Theoretical Study Of The Mechanism Of Aluminum Carbenoid Promoted Cyclopropanation Reactions With Ethylene

Cyclopropane-containing molecules are found in a wide range natural and unnatural compounds that display important biological activities and in many substances used as starting materials and intermediates in organic synthesis. Since the reported use of diiodomethane and a Zn/Cu couple to react with olefins to form cyclopropane units by Simmons and Smith in 1985, much research has been done to develop improvements or alternative techniques to form active reagents similar to the Simmons-Smith reagent that can make cyclopropane-containing products from olefins with high efficiency and stereoselectivity.To our knowledge, there have been only a few theoretical computational studies of the cyclopropanation reactions for the carbenoid species. In this paper, we chose several typical reactions that have been carefully studied using quantum methods, obtained some interesting results. On the basis of the molecular orbital theory, the tradition transition state theory as well as quantum chemistry theory, the systems chose have been investigated using Density Functional Theory (DFT), the Moller-Plesset correlation energy correction MPn, the polarized continuum model (PCM) and the natural bond orbital (NBO). The structures of the reagents, the reaction products and transition states along the reaction paths have been obtained, then obtained the reaction surfaces, the spectrum datum, thermodynamic datum as well as the information of orbitals. The reaction mechanism has been argued deeply using these data.The whole paper consists of five chapters. Chapter 1 mainly reviews the evolution of cyclopropanation reaction of ethylene with metal carbenoid. The second chapter summarizes the theory of quantum chemistry and calculation methods of this paper. The contents of two chapters were the basis and background of our studies and offer us with userful and reliable quantum methods. In chapter 3, Density functional theory calculations are reported for the cyclopropanation reactions of selected aluminum carbenoids with ethylene for two reaction channels: methylene transfer and carbometalation. The aluminum carbenoids react with ethylene via an asynchronous attack on one CH₂ group of ethylene with a relatively high barrier (11-15 kcal/mol). In contrast, the reaction barriers for cyclopropanation via the carbometalation are much higher (about 30 kcal/mol). These computational results are in good agreement with experimental results and this suggests that the methylene transfer process is favored and the competition from the carbometalation pathway is negligible. The (CH₃)₂AlCH₂Cl carbenoid (reaction barrier of 11.3 kcal/mol) is found to be the most reactive carbenoid in the (CH₃)₂AlCH₂X (X=Cl, Br, I) series of carbenoids and the (CH₃)₂AlCH₂I carbenoid is the least reactive one. The present computational results are briefly compared with previously reported results for related lithium, samarium and zinc carbenoids.Chapter 4, the cyclopropanation reaction of ethene with aluminum carbenoid has been studied by means of the B3LYP hybrid density functional method. The reaction goes through two pathways: methylene transfer and carbometalation. In methylene transfer pathway, a quantum-chemical investigation shows that the reactions of the carbenoids X₂AlCH)2X 1-X, (X =F, Cl, Br, I) with ethene 2 to cyclopropane 3+AlX₃ profit from a weakening of the C-X bonds by the C-Al bonds in the carbenoids 1-X and in the complex [1-X*2]. The C-F bond is more affected than the C-I bond. Since in the transition states ³[1-X*2]^? AlHal is strongly decomplexed, the cleavage of the C-Hal bond is essential compensated by the formation of the Al-Hal bonds, which leads to almost equal transition state energy for the reactions of 1-X with 2. In contrast with methylene transfer, the cyclopropanation reaction of the carbometalation pathway profit from a weakening of the C-Al bonds by the C-X bonds.Chpter 5, mechanisms for the cyclopropanation reactions of Al carbenoid with an allylic alcohol have been characterized in detail using density functional theory. The allylic alcohol reaction was modeled with intermolecular reaction and intramolecular reaction (alkoxide complex ICH₂AlOCH₂CH=C(CH₃)₂ formed from the Al reagent and allylic alcohol). Two modes of acceleration were found. The first involves the methylene transfer and carbometalation. The second involves the well-accepted mechanism of 1, 2-iodine migration. The former was found to be more facile than the latter.