Date: Wed, 6 Oct 1999 05:14:43 -0500 (CDT) Reply-To: CH412F99-L@po.missouri.edu Sender: owner-CH412F99-L@po.missouri.edu Precedence: bulk From: Zhengyu Wu
To: CH412F99-L@po.missouri.edu Subject: Paper of the week MIME-Version: 1.0 X-Sender: email@example.com Hello, everyone, Here you will find the introduction of the paper for this week. The attachment is the Word file. I would like to hear the comments from you and answer your questions as much as I can. Best. Martin Paper of the week 4 The paper is from J.Phys.Chem. B 1999, 103, 7145-7150. The name of the paper is "Competition between Wolff Rearrangement and 1,2-Hydrogen Shift in b-Oxy-a-ketocarbenes: Electrostatic and Specific Solvent Effects". The first author is Calvo-Losada from the Departamento de Quimica Fisica. Stereoselective Wolff rearrangement of b-Oxy-a-ketocarbenes gives 1-alkyl-2-oxy synthons. This is a useful pathway for the synthesis of some antibiotic macrolides. 1,2-hydrogen shift reaction is a competitive pathway in this reaction process and it becomes dominant in the case of b-hydroxyl-a-ketocarbenes. One can get 100% yield of the vinyl-ketone product, which means the 1,2-hydrogen shift is 100% selective in this case. The authors are interested in understanding the reason for this fact and thus pursued a theoretical analysis to find the factors governing the competition between Wolff rearrangement and 1,2-hydrogen shift processes. The analysis is concentrated on the calculation of the activation barriers of the two reaction processes (W-rearrangement and 1,2-H shift) because the reactions and percent yields are kinetic-controlled. The quantum mechanical calculations were carried out using the Gaussian 94 program (which is what we will use in this course! :-) at B3LYP and HF levels with 6-31G* and 6-31G** basis sets. Firstly, the activation Gibbs energies of the two reaction processes in gas-phase were calculated. The structures of RKC, TSH and TSW ( ketocarbene, transition state of 1,2-H shift, transition state of Wolff rearrangement respectively) were optimized, too. From the optimized structures, one can tell easily that the reason that 1,2-H shift is preferred is because of the H-bond between the hydroxyl group hydrogen and the carbonyl oxygen which keep the rotation of carbonyl group which is required in Wolff rearrangement process. A 96% yield of vinyl-ketone product was calculated from the Gibbs energy difference, which is 1.47kcal/mol. Secondly, the authors tried to reproduce the experimental result (100% yield for 1,2-H shift) in solution by adding the pure electrostatic effect of water as solvent. The result is 60% yield, which means this model failed. Then they came up with the association complexes model, which means at this time they did not consider only the pure electrostatic effect of water, but the association process of the each solute molecule with several (here it is 3) water molecules as well. To find the reasonable association complex, they first calculate the energies of various complexes between one water molecule and the ketocarben intermediate to find the most important so-called "specific water-ketocarbene interaction sites". Then they built the association complex model with one ketocarbene molecule and three water molecules because they found three important interaction sites. Following this step is are the optimizations of the RKC, TSH and TSW structures and their energy calculations. The most interesting result is an extra H-bonding stabilization in TSH. The percent selectivity of 1,2-H shift is 100% according to the energy calculation of this model. By comparing the structures and energy values between the association model and SCRF calculation, the authors came up with their conclusion that specific H-bond interactions among the keocarbene and the solvent molecules. Finally, the contribution of different energy components to the relative solvent-solute ineraction energies in the association complexes was examined by means of the generalized molecular interaction potential with polarization (GMIPp) method. From the calculation results, we can find that he electrostatic effect plays the most important role in lowering the activation barrier of 1,2-H shift, while the van der Waals interaction favors Wolff rearrangement. One can certainly expect a 100% selectivity of the Wolff rearrangement in a non-polar solvent. The reason I chose this paper is that the idea of the author is clear and straightforward. The calculation process is systematic and they used the potential energy surface knowledge that we learned recently. It is easy to understand why they performed optimizations and energy calculations. The data analysis also makes sense. It is a good example on how to solve a problem from the very beginning. We may not be very clear on the exact meaning of different calculation models. But it is not the point of the paper and should not affect our understanding of this paper. We still do not know how to choose proper calculation methods for specific cases. I believe we will learn it later this semester.