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Mechanistic Insights Into the Methylenation of Ketone by A Article pubs.acs.org/Organometallics Mechanistic Insights into the Methylenation of Ketone by a Trinuclear Rare-Earth-Metal Methylidene Complex † ‡ † † † ‡ Gen Luo, , Yi Luo,*, Jingping Qu, and Zhaomin Hou*, , † State Key Laboratory of Fine Chemicals, School of Pharmaceutical Science and Technology, Dalian University of Technology, Dalian 116024, People’s Republic of China ‡ Advanced Catalysis Research Group, RIKEN Center for Sustainable Resource Science, and Organometallic Chemistry Laboratory, RIKEN, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan *S Supporting Information ABSTRACT: Trinuclear rare-earth-metal methylidene 2− (CH2 ) complexes are an emerging class of compounds that serve as methylidene transfer agents for methylenation of carbonyl compounds. Herein, the reaction of a trinuclear scandium methylidene complex with acetophenone was used as a model reaction of the multimetallic-cooperating methylidene transfer case, and its detailed mechanism has been investigated by the DFT approach. The analyses of Wiberg bond index, electron occupation, the frontier molecular orbital, and natural charge provide us a clear and 2− 2− comprehensive understanding of the CH2 /O group interchange process assisted by cooperating multimetal sites. The mechanism presented here is markedly different from μ conventional Wittig and transition-metal-mediated Wittig-type reactions. In addition, the behavior of 3-CH2 in a multinuclear complex system is also demonstrated. This study not only enriches the chemistry of metal Wittig-type reactions but also sheds light on the intermetallic cooperation for methylidene transfer. ■ INTRODUCTION Chart 1. Homometallic Trinuclear Rare-Earth-Metal Methylidene Complexes The terminal alkene is an essential motif in many natural products, and the methods toward its synthesis have been investigated intensively in the past decades. Some organo- metallic carbene/methylidene complexes, such as the Tebbe μ μ 1,2 reagent Cp2Ti[( 2-CH2)( 2-Cl)AlMe2], have been proved to be powerful methylidene transfer reagents for this purpose.3 Due to the highly nucleophilic character of the unhindered methylidene ligand, a Lewis acid is always needed to stabilize the methylidene group, and mononuclear terminal methylidene complexes are rarely isolated and structurally characterized.4 For example, the isolation and characterization of mononuclear complexes have shown unique reactivity toward carbonyl terminal methylidene complexes of group 4 metals is reported complexes, acting as methylidene transfer reagents, leading to 5 μ just very recently by Mindiola et al., although the methylidene terminal alkenes and rare-earth-metal 3-oxygen complexes (a 6,11−13 complexes have been investigated for decades. In particular, for metal Wittig-type reaction). An understanding of the rare-earth-metal methylidene chemistry, even Lewis acid (such exact reaction mechanism is an essential aspect of chemistry in as alkylaluminum)-stabilized methylidene complexes of the general, which would be helpful for improving the reactivity − rare-earth metals are quite rare,6 10 and no mononuclear and selectivity of the reactions, as well as for designing more ffi terminal rare-earth-metal methylidene complex was reported e cient reagents/reactions. As we know, the mechanism of the hitherto. Alternatively, a multimetal center is effective for conventional Wittig reaction (metal-free reaction) is one of the stabilizing the methylidene group instead of using a Lewis acid. great long-running investigations of organic chemistry, and the Thus, a series of structurally characterized homometallic salt-free Wittig reaction is generally considered to follow a two- trinuclear rare-earth-metal methylidene complexes, which step mechanism, viz., the initial addition and subsequent “ μ ” have a striking structural feature with a Ln3( 3-CH2) (Ln = rare-earth metal) motif, have been documented (Chart Received: November 19, 2014 − 1).6,11 18 Moreover, most of these trinuclear methylidene Published: December 19, 2014 © 2014 American Chemical Society 366 dx.doi.org/10.1021/om501171w | Organometallics 2015, 34, 366−372 Organometallics Article 19 2− 2− elimination (Scheme 1a). The metal Wittig-type carbonyl of the carbonyl methylenation process (the CH2 /O group methylenation by Tebbe reagent (monometal-mediated)3a,b,d interchange) assisted by trinuclear rare-earth-metal methylidene μ complexes and the behavior of the metal-connected 3-CH2 Scheme 1. Several Strategies for Methylenation of Ketones group during the transfer process. This work is the first example of a mechanistic study on multimetal-cooperating methylidene fi (CH2) transfer leading to terminal ole n and demonstrates new insights into Wittig-type chemistry. The results show that the 2− 2− mechanism of the CH2 /O group interchange process in such trinuclear complexes goes through a three-step mecha- nism, in sharp contrast to the known two-step mechanism (Scheme 1a−c). Additionally, the current results provide us a better understanding of the behavior of intermetallic cooper- ation for methylidene (CH ) transfer in such newly arising − 2 trinuclear complexes.6,11 13 ■ COMPUTATIONAL DETAILS Our previous study suggested that the ligand model could significantly affect the energy profile in such a system.15 Thus, the full ligand model was used in this study. Due to the huge molecular size (more than 250 atoms), however, the two-layer ONIOM (TPSSTPSS/GenECP:HF/ LanL2DZ) approach was used in the geometrical optimizations. As shown in Chart 2, the part shown in black represents the inner layer, Chart 2. Division of the ONIOM Layers and gem-dizinc reagent (dimetal-mediated)20 is also proposed to occur via the two-step reaction (Scheme 1b and c). However, the mechanism of the metal Wittig-type reaction mediated by multinuclear complexes has remained unclear to date (Scheme 1d). During our theoretical studies on the multinuclear organo- metallic systems,15,17,21 it has been found that the metal- μ connected terminal methyl ( 1-CH3) is more reactive than the μ μ edge-bridging 2-CH3 and face-capping 3-CH3 groups in a μ trinuclear thulium polymethyl complex, and therefore the 2- μ and the one in red was included in the outer layer. The inner layer was CH3 group tends to change to 1-CH3, being more capable of detaching after acceptance of a hydrogen atom.17 This finding calculated at the higher level. In the higher-level calculations, the TPSSTPSS functional22,23 was applied, the 6-31G(d) basis set was drove us to wonder about the behavior of the metal-connected ff μ used for C, H, N, and O atoms, and the e ective core potentials 3-CH2 methylidene group during the transfer process in (ECP) of Hay and Wadt with double-ζ valence basis set (LanL2DZ)24 homometallic trinuclear complexes. were used for the Sc atoms. The outer layer was involved in lower-level Although a series of homometallic trinuclear rare-earth-metal calculations. In the lower-level calculations, the Hartree−Fock (HF) methylidene complexes and their reactivity as methylidene method was utilized and the LanL2MB basis set was used for all atoms. − transfer agents have been explored experimentally,6,11 18 the Each optimized structure was subsequently analyzed by harmonic related theoretical study is still in its infancy possibly due to the vibration frequencies at the same level of theory for characterization of huge computational consumption and complexities in the a minimum (Nimag = 0) or a transition state (Nimag = 1). The Berny algorithm as implemented in the Gaussian program (keyword “Opt = calculations of multinuclear systems. Herein, the reaction of 3a ” (as a metal Wittig reagent) with acetophenone to give α- TS ) was used for locating transition states. To obtain more reliable μ relative energies, single-point energy calculations were carried out by methylstyrene and trinuclear 3-oxygen complex P (Scheme 13 using pure DFT method (single-layer) on the basis of optimized 2) was used as a model reaction to investigate the mechanism structures. In such single-point calculations, the M06-L functional,25 which often shows better performance in the treatment of transition- Scheme 2. Reaction of 3a with PhMeCO To Form metal systems,26,27 was used together with the CPCM model28 (in 29 PhMeC CH2 and P toluene solution with UFF atomic radii ) for considering the solvation effect, the Stuttgart/Dresden ECP together with associated basis sets30 was used for Sc atoms, and the 6-31G(d,p) was used for the remaining atoms. The free energies in solvation (enthalpies given in parentheses), including corresponding energy corrections obtained from gas-phase calculation, were presented in the computed energy profile. In this paper, the relative free energies in solution are used to analyze the reaction mechanism. Considering the oxygen atom of the carbonyl group may be sensitive to the basis set, a larger basis set, 6- 31+G(d), including polarization and diffusion functions was used for 367 dx.doi.org/10.1021/om501171w | Organometallics 2015, 34, 366−372 Organometallics Article Figure 1. Computed Gibbs free energy profile (kcal/mol, enthalpies given in parentheses) for the reaction of 3a with PhMeCO. All the energies are relative to the energy sum of 3a and PhMeCO. Figure 2. Structures (distances in Å) of optimized stationary points involved in the favorable pathway for the reaction of 3a with PhMeCO. All PhC[NC6H4(iPr-2,6)2]2 ligands and the H atoms of methyl groups are omitted for clarity. the oxygen atom to test the basis set effect on the energy profile and respectively. As shown in Figure 1, the reaction starts with the the structures (see more details in the Supporting Information). The coordination of the oxygen atom of PhMeCO to the Sc1 results shown in Figure 1 (vide infra) and Figures S3 and S4 suggest center of 3a to form complex B. Due to the coordination of that, in the current system, the larger basis set of the oxygen atom has − no significant effect on both the energy profile and the structures PhMeC O, the Sc1 C1 bond in complex B became weaker calculated.
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