Article pubs.acs.org/Organometallics
Reactivities of Adamantyl-Substituted Metallenes with a CE (E = C, Si, Ge, Sn, and Pb) Double Bond. A Theoretical Study Bo-Ying Li and Ming-Der Su* Department of Applied Chemistry, National Chiayi University, Chiayi 60004, Taiwan, Republic of China S Supporting Information *
ABSTRACT: The potential energy surfaces for the chemical reactions of adamantyl-substituted compounds containing the CE double bond, Rea-CE, where E = C, Si, Ge, Sn, and Pb, were studied using density functional theory (B3LYP/ LANL2DZ). Three kinds of chemical reactionsmethanol addition, [2 + 4] Diels−Alder cycloaddition with 1methoxybutadiene, and [2 + 2] cycloaddition with a ketonewere used to study the chemical reactivity of these Rea-CE molecules. Our theoretical findings reveal that the smaller the singlet−triplet splitting of the Rea-CE, the lower its activation barriers and, in turn, the more rapid its chemical reactions with other chemical molecules. Theoretical studies suggest that the relative chemical reactivity increases in the order CC ≪ CSi < CGe < CSn < CPb. That is, the smaller the atomic weight of the group 14 atom (E), the smaller the atomic radius of E and the more stable its adamantyl-substituted Rea-CE to chemical reaction. It is predicted that the adamantyl-substituted Rea-CE (E = C and Si) compound should be stable and readily synthesized and isolated at room temperature. Our computational results are in accordance with the available experimental observations. Moreover, our theoretical findings demonstrate that both electronic and steric factors play a key role in determining the chemical reactivity of the group 14 adamantyl-substituted Rea-CE molecules, both kinetically and thermodynamically. The present understanding of the reactivity of adamantyl-substituted doubly bonded CE molecules provides a useful building block for a future, deeper understanding of this field of organometallic chemistry. Through the elegant studies, performed by Apeloig and many co-workers, a series of novel silenes has been kinetically stabilized that contain two alkyl substituents at carbon and trialkylsilyl substituents at silicon (1). These have been isolated and structurally characterized.6,7 It has also been reported that such adamantyl silenes can undergo 1,2-addition of alcohols and water. They also react with 1-methoxybutadiene, to produce the expected Diels−Alder [2 + 4] cycloaddition products.6,7 However, little definitive information (such as activation energies and reaction mechanisms) has been determined about these chemical reactions, due to difficulties in probing these reactions experimentally. 8 Additionally, attempts to experimentally isolate the other stable analogues, possessing the CC, CGe, CSn, and CPb double bonds, have not been reported, let alone a systematic study of the effects of group 14 elements on the reactivities of doubly bonded CE (E = C, Si, Ge, Sn, and Pb) species. Indeed, our understanding of the various facets of the doubly bonded CE chemistry is still inferior to out knowledge of alkenes. This study aims to enlarge upon previous experimental observations by studying the reactivities of the adamantyl-
I. INTRODUCTION Since the first isolation of a thermally stable molecular compound containing a carbon−silicon double bond, Ad(Me3SiO)CSi(SiMe3)2, in 1981,1 the chemistry of the silenes has experienced rapid development, as reflected in the number of recent review articles,2 wherein many other novel stable silenes containing various substituents such as alkyl, aryl, and silyl groups have been synthesized and fully characterized.3,4 In fact, due to the substantial differences in electronegativity between carbon and silicon, the CSi bond is quite polar and strongly electrophilic. As a result, silenes exhibit high reactivity toward nucleophiles. The silicon atom is the site that is attacked by water, alcohols, amines, and other nucleophiles, which are quite important, from both a fundamental and an applied viewpoint.2,5 The quest for stable CSi double-bond compounds and the study of their related chemical reactions has been a focus of attention for chemists for many decades.2
Received: August 3, 2011 Published: October 31, 2011 © 2011 American Chemical Society
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values of the spin-squared operator, ⟨S 2⟩, were in the range 2.001− 2.007 for all of the triplet species considered here. They were, therefore, very close to the correct value of 2.0 for pure triplets, so that their geometries and energetics are acceptable for this study. Frequency calculations were performed on all structures to confirm that the reactants and products had no imaginary frequencies and that the transition states possessed only one imaginary frequency. The relative energies, at 0 K, were thus corrected for vibrational zero-point energies (ZPE, not scaled). The thermodynamic corrections to 298 K, ZPE corrections, heat capacity corrections, and entropy corrections (ΔS) obtained were applied at the B3LYP/LANL2DZ level. Thus, the relative free energy (ΔG), at 298 K, was also calculated at the same level of theory. Cartesian coordinates, calculated for the stationary points at the B3LYP level, are available as Supporting Information. As suggested by one reviewer, for comparison, we have used the Møller−Plesset perturbation (MP2) theory12 with the 6-31G(d) basis set. That is, single-point MP2 calculations were performed on all B3LYP-optimized structures, i.e., MP2/6-31G(d)//B3LYP/ LANL2DZ, whose relative energetics are collected in the Supporting Information. All of the DFT and MP2 calculations were performed using the GAUSSIAN 03 package of programs.13
substituted compounds featuring the CE double bond, in which one silicon atom is replaced, at a double bond of silene, by another group 14 element, E. To achieve this aim, the transition-state (TS) geometries of the molecules as well as the ground-state energies of the products of reactions must be determined. To better understand the effect of the group 14 element on reactivities of the heavier analogues bearing a CE (E = the group 14 element) double bond, this study undertakes the first systematic computational examination of these compounds, using density functional theory (DFT). The three kinds of chemical reactions are represented as eqs 1−3.
III. RESULTS AND DISCUSSION 1. Geometries of (adamantyl)CSi(SiMe 2 t-Bu)(SiMe3). Prior to consideration of the geometrical optimization and the potential energy surfaces for the chemical reactions of the molecules featuring the CE (E = C, Si, Ge, Sn, and Pb) double bonds, the geometrical structure of 1-tertbutyldimethylsilyl-1-trimethylsilyl-2-adamantylidenesilane (1a) is examined, since it was already isolated and structurally characterized experimentally.6 The optimized geometries for the silene 1a species were computed at the B3LYP/LANL2DZ level of theory. As shown in Figure 1, DFT calculations indicate that the double CSi bond length in the 1a molecule is 1.741 Å, which agrees well with experimental data (1.761 Å). Also, the computed C−adamantyl and Si−Si bond lengths in 1a (average 1.531 and 2.369 Å at B3LYP) compare favorably with average C−adamantyl and Si−Si bond lengths determined from X-ray data for 1a (1.518 and 2.387 Å) reported by Apeloig et al.6 Similarly, the ∠(adamantly)−C−(adamantyl) and ∠Si−Si− Si angles in 1a were calculated to be 110.6° and 122.6°, which are in good agreement with the experimental values (110.6° and 122.9°, respectively), as given in Figure 1. It should be noted that working with larger basis sets, to examine transition-state structures and the barrier heights of the potential energy surfaces, would be quite prohibitively expensive in terms of available computation time and disk space. In view of these constraints and considering the good agreement between the B3LYP method with the shorter basis set (LANL2DZ) and the available experimental data,6 it is expected that the same relative accuracy should also apply to the geometries and energetics predicted for the other adamantyl-substituted molecules containing the CE (E = group 14 elements) double bond. This, in turn, should provide reliable information for the discussion of their reactivities and reaction mechanisms, for which experimental data are still not available. In light of these assumptions, this study uses the B3LYP/LANL2DZ level of theory from now on. 2. Geometries and Electronic Structures of Adamantyl-Substituted Doubly Bonded CE Molecules. This section examines the geometries and electronic structures of the reactants, i.e., (adamantyl)CE(SiMe2t-Bu)(SiMe3) (E = C, Si, Ge, Sn, and Pb). Reactants Rea-CC, Rea-CSi, ReaCGe, Rea-CSn, and Rea-CPb were calculated in both the singlet and triplet states at the B3LYP/LANL2DZ level of
In this paper, the reactions of doubly bonded (adamantyl)CE(SiMe2t-Bu)(SiMe3) compounds can be sorted into three classes: 1,2-addition of methanol, [2 + 4] Diels−Alder reaction of 1-methoxybutadiene, and [2 + 2] cycloaddition of acetone. These reactions have been chosen, because they represent various doubly bonded CE molecule reactions that have already been investigated in some previous papers.6,7 As experimental values and trends are not readily available for molecules containing the CE (E = the group 14 element) double bond, computational analysis thus plays a crucial role. This paper aims to explore (i) the influence of different group 14 atomic centers upon the geometries and energies of the transition states, (ii) the electronic effects on the reactivities in numerous variations in the group 14 centers, and (iii) the determining factor that controls the activation barrier for these chemical reactions. The predicted molecular parameters, presented in this paper, can serve as a guide for any future experimental investigations of unknown molecular compounds containing the CE double bond. As a result of this study, it is hoped that a logical framework can be constructed that allows identification of the factors determining the fundamental chemical properties of compounds possessing the adamantylsubstituted CE double bond in order to achieve more precise control of the total reaction process.
II. THEORETICAL METHODS All geometries were fully optimized, without imposing any symmetry constraints, although several optimized structures showed various elements of symmetry. For DFT calculations, the hybrid, gradientcorrected exchange function, proposed by Becke,9 was used in combination with the gradient-corrected correlation function of Lee, Yang, and Parr.10 Thus, the geometries of all the stationary points were fully optimized, at the B3LYP level of theory. These B3LYP calculations were carried out using pseudorelativistic effective core potentials of group 14 elements, modeled using the double-ζ (DZ) basis sets.11 Accordingly, B3LYP calculations are denoted B3LYP/ LANL2DZ. It is noted that the model compounds (adamantyl)C E(SiMe2t-Bu)(SiMe3) have 272 (164 electrons) basis functions, for E = C, Si, Ge, Sn, and Pb. A spin-unrestricted (UB3LYP) formalism was used for the open-shell (triplet) species. The computed expectation 6190
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Figure 1. Selected geometrical parameters (in Å and deg) and Mulliken charges (QC and QE) of the reactants (adamantyl)C E(SiMe2t-Bu)(SiMe3) (Rea-CE, E = C, Si, Ge, Sn, and Pb) at both singlet and triplet states calculated at the B3LYP/LANL2DZ level of theory, compared with the experimental values (ref 8). Hydrogens are omitted for clarity.
Figure 2. Calculated frontier molecular orbital for the singlet Rea-C E (E = C, Si, Ge, Sn, and Pb) species. For more information see the text.
molecules. This phenomenon can be explained by their electronic structures (vide infra). In addition, our DFT computations indicate in the triplet state the planes of R 2C and ER′2 (R2 = adamantly group and R′2 = Si(SiMe2tBu)(SiMe3)) are perpendicular to each other and that the ER′2 fragment has some degree of pyramidality. The charge distribution in the reactant Rea-CE species is also examined. As can be seen in Figure 1, the effect of the substituted elements (E) on the charge distribution is very large. For heteronuclear combinations −CE− (E = Si, Ge, Sn, and Pb), the charge distributions on both doubly bonded atoms are strongly dependent on the electronegativities of the group 14 elements as well as those of the attached substituent groups. As a result, in all five reactants, only the carbon atom attached to the silyl groups is the electrophilic site of the CC bond in the Rea-CC molecule, whereas for the other group 14 elements (E) the CE double bond is a nucleophilic site. Our calculations show that the total polarity, |Δ(t)|, i.e., the absolute value of the charge on the group 14 element (E) minus the charge on the carbon atom, decreases in the following order: Rea-CC > Rea-CSn > Rea-CSi > ReaCPb > Rea-CGe. This polarity result, however, is not consistent with their corresponding double-bond lengths, whose trend follows that of the atomic weight of E, i.e., ReaCPb > Rea-CSn > Rea-CGe > Rea-CSi > Rea-C C.
theory, and their selected geometrical parameters and Mulliken charges (QC and QE) are shown in Figure 1. The valence molecular orbitals, based on the same theoretical calculations, are presented in Figure 2. The calculations shown in Figure 1 indicate that the calculated CE double-bond length in such adamantylsubstituted species increases in the order 1.378 Å (Rea-C C) < 1.761 Å (Rea-CSi) < 1.830 Å (Rea-CGe) < 2.002 Å (Rea-CSn) < 2.088 Å (Rea-CPb) and 1.482 Å (Rea-C C) < 1.876 Å (Rea-CSi) < 1.959 Å (Rea-CGe) < 2.130 Å (Rea-CSn) < 2.200 Å (Rea-CPb) for the singlet and triplet states, respectively. That is, substitution of a heavy group 14 element causes a large increase in the CE double-bond length of the reactants in both the singlet and triplet states. These observations can be explained in terms of the expected atomic size of the central atom E, which increases as E changes from carbon to lead. It is noteworthy that our computed lengths for the singlet CC, CSi, CGe, and CSn double bond are 1.378, 1.761, 1.830, and 2.002 Å, which are in reasonable agreement with the available experimentally determined lengths for the double bonds, CC (1.33 Å),14 CSi (1.702−1.764 Å),15 CGe (1.803−1.827 Å),15 and CSn (2.025−2.379 Å),15 bearing in mind that our calculated molecules contain much bulkier substituents. Moreover, the theoretical investigations also demonstrate that the CE bond distance is shorter for the singlet than for its triplet Rea-CE 6191
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Figure 2 is a molecular orbital correlation diagram for the valence orbitals of the adamantyl-substituted molecules possessing the CE (E = the group 14 element) double bond. The nature of HOMO and LUMO in the reactants (ReaCE), which are primarily p−π orbitals on the CE double bond, agrees well with that encountered in most group 14 alkene compounds.16 That is, the replacement of one silicon atom at a CSi double bond in 1a by the other group 14 atom (E) increases the energy of the bonding π(CE) orbitals (HOMO), while the energy of the antibonding π*(CE) orbitals (LUMO) is decreased. Here, the HOMOs are essentially the bonding π(CE) orbitals, whereas the LUMOs are essentially antibonding π*(CE) orbitals. In addition, it is apparent from Figure 2 that the triplet state is a diradical where the π-bond is broken and the two electrons reside in two degenerate, perpendicular SOMO orbitals, leading actually to a C−E single bond.17 As a result, it is expected that the bond distance, r(CE), is longer for the triplet than the singlet state. This prediction is confirmed by our B3LYP/ LANL2DZ results for all cases, as already detailed in Figure 1. The singlet−triplet energy separations of the Rea-CE molecules have some noteworthy properties. In Figure 2, it is readily seen that the HOMO−LUMO energy gap in Rea-CE decreases as the atomic number of the group 14 atom (E) increases. This strongly implies that the corresponding singlet− triplet energy gap should also decrease, as the E atom gets heavier. Indeed, this prediction is consistent with what is observed in the theoretical computations. Indeed, the DFT computations indicate that the singlet−triplet energy splittings, ΔEst (kcal/mol), of Rea-CE are 47 (Rea-CC) > 31 (ReaCSi) > 26 (Rea-CGe) > 18 (Rea-CSn) > 15 (Rea-C Pb). In the following sections, these results will be used to account for the origin of barrier heights and reaction enthalpies, for the three chemical reactions considered (eqs 1−3) as well as the reactivity of the Rea-CE compounds. 3. 1,2-Addition Reactions of Adamantyl-Substituted Doubly Bonded CE Molecules with Methanols. It is well understood that silenes undergo nucleophilic addition of alcohols and that this could be one of the reactions that complicates their isolation.18 This reaction with alcohols has been more extensively studied, from a mechanistic standpoint, during the last three decades. Nevertheless, the factors that govern the reactivity of silenes in such 1,2-addition of nucleophiles are still not fully understood, especially for the adamantyl-substituted, doubly bonded CE molecules studied in this work. Accordingly, in order to investigate the factors that control the reactivity of adamantyl-substituted compounds containing the CE double bond (E = C, Si, Ge, Sn, and Pb), we consider the 1,2-addition of methanol to these substituted, doubly bonded molecules, which proceeds via eq 1. Previous kinetic data for the reactions of adamantyl-substituted silene with methanols8 show that the actual reaction mechanism is a reaction of the dimeric form of methanol with the CSi double bond, rather than the traditional addition of a single molecule of methanol across the CSi bond. The dimer addition mechanism was investigated in detail using the B3LYP/LANL2DZ level of theory, comparing the energetics with those associated with the addition of the methanol monomer at DFT.19 The two competing processes for methanol addition are schematically represented in Figure 3. The first route indicates that the final product (Pro-CEMeOH) may form directly from the transition state (TS-CEMeOH), in which the oxygen atom of monomeric methanol is
Figure 3. Two reaction pathways (path I and path II) for the methanol addition reaction between reactants Rea-CE (E = C, Si, Ge, Sn, and Pb) and MeOH including the precursor complexes (Pcx-CEMeOH and Pcx-CE-(MeOH)2), transition states (TS-CEMeOH and TS-CE-(MeOH)2), and product (Pro-CE-MeOH). For selected geometrical parameters and relative energies optimized at the B3LYP/LANL2DZ level of theory for each species see Table 1. Some hydrogens are omitted for clarity.
attached to the electropositive group 14 atom, E, followed by intramolecular proton transfer (path I). The second route shows that the O−H addition process may involve formation of the same final product (Pro-CE-MeOH), in which the oxygen atom of dimeric MeOH is bonded to the electropositive M atom, forming through a precursor complex (Pcx-CE(MeOH)2) and a transition state (TS-CE-(MeOH)2) (path II). Selected geometrical parameters and relative energies, at the B3LYP/LANL2DZ level of theory, are summarized in Table 1. Figure 3 and Table 1 reveal several noteworthy features. (a) Basically, our computational results based on the 1,2addition reactions of the monomeric MeOH and dimer of the alcohol, (MeOH)2, with 1a using the B3LYP/LANL2DZ method are quite similar to the previous theoretical work studied at both the B3LYP/6-31G(d) and MP2/6-31G(d) levels of theory by Leigh, Bendikov, Apeloig, and co-workers. 8 For instance (see Table 1), our calculated CSi bond lengths in 1a (1.761 Å), Pcx-CSi-MeOH (1.766 Å), Pcx-CSi(MeOH)2 (1.767 Å), TS-CSi-MeOH (1.820 Å), TS-CSi(MeOH)2 (1.798 Å), and Pro-CSi-MeOH (1.936 Å) compare favorably with those determined from B3LYP/631G(d) data (1.755, 1.757, 1.759, 1.807, 1.804, and 1.902 Å, respectively).8 Also, according to the detailed experimental− theoretical study of methanol addition to 1a,8 it was concluded that a methanol dimer reacts faster than a monomer, which agrees well the present results based on the B3LYP/LANL2DZ method (see below). Again, these comparisons give us 6192
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Table 1. Selected Geometrical Parameters (bond distances in Å), Relative Energies (zero-point corrected; kcal mol −1), and Relative Gibbs Free Energies (kcal mol−1) at 298 K at the B3LYP/LANL2DZ Level of Theory for the Optimized Stationary Points on the Studied 1,2-Addition Channelsa geometrical parameters
a b
system
CE
Rea-CC Pcx-CC-MeOH TS-CC-MeOH Pcx-CC-(MeOH)2 TS-CC-(MeOH)2 Pro-CC-MeOH Rea-CSi Pcx-CSi-MeOH TS-CSi-MeOH Pcx-CSi-(MeOH)2 TS-CSi-(MeOH)2 Pro-CSi-MeOH Rea-CGe Pcx-CGe-MeOH TS-CGe-MeOH Pcx-CGe-(MeOH)2 TS-CGe-(MeOH)2 Pro-CGe-MeOH Rea-CSn Pcx-CSn-MeOH TS-CSn-MeOH Pcx-CSn-(MeOH)2 TS-CSn-(MeOH)2 Pro-CSn-MeOH Rea-CPb Pcx-CPb-MeOH TS-CPb-MeOH Pcx-CPb-(MeOH)2 TS-CPb-(MeOH)2 Pro-CPb-MeOH
1.378 1.381 1.488 1.382 1.476 1.597 1.761 1.766 1.820 1.767 1.798 1.936 1.830 1.835 1.892 1.837 1.866 2.016 2.002 2.019 2.035 2.016 2.026 2.189 2.088 2.090 2.120 2.108 2.121 2.255
C−H
E−H
3.094 1.137 2.656 1.210 1.101
2.857
3.314 1.682 2.693 1.834 1.107
2.851
3.351 1.746 2.676 1.864 1.106
2.867
3.862 2.083 2.563 2.480 1.106
2.895
4.255 1.936 1.936 2.559 1.104
2.911 2.577 2.962
E−O
2.262 2.480 2.302 1.535
2.216 2.712 2.220 1.757
2.246 2.763 2.384 1.847
2.434 2.914 2.905 1.979
2.355 2.067
energetics O−H
b
0.9838 1.622 0.9860 1.498 (1.182)
0.9849 1.098 0.9896 1.088 (1.040)
0.9844 1.085 0.9903 1.041 (1.031)
0.9857 1.021 0.9924 0.9932 (1.005)
0.9857 1.029 0.9968 1.001 (1.009)
ΔE
ΔG
0.000 15.11 70.17 −10.38 33.04 14.10 0.000 8.982 18.42 −13.61 −2.401 −41.58 0.000 3.249 11.51 −14.09 −5.641 −42.49 0.000 1.080 3.979 −14.73 −11.66 −44.74 0.000 −0.7643 1.645 −14.81 −12.19 −51.15
0.000 26.70 83.24 9.888 57.81 27.83 0.000 18.90 31.32 5.924 22.35 −29.11 0.000 15.62 23.30 5.912 19.31 −30.40 0.000 12.37 15.51 5.564 11.41 −33.80 0.000 9.890 13.26 6.328 10.19 −39.67
For structures, see Figures 1 and 3. The C−O and O−H bond lengths in parent MeOH were calculated to be 1.460 and 0.9793 Å, respectively. H−O1 and H−O2 (in parentheses) bond distances.
complexes formed by monomeric MeOH with Rea-CE species exist in both gas and solvent phase at room temperature. In path II (the dimeric MeOH attack), only one weakly bonded complex (Pcx-CE-(MeOH)2) was found for all model reactants. These are significantly more stable than the corresponding Pcx-CE-MeOH complexes, as shown in Table 1. These phenomena are explained by the higher acidity of the dimeric form of methanol, compared to that of the monomer. Additionally, the CE centered bond distance and the O−H bond length in the (MeOH)2 complexes are slightly larger than those in the free species in all cases. The B3LYP/ LANL2DZ results demonstrate that the Gibbs free energies of these dimeric methanol complexes are all 10−5.6 kcal/mol greater than those of their corresponding reactants. Again, these calculations imply that such a precursor complex should not exist at room temperature and that experimental detection of the complex formed during the reaction is unlikely. (c) Starting from the weakly bonded complex, two reaction pathways (paths I and II) are possible. For reaction path I, the transition state (TS-CE-MeOH) was located, for each of the group 14 elements, using DFT theory, along with the imaginary frequency eigenvector. The DFT frequency calculations for the transition states TS-CC-MeOH, TS-CSi-MeOH, TS-C
confidence that the B3LYP/LANL2DZ level employed in this work should provide reasonable molecular geometries and energetics for those chemical reactions studied in this work, for which experimental data are not available. (b) Similarly to previous studies of the process of methanol addition to silenes,8,18 the initial step in methanol addition reactions is predicted to be the formation of a weakly bonded precursor complex. In path I (the monomeric MeOH attack), our computed structures are quite similar to those reported by Leigh, Bendikov, Apeloig, and co-workers8 and are consistent with a weak hydrogen-bonding interaction between the methanol molecule and the CE double bond of the ReaCE species. Various groups18 have found no interaction between the group 14 element, E, and the oxygen atom of methanol in such complexes. Table 1 shows that the structures of the reactant molecules in the Pcx-CE-MeOH species are almost unchanged, although both the CE bond in the ReaCE component and the O−H bond in methanol are slightly elongated. On the basis of the B3LYP calculations, it can be readily seen that their binding energies, relative to their corresponding reactants, are between 15 and 1.1 kcal/mol. Also, the relative Gibbs free energies for the formation of these complexes are always positive, in the range 27−12 kcal/mol. In light of these theoretical results, it seems unlikely that the 6193
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Ge-MeOH, TS-CSn-MeOH, and TS-CPb-MeOH suggest that the single imaginary frequency values are 644i, 605i, 478i, 198i, and 184i cm−1, respectively. All TS-CE-MeOH geometries are quite similar, and they all have a fourmembered-ring structure containing the CE bond and H− O bond. As a result, electrons flow from the σ orbital of the H− O group to the π*(CE) orbital. This can lead to a longer CE bond distance in the transition state (TS-CE-MeOH), as shown in Table 1. For instance, the theoretical calculations predict that the CE bond distances are as follows: 1.488 Å (TS-CC-MeOH), 1.820 Å (TS-CSi-MeOH), 1.892 Å (TS-CGe-MeOH), 2.035 Å (TS-CSn-MeOH), and 2.120 Å (TS-CPb-MeOH). These theoretical data also reveal that the heavier the group 14 element, E, involved in the Rea-CE molecule, the longer the CE bond length in the TS. Conversely, for path II, our B3LYP frequency calculations for the transition states TS-CC-(MeOH) 2 , TS-CSi(MeOH)2, TS-CGe-(MeOH)2, TS-CSn-(MeOH)2, and TS-CPb-(MeOH) 2 predict that the single imaginary frequency values are 641i, 205i, 121i, 162i, and 90.3i cm −1, respectively. The normal modes associated with the single imaginary frequency are consistent with the MeO−H bondbreaking process, while the oxygen atom of the other MeOH species is attached to the electropositive site of the CE double bond. This certainly decreases the activation barrier for the methanol dimer addition reaction, since the entropy of activation for the addition of dimeric MeOH is apparently more positive than for addition of the monomer. Accordingly, from Table 1, it is readily seen that the activation barrier for path II is smaller than that for path I. For example, the B3LYP free energy results (kcal/mol) demonstrate that 58 (TS-CC-(MeOH)2) < 83 (TS-CC-MeOH), 22 (TS-CSi-(MeOH)2) < 31 (TS-CSi-MeOH), 19 (TSCGe-(MeOH)2) < 23 (TS-CGe-MeOH), 11 (TS-C Sn-(MeOH)2) < 16 (TS-CSn-MeOH) and 10 (TS-CPb(MeOH)2) < 13 (TS-CPb-MeOH). These computed figures anticipate a lower barrier for the reaction of hydrogen-bonded MeOH dimer than for monomeric MeOH addition. They also show that the greater the atomic number of the group 14 element, E, contained in the Rea-CE molecule, the smaller the activation barrier for its methanol addition reaction. It is thus expected that the presence, in such reactions, of a substantially more polar solvent and a heavier Rea-CE molecule will promote a further change in the mechanism. (d) The two pathways (paths I and II) for the reaction of the adamantyl-substituted Rea-CE double-bond molecules with methanol, which yield the same final product, appear in Figure 3. The key geometrical parameters of Pro-CC-MeOH, ProCSi-MeOH, Pro-CGe-MeOH, Pro-CSn-MeOH, and Pro-CPb-MeOH are given in Table 1. From the theoretical results depicted in this table, it should be noted that the TS of path II lies early in the reaction coordinate, as judged by the length of the forming E−O bonds, relative to the E−O bond length in the final addition product (Pro-CE-MeOH). That is, the newly forming E−O bonds in the transition structures (TS-CE-MeOH) of path I are stretched by 47.4% (C), 26.1% (Si), 21.6% (Ge), 23.0% (Sn), and 24.7% (Pb) relative to their final equilibrium values in the corresponding products (Pro-CE-MeOH), while those (TS-CE-(MeOH)2) for path II are stretched by 50.0% (C), 26.4% (Si), 29.1% (Ge), 46.8% (Sn), and 23.8% (Pb), respectively. On the basis of the Hammond postulate,20 path II is more favorable than path I, with a smaller activation barrier, as well as being more
exothermic. Indeed, this is supported by our earlier predictions, as already mentioned in Table 1. In consequence, our theoretical findings reveal that the adamantyl-substituted ReaCE molecules react with dimeric methanol, rather than monomeric methanol, to undergo the 1,2-addition reaction. These theoretical results are in reasonable agreement with
previous observations reported by Leigh, Bendikov, Apeloig, et al.8 (e) This study attempts to provide a theoretical model to explain the activation energy and the reaction enthalpy, as stated above. This explanation will embrace the general conclusions that Pross and Shaik arrived at, with the aid of the configuration mixing (CM) model.21,22 It is necessary to emphasize, here, the importance of the status of the triplet state for the Rea-CE and MeOH reactants. Since two new covalent bonds are formed in the product, i.e., the C−H and E−O bonds, the bond-prepared Rea-CE state must have at least two open shells. The lowest state of this type is the triplet state. Thus, from the valence-bond point of view, the bonding in the product can be identified as that between the triplet ReaCE state and triplet H−OCH3 (overall singlet) (see 2). This is similar to the bonding in a water molecule, which can be considered as that between a triplet oxygen atom and two doublet hydrogen atoms.23 Therefore, according to the conclusions of the CM model, both the energy barriers and the reaction enthalpies governing the chemical processes are proportional to the energy gaps for the reactants, that is, ΔEst (= Etriplet − Esinglet for Rea-CE) + ΔEσσ* (= Etriplet − Esinglet for CH 3 OH). From the free energy calculations for the aforementioned five reactants on path II (Table 1), a linear relationship is evident between the activation barrier and ΔEst (units in kcal/mol; r 2 is the correction coefficient): ΔE ⧧II = 1.53x − 18.0 (x = ΔEst, ΔE ⧧II = the activation energy; r 2 = 0.947). Likewise, there is also a linear correlation between ΔEst and the reaction enthalpy (ΔH), at the same level of theory: ΔH = 2.02x − 76.1 (r 2 = 0.855). A similar linear relationship also exists between the singlet−triplet splitting (ΔEst) of the Rea-CE species and the barrier heights (ΔE ⧧I), for path I. These theoretical investigations show that the singlet−triplet energy separation (ΔEst) of the adamantyl-substituted Rea-C E molecules is responsible for the order of the energies of their activation barriers and reaction enthalpies.21,22 (f) As pointed out by one reviewer, we have performed single-point MP2 calculations based on the B3LYP/LANL2DZoptimized geometries (i.e., MP2/6-31G(d)//B3LYP/ LANL2DZ). The relative energetics based on the MP2 computations are given in Table A (Supporting Information). One sees in this table that the MP2 calculations predict lower activation barriers by at least ca. 0.2 kcal/mol and more exothermic reactions at least by ca. 13 kcal/mol than do the B3LYP models. Indeed, such observations are commonplace when the B3LYP and MP2 models are compared.24 Moreover, it is well known that hybrid DFT calculations inherently favor spin states of high multiplicities owing to the explicit consideration of Fermi correlation through exchange admixture.25 Therefore, as expected on the basis of the overemphasis 6194
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of triplet-state stability in the B3LYP calculations, the ΔEst was computed to be smaller by this method than by the MP2 method (see Table A). (g) Reviewers also suggested to consider the role played by the adamantyl substituent if this group is replaced by two simple alkyl substituents. We thus chose the simplified model molecules (H2CE(SiH3)2, E = C, Si, Ge, Sn, and Pb) to study their MeOH addition reactions using the B3LYP/ LANL2DZ level of theory. The relative energetics for both the activation barrier and the reaction enthalpy are given in Table B (Supporting Information), compared with the data from Table 1 for the cases of the Rea-CE species. From Table B, one can see that the steric bulks of the adamantyl substituent and silylalkyl groups raise the transition-state barriers in the range 11−31 kcal/mol, compared to those for H 2CE(SiH3)2 molecules. Moreover, the overall reactions for the simplified compounds are quite exothermic, i.e., in the range −12 to −67 kcal/mol. Therefore, these theoretical examinations show that the smaller attached substituents, which can reduce the activation energies and lead to larger exothermicities, can significantly enhance the MeOH addition reactions. In short, our theoretical investigations reveal that both electronic and steric effects can greatly affect the activation barrier heights and reaction enthalpies as well as the trends in the reactivities of the Rea-CE molecules. 4. [2 + 4] Cycloaddition Reactions of AdamantylSubstituted Doubly Bonded CE Molecules with 1Methoxybutadiene. It is well known that cycloaddition reactions, involving olefins or alkynes, play an exceedingly important role in organic chemistry. 26 The adamantylsubstituted Rea-CE (E = C, Si, Ge, Sn, and Pb) systems were applied, as model reactants, to study their cycloaddition reactions.27 The [2 + 4] Diels−Alder reactions of the Rea-C E compound with 1-methoxybutadiene, which have already been studied experimentally (see eq 2),6 are studied. In this work, the Diels−Alder reaction mechanisms can be considered to proceed as follows: reactants (Rea-CE + 1-methoxybutadiene) → transition state (TS-CE-Methoxybutadiene) → cycloaddition [2 + 4] product (Pro-CE-Methoxybutadiene), which is schematically outlined in Figure 4. Selected geometrical parameters for these stationary points and their relative energies, calculated at the B3LYP/LANL2DZ level of theory, are also shown in Figure 4. The major conclusions to be drawn from the current study can be summarized as follows. (a) As seen in Figure 4, the Diels−Alder transition states are represented by TS-CC-Methoxybutadiene, TS-CSi-Methoxybutadiene, TS-CGe-Methoxybutadiene, TS-CSnMethoxybutadiene, and TS-CPb-Methoxybutadiene, which all show a marked asymmetry. However, these transition states still have a six-membered-ring structure. It can be observed that the main components of the transition vector correspond to the motion of the ring cycloaddition between the centered CE bond and the carbon atoms of 1-methoxybutadiene, whose eigenvalue gives an imaginary frequency of 425i (C), 199i (Si), 139i (Ge), 117i (Sn), and 106i (Pb) cm−1. Indeed, inspection of the transition vector shows clearly that the reaction proceeds toward formation of the cycloaddition product. Additionally, our theoretical data indicate that all of the transition states for such Diels−Alder cycloadditions are asynchronous. For convenience, the carbon bonding to carbon will have the symbol C1 and that bonding to E is C4. For instance, the forming C−C1 bond is greatly stretched, i.e., TS-CCMethoxybutadiene, 2.666 Å; TS-CSi-Methoxybutadiene,
Figure 4. B3LYP/LANL2DZ-optimized geometries (in Å) of the transition state (TS-CE-Methoxybutadiene) and [2 + 4] cycloaddition product (Pro-CE-Methoxybutadiene) for the Diels−Alder reaction between reactants Rea-CE (E = C, Si, Ge, Sn, and Pb) and 1-methoxybutadiene. Selected geometrical parameters and relative energies for each species (energy relative to the corresponding reactants) are given as well. Some hydrogens are omitted for clarity.
3.377 Å; TS-CGe-Methoxybutadiene, 3.478 Å; TS-CSnMethoxybutadiene, 3.665 Å; and TS-CPb-Methoxybutadiene, 3.297 Å, whereas the other (M−C4) is stretched only a little, i.e., TS-CC-Methoxybutadiene, 2.009 Å; TS-CSiMethoxybutadiene, 2.456 Å; TS-CGe-Methoxybutadiene, 2.448 Å; TS-CSn-Methoxybutadiene, 2.520 Å; and TS-C Pb-Methoxybutadiene, 2.507 Å. Although these transition structures are formed asynchronously, calculations indicate that the cycloadditions of adamantyl-substituted Rea-CE with 1methoxybutadiene are still concerted, since no energy minimum, corresponding to an intermediate between the transition state and the products, is evident. In consequence, our theoretical investigations suggest that Diels−Alder cycloadditions of adamantyl-substituted Rea-CE species should retain stereochemical integrity in the CE skeleton. As there are no relevant experimental or theoretical data on such systems, this result is a prediction. (b) It is already known that the Diels−Alder reaction of the parent molecules, 1,3-butadiene and ethane, is difficult, requiring high temperature and pressure.26 Nevertheless, the reaction can be accelerated by the substitution of π-electron donors on the diene moiety and by the presence of electronwithdrawing substituents on the alkene. This is the case studied in this work. The methoxy group, which is attached to the diene system, is a π-electron donor and the adamantyl-substituted 6195
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Rea-CE reactant is a well-known species of electrophile, which then becomes a good dienophile. Therefore, it is expected that the [2 + 4] cycloadditions between adamantylsubstituted Rea-CE and 1-methoxybutadiene will be fast and easy reactions. This qualitative prediction will be confirmed by the theoretical investigations (vide infra). According to the available experimental observation about the Diels−Alder reaction of 1 and 1-methoxybutadiene,6 no isomeric regioisomers were detected. From Figure 4, the reason for C1 attacking carbon and C4 attacking the E element to produce the regioselective product is simply because of the electronic effects. On the basis of the present theoretical calculations (B3LYP/LANL2DZ), it was found that the 1methoxybutadiene molecule bears the less negative charge (−0.058) and more negative charge (−0.575) on the C 1 and C4 atoms, respectively. As a result, it is reasonable to predict that the C1 of the 1-methoxybutadiene would attack on the C-end of the CE bond, while the C4 would attack on its E-end due to the C and E elements of the CE bond featuring negative and positive charges, respectively. In other words, the trends in the reaction barriers as well as reaction enthalpies are mainly due to changes in electronic effects, which can be easily understood by the frontier orbital (FMO) theory, i.e., changes in HOMO(diene)−LUMO(dienophile) energy difference and interacting orbital coefficients.28,29 Interested readers can find the related data in the Supporting Information. (c) As already mentioned, in FMO theory,29 it has been found that the CE double-bond lengths in the TS-CEMethoxybutadiene species are longer than that predicted for free Rea-CE. For example, the results of calculations, given in Figure 4, show that TS-CC-Methoxybutadiene (1.474 Å) > Rea-CC (1.387 Å), TS-CSi-Methoxybutadiene (1.811 Å) > Rea-CSi (1.761 Å), TS-CGe-Methoxybutadiene (1.877 Å) > Rea-CGe (1.830 Å), TS-CSn-Methoxybutadiene (2.038 Å) > Rea-CSn (2.002 Å), and TS-CPbMethoxybutadiene (2.105 Å) > Rea-CPb (2.088 Å). It should be noted that the order of the CE double-bond length in the TS-CE-Methoxybutadiene species follows the same trend as that for the atomic number of the group 14 element, E, from carbon to lead. Further, the DFT calculations suggest that the CE double bond is stretched by 6.3%, 2.8%, 2.5%, 1.8%, and 0.81%, for TS-CC-Methoxybutadiene, TSCSi-Methoxybutadiene, TS-CGe-Methoxybutadiene, TS-CSn-Methoxybutadiene, and TS-CPb-Methoxybutadiene, respectively, relative to their corresponding reactant, Rea-CE. These results strongly imply that, according to the Hammond postulate,20 the transition state for an adamantylsubstituted Rea-CE doubly bonded molecule with a heavier group 14 atom, E, should take on a more reactant-like character and the barrier should be encountered earlier than that for a Rea-CE analogue with a lighter group 14 atom, E. Our theoretical model calculations confirm this prediction. As demonstrated in Figure 4, the Gibbs activation barriers (kcal/ mol) for such [2 + 4] Diels−Alder increase as the atomic weight of the atom E decreases. This, in turn, leads to a stable transition state and a lower activation barrier. It must be emphasized that, in comparison with the methanol addition reaction discussed earlier (eq 1), the Diels−Alder cycloaddition (eq 2) studied in this work requires a considerably greater activation energy. (d) The computationally predicted cycloaddition products (Pro-CE-Methoxybutadiene) of the Diels−Alder reactions between the adamantyl-substituted Rea-CE (E = C, Si, Ge,
Sn, and Pb) molecule and 1-methoxybutadiene are observed to adopt a half-chair formation in order to minimize steric repulsions. Similarly to the activation barrier shown in Figure 4, the energetic ordering of such Diels−Alder cycloadditions shows that the Gibbs free enthalpy (kcal/mol) for the process decreases as the atomic weight of the atom E increases. Again, this result is in good agreement with the observations stressed earlier, in which the saddle point bearing group 14 element E with heavier atomic weight lies much closer to the reactants than the products. Furthermore, our theoretical investigations indicate that the Gibbs free energies of Pro-CE-Methoxybutadiene (E = Si, Ge, Sn, and Pb) are less than those of their corresponding reactants, except for the case of Pro-CCMethoxybutadiene. These results strongly imply that this type of reaction is energetically favorable for adamantyl-substituted Rea-CE compounds possessing a heavier group 14 element, E, and that the reactions are exothermic at room temperature. (e) Again, all of these computational results can be explained using a CM model as stated previously.21,22 According to this model, the stabilization of the transition state of a [2 + 4] Diels−Alder cycloaddition reaction depends on the singlet− triplet splitting ΔEst (= Etriplet − Esinglet) of the reactant, ReaCE; that is, a smaller ΔEst results in a more stable transition state, a lower activation energy, and a faster cycloaddition reaction. Our model calculations confirm this prediction and suggest a decreasing trend in ΔEst (kcal/mol) for reactant molecules (Rea-CE). As previously demonstrated, it can be seen that this result is in strong agreement with the trend in activation energies and reaction enthalpies for the Diels−Alder cycloadditions of the adamantyl-substituted Rea-CE systems. Accordingly, our theoretical findings strongly suggest that the greater the atomic number of the group 14 atom, E, in the ReaCE compound, the smaller the activation energy and the more exothermic (or the less endothermic) the [2 + 4] cycloaddition reaction. This theoretical work also shows that the singlet−triplet splitting of a Rea-CE species can be used as a diagnostic tool to predict the reactivities of various ReaCE analogues in Diels−Alder cycloaddition reactions. 5. [2 + 2] Cycloaddition Reactions of AdamantylSubstituted Doubly-Bonded CE Molecules with Ketone. The mechanisms for the addition of other nucleophiles, such as ketones, to the molecules bearing the CE double bond have been much less extensively investigated either experimentally or theoretically30 than that for alcohol addition (eq 2). Therefore, it is of interest to investigate the [2 + 2] cycloaddition reaction between the adamantyl-substituted Rea-CE (E = C, Si, Ge, Sn, and Pb) molecule and a ketone (eq 3). Similarly to the Diels−Alder reaction, such [2 + 2] cycloaddition mechanisms can be represented as follows: reactants (Rea-CE + (CH3)CO(CH3)) → transition state (TS-CE-Ketone) → [2 + 2] product (Pro-CE-Ketone), which is schematically shown in Figure 5. Selected geometrical parameters for these stationary points and their relative energies, calculated at the B3LYP/ LANL2DZ level of theory, compared with the corresponding reactants, are also summarized in Figure 5. There are several important conclusions to be drawn from these results. (a) According to the Woodward−Hoffmann rules for cycloaddition reactions,31 a concerted supra−supra process ([2π s + 2π s]) is thermally forbidden. Nevertheless, it is generally believed that the polarization of the double bond results in a relaxation of these rules.32 Indeed, several groups have reported the dimerization of silene (H2CSiH2) by ab 6196
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the single imaginary frequency values are 1234i (CC), 641i (CSi), 482i (CGe), 328i (CSn), and 249i (CPb) cm−1, respectively. As seen in Figure 5, the major component of the TS-CE-Ketone vibrational mode corresponds to the displacement of the ketone toward a double bond in the adamantyl-substituted Rea-CE compound during the formation of a four-membered-ring cycloadduct. Figure 5 illustrates two interesting geometrical features of the transition state. The first is that it is, indeed, planar, thus confirming the belief that the Rea-CE and ketone [2 + 2] cycloaddition reactions proceed through a concerted [2π s + 2π s] mechanism. The second point of interest is the CE and CO bond lengths and the C···C and O···E interaction distances. DFT calculations indicate that the CE bond is stretched by 6.2%, 2.8%, 3.2%, 2.9%, and 3.1% for TS-CCKetone, TS-CSi-Ketone, TS-CGe-Ketone, TS-CSnKetone, and TS-CPb-Ketone, respectively, relative to its value in the corresponding adamantyl-substituted Rea-CE reactant. Also, it is found that the CO bond in the carbonyl unit is longer, by 11% (CC), 4.6% (CSi), 4.4% (CGe), 3.4% (CSn), and 3.0% (CPb), respectively, relative to that in the primitive ketone. This theoretical evidence indicates that the structures of the transition states are much as one would expect from Hammond’s postulate.20 That is, the [2 + 2] reaction for the adamantyl-substituted Rea-CE molecule possessing a heavy group 14 element, E, is exothermic (vide infra), so the transition state should resemble the reactants more closely than the products. In consequence, the barrier for the [2 + 2] process is encountered earlier the heavier the group 14 atom, E, which is already confirmed by our B3LYP calculations, as given in Figure 5. (c) The equilibrium geometries for the pericyclic [2 + 2] products (i.e., Pro-CC-Ketone, Pro-CSi-Ketone, ProCGe-Ketone, Pro-CSn-Ketone, and Pro-CPb-Ketone) are presented in Figure 5. These theoretical results show that all of the [2 + 2] products, Pro-CE-Ketone, adopt a planar, four-membered-ring geometry. Unfortunately, experimentally verified structures for these cycloaddition products are not yet known. As discussed above, an adamantylsubstituted Rea-CE reactant with an element E of heavier atomic weight reaches the transition state relatively early, whereas an element E with a lighter atomic weight arrives relatively late. The former is therefore predicted to undergo a more exothermic cycloaddition, which is borne out by our theoretical calculations. For instance, the order of Gibbs free enthalpy (kcal/mol), in Figure 5, follows the same trend as that of activation energy. In consequence, considering both the calculated activation barriers and reaction enthalpies, it can be concluded that, for the [2 + 2] pericyclic reaction of an adamantyl-substituted Rea-CE molecule, the order of reactivity is CC ≪ CSi < CGe < CSn < CPb. This is a consequence of the strength of the carbon−E bond. That is to say, the heavier the group 14 atom E, the larger the atomic radius of E and the smaller the CE bond-breaking energy. This, in turn, leads to a decrease in the barrier height and an increase in the exothermicity of its bimolecular pericyclic reaction (eq 3). (d) The [2 + 2] computational results can be understood with reference to the CM model, cited previously.21,22 Using this model, the barrier height (ΔE ⧧) and the reaction enthalpy (ΔH) can be expressed in terms of the singlet−triplet splitting ΔEst (= Etriplet − Esinglet) of the reactants. Accordingly, the smaller the value of ΔEst for the adamantyl-substituted Rea-
Figure 5. B3LYP/LANL2DZ-optimized geometries (in Å) of the transition state (TS-CE-Ketone) and cycloaddition product (ProCE-Ketone) for the [2 + 2] reaction between reactants Rea-CE (E = C, Si, Ge, Sn, and Pb) and ketone. Selected geometrical parameters and relative energies for each species (energy relative to the corresponding reactants) are given as well. Hydrogens are omitted for clarity.
initio methods33 and concluded that the process proceeds by a concerted, [2π s + 2π s] cycloaddition mechanism, made kinetically favorable by the relaxation of orbital symmetry restrictions, due to the high polarity of the CSi bond. On the basis of this conclusion, this reasoning can also be applied to the systems studied in this work. That is to say, the considerable polarity of the CE double bond in the adamantyl-substituted Rea-CE (E = C, Si, Ge, Sn, and Pb) reactant can relieve the symmetry restriction, and its bimolecular, pericyclic [2 + 2] reaction with a ketone is no longer forbidden (see below).34 (b) As stated earlier, the electronegative oxygen of a ketone attacks the electropositive group 14 element, E, of the adamantyl-substituted Rea-CE reactant. Simultaneously, the electropositive carbon of the carbonyl group attacks the electronegative carbon of the centric CE double bond. This formation leads to a four-membered-ring transition-state structure (TS-CE-Ketone), as shown in Figure 5. The transition states (TS-CC-Ketone, TS-CSi-Ketone, TSCGe-Ketone, TS-CSn-Ketone, and TS-CPb-Ketone) were located for each CE species at DFT, along with the imaginary frequency eigenvector. These reactions appear to be concerted. It was possible to locate only one transition state for each reaction and to confirm that it is a true transition state on the basis of frequency analysis. The B3LYP/LANL2DZ frequency calculations for these transition states suggest that 6197
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CE reactant, the lower its barrier height, the greater its exothermicity, and the faster the [2 + 2] reaction with the ketone. With reference to the DFT Gibbs free energy calculations for the aforementioned five systems, detailed in Figure 5, the following correlations are noted (units in kcal/ mol; r 2 is the correction coefficient):
Further, this work demonstrates that the computational results can be explained using a simple CM model. The concepts of the CM model, which focus on the singlet−triplet splitting in the reactants, allow speedy estimation of the relative reactivity of a variety of adamantyl-substituted Rea-CE species, without specific knowledge of the actual energies of the interactions involved. The predictions provided by this study will be useful as a diagnostic tool in future efforts at synthesis and to indicate problems that merit further study, both experimentally and theoretically. It is ultimately hoped that the present work can stimulate further research into this subject.
(A) (B)
As one can see in eqs A and B, a linear correlation exists between ΔEst and ΔE ⧧ (the activation barrier) as well as ΔH (the reaction enthalpy). Consequently, our model calculations provide strong evidence that electronic factors, resulting from the presence of the group 14 element, play a key role in determining the reactivity of the adamantyl-substituted ReaCE species. (e) Again, the reviewers suggested estimating the steric effects for the [2 + 2] cycloaddition reactions of adamantylsubstituted doubly bonded CE molecules. We thus also used the simplified model molecules (H2CE(SiH3)2; E = C, Si, Ge, Sn, and Pb) to study the steric effect of their [2 + 2] cycloaddition reactions with ketone using the B3LYP/ LANL2DZ level of theory. The relative energetics for both the activation barrier and the reaction enthalpy are represented in Table C (Supporting Information), compared with the values from Figure 5 for the cases of the Rea-CE species. Once again, the data in Table C indicate that attaching bulky substituents to the group 14 elements raises the reaction barriers and lowers the exothermicity. These facts can be easily understood because the cycloaddition transition state and the cycloproduct put several bulky substituents in proximity to each other, which can greatly increase the steric repulsions between the two molecules. Simply speaking, from the above study, our theoretical findings indicate that both electronic and steric effects play a pivotal role in determining the barrier heights and reaction enthalpies as well as the trends in the reactivities of the Rea-CE molecules.
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ASSOCIATED CONTENT
S Supporting Information *
This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author *E-mail:
[email protected].
ACKNOWLEDGMENTS
The authors are grateful to the National Center for HighPerformance Computing of Taiwan for generous amounts of computing time. They also thank the National Science Council of Taiwan for the financial support. Special thanks are also due to the reviewers for very helpful suggestions and comments.
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REFERENCES
(1) (a) Brook, A. G.; Abdesaken, F.; Gutekunst, B.; Gutekunst, G.; Kallury, R. K. J. Chem. Soc., Chem. Commun. 1981, 191. (b) Brook, A. G.; Nyburg, S. C.; Abdesaken, F.; Gutekunst, B.; Gutekunst, G.; Kallury, R. K. M. R.; Poon, Y. C.; Chang, Y.-M.; Wong-Ng, W. J. Am. Chem. Soc. 1982, 104, 5667. (2) For recent comprehensive reviews of silenes, see: (a) Brook, A. G.; Brook, M. A. Adv. Organomet. Chem. 1996, 39, 71. (b) Müller, T.; Ziche, W.; Auner, N. In The Chemistry of Organic Silicon Compounds; Rappoport, Z., Apeloig, Y., Eds.; Wiley: New York, 1998; Vol. 2, Chapter 16. (c) Leigh, W. J. Pure Appl. Chem. 1999, 71, 453. (d) West, R. Polyhderon 2001, 21, 467. (e) Morkin, T. L.; Leigh, W. J. Acc. Chem. Res. 2001, 34, 129. (f) Gusel’nikov, L. E. Coord. Chem. Rev. 2003, 244, 149. (g) Ottosson, H.; Eklöf, A. M. Coord. Chem. Rev. 2008, 252, 1287. (h) Scheschkewitz, D. Chem.Eur. J. 2009, 15, 2476. (3) For instance, see: (a) Brook, A. G.; Abdesaken, F.; Gutekunst, G.; Plavac, N. Organometallics 1982, 1, 994. (b) Wiberg, N.; Wagner, G.; Müller, G. Angew. Chem., Int. Ed. Engl. 1985, 24, 229. (c) Miracle, G. E.; Ball, J. L.; Powell, D. R.; West, R. J. Am. Chem. Soc. 1993, 115, 11598. (d) Trommer, M.; Miracle, G. E.; Eichler, B. E.; Powell, D. R.; West, R. Organometallics 1997, 16, 5737. (e) Sakamoto, K.; Ogasawara, J.; Sakurai, H.; Kira, M. J. Am. Chem. Soc. 1997, 119, 3405. (f) Veszprémi, T.; Takahashi, M.; Hajgató, B.; Ogasawara, J.; Sakamoto, K.; Kira, M. J. Phys. Chem. A 1998, 102, 10530. (g) Boesveld, W. M.; Gehrhus, B.; Hitchcock, P. B.; Lappert, M. F.; von Ragué Schleyer, P. Chem. Commun. 1999, 755. (h) Sakamoto, K.; Ogasawara, J.; Kon, Y.; Sunagawa, T.; Kabuto, C.; Kira, M. Angew. Chem., Int. Ed. 2002, 41, 1402. (i) EI-Sayed, I.; Guliashvili, T.; Hazell, R.; Gogoll, A.; Ottosson, H. Org. Lett. 2002, 4, 1915. (j) Toulokhonova, I. S.; Guzei, I. A.; West, R. J. Am. Chem. Soc. 2004, 126, 5336. (4) For instance, see: (a) Baines, K. M.; Brook, A. G.; Ford, P. R.; Lickiss, P. D.; Saxena, A. K.; Chatterton, W. J.; Sawyer, J. F.; Behnam, B. A. Organometallics 1989, 8, 693. (b) Brook, A. G.; Baumegger, A.; Lough, A. J. Organometallics 1992, 11, 3088. (c) Ohshita, J.; Masaoka, Y.; Masaoka, S.; Ishikawa, M.; Tachibana, A.; Yano, T.; Yamabe, T. J. Organomet. Chem. 1994, 473, 15. (d) Ohshita, J.; Masaoka, S.; Masaoka, Y.; Hasebe, H.; Ishikawa, M.; Tachibana, A.; Yano, T.;
IV. CONCLUSION This work studied the mechanisms for three kinds of chemical reactions of adamantyl-substituted Rea-CE (E = C, Si, Ge, Sn, and Pb) species featuring the CE double bond using density functional theory. It must be mentioned here that this study provides the first theoretical demonstration of the reaction trajectory and theoretical estimation of the activation energy and reaction enthalpy for these chemical processes. Our theoretical examinations demonstrate that the chemical reactivity of adamantyl-substituted Rea-CE compounds increases in the order Rea-CC ≪ Rea-CSi < Rea-C Ge < Rea-CSn < Rea-CPb. From a mechanistic viewpoint, our theoretical findings confirm the general belief that one of the crucial influences on the isolability of a doubly bonded group 14 Rea-CE molecule is its centric CE double bond.2 That is to say, our theoretical investigations strongly imply that adamantyl-substituted, doubly bonded C E molecules with a lighter group 14 element (such as E = C and Si) should be stable and able to be readily synthesized and isolated at room temperature. In short, electronic as well as steric effects play a decisive role in determining the chemical reactivity of the group 14 adamantyl-substituted Rea-CE molecules, both kinetically and thermodynamically. 6198
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Organometallics
Article
Chem. 1984, 273, 141. (c) Apeloig, Y.; Karni, M. J. Am. Chem. Soc. 1984, 106, 6676. (d) Brook, A. G.; Safa, K. D.; Lickiss, P. D.; Baines, K. M. J. Am. Chem. Soc. 1985, 107, 4338. (e) Jones, P. R.; Bates, T. F. J. Am. Chem. Soc. 1987, 109, 913. (f) Steinmetz, M. G.; Udayakumar, B. S.; Gordon, M. S. Organometallics 1989, 8, 530. (g) Kira, M.; Maruyama, T.; Sakurai, H. J. Am. Chem. Soc. 1991, 113, 3986. (h) Sluggett, G. M.; Leigh, W. J. J. Am. Chem. Soc. 1992, 114, 1195. (i) Leigh, W. J.; Sluggett, G. M. J. Am. Chem. Soc. 1994, 116, 10468. (j) Leigh, W. J.; Bradaric, C. J.; Kerst, C.; Banisch, J. H. Organometallics 1996, 15, 2246. (k) Baradarci, C. J.; Leigh, W. J. J. Am. Chem. Soc. 1996, 118, 8971. (l) Baradarci, C. J.; Leigh, W. J. Can. J. Chem. 1997, 75, 1393. (m) Veszprémi, T.; Takahashi, M.; Ogasawara, J.; Sakamoto, K.; Kira, M. J. Am. Chem. Soc. 1998, 120, 2408. (n) Leigh, W. J.; Boukherroub, R.; Kerst, C. J. Am. Chem. Soc. 1998, 120, 9504. (o) Leigh, W. J.; Kerst, C.; Boukherroub, R.; Morkin, T. L.; Jenkins, S. L.; Sung, K.; Tidwell, T. T. J. Am. Chem. Soc. 1999, 121, 4744. (p) Leigh, W. J.; Toltl, N. P.; Apodaca, P.; Castruita, M.; Pannell, K. H. Organometallics 2000, 19, 3232. (q) Morkin, T. L.; Owens, T. R.; Leigh, W. J. In The Chemistry of Organic Silicon Compounds; Rappoport, Z., Apeloig, Y., Eds.; Wiley: Chichester, UK, 2001; Vol. 3, Chapter 9. (r) Morkin, T. L.; Leigh, W. J.; Tidwell, T. T.; Allen, A. D. Organometallics 2001, 20, 5707. (s) Takahashi, M.; Sakamoto, K.; Kira, M. Int. J. Quantum Chem. 2001, 84, 198. (t) Leigh, W. J.; Li, X. Organometallics 2002, 21, 1197. (u) Bendikov, M.; Quadt, S. R.; Rabin, O.; Apeloig, Y. Organometallics 2002, 21, 3930. (v) Owens, T. R.; Harrington, C. R.; Pace, T. C. S.; Leigh, W. J. Organometallics 2003, 22, 5518. (w) Oláh, J.; Veszprémi, T. Organometallics 2008, 27, 2723. (x) Guliashvili, T.; Tibbelin, J.; Ryu, J.; Ottosson, H. Dalton Trans. 2010, 39, 9379. (19) It is proposed to proceed by initial nucleophilic attack at group 14 atom M to yield a weaker bonded complex, which collapses to an alkoxy-substituted molecule by proton transfer from oxygen to the center carbon. Proton transfer takes place by two competing pathways: one involving intramolecular migration within the weaker bonded complex, and the other involving a second molecule of alcohol. For more details, see ref 18. (20) Hammond, G. S. J. Am. Chem. Soc. 1955, 77, 334. (21) For details, see: (a) Shaik, S.; Schlegel, H. B.; Wolfe, S. In Theoretical Aspects of Physical Organic Chemistry; John Wiley & Sons Inc.: New York, USA, 1992. (b) Pross, A. In Theoretical and Physical Principles of Organic Reactivity; John Wiley & Sons Inc.: New York, USA, 1995. (c) Shaik, S. Prog. Phys. Org. Chem. 1985, 15, 197. (d) Shaik, S.; Hiberty, P. C. In A Chemist’s Guide to Valence Bond Theory; Wiley, Interscience: New York, USA, 2008. (22) (a) For the first paper that originated the CM model see: Shaik, S. J. Am. Chem. Soc. 1981, 103, 3692. (b) About the most updated review of the CM model: Shaik, S.; Shurki, A. Angew. Chem., Int. Ed. 1999, 38, 586. (23) Siegbahn, P. E. M. J. Am. Chem. Soc. 1996, 118, 1487. (24) For example, see: Foresman, J. B.; Frisch, A. Exploring Chemistry with Electronic Structure Methods; Gaussian, Inc.: Pittsburgh, PA, 2003; Chapters 6 and 8. (25) For instance, see: (a) Reiher, M.; Sundermann, A. Eur. J. Inorg. Chem. 2002, 1854. (b) Reiher, M.; Salomon, O.; Hess, B. A. Theor. Chim. Acta 2001, 107, 48. (26) For contemporary reviews, see: Ma, S., Ed. Handbook of Cycloaddition Reactions, Vols. 1 and 2; Wiley-VCH: Weinheim, Germany, 2009. (27) Diels−Alder cycloadditions to silenes: (a) Ishikawa, M.; Fuchikami, T.; Kumada, M. J. Organomet. Chem. 1978, 149, 37. (b) Brook, A. G.; Harris, J. W.; Lennon, J.; El Sheikh, M. J. Am. Chem. Soc. 1979, 101, 83. (c) Wiberg, N. J. Organomet. Chem. 1984, 273, 141. (d) Wiberg, N.; Wagner, G. Chem. Ber. 1986, 119 (1455), 1467. (e) Jones, P. R.; Lee, M. E.; Lin, L. T. Organometallics 1983, 2, 1039. (f) Jones, P. R.; Lim, T. F. O.; Pierce, R. A. J. Am. Chem. Soc. 1980, 102, 4970. (g) Brook, A. G.; Vorspohi, K.; Ford, R. R.; Hesse, M.; Chatterton, W. J. Organometallics 1987, 6, 2128. (h) Sewald, N.; Ziche, W.; Wolff, A.; Auner, N. Organometallics 1993, 12, 4123.
Yamabe, T. Organometallics 1996, 15, 3136. (e) Ohshita, J.; Masaoka, S.; Morimoto, Y.; Sano, M.; Ishikawa, M. Organometallics 1997, 16, 1123. (f) Ohshita, J.; Sakurai, H.; Tokunaga, Y.; Kunai, A. Organometallics 1999, 18, 4545. (g) Ohshita, J.; Sakurai, H.; Masaoka, S.; Tamai, M.; Kunai, A.; Ishikawa, M. J. Organomet. Chem. 2001, 633, 131. (h) Ohshita, J.; Tamai, M.; Sakurai, H.; Kunai, A. Organometallics 2001, 20, 1065. (i) Guliashvili, T.; EI-Sayed, I.; Fischer, A.; Ottosson, H. Angew. Chem., Int. Ed. 2003, 42, 1640. (j) Inoue, S.; Ichinohe, M.; Sekiguchi, A. Angew. Chem., Int. Ed. 2007, 46, 3346. (k) Bejan, I.; Güclii, D.; Inoue, S.; Ichinohe, M.; Sekiguchi, A.; Scheschkewitz, D. Angew. Chem., Int. Ed. 2007, 46, 3349. (5) For instance, see: (a) Batsanov, A. S.; Clarkson, I. M.; Howard, J. A. K.; Steel, P. G. Tetrahedron Lett. 1996, 37, 2491. (b) Ichinohe, M.; Tanaka, T.; Sekiguchi, A. Chem. Lett. 2001, 30, 1074. (c) Berry, M. B.; Griffiths, R. J.; Sanganee, M. J.; Steel, P. G.; Whelligan, D. K. Tetrahedron Lett. 2003, 44, 9135. (d) Showell, G. A.; Mills, J. S. Drug Discovery Today 2003, 8, 551. (e) Bains, W.; Tacke, R. Curr. Opin. Drug Discovery Dev. 2003, 6526. (f) Berry, M. B.; Griffiths, R. J.; Sanganee, M. J.; Steel, P. G.; Whelligan, D. K. Org. Biomol. Chem. 2004, 2, 2381. (g) Mills, J. S.; Showell, G. A. Exp. Opin. Invest. Drugs 2004, 13, 1149. (h) Sellars, J. D.; Steel, P. G.; Turner, M. J. Chem. Commun. 2006, 2385. (i) Sellars, J. D.; Steel, P. G. Org. Biomol. Chem. 2006, 4, 3223. (j) Ottosson, H.; Stell, P. G. Chem.Eur. J. 2006, 12, 1576. (k) Pooni, P. K.; Showell, G. A. Mini-Rev. Med. Chem. 2006, 6, 1169. (6) Apeloig, Y.; Bendikov, M.; Yuzefovich, M.; Nakash, M.; BravoZhivotovskii, D.; Bläser, D.; Boese, R. J. Am. Chem. Soc. 1996, 118, 12228. (7) (a) Bravo-Zhivotovskii, D.; Braude, V.; Stanger, A.; Kapon, M.; Apeloig, Y. Organometallics 1992, 11, 2326. (b) Apeloig, Y.; BravoZhivotovskii, D.; Zharov, I.; Panov, V.; Leigh, W. J.; Sluggett, G. W. J. Am. Chem. Soc. 1998, 120, 1398. (c) Bravo-Zhivotovskii, D.; PelegVasserman, H.; Kosa, M.; Molev, G.; Botoshanskii, M.; Apeloig, Y. Angew. Chem., Int. Ed. 2004, 43, 745. (d) Bravo-Zhivotovskii, D.; Melamed, S.; Kapon, M.; Apeloig, Y. Organometallics 2002, 21, 2049. (8) Leigh, W. J.; Owens, T. R.; Bendikov, M.; Zade, S. S.; Apleoig, Y. J. Am. Chem. Soc. 2006, 128, 10772. (9) (a) Becke, A. D. Phys. Rev. A 1988, 38, 3098. (b) Becke, A. D. J. Chem. Phys. 1993, 98, 5648. (10) Lee, C.; Yang, W.; Parr, R. G. Phys. Rev. B 1988, 37, 785. (11) (a) Dunning, T. H., Jr.; Hay, P. J. In Modern Theoretical Chemistry; Schaefer, H. F., III, Ed.; Plenum: New York, 1976; pp 1− 28. (b) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 270;(c) Ibid. 1985, 82, 284;(d) Ibid. 1985, 82, 299. (12) Hehre, W. J., Random, L., Schleyer, P. v. R., Pople, J. A. Ab-initio Molecular Orbital Theory; Wiley Interscience: New York, 1986; and references therein. (13) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Zakrzewski, V. G.; Montgomery, J. A., Jr.; Stratmann, R. E.; Burant, J. C.; Dapprich, S.; Millam, J. M.; Daniels, A. D.; Kudin, K. N.; Strain, M. C.; Farkas, O.; Tomasi, J.; Barone, V.; Cossi, M.; Cammi, R.; Mennucci, B.; Pomelli, C.; Adamo, C.; Clifford, S.; Ochterski, J.; Petersson, G. A.; Ayala, P. Y.; Cui, Q.; Morokuma, K.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Cioslowski, J.; Ortiz, J. V.; Baboul, A. G.; Stefanov, B. B.; Liu, Liashenko, G.; Piskorz, A.; Komaromi, P.; I.; Gomperts, R.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Gonzalez, C.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Andres, J. L.; Gonzalez, C.; Head-Gordon, M.; Replogle, E. S. Pople, J. A. GAUSSIAN 03; Gaussian, Inc.: Pittsburgh, PA, 2003. (14) Wade, Jr. L. G. In Organic Chemistry; Pearson Education Inc.: New York, 2009; p 282. (15) Power, P. P. Chem. Rev. 1999, 99, 3463. (16) Albright, T. A.; Burdett, J. K.; Whangbo, M.-H. Orbital Interactions in Chemistry; John Wiley & Sons: New York, 1985; p164. (17) We thank reviewer 3 for this useful comment. (18) The addition of alcohols to silenes: (a) Nagase, S.; Kudo, T. J. Chem. Soc., Chem. Commun. 1983, 363. (b) Wiberg, N. J. Organomet. 6199
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Article
(28) Kahn, S. D.; Pau, C. F.; Overman, L. E.; Hehre, W. J. J. Am. Chem. Soc. 1986, 108, 7381. (29) (a) Fleming, I. Frontier Orbitals and Organic Chemical Reactions; John Wiley & Sons: New York, 1976. (b) Albright, T. A.; Burdett, J. K.; Whangbo, M.-H. Orbital Interactions in Chemistry; John Wiley & Sons: New York, 1985; p164. (30) [2 + 2] cycloaddition of ketones to silenes: (a) Wiberg, N.; Link, M. Chem. Ber. 1995, 128, 1231. (b) Reference 2f. (31) Woodward, R. B.; Hoffmann, R. Angew. Chem., Int. Ed. Engl. 1969, 8, 781. (32) (a) Gusel’nikov, L. E.; Nametkin, N. S. Chem. Rev. 1979, 79, 529. (b) Baines, K. M.; Brook, A. G. Adv. Organomet. Chem. 1986, 25, 1. (33) (a) Ahlrichs, R.; Heinzmann. J. Am. Chem. Soc. 1977, 99, 7452. (b) R. Seidl, E. T.; Grev, R. S.; Schaefer, H. F. III. J. Am. Chem. Soc. 1992, 114, 3643. (34) As suggested by one reviewer, such [2 + 2] cycloadditions might undergo a biradical mechanism. However, repeated attempts to locate the biradical transition state for a [2 + 2] cycloaddition of ReaCE and (CH3)CO(CH3) using the B3LYP methodology were unsuccessful. We may therefore conclude that no biradical transition states exist on the B3LYP surface for the [2 + 2] cycloadditions of adamantyl-substituted group 14 metallene species. For related biradical mechanisms of [2 + 2] cycloadditions, see: (a) Milnes, K. K.; Baines, K. M. Organometallics 2007, 26, 2392. (b) Hardwick, J. A.; Baines, K. M. Organometallics 2010, 29, 1305.
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