Quantum-Chemical Study of Neutral Lewis Base Catalyzed Allylation

Jan 21, 2010 - Ken Sakata*,† and Hiroshi Fujimoto‡,§. †Faculty of ... structure to facilitate the coordination of DMF or other neutral Lewis ba...
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Organometallics 2010, 29, 1004–1011 DOI: 10.1021/om901086q

Quantum-Chemical Study of Neutral Lewis Base Catalyzed Allylation of Aldehydes with Allyltrichlorosilanes Ken Sakata*,† and Hiroshi Fujimoto‡,§ †

Faculty of Pharmaceutical Sciences, Hoshi University, Ebara, Shinagawa-ku, Tokyo 142-8501, Japan, and ‡ Department of Molecular Engineering, Graduate School of Engineering, Kyoto University, Katsura, Nishikyo-ku, Kyoto 615-8510, Japan. §Emeritus Professor. Received December 18, 2009

Calculations at the MP2/6-31G** level of theory show that the activation barrier of the allylation reaction between benzaldehyde and allyltrichlorosilane with the aid of a neutral Lewis base, N,Ndimethylformamide (DMF) or hexamethylphosphoric triamide (HMPA), is considerably lower than that of the reaction in the absence of the Lewis base. An orbital analysis indicates that the electrondonating ability of the γ-carbon in the allyl group is enhanced by the coordination of a DMF or HMPA molecule. The silicon having a hexacoordinated bond arrangement shows an electronaccepting ability or Lewis acidity significantly stronger than that in silicon having a pentacoordinated structure to facilitate the coordination of DMF or other neutral Lewis bases. On the other hand, the electron-accepting level of the orbital that is left for the bond formation with the attacking aldehyde is elevated by the preceding coordination of a neutral base. The orbital is seen, however, to be localized more efficiently on the silicon center in the hexacoordinated transition state. The two Si-Cl bonds that are approximately perpendicular to the Si-O bond being formed and the Si-C bond being broken are strongly polarized at the transition state, reducing the repulsive interaction with the attacking aldehyde. These make up for the weakened electron-accepting ability of the silicon and, therefore, the combined coordination of the aldehyde and a neutral base recovers most of the destabilization associated with the changes in reagent and reactant structures to give the hexacoordinated transition state.

Introduction Allylation of aldehydes and ketones with allylsilanes is one of the most useful tools in organic synthesis.1-5 Hosomi et al. found that a variety of aldehydes and ketones are allylated with allyltrimethylsilane in the presence of a stoichiometric amount of Lewis acids to afford the corresponding homoallyl alcohols. The reaction provides a very popular strategy for the formation of C-C bonds.6-9 Kira et al. reported, on the other hand, that highly stereoselective allylations of aldehydes are achieved by introducing strongly electronegative *To whom correspondence should be addressed. E-mail: sakata@ hoshi.ac.jp. (1) Yamamoto, Y.; Asao, N. Chem. Rev. 1993, 93, 2207–2293. (2) Denmark, S. E.; Fu, J. Chem. Rev. 2003, 103, 2763–2793. (3) Dilman, A. D.; Ioffe, S. L. Chem. Rev. 2003, 103, 733–772. (4) Kennedy, J. W. J.; Hall, D. G. Angew. Chem., Int. Ed. 2003, 42, 4732–4739. (5) Miura, K.; Hosomi, A. In Main Group Metals in Organic Synthesis; Yamamoto, H., Oshima, K., Eds. Wiley-VCH: Weinheim, Germany, 2004; Vol. 2, p 409. (6) Hosomi, A.; Sakurai, H. Tetrahedron Lett. 1976, 1295–1298. (7) Hosomi, A.; Shirahata, A.; Sakurai, H. Tetrahedron Lett. 1978, 3043–3046. (8) Hosomi, A. Acc. Chem. Res. 1988, 21, 200–206. (9) Fleming, I. In Comprehensive Organic Synthesis: Selectivity, Strategy, and Efficiently in Modern Organic Chemistry; Trost, B. M., Fleming, I., Eds.; Pergamon Press: Oxford, U.K., 1991; Vol. 2, p 563. (10) Sakurai, H. Synlett 1989, 1, 1–8. (11) Kira, M.; Kobayashi, M.; Sakurai, H. Tetrahedron Lett. 1987, 28, 4081–4084. pubs.acs.org/Organometallics

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fluoride and/or alkoxy ligands to allylsilanes.10-16 Theoretical calculations suggested that the reaction should start from pentacoordinate allylsilicates and proceed via a chairlike six-membered cyclic transition state.17,18 This is in contrast with the Lewis acid promoted allylation reactions reported by Hosomi et al., for which an acyclic transition state between a Lewis acid catalyzed aldehyde or ketone species and an allylsilane molecule has been proposed.19,20 Notable progress in allylation reactions promoted by Lewis bases has been made by Kobayashi et al.21,22 They (12) Kira, M.; Sato, K.; Sakurai, H. J. Am. Chem. Soc. 1988, 110, 4599–4602. (13) Sato, K.; Kira, M.; Sakurai, H. J. Am. Chem. Soc. 1989, 111, 6429–6431. (14) Kira, M.; Hino, T.; Sakurai, H. Tetrahedron Lett. 1989, 30, 1099–1102. (15) Kira, M.; Sato, K.; Sakurai, H. J. Am. Chem. Soc. 1990, 112, 257–260. (16) Kira, M.; Zhang, L. C.; Kabuto, C.; Sakurai, H. Organometallics 1996, 15, 5335–5341. (17) Kira, M.; Sato, K.; Sakurai, H.; Hada, M.; Izawa, M.; Ushio, J. Chem. Lett. 1991, 387–390. (18) Hada, M.; Nakatsuji, H.; Ushio, J.; Izawa, M.; Yokono, H. Organometallics 1993, 12, 3398–3404. (19) Bottoni, A.; Costa, A. L.; Tommaso, D. D.; Rossi, I.; Tagliavini, E. J. Am. Chem. Soc. 1997, 119, 12131–12135. (20) Organ, M. G.; Dragan, V.; Miller, M.; Froese, R. D. J.; Goddard, J. D. J. Org. Chem. 2000, 65, 3666–3678. (21) Kobayashi, S.; Nishio, K. Tetrahedron Lett. 1993, 34, 3453– 3456. (22) Kobayashi, S.; Nishio, K. J. Org. Chem. 1994, 59, 6620–6628. r 2010 American Chemical Society

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reported that allyltrichlorosilanes react with aldehydes in the absence of catalyst when N,N-dimethylformamide (DMF) is used as the solvent. Meanwhile, Denmark et al. found that allylation of aldehydes with allyltrichlorosilane is promoted by adding a substoichiometric amount of phosphoramide in dichloromethane. The work provided also an important clue to asymmetric allylation reactions.23 Thereafter, chiral Lewis base catalyzed reactions were vigorously developed.24-32 Kobayashi et al. regarded these neutral Lewis bases as neutral coordinate organocatalysts33 and applied them to other types of reactions, such as allenylations and propargylations of aldehydes,34 allylations of hydrazones,33,35-39 and allenylations and propargylations of hydrazones.40,41 Most of the theoretical studies carried out so far on the allylation reactions using allylsilane support the mechanism in which a chairlike six-membered cyclic transition state intervenes in the course of the reaction,17,18,42,43 except for y and cothe Lewis acid promoted cases.19,20 Kocovsk workers recently performed kinetic and DFT studies of the allylation of aldehydes with trichlorosilane catalyzed by QUINOX and showed that the reaction is likely to proceed via a hexacoordinated transition state involving a QUINOX molecule.32d The allylation reaction promoted by a fluoride ion has also been investigated theoretically.17,18,44 The (23) Denmark, S. E.; Coe, D. M.; Pratt, N. E.; Griedel, B. D. J. Org. Chem. 1994, 59, 6161–6163. (24) Dalko, P. I.; Moisan, L. Angew. Chem., Int. Ed. 2004, 43, 5138– 5175. (25) Rendler, S.; Oestreich, M. Synthesis 2005, 1727–1747. (26) Orito, Y.; Nakajima, M. Synthesis 2006, 1391–1401. (27) Denmark, S. E.; Beutner, G. L. Angew. Chem., Int. Ed. 2008, 47, 1560–1638. (28) (a) Denmark, S. E.; Fu, J. J. Am. Chem. Soc. 2000, 122, 12021– 12022. (b) Denmark, S. E.; Fu, J. J. Am. Chem. Soc. 2001, 123, 9488–9489. (c) Denmark, S. E.; Fu, J.; Coe, D. M.; Su, X.; Pratt, N. E.; Griedel, B. D. J. Org. Chem. 2006, 71, 1513–1522. (29) Iseki, K.; Mizuno, S.; Kuroki, Y.; Kobayashi, Y. Tetrahedron 1999, 55, 977–988. (30) Nakajima, M.; Saito, M.; Shiro, M.; Hashimoto, S. J. Am. Chem. Soc. 1998, 120, 6419–6420. (31) Shimada, T.; Kina, A.; Ikeda, S.; Hayashi, T. Org. Lett. 2002, 4, 2799–2801. (32) (a) Malkov, A. V.; Orsini, M.; Pernazza, D.; Muir, K. W.; Langer, V.; Meghani, P.; Kocovsky, P. Org. Lett. 2002, 4, 1047–1049. (b) Malkov, A. V.; Bell, M.; Orsini, M.; Pernazza, D.; Massa, A.; Herrmann, P.; Meghani, P.; Kocovsky, P. J. Org. Chem. 2003, 68, 9659–9668. (c) Malkov, A. V.; Dufkova, L.; Farrugia, L.; Kocovsky, P. Angew. Chem., Int. Ed. 2003, 42, 3674–3677. (d) Malkov, A. V.; Ramírez-Lopez, P.; Biedermannova, L.; Rulísek, L.; Dufkova, L.; Kotora, M.; Zhu, F.; Kocovsky, P. J. Am. Chem. Soc. 2008, 130, 5341–5348. (33) Kobayashi, S.; Ogawa, C.; Konishi, H.; Sugiura, M. J. Am. Chem. Soc. 2003, 125, 6610–6611. (34) Kobayashi, S.; Nishio, K. J. Am. Chem. Soc. 1995, 117, 6392– 6393. (35) Hirabayashi, R.; Ogawa, C.; Sugiura, M.; Kobayashi, S. J. Am. Chem. Soc. 2001, 123, 9493–9499. (36) Ogawa, C.; Sugiura, M.; Kobayashi, S. J. Org. Chem. 2002, 67, 5359–5364. (37) Kobayashi, S.; Sugiura, M.; Ogawa, C. Adv. Synth. Catal. 2004, 346, 1023–1034. (38) Sugiura, M.; Kobayashi, S. Angew. Chem., Int. Ed. 2005, 44, 5176–5186. (39) Ogawa, C.; Sugiura, M.; Kobayashi, S. Angew. Chem., Int. Ed. 2004, 43, 6491–6493. (40) Schneider, U.; Sugiura, M.; Kobayashi, S. Tetrahedron 2006, 62, 496–502. (41) Schneider, U.; Sugiura, M.; Kobayashi, S. Adv. Synth. Catal. 2006, 348, 323–329. (42) Omoto, K.; Sawada, Y.; Fujimoto, H. J. Am. Chem. Soc. 1996, 118, 1750–1755. (43) Omoto, K.; Fujimoto, H. J. Am. Chem. Soc. 1997, 119, 5366–5372. (44) For the enhanced reactivity of pentacoordinated anionic silicon species, see: Deiters, J. A.; Holmes, R. R. J. Am. Chem. Soc. 1990, 112, 7197–7202.

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basicity of the neutral Lewis bases should be much lower compared with that of anionic Lewis bases, such as a fluoride ion. It appears, therefore, to be worthwhile to examine how the allylation of aldehydes and ketones by allyltrichlorosilanes is promoted in the presence of those neutral Lewis bases. Here, we attempt to investigate first the roles of DMF and hexamethylphosphoric triamide (HMPA) in allylation reactions, by comparing the two models of the reaction given by eqs 1 and 2.

Computational Details We used in this study ab initio molecular orbital methods.45 Geometry optimization and analytical vibrational frequency analysis were performed by applying the MP2 theory46 with the 6-31G** basis set47 (MP2/6-31G**). The energies of intermediates and transition states were compared also by using the CCSD(T) method48 with the 6-31þG** basis set47 (CCSD(T)/631þG**) for the reaction of allyltrichlorosilane (1) with formaldehyde in the presence and absence of DMF, where structures optimized at the MP2/6-31G** level of theory were used. The calculations were carried out with the Gaussian0349 and GAMESS50 program packages.

Results and Discussion Structures and Energetics. First of all, we will examine the reaction in the absence of Lewis base. The transition state, TS1, for reaction 1, in which 1 reacts with benzaldehyde, has a six-membered cyclic form, as illustrated in Figure 1. The silicon atom provides a pentacoordinated arrangement, where the carbonyl oxygen and the Cl1 atom each have apical positions. The distance between the silicon and the carbonyl oxygen is 1.849 A˚. The apical Si-Cl1 bond, 2.148 A˚, is somewhat longer than the equatorial Si-Cl bonds, 2.073 A˚ for the Si-Cl2 bond and 2.091 A˚ for the Si-Cl3 bond.51 The allylic R-carbon departs from the silicon center (45) In our preliminary examination, the DFT calculations with the B3LYP functional gave rather a poor description of the interaction energies between allyltrichlorosilane and DMF. DFT method including the empirical dispersion parameters [DFT(þD)] was used for the recent theoretical study by Kocovsky and co-workers.32d (46) Møller, C.; Plesset, M. S. Phys. Rev. 1934, 46, 618–622. (47) Hehre, W. J.; Radom, L.; Schleyer, P. v. R.; Pople, J. A. Ab Initio Molecular Orbital Theory; Wiley: New York, 1986.  zek, J. Adv. Chem. Phys. 1969, 14, 35–89. (b) Raghavachari, (48) (a) Cı´ K.; Trucks, G. W.; Pople, J. A.; Head-Gordon, M. Chem. Phys. Lett. 1989, 157, 479–483. (49) Frisch, M. J., et al. Gaussian 03, Revision D.02; Gaussian, Inc., Wallingford, CT, 2004. (50) Schmidt, M. W.; Baldridge, K. K.; Boatz, J. A.; Elbert, S. T.; Gordon, M. S.; Jensen, J. H.; Koseki, S.; Matsunaga, N.; Nguyen, K. A.; Su, S.; Windus, T. L.; Dupuis, M.; Montgomery, J. A. J. Comput. Chem. 1993, 14, 1347–1363. (51) For the hypervalent bonding in pentacoordinate silicon compounds, see: (a) Gimarc, B. M. Molecular Structure and Bonding: The Qualitative Molecular Orbital Approach; Academic Press: New York, 1979. (b) Tandura, S. N.; Voronkov, M. G.; Alekseev, N. V. Top. Curr. Chem. 1986, 131, 99–189.

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Figure 1. Relative energy diagram with optimized structures for reaction 1 at the MP2/6-31G** level of theory. Energies are in kcal/mol, and bond lengths are in A˚.

Figure 2. Relative energy diagram with optimized structures for reaction 2 in the presence of DMF at the MP2/6-31G** level of theory. Energies are in kcal/mol, and bond lengths are in A˚.

in one of the equatorial positions. The breaking Si-CR bond length is 2.055 A˚, and the bond that is formed between the allylic γ-carbon and the carbonyl carbon is 1.974 A˚. At the MP2/6-31G** level of theory, the activation energy of the reaction has been calculated to be 14.3 kcal/mol and the reaction is exothermic by 34.9 kcal/mol. The activation energy for the reaction of 1 with formaldehyde has been calculated to be very similar, being 13.3 and 17.1 kcal/mol at the MP2/6-31G** and CCSD(T)/6-31þG**//MP2/6-31G** levels of theory, respectively (see the Supporting Information). Reaction 2 includes a DMF molecule that is coordinated to allyltrichlorosilane. Kobayashi et al. examined 29Si NMR spectra of crotyltrichlorosilane in DMF solvent and suggested that DMF molecules are coordinated to the silicon atom to give a penta- or hexacoordinated silicon species.21,22 Here we focus our attention on the most stable configuration of the complex in which the silicon atom in 1 interacts with

the oxygen atom in a DMF molecule, Cx1, shown in Figure 2. The distance between the silicon and the oxygen in the DMF has been found, however, to be very long (3.299 A˚), the silicon atom retaining practically a tetrahedral bond arrangement.52 This is in clear contrast to the fluoride ion promoted case, in which a large amount of stabilization energy is created by forming a tight pentacoordinated complex.17,18 To obtain insight into this weak coordination of the DMF molecule, we examined a complex structure in which the Si-O(DMF) is fixed at a shorter length, 2.0 A˚. This imaginary complex structure was shown to be less stable by 1.6 kcal/mol, after the optimization of other geometrical parameters, relative to the two fragments 1 and DMF in an isolated state. The deformation of the two species on going from an isolated state to this complex structure causes (52) The distance between the silicon and the oxygen in DMF has been calculated to be 3.187 A˚ at the MP2/6-31þG** level of theory.

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Table 1. Relative Energies ΔE and Relative Gibbs Free Energies ΔG at the MP2/6-31G** Level of Theory in the Allylation of Benzaldehyde in the Absence and in the Presence of DMF (kcal/mol)a ΔE

ΔG298 K

ΔG273 K

ΔG195 K

Reaction 1 (Benzaldehyde) 1 þ PhCHO TS1 P1

0.0 þ14.3 -34.9

0.0 þ31.5 -20.3

0.0 þ30.2 -21.3

0.0 þ26.1 -24.4

0.0 þ5.0 þ25.7 þ25.7 þ31.3 -16.1

0.0 þ2.9 þ19.4 þ18.9 þ23.9 -21.5

0.0 (0.0) þ28.7 (þ32.6)

0.0 (0.0) þ25.0 (þ28.8)

0.0 (0.0) þ31.2 (þ31.0)

0.0 (0.0) þ24.1 (þ23.9)

Reaction 2 (Benzaldehyde) 1 þ DMF þ PhCHO Cx1 þ PhCHO TS2 R2 TS3 P2

0.0 -3.6 þ1.4 -1.2 þ2.3 -38.6

0.0 þ5.7 þ27.6 þ27.8 þ33.7 -14.4 Reaction 1 (Formaldehyde)

1 þ CH2O TS10

0.0 (0.0) þ13.3 (þ17.1)

0.0 (0.0) þ30.0 (þ33.8) Reaction 2 (Formaldehyde)

1 þ DMF þ CH2O TS30 a

0.0 (0.0) þ2.8 (þ2.6)

0.0 (0.0) þ33.5 (þ33.3)

ΔE and ΔG values at the CCSD(T)/6-31þG**//MP2/6-31G** level of theory are given in parentheses.

destabilization of the system, 27.2 kcal/mol, which is higher than the stabilization brought about by the interaction between a DMF molecule and 1, 25.6 kcal/mol, at the MP2/6-31G** level of theory. That is, the present calculation suggests that the coordination of a DMF molecule is not so strong as to transform the tetrahedral arrangement of bonds around the silicon center into a pentacoordinated arrangement.53 The attack of benzaldehyde at Cx1 gives the hexacoordinated complex R2 via the transition state TS2, as shown in Figure 2.54 Very interestingly, the DMF molecule is seen to come closer to the silicon center upon attack of the aldehyde. In the TS2 structure, the distance between the silicon and the carbonyl oxygen of benzaldehyde is still long (2.872 A˚), while the distance between the silicon and the oxygen atom in DMF has been shortened from 3.299 A˚ in Cx1 to 1.991 A˚. The reaction goes through the transition state TS3, which has a six-membered cyclic structure and has a hexacoordinated arrangement of bonds around the silicon. The Si-O(benzaldehyde) bond length has now been shortened to 1.851 A˚, which is comparable to that in the transition state of the reaction without DMF. The Si-O(DMF) bond is now 1.930 A˚. The result suggests that both benzaldehyde and DMF molecules are coordinated approximately in the same strength to the silicon center at TS3. The deformation from a pentacoordinated structure to a hexacoordinated structure costs an additional energy amounting to a total of 80.9 kcal/ mol on going from 1 in an isolated state to the hexacoordinated geometry at TS3. The deformation of the aldehyde and DMF fragments amounts to 13.6 kcal/mol. The destabilization is compensated now for the most part by the interaction (53) Theoretical studies indicated that the destabilization energy governs the stability of the donor-acceptor complexes of silanes. See: (a) Fleisher, H. Eur. J. Inorg. Chem. 2001, 393–404. (b) Davydova, E. I.; Timoshkin, A. Y.; Sevastianova, T. N.; Suvorov, A. V.; Frenking, G. J. Mol. Stuct. (THEOCHEM) 2006, 767, 103–111. (54) In the case of formaldehyde, there appears another transition state between TS2 and R2 in which the aldehyde molecule has a strongly pyramidized structure. It may not be essential to the allylation reaction. See the Supporting Information.

with the attacking benzaldehyde (-44.7 kcal/mol)55 and by the strengthened interaction with the attached DMF (-38.8 kcal/mol).56 The activation barrier is lowered in reaction 2 compared with that in reaction 1, owing to the associated coordination of DMF. The Si-CR bond length (2.025 A˚) is shorter in TS3 than in TS1, while the bond which is formed between the allylic γcarbon and the carbonyl carbon of benzaldehyde (2.105 A˚) is longer than that in TS1. This signifies that the reaction reaches the transition state at an earlier stage of the reaction course in the presence of DMF. Calculations at the MP2 level show that TS3 is located only 2.3 kcal/mol above the initial reactant state, (1 þ DMF þ PhCHO), as illustrated in Figure 2. In the case of the reaction with formaldehyde, the relative energy ΔE of the six-membered cyclic transition state (TS30 ) has been calculated to be þ2.8 and þ2.6 kcal/mol at the MP2/6-31G** and CCSD(T)/6-31þG**//MP2/6-31G** levels of theory, respectively, demonstrating that the activation barrier is not high. It is suggested above that reaction 2, which includes a DMF molecule, is preferred to reaction 1, from an energetic point of view. The relative Gibbs free energies ΔG are given in Table 1. At a temperature of 298 K, the ΔG value of TS3 in reaction 2 is larger than that of TS1 in reaction 1 by 2.2 kcal/ mol. Interestingly, the relative free energy of TS3 becomes lower than that of TS1 at a lower temperature: for example, by 2.2 kcal/mol at 195 K. A similar result is obtained for the reaction of 1 with formaldehyde at the MP2/6-31G** level. Further calculations at the CCSD(T)/6-31þG**//MP2/631G** level of theory show that the ΔG value of the transition state (TS30 ) in the presence of DMF is smaller than that of the transition state (TS10 ) in the absence of DMF by 0.5 kcal/mol at a temperature of 298 K. These results indicate that reaction 2, in which a DMF molecule participates, is preferred to reaction 1 without DMF, at the low temperature at which most of the experiments have been carried out. (55) Estimated by the energy difference between TS3 and (the DMFcoordinated allylsilane þ benzaldehyde) frozen to the structures in TS3. (56) The energy difference between TS3 and (the benzaldehydecoordinated allylsilane þ DMF).

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Figure 3. Optimized structures for reaction 2 in the presence of HMPA at the MP2/6-31G** level of theory. Bond lengths are given in A˚. Table 2. Relative Energies ΔE and Relative Gibbs Free Energies ΔG at the MP2/6-31G** Level of Theory in the Allylation of Benzaldehyde in the Presence of HMPA (kcal/mol) ΔE

ΔG298 K

ΔG273 K

ΔG195 K

0.0 þ8.6 þ19.9 þ24.8

0.0 þ5.3 þ12.9 þ17.0

Reaction 2 (Benzaldehyde) 1 þ HMPA þ PhCHO Cx100 þ PhCHO R200 TS300

0.0 -4.7 -7.8 -5.4

0.0 þ9.7 þ22.2 þ27.3

Calculations at the MP2/6-31G** level show that HMPA, whose basicity is slightly stronger than that of DMF,57 gives a stable pentacoordinated complex with 1, with an Si-O(HMPA) length of 1.905 A˚, as illustrated in Figure 3. The complex Cx100 , corresponding to Cx1 in reaction 2 with DMF, is shown to be 4.7 kcal/mol lower than the initial reactant state in this case, and the hexacoordinated complex R200 and transition state TS300 are 3.1 and 0.7 kcal/mol lower than the complex, respectively, as given in Table 2. At a temperature of 298 K, the ΔG value for the transition state is 27.3 kcal/mol, which is lower by 6.4 kcal/mol than the barrier height calculated above for the reaction with DMF. In this case, the deformation of the fragments leads to a destabilization of the reacting system by 99.8 kcal/mol, while the interaction between 1 and HMPA and the interaction between (1 þ HMPA) and benzaldehyde give rise to stabilizations of the system by 64.7 and 40.5 kcal/mol, respectively. The main role of neutral coordinate organocatalysts appears to be to collaborate with aldehydes to provide hexacoordinated transition states that are significantly lower in energy than the pentacoordinated transition state. Denmark et al. have recently suggested that the major pathway in the allylation promoted by chiral phosphoramide involves two phosphoramide and that the transition state is a cationic species.28c The calculation at the MP2/6-31G** level showed that two DMF molecules coordinate to allylsilane to give a hexacoordinated complex, located lower in energy by 5.4 kcal/mol relative to (1 þ 2DMF) in an isolated state (see the Supporting Information). This result is in agreement with the NMR observation by Kobayashi that DMF molecules are coordinated to crotyltrichlorosilane to give a penta- or hexacoordinated silicon species.21,22 The complex is transformed to a hexacoordinated structure such as TS3 in reaction 2 with DMF but is accompanied by a higher barrier height of 12.7 kcal/mol, due probably to the lower electronaccepting ability of the CdO π* orbital. Phosphoramide also (57) (a) Maria, P.-C.; Gal, J.-F. J. Phys. Chem. 1985, 89, 1296–1304. (b) Bassindale, A. R.; Stout, T. Tetrahedron Lett. 1985, 26, 3403–3406.

gives a hexacoordinated silicon complex with 1 which is similar to that obtained above for DMF.58 One of the electronegative chloride ligands may be freed from a hexacoordinated complex, leaving a positive charge on the silicon. In this case, the reaction with the aldehyde would proceed via a cationic transition state. On the other hand, Kocovsk y and co-workers showed recently that the allylation of aldehydes with trichlorosilane catalyzed by QUINOX is likely to proceed via a neutral hexacoordinated transition state involving QUINOX.32d,59 It is not easy at present to estimate very accurately the relative stabilities of the neutral and cationic transition states in solution or the stability of solvated ion pairs by theoretical calculations. The present calculations suggest that allylation reactions may take place via a neutral hexacoordinated transition state without going through a high-energy barrier in the presence of DMF or HMPA. Lewis Acidity of the Hexacoordinated Silicon Atom in Allyltrichlorosilanes. In order to see the role of the Lewis base in allylation reactions, we have studied orbital interactions in terms of the interaction frontier orbitals (IFOs).60 The analysis shows that two pairs of orbitals (φ0 1; ψ0 1) and (φ0 2; ψ0 2) of the two fragments, i.e., 1 and benzaldehyde, illustrated in Figure 4 governs electron delocalization at TS1 in reaction 1. In the first orbital pair, the orbital φ0 1 is localized well on the silicon center consisting of the lowest unoccupied (LU) MO and of several other low-lying unoccupied canonical MOs of 1, while the orbital ψ0 1 is localized on the oxygen of benzaldehyde being made of several σ-type occupied canonical MOs of the aldehyde. Electron delocalization from the latter orbital to the former is responsible for the formation of the Si-O bond. In the second pair, the orbital φ0 2 looks like an occupied π-like orbital having a large amplitude on the CγdCβ bond in 1, while the orbital ψ0 2 is practically the unoccupied π* orbital of the CdO bond in the aldehyde. Electron delocalization from the former to the latter generates the new Cγ-C(benzaldehyde) bond. At TS3, electron delocalization from the allylsilane fragment coordinated by a DMF molecule to benzaldehyde is (58) Recently, Denmark et al. carried out synthesis and characterization of the solution and solid-state structures of HMPA adducts of silicon tetrachloride. See: Denmark, S. E.; Eklov, B. M. Chem. Eur. J. 2008, 14, 234–239. (59) The experimental and theoretical study performed by Hrdina et al. indicated that the allylation reaction of aldehydes catalyzed by bipyridine N,N0 -dioxides proceeds via two different reaction mechanisms depending on the solvent. See: Hrdina, R.; Opekar, F.; Roithova, J.; Kotora, M. Chem. Commun. 2009, 2314–2316. (60) (a) Fukui, K.; Koga, N.; Fujimoto, H. J. Am. Chem. Soc. 1981, 103, 196–197. (b) Fujimoto, H. Acc. Chem. Res. 1987, 20, 448–453.

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Figure 4. Pairs of interacting orbitals for TS1 calculated at the RHF/6-31G**//MP2/6-31G** level of theory: (a) (φ0 1; ψ0 1); (b) (φ0 2; ψ0 2). The positive and negative regions of orbitals are discriminated by depth in black. The two fragments possess the same structures as those at the transition state, but their relative positions are held separated, ensuring the two orbitals are not mixed. The orbitals φ0 1, ψ0 1, φ0 2, and ψ0 2 are located at þ0.156, -0.529, -0.369, and þ0.125 au in energy, respectively.

shown to take place between a pair of orbitals that look very similar in shape to those in the second orbital pair (φ0 2, ψ0 2) of TS1 (see the Supporting Information). The occupied orbital φ0 2 is elevated by 0.037 au in the DMF-coordinated allylsilane fragment, while the orbital ψ0 2 of the aldehyde part remains almost the same in energy. This result signifies that the interaction to produce the Cγ-C(benzaldehyde) bond is strengthened by an attachment of DMF. Electron delocalization from the aldehyde part to 1 with DMF at TS3 is represented by a pair of orbitals that looks very similar to (φ0 1; ψ0 1) of TS1. The unoccupied orbital φ0 1 of the allylsilane fragment is elevated by 0.031 au in TS3 above that in TS1, indicating that the electron-accepting ability of the silicon center is somewhat weakened by receiving an electronic charge from the attached DMF molecule, 0.222 e estimated by the Mulliken population analysis61 at the RHF/6-31G**// MP2/6-31G** level. The orbital φ0 1 is given practically by a hybrid of s, p, and d AO functions of Si, their contributions being 18.7%, 44.6%, and 16.0%, respectively, as estimated by the partial Mulliken population in that orbital.62 That is, φ0 1 is localized up to 79% on the Si center, while the extent of localization of φ0 1 is lower in TS1, being 74%. The enhanced localization is related to the longer Si-Cl1 bond extending on the backside of the Si-O(benzaldehyde) bond, Cl1 being repelled away from the four ligands in TS3, instead of three in TS1. The higher localization of the orbital compensates in part the weakened acidity of the silicon center. (61) Mulliken, R. S. J. Chem. Phys. 1955, 23, 1833-1840, 1841-1846, 2338-2342, 2343-2346. (62) The bonding contribution of the s-type orbitals in electron delocalization is counterbalanced by their antibonding participation in the overlap repulsion between the occupied MOs of the two fragments, and therefore, the s-type orbital functions of Si do not participate in the Si-O(benzaldehyde) bond both at TS1 and at TS3.

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The Si-Cl2 and Si-Cl3 bonds have already been polarized strongly in 1 in an isolated state,63 but the Mulliken population analysis, as well as the natural population analysis,64 demonstrates that the charge polarization in those Si-Cl bonds is strengthened further upon an attack of the aldehyde molecule at 1. This behavior is in line with the Gutmann empirical rule that predicts the charge density redistribution in the donoracceptor interactions.27,65 A configuration analysis of the wave function66 indicates that a part of the electron population of Si is shifted to the two Cl atoms through an intermixing of the occupied (bonding) and unoccupied (antibonding) three-centered Cl-Si-Cl MOs, each composed of the Si s AOs and the chlorine p AOs directed toward the Si atom. A slightly larger amount of s electron population has been lifted from Si in TS3 than in TS1. The shift of electron population in the p-type orbitals from Si to the chlorine atoms is of greater significance and is considerably stronger in the hexacoordinated transition state TS3. The Si-Cl2 and Si-Cl3 bonds are elongated in TS3, as shown in Figure 2, and therefore, the bonding interactions between the Si p orbitals and the chlorine p orbitals, in particular of σ-type, are weakened.67 As a result, a significant part of electron population in the Si p orbitals is shifted to the chlorine p orbitals having lower orbital energies. Thus, in spite of the donation of an electronic charge from the attached DMF molecule to 1, the Si center carries a larger net positive charge of þ1.466 in the Mulliken population analysis and þ1.876 in the natural population analysis in TS3 as compared to that in TS1, þ1.211 and þ1.774, respectively. The overlap repulsion between the silicon center in 1 and the attacking aldehyde is reduced in TS3, to compensate in part the weakened acidity of the silicon center in the presence of DMF described above. Thus, the calculations show that the stabilization arising from the interaction of benzaldehyde with the DMF-coordinated allylsilane, -44.7 kcal/mol, is very similar to that coming from the interaction of the aldehyde with 1 in the absence of DMF, -44.8 kcal/mol. Let us now examine the interaction between 1 and DMF in the DMF-coordinated allylsilane fragment, frozen to the same geometry as that in the hexacoordinated transition state TS2, to examine an early stage of the reaction. Electron delocalization from DMF to allylsilane is described by the pair of orbitals (φ0 3; j0 3) illustrated in Figure 5. The orbital φ0 3 of 1 is localized well on the silicon center, while j0 3 of the attached DMF molecule is the orbital for a lone pair of electrons. They are located at þ0.116 and -0.504 au in energy, respectively. The orbital φ0 3 was located at 0.144 au in the pentacoordinated complex structure, where the Si-O(DMF) length was fixed at 2.0 A˚. This demonstrates that the electron-accepting orbital is lowered in energy and, therefore, the Lewis acidity of the silicon center has been enhanced on going (63) A recent DFT study of the octahedral complex of dichlorosilane with substituted pyridines suggested the strongly polar Si-Cl bonds. See: Fester, G. W.; Wagler, J.; Brendler, E.; B€ ohme, U.; Roewer, G.; Kroke, E. Chem. Eur. J. 2008, 14, 3164–3176. (64) Reed, A. E.; Weinstock, R. B.; Weinhold, F. J. Chem. Phys. 1985, 83, 735–746. (65) (a) Gutmann, V. The Donor-Acceptor Approach to Molecular Interactions; Plenum Press: New York, 1978. (b) Jensen, W. B. The Lewis Acid-Base Concepts; Wiley: New York, 1980. (66) Fujimoto, H.; Kato, S.; Yamabe, S.; Fukui, K. J. Chem. Phys. 1974, 60, 572–578. (67) It appears that the Si-Cl2 and Si-Cl3 bonds share one 3p AO in TS3, while the two Si-Cl bonds share two 3p AOs with the Si-CR bond in TS1, in a sense of the minimal basis set argument.51a The s-type orbitals are shown to contribute significantly, however, to the Si-CR bond not only in TS1 but also in TS3.

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Sakata and Fujimoto Table 3. Relative Solvation Energies ΔΔGsolv Estimated by PCM Calculations (kcal/mol) acetonitrile (ε = 36.64)

dichloromethane (ε = 8.93)

Reaction 1 1 þ PhCHO TS1

0.0 -4.13

0.0 -2.53

Reaction 2 1 þ DMF þ PhCHO TS3

Figure 5. A pair of orbitals (φ0 3; j0 3) of the DMF-coordinated allylsilane fragment at TS2 calculated at the RHF/6-31G**// MP2/6-31G** level of theory. The positive and negative regions of orbitals are discriminated by depth in black. The two fragments possess the same structures as those at the transition state, but their relative positions are held separated, ensuring the two orbitals are not mixed.

from a pentacoordinated structure to a hexacoordinated structure. The influence of the attacking aldehyde is still small in an early stage of the reaction, and therefore, the attached DMF molecule comes nearer to allylsilane by making use of the enhanced Lewis acidity of silicon to yield a structure such as TS2. Calculations show that the electron-accepting orbital φ0 1 localized on Si of (1 þ HMPA) has been elevated even higher than that of (1 þ DMF), being 0.214 au.68 Accordingly, the interaction between benzaldehyde and (1 þ HMPA) is weakened further, compared with that between the aldehyde and (1 þ DMF). On the other hand, the interaction between 1 and HMPA brings a larger stabilization to the reacting system, as discussed above. The results obtained here signifies altogether that the role of neutral coordinate organocatalysts is to provide, in collaboration with the aldehyde, the hexacoordinated transition states that are located significantly lower in energy than the pentacoordinated transition state. This is in clear contrast to the allylation reaction promoted by a fluoride ion, in which the pentacoordinated complex corresponding to Cx1 in Figure 2 is strongly stabilized, while any hexacoordinated complex similar to R2 has not been located.17,18 The present analysis strongly suggests that the Lewis acidity of the silicon is weakened significantly by the coordination of the base and, accordingly, the hexacoordinated transition state is not as favored as in the case of the allylation reactions catalyzed by neutral Lewis bases. Effect of Solvents. We examined finally the effect of solvents by using single-point IEF-PCM calculations.69,70 (68) The extent of localization of the orbital φ0 1 on the Si center has been found to be 79%, the same magnitude as that in TS3. (69) (a) Cances, E.; Mennucci, B.; Tomasi, J. J. Chem. Phys. 1997, 107, 3032–3041. (b) Mennucci, B.; Cances, E.; Tomasi, J. J. Phys. Chem. B 1997, 101, 10506–10517. (c) Mennucci, B.; Tomasi, J. J. Chem. Phys. 1997, 106, 5151–5158. (d) Cossi, M.; Scalmani, G.; Rega, N.; Barone, V. J. Chem. Phys. 2002, 117, 43–54. (70) The solvation energy ΔGsolv consists of the electrostatic term ΔGel and the nonelectrostatic term ΔGnonel. ΔGel is the energy difference between the PCM-MP2 energy and the MP2 energy in vacuo and ΔGnonel is the sum of the cavitation, dispersion, and repulsion energies in the SCRF calculations. The united atom topological model applied to radii optimized at the RHF/6-31G* level of theory (UAHF) was utilized for the PCM calculations.

0.0 -3.50

0.0 -0.87

Table 3 presents how the solvation energy, ΔGsolv, influences the barrier height, taking the initial state as the reference in each system. In acetonitrile, whose dielectric constant is close to that of DMF, the change in solvation energy, ΔΔGsolv, is seen to be -4.13 kcal/mol at TS1 and -3.50 kcal/mol at TS3, the difference being very small. Accordingly, the solvent effect does not seem to be of significance in the allylation reaction carried out in acetonitrile at a low temperature. On the other hand, the ΔΔGsolv values of TS1 and TS3 in dichloromethane are -2.53 and -0.87 kcal/mol, respectively. The advantage of reaction 2 over reaction 1 may be lost in dichloromethane. These results suggest a possible dependence of the Lewis-base promotion of allylation reactions on the reaction conditions, such as temperature and solvents.

Conclusion We have studied theoretically the Lewis base catalyzed allylation reaction of benzaldehyde with allyltrichlorosilane. The reaction is likely to take place via a hexacoordinated transition state in the presence of DMF and/or HMPA. The coordination of DMF covers a significant part of the energy that is needed to deform the allylsilane from the initial tetravalent structure to that in the hexacoordinated transition state. Calculations show that the Lewis acidity of the silicon center is enhanced in a hexacoordinated structure to facilitate the coordination of DMF to allylsilane. On the other hand, the electron-accepting level of the orbital that is used for the formation of the Si-O(aldehyde) bond is slightly elevated in the DMF-coordinated allylsilane, caused by the migration of an electronic charge from DMF to the allylsilane fragment. It has been shown, however, that the orbital is localized more effectively on the Si center in the hexacoordinated transition state than in the pentacoordinated transition state. In addition, the two Si-Cl bonds that are not involved directly in the formation and breaking of chemical bonds play a role in favoring the reaction by shifting electron populations from the silicon atom to the chlorine atoms to reduce the overlap repulsion between the allylsilane moiety and the incoming aldehyde molecule. This effect is more significant in the hexacoordinated transition state, in which the two Si-Cl bonds are collinear. Thus, a stronger localization of the reactive orbital on Si and the reduced overlap repulsion make up for the weakened ability of the Si orbital for electron acceptance. A neutral Lewis base, DMF and also HMPA, strengthens the electron-donating ability of the γcarbon in the allyl group, but it does not enhance electronically the reactivity of the silicon center. It provides a substantial amount of stabilization to the reacting system to compensate, together with the coordination of the

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aldehyde, a large portion of destabilization arising from deformation of the reagent and the reactant to reach the hexacoordinated transition state. These seem to provide a detailed description of the “Lewis base activation of Lewis acid” concept which Denmark has developed on the basis of the Gutmann empirical rules.27 It is suggested that the Lewis base promotion of the allylation reaction is affected significantly by the reaction conditions, such as temperature and solvents.

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Acknowledgment. Some of the calculations were carried out at the Research Center for Computational Science, Okazaki, Japan. K.S. is grateful to the Center for generous permission to use its computing facilities. Supporting Information Available: Tables giving energies and geometries and text giving the complete ref 49. This material is available free of charge via the Internet at http://pubs. acs.org.