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Mechanistic Insights into the Methylenation of 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 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 /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 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 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 PhMeCO 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 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 PhMeCO. All the energies are relative to the energy sum of 3a and PhMeCO.

Figure 2. Structures (distances in Å) of optimized stationary points involved in the favorable pathway for the reaction of 3a with PhMeCO. 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 PhMeCO 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. All calculations were performed with the Gaussian 09 in comparison with 3a, as suggested by the bond length (2.30 Å 31 software package. in 3a and 2.44 Å in B, Figure 2) as well as the decrease of Wiberg bond indexes (WBI of 0.481 in 3a and 0.363 in B, ■ RESULTS AND DISCUSSION Table 1). Complex B subsequently undergoes a nucleophilic The energy profile for the reaction of 3a with PhMeCO and addition reaction via a transition state, TSBC, leading to an their corresponding structures are shown in Figures 1 and 2, intermediate C. This process with a free energy barrier of 14.0

368 dx.doi.org/10.1021/om501171w | Organometallics 2015, 34, 366−372 Organometallics Article

a Table 1. Selected Wiberg Bond Indexes (WBIs) for the Stationary Points Involved in the Reaction Pathway

entry bond 3a B TSBC CTSCD DTSDE ETSEP P 1 Sc1−Sc2 0.179 0.179 0.174 0.165 0.170 0.222 0.215 0.197 0.199 0.193 2 Sc1−Sc3 0.225 0.180 0.190 0.163 0.146 0.147 0.148 0.167 0.180 0.194 3 Sc2−Sc3 0.225 0.241 0.231 0.212 0.223 0.216 0.202 0.161 0.172 0.194 4 Sc1−C1 0.481 0.363 0.344 0.120 0.055 0.059 0.055 0.035 0.035 5 Sc2−C1 0.499 0.602 0.528 0.359 0.339 0.277 0.203 0.034 0.034 6 Sc3−C1 0.735 0.647 0.622 0.364 0.366 0.405 0.442 0.517 0.398 7 Sc1−C3 0.427 0.414 0.358 0.334 0.340 0.419 0.412 0.388 0.384 0.392 8 Sc2−C3 0.400 0.406 0.467 0.458 0.405 0.338 0.334 0.331 0.320 0.383 9 Sc1−C4 0.423 0.403 0.371 0.325 0.326 0.334 0.338 0.355 0.339 0.384 10 Sc3−C4 0.437 0.411 0.442 0.463 0.432 0.426 0.416 0.409 0.425 0.393 11 Sc2−C5 0.431 0.438 0.447 0.425 0.391 0.364 0.366 0.369 0.329 0.392 12 Sc3−C5 0.422 0.421 0.406 0.385 0.393 0.420 0.403 0.383 0.422 0.385 13 Sc1−C6 0.299 0.227 0.241 0.193 0.213 0.275 0.281 0.290 0.281 0.272 14 Sc2−C6 0.295 0.347 0.348 0.355 0.331 0.298 0.284 0.262 0.246 0.273 15 Sc3−C6 0.319 0.338 0.319 0.329 0.336 0.322 0.311 0.257 0.278 0.275 16 Sc1−O 0.342 0.333 0.649 0.624 0.388 0.391 0.352 0.412 0.507 17 Sc2−O 0.012 0.018 0.031 0.098 0.363 0.349 0.316 0.363 0.507 18 Sc3−O 0.011 0.015 0.027 0.050 0.030 0.038 0.153 0.239 0.512 19 C2−O 1.758b 1.509 1.422 0.904 0.868 0.855 0.842 0.758 0.463 20 C2−C1 0.060 0.148 0.955 0.972 0.985 0.991 1.039 1.253 1.856c aAtom labeling is defined in Figure 1. bThe value is obtained from the CO bond of isolated PhMeCO. cThe value is obtained from the CC  bond of isolated PhMeC CH2. kcal/mol involves three events (C2O double-bond addition Wittig reaction or mono- and dimetalllic methylenation yielding a C2−O single bond, C1−Sc1 bond cleavage, and C1− reagents (vide infra). Interestingly, it is noteworthy that, during C2 single-bond formation) and is exothermic by 10.4 kcal/mol. the reaction, the change in coordination manner of the oxygen μ → μ → μ In contrast to the four-membered-ring intermediate involved in atom follows the order 1 2 3 and that of the CH2 μ → μ → μ the conventional Wittig reaction and metal-mediated carbonyl group (leaving group) follows a reverse trend, 3 2 1. − methylenation (Scheme 1a c), complex C shows a bridging This is similar to the CS2 activation by a trinuclear structure supported by three metal centers and could not phosphinidene cluster, during which the change of coordination fi μ → μ → μ directly give the ole nation product via elimination. Similar to manner of the sulfur atom follows the order 1 2 3, and μ → the activation of CS2 by a trinuclear rare-earth-metal the PPh group (leaving ligand) follows the reverse trend, 3 15 μ → μ 15 μ phosphinidene complex, the O atom in C would further 2 1. Additionally, the behavior of -CH2 in the current μ μ μ change its coordination mode from 1-to 2-form. Due to the system and our previous studies on the detaching group ( - 15 μ 17 asymmetrical of C2 (methyl and phenyl), there are PPh and -CH3 ) for the trinuclear clusters give us a general two possible pathways for the change of coordination mode of sense that the detaching groups first change their coordination − μ − the O atom, viz., from the phenyl side to form a Sc1 ( 2-O) manner to terminal form, being more capable of detaching from − μ − Sc2 frame and from the methyl side to form a Sc1 ( 2-O) the metal center of the clusters. Sc3 frame. Both possibilities were calculated, and the energy To assess the possibility of the methylation of the carbonyl fi pro le (Figure 1) indicates that the former case (via TSCD)is functionality in the current system, the transfer ′  more favorable than the latter (via TS CD) both kinetically and event in the reaction of 3a with PhMeC O has also been energetically. The favorable pathway overcomes only ΔG⧧ = investigated. In complex 3a, there are two kinds of bridged μ 4.9 kcal/mol (TSCD), and the corresponding product D with a methyl group, viz., 2-CH3 (such as the C4 methyl group) and μ μ 2-O moiety is more stable than C by 6.6 kcal/mol. Then, the 3-CH3 (C6 methyl group). Both possibilities were calculated, − cleavage of Sc2 C1 in D occurs via a transition state TSDE, and the transition states of TS1 (C4 group transfer) and TS2 with ΔG⧧ = 0.4 kcal/mol, leading to more stable complex E (C6 group transfer) and their corresponding relative energies with a single-metal-bound CH2R (like a terminal form) group are shown in Figure 3. The relative free energies of TS1 (50.0 for detaching. The subsequent elimination occurs via a kcal/mol) and TS2 (45.7 kcal/mol) are significantly higher in multimetal-assisted four-center transition state, TSEP, leading comparison with TSBC (14.0 kcal/mol, Figure 1), suggesting fi α μ to the nal product -methylstyrene and trinuclear 3-oxygen that methyl group transfer in such a methylidene complex complex P, which was structurally identified experimentally.13 seems unlikely. Therefore, methylidene transfer rather than Such an elimination process, involving the formation of C1 methyl transfer occurs to achieve methylenation of the carbonyl C2 and Sc3−O bonds and the cleavage of C2−O and Sc3−C1 functionality, which is consistent with the experimental bonds simultaneously, has an energy barrier of 3.6 kcal/mol and observation.13 is significant exothermic by 40.6 kcal/mol. From the point of To get more information on the bonding during the reaction, view of the whole reaction, the energy barriers of all steps are selected Wiberg bond indexes (WBIs) are listed in Table 1. As less than 15 kcal/mol and the whole reaction is significantly shown in this table, the WBIs of Sc−Sc bonds are 0.146−0.241 exergonic, ca. 67 kcal/mol. The mechanism proposed here (entries 1−3), suggesting that the three metal centers retain (multinuclear-cooperating Wittig-type reaction) is more bond interactions between each other during the whole complicated than carbonyl methylenation by conventional reaction. The WBIs also clearly display that the interaction of

369 dx.doi.org/10.1021/om501171w | Organometallics 2015, 34, 366−372 Organometallics Article

Figure 3. Two possible methyl transfer transition states for the reaction of 3a with PhMeCO. The energies (kcal/mol) are relative to the energy sum of 3a and PhMeCO. In structures of TS1 and TS2 (distances in Å), all PhC[NC6H4(iPr-2,6)2]2 ligands (L) and the H atoms of methyl groups are omitted for clarity.

Sci−C1 (i =1−3) becomes weaker and the interaction of Sci− Figure 4. Changes of (a) Wiberg bond indexes (WBIs) and (b) the O(i =1−3) gets stronger along with the reaction coordinates electron occupations (EOs) for C2−O and C2−C1 bonds along the (entries 4−6, 16−18). The WBIs of other Sc−Ci (i =3−6) reaction pathway. bonds have no significant changes (entries 7−15). It is noteworthy that, during the whole reaction, two essential is computed to be 3.91. These results could give us an intuitive 2− 2− events are involved in the change of the chemical bonds. One is understanding of the CH2 /O interchange event assisted by the cleavage of the CO of PhMeCO, and the cooperating multimetal sites. other is the formation of new CC double bond of PhMeC As aforementioned, the cleavage of the CO bond and  CH2. Along with the reaction pathway, the changes of the WBIs formation of the C C bond mainly occur in stages (a) and (also see entries 19 and 20 in Table 1) and the electron (c). Thus, the two corresponding transition states TSBC and ··· ··· 2− 2− occupations (EOs) of C2 O and C2 C1 are illustrated in TSEP play an important role in the CH2 /O group Figure 4. Interestingly, the changes of WBIs in Figure 4a interchange process. To get more detailed information on obviously demonstrate that the whole reaction occurs via three these two important steps, the frontier molecular orbitals of  − stages: (a) weakening C2 OtoaC2 O single bond and TSBC and TSEP are analyzed. As shown in Figure 5, the HOMO − formation of a C2 C1 single bond (nucleophilic addition, from of TSBC is mainly contributed by the 2p orbital of the C1 atom 3a to C), (b) intramolecular isomerization retaining single- (51.2%). In TSBC, the molecular orbital shows that the C1 atom bond characters of C2−O and C2−C1 bonds (from C to E), attacks the C2 center assisted by cooperation of three metal and (c) cleavage of a C2−O single bond and formation of a centers (3d orbitals of metals) to form a C1−C2 σ-bond. In  fi − C2 C1 double bond (ole nation elimination, from E to P). TSEP, the HOMO 1 is also mainly contributed by the 2p ff − This result is markedly di erent from the cases of previous orbital of the C1 atom (45.4%). The HOMO 1ofTSEP shows carbonyl methylenation, which is generally considered as a two- that the formation of the C1C2 double bond is achieved by σ − 2 step reaction, viz., the initial addition and subsequent breaking the -bond of Sc3 C1 (2p orbital of C1 and 3dz 2− 2− π  − π elimination step. In the current system, the CH2 /O orbital of Sc3) and forming the -bond of C1 C2 (p p - group interchange process (the bond-breaking and -forming orbital of C1 and C2). These frontier molecular orbitals of the of C2O and C2C1 bonds) mainly occurs in stage (a) and two transition states clearly present the process of bond- (c). During stage (b), with the assistance of the cooperating breaking and -forming of the C2O and C2C1 double multimetal sites,15,17 the complex undergoes intramolecular bonds. − isomerization for further bond interchange. The change of It is known that the CH2 group in metal carbene complexes bonds can be further corroborated by the value of EOs of the often acts as a nucleophilic group, and it could react with intermediates during the reaction. As shown in Figure 4b, at the electrophilic substrates. In the current reaction, the CH2 of beginning of the reaction, the EO of the PhMeC2O double trinuclear scandium methylidene complex 3a (as a nucleophilic bond is computed to be 3.97. In the middle stage, the EOs of group) first attacks PhMeCO (as an electrophilic substrate) both C2−O and C2−C1 bonds are computed to be ca. 1.95− to form a C−C single bond (Figure 4) and finally achieves the − − 2− 2− 1.97, suggesting single bonds of C2 O and C2 C1. In the CH2 /O group interchange process. However, the detailed product of PhMeC2C1, the EO of the C2C1 double bond behavior of charge transfer is unclear. To further clarify the

370 dx.doi.org/10.1021/om501171w | Organometallics 2015, 34, 366−372 Organometallics Article ■ CONCLUSION In summary, 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 result indicates that such an event assisted by cooperating multimetal sites is mechanistically different from carbonyl methylenation by the conventional Wittig reaction or other methylidene reagents. The analysis of Wiberg bond indexes and the electron occupations clearly indicates that the reaction occurs via three stages, viz., (a) nucleophilic addition, (b) intramolecular isomerization, and (c) olefination elimi- 2− 2− nation. The CH2 /O group interchange process (the bond- breaking and -forming of CO and CC bonds) mainly occurs at stages (a) and (c). During stage (b), the complex undergoes intramolecular isomerization with the assistance of cooperating multimetal sites in preparation for further bond interchange. The frontier molecular orbital analysis of the two − key transition states gives us a better understanding of bond- Figure 5. HOMO of TSBC and HOMO 1ofTSEP (left, isovalue = breaking and -forming processes. The natural charges clearly 0.03) and their schematic representations (right). Atomic orbital display that the electron transfer occurs at stages (a) and (c), contributions are reported in parentheses. All hydrogen atoms were omitted for clarity. The atom labeling is the same as in Figure 1. and no electron transfer occurs in middle stage (b), in sharp contrast to other carbonyl olefination reactions. This study not charge transfer process, the change of the natural charges of the only enriches the chemistry of metal Wittig-type reactions but also sheds light on the intermetallic cooperation for C1 atom (nucleophilic center) and C2 atom (electrophilic fi center) along with the reaction coordinates is illustrated in methylidene (CH2) transfer leading to a terminal ole n. Figure 6. Interestingly, the change of natural charges also ■ ASSOCIATED CONTENT *S Supporting Information Table of bond distances computed at the B3LYP and BP86 theories, figure giving the energy profile at the M06// ONIOM(TPSSTPSS:HF) level, and an .xyz file giving all optimized Cartesian coordinates of stationary points. This material is available free of charge via the Internet at http:// pubs.acs.org. ■ AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] (Y. Luo). *E-mail: [email protected] (Z. Hou). Notes The authors declare no competing financial interest. Figure 6. Change of natural charges for C1 and C2 atoms along with ■ ACKNOWLEDGMENTS the reaction pathway. This work was partly supported by the NSFC (Nos. 21174023, fi indicates that the reaction includes three stages as demon- 21137001, 21429201) and a Grant-in-Aid for Scienti c strated in Figure 4. As shown in Figure 6, the change of natural Research (S) from the JSPS (No. 21225004). Y.L. thanks the charges on C1 and C2 atoms shows a complementary trend. At Fundamental Research Funds for the Central Universities the first stage (from 3a to C), the negatively charged C1 attacks (DUT13ZD103). The authors also thank the RICC (RIKEN the positively charged C2 to form the C1−C2 bond, during Integrated Cluster of Clusters) and the Network and which the C1 atom serves as an electron donor and the C2 Information Center of the Dalian University of Technology atom serves as an electron acceptor. At the second stage, the for part of the computational resources. charges on both C1 and C2 atoms remain essentially constant REFERENCES during the conversion of C to E. At the third stage (from E to ■ P), the negatively charged C1 atom further donates electron to (1) (a) Tebbe, F. N.; Parshall, G. W.; Reddy, G. S. J. Am. Chem. Soc. fi 1978, 100, 3611−3613. (b) Tebbe, F. N.; Parshall, G. W.; Ovenall, D. the positively charged C2 atom and nally completes the − reaction. Similar to the changes of WBIs and EOs shown in W. J. Am. Chem. Soc. 1979, 101, 5074 5075. (2) For recent structural elucidation of the Tebbe reagent, see: Figure 4, the charge transfer process also mainly occurs at Thompson, R.; Nakamaru-Ogiso, E.; Chen, C.-H.; Pink, M.; Mindiola, stages (a) and (c), and no electron transfer occurs in the D. J. Organometallics 2014, 33, 429−432. middle stage (b). This is in contrast to the case of carbonyl (3) (a) Schiøtt, B.; Jørgensen, K. A. J. Chem. Soc., Dalton Trans. 1993, methylenation by a gem-dizinc reagent, in which the charge 337−344. For reviews, see: (b) Hartley, R. C.; McKiernan, G. J. J. changes monotonically from the reactant to the product.20 Chem. Soc., Perkin Trans. 1 2002, 2763−2793. (c) Rana, K. C. Synlett

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A comparison of the geometrical parameters among the DFTs’ results indicates that the

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