Transition Metal鈭扖arboryne Complexes: Synthesis, Bonding, and Reactivity
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- 作者：Zaozao Qiu ; Shikuo Ren ; Zuowei Xie
- 刊名：Accounts of Chemical Research
- 出版年：2011
- 出版时间：April 19, 2011
- 年：2011
- 卷：44
- 期：4
- 页码：299-309
- 全文大小：1239K
- 年卷期：v.44,no.4(April 19, 2011)
- ISSN：1520-4898
文摘
The construction and transformation of metal鈭抍arbon (M鈭扖) bonds constitute the central themes of organometallic chemistry. Most of the work in this field has focused on traditional M鈭扖 bonds involving tetravalent carbon: relatively little attention has been paid to the chemistry of nontraditional metal鈭抍arbon (M鈭扖cage) bonds, such as carborane cages, in which the carbon is hypervalent. We therefore initiated a research program to study the chemistry of these nontraditional M鈭扖cage bonds, with a view toward developing synthetic methodologies for functional carborane derivatives. In this Account, we describe our results in constructing and elucidating the chemistry of transition metal鈭抍arboryne complexes.
Our work has shown that the M鈭扖cage bonds in transition metal鈭抍arboranyl complexes are generally inert toward electrophiles, and hence significantly different from traditional M鈭扖 bonds. This lack of reactivity can be ascribed to steric effects resulting from the carboranyl moiety. To overcome this steric problem and to activate the nontraditional M鈭扖cage bonds, we prepared a series of group 4 and group 10 transition metal鈭抍arboryne complexes (where carboryne is 1,2-dehydro-o-carborane), because the formation of metallacyclopropane opens up the coordination sphere and creates ring strain, facilitating the reactions of M鈭扖cage bonds with electrophiles. Structural and theoretical studies on metal鈭抍arboryne complexes suggest that the bonding interaction between the metal atom and the carboryne unit is best described as a resonance hybrid of the M鈭扖 蟽 and M鈭扖 蟺 bonds, similar to that observed in metal鈭抌enzyne complexes. The nickel鈭抍arboryne complex (畏2-C2B10H10)Ni(PPh3)2 can (i) undergo regioselective [2 + 2 + 2] cycloaddition reactions with 2 equiv of alkyne to afford benzocarboranes, (ii) react with 1 equiv of alkene to generate alkenylcarborane coupling products, and (iii) also undergo a three-component [2 + 2 + 2] cyclotrimerization with 1 equiv of activated alkene and 1 equiv of alkyne to give dihydrobenzocarboranes.
The reaction of carboryne with alkynes is also catalyzed by Ni species. Subsequently, a Pd/Ni co-catalyzed [2 + 2 + 2] cycloaddition reaction of 1,3-dehydro-o-carborane with 2 equiv of alkyne was developed, leading to the efficient formation of C,B-substituted benzocarboranes in a single process. In contrast, the zirconium鈭抍arboryne species, generated in situ from Cp2Zr(渭-Cl)(渭-C2B10H10)Li(OEt2)2, reacts with only 1 equiv of alkyne or polar unsaturated organic substrates (such as carbodiimides, nitriles, and azides) to give monoinsertion metallacycles, even in the presence of excess substrates. The resultant five-membered zirconacyclopentenes, incorporating a carboranyl unit, are an important class of intermediates for the synthesis of a variety of functionalized carboranes. Transmetalation of zirconacyclopentenes with other metals, such as Ni and Cu, was also found to be a very useful tool for various chemical transformations.
Studies of metal鈭抍arboryne complexes remain a relatively young research area, particularly in comparison to the rich literature of metal鈭抌enzyne complexes. Other transition metal鈭抍arborynes are expected to be prepared and structurally characterized as the field progresses, and the results detailed here will further that effort by providing easy access to a wide range of functionalized carborane derivatives.
Our work has shown that the M鈭扖cage bonds in transition metal鈭抍arboranyl complexes are generally inert toward electrophiles, and hence significantly different from traditional M鈭扖 bonds. This lack of reactivity can be ascribed to steric effects resulting from the carboranyl moiety. To overcome this steric problem and to activate the nontraditional M鈭扖cage bonds, we prepared a series of group 4 and group 10 transition metal鈭抍arboryne complexes (where carboryne is 1,2-dehydro-o-carborane), because the formation of metallacyclopropane opens up the coordination sphere and creates ring strain, facilitating the reactions of M鈭扖cage bonds with electrophiles. Structural and theoretical studies on metal鈭抍arboryne complexes suggest that the bonding interaction between the metal atom and the carboryne unit is best described as a resonance hybrid of the M鈭扖 蟽 and M鈭扖 蟺 bonds, similar to that observed in metal鈭抌enzyne complexes. The nickel鈭抍arboryne complex (畏2-C2B10H10)Ni(PPh3)2 can (i) undergo regioselective [2 + 2 + 2] cycloaddition reactions with 2 equiv of alkyne to afford benzocarboranes, (ii) react with 1 equiv of alkene to generate alkenylcarborane coupling products, and (iii) also undergo a three-component [2 + 2 + 2] cyclotrimerization with 1 equiv of activated alkene and 1 equiv of alkyne to give dihydrobenzocarboranes.
The reaction of carboryne with alkynes is also catalyzed by Ni species. Subsequently, a Pd/Ni co-catalyzed [2 + 2 + 2] cycloaddition reaction of 1,3-dehydro-o-carborane with 2 equiv of alkyne was developed, leading to the efficient formation of C,B-substituted benzocarboranes in a single process. In contrast, the zirconium鈭抍arboryne species, generated in situ from Cp2Zr(渭-Cl)(渭-C2B10H10)Li(OEt2)2, reacts with only 1 equiv of alkyne or polar unsaturated organic substrates (such as carbodiimides, nitriles, and azides) to give monoinsertion metallacycles, even in the presence of excess substrates. The resultant five-membered zirconacyclopentenes, incorporating a carboranyl unit, are an important class of intermediates for the synthesis of a variety of functionalized carboranes. Transmetalation of zirconacyclopentenes with other metals, such as Ni and Cu, was also found to be a very useful tool for various chemical transformations.
Studies of metal鈭抍arboryne complexes remain a relatively young research area, particularly in comparison to the rich literature of metal鈭抌enzyne complexes. Other transition metal鈭抍arborynes are expected to be prepared and structurally characterized as the field progresses, and the results detailed here will further that effort by providing easy access to a wide range of functionalized carborane derivatives.