1,2-Addition Reaction of Monosubstituted Disilenes: An Ab Initio Study

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Organometallics 2004, 23, 5768-5778

1,2-Addition Reaction of Monosubstituted Disilenes: An Ab Initio Study Masae Takahashi,*,† Tama´s Veszpre´mi,*,‡ and Mitsuo Kira*,§ Photodynamics Research Center, The Institute of Physical and Chemical Research (RIKEN), 519-1399, Aoba, Aramaki, Aoba-ku, Sendai 980-0845, 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 July 28, 2004

Mechanisms of 1,2-addition reactions of the monosubstituted disilenes H2SidSiHMe, H2SidSiHF, H2SidSiH(CtCH), and H2SidSiH(NH2) were investigated in detail by the ab initio MO method. All reactions start from the electrophilic and nucleophilic initial complexes CE and CN. Four initial complexes and, therefore, four reaction channels were found for the reactions of methyldisilene with water and of ethynyldisilene with water. Only two complexes were found, however, in the reactions of fluorodisilene and aminodisilene with water/hydrogen fluoride. The reaction of the polar substrate aminodisilene shows reaction profiles similar to that of the polar substrate silene. In addition, product switching of the stereochemistry, depending on the acidity of the reagent, was found for the reaction of aminodisilene. Introduction The synthesis of the first stable disilene, tetramesityldisilene, in 19811 and several other disilene derivatives has stimulated the mechanistic studies of addition reactions of nucleophiles or electrophiles to unsaturated silicon compounds.2 One of the most studied reactions has been the 1,2-addition of alcohols to the SidSi bond.2 Alcohol addition to alkoxy- and amino-substituted disilenes has shown high regioselectivity, because the reaction seems to be governed by electrophilic factors in terms of a strong π-donation effect.3 In the reaction mechanisms of disilenes with alcohols, a four-centered cyclic mechanism, including a nucleophilic attack of the alcoholic oxygen at the unsaturated silicon, has been first proposed on the basis of the theoretical and experimental studies.4,5 However, Apeloig and Nakash found experimentally that an electrophilic attack prompts the reaction of disilene with phenols * To whom correspondence should be addressed. E-mail: masae@ imr.edu(M.T.);[email protected](T.V.);[email protected] (M.K.). † The Institute of Physical and Chemical Research. Current address: Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan. ‡ Technical University of Budapest. § Tohoku University. (1) West, R.; Fink, M. J.; Michl, J. Science 1981, 214, 1343. (2) For recent 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 3. (e) Tsumuraya, T.; Batcheller, S. A.; Masamune, S. Angew. Chem., Int. Ed. Engl. 1991, 30, 902. (f) Weidenbruch, M. Coord. Chem. Rev. 1994, 130, 275. (g) Okazaki, R.; West, R. Adv. Organomet. Chem. 1996, 39, 231. (h) Kira, M. Pure Appl. Chem. 2000, 72, 2333. (3) Sakurai, H. In The Chemistry of Organic Silicon Compounds; Rappoport, Z., Apeloig, Y., Eds.; Wiley: New York, 1998; Vol. 2, Part 1, Chapter 15. (4) 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; pp 249-267. (5) Sekiguchi, A.; Maruki, I.; Sakurai, H. J. Am. Chem. Soc. 1993, 115, 11460.

having electron-withdrawing substituents.6 In our previous paper, we reinvestigated the water addition reaction to disilene by ab initio quantum-mechanical calculations and have found both an electrophilic and a nucleophilic channel, as shown in Scheme 1.7 The first step of both channels is the appearance of the initial complexes CE and CN. The structure of these complexes can be understood with the help of FMO theory,8 as they are formed by an electrophilic interaction between the LUMO of water and the HOMO of disilene (electrophilic channel) or by a nucleophilic interaction between the HOMO of water and the LUMO of disilene (nucleophilic channel). CE and CN determine the whole reaction path and finally lead to different products, syn and anti adducts, respectively, via two different transition states, TE and TN. The path from the Lewis adduct CL is common in the two reaction channels and is similar to that proposed by Nagase, Kudo, and Ito.4 Rate-determining steps are before CL. The two pathways explain the experimentally observed solvent-dependent anti product formation6c and also the kinetic isotope effects of the electrophilic channel.6a In our subsequent work, we observed the sensitivity of the reaction mechanism to the acidity of the reagents. In case of acidic alcohol (CF3OH)7,9a or hydrogen halides9b only electrophilic channels could be found. This mechanism is, however, different from that of the usual addition reactions to CdC bond in a few steps. The reaction profile of water addition to silene having a polar double bond is also different from the case of disilene.9a (6) (a) Apeloig, Y.; Nakash, M. J. Am. Chem. Soc. 1996, 118, 9798. (b) Apeloig, Y.; Nakash, M. Organometallics 1998, 17, 2307. (c) Apeloig, Y.; Nakash, M. Organometallics 1998, 17, 1260. (7) (a) Takahashi, M.; Veszpre´mi, T.; Hajgato´, B.; Kira, M. Organometallics 2000, 19, 4660. (b) Takahashi, M.; Veszpre´mi, T.; Kira, M. Int. J. Quantum Chem. 2001, 84, 192. (8) Fukui, K. Acc. Chem. Res. 1971, 4, 57. (9) (a) Veszpre´mi, T.; Takahashi, M.; Hajgato´, B.; Kira, M. J. Am. Chem. Soc. 2001, 123, 6629. (b) Hajgato´, B.; Takahashi, M.; Kira, M.; Veszpre´mi, T. Chem. Eur. J. 2002, 8, 2126.

10.1021/om049418m CCC: $27.50 © 2004 American Chemical Society Publication on Web 10/22/2004

Monosubstituted Disilenes

Organometallics, Vol. 23, No. 24, 2004 5769 Scheme 1. Two Pathways in the Disilene + Water Reaction

Scheme 2. Two Pathways in the Silene + Water Reaction

Initial nucleophilic and electrophilic complexes, C′E and C′N, were also found to lead to two reaction channels (Scheme 2). The reaction from C′E proceeds in one step, giving silylmethanol via a four-centered, energy-rich transition state, T′L. On the other hand, the reaction from C′N occurs in two steps with an intervening Lewis adduct, C′L. The latter reaction channel is a facile process with low activation energy. This result immediately explains the experimental fact: the product of alcohol addition to silene is always an alkoxysilane.10 The regioselectivity of these reactions is caused by an inductive effect (I effect), due to the difference of electronegativity between silicon and carbon. (10) (a) Kira, M.; Maruyama, T.; Sakurai, H. J. Am. Chem. Soc. 1991, 113, 3986. (b) Sluggett, G. W.; Leigh, W. J. J. Am. Chem. Soc. 1992, 114, 1195. (c) Leigh, W. J.; Sluggett, G. W. J. Am. Chem. Soc. 1994, 116, 10468. (d) Bradaric, C. J.; Leigh, W. J. J. Am. Chem. Soc. 1996, 118, 8971.

A new question is the possible variation of the reaction mechanism of 1,2-additions to substituted disilenes. In these cases, the two silicon atoms are different, which indicates four theoretically possible products (Scheme 3) with different product ratios depending on the effects of the substituent. The study of the substituent effects may explain and predict the variation in the regioselectivity and stereoselectivity and may help in the synthesis of the required derivatives. To study the 1,2-addition reaction of the polar substrate with a homogeneous double bond, water additions to the following disilenes have been investigated: 1-methyldisilene (R1), 1-fluorodisilene (R2), 1-ethynyldisilene (R3), and 1-aminodisilene (R4). The expected effects of these substituents are as follows: methyl group, weak inductive (-I); fluorine, strong inductive (-I) and weak conjugative (+M); ethynyl group, inductive (-I) and

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Scheme 3. Four Possible Products in 1,2-Addition Reactions to Monosubstituted Disilenes

Takahashi et al. Table 1. Electronic Properties of the Disilene Derivatives and the Corresponding Ethylene Analogues at the MP2(full)/6-311++G(d,p) Geometry

σ electronsa π electronsa E1 E2 E1 E2 disilene methyldisilene fluorodisilene ethynyldisilene aminodisilene ethylene methylethylene fluoroethylene ethynylethylene aminoethylene

π-conjugative (-M); amino group, strong inductive (-I) and strong conjugative (+M) effects. To clarify the reaction of monoaminodisilene, a reaction with the acidic reagent hydrogen fluoride has also been examined (R5). For comparison, the H2SidSiH2 + HF reaction has also been investigated (R6). Some of the important initial complexes of reactions R1-R6 were studied elsewhere.11

R1: H(Me)SidSiH2 + H2O R2: HFSidSiH2 + H2O R3: H(CtCH)SidSiH2 + H2O R4: H(H2N)SidSiH2 + H2O R5: H(H2N)SidSiH2 + HF R6: H2SidSiH2 + HF Calculations Quantum-chemical calculations were performed using the Gaussian 98 program package.12 In our previous calculations investigating the water addition reactions of silene and silatriafulvene,13 we studied the effect of electron correlation using various correlation methods such as MP2, MP3, MP4SDQ, and QCISD. We learned that all of the important effects during the reactions studied could be described properly using the MP2 level. Also, in our preceding study concerning the (11) Takahashi, M.; Veszpre´mi, T.; Sakamoto, K.; Kira, M. Mol. Phys. 2002, 100, 1703. (12) (a) Hehre, W. J.; Radom, L.; Schleyer, P. v. R.; Pople, J. A. Ab Initio Molecular Orbital Theory; Wiley: New York, 1986. (b) All calculations were performed with Gaussian 98: 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.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, 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. G.; Chen, W.; Wong, M. W.; Andres, J. L.; HeadGordon, M.; Replogle, E. S.; Pople, J. A. Gaussian 98, revisions A3A11; Gaussian, Inc.: Pittsburgh, PA, 1998. (13) Veszpre´mi, T.; Takahashi, M.; Ogasawara, J.; Sakamoto, K.; Kira, M. J. Am. Chem. Soc. 1998, 120, 2408.

0.978 0.963 0.904 0.945 0.480 0.998 1.011 0.997 1.030 0.976

0.978 0.986 0.999 0.987 1.456 0.998 0.980 0.998 0.957 1.014

0.987 0.897 0.826 0.969 1.150 1.000 0.949 0.937 1.012 0.905

0.987 1.065 1.103 0.964 0.794 1.000 1.034 1.059 0.932 1.082

HOMO/ au

LUMO/ au

-0.281 06 -0.269 99 -0.296 18 -0.276 70 -0.270 57 -0.375 81 -0.357 28 -0.385 83 -0.343 48 -0.318 74

0.016 25 0.025 88 0.003 48 0.011 22 0.026 60 0.049 29 0.042 35 0.045 49 0.040 12 0.042 31

a Number of electrons occupying the σ and π bonds at each silicon and carbon atom by natural bond analysis.

mechanism of water and alcohol addition to doubly bonded systems9a excellent results could be gained by the CBS-Q method for the thermodynamic data14 and by MP2 and B3LYP methods15,16 with the 6-311++G(d,p) basis set for the molecular geometry. Therefore, in the present work we used the same level of theory. In all cases frequency analysis was performed and the existence of only one imaginary frequency was checked for transition states. All the calculations were corrected by the zero-point energy (ZPE). IRC calculations were performed for all the paths at the MP2(full)/6-311++G(d,p) level to confirm the reaction coordinates from transition states to stable products. Natural population analysis (NPA) charges were obtained with the NBO program in Gaussian 98.17Relative orbital energies of disilene and water were calculated by the outer valence Green’s function (OVGF)18 method using the 6-311++G(d,p) wave function for the MP2(full)/6-311++G(d,p) geometry.

Results and Discussion Since our previous experiences13,19 suggested that the reaction mechanism sensitively varies with the charge polarization of the SidX bond, first we investigated the σ and π charge on the SidSi and the CdC bonds of the substituted disilene and ethylene derivatives (Table 1). It can be seen that the inductive effect of the substituents is effective to the neighboring atom. The calculated data suggest the expected interactions: a +I effect for the methyl and ethynyl groups and a -I effect for the fluorine and amino groups on ethylene. In the case of disilene derivatives, all substituents show -I effects. On the other hand, the terminal atom of the conjugating (14) Petersson, G. A.; Tensfeldt, T. G.; Montgomery, J. A., Jr. J. Chem. Phys. 1991, 94, 6091. (15) (a) Becke, A. D. Phys. Rev. A 1988, 38, 3098. (b) Becke, A. D. J. Chem. Phys. 1993, 98, 5648. (16) Lee, C.; Yang, W.; Parr, R. G. Phys. Rev. B 1988, 37, 785. (17) (a) Foster, J. P.; Weinhold, F. J. Am. Chem. Soc. 1980, 102, 7211. (b) Reed, A. E.; Weinstock, R. B.; Weinhold, F. J. Chem. Phys. 1985, 83, 735. (c) Reed, A. E.; Weinhold, F. J. Chem. Phys. 1985, 83, 1736. (d) Reed, A. E.; Curtiss, L. A.; Weinhold, F. Chem. Rev. 1988, 88, 899. (18) (a) Cederbaum, L. S. J. Phys. B 1975, 8, 290. (b) Ortiz, J. V. J. Chem. Phys. 1988, 89, 6348. (c) Zakrzewski, V. G.; Niessen, W. v. J. Comput. Chem. 1993, 14, 13. (19) (a) Sakamoto, K.; Ogasawara, J.; Sakurai, H.; Kira, M. J. Am. Chem. Soc. 1997, 119, 3405. (b) Sakamoto, K.; Ogasawara, J.; Kon, Y.; Sunagawa, T.; Kabuto, C.; Kira, M. Angew. Chem., Int. Ed. 2002, 41, 1402.

Monosubstituted Disilenes

system is more sensitive to the mesomeric effects. A +M effect can be observed in the case of methyl, fluoro, and amino groups and a -M effect for the ethynyl-substituted ethylene. Mesomeric effects of disilene derivatives are also different: methyl, fluoro, ethynyl, and amino cause +M, +M, -M, and -M, respectively. Because of the large polarization caused by the mesomeric effects in fluoro- and aminodisilene, a drastic difference in the mechanism of the reactions from those of parent disilene is expected. The energy levels of the HOMO and LUMO in part govern the initial interaction at an early stage of the reaction. The thermodynamic data calculated by the CBS-Q, MP2, and DFT methods are listed in Table 2. The MP2computed thermodynamic data are generally closer to the highly accurate CBS-Q than to the DFT results. Selected geometric parameters at the MP2 level are given in Table 3. As the MP2 and B3LYP methods give very similar molecular geometry, only the MP2 results are presented in Table 3. Similarly to our previous studies7,9 the initial step of all the investigated addition reactions is the formation of weakly bonded complexes that carry several characteristic features. The geometry of the reactant molecules in the complexes is almost unchanged compared to that of the initial reactants (Figure 1, Table 3). The characteristic distance between the two reactants depends on the orientation of the two molecules and the substituent of disilene. While the energy gain relative to the initial state is between 0.11 and 6.78 kcal/mol (at the CBS-Q level), the Gibbs free energy is always positive, due to the negative entropy factors (Table 2). The orientation of the reacting agents in the initial complexes clearly indicates the electrophilic or nucleophilic character of the attack. A small but systematic charge transfer between the two agents also supports this character (Table 3). In the complexes formed by electrophilic interaction (CE), the NBO charge of the water or hydrogen fluoride moiety is negative. In the other complexes formed by nucleophilic interactions (CN), the NBO charge is positive. The shape of the molecular orbitals in the complexes is usually very similar to that of the separated component molecules. Remarkable polarization of the π-charge would affect the orbitalcontrolled reaction pathways of disilene derivatives. The shift of the orbital energies, however, reflects the interaction, which is also understandable on the basis of the FMO theory. H2SidSiHMe + H2O (R1). Two different initial complexes, an “electrophilic CE” and a “nucleophilic CN” type, were found in the reaction of unsubstituted disilene with water. The formation of these complexes is the first step of two different reaction channels that finally produced two stereochemically different products. Because of the obvious lack of symmetry compared to the parent molecule, the original reaction channels of methyldisilene are expected to be doubled (see Scheme 3). Indeed, four different initial complexes, two electrophilic and two nucleophilic ones, have been found in R1: electrophilic and nucleophilic complexes, CE1 and CN1, demonstrate the attack of the silicon on the substituent side (Si1) and an electrophilic and nucleophilic type complexes, CE2 and CN2, show the attack on the other side (Si2).

Organometallics, Vol. 23, No. 24, 2004 5771 Table 2. Thermodynamic Data of Stationary Points (in kcal/mol) CBS-Q reacn R1

R2

R3

R4

R5

R6c

reagent CE1 TE1 CN1 TN1 CL1 TL1 PF1 CE2 TE2 CN2 CL2 TL2 PF2 reagent CN1 TN1 CL1 TL1 PF1 CN2 TN2 CL2 TL2 PF2 reagent CE1 TE1 CN1 TN1 CL1 TL1 PF1 CE2 TE2 CN2 CL2 TL2 PF2 reagent CN1 TN1 CL1 TL1 PF1 CE2 CL2 TL2 PF2 reagent CE1 T1 PF1 CE2 T2 PF2 reagent CE T PF

MP2a

B3LYPa

∆E

∆G298

∆Eb

∆G298

∆Eb

∆G298

0.00 -2.49 -1.10 -2.16 0.91 -4.03 -4.65 -66.86 -5.24 -2.16 -0.11 2.14 0.55 -64.08 0.00 -2.01 -0.88 -9.38 -10.57 -74.64 -2.44 3.42 0.40 -1.13 -65.56 0.00 -1.65 -0.59 -3.06 -1.76 -5.93 -6.15 -66.65 -3.77 -4.03 -3.25 -1.38 -1.65 -64.02 0.00 -6.78 -4.31 -5.85 -5.23 -67.48 -6.23 6.09 4.03 -59.73 0.00 -3.95 -5.58 -77.49 -2.73 9.54 -68.64 0.0 -2.2 4.0 -72.1

0.00 3.61 7.44 6.66 10.58 5.21 5.40 -57.40 1.96 6.66 6.19 11.32 10.49 -54.75 0.00 4.56 6.06 -0.66 -1.16 -65.45 4.46 11.20 9.08 8.37 -56.58 0.00 4.43 7.79 5.63 7.48 2.85 3.33 -57.39 1.38 1.36 3.92 7.29 7.91 -54.92 0.00 1.87 4.86 2.56 4.36 -58.46 2.28 14.75 13.56 -50.99 0.00 3.40 3.22 -68.83 4.31 18.00 -60.93 0.0 2.9 12.6 -63.9

0.00 -2.85 -1.19 -3.30 2.33 -3.60 -4.28 -64.76 -2.58 3.63 -1.50 2.31 0.75 -62.24 0.00 -2.97 -1.59 -8.75 -8.85 -73.59 -2.70 3.91 -0.44 -1.79 -65.27 0.00 -4.33 -0.76 -3.94 2.29 -2.68 -3.06 -64.31 -1.48 -0.80 -2.50 -1.12 -1.77 -61.77 0.00 -5.66 -3.74 -5.75 -4.92 -65.69 -3.37 5.70 4.17 -58.02 0.00 -2.97 -1.03 -73.58 -1.75 12.72 -64.80 0.0 -0.9 6.2 -68.8

0.00 4.23 7.41 4.21 11.81 5.65 5.51 -55.63 3.62 12.55 5.44 11.41 10.38 -52.81 0.00 4.09 5.99 -0.11 0.77 -64.38 4.52 12.38 8.54 7.76 -56.42 0.00 3.45 7.79 4.24 11.74 6.35 6.62 -55.24 4.09 5.55 4.98 7.78 7.78 -53.24 0.00 2.87 5.74 2.91 4.75 -56.58 3.36 14.65 13.64 -49.27 0.00 1.94 7.90 -64.99 2.94 21.31 -56.70 0.0 5.2 14.5 -60.9

0.00 -1.53 -0.15 -2.09 3.93 -2.78 -3.30 -60.40 -1.25 4.32 -0.21 2.62 1.12 -58.01 0.00 -7.96 -0.71 -7.21 -7.90 -67.69 1.91 3.41 -1.03 -2.15 -59.83 0.00 -3.31 0.48 -2.30 4.19 3.77 -2.29 -59.56 -0.46 -1.56 -0.65 -1.37 -57.28 0.00 -3.57 -1.19 -3.82 -2.89 -59.65 -2.36 6.25 4.77 -52.34 0.00 -5.00 -2.36 -69.91 -2.68 10.21 -61.70 0.0 -2.4 3.6 -66.7

0.00 5.13 8.35 5.67 13.39 6.44 6.50 -51.14 4.39 13.12 6.79 11.74 10.75 -48.92 0.00 0.54 6.34 1.54 1.81 -58.46 9.10 11.58 7.96 7.43 -50.92 0.00 4.32 8.26 5.94 13.51 12.77 7.37 -50.44 4.74 5.94 8.35 8.22 -48.39 0.00 4.77 8.05 4.88 6.79 -50.63 3.25 15.17 14.22 -43.48 0.00 1.31 6.60 -61.27 3.39 18.72 -53.45 0.0 3.8 11.9 -58.8

a Using 6-311++G(d,p) basis. b With zero point energy correction. c Reference 9b.

Both CE1 and CE2 clearly suggest the electrophilic interaction between the water hydrogen and the HOMO of disilene. Despite the long H-Si distance (2.96-2.99 Å) the group charge on water is slightly negative (Figure 1, Table 3), indicative of charge transfer between the reactant molecules. The complexes are only slightly stabilized in comparison to the energy of the initial molecules, and CE2 is somewhat more stable than CE1. The stabilization energies of CN1 and CN2 are even smaller than those of the electrophilic complexes, and in this case, CN1 is somewhat more stable than CN2 (the

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Takahashi et al.

Table 3. Selected Geometric Dataa geometric paramb r(Si1Si2)

reacn R1

R2

R3

R4

R5

R6

reagent CE1 TE1 CN1 TN1 CL1 TL1 PF1 reagent CE2 TE2 CN2 CL2 TL2 PF2 reagent CN1 TN1 CL1 TL1 PF1 reagent CN2 TN2 CL2 TL2 PF2 reagent CE1 TE1 CN1 TN1 CL1 TL1 PF1 reagent CE2 TE2 CN2 CL2 TL2 PF2 reagent CN1 TN1 CL1 TL1 PF1 reagent CE2 CL2 TL2 PF2 reagent CE1 T1 PF1 reagent CE2 T2 PF2 reagent CE T PF

2.167 2.171 2.200 2.193 2.328 2.301 2.329 2.350 2.167 2.165 2.211 2.179 2.306 2.334 2.352 2.195 2.206 2.200 2.295 2.330 2.336 2.195 2.218 2.243 2.379 2.383 2.345 2.173 2.174 2.194 2.203 2.353 2.302 2.333 2.337 2.173 2.172 2.165 2.195 2.311 2.339 2.340 2.223 2.308 2.348 2.320 2.340 2.337 2.223 2.234 2.393 2.404 2.346 2.223 2.234 2.321 2.340 2.223 2.231 2.323 2.355 2.163 2.162 2.267 2.342

r(Si1X)

r(Si2H)

3.503 2.584 2.533 1.975 1.991 1.869 1.677

2.992 3.016 4.260 2.761 2.499 1.975 1.480

4.017 2.346 2.768 1.962 1.872 1.680

2.957 2.898 5.105 2.410 2.037 1.489

2.924 3.394 1.970 1.873 1.653

3.513 3.034 2.660 2.001 1.476

2.589 2.505 1.939 1.861 1.672

5.123 3.014 2.392 2.003 1.473

4.064 2.600 2.372 1.934 1.967 1.848 1.667

4.869 3.359 4.289 2.884 2.576 1.895 1.477

3.913 3.273 2.496 1.958 1.852 1.673

3.039 4.771 4.718 2.551 1.980 1.475

2.102 2.042 2.075 1.883 1.675

3.062 2.837 2.581 1.953 1.476

3.664 1.946 1.876 1.673

2.971 2.317 2.017 1.477

3.190 2.081 1.637

2.507 2.009 1.476

3.733 2.173 1.637

3.011 2.176 1.476

3.774 2.107 1.632

2.743 2.195 1.478

chargec r(XH) 0.959 0.963 0.965 0.963 0.972 0.990 1.135 0.959 0.962 0.970 0.961 1.002 1.106 0.959 0.960 0.961 0.979 1.118 0.959 0.962 0.965 0.999 1.107 0.959 0.959 0.963 0.963 0.978 0.985 1.128 0.959 0.962 0.960 0.962 0.987 1.132 0.959 0.970 0.978 0.980 1.149 0.959 0.965 1.011 1.103 0.916 0.930 1.037 0.916 0.924 0.985 0.916 0.925 0.988

a(XSi1Si2) 82.8 91.4 128.2 92.1 89.5 82.8 111.0 60.4 93.0 144.1 88.1 83.2 106.2 79.6 74.8 92.2 82.8 106.8 152.9 88.2 85.2 80.5 103.4 110.3 96.7 132.6 96.5 90.2 82.7 107.7 76.1 111.6 138.7 89.2 82.2 106.5 101.2 95.9 88.7 81.5 103.7 61.6 84.2 80.7 106.0 62.1 79.5 107.3 74.0 80.8 108.9 69.9 83.7 109.5

δ(Si1)

δ(Si2)

0.679 0.751 1.015 0.898 1.172 1.135 1.165 1.362 0.253 0.211 0.724 0.404 0.861 0.897 1.132 1.122 1.170 1.225 1.449 1.479 1.652 0.154 0.341 0.641 0.797 0.829 1.092 0.641 0.624 0.935 0.877 1.147 1.116 1.151 1.342 0.340 0.376 0.538 0.577 0.908 0.939 1.158 1.092 1.309 1.302 1.306 1.333 1.504 0.041 -0.051 0.458 0.575 0.966 1.092 1.171 1.337 1.555 0.041 0.013 0.789 1.158 0.304 0.383 0.905 1.211

0.253 0.171 -0.057 0.070 -0.117 -0.172 0.002 0.510 0.679 0.703 0.234 0.559 0.162 0.293 0.783 0.154 0.152 0.036 -0.200 -0.035 0.503 1.122 0.961 0.681 0.566 0.709 1.180 0.340 0.361 0.042 0.118 0.001 -0.131 0.040 0.560 0.641 0.588 0.442 0.442 0.139 0.301 0.774 0.041 -0.086 -0.061 -0.187 0.000 0.531 1.092 1.172 0.789 0.814 1.130 0.041 -0.042 -0.046 0.510 1.092 1.120 0.532 0.994 0.304 0.296 -0.056 0.517

δ(X′H)b -0.008 0.030 0.048 0.154 0.095 -0.141 -0.004 0.046 0.025 0.050 -0.130

0.010 -0.010 0.117 -0.118

0.041 0.032 0.066 -0.142 -0.001 0.036 0.073 0.157 0.115 -0.123 -0.007 0.004 0.051 0.108 -0.135

0.115 0.122 0.094 -0.164 -0.009 0.023 -0.143 -0.038 -0.159 -0.011 -0.126 -0.024 -0.082

a Calculated at the MP2/6-311++G(d,p) level. Distances are given in angstroms and angles in degrees. b X ) O, F; X′ ) OH, F. c Natural population analysis (NPA) charge.

Gibbs free energy shows the opposite tendency). The Si-O distances in CN1 and CN2 are 2.53 and 2.77 Å, respectively. The orientation of CN1 and CN2 indicates a nucleophilic interaction. In the language of FMO theory, the lone pair of water oxygen approaches the

LUMO of disilene. The nucleophilic interaction is also proved by the positive charge on the water molecule (Table 3). Figure 2 shows the energy profile of the four reaction channels of R1 starting from the four initial complexes.

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Figure 1. Optimized structures of initial complexes at the MP2(full)/6-311++G(d,p) level. Bond distances are in Å. NBO charges are shown in italics. δH2O and δHF are the natural population analysis (NPA) charges of the H2O and HF moieties. CN and CE are the nucleophilic and electrophilic initial complexes, respectively. The suffixes 1 and 2 denote 1- and 2-addition, respectively.

As in the reaction of the parent disilene with water, in all cases before finding the final product, a second stable complex can be reached. CN1 and CE1 and also CN2 and CE2 lead to the same second complexes: CL1 and CL2, respectively. Although the activation energies on the paths to CL1 and CL2 are only slightly different, the structures of the transition states on the electrophilic and the nucleophilic paths suggest essentially different processes. In the electrophilic channels CE-TE-CL (regardless of 1- or 2-addition) the first electrophilic interaction is followed by a nucleophilic attack. For this, the oxygen lone pair approaches silicon with rotation of a water molecule around the Si-H axis. The transition states can be characterized by small geometric changes. The Si-O distances are 2.58 Å (TE1) and 2.35 Å (TE2); the Si-Si bond lengths elongate slightly to 2.20-2.21 Å, which indicates a considerable double bond character at this stage. The CE-TE-CL process is

accompanied by a charge shift from the positive disilene to the negative water. These reaction channels end in syn orientations of the products. The nucleophilic channels CN-TN-CL, on the other hand, give stereochemically opposite products (anti). From the initial complex, one of the water hydrogens turns toward the HOMO lobe of disilene (antarafacial approach). This step is accompanied by the rotation of the SiH2 (or SiHMe) group by 180° around the Si-Si bond, and the product is an anti silanol. At the transition state, TN, the rotation is close to 90°. Obviously, at this stage the π-bond has already disappeared and the Si-Si bond is extremely long. The Si-O distance is also shorter than in the case of TE, indicating the enhanced interaction. The rotating and strongly pyramidal silicon carries both electrons of the original π-bond in a lone electron pair.7 Despite the drastic geometric change, the activation energy is, however, surprisingly small, which indicates

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Figure 2. Free energy diagram at 298 K by the CBS-Q method in kcal/mol for methyldisilene + H2O (R1). Imaginary frequency modes of the transition structures are indicated by arrows.

that this type of reaction channel is also feasible. Unfortunately, we could not localize the transition state, TN2, for the 2-addition channel, but from our previous experience7,9a we anticipate the same rotation of the SiHMe group and finally the anti-PF2 species. As without clear evidence we cannot exclude any exception from the rule, we place anti-PF2 in parentheses in Figure 2. The geometry around the attacked silicon of CL1 and CL2 becomes almost planar, while that of the neighboring silicon is strongly pyramidal. Although these complexes are far from the final products in structure and energy, in this period of the reaction the Si-Si bond is close to the final Si-Si single bond. One of the water hydrogens turns toward a silicon atom. The O-H bond is slightly longer than at the beginning of the reaction, and the distance between the water hydrogen and the attacked silicon is long (2.4-2.5 Å) but is less than the sum of van der Waals radii of Si and H (3.3 Å). Considering the charge distribution of the molecule, this stage of the reaction is in accord with the zwitterionic structure suggested on the basis of experimental results.6c The most important process of the next transition state (TL1 and TL2) is the beginning of O-H bond breaking and a simultaneous Si-H bond formation. The Si-O and Si-H distances are shortened and the O-H distance is elongated to 1.11-1.14 Å, while the Si-Si bond length practically reaches its final value. It forms a four-membered-ring shape, which was suggested in previous studies.4 Since CL2 is higher in energy than all the transition states leading to the 1-adduct (TE1, TN1, and TL1), the formation of the 1-adduct is more preferable than that of the 2-adduct. TE1 is lower in energy than TN1, and thus the syn product is somewhat more preferable among 1-adducts. H2SidSiHF + H2O (R2). Because of the strongly electronegative substituent, both the HOMO and LUMO of fluorodisilene are stabilized (Table 1). Therefore, only the nucleophilic interaction is expected to be effective:

indeed, only two nucleophilic initial complexes, CN1 and CN2, can be found (Figure 1). The mutual orientation of the molecules and the positive charge on the water in CN1 (0.010) and CN2 (0.041) give evidence for the nucleophilic interaction. The stabilities of CN1 and CN2 are comparable: 2.01 and 2.44 kcal/mol, respectively, relative to that of the reagent molecules. The structure of CN2 is very similar to those in the unsubstituted disilene + water reaction and in R1. CN1 is, however, somewhat different, as the water molecule attacks disilene from the opposite direction of the fluorine substituent and the oxygen lone pair is located in the pseudo π-plane. The structure of CN1 is similar to the charge-controlled initial complex found in the reaction of parent disilene with water.9a The long O-Si distances (2.924 and 2.589 Å for CN1 and CN2, respectively) indicate the weakness of the bond. Figure 3 shows the free energy profile of the two different reaction channels of R2 starting from the two initial complexes. Both are two-step reactions via Lewis adducts. In R1 and the unsubstituted disilene + water reaction, the most crucial step of the nucleophilic channels is the silyl group rotation around the Si-Si axes. This rotation is the fundamental step for the antioriented product formation. In R2, however, the rotation around the Si-Si bond is missing. Instead of the rotation, an inversion motion of the SiHF group can be observed at the path from CN2 to TN2; consequently, the final product should be syn oriented. The path from CN1 to TN1 is a motion of the water molecule, driving the reaction also to syn product. Among the two, the formation of a 1-adduct is energetically preferable. Since the fluorine substituent inhibits the electrophilic channels and prefers the 1-addition, it evokes a strong regioand stereoselectivity of the reaction. H2SidSiH(CtCH) + H2O (R3). All four expected initial complexes, CN1, CE1, CN2, and CE2, are found in the reaction of ethynyldisilene with water (Figure 1). The structures of CN1, CN2, and CE2 are similar to those

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Figure 3. Free energy diagram at 298 K by the CBS-Q method in kcal/mol for fluorodisilene + H2O (R2). Imaginary frequency modes of the transition structures are indicated by arrows.

Figure 4. Free energy diagram at 298 K by the CBS-Q method in kcal/mol for ethynyldisilene + H2O (R3). Imaginary frequency modes of the transition structures are indicated by arrows.

in the unsubstituted disilene + water reaction. The structure of CE1 is rather unusual, since the interaction of hydrogen with the ethynyl group is more effective than with silicon, although the reason is not well understood at present.

Figure 4 shows the energy profile of the reactions starting from the four initial complexes. The channels from CE1 and CN1 are similar to those of the reaction of disilene with H2O, and therefore, the products are syn and anti adducts, respectively. The 2-addition channels

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Figure 5. Free energy diagram at 298 K by the CBS-Q method in kcal/mol for aminodisilene + H2O (R4). Imaginary frequency modes of the transition structures are indicated by arrows.

seem to be also similar to the reaction of parent disilene with water. Unfortunately, we could not localize the transition state, TN2, but from our previous experiences7,9a we anticipate the rotation of the SiH(CtCH) group around the SidSi double bond similarly to the nucleophilic path of the parent reaction and finally the antioriented product. Without clear evidence, however, we cannot exclude any exception from the rule. The rate-determining step for the 1-addition path is the barrier before CL, while that for 2-addition is the barrier after CL. The barrier heights controlling the four reaction channels are almost the same. In comparison to the energetics of the parent reaction, this indicates that the ethynyl substituent reduces the barrier of the nucleophilic channels selectively (by about 5 kcal/mol for nucleophilic channels and by about 0.8 kcal/mol for electrophilic channels). H2SidSiH(NH2) + H2O (R4). Since the substituent effects of the amino group on the energy levels of the HOMO and LUMO are insignificant and are similar to those of the methyl group (Table 1), four initial complexes and four successive reaction pathways are expected. However, two pathways were found from two initial complexes (Figure 5). This pattern is similar to the reaction of the polar substrate silene with water,9a where the nucleophilic channel starts from the attack of the positively charged silicon and the electrophilic channel from the attack of the negative carbon. The electrophilic channel has a very high activation energy. The charge difference between Si1 and Si2 of the aminodisilene (∆(Si1-Si2) ) 1.051) is similar to that of fluorodisilene (0.968), and it is quite large in comparison to the other two monosubstituted disilenes (∆(Si1-Si2)

) 0.426 for methyldisilene and 0.301 for ethynyldisilene). The inductive and conjugative effects of the amino group reduce the σ-electron density at Si1 and the π-electron density at Si2 and increase the σ-electron density at Si2 and π-electron density at Si1, respectively (Table 1). The Si-O distance in CN1 is quite short (2.102 Å) compared to other CN’s, which indicates a strong interaction between Si1 and oxygen. The next step is the expected rotation around the SidSi double bond, which finally leads to an anti-1-hydroxy product via the Lewis adduct CL1 and the four-membered-ring transition state TL1. We could not find the transition states between CE2 and CL2 in the 2-addition pathway. However, the energy-rich Lewis adduct CL2 and the fourmembered-ring transition state TL2 (free energies are 14.8 and 13.6 kcal/mol, respectively) were found before the syn-oriented product. From the large energy difference between the two competitive channels we may predict the strong regio- and stereoselectivity and the most likely 1-addition and anti-oriented product. The experimental facts fully support our finding: alcohol addition to amino-substituted disilenes gives 100% 1-adduct.3 Unfortunately, there are no data for the stereochemistry of the product. H2SidSiH (NH2) + HF (R5). Only the electrophilic channel is expected in the reaction with the highly acidic hydrogen fluoride. The NPA charge of the HF part in the complex in the unsubstituted disilene + HF reaction (R6) is negative (-0.024), indicative of the charge transfer from the disilene moiety to the HF part by electrophilic interaction (Table 3). The next step is a nucleophilic attack of fluorine at silicon.9b,11 No Lewis adduct is found, and

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of CE1 and CE2 are -0.038 and -0.011, respectively, which confirms the electrophilic character. The structures of the two complexes are similar; in both cases the reacting hydrogen points to the less positive Si2 (Figure 1). The main difference between the two is that in CE1 the hydrogen turns toward the lone pair of silicon, while in CE2 it attacks from the back side of the lone pair. The structure of CE1 is similar to that of CE of R6 with some geometrical differences: the longer H-F bond (0.930 Å) and the shorter F-Si and H-Si bonds (3.190 and 2.507 Å) compared to those of CE in R6 (0.925, 3.774, and 2.743 Å) indicate a somewhat enhanced interaction.

Figure 6. Free energy diagram at 298 K by the CBS-Q method in kcal/mol for disilene + HF (R6). Imaginary frequency modes of the transition structures are indicated by arrows.

the reaction can be characterized as an electrophilic onestep mechanism (Figure 6). The activation free energy is low (12.6 kcal/mol by the CBS-Q method), which makes the reaction feasible (Table 2). A similar mechanism occurs in the reaction of H2SidSiH2 with acidic alcohols such as CF3OH.7,9a Two different electrophilic initial complexes, CE1 and CE2, were obtained in R5. The charges of the HF moiety

The two different reaction pathways starting from the individual initial complexes give two different products (Figure 7). A one-step reaction from CE1 with the low energy transition state (T1) gives the syn 1-adduct, while a similar mechanism from CE2 via a high-energy transition state (T2) produces syn-PF2. The lack of nucleophilic channels and the considerable energy difference between the two sides sustain the expected regio- and stereoselectivity. A remarkable regioselectivity was found in the reaction of the polar substrate aminodisilene (R4 and R5), the same as in the reaction of the polar substrate silene with water: the 1-addition pathway is remarkably preferable. The difference from R4 is the stereoselectivity: the 1-addition product is anti-oriented in R4 and syn-oriented in R5, respectively. The results of R4 and R5 suggest that the stereoselectivity of aminodisilene could be controlled by the acidity of the reagent.

Figure 7. Free energy diagram at 298 K by the CBS-Q method in kcal/mol for aminodisilene + HF (R5). Imaginary frequency modes of the transition structures are indicated by arrows.

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Conclusions To study the observed regio- and stereoselectivity in the bimolecular addition reaction of disilenes, we investigated all the possible paths of the reactions of several monosubstituted disilenes. At the early stage of all the reactions, a weakly bonded initial complex was found which determined the further mechanism of the reaction. The formation of these complexes from the initial reactants is endergonic and not spontaneous at room temperature because of the positive Gibbs free energies. In R1 and R3, four initial complexes and therefore four reaction channels were found. Only two complexes were found, however, in R2, R4, and R5. The reactions of the polar substrate aminodisilene in R4 and R5 show reaction profiles similar to that of the polar substrate silene.9a Considering the competition of reaction channels, we can attempt to estimate the most probable product and the product ratio of the reactions. This seems to be easy in R4 and R5, as the energy difference between the appropriate transition states is large. In case of the other three reactions (R1-R3), however, the energy difference is not large enough to exclude the presence of any possible product. Comparing the calculated barrier heights to those of the parent disilene + water reaction, we can estimate some qualitative substituent increments which can help to estimate the product ratio in the reaction of di-, tri-, or tetrasubstituted disilenes (Table 4). The general conclusion from Table 4 is that the substituents promote the nucleophilic paths. This indicates that the anti product ratio of 1-addition is expected to be high. It is important to consider the effects of bulky substituents on the reaction profiles, especially on the importance of initial complexes, because experimentally synthesized disilenes are usually substituted by much bulkier groups. When the structures of the two types of initial

Takahashi et al. Table 4. Increment ) ∆G298(TS) - ∆G298(TS)parent of Rate-Determining Step electrophilic

nucleophilic

substituent

1-addition

2-addition

1-addition

2-addition

Me F CtCH NH2

-1.2

-1.9

-0.8

-7.2 6.2

-2.2 -6.7 -5.3 -7.9

-6.6 -1.6 -8.9

complexes are compared, the attacked site of disilenes with bulky substituents seems to be less crowded in the nucleophilic complex CN and thus it is easier to form CN than CE. This would suggest a high anti product ratio in disilenes with bulky substituents. Special attention has been focused on the reaction of ethynyldisilene, since the ethynyl group simulates the effect of the practically important and frequently used phenyl group. To prove the comparability, several stationary points for the reaction of phenyldisilene with water were calculated at the MP2/6-311++G(d,p)//MP2/ 6-31+(d,p) + ZPE level. The barrier heights are 2.50 (2.29), -0.78 (-0.76), and 0.61 (-1.77) kcal/mol for TN1, TE1, and TL2, respectively (calculated data for the ethynyldisilene + water reaction using the same level are given in parentheses). From the data it is clear that the effects of the phenyl group can be simulated well by the ethynyl group. Acknowledgment. This work was supported in part by the OTKA (Grant No. T 034768) and in part by Hayashi Memorial Foundation. Supporting Information Available: Tables giving Cartesian coordinates and absolute energies of all calculated structures. This material is available free of charge via the Internet at http://pubs.acs.org. OM049418M