烷基(卡宾)络合物(即M==CRZ)作为活性载链催化剂，它与烯烃反应生成金属环丁烷中间体(方案I的a)，该中间体分解形成产物烯烃(方案I的b)。这一机制是由Herison和Chauvin4提出的，并通过对同位素扰乱的详细(和巧妙)研究(GRubbs5和Katz6)和间接证据建立起来，例如合成金属亚烷基(Schrock‘)和金属环丁烷(Green：Puddephatt？Whiteside，Lo和Ibers“)和环烯烃聚合产物的特性分析(Caseylz)。此外，Casey13还合成了低价金属卡宾络合物[W(CO)，CPH，]，这些络合物的化学计量比是偏位大小的烯烃。潜在的链终止副反应是金属环的单分子分解(还原消除)，形成环丙烷和还原的金属络合物(方案I的c)，以及卡宾络合物的双分子分解(方案I的d)。如上所述，用于机械研究烯烃歧化反应的络合物有两类：(1)高价(一般为DO)的Mo、W和Re络合物(这些络合物通常是催化的，通常具有重要的工业意义)和(2)低价的Cr、Mo和W络合物(这些络合物在确定歧化反应的一般机理方面具有重要意义，但通常不是催化的，对工业意义不大)。高价和低价体系之间存在着显著的差异，特别是在金属-CR键的性质上。由于只有高价的复分解络合物被确定为真正具有催化作用，因此本文将专门讨论这类化合物。活性均相~S~高价钼、钨和稀土复分解催化剂的典型配方包括在各种条件下将各种金属来源[M(O)到M(VI)]与路易斯酸(如**M*)相结合。这种多样性的结果是对配体环境的详细了解，甚至对Bantrell For，198C-1981。科罗拉多州立大学化学系，科罗拉多州柯林斯堡，80523。方案一
The results of an ab initio theoretical mechanistic study are used to suggest that oxo-alkylidene complexes are the active, chain-carrying metathesis catalysts for high-valent Mo, W, and Re complexes and that the oxygen ligand is intimately involved in the catalytic process. Furthermore, we suggest that oxo-alkylidene complexes are formed on supported molybdate and tungstenate catalysts and that dioxo precursors can provide a convenient route to formation of well-defined surface catalysts for olefin metathesis and oxidation reactions. The spectator oxo group is suggested to play a central role in stabilizing the critical intermediate in these reactions and may be important in other reactions of metal oxides (e.g., Mn04-, OsO,, Ru04, supported transition-metal oxides). One of the most intriguing and best studied catalytic reactions in organometallic chemistry is the olefin metathesis reaction.' It is of potential synthetic utilityZ and is of industrial importances3 Formally, the reaction involves a simultaneous cleavage of two olefin double bonds followed by the formation of the alternate double bonds c=c + c=c 2 c = c (A) where M is an appropriate metal complex. The currently accepted mechanism involves a metal-alkylidene (carbene) complex (that is, M==CRz) as the active chain-carrying catalyst that reacts with an olefin to form a metallacyclobutane intermediate (a of Scheme I) that decomposes to form the product olefin (b of Scheme I). This mechanism was proposed by Herrison and Chauvin4 and has been established through detailed (and ingenious) study of isotopic scrambling (Grubbs5 and Katz6) and by indirect evidence such as the synthesis of metalloalkylidenes (Schrock') and metallacyclobutanes (Green: Puddephatt? Whitesides,lo and Ibers") and by the analysis of the character of polymeric products of cycloolefins (Caseylz). Furthermore, Casey13 has synthesized lowvalent metallocarbenoid complexes [W(CO),CPh,] that stoichiometrically metathesize olefins. Potential chain-terminating side reactions are the unimolecular decomposition of the metallacycle (reductive elimination), forming a cyclopropane plus the reduced metal complex (c of Scheme I), and the bimolecular decomposition of the carbene complex (d of Scheme I). As indicated above, that are two general classes of complexes used to mechanistically investigate olefin metathesis: (1) highvalent (generally do) complexes of Mo, W, and Re (these are generally catalytic and often industrially important) and (2) low-valent complexes of Cr, Mo, and W (oxidation state zero) (these have been significant in determining the general mechanism for the metathesis reaction but are generally not catalytic and have been of little industrial importance). There are significant differences between the highand low-valent systems, particularly in the nature of the metal-CR, bond. Since only the high-valent class of metathesis complexes have been established to be truly catalytic, this paper will deal exclusively with this type. Typical recipes for active homogeneo~s '~ high-valent Mo, W, and Re metathesis catalysts involve a combination of various sources of metal [M(O) through M(VI)] with Lewis acids such as * * M * under a variety of conditions. A result of this diversity is that a detailed understanding of the ligand environment and even the Bantrell Fellow, 198C-1981. Department of Chemistry, Colorado State University, Fort Collins, Colorado 80523. Scheme I