NMR Spectroscopic Characterization and DFT Calculations of Zirconium(IV)-3,3′-Br<sub>2</sub>–BINOLate and Related Complexes Used in an Enantioselective Friedel–Crafts Alkylation of Indoles with α,β-Unsaturated Ketones
作者:Gonzalo Blay、Joan Cano、Luz Cardona、Isabel Fernández、M. Carmen Muñoz、José R. Pedro、Carlos Vila
DOI:10.1021/jo3013594
日期:2012.12.7
Experimental and theoretical studies on the structure of several complexes based on (R)-3,3′-Br2–BINOL ligand and group (IV) metals used as catalysts in an enantioselectiveFriedel–Craftsalkylation of indoles with α,β-unsaturated ketones have been carried out. NMR spectroscopic studies of these catalysts have been performed, which suggested that at room temperature the catalysts would form a monomeric
基于(R)-3,3'-Br 2 -BINOL配体和第(IV)族金属的几种配合物结构的实验和理论研究,用于对映体的α,β-不饱和吲哚的对映选择性Friedel-Crafts烷基化反应酮已被进行。已经对这些催化剂进行了NMR光谱研究,这表明在室温下,对于Ti IV,催化剂将形成单体结构,对于Zr IV和Hf IV,催化剂将形成二聚结构。密度泛函理论(DFT)的计算清楚地证实了这些实验光谱学研究的结论。具有双桥基序的二聚体结构[Zr IV 2(μ-(R)-3,3'-Br 2 -BINOL)2 ],其中每个联萘酚配体充当金属中心之间的桥(Novak模型)比带有双桥基序的二聚体结构[Zr IV 2(μ-O )更稳定t Bu)2 ],其中叔丁醇基团起桥连配体的作用(小林模型)。研究了Friedel-Crafts烷基化的吲哚结构。最后,提出了Friedel-Crafts反应的合理机理和催化剂作用方式的立
Enantioselective Friedel−Crafts Alkylations Catalyzed by Bis(oxazolinyl)pyridine−Scandium(III) Triflate Complexes
作者:David A. Evans、Keith R. Fandrick、Hyun-Ji Song、Karl A. Scheidt、Risheng Xu
DOI:10.1021/ja072976i
日期:2007.8.1
The enantioselective Friedel-Crafts addition of a variety of indoles catalyzed by bis(oxazolinyl)pyridine-scandium(III) triflate complexes (Sc(III)-pybox) was accomplished utilizing a series of beta-substituted alpha,beta-unsaturated phosphonates and alpha,beta-unsaturated 2-acyl imidazoles. The acyl phosphonate products were efficiently transformed into esters and amides, whereas the acyl imidazole adducts were converted to a broader spectrum of functionalities such as esters, amides, carboxylic acids, ketones, and aldehydes. The sense of stereoinduction and level of enantioselectivity were found to be functions of the size of the substrate employed, the substitution on the ligand, and the catalyst loading. Molecular modeling of the catalyst with the bound substrates was performed based on the crystal structures of the catalyst complexes and the sense of stereoinduction observed in the addition reaction. Nonlinear effects over a range of catalyst concentrations implicate a mononuclear complex as the active catalyst.
Can Simple Enones Be Useful Partners for the Catalytic Stereoselective Alkylation of Indoles?
A new catalytic system for the first example of enantioselective Friedel-Crafts-type (FC) addition of indoles to simple enones is described. The use of an equimolar amount of chiral [AI(salen)Cl] and 2,6-lutidine (10 mol %) was found to be effective in promoting the conjugate addition of indoles to (E)-arylcrotyl ketones, furnishing the corresponding beta-indolyl ketones in excellent yield and high enantioselectivity (ee up to 89%). The role of the base was investigated through spectroscopic as well as computational analyses, which suggested that in situ formation of a new chiral (base(.)[Al(salen)]) complex was operating under our reaction conditions. In particular, a stable cationic [Al(salen)] hexacoordinate trans complex with the additive base and the enone is suggested as being responsible for the stereocontrolled reaction. Finally, detailed monitoring of the reaction course was carried out showing that a conventional FC pathway induced by [Al(salen)Cl] acting as a Lewis acid is operating.