Ligand Effects on Rates and Regioselectivities of Rh(I)-Catalyzed (5 + 2) Cycloadditions: A Computational Study of Cyclooctadiene and Dinaphthocyclooctatetraene as Ligands

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• 作者：Xiufang Xu ; Peng Liu ; Adam Lesser ; Lauren E. Sirois ; Paul A. Wender ; K. N. Houk
• 刊名：The Journal of the American Chemical Society
• 出版年：2012
• 出版时间：July 4, 2012
• 年：2012
• 卷：134
• 期：26
• 页码：11012-11025
• 全文大小：803K
• 年卷期：v.134,no.26(July 4, 2012)
• ISSN：1520-5126

文摘

The first theoretical study on the effects of ligands on the mechanism, reactivities, and regioselectivities of Rh(I)-catalyzed (5 + 2) cycloadditions of vinylcyclopropanes (VCPs) and alkynes has been performed using density functional theory (DFT) calculations. Highly efficient and selective intermolecular (5 + 2) cycloadditions of VCPs and alkynes have been achieved recently using two novel rhodium catalysts, [Rh(dnCOT)]⁺SbF₆^{鈥?/sup> and [Rh(COD)]+SbF₆鈥?/sup>, which provide superior reactivities and regioselectivities relative to that of the previously reported [Rh(CO)₂Cl]₂ catalyst. Computationally, the high reactivities of the dnCOT and COD ligands are attributed to the steric repulsions that destabilize the Rh-product complex, the catalyst resting state in the catalytic cycle. The regioselectivities of reactions with various alkynes and different Rh catalysts are investigated, and a predictive model is provided that describes substrate鈥搒ubstrate and ligand鈥搒ubstrate steric repulsions, electronic effects, and noncovalent 蟺/蟺 and C鈥揌/蟺 interactions. In the reactions with dnCOT or COD ligands, the first new C鈥揅 bond is formed proximal to the bulky substituent on the alkyne to avoid ligand鈥搒ubstrate steric repulsions. This regioselectivity is reversed either by employing the smaller [Rh(CO)₂Cl]₂ catalyst to diminish the ligand鈥搒ubstrate repulsions or by using aryl alkynes, for which the ligand鈥搒ubstrate interactions become stabilizing due to 蟺/蟺 and C鈥揌/蟺 dispersion interactions. Electron-withdrawing groups on the alkyne prefer to be proximal to the first new C鈥揅 bond to maximize metal鈥搒ubstrate back-bonding interactions. These steric, electronic, and dispersion effects can all be utilized in designing new ligands to provide regiochemical control over product formation with high selectivities. The computational studies reveal the potential of employing the dnCOT family of ligands to achieve unique regiochemical control due to the steric influences and dispersion interactions associated with the rigid aryl substituents on the ligand.}