A Combined Theoretical and Experimental Study of Efficient and Fast Titanocene-Catalyzed 3-exo Cyclizations
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- 作者：Joachim Friedrich ; Michael Dolg ; Andreas Gansä ; uer ; Daniel Geich-Gimbel ; and Thorsten Lauterbach
- 刊名：Journal of the American Chemical Society
- 出版年：2005
- 出版时间：May 18, 2005
- 年：2005
- 卷：127
- 期：19
- 页码：7071 - 7077
- 全文大小：188K
- 年卷期：v.127,no.19(May 18, 2005)
- ISSN：1520-5126
文摘
The mechanism of titanocene mediated 3-exo cyclizations was investigated by a combinedtheoretical and experimental study. A gradient corrected density functional theory (DFT) method has beenscaled against titanocene dichloride, the parent butenyl radical, and in bond dissociation energy (BDE)calculations. The BP86 method using density fitting, and a basis set of triple-
quality emerged as a highlyreliable tool for studying titanocene mediated radical reactions. The computational results revealed importantkinetic and thermodynamic features of cyclopropane formation. Surprisingly, the 
-titanoxy radicals, thefirst intermediates of our investigations, were demonstrated to possess essentially the same thermodynamicstabilization as the corresponding alkyl radicals by comparison of the calculated BDEs. In contrast tosuggestions for samarium mediated reactions, the cyclization was shown to be thermodynamically favorablein agreement with earlier kinetic studies. It was established that stereoselectivity of the cyclization is governedby the stability of the intermediates and thus the trans disubstituted products are formed preferentially.The observed ratios of products are in good to excellent agreement with the DFT results. By a combinationof computational and experimental results, it was also shown that for the completion of the overallcyclopropane formation the efficiency of the trapping of the cyclopropylcarbinyl radicals is decisive.
quality emerged as a highlyreliable tool for studying titanocene mediated radical reactions. The computational results revealed importantkinetic and thermodynamic features of cyclopropane formation. Surprisingly, the -titanoxy radicals, thefirst intermediates of our investigations, were demonstrated to possess essentially the same thermodynamicstabilization as the corresponding alkyl radicals by comparison of the calculated BDEs. In contrast tosuggestions for samarium mediated reactions, the cyclization was shown to be thermodynamically favorablein agreement with earlier kinetic studies. It was established that stereoselectivity of the cyclization is governedby the stability of the intermediates and thus the trans disubstituted products are formed preferentially.The observed ratios of products are in good to excellent agreement with the DFT results. By a combinationof computational and experimental results, it was also shown that for the completion of the overallcyclopropane formation the efficiency of the trapping of the cyclopropylcarbinyl radicals is decisive.