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Theoretical Study on Stereochemical Diversity in the Addition of Water to Disilene Masae Takahashi,† Tama´s Veszpre´mi,‡ Balazs Hajgato´,‡ and Mitsuo Kira*,§ Photodynamics Research Center, The Institute of Physical and Chemical Research (RIKEN), 519-1399, Aoba, Aramaki, Aoba-ku, Sendai 980-0868, Japan, Department of Inorganic Chemistry, Technical University of Budapest, H-1521 Budapest, Hungary, and Department of Chemistry, Graduate School of Science, Tohoku University, Aoba-ku, Sendai 980-8578, Japan Received May 5, 2000 Summary: An ab initio MO study of the mechanisms for the addition of water to disilene (H2SidSiH2) has shown that two bimolecular addition pathways are feasible: one proceeds in a syn-addition manner, while the other proceeds in an anti-addition manner. In the latter pathway, the intracomplex electrophilic attack of a water hydrogen in an initial weak nucleophilic complex between disilene and water occurs with the antarafacial approach according to the Woodward-Hoffmann rules. In contrast to CdC double bonds, SidSi double bonds have been known to react with water and various alcohols very smoothly without catalysts to give the corresponding adducts.1 Although much attention has been focused recently on the mechanisms of the unique addition reactions of disilenes,2 the origin of the diverse stereochemical outcomes is still controversial. Young, Fink, West, and Michl first observed that the reaction of a stereoisomeric disilene, (E)-1,2-di-tert-butyl-1,2-dimesityldisilene, with various alcohols gave a mixture of two diastereomers.3 Sekiguchi, Maruki, and Sakurai found that the diastereoselectivity of the reactions of the transient disilenes (E)- and (Z)-PhMeSidSiMePh with alcohols was controlled by the concentration of the alcohols.4 They concluded that syn and anti adducts, which were preferred at the low and high concentrations of the alcohols, respectively, were formed via intra- and intermolecular transfer of an alcoholic hydrogen atom, respectively, at a four-membered cyclic intermediate. Recent studies by Apeloig and Nakash on the mechanisms of the addition of alcohols (phenols) to stable disilenes have been most intriguing for us,5 because the addition of p-methoxyphenol to (E)-1,2-di-tert-butyl-1,2dimesityldisilene was shown to be syn selective in * To whom correspondence should be addressed. E-mail: mkira@ si.chem.tohoku.ac.jp. Fax: +81-22-217-6589. † The Institute of Physical and Chemical Research (RIKEN). ‡ Technical University of Budapest. § Tohoku University. (1) For reviews on disilenes, see: (a) West, R. Pure Appl. Chem. 1984, 56, 163. (b) Raabe, G.; Michl, J. Chem. Rev. 1985, 85, 419. (c) West, R. Angew. Chem., Int. Ed. Engl. 1987, 26, 1201. (d) Raabe, G.; Michl, J. In The Chemistry of Organic Silicon Compounds; Patai, S., Rappoport, Z., Eds.; Wiley: New York, 1989; Part 2, Chapter 17. (e) Tsumuraya, T.; Batcheller, S. A.; Masamune, S. Angew. Chem., Int. Ed. Engl. 1991, 30, 902. (f) Grev, R. S. Adv. Organomet. Chem. 1991, 33, 125. (g) Okazaki, R.; West, R. Adv. Organomet. Chem. 1996, 39, 231. (2) For a recent review of the mechanisms of alcohol addition to disilenes, see: Sakurai, H. In The Chemistry of Organic Silicon Compounds; Rappoport, Z., Apeloig, Y., Eds.; Wiley: New York, 1998; Vol. 2, Part 1, Chapter 15. (3) De Young, D. J.; Fink, M. J.; West, R.; Michl, J. Main Group Met. Chem. 1987, 10, 19. (4) Sekiguchi, A.; Maruki, I.; Sakurai, H. J. Am. Chem. Soc. 1993, 115, 11460.
toluene but anti selective in the polar THF, even at low alcohol concentrations (Scheme 1).5b These results indicate that the bimolecular reaction of a disilene with an alcohol can afford an anti adduct, in contrast to the conventional idea. Although the stereochemical diversity has been explained by the competition of the rotation around the Si-Si bond in the zwitterionic intermediate with the intramolecular proton transfer, a clear-cut explanation for the remarkable solvent effects has not been afforded. To elucidate the origin of the remarkable stereochemical diversity, we have examined the detailed reaction pathways from reactants to products using ab initio MO calculations and found two bimolecular addition pathways are feasible: one proceeds in a syn addition manner (syn pathway), while the other proceeds in an anti addition manner (anti pathway).6,7 An important issue of the present calculations is the discovery of the bimolecular anti addition pathway, where from an initial weak nucleophilic complex between water and disilene, the intracomplex electrophilic attack of a water hydrogen atom occurs via the non-least-motion antarafacial approach according to the Woodward-Hoffmann rules. Our ab initio MO calculations7 revealed that two van der Waals complexes were formed at the initial stage of the reaction of disilene with water. These two complexes were derived by the following procedure. As shown in Figure 1, the HOMO and LUMO of trans bent disilene10 are π- and π*-type orbitals, respectively, while the HOMO and LUMO of water are the oxygen lone pair (n) and σ*(O-H) orbitals, respectively.11 According to the frontier MO (FMO) theory,12 LUMO(π*)-HOMO(n) and HOMO(π)-LUMO(σ*) interactions should play important roles during the initial encounter of the two reagents; the former corresponds to the nucleophilic (5) (a) Apeloig, Y.; Nakash, M. J. Am. Chem. Soc. 1996, 118, 9798. (b) Apeloig, Y.; Nakash, M. Organometallics 1998, 17, 1260. (c) Apeloig, Y.; Nakash, M. Organometallics 1998, 17, 2307. (6) Nagase et al. first carried out a theoretical investigation of the reaction of the parent disilene with water at the HF/6-31G* level and found the syn addition pathway via the four-centered cyclic transition state: Nagase, S.; Kudo, T.; Ito, K. In Applied Quantum Chemistry; Smith, V. H., Jr., Schaefer, H. F., Morokuma, K., Eds.; Reidel: Dordrecht, The Netherlands, 1986. Apeloig and Nakash have recently investigated in detail the addition reactions of CH3OH and CF3OH to Me2SidSiMe2 at the MP3/6-31G*//HF/6-31G* level.5c However, only pathways leading to the syn adducts were found in these calculations. (7) Our calculations were performed using the Gaussian 94 programs.8 The geometries of all stationary points along the reaction pathway were completely optimized at the MP2(full)/6-311++G** level.9 Minima and transition states were verified by establishing the matrixes of energy: second derivatives have zero and one negative eigenvalue, respectively. Intrinsic reaction coordinate (IRC) calculations confirmed all the reaction pathways from the saddle points.
10.1021/om000385u CCC: $19.00 © 2000 American Chemical Society Publication on Web 10/07/2000
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Organometallics, Vol. 19, No. 23, 2000 4661
Figure 1. Relative orbital energies of disilene and water calculated by the OVGF (outer valence Green’s function)14 method using a 6-311++G** basis set for the MP2(full)/ 6-311++G** geometry. MO drawings of the HOMO and LUMO of disilene show the MP2 natural orbitals, with the threshold level being 0.083. Scheme 1
encounter of water with disilene and the latter the electrophilic encounter. Inspection of the shape of these FMOs suggests that, in the nucleophilic encounter, the HOMO(n) of water approaches from the direction shown by a solid arrow in Figure 1, while in the electrophilic encounter, a water hydrogen atom with large orbital coefficient in the LUMO(σ*) approaches from the direction shown by a dotted arrow. From these two types of (8) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Gill, P. M. W.; Johnson, B. G.; Robb, M. A.; Cheeseman, J. R.; Keith, T.; Petersson, G. A.; Montgomery, J. A.; Raghavachari, K.; Al-Laham, M. A.; Zakrzewski, V. G.; Ortiz, J. V.; Foresman, J. B.; Cioslowski, J.; Stefanov, B. B.; Nanayakkara, A.; Challacombe, M.; Peng, C. Y.; Ayala, P. Y.; Chen, W.; Wong, M. W.; Andres, J. L.; Replogle, E. S.; Gomperts, R.; Martin, R. L.; Fox, D. J.; Binkley, J. S.; Defrees, D. J.; Baker, J.; Stewart, J. P.; Head-Gordon, M.; Gonzalez, C.; Pople, J. A. Gaussian 94; Gaussian, Inc.: Pittsburgh, PA, 1995. (9) To correct the basis set superposition error (BSSE), the counterpoise method is used at the MP2(fc)/6-311++G(3df,2p)//MP2(full)/ 6-311++G** level: Boys, S. F.; Bernardi, F. Mol. Phys. 1970, 19, 553. (10) For a review of the theoretical calculations on the geometry of disilenes, see: Apeloig, Y. In The Chemistry of Organic Silicon Compounds; Patai, S., Rappoport, Z., Eds.; Wiley: New York, 1989; Part 1, Chapter 2.
encounters, two van der Waals complexes were produced: CN from the nucleophilic encounter and CE from the electrophilic encounter. The stabilization energies of these complexes were less than 1 kcal mol-1, and the geometry of each reactant was almost unchanged in these complexes. The NPA (natural population analysis) group charge of the water part (δ(H2O)) is positive (0.061) in CN and negative (-0.009) in CE, indicative of a small extent of charge transfer between water and the disilene caused by the nucleophilic and electrophilic interaction, respectively.13 Two different reaction pathways, anti and syn pathways, were derived from CN and CE, respectively, as shown in Scheme 2.15 The electrophilic approach of a water hydrogen atom to a silicon in CN should involve an important interaction between the water LUMO(σ*) and the disilene HOMO(π), as shown in Figure 2a.16 According to the orbital symmetry consideration, the antarafacial approach of the water hydrogen atom to the pπ lobe of an opposite side of the disilene π plane is allowed, but the least motion suprafacial approach is forbidden, in this electrophilic approach; this situation is similar to that in the [2 + 2] cycloaddition.17 Actually, the electrophilic approach followed by ab initio MO calculations led to a transition state (TSE) with twisting around the Si-Si bond, which produced the anti adduct (PA) through another intermediate complex (a Lewis adduct, CL).18 The two H2Si planes in TSE were almost perpendicular to each other with a dihedral angle of 83.5°. On the other hand, from CE, the nucleophilic attack of water at the disilene should occur via the interaction of the water HOMO(n) with the disilene LUMO(π*)16 (Figure 2b). Since the oxygen n orbital was almost orthogonal to the pπ orbital at the approaching Si atom in CE, the least motion syn approach of the oxygen was chosen to lead to a four-membered cyclic transition state (TSN). From TSN, the syn adduct PS was derived through (11) Although the frontier orbitals of disilene and water are drawn as MP2 natural orbitals in Figure 1, a similar shape for the orbitals was obtained by the HF calculations. Whereas there are two σ*(OH) orbitals (A1 and B2) in C2v water, only a σ*(OH) orbital with A1 symmetry is shown in Figure 1 for simplicity; the discussion in this paper is not altered if the other σ*(OH) orbital is taken into account. Although the lowest excited state of water is known to be a valenceRydberg state as suggested by a reviewer, the LUMO in the groundstate molecular orbitals of water at the HF/6-311++G** level is the σ*(OH) orbital, which is taken to be the appropriate LUMO for FMO considerations. (12) (a) Fukui, K. Acc. Chem. Res. 1971, 4, 57. (b) Fleming, I. Frontier Orbital and Organic Chemical Reactions; Wiley-Interscience: New York, 1976. (13) For a review of the van der Waals complexes stabilized by weak charge-transfer interactions, see: Reed, A. E.; Curtiss, L. A.; Weinhold, F. Chem. Rev. 1988, 88, 899. (14) (a) Ortiz, J. V. Chem. Phys. 1988, 89, 6348. (b) Cederbaum, L. S. J. Phys. 1975, B8, 290. (c) Niessen, W. v.; Schirmer, J.; Cederbaum, L. S. Comput. Phys. Rep. 1984, 1, 57. (d) Zakrzewski, V. G.; Niessen, W. v. J. Comput. Chem. 1993, 14, 13. (15) In the present model reaction, there is no discrimination of the stereochemistry of the adduct (disilanol). Herein, the products formed by the formal syn and anti addition of water to the starting disilene are referred to as the syn adduct and anti adduct, respectively. For convenience, hydrogens are labeled in Scheme 2. All the structures of the stationary points found during the reactions are shown in Figure S1 in the Supporting Information. (16) Strictly speaking, the frontier orbitals of the van der Waals complex should be considered. However, the pertinent frontier orbitals of water and disilene can be used approximately, since the interaction of water with disilene in the complex is very weak, as suggested by the very small stabilization energy. (17) Woodward, R. B.; Hoffmann, R. The Conservation of Orbital Symmetry; Academic Press: New York, 1970.
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Communications Scheme 2
Figure 2. Schematic representation of the FMO interaction in the weak complexes (a) CN and (b) CE leading to CL and CL′, respectively.
the Lewis adduct CL′ identical in structure to CL (Scheme 2). The energy profiles for the two pathways are shown in Figure S2 in the Supporting Information. The energy barrier for the anti pathway was 5.16 kcal mol-1, while the barrier for the syn pathway was 2.92 kcal mol-1.19 Whereas in the model disilene-water addition reaction, the syn pathway is more favorable than the anti pathway, the preference of the two pathways in the actual reactions of disilenes with various alcohols will depend on the reaction conditions: the electronic and steric effects of substituents on disilene, the nature of the nucleophiles, solvents, etc. Our theoretical results may give a key to understand the bimolecular anti addition pathway found by Apeloig and Nakash,5b while the present model reaction is far remote from the experimental systems. The present calculations suggest that the intermediates determining the stereochemistry are not the zwitterionic intermediates, as suggested by Apeloig and Nakash,5b but two types of van der Waals complexes, CN and CE. The remarkable solvent effects observed experimentally5b are (18) The complex CL was characterized as a Lewis adduct, because the Si-O distance was much shorter than those of weak complexes CN and CE (Figure S1). A significantly shorter Si-H(O) distance accompanied by the elongation of the pertinent O-H bond distance (0.989 Å) suggests a significant Si- - -H(O) interaction in CL. Although there was a transition state (TSL) between CL and the final product PA, the process from CL to PA (CL′ to PS) proceeds with almost no barrier. The transition state TSL seems to correspond to the transition state found by Nagase et al.6 (19) Calculations with various electron correlations gave similar reaction profiles, while the relative energies of the transition states and intermediate complexes depend significantly on the order of perturbation. As suggested by a reviewer, the major part of the energy difference between TSN and TSE may come from the difference in their ZPE energies. See Tables S1 and S3 in the Supporting Information.
in good accord with the nature of the transition state for the anti pathway (TSE) being more polar than that for the syn pathway (TSN).20 Detailed experimental studies are required to confirm the validity of the above mechanistic view. The two pathways revealed by our calculations do not cover all the features of the reactions of disilenes with hydroxylic compounds. Apeloig and Nakash have actually found, using ab initio calculations at the MP3/631G*//HF/6-31G* level,5c that the reaction of CF3OH with Me2SidSiMe2 proceeds concertedly via a TSL-like transition state. In good accord with their results, our calculations showed that only the CE-TSL-PS pathway was obtained for the addition of CF3OH to H2SidSiH2;21 a similar concerted pathway was found in the HFaddition reaction to disilene. The third pathway involving TSL at the rate-determining step seems to be characteristic of the addition of a compound having a highly acidic hydrogen to disilenes.22 Further work is in progress. Acknowledgment. This work was supported in part by OTKA 0229976 (T.V.). Supporting Information Available: Tables and figures giving relative energies and structural parameters of complexes (CN, CE, CL) and transition states (TSE, TSN, TSL) at various levels and deuterium isotope effects on the transition states calculated at the CBS-Q level. This material is available free of charge via the Internet at http://www.pub.acs.org. OM000385U (20) The dipole moment of TSE (6.56 D) is much larger than that of TSN (3.51 D) at the MP2(full)/6-311++G** level. (21) Expectedly, at the MP2(full)/6-311++G** level, the reaction of CH3OH with H2SidSiH2 was found to proceed through the syn and anti pathways, which are similar to those in the reaction of water with H2SidSiH2. The pathway for the reaction of CH3OH with Me2SidSiMe2 found by Apeloig and Nakash5c is similar to the syn pathway, where TS1 and TS2 in their paper may correspond to TSN and TSL, respectively, in our paper. (22) By calculating the reaction of HOD with H2SidSiH2 to give H2DSi-SiH2OH, kinetic isotope effects (kH/kD) on the three elementary reactions characterized by the three transition states TSN, TSE, and TSL were estimated to be 0.96, 0.95, and 5.92, respectively (see the Supporting Information). The third CE-TSL-PS pathway, which has TSL as the rate-determining transition state, is expected to show large kinetic isotope effects, while there are no significant kinetic isotope effects for both syn and anti pathways in the water-disilene reaction. The large kH/kD value of 5.27 was actually observed by Apeloig and Nakash for the addition of p-CF3C6H4OH(D) to tetramesityldisilene,5a indicative of the CE-TSL-PS pathway.
