Hyperconjugation Effect in Substituted Methyl Boranes: An Orbital Deletion Procedure Analysis Yirong Mo,*,† Haijun Jiao,‡ and Paul von Rague´ Schleyer§ Department of Chemistry, Western Michigan University, Kalamazoo, Michigan 49008, Leibniz-Institut fu¨ r Organische Katalyse an der Universita¨ t Rostock e.V., Buchbinderstrasse 5-6, 18055 Rostock, Germany, and Department of Chemistry, University of Georgia, Athens, Georgia 30602
[email protected] Received November 24, 2003
The hyperconjugation effect in the substituted methyl boranes, XCH2BH2 (X ) H, CH3, NH2, PH2, OH, SH, F, Cl, Br), has been quantitatively evaluated by using the orbital deletion procedure (ODP), where the pπ orbital on boron is deactivated. Except for the case of X ) NH2, which forms a threemembered ring, the magnitude of the hyperconjugative stabilization in all other substituted methylborane ranges from 6.8 to 3.4 kcal/mol. Significant structural changes are observed, particularly the shortening of the central B-C bond distance and the reducing of the corresponding XCB and HCB bond angles. In general, the strength of the hyperconjugative interaction between the occupied σC-X bond and the vacant pπ orbital on boron is correlated to the electronegativity of X, and the competition between the donation ability of the σC-X and the σC-H bonds determines the preference of the staggered or eclipsed structure as the energy minimum state. When the donation abilities of the C-X and C-H bonds are comparable, other factors such as electron correlation and steric effect may play elaborate roles in the geometrical propensity of the most stable structures. Introduction Due to electron deficiency of the vacant p-orbitals, boranes are used not only as Lewis acids but also as intermediates in synthetic chemistry.1 Apart from the boron clusters which are characteristic of three-dimensional delocalized multicenter-multielectron bondings, one of the classic example for the interaction between vacant p-orbitals on boron atoms and vicinal bonds is borirene (A) in which the unoccupied boron p-orbital allows π electrons in the CdC double bond to delocalize over the whole ring as its isoelectronic aromatic cyclopropenyl cation (B).2
In addition to the conjugation effect, hyperconjugation between the adjacent C-X σ bonds and the empty boron †
Western Michigan University. Leibniz-Institut fu¨r Organische Katalyse an der Universita¨t Rostock e.V. § University of Georgia. (1) (a) Siebert, W. Advances in Boron Chemistry; The Royal Society of Chemistry:, Cambridge, England, 1997. (b) Pelter, A.; Smith, K.; Brown, H. C. Borane Reagents; Academic Press: New York, 1988. (c) Goldfuss, B.; Knochel, P.; Bromm, L. O.; Knapp, K. Angew. Chem., Int. Ed. 2000, 39, 4136. (2) (a) Volpin, M. E.; Koreshkov, Y. D.; Dulova, V. G.; Kursanov, D. N. Tetrahedron 1962, 18, 107. (b) Pittman, C. U.; Kress, A.; Patterson, T. B.; Walton, P.; Kispert, L. D. J. Org. Chem. 1974, 39, 373. (c) Allinger, N. L.; Siefert, J. H. J. Am. Chem. Soc. 1975, 97, 752. (d) Krogh-Jespersen, K.; Cremer, D.; Dill, J. D.; Pople, J. A.; Schleyer, P. v. R. J. Am. Chem. Soc. 1981, 103, 2589.
p-orbitals3 is of considerable interest to experimental and theoretical studies.4 A thorough retrospect of the hyperconjugation effect can be found by a recent paper.5 The delocalization of σ electrons into the empty pπ orbitals on borons is much similar to the substituted carbocations,6 although the magnitude of the hyperconjugative interaction in boranes is expected to be much smaller than in carbocations, where the quest to relieve the accumulation of the positive charge and enhance the molecular stability drives a significant amount of adjacent bond electrons to the cationic center. Qualitatively, it has been established that the donor ability of a C-X bond depends on the electronegativity of atom X.7 This is understandable as the magnitude of the hyperconjugation effect is determined by the energy gap between the donor (the C-X bond) and the acceptor (the vacant p orbital on boron for boranes) as well as their overlap. Experimentally, the hyperconjugation effect resulting in structural changes is reflected by the decreased vibration frequencies8 and downshifts of NMR signal.9 Although the structural variations can be roughly compared with reference systems, the hyperconjugative stabilization
‡
(3) (a) Conference on Hyperconjugation. Tetrahedron 1959, 5, 105. (b) Dewar, M. J. S. Hyperconjugation; Ronald Press: New York, 1962. (4) (a) Wiberg, K. B.; Castejon, H. J. Am. Chem. Soc. 1994, 116, 10489. (b) Schneider, W. F.; Nance, B. I.; Wallington, T. J. J. Am. Chem. Soc. 1995, 117, 478. (c) Tostes, J. G. R.; Seidl, P. R.; Sota, M. M.; Carneiro, J. W. M.; Lie, S. K.; Taft, C. A.; Brown, W.; Lester, W. A., Jr. Chem. Phys. Lett. 1995, 237, 33. (d) Cramer, C. J. In Encyclopedia of Computational Chemistry; Schleyer, P. v. R., Ed.; John Wiley & Sons: Berlin, 1998; pp 1294. (5) Alabugin, I. V.; Zeidan, T. A. J. Am. Chem. Soc. 2002, 124, 3175. (6) Olah, G. A. J. Org. Chem. 2001, 66, 5943. (7) Hoffmann, R.; Radom, L.; Pople, J. A.; Schleyer, P. v. R. J. Am. Chem. Soc. 1972, 94, 6221.
10.1021/jo035724i CCC: $27.50 © 2004 American Chemical Society
Published on Web 04/09/2004
J. Org. Chem. 2004, 69, 3493-3499
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Mo et al. SCHEME 1
energies cannot be obtained directly from experimental data. Approximately, however, isodemic reactions can be designed to estimate the magnitude of hyperconjugative interactions. Computationally, the natural bond orbital (NBO) method10 has been broadly employed to investigate the hyperconjugation effect in various systems.11-13 Alternatively, Mo et al. proposed the orbital deletion procedure (ODP) to analyze the rotation barrier in B2X4 (X ) H, F, OH, NH2, CH3) systems14 and to quantify the hyperconjugation in cyclopropylcarbinyl cation and cyclopropylborane15 and cyclic electron delocalization (aromaticity) in planar carbocations.16 The idea of ODP is quite simple; for example, in carbocations, ODP can be used to “inactivate” or “quench” the π atomic orbitals (AOs) on a positively charged carbon in delocalized systems. The resulting hypothetical localized structure can be subsequently optimized. This generates a localized geometry and a corresponding localized wave function ΨLoc with a total energy E(ΨLoc) where the effects of conjugation or hyperconjugation are excluded. In the meantime, the usual HF optimization with all AOs generates the delocalized geometry and wave function ΨDel and its corresponding total energy E(ΨDel). The difference between E(ΨLoc) and E(ΨDel) is consequently ascribed to the effects of conjugation or hyperconjugation. It should be noted that both ΨLoc and ΨDel are optimized self-consistently. In this paper, we probe the hyperconjugation effect on the substituted methylboranes (XCH2BH2; X ) H, CH3, NH2, PH2, OH, SH, F, Cl, Br) by employing the ODP approach systematically. In the substituted boranes, where the C-X σ bond is aligned parallel with the vacant pπ AO on boron, the hyperconjugation effect can be illustrated by Scheme 1, where a substituted borane II can be described as the resonance between the two valence bond structures I and III. Structure I without electronic occupation in the boron π-AO can be regarded (8) (a) Hernandez, V.; Castiglioni, C.; Zerbi, G. J. Mol. Struct. 1994, 324, 189. (b) Mesic, M.; Novak, I.; Sunko, D. E.; Vancik, H. J. Chem. Soc., Perkin Trans. 2 1998, 2371. (9) Siehl, H.-U.; Mu¨ller, T.; Gauss, J.; Buzek, P.; Schleyer, P. v. R. J. Am. Chem. Soc. 1994, 116, 6384. (10) (a) Reed, A. E.; Weinhold, F. J. Chem. Phys. 1983, 78, 4066. (b) Reed, A. E.; Weinstock, R. B.; Weinhold, F. J. Chem. Phys. 1985, 83, 735. (c) Reed, A. E.; Curtiss, L. A.; Weinhold, F. Chem. Rev. 1988, 88, 899. (11) (a) Reed, A. E.; Schleyer, P. v. R. J. Am. Chem. Soc. 1990, 112, 1434. (b) Salzner, U.; Schleyer, P. v. R. Chem. Phys. Lett. 1992, 190, 401. (c) Salzner, U.; Schleyer, P. v. R. J. Am. Chem. Soc. 1993, 115, 10231. (d) Salzner, U.; Schleyer, P. v. R. J. Org. Chem. 1994, 59, 2138. (12) (a) Reed, A. E.; Weinhold, F. Isr. J. Chem. 1991, 31, 277. (b) Pophristic, V.; Goodman, L. Nature 2001, 411, 565. (c) Weinhold, F. Angew. Chem., Int. Ed. 2003, 42, 4188. (13) Bickelhaupt, F. M.; Baerends, E. J. Angew. Chem., Int. Ed. 2003, 42, 4183. (14) (a) Mo, Y.; Lin, Z. J. Chem. Phys. 1996, 105, 1046. (b) Mo, Y.; Lin, M.; Wu, W.; Zhang, Q.; Schleyer, P. v. R. Sci. Sinica 1999, B42, 253. (c) Mo, Y.; Wu, W.; Zhang, Q. J. Chem. Phys. 2003, 119, 6448. (15) Mo, Y.; Schleyer, P. v. R.; Jiao, H.; Lin, Z. Chem. Phys. Lett. 1997, 280, 439. (16) (a) Mo, Y.; Jiao, H.; Lin, Z.; Schleyer, P. v. R. Chem. Phys. Lett. 1998, 289, 383. (b) Jiao, H.; Schleyer, P. v. R.; Mo, Y.; McAllister, M. A.; Tidwell, T. T. J. Am. Chem. Soc. 1997, 119, 7075.
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as the reference structure (or the most stable resonance structure in the terminology of the resonance theory) to evaluate the hyperconjugation effect in II, whose wave function is a combination of the resonance structures I and III. The ionic structure III is the radical presentation of “no-bond” (between C and X+) and “double bond” (Cd B) for hyperconjugation whose energy lies much higher than the neutral resonance structure I. Due to the high energy gap between I and III, the hyperconjugation effect can be studied by the comparison of the covalent structure I and delocalized structure II, which makes the ODP applicable in these systems.
Computational Methods The calculations were performed with the Gaussian 94 programs.17 For the normal substituted methyl boranes XCH2BH2, both the perpendicular and eclipsed conformations were optimized at the levels of HF/6-311G** and B3LYP/6-311+G** density functional theory.18 The nature of each stationary point was characterized by a frequency calculation at the same levels, and the relative energies have been corrected for the zero-point energy (ZPE, scaled by 0.89 for HF method). The final relative energies were computed at the CCSD(T) level with the 6-311+G** basis set and the B3LYP/6-311+G** geometries including ZPE corrections. All these data are summarized in the Supporting Information. Since none of the standard quantum chemistry software can perform ODP calculations in a straightforward manner, a slightly modified Gaussian 94 program was used.19 The unique feature of our ODP method is the deletion of the pπ (and dπ when d-polarization functions are employed) basis functions on the boron atom (and pπ orbitals on the two hydrogen atoms attached to the boron atom when p-polarization functions for hydrogen are used) to evaluate the effect of hyperconjugation on the conformations and energies of the XCH2BH2 systems. It should be pointed out here that the current ODP scheme is still limited. For example, the local symmetry is constrained to Cs for XCH2BH2 in which the BH2 and C should lie in the same plane. This is perceivable since without hyperconjugation the pπ AOs will be completely empty and the three bonds around boron could make use of only σ and pσ AOs on the boron atom. Although the current ODP approach is established at the HF level where the electron correlation is not taken into account, the electron correlation is expected to play a limited role in the estimation of the hyperconjugation energy (HCE) as the HCE is a relative quantity and defined as the energy difference between the optimized structures without and with the ODP scheme. This is proved by the case of the allyl cation since the ODP calculations and the ab initio valence bond calculations lead to very close data.20 Another concerning issue in the ODP approach is the basis set superposition error (17) Gaussian 94: Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Gill, P. M. W.; Johnson, B. G.; Robb, M. A.; Cheeseman, J. R.; Keith, T. A.; Petersson, G. A.; Montgomery, J. A.; Raghavachari, K.; Al-Laham, M. A.; Zakrewski, 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.; Stewart, J. P.; Head-Gordon, M.; Gonzalez, C.; Pople, J. A. Gaussian Inc., Pittsburgh, PA, 1995. (18) Foresman, J. B.; Frisch, Æ. Exploring Chemistry with Electronic Structure Methods, 2nd ed.; Gaussian, Inc.: Pittsburgh, 1996. (19) To make these specific basis functions deactivated in the molecular orbitals, we simply set their one-electron integrals a very high positive value (e.g., 5000 au) and assign zero to their overlap integrals with all other basis functions. Consequently, their molecular orbital coefficients in the occupied molecular orbitals become negligible and their contribution to the molecular energy vanishes. (20) (a) Mo, Y.; Lin, Z.; Wu, W.; Zhang, Q. J. Phys. Chem. 1996, 100, 6469. (b) Mo, Y.; Peyerimhoff, S. D. J. Chem. Phys. 1998, 109, 1687.
Hyperconjugation Effect in Substituted Methyl Boranes TABLE 1. Relative Energies (Erel, kcal/mol) and Hyperconjugation Energies (HCE, kcal/mol) for Different Conformers of the Substituted Methyl Boranes BH2CH2X X H CH3
NH2 PH
OH
SH
F Cl Br
species
Erela
Erelb
1a 1b 2a 2b 2c 2d 2e 2f 3a 4a 4b 4c 4d 5a 5b 5c 5d 6a 6b 6c 6d 7a 7b 8a 8b 9a 9b
0.0 0.0 0.0 2.0 2.0 3.8 0.2 -15.0 0.0 0.0 5.3 5.4 7.0 0.0 -0.3 6.0 8.1 0.0 0.4 2.5 0.7 0.0 -6.9 0.0 -0.4 0.0 2.1
0.0 0.0 0.0 2.2 1.7 3.0 0.2 -13.5 0.0 0.0 4.8 5.5 7.3 0.0 0.1 4.9 7.7 0.0 0.6 1.9 -0.2 0.0 -5.9 0.0 1.4 0.0 3.5
Erelc
HCEc
0.0 0.0 0.0 2.5 1.5 4.2
4.9 4.9 5.1 5.3 4.7 4.9
-13.3 0.0 0.0 3.8 3.0 4.9 0.0 0.7 4.6 8.4 0.0 1.0 2.1 1.9 0.0 -6.0 0.0 -0.2 0.0 0.8
7.5 6.8 5.7 4.8 5.0 5.2 5.8 3.4 4.1 5.0 5.2 4.7 5.8 3.6 5.2 4.7 4.6 5.9 4.6
a At the B3LYP/6-311+G** level plus the ZPE (B3LYP/6311+G**) correction. b At the CCSD(T)/6-311+G**//B3LYP/6311+G* level plus the ZPE (B3LYP/6-311+G**) correction. c At the HF/6-311G** level plus the ZPE correction scaled by 0.89.
(BSSE), since the basis set in the HF calculations is larger than that in the ODP calculations. Without the proper removal of the BSSE, the HCE as defined in the above may be overestimated. Fortunately, our previous detailed comparison with a range of basis sets suggests that the BSSE is not significant as the computed HCE is essentially independent of the basis sets.14a The HCEs for various conformations of the substituted methyl boranes are listed in Table 1, where ZPE corrections have been taken into account.
Results and Discussion H3CBH2 (1). The methyl hyperconjugation has attracted the most intensive attentions. Radom21 examined the variations of the methyl tilt angles and the C-H bond lengths in terms of two-electron interaction alone, while Boggs22 interpreted their results in terms of a bond-bond repulsion model. At both the HF/6-311G** and B3LYP/ 6-311+G** levels, the staggered conformation 1a is the energy minimum state and the eclipsed form 1b is the transition state for the BH2 rotation around the B-C bond. However, after the ZPE correction, no rotation barrier can be observed at the HF/6-311G** and B3LYP/ 6-311+G** levels or at the CCSD(T)/6-311+G** level with the B3LYP/6-311+G** geometries. This means that the methyl group essentially can rotate freely in methylborane, which is in contrast to ethane.12,13 The absence of a rotational barrier in methyl borane can be explained (21) Pross, A.; Radom, L.; Riggs, N. V. J. Am. Chem. Soc. 1980, 102, 2253. (22) (a) Flood, E.; Pulay, P.; Boggs, J. E. A. J. Am. Chem. Soc. 1997, 99, 55570. (b) McKean, D. C.; Boggs, J. E.; Scha¨fer, L. J. Mol. Struct. 1984, 116, 313.
FIGURE 1. Optimized delocalized (localized) structural parameters of methylborane (1) and ethylborame (2).
in terms of the square of the overlap between a CH bond and a BH bond which is related to both the steric repulsion and hyperconjugative stabilization between the methyl group and the BH2 moiety. If the methyl group is of a perfect C3v local symmetry, the sum of the squares of the projections of the three C-H bond to any plane (namely the orientation of the planar H-B(C)-H) can be proved to be a constant. The comparison between HF and ODP computations shows that the hyperconjugation effect plays a significant role in the molecular structure of methylborane. The optimized structural parameters for both with (in parentheses) and without ODP scheme are given in Figure 1. Relative to its localized form 1a* after ODP, the B-C bond length in 1a is shortened by 0.024 Å, and the vertical C-H1 bond is elongated from 1.090 Å to 1.096 Å, which is longer than the two out-of-plane C-H2 or C-H3 bonds (1.086 Å), and the BCH1 angle reduces 3.9° at the HF/6-311G** level. It is interesting to note that the eclipsed C-H1 bond in 1a* is still longer than the gauche bonds (C-H2 and C-H3), which can be well explained in terms of steric effects. The repulsion between the methyl group and BH2 moiety pushes the C-H1 bond toward the vertical position of larger pchacater. B3LYP/6-311+G** optimization results in a further shorter B-C bond (1.552 Å) and larger bond angle difference between BCH1 and BCH2 in 1a (10.8°) compared with the HF/6-311G** geometry (1.569 Å vs 8.7°). This suggests that the B3LYP functional including electron correlation favors more hyperconjugated strucJ. Org. Chem, Vol. 69, No. 10, 2004 3495
Mo et al. TABLE 2. Stretching Frequencies (ν, cm-1) and Intensity (kM/mol) Of the Methyl Group for the Delocalized (Localized) Staggered (1a) and Eclipsed (1b) Conformation of Methylboranea structure
symmetry
frequency
intensity
vibrational mode
1a (1a*)
a′ a′ a′′ a′ a′′ a′
ν1 ) 3104 (3146) ν2 ) 3190 (3203) ν3 ) 3232 (3222) ν1 ) 3127 (3147) ν2 ) 3168 (3198) ν3 ) 3232 (3223)
9.4 (15.4) 26.6 (44.6) 33.5 (38.2) 4.7 (14.1) 25.2 (45.5) 38.6 (39.0)
mainly the stretching of the in-plane C-H1 bond symmetric stretching of C-H2 and C-H3 bonds asymmetric stretching of C-H2 and C-H3 bonds symmetric stretching of the three C-H bonds asymmetric stretching of the two out-of-plane C-H bonds mainly the stretching of the in-plane C-H1 bond
1b (1b*)
a The numbers in parentheses are computed with the ODP approach and thus correspond to the situations where the hyperconjugation effect is quenched.
FIGURE 2. Electron density difference (EDD) maps for methylborane (isodensity ) 0.0008 au): (a) staggered conformation (1a); (b) eclipsed conformation (1b). Red means a gain and blue represents a loss in electrons.
ture than HF calculations. Apart from the structural changes, the ODP computation also shows that the hyperconjugative interaction stabilizes 1a by 4.9 kcal/ mol. The HCE derived by the ODP approach can be justified by the energy difference between the following bond exchange reactions 1 and 2.
C2H6 + BH3 f H3C-BH2 (1a) + CH4
(1)
C2H6 + BH4- f H3C-BH3- + CH4
(2)
The above reactions are related to the exchange of C-C and C-B bonds, but the first reaction encompasses the hyperconjugation effect in 1a, and consequently, the energy difference between them can be approximately used to estimate the hyperconjugation effect. At the CCSD(T)/6-311+G** level, the computed reaction enthalpy is -13.2 kcal/mol for eq 1 and -6.8 kcal/mol for eq 2. As a result, the calculated HCE for 1a is 6.4 kcal/ mol, which is reasonably close to the value (4.9 kcal/mol) from ODP calculation. This consistency confirms that the ODP approach at the HF level can be rationally adopted to analyze the hyperconjugation effect. Since 1a and 1b are close in energy, the magnitude of the hyperconjugation effect in 1b and 1a should be similar as there are two out-of-plane C-H bonds involved in the hyperconjugative interaction in 1b. Indeed, the alignment between 1b and 1b* reveals the shortening of the C-B bond by 0.022 Å, which is close to the case of 1a. The hyperconjugation effect of the out-of-plane hydrogen atoms pushes the hydrogen atom (H1) 1.4° further away from the BH2 moiety, and the two out-of-plane C-H2/C-H3 bonds (1.091 Å) are longer than the in-plane C-H1 bond (1.084 Å) at the HF/6-311G** level. The H1CB and H2BC difference in 1b (5.6°) is larger than in 1b* (3.2°). 3496 J. Org. Chem., Vol. 69, No. 10, 2004
As the vibrational frequencies and intensity patterns provide a direct evidence for the existence of the hyperconjugation phenomenon in molecules, we analyzed the vibrational spectra for the staggered (1a) and eclipsed (1b) methylborane by means of HF and ODP computations. Table 2 listed the stretching frequencies and intensities corresponding to the three C-H bonds in the methyl group. Obviously, the electron flow from the methyl group to the adjacent vacant boron pπ orbital lowers the stretching frequencies and significantly reduces their intensities, as evidenced by the first two vibrations (ν1 and ν2) in Table 2. These findings are in good agreement to experimental results.8 The hyperconjugation effect in methylborane accompanying the electron flow from the methyl group to the vacant pπ orbital on boron can be visualized by the electron density difference (EDD) map between the delocalized HF wave function ΨDel and localized ODP wave function ΨLoc. Figure 2 shows the EDD maps for the two conformations of methylborane. The magnitude of the electron flow can be further approximated by the population analysis. In 1a (1a*), natural population analyses10 estimate that the pπ orbital on boron has a population of 0.054e and 0.040e, respectively, whereas the amount in 1b (1b*) is 0.051e or 0.037e. CH3CH2BH2 (2). At the HF/6-311G** level, the most stable conformation for ethylborane is 2a in which the methyl group is as expected staggered, but the BH2 unit is unexpectedly eclipsed to the ethylene (CH2) group, and 2c is the transition state for the BH2 rotation which has a barrier of 1.5 kcal/mol after the correction of ZPE. These findings are somewhat different from the case of methylborane (1a and 1b). The comparison between methylborane and ethylborane reveals that the hyperconjugation of the two out-of-plane C-H bonds is more
Hyperconjugation Effect in Substituted Methyl Boranes
FIGURE 3. Optimized delocalized (localized) structures of dimethylborane (2f and 2f*).
pronounced than the only in-plane C-C bond. Indeed, we find that the hyperconjugation of the C-C bond to the vacant pπ orbital on boron is about 0.4 kcal/mol lower than that of the C-H bond (Table 1). If we rotate the methyl group in 2a by 180° to an eclipsed structure 2b, the system will be destabilized by 2.5 kcal/mol, which is comparable to the energy difference (2.7 kcal/mol) between 2c and 2d as well as the ethane rotation barrier.12,13 When electron correlation is taken into account at the B3LYP/6-311+G** level, frequency analysis indicates that 2a is a transition state as it possesses an imaginary vibration mode and instead 2c is a minimum structure, although 2c is of higher energy than 2a. However, 2a is not the transition state for 2c. Following the imaginary vibration mode of 2a, we located the energy minimum state structure 2e of C1 symmetry (not shown as it is not available at the HF level). Relative to 2a, the BH2 group in 2e is slightly rotated out of the plane, and 2e can also be regarded as a gauche conformation in which the two C-H2/C-H3 bonds (1.108 vs 1.099 Å) and the corresponding HCB angles (102.9° vs 110.1°) of the methylene group differ noticeably. Interestingly, the energetic change between 2a and 2e is negligible and totally comes from the ZPE correction at both B3LYP/6-311+G** and CCSD(T)/6-311+G** levels. The energy difference between 2e and 2c is 1.5 kcal/mol at the CCSD(T)/6-311+G** level after the ZPE correction (see Table S2, Supporting Information). It is valuable to analyze the isomerization reaction from ethylborane (CH3CH2BH2, 2e) to dimethylborane (CH3BHCH3, 2f as shown in Figure 3) and examine the roles of the hyperconjugation effect and the B-C bond exchange in the reaction
CH3CH2BH2 (2e) f CH3BHCH3 (2f)
(3)
The dimethylborane minimum (2f) is of C2 symmetry, and both methyl groups adopt a staggered conformation which enables the enhanced hyperconjugative interaction between C-H2 or C-H2′ bond and the pπ orbital on boron. As shown in Figure 3, for example, the C-H2 bond is longer than the other two C-H3 and C-H4 bonds and the corresponding H2CB bond angle is reversely smaller than the others. When the hyperconjugation effect is quenched, the C-H2 bond length gets close to the bond distances of C-H3 and C-H4, with the increased H2CB bond angle. Similar to methylborane and ethylborane, the B-C bond is sensitive to the hyperconjugative
interaction and its length will changes from 1.576 to 1.595 Å if the hyperconjugation effect, which is measured as 7.5 kcal/mol at the HF level, is deactivated. At the CCSD(T)/6-311+G** level, the isomerization reaction 3 is exothermic by 13.6 kcal/mol which includes not only the bond exchange energy from C-C to C-B, but also the additional hyperconjugation stabilization. After extracting the C-B bond exchange energy of 6.8 kcal/mol from the reaction 2, the net hyperconjugation of the two methyl groups is 6.8 kcal/mol at the CCSD(T)/6-311+G** level. This result agrees with the value of 7.5 kcal/mol deduced from the ODP computation. XCH2BH2 (X ) NH2 (3), PH2 (4), OH (5), SH (6)). Figure 4 shows the optimized delocalized and localized structural parameters for the above four substituted methylboranes. It has been found that the amino group is such a good π-donor that it can interact significantly with the empty pπ orbital on boron to form a threemembered ring in aminoborylamethane.23 Indeed, 3a is the global minimum for BH2CH2NH2 and the interaction between the amino group and boron has well gone beyond the hyperconjugation effect, and as a matter of fact, there is a strong dative bond between them. With the deactivation of pπ orbital on boron, the N-B dative bond is prohibited and the ODP optimization leads to three structures (3b*-3d*). The energetic order of these three structures seems to be determined by the steric effect as the energies are closely correlated with the C-B bond lengths. Compared with its localized form 3c*, the energy of the N-B dative binding in 3a is 12.8 kcal/mol, which is much smaller than the binding energy between of NH3 and BH3 (29 kcal/mol). This discrepancy can be ascribed to the considerable strain of the three-membered ring in 3a. As the electronegativity of phosphorus is much lower than that of nitrogen, it may be anticipated that the PH2 group can interact with BH2 strongly. However, this is not the case and the most stable conformation for BH2CH2PH2 is 4a, in which the C-P bond is parallel to the pπ orbital of boron, and the electron lone pair on the phosphorus atom points to the opposite direction of BH2 moiety. At the HF level, 4c is the transition state for the BH2 rotation and the barrier is 2.9 kcal/mol, the HCE is 6.8 kcal/mol for the C-P (4a), and 4.8 for the two C-H bonds (4c). At the B3LYP/6-311+G** level, 4a represents a transition state, but the energy difference between 4a and 4b is negligible, and 4c represents a local minimum. Similarly, at the CCSD(T) level, 4a and 4b have the similar energy, while 4c is less stable by 4.8 kcal/mol. Like ethylborane (2, X ) CH3), 5a (X ) OH) and 6a (X ) SH) are the most stable conformer in which X is in eclipsed conformation to the BH2 group, and this indicates that the C-H bond donation ability is stronger than the O-C and S-C bonds. As the electronegativity of sulfur is lower than that of oxygen, the hyperconjugation of the S-C bond (6c) to the adjacent vacant pπ orbital on boron is stronger than that of the O-C bond (5c) by 1.3 kcal/mol. However, the large rotation barrier around the B-C bond in BH2CH2OH (namely the energy difference between 5a and 5c) implies that other forces such induction or steric effects have more profound impact on the rotation barrier than the hyperconjugation effect. (23) Pasto, D. J. J. Am. Chem. Soc. 1988, 110, 8164.
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FIGURE 4. Optimized delocalized (localized) structural parameters of BH2CH2X; X ) NH2 (3), PH2 (4), OH (5), SH (6).
XCH2BH2 (X ) F (7), Cl (8), Br (9)). The optimized delocalized and localized geometries for the substituted methylboranes with halides are shown in Figure 5. This series of compounds can give more logic and systematic clues for the understanding of the hyperconjugation effect than the rest of substituted methylboranes studied above since there are no other atoms attached to halides. In other words, the steric effect which may implicate the analysis on the hyperconjugation effect is much less severe in halide substituted methylboranes. In the staggered conformations 7a-9a, with the decreasing of the electronegativity of halides from F to Br, the B-C bond length decreases progressively from 1.600 to 1.580, to 1.573 Å, while the ∠BCX angle decreases from 103.1 to 97.5 to 95.2°. Energetically, the HCE in the staggered conformations steadily increases from 3.6 3498 J. Org. Chem., Vol. 69, No. 10, 2004
to 4.7 to 5.9 kcal/mol, in the order of decreasing electronegativity. However, whether the energy minimum state is of a staggered or eclipsed conformation dominantly depends on the competition between the hyperconjugative donor abilities of C-H or C-X (X ) halide) bonds. For BH2CH2Br, the HCE plays such a prominent role that the staggered conformation is more stable than the eclipsed conformation at all levels. On the other hand, as fluorine has the highest electronegativity, the C-F bond is a much poorer donor than the C-H bond, and consequently the eclipsed conformation where the C-F bond lies in the plane of the BH2 moiety is strongly favored over the staggered conformation and the rotation barrier ranges from 5.9 to 6.9 kcal/mol depending on the calculation levels. Between the fluorine and bromine, things are a little tricky as the C-Cl bond has compa-
Hyperconjugation Effect in Substituted Methyl Boranes
from the normal value (1.593 Å in BH2CH3). The more electronegative the substituted atom is, the large the deviation. Another proof is that the ∠BCH angle increases and the C-H2 (or H3) bond length decreases systematically from F to Cl to Br. In the eclipsed conformations, hyperconjugation is solely resulted from the C-H bonds and if the hyperconjugation effect is excluded, the central B-C bond will be lengthened to distance similar to the value in BH2CH3, although the HCE in the eclipsed BH2CH2F (5.2 kcal/mol) is a little larger than the values in BH2CH2Cl and BH2CH2Br or in BH2CH3 (1b). Similar to the case of BH2CH2OH (5a and 5b), this enhancement may come from the inductive effect due to the highest electronegativity of fluorine. Conclusion
FIGURE 5. Optimized delocalized (localized) structural parameters of BH2CH2X; X ) F (7), Cl (8), Br (9).
rable hyperconjugation ability with the C-H bond. Thus, the electron correlation plays a very elaborate role to determine the energy minimum state of chlorine substituted methylborane. At both the HF and B3LYP level, the eclipsed structure has a slight edge over the staggered structure after the ZPE correction, whereas the CCSD(T) level prefers a staggered conformation. The strength of hyperconjugation of the C-X bonds (X ) halide) also can be inferred from the variation of the C-X bond lengths between the delocalized structures and the localized structures optimized with the ODP scheme. The loss of electron from the C-X bond to the empty pπ orbital of boron lengthens the C-X bond from 0.007 Å (F) to 0.011 Å (Cl) to 0.019 Å (Br). These results are in accord with our previous qualitative knowledge about hyperconjugation. It should be pointed out that in the staggered conformations, some kind of repulsion or inductive interactions apparently exist since even in the localized structures, the B-C bond length still obviously deviates
The orbital deletion procedure (ODP) approach provides a simple yet quantitative way to measure the hyperconjugation effect on substituted methylboranes XCH2-BH2 structurally and energetically. Although the computed hyperconjugation energies are modest and less than 7 kcal/mol, the hyperconjugation effect does have a remarkable impact on the molecular structures. Of particular, the central B-C bond and its corresponding bond angles are very sensitive to the interaction between the C-X bond and the vicinal vacant pπ orbital on boron. In average, the B-C bond shortens about 0.02 Å due to the hyperconjugation effect. The electron flow from the C-X bond to the boron atom also lengthens the C-X bond. The donor ability of the C-X bond increases with the decreasing of the electronegativity of X, which is in alignment with our conventional understanding. For example, the hyperconjugative ability is of the order of X ) Br > Cl > F for the halide substituted methylboranes, whereas the donor ability of the C-Cl bond is close to that of the C-H bond. In most cases, the difference between the C-X and C-H donor abilities controls the energy minimum state structure, i.e., a staggered conformation would be preferred over an eclipsed conformation if the C-X bond has higher hyperconjugative ability than the C-H bond. However, in some cases such as X ) Cl, the closeness between the C-X and C-H donor abilities leaves other factors such as electron correlation and steric effect to determine the energy minimum state structure, albeit that the energy difference between the two conformations would be small.
Acknowledgment. This work was supported by Western Michigan University (Y.M.) and the Funds der Chemischen Industry (at Erlangen, P.v.R.S.). Supporting Information Available: Total electronic energies and zero-point energies as well number of imaginary frequencies for all systems are summarized. This material is available free of charge via the Internet at http://pubs.acs.org. JO035724I
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