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 (CH22−) 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 comprehensive understanding of the CH22−/O2− group interchange process assisted by cooperating multimetal sites. The mechanism presented here is markedly different from conventional Wittig and transition-metal-mediated Wittig-type reactions. In addition, the behavior of μ3-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.
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INTRODUCTION 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 organometallic carbene/methylidene complexes, such as the Tebbe reagent Cp2Ti[(μ2-CH2)(μ2-Cl)AlMe2],1,2 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 terminal methylidene complexes of group 4 metals is reported just very recently by Mindiola et al.,5 although the methylidene complexes have been investigated for decades. In particular, for rare-earth-metal methylidene chemistry, even Lewis acid (such as alkylaluminum)-stabilized methylidene complexes of the rare-earth metals are quite rare,6−10 and no mononuclear terminal rare-earth-metal methylidene complex was reported hitherto. Alternatively, a multimetal center is effective for stabilizing the methylidene group instead of using a Lewis acid. Thus, a series of structurally characterized homometallic trinuclear rare-earth-metal methylidene complexes, which have a striking structural feature with a “Ln3(μ3-CH2)” (Ln = rare-earth metal) motif, have been documented (Chart 1).6,11−18 Moreover, most of these trinuclear methylidene © XXXX American Chemical Society
Chart 1. Homometallic Trinuclear Rare-Earth-Metal Methylidene Complexes
complexes have shown unique reactivity toward carbonyl complexes, acting as methylidene transfer reagents, leading to terminal alkenes and rare-earth-metal μ3-oxygen complexes (a metal Wittig-type reaction).6,11−13 An understanding of the exact reaction mechanism is an essential aspect of chemistry in general, which would be helpful for improving the reactivity and selectivity of the reactions, as well as for designing more efficient reagents/reactions. As we know, the mechanism of the conventional Wittig reaction (metal-free reaction) is one of the great long-running investigations of organic chemistry, and the salt-free Wittig reaction is generally considered to follow a twostep mechanism, viz., the initial addition and subsequent Received: November 19, 2014
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elimination (Scheme 1a).19 The metal Wittig-type carbonyl methylenation by Tebbe reagent (monometal-mediated)3a,b,d
of the carbonyl methylenation process (the CH22−/O2− group interchange) assisted by trinuclear rare-earth-metal methylidene complexes and the behavior of the metal-connected μ3-CH2 group during the transfer process. This work is the first example of a mechanistic study on multimetal-cooperating methylidene (CH2) transfer leading to terminal olefin and demonstrates new insights into Wittig-type chemistry. The results show that the mechanism of the CH22−/O2− group interchange process in such trinuclear complexes goes through a three-step mechanism, 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 cooperation for methylidene (CH2) transfer in such newly arising trinuclear complexes.6,11−13
Scheme 1. Several Strategies for Methylenation of Ketones
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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 organometallic systems,15,17,21 it has been found that the metalconnected terminal methyl (μ1-CH3) is more reactive than the edge-bridging μ2-CH3 and face-capping μ3-CH3 groups in a trinuclear thulium polymethyl complex, and therefore the μ2CH3 group tends to change to μ1-CH3, being more capable of detaching after acceptance of a hydrogen atom.17 This finding drove us to wonder about the behavior of the metal-connected μ3-CH2 methylidene group during the transfer process in homometallic trinuclear complexes. Although a series of homometallic trinuclear rare-earth-metal methylidene complexes and their reactivity as methylidene transfer agents have been explored experimentally,6,11−18 the related theoretical study is still in its infancy possibly due to the huge computational consumption and complexities in the calculations of multinuclear systems. Herein, the reaction of 3a (as a metal Wittig reagent) with acetophenone to give αmethylstyrene and trinuclear μ3-oxygen complex P (Scheme 2)13 was used as a model reaction to investigate the mechanism
and the one in red was included in the outer layer. The inner layer was calculated at the higher level. In the higher-level calculations, the TPSSTPSS functional22,23 was applied, the 6-31G(d) basis set was used for C, H, N, and O atoms, and the effective core potentials (ECP) of Hay and Wadt with double-ζ valence basis set (LanL2DZ)24 were used for the Sc atoms. The outer layer was involved in lower-level calculations. In the lower-level calculations, the Hartree−Fock (HF) method was utilized and the LanL2MB basis set was used for all atoms. Each optimized structure was subsequently analyzed by harmonic vibration frequencies at the same level of theory for characterization of a minimum (Nimag = 0) or a transition state (Nimag = 1). The Berny algorithm as implemented in the Gaussian program (keyword “Opt = TS”) was used for locating transition states. To obtain more reliable relative energies, single-point energy calculations were carried out by using pure DFT method (single-layer) on the basis of optimized structures. In such single-point calculations, the M06-L functional,25 which often shows better performance in the treatment of transitionmetal systems,26,27 was used together with the CPCM model28 (in toluene solution with UFF atomic radii29) 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, 631+G(d), including polarization and diffusion functions was used for
Scheme 2. Reaction of 3a with PhMeCO To Form PhMeCCH2 and P
B
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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 the structures (see more details in the Supporting Information). The results shown in Figure 1 (vide inf ra) and Figures S3 and S4 suggest 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 calculated. All calculations were performed with the Gaussian 09 software package.31
respectively. As shown in Figure 1, the reaction starts with the coordination of the oxygen atom of PhMeCO to the Sc1 center of 3a to form complex B. Due to the coordination of PhMeCO, the Sc1−C1 bond in complex B became weaker in comparison with 3a, as suggested by the bond length (2.30 Å 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, Table 1). Complex B subsequently undergoes a nucleophilic addition reaction via a transition state, TSBC, leading to an intermediate C. This process with a free energy barrier of 14.0
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RESULTS AND DISCUSSION The energy profile for the reaction of 3a with PhMeCO and their corresponding structures are shown in Figures 1 and 2, C
dx.doi.org/10.1021/om501171w | Organometallics XXXX, XXX, XXX−XXX
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Table 1. Selected Wiberg Bond Indexes (WBIs) for the Stationary Points Involved in the Reaction Pathwaya
a
entry
bond
3a
B
TSBC
C
TSCD
D
TSDE
E
TSEP
P
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
Sc1−Sc2 Sc1−Sc3 Sc2−Sc3 Sc1−C1 Sc2−C1 Sc3−C1 Sc1−C3 Sc2−C3 Sc1−C4 Sc3−C4 Sc2−C5 Sc3−C5 Sc1−C6 Sc2−C6 Sc3−C6 Sc1−O Sc2−O Sc3−O C2−O C2−C1
0.179 0.225 0.225 0.481 0.499 0.735 0.427 0.400 0.423 0.437 0.431 0.422 0.299 0.295 0.319
0.179 0.180 0.241 0.363 0.602 0.647 0.414 0.406 0.403 0.411 0.438 0.421 0.227 0.347 0.338 0.342 0.012 0.011 1.509 0.060
0.174 0.190 0.231 0.344 0.528 0.622 0.358 0.467 0.371 0.442 0.447 0.406 0.241 0.348 0.319 0.333 0.018 0.015 1.422 0.148
0.165 0.163 0.212 0.120 0.359 0.364 0.334 0.458 0.325 0.463 0.425 0.385 0.193 0.355 0.329 0.649 0.031 0.027 0.904 0.955
0.170 0.146 0.223 0.055 0.339 0.366 0.340 0.405 0.326 0.432 0.391 0.393 0.213 0.331 0.336 0.624 0.098 0.050 0.868 0.972
0.222 0.147 0.216 0.059 0.277 0.405 0.419 0.338 0.334 0.426 0.364 0.420 0.275 0.298 0.322 0.388 0.363 0.030 0.855 0.985
0.215 0.148 0.202 0.055 0.203 0.442 0.412 0.334 0.338 0.416 0.366 0.403 0.281 0.284 0.311 0.391 0.349 0.038 0.842 0.991
0.197 0.167 0.161 0.035 0.034 0.517 0.388 0.331 0.355 0.409 0.369 0.383 0.290 0.262 0.257 0.352 0.316 0.153 0.758 1.039
0.199 0.180 0.172 0.035 0.034 0.398 0.384 0.320 0.339 0.425 0.329 0.422 0.281 0.246 0.278 0.412 0.363 0.239 0.463 1.253
0.193 0.194 0.194
1.758b b
0.392 0.383 0.384 0.393 0.392 0.385 0.272 0.273 0.275 0.507 0.507 0.512 1.856c
c
Atom labeling is defined in Figure 1. The value is obtained from the CO bond of isolated PhMeCO. The value is obtained from the CC bond of isolated PhMeCCH2.
Wittig reaction or mono- and dimetalllic methylenation reagents (vide inf ra). Interestingly, it is noteworthy that, during the reaction, the change in coordination manner of the oxygen atom follows the order μ1 → μ2 → μ3 and that of the CH2 group (leaving group) follows a reverse trend, μ3 → μ2 → μ1. This is similar to the CS2 activation by a trinuclear phosphinidene cluster, during which the change of coordination manner of the sulfur atom follows the order μ1 → μ2 → μ3, and the PPh group (leaving ligand) follows the reverse trend, μ3 → μ2 → μ1.15 Additionally, the behavior of μ-CH2 in the current system and our previous studies on the detaching group (μPPh15 and μ-CH317) for the trinuclear clusters give us a general sense that the detaching groups first change their coordination manner to terminal form, being more capable of detaching from the metal center of the clusters. To assess the possibility of the methylation of the carbonyl functionality in the current system, the methyl group transfer event in the reaction of 3a with PhMeCO has also been investigated. In complex 3a, there are two kinds of bridged methyl group, viz., μ2-CH3 (such as the C4 methyl group) and μ3-CH3 (C6 methyl group). Both possibilities were calculated, and the transition states of TS1 (C4 group transfer) and TS2 (C6 group transfer) and their corresponding relative energies are shown in Figure 3. The relative free energies of TS1 (50.0 kcal/mol) and TS2 (45.7 kcal/mol) are significantly higher in comparison with TSBC (14.0 kcal/mol, Figure 1), suggesting that methyl group transfer in such a methylidene complex seems unlikely. Therefore, methylidene transfer rather than methyl transfer occurs to achieve methylenation of the carbonyl functionality, which is consistent with the experimental observation.13 To get more information on the bonding during the reaction, selected Wiberg bond indexes (WBIs) are listed in Table 1. As shown in this table, the WBIs of Sc−Sc bonds are 0.146−0.241 (entries 1−3), suggesting that the three metal centers retain bond interactions between each other during the whole reaction. The WBIs also clearly display that the interaction of
kcal/mol involves three events (C2O double-bond addition yielding a C2−O single bond, C1−Sc1 bond cleavage, and C1− C2 single-bond formation) and is exothermic by 10.4 kcal/mol. In contrast to the four-membered-ring intermediate involved in the conventional Wittig reaction and metal-mediated carbonyl methylenation (Scheme 1a−c), complex C shows a bridging structure supported by three metal centers and could not directly give the olefination product via elimination. Similar to the activation of CS2 by a trinuclear rare-earth-metal phosphinidene complex,15 the O atom in C would further change its coordination mode from μ1- to μ2-form. Due to the asymmetrical substituents of C2 (methyl and phenyl), there are two possible pathways for the change of coordination mode of the O atom, viz., from the phenyl side to form a Sc1−(μ2-O)− Sc2 frame and from the methyl side to form a Sc1−(μ2-O)− Sc3 frame. Both possibilities were calculated, and the energy profile (Figure 1) indicates that the former case (via TSCD) is more favorable than the latter (via TS′CD) both kinetically and energetically. The favorable pathway overcomes only ΔG⧧ = 4.9 kcal/mol (TSCD), and the corresponding product D with a μ2-O moiety is more stable than C by 6.6 kcal/mol. Then, the cleavage of Sc2−C1 in D occurs via a transition state TSDE, with ΔG⧧ = 0.4 kcal/mol, leading to more stable complex E with a single-metal-bound CH2R (like a terminal form) group for detaching. The subsequent elimination occurs via a multimetal-assisted four-center transition state, TSEP, leading to the final product α-methylstyrene and trinuclear μ3-oxygen complex P, which was structurally identified experimentally.13 Such an elimination process, involving the formation of C1 C2 and Sc3−O bonds and the cleavage of C2−O and Sc3−C1 bonds simultaneously, has an energy barrier of 3.6 kcal/mol and is significant exothermic by 40.6 kcal/mol. From the point of view of the whole reaction, the energy barriers of all steps are less than 15 kcal/mol and the whole reaction is significantly exergonic, ca. 67 kcal/mol. The mechanism proposed here (multinuclear-cooperating Wittig-type reaction) is more complicated than carbonyl methylenation by conventional D
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Figure 3. Two possible methyl transfer transition states for the reaction of 3a with PhMeCO. The energies (kcal/mol) are relative to the energy sum of 3a and PhMeCO. 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. Figure 4. Changes of (a) Wiberg bond indexes (WBIs) and (b) the electron occupations (EOs) for C2−O and C2−C1 bonds along the reaction pathway.
Sci−C1 (i = 1−3) becomes weaker and the interaction of Sci− O (i = 1−3) gets stronger along with the reaction coordinates (entries 4−6, 16−18). The WBIs of other Sc−Ci (i = 3−6) bonds have no significant changes (entries 7−15). It is noteworthy that, during the whole reaction, two essential events are involved in the change of the chemical bonds. One is the cleavage of the CO double bond of PhMeCO, and the other is the formation of new CC double bond of PhMeC CH2. Along with the reaction pathway, the changes of the WBIs (also see entries 19 and 20 in Table 1) and the electron occupations (EOs) of C2···O and C2···C1 are illustrated in Figure 4. Interestingly, the changes of WBIs in Figure 4a obviously demonstrate that the whole reaction occurs via three stages: (a) weakening C2O to a C2−O single bond and formation of a C2−C1 single bond (nucleophilic addition, from 3a to C), (b) intramolecular isomerization retaining singlebond characters of C2−O and C2−C1 bonds (from C to E), and (c) cleavage of a C2−O single bond and formation of a C2C1 double bond (olefination elimination, from E to P). This result is markedly different from the cases of previous carbonyl methylenation, which is generally considered as a twostep reaction, viz., the initial addition and subsequent elimination step. In the current system, the CH22−/O2− group interchange process (the bond-breaking and -forming of C2O and C2C1 bonds) mainly occurs in stage (a) and (c). During stage (b), with the assistance of the cooperating multimetal sites,15,17 the complex undergoes intramolecular isomerization for further bond interchange. The change of bonds can be further corroborated by the value of EOs of the intermediates during the reaction. As shown in Figure 4b, at the beginning of the reaction, the EO of the PhMeC2O double bond is computed to be 3.97. In the middle stage, the EOs of both C2−O and C2−C1 bonds are computed to be ca. 1.95− 1.97, suggesting single bonds of C2−O and C2−C1. In the product of PhMeC2C1, the EO of the C2C1 double bond
is computed to be 3.91. These results could give us an intuitive understanding of the CH22−/O2− interchange event assisted by cooperating multimetal sites. As aforementioned, the cleavage of the CO bond and formation of the CC bond mainly occur in stages (a) and (c). Thus, the two corresponding transition states TSBC and TSEP play an important role in the CH22−/O2− group interchange process. To get more detailed information on these two important steps, the frontier molecular orbitals of TSBC and TSEP are analyzed. As shown in Figure 5, the HOMO of TSBC is mainly contributed by the 2p orbital of the C1 atom (51.2%). In TSBC, the molecular orbital shows that the C1 atom attacks the C2 center assisted by cooperation of three metal centers (3d orbitals of metals) to form a C1−C2 σ-bond. In TSEP, the HOMO−1 is also mainly contributed by the 2p orbital of the C1 atom (45.4%). The HOMO−1 of TSEP shows that the formation of the C1C2 double bond is achieved by breaking the σ-bond of Sc3−C1 (2p orbital of C1 and 3dz2 orbital of Sc3) and forming the π-bond of C1C2 (p−p πorbital of C1 and C2). These frontier molecular orbitals of the two transition states clearly present the process of bondbreaking and -forming of the C2O and C2C1 double bonds. It is known that the CH2 group in metal−carbene complexes often acts as a nucleophilic group, and it could react with electrophilic substrates. In the current reaction, the CH2 of trinuclear scandium methylidene complex 3a (as a nucleophilic group) first attacks PhMeCO (as an electrophilic substrate) to form a C−C single bond (Figure 4) and finally achieves the CH22−/O2− group interchange process. However, the detailed behavior of charge transfer is unclear. To further clarify the E
dx.doi.org/10.1021/om501171w | Organometallics XXXX, XXX, XXX−XXX
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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 elimination. The CH22−/O2− group interchange process (the bondbreaking and -forming of CO and CC 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 bondbreaking and -forming processes. The natural charges clearly display that the electron transfer occurs at stages (a) and (c), and no electron transfer occurs in middle stage (b), in sharp contrast to other carbonyl olefination reactions. This study not only enriches the chemistry of metal Wittig-type reactions but also sheds light on the intermetallic cooperation for methylidene (CH2) transfer leading to a terminal olefin.
Figure 5. HOMO of TSBC and HOMO−1 of TSEP (left, isovalue = 0.03) and their schematic representations (right). Atomic orbital contributions are reported in parentheses. All hydrogen atoms were omitted for clarity. The atom labeling is the same as in Figure 1.
charge transfer process, the change of the natural charges of the C1 atom (nucleophilic center) and C2 atom (electrophilic center) along with the reaction coordinates is illustrated in Figure 6. Interestingly, the change of natural charges also
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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.
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AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected] (Y. Luo). *E-mail:
[email protected] (Z. Hou). Notes
The authors declare no competing financial interest.
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Figure 6. Change of natural charges for C1 and C2 atoms along with the reaction pathway.
ACKNOWLEDGMENTS This work was partly supported by the NSFC (Nos. 21174023, 21137001, 21429201) and a Grant-in-Aid for Scientific Research (S) from the JSPS (No. 21225004). Y.L. thanks the Fundamental Research Funds for the Central Universities (DUT13ZD103). The authors also thank the RICC (RIKEN Integrated Cluster of Clusters) and the Network and Information Center of the Dalian University of Technology for part of the computational resources.
indicates that the reaction includes three stages as demonstrated in Figure 4. As shown in Figure 6, the change of natural charges on C1 and C2 atoms shows a complementary trend. At the first stage (from 3a to C), the negatively charged C1 attacks the positively charged C2 to form the C1−C2 bond, during which the C1 atom serves as an electron donor and the C2 atom serves as an electron acceptor. At the second stage, the charges on both C1 and C2 atoms remain essentially constant during the conversion of C to E. At the third stage (from E to P), the negatively charged C1 atom further donates electron to the positively charged C2 atom and finally completes the reaction. Similar to the changes of WBIs and EOs shown in Figure 4, the charge transfer process also mainly occurs at stages (a) and (c), and no electron transfer occurs in the middle stage (b). This is in contrast to the case of carbonyl methylenation by a gem-dizinc reagent, in which the charge changes monotonically from the reactant to the product.20
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REFERENCES
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Organometallics
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dx.doi.org/10.1021/om501171w | Organometallics XXXX, XXX, XXX−XXX
