Computational Studies of Pericyclic Reactions of Aminoborane (F3C

778

Organometallics 2011, 30, 778–791 DOI: 10.1021/om101018e

Computational Studies of Pericyclic Reactions of Aminoborane (F3C)2BdN(CH3)2: Ene Reactions vs Hydrogen Transfers and Regiochemical and Conformational Preferences Brendan C. Dutmer and Thomas M. Gilbert* Department of Chemistry and Biochemistry, Northern Illinois University, DeKalb, Illinois 60115, United States Received October 28, 2010

The aminoborane (F3C)2BdN(CH3)2 (1) undergoes olefin-like concerted pericyclic ene-type and hydrogen transfer reactions with alkenes. Representative examples of each were studied computationally using density functional, ab initio, and composite models. Both types of reactions proceed through six-membered cyclic chairlike transition states. Energy barriers for reactions observed experimentally are computed to be e48 kJ/mol (OG2R3 composite model); therefore, this value appears to be an upper limit for viable reactions under the experimental conditions. All reactions exhibit substantial exothermicities, suggesting that neither pathway should be reversible. However, the experimental reaction between 1 and H2CdC(CH3)(t-Bu) provides a mixture of hydrogen transfer and ene-type products, favoring the former. Computations indicate that the ene-type pathway requires a lower barrier than the hydrogen transfer pathway but is less exothermic; thus, the experimental result is rationalized on the basis of the ene-type pathway being reversible. Although 1 reacts with most alkenes of the formula H2CdC(H)(CHR1R2) by one of the two pathways, it reportedly does not react with H2CdC(H)(CHMeCl). Computations indicate that this results from a slight increase in the barriers for the pericyclic reactions, presumably stemming from the presence of the electron-withdrawing chloride. Warming the reactants should allow formation of the ene-type product.

B€ urger, Brauer, and Pawelke described myriad reactions of the aminoborane (F3C)2BdN(CH3)2 (1) in a review.1 The matched electron-withdrawing/electron-donating characteristics of the peripheral groups, combined with their steric bulk,2 make this the most olefin-like aminoborane known, in terms of reactivity.1,3 It undergoes cycloaddition reactions common to olefins, as well as cyclizations less well-established for olefins.4 Aminoborane 1 undergoes pericyclic reactions that involve “cycloaddition” in transition states but that provide acyclic products. Germane to this work are ene-type and hydrogen transfer processes (Scheme 1). Both involve transfer of hydrogen from one substrate to the other. During the ene-type reaction, hydrogen is transferred from the organic substrate to the aminoborane nitrogen. This typically requires that the organic species contain a methyl group adjacent to a double bond; however, variations are known. For example, 1 reacts in an ene-type fashion with nitriles of (1) Pawelke, G.; B€ urger, H. Appl. Organomet. Chem. 1996, 10, 147– 174. (2) Hausser-Wallis, R.; Oberhammer, H.; B€ urger, H.; Pawelke, G. J. Chem. Soc., Dalton Trans. 1987, 1839–1845. (3) Brauer, D. J.; B€ urger, H.; Dittmar, T.; Pawelke, G. J. Organomet. Chem. 1995, 493, 167–173. (4) (a) Ansorge, A.; Brauer, D. J.; B€ urger, H.; D€ orrenbach, F.; Hagen, T.; Pawelke, G.; Weuter, W. J. Organomet. Chem. 1991, 407, 283–300. (b) Brauer, D. J.; Buchheim-Spiegel, S.; B€ urger, H.; Gielen, R.; Pawelke, G.; Rothe, J. Organometallics 1997, 16, 5321–5330. pubs.acs.org/Organometallics

Published on Web 02/03/2011

the form RCH2CtN (R = Cl, CH3, CH2CH3) and with carbonyls of the form R2C(dO)CH2R1 (R1 = Cl, alkyl chain, R2 = alkyl, alkoxy, dialkylamino).5 During a hydrogen transfer reaction, hydrogen transfers from the aminoborane nitrogen bound methyl substituent to the organic substrate. This represents the only reaction option when the organic species does not contain the necessary R-hydrogen to allow an ene-type process. Examples include reactions between 1 and CF3CtN, (CF3)2C(dO), or (CF3)C(dO)Ph.1 However, hydrogen transfer reactions sometimes occur even when R-hydrogens are present. Most of these cases include bulky substituents. For example, alkenes of the form H2CdC(CH2R1)(R2) containing one large R group (R1 = Ph, n-Pr, i-Pr; R2 = CH2(t-Bu), Ph, CHC(CH3)2, s-Bu, Mes, Si(CH3)3, Si(CH2CH3)3) react with 1 via hydrogen transfer, although not exclusively.6,7 In some cases, competition between ene-type and hydrogen transfer reactions is observed. One such case involved reaction between 1 and the sterically congested alkene H2CdC(CH3)(t-Bu), yielding 20% ene-type product and 80% hydrogen transfer product.6 Similarly, treating 1 with (5) Ansorge, A; Brauer, D. J.; B€ urger, H.; Hagen, T.; Pawelke, G. J. Organomet. Chem. 1993, 444, 5–14. (6) B€ urger, H.; Hagen, T.; Pawelke, G. J. Organomet. Chem. 1993, 456, 19–24. (7) B€ urger, H.; Hagen, T.; Pawelke, G. J. Fluorine Chem. 1991, 55, 323–327. r 2011 American Chemical Society

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Organometallics, Vol. 30, No. 4, 2011

779

Scheme 1

the alkynes HCtCCH2Pr and HCtCCH2Bu gives 30% ene-type product and 70% hydrogen transfer product.1 We previously described computational studies of [4 þ 2] and [2 þ 2] cyclizations between 1 and alkenes and alkynes and competition between these processes and ene reactions.8,9 Of particular interest was the observation that, while computations predict 1 to undergo exothermic [2 þ 2] cyclization with ethene, the analogous reaction between 1 and propene is endothermic. Hirao and Fujimoto attributed this to the enhanced Lewis acidity of the CF3-subtituted boron in 1.10 The [4 þ 2] ene-type reaction with propene, however, was predicted to be exothermic and to exhibit a lower transition state barrier than the cyclization. This work extends that above to a broader range of ene reactions and to hydrogen transfer processes. Motivation for this arises from observations of mixtures of products from the two types of reactions when 1 was treated with certain alkenes and alkynes.1 It was of interest to determine whether this occurred as a result of kinetic behavior (similar barriers (8) Gilbert, T. M. Organometallics 1998, 17, 5513–5520. (9) Bissett, K. M.; Gilbert, T. M. Organometallics 2004, 23, 850–854. (10) Hirao, H.; Fujimoto, H. J. Phys. Chem. A 2000, 104, 6649–6655. (11) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, Jr., J. A.; Vreven, T.; Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.; and Pople, J. A. Gaussian 03, revision E.01; Gaussian, Inc., Wallingford, CT, 2004. (12) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A.; Nakatsuji, H.; Caricato, M.; Li, X.; Hratchian, H. P.; Izmaylov, A. F.; Bloino, J.; Zheng, G.; Sonnenberg, J. L.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Vreven, T.; Montgomery, Jr., J. A.; Peralta, J. E.; Ogliaro, F.; Bearpark, M.; Heyd, J. J.; Brothers, E.; Kudin, K. N.; Staroverov, V. N.; Kobayashi, R.; Normand, J.; Raghavachari, K.; Rendell, A.; Burant, J. C.; Iyengar, S. S.; Tomasi, J.; Cossi, M.; Rega, N.; Millam, N. J.; Klene, M.; Knox, J. E.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Martin, R. L.; Morokuma, K.; Zakrzewski, V. G.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Dapprich, € Foresman, J. B.; Ortiz, J. V.; Cioslowski, J.; S.; Daniels, A. D.; Farkas, O.; Fox, D. J. Gaussian 09, Revision A.02, Gaussian, Inc., Wallingford CT, 2009.

for the processes) or thermodynamic behavior (low barriers allowing establishment of equilibrium mixtures). We find that both contribute but that the observation of mixtures appears to reflect thermodynamic behavior and establishment of equilibria rather than near-identical activation energies.

Computational Methods The Gaussian suite (G0311 and G0912) was employed for all calculations. Structures were initially optimized without constraints using the density functional theory (DFT) MPWPW9113 model and various basis sets. Diborylated 5 was an exception, in that it was optimized in C2 and Cs symmetry; the former was of lower energy at the MPWPW91/6-31þG(d) level, and therefore it was used for subsequent optimizations. Each stationary point was used as a starting point for reoptimizations using larger basis sets, culminating at the MPWPW91/6311G(2d, 2p) level. The triple-ζ 6-311G(2d,2p) basis set was chosen as the most complete basis set available for timely completion of jobs; extra polarization functions appear to improve calculated geometries.14 Ground state and ene-type transition state structure optimizations were generally well behaved. Hydrogen transfer transition state structure optimizations proved more challenging, owing to the presence of higher order saddle points near the desired ones. Frequency calculations were performed at this level to confirm that ground states were minima (no imaginary frequencies) and that transition states were first-order saddle points (one imaginary frequency). That the transition states connected the reactants and products of interest was confirmed by visualizing the imaginary vibration using GaussView.15 Pictorial representations of molecules in the figures were created using Molecule for Macintosh.16 Structures were then reoptimized at the MPW1K/6-311þþG(2d,2p) level. The MPW1K17 model was employed, because it was designed specifically to reproduce transition state geometries and energies, providing results for both comparable to those from more demanding models.18 Diffuse functions were used to properly describe the weak interactions between components of the transition state.19 These structures were then used for several different types of single-point energy determinations, (13) Adamo, C.; Barone, V. J. Chem. Phys. 1998, 108, 664–675. (14) Boese, A. D.; Martin, J. M. L.; Handy, N. C. J. Chem. Phys. 2003, 119, 3005–3014. (15) GaussView, version 4.1.2; Gaussian, Inc., Wallingford, CT, 2007. (16) van Eikema Hommes, N. Molecule for Macintosh, version 1.3.5d9; University of Erlangen-N€ urnberg, Erlangen, Germany, 1999. (17) (a) Lynch, B. J.; Truhlar, D. G. J. Phys. Chem. A 2001, 105, 2936–2941. (b) Lynch, B. J.; Fast, P. L.; Harris, M.; Truhlar, D. G. J. Phys. Chem. A 2000, 104, 4811–4815. (18) Gilbert, T. M. J. Phys. Chem. A 2004, 108, 2550–2554. (19) Lynch, B. J.; Zhao, Y.; Truhlar, D. G. J. Phys. Chem. A 2003, 107, 1384–1388.

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including (1) MP220/6-311þþG(2d, 2p), denoted MP2//MPW1K in the tables, (2) ONIOM G2R3 (OG2R3) composite, denoted OG2R3//MPW1K21 and meant to approximate CCSD(T)/6-311þG(2df, 2p) energies in the high ONIOM layer, and (3) a modified version of the G3(MP2) approach, denoted G3(MP2)mod//MPW1K,22 for the smaller molecules/transition states only. A graphic illustrating the separation of layers for the ONIOM approach appears in the Supporting Information. All these methods are abbreviated to model//MPW1K in the tables and text below. They provide perturbation theory based comparisons with the DFT results.18 Reaction energetics in the tables below were corrected using unscaled zero-point energies from the frequency analyses. It is relevant to comment on computational “errors” and models. Cramer’s23 performance overview for numerous DFT methods and wave function based methods indicates that absolute errors in calculated bond lengths and angles for MP2 and DFT models are around 1 pm and 1°, respectively. The error in minimum energies reported using MP2 and DFT models averages around 20 kJ/mol. However, because DFT methods generally suffer from insufficient medium-range correlation and inadequate treatment of van der Waals interactions,24 their ability to predict reaction energies worsens with molecule size.18,25,26 Germane to the ene-type and hydrogen transfer reactions in this work, it appears DFT/triple-ζ basis set model chemistries (the MPW1K model particularly) show large errors for formation energies when the reaction converts a π bond to a σ bond.27 The MP2 model tends not to exhibit such behavior and predicts reaction energies well over a range of compound types. However, MP2 often exhibits sizable errors when predicting barrier energies for cyclizations,28 while MPW1K typically gives reaction barrier energies accurate to less than 10 kJ/mol.17 This dichotomy is expressed in the data in the tables; the reader will note that the MPW1K and MP2// MPW1K values differ, often wildly, for the same species. It is thus crucial to calibrate calculations against experimental data or against high-level calculations when experimental data are unavailable. At the very least, one should examine any molecule or reaction using multiple models, to provide “error bars” for the associated energetics. (20) M€ oller, C.; Plesset, M. S. Phys. Rev. 1934, 46, 618–622. (21) (a) Gilbert, T. J. Comput. Chem., in press. (b) (a) Vreven, T.; Morokuma, K. J. Phys. Chem. A 2002, 106, 6167–6170. (b) Vreven, T.; Morokuma, K. J. Chem. Phys. 1999, 111, 8799–8803. (22) (a) Curtiss, L. A.; Redfern, P. C.; Raghavachari, K.; Rassolov, V.; Pople, J. A. J. Chem. Phys. 1999, 110, 4703–4709. It should be noted that this composite as implemented in Gaussian employs QCISD(T, FC)/6-31G(d) single-point energies, MP2(FU)/6-31G(d)-optimized structures, and scaled HF/6-31G(d) zero-point energies. Our G3(MP2)mod//MPW1K approach here employs CCSD(T,FC)/6-31G(d) single-point energies, MPW1K/6-311þþG(2d,2p)-optimized structures, and unscaled MPWPW91/6-311G(2d,2p) zero-point energies. G3(MP2)mod//MPW1K is more related to a G3XCC(MP2) calculation than to the implemented G3(MP2) version; as such, it represents a more accurate and more robust version of G3(MP2).22b,c It would be incorrect in our work to use the high-level corrections (HLCs) of the published composite; fortunately, as we only study reactions of molecular systems here, these cancel between reactions and products. (b) Curtiss, L. A.; Raghavachari, K.; Redfern, P. C.; Baboul, A. G.; Pople, J. A. Chem. Phys. Lett. 1999, 314, 101–107. (c) Curtiss, L. A.; Redfern, P. C.; Raghavachari, K. J. Chem. Phys. 2007, 126, 084108-1–084108-12. (23) Cramer, C. J. Essentials of Computational Chemistry: Theories and Models, 2nd ed.; Wiley: West Sussex, England, 2004. (24) Grimme, S. Angew. Chem., Int. Ed. 2006, 45, 4460–4464. (25) Schreiner, P. R. Angew. Chem., Int. Ed. 2007, 46, 4217–4219. (26) (a) Curtiss, L. A.; Raghavachari, K.; Redfern, P. C.; Pople, J. A. J. Chem. Phys. 2000, 112, 7374–7383. (b) Curtiss, L. A.; Redfern, P. C.; Raghavachari, K. J. Chem. Phys. 2005, 123 (124107), 1–12. (27) Pieniazek, S. N; Clemente, F. R.; Houk, K. N. Angew. Chem., Int. Ed. 2008, 47, 7746–7749. (28) Gilbert, T. M.; Bachrach, S. M. Organometallics 2007, 26, 2672– 2678.

Dutmer and Gilbert Table 1. Deviations between Energy Predictions Using the G3(MP2)mod//MPW1K Composite and Other Models (kJ mol-1) for Reactions of 1 with Propene, 2-Methylpropene, and 2,3,3-Trimethylbutene ASEa d

e

AAEb

RMSEc

f

cum TS RXN cum TS RXN cum TS RXN MPWPW91 -25 3 -52 21 7 52 36 9 56 -30 27 -34 26 27 34 32 30 38 MPW1Kh 12 -14 10 12 14 10 14 16 11 MP2//MPW1Ki 4 -4 3 4 4 4 4 4 4 OG2R3//MPW1Kj P a b Average absolute P sgned error = ( (Eexptl - Ecalcd))/N. Average P error = ( |(Eexptl - Ecalcd)|)/N. c Root mean square error = (( (Eexptl 2 1/2 d - Ecalcd) )/(N - 1)) . Cumulative errors (barrier energies and reaction energies). e Errors for barrier energies only. f Errors for overall reaction energies only. g MPWPW91/6-311G(2d, 2p) optimized energies. h MPW1K/ 6-311þþG(2d, 2p)-optimized energies i MP2/6-311þþG(2d, 2p)// MPW1K/6-311þþG(2d, 2p) single point energies j OG2R3//MPW1K/ 6-311þþG(2d, 2p) single point energies g

We calibrated this work by determining G3(MP2)mod// MPW1K barriers and reaction energies for reactions between 1 and propene, 2-methylbutene, and 2,3,3-trimethylbutene (only the four most important structures for the last). These are shown in the relevant tables; we assumed that this composite model would give the most accurate results. In Table 1, the energies from these calculations are compared with those from the others. Three statistical parameters were evaluated: the average signed error (ASE), used to assess whether a model systematically under- or overestimates the dissociation energy, the average absolute error (AAE), used to assess the general accuracy of the models, and the root-mean-square error (RMSE), used to assess the “worst case” errors of the models. A total of 18 comparisons were available: 10 barrier energies and 8 reaction energies. The data indicate that both DFT approaches overall performed poorly. The ASEs show that the MPW1K model systematically overestimated transition state barriers by an average of 27 kJ mol-1 and underestimated reaction energies by an average of 34 kJ mol-1, rendering it untrustworthy for either. The AAEs and RMSEs support this view. Surprisingly, the ASEs show that the poor performance of the MPWPW91 model arose entirely from its underestimation of reaction energies; barriers were predicted rather well. This is in notable contrast to normal behavior, as noted above; one expects barriers from the hybrid MPW1K model to be more accurate than those from the pure DFT model. As expected, the MP2//MPW1K model systematically underestimated barriers by 14 kJ mol-1; distressingly, it performed only slightly better (errors ca. 10 kJ mol-1) in predicting (systematically overestimated) reaction energies. The OG2R3// MPW1K model performed best, in that it predicted barriers and reaction energies equally well, to within 4 kJ mol-1. This is unsurprising, given that the G3(MP2)mod and OG2R3 composites are similar in construction; the former approximates a CCSD(T)/G3Large calculation for the entire molecule, while the latter approximates a CCSD(T)/6-311þG(2df, 2p) calculation for the high layer. For first- and second-row atoms, the two basis sets are nearly identical; therefore, the energies calculated by both composites will be similar so long as the high layer in the ONIOM calculation reflects the most chemically active part of the molecule. The similar energies predicted by the two validates the use of the OG2R3 approach; its utility is denoted by the fact that calculations on the larger molecules required only a few hours, while those using the G3(MP2)mod approach required 1-12 days. Consequently, while values from all models appear in the tables, we will focus on the OG2R3//MPW1K results. When they are relevant in supporting these, we will note MPWPW91 transition state barriers and MP2//MPW1K reaction energies.

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781

Scheme 2

Results and Discussion Ene and Hydrogen Transfer Reactions between (F3C)2Bd N(CH3)2 (1) and Propene. Aminoborane 1 reacts with a variety of olefins to form ene-type products.6,29 Propene represents the simplest olefin capable of this reaction (Scheme 2), although as far as we are aware, interaction between 1 and propene has not been reported. As a result, the ene-type and hydrogen transfer reactions between 1 and propene represent benchmarks for studying rearrangements involving 1. We previously reported studies of the ene-type reaction between 1 and propene, using smaller basis sets.9 With the more demanding MPW1K/6-311þþG(2d,2p) model, the structural parameters of the “six-membered-ring chair” transition state 2ts30 and the acyclic product 2 (Figure 1), and the associated barrier and reaction energetics (Table 1), are nonetheless comparable.9 The distances using the current model are very slightly shorter. The transition state is positionally asymmetric, in that, from the perspective of the forming B-C bond, the transition state is rather late (the B-C distance in 2ts is only 17 pm longer than in 2), while from the perspective of the forming N-H bond, it is early (the N-H distance in 2ts is 54 pm longer than in 2). We investigated the possibility that the process is actually stepwise rather than concerted. However, using the MPW1K model, we found no transition state for B-C formation free (29) Interestingly, in many cases the expected ene-type product rearranges to an alternative that places a substituted carbon adjacent to the boron.1 For propene, the rearrangement is degenerate. (30) Transition states and products are labeled systematically in the text and tables as follows. The first digit indicates a transition state or product derived from 1. If the transition state or product is derived from the ene-type reaction, no further indicators are used, save that the transition state is tagged ts. Thus, 2ts and 2 are the transition state and product of the ene-type reaction between 1 and propene. For hydrogen transfer reactions, this pattern is followed, with two additions. First, a and b denote attachment of the boron to the least and most substituted alkene carbon atoms, respectively (corresponding to hydrogen transfer from the amine methyl group in 1 to the most and least substituted alkene carbon atom, respectively). Second, ax and eq indicate whether the largest alkene alkyl substituent in the six-membered cyclic transition state adopts an axial or equatorial position. (This distinction is unnecessary for 6ats/6bts, because the substituted alkene carbon atom has identical methyl substituents). Therefore, product 7a forms from hydrogen transfer from the amine methyl group in 1 to the silyl-substituted alkene carbon in H2CdCH(SiMe3), through transition states 7ats ax, where the trimethylsilyl group occupies an axial position in the cycle, and 7ats eq, where the trimethylsilyl group occupies an equatorial position in the cycle. Similarly, product 7b forms from hydrogen transfer from the amine methyl group in 1 to the unsubstituted alkene carbon in H2CdCH(SiMe3), through transition states 7bts ax, where the trimethylsilyl group is axial, and 7bts eq, where the trimethylsilyl group is equatorial. (31) A reviewer wondered whether a stepwise mechanism might be preferred if solvent were included in the calculations. We did not examine this extensively, but we did optimize transition states 10ts, 11ats, and 11bts using the polarizable continuum models implemented in Gaussian 09 for CH2Cl2 and pentane (MPW1K/6-311þþG(2d,2p) level). The transition state structures and their energies with respect to the solvated reactants changed little in these calculations; thus, we consider it unlikely that modestly polar solvents would cause the reactions to become stepwise rather than concerted.

Figure 1. Optimized (MPW1K/6-311þþG(2d,2p)) structures (distances in pm) of 2ts and 2 from the ene reaction between 1 and propene. Some hydrogen and fluorine atoms were removed from 2ts for clarity. Ring bond angles in 2ts (deg): N-B-C, 113.7; B-N-H, 96.8; B-C-C, 100.8; N-H-C, 153.2; C-C-C, 121.4; C-C-H, 92.3.

of the N 3 3 3 H 3 3 3 C interaction observed for 2ts, suggesting a concerted rearrangement.31 As is evident in Table 2, the different models predict an array of energetics. We refer the reader to the Computational Methods for the reasons we will only discuss OG2R3 energies hereafter. The OG2R3 approach suggests a barrier for the ene-type reaction of ca. 50 kJ mol-1, with an exothermicity of 70 kJ mol-1. This indicates a barrier for 2 undergoing the retro-ene-type reverse reaction of g120 kJ/mol; therefore, this requires substantially more energy than the forward process. However, it might be possible at temperatures slightly above 25 °C (see below, in the discussion of 2,3,3trimethylbutene). The hydrogen transfer reactions exhibit two pairs of transition states. The hydrogen can move from amine methyl carbon to olefin CH(Me) (forming 3a) or to olefin CH2 (forming 3b), and in each case, the six-membered-ring transition state can orient the olefin methyl substituent in an axial (3ats ax and 3bts ax) or equatorial (3ats eq and 3bts eq) position.30 Structures of these appear in Figures 2 and 3. One sees that 3ats ax and 3ats eq adopt nearly identical structures, with the only apparent difference being that the latter displays a forming C-H distance 1.3 pm longer than the former. This indicates that the single methyl substituent scarcely affects the formation of the transition state, and in fact the associated barrier energies are identical within 1 kJ mol-1. Product 3a shows distinct distance differences between the N-C and NdC bonds but otherwise does not differ much from 2 in structural terms.

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Dutmer and Gilbert

Table 2. Energetics (kJ/mol) for Ene-Type and Hydrogen Transfer Reactions between (F 3 C)2 BdN(CH 3 )2 (1) and Propene a reacn

model

ΔEts

ΔE

ene-type forming 2

G3(MP2)mod//MPW1K MPWPW91 MPW1K MPW1K/6-311þþG(d,p)9 MP2//MPW1K MP2/6-311þþG(d,p)9 OG2R3//MPW1K

53 59 74 66 38 33 50

-68 -19 -41 -32 -79 -70 -71

reacn

model

ΔEts(ax)

ΔEts(eq)

ΔE

G3(MP2)mod//MPW1K 64 65 -67 MPWPW91 55 58 -27 MPW1K 83 84 -44 MP2//MPW1K 52 53 -79 OG2R3//MPW1K 60 61 -70 G3(MP2)mod//MPW1K 83 89 -53 H transfer to CH2 forming 3b MPWPW91 82 88 -7 MPW1K 112 119 -21 MP2//MPW1K 64 71 -65 OG2R3//MPW1K 79 84 -55 a ΔEts, ΔEts(ax), and ΔEts(eq) denote transition state barriers, with ax and eq denoting cases where the methyl substituent occupies axial and equatorial positions, respectively, in the six-membered-ring transition states. ΔE denotes the overall reaction energy.

H transfer to CHC forming 3a

Figure 2. Optimized (MPW1K/6-311þþG(2d,2p)) structures (distances in pm) of 3ats ax, 3ats eq, and 3a from the (H f CHC) hydrogen transfer reaction between 1 and propene. Some hydrogen and fluorine atoms were removed from 3ats ax and 3ats eq for clarity. Ring bond angles in 3ats ax (deg): N-B-C, 105.5; B-N-C, 115.7; B-C-C, 116.1; N-C-H, 106.2; C-C-H, 102.6; C-H-C, 146.2. Ring bond angles in 3ats eq (deg): N-B-C, 105.9; B-N-C, 114.5; B-C-C, 115.1; N-C-H, 106.4; C-C-H, 102.8; C-H-C, 146.8.

Figure 3. Optimized (MPW1K/6-311þþG(2d,2p)) structures (distances in pm) of 3bts ax, 3bts eq, and 3b from the (H f CH 2) hydrogen transfer reaction between 1 and propene. Some hydrogen and fluorine atoms were removed from 3bts ax and 3bts eq for clarity. Ring bond angles in 3bts ax (deg): N-B-C, 104.5; B-N-C, 114.8; B-C-C, 109.2; N-C-H, 105.3; C-C-H, 105.7; C-H-C, 145.8. Ring bond angles in 3bts eq (deg): N-B-C, 103.8; B-N-C, 115.3; B-C-C, 110.7; N-C-H, 105.2; C-C-H, 107.6; C-H-C, 146.3.

Article


783

Scheme 3

The more sterically hindered 3bts ax and 3bts eq display notably longer B-C forming distances than those in 2ts, 3ats ax, and 3ats eq. Interestingly, this is countered by their having C-H forming distances that are shorter by ca. 3 pm. This supports the hypothesis that the process is concerted, in that the transition state “compensates” for having a weaker B-C interaction by increasing the C-H interaction, a situation inconsistent with stepwise behavior. Surprisingly, despite the fact that the boron-bound carbon is disubstituted in 3b rather than monosubstituted as in 2 and 3a, the B-C bond distance in 3b is only 1-3 pm longer. Nonetheless, this denotes control of steric effects over electronic ones for these borane complexes. If electronic characteristics dominated, one would expect the shortest B-C bond in 3b, as the Lewis acidic boron should bind the more Lewis basic dimethylsubstituted carbon more tightly. In this regard, we note that borane-imines 3a,b exhibit B-N bonds ca. 2 pm shorter than does borane-amine 2. The difference reflects the different geometry around nitrogen (trigonal planar as opposed to tetrahedral) and the associated different hybridization and increased Lewis basicity. The consequences of steric repulsion in the pathway leading to 3b appear in the barriers;forming 3bts ax/3bts eq requires some 20 kJ mol-1 more energy than forming 3ats ax/3ats eq (Table 2). As forming 3b is 15 kJ mol-1 less exothermic than forming 3a, it is evident that the latter route is highly preferred to the former. For either path, the reverse hydrogen transfer faces a barrier of ca. 130 kJ mol-1, suggesting difficulty accomplishing this. Competition in the reaction between 1 and propene thus is restricted to formation of 2 vs 3a. The data suggest that the ene-type process faces the smallest barrier and possibly is more exothermic, and so it represents the preferred path. However, the barrier to forming hydrogen transfer transition states 3ats ax/3ats eq is only 10 kJ/mol higher; thus, it is possible that at a reasonably high temperature the two pathways would compete. As the two exhibit essentially identical exothermicities, it would not be surprising if treatment of 1 with propene gave a mixture of products 2 and 3a, with the former dominating. Observation of 3b is unlikely. Ene and Hydrogen Transfer Reactions between (F3C)2Bd N(CH3)2 (1) and 2-Methylpropene. The reaction between 1 and 2-methylpropene is of interest for two reasons. First, although plausibly both ene-type and hydrogen transfer products could form, experimentally only the former does. Second, the monoborylated product 4 reacts with a second equivalent of 1 to form the diborylated product 5 (Scheme 3).1,6 The mono- and diborylated compounds were obtained as a 5%:83% yield mixture when the reaction was conducted in pentane at -10 °C and could be separated by fractional sublimation. Formation of diborylated 5 occurred readily even when excess 2-methylpropene was used, indicating that the second ene reaction is faster than the first. This is surprising, given than 4 is more sterically congested than 2-methylpropene itself. Of course, if formation of 4 is

Figure 4. Optimized (MPW1K/6-311þþG(2d,2p)) structures (distances in pm) of 4ts and 4 from the initial ene reaction between 1 and 2-methylpropene. Some hydrogen and fluorine atoms were removed from 4ts for clarity. Ring bond angles in 4ts (deg): N-B-C, 109.1; B-N-H, 96.7; B-C-C, 105.3; N-H-C, 152.4; C-C-C, 118.0; C-C-H, 93.8.

exothermic, then the barrier to 5ts with respect to 4 and a second equivalent of 1 could be lower than the barrier to 4ts with respect to 1 and 2-methylpropene; thus, the process would already have enough energy to traverse the 5ts barrier once it traversed the 4ts barrier. Under such circumstances, one expects to form only 5; therefore, it is curious that any 4 is isolated. This suggests either that a remarkable potential energy surface exists where the barrier to 5ts from 4 is nearly identical with the barrier for the retro-ene process for 4 (i.e., the barrier to 4ts from 4) or that the experiment was carried out with a deficiency of 1. The experimental report6 suggests the latter (12 mmol of 2-methylpropene to 10 mmol of 1); it is unclear whether the reaction was ever performed with an excess of 1. From a computational standpoint, 2-methylpropene represents propene with an electron-donating methyl group adjacent to the reactive carbon; therefore, it allows study of the effect of that perturbation on reaction energetics. Structures of the transition state 4ts and product 4 for the monoborylation ene reaction appear in Figure 4, and those

784


Dutmer and Gilbert

Figure 5. Optimized (MPW1K/6-311þþG(2d,2p)) structures (distances in pm) of 5ts and 5 from the second ene reaction between 1 and 2-methylpropene. Some hydrogen and fluorine atoms were removed from 5ts for clarity. Ring bond angles in 5ts (deg): N-B-C, 109.2; B-N-H, 96.4; B-C-C, 105.6; N-H-C, 153.0; C-C-C, 116.6; C-C-H, 95.1.

Table 3. Energetics (kJ/mol) for the Ene-Type and Hydrogen Transfer Reactions between (F3C)2BdN(CH3)2 (1) and 2-Methylpropenea reacn ene-type forming 4

model

G3(MP2)mod//MPW1K MPWPW91 MPW1K MP2//MPW1K OG2R3//MPW1K ene-type forming 5 from 4 MPWPW91 MPW1K MP2//MPW1K OG2R3//MPW1K H transfer to C(CH3)2 forming 6a G3(MP2)mod//MPW1K MPWPW91 MPW1K MP2//MPW1K OG2R3//MPW1K G3(MP2)mod//MPW1K H transfer to CH2 forming 6b MPWPW91 MPW1K MP2// MPW1K OG2R3//MPW1K

ΔEts ΔE 38 48 60 23 34 48 61 3 35 60 60 85 51 56 112 121 156 91 107

-67 -14 -35 -66 -71 3 -19 -80 -56 -54 -4 -21 -64 -58 -41 23 9 -53 -47

a ΔEts denotes the transition state barrier in the six-membered-ring transition states. ΔE denotes the overall reaction energy.

for the diborylation (5ts and 5) appear in Figure 5. The geometries are similar to those for the reaction between 1 and propene. The most notable exception is that transition state structures 4ts and 5ts exhibit B-C forming bond distances that are 4-5 pm shorter than that for 2ts; all other distances

Figure 6. Relative energies (kJ mol-1, OG2R3/MPW1K) for the mono- and diborylation reactions between 1 and H2CdC(CH3)2.

are within 2 pm and do not appear to show a pattern. The shorter B-C distances are interesting in that one might anticipate that steric repulsions between the 2-methylpropene methyl substituent and the trifluoromethyl substituents would lengthen this distance in 4ts and 5ts; that this does not occur suggests the electron-donating capability of the methyl group overrides steric difficulties here. Steric constraints clearly play a role in the formation of diborylated, C2-symmetric 5, which orients its amine-borane substituents on opposite sides of the alkene core. The OG2R3//MPW1K model predicts that the first ene reaction is as exothermic as that for propene, with a significantly lower barrier to transition state 4ts (Table 3). This presumably reflects the greater Lewis basicity of 2-methylpropene vs propene resulting from the presence of the electron-donating methyl substituent. It is thus expected that reaction occurs. The retro-ene process faces a barrier 4 f 4ts of 105 kJ mol-1, while the barrier 4 f 5ts is only 35 kJ mol-1, shown graphically in Figure 6. These observations confirm that intermediate monoborylated 4 has adequate energy to cross the second barrier once the first has been traversed; therefore, it is reasonable that more 5 than 4 formed experimentally and surprising that B€ urger et al. isolated any monoborylated 4.6 Given the minimal experimental details reported, we speculate as noted that the experimenters used too little 1 for complete formation of 5. Structures of the components of the experimentally unobserved hydrogen transfer reaction between 1 and 2-methylpropene

Article

Figure 7. Optimized (MPW1K/6-311þþG(2d,2p)) structures(distances in pm) of 6ats and 6a from the (H f CC2) hydrogen transfer reaction between 1 and 2-methylpropene. Some hydrogen and fluorine atoms were removed from 6ats for clarity. Ring bond angles in 6ats (deg): N-B-C, 107.0; B-N-C, 114.9; B-C-C, 117.7; N-C-H, 106.7; C-C-H, 101.2; C-H-C, 147.4.

appear in Figures 7 and 8. As with the ene reactions, the geometries are similar to those for the analogous reactions between 1 and propene. Epitomizing the theme of steric vs electronic considerations, the forming B-C distance in 6ats is 3 pm shorter than that in analogue 3ats, reflecting the electron richness engendered by the additional methyl group in the former, while the B-C distance in 6bts is 10 pm longer than that in 3bts, reflecting the steric constraints imposed by the imminent formation of a tert-butyl substituent in the former. Thus, the difference in this distance between 6ats and 6bts is 22 pm, remarkable for identical “core” transition states. It is notable, as above, that these issues do not significantly affect other distances within the transition states, including the distances involving moving the hydrogen from one atom to another. It is also interesting that the B-C distances in products 6a,b differ by only 4 pm, given that in the former the carbon is primary, while in the latter it is tertiary. The energetics associated with hydrogen transfer (Table 2) bear out experimental observations. Formation of 6b is exothermic but requires traversing a barrier nearly twice that for 6a; therefore, observing it in an experiment is exceedingly unlikely. Moreover, while the barrier associated with 6ats is accessible at 56 kJ mol-1, it is 22 kJ/mol greater than that for 4ts. Recall that the difference between 3ats and 2ts was 10 kJ/mol. The additional methyl group imposes a steric constraint that substantially destabilizes 6ats with respect to 4ts, leading to exclusive use


785

Figure 8. Optimized (MPW1K/6-311þþG(2d,2p)) structures (distances in pm) of 6bts and 6b from the (H f CH2) hydrogen transfer reaction between 1 and 2-methylpropene. Some hydrogen and fluorine atoms were removed from 6bts for clarity. Ring bond angles in 6bts (deg): N-B-C, 104.6; B-N-C, 115.2; B-C-C, 106.2; N-C-H, 104.6; C-C-H, 109.4; C-H-C, 148.7.

of the ene-type reaction path when 1 reacts with 2-methylpropene. The fact that 4 forms 13 kJ mol-1 more exothermically than does 6a simply increases the preference for the ene-type path, as well as guaranteeing that the reaction will not form an equilibrium mixture of the two. Hydrogen Transfer Reaction between (F3C)2BdN(CH3)2 (1) and (Trimethylsilyl)ethene. Some alkenes with large peripheral groups such as Si(CH3)3, s-Bu, Ph, Mes, and SiEt3 react exclusively via hydrogen transfer reactions (Scheme 1). We examined the case of H2CdC(H)(Si(CH3)3), which lacks the β-hydrogen required for an ene-type reaction and contains the bulky Si(CH3)3 substituent. Experimentally, only hydrogen transfer to the more substituted carbon and formation of a bond between boron and the less substituted carbon are observed (1 f 7a in Scheme 4). This minimizes steric repulsions between substituents on boron and on the adjacent carbon atom. Structures for the two cases appear in Figures 9 and 10. Several differences among the four transition states are apparent. Comparing 7ats ax and 7ats eq, one sees that the latter is a slightly later transition state, in that the forming C-H bond is 5 pm longer than in the former. Other distances are little affected; thus, it appears likely that this represents greater steric repulsions between the silyl methyls and the transferring hydrogen in the equatorial conformer. The hypothesis is supported by comparing 7bts ax and 7bts eq, where the

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Dutmer and Gilbert

forming C-H distance in the latter is 20 pm longer than in the former. However, several other distances change significantly between these two, so that associating the change with steric issues is less clear-cut. It is noteworthy that the structures of trimethylsilyl-substituted 7ats ax and 7ats eq and of 7bts ax and 7bts eq differ only slightly from those of methyl-substituted 3ats ax and 3ats eq, and of 3bts ax and 3bts eq, respectively, save for the exceptionally long forming C-H distance in 7bts eq. This suggests that, for these systems, the steric bulk of the SiMe3 substituent is similar to that of the CH3 substituent; i.e., the long alkene C-Si bond in the former cancels the effect of it having methyl groups rather than hydrogens as in CH3. In addition to the differences noted above, the structures of sterically hindered 7bts ax/

7bts eq differ from those of the less hindered 7ats ax/7ats eq when B-C bond forming distances are compared; unsurprisingly, those for the former are longer than those for the latter. On comparison of products 7a,b, steric considerations result in the B-C bond being 2.3 pm longer in the latter, although it is possible that the trimethylsilyl group here acts as an electron acceptor, lowering the Lewis basicity of the carbon and decreasing its binding to boron. Experimentally, only 7a is isolated from the reaction, and the calculated energetics support this (Table 4). The barriers to 7ats ax and 7ats eq are lower than those to 7bts ax and 7bts eq, and formation of 7a is 12 kJ mol-1 more exothermic than formation of 7b. The differences certainly result from the steric effects noted. It is notable that the barrier to 7ats ax

Figure 9. Optimized (MPW1K/6-311þþG(2d,2p)) structures (distances in pm) of 7ats ax, 7ats eq, and 7a from the (H f CHSi) hydrogen transfer reaction between 1 and (trimethylsilyl)ethene. Some hydrogen and fluorine atoms were removed for clarity. Ring bond angles in 7ats ax (deg): N-B-C, 104.4; B-N-C, 116.1; B-C-C, 114.9; N-C-H, 106.1; C-C-H, 104.2; C-H-C, 146.7. Ring bond angles in 7ats eq (deg): N-B-C, 104.9; B-N-C, 116.1; B-C-C, 113.7; N-C-H, 106.0; C-C-H, 103.0; C-H-C, 144.0.

Figure 10. Optimized (MPW1K/6-311þþG(2d,2p)) structures (distances in pm) of 7bts ax, 7bts eq, and 7b from the (H f CH2) hydrogen transfer reaction between 1 and (trimethylsilyl)ethene. Some hydrogen and fluorine atoms were removed for clarity. Ring bond angles in 7bts ax (deg): N-B-C, 106.6; B-N-C, 116.0; B-C-C, 109.7; N-C-H, 105.9; C-C-H, 108.0; C-H-C, 145.5. Ring bond angles in 7bts eq (deg): N-B-C, 101.0; B-N-C, 116.0; B-C-C, 113.7; N-C-H, 107.7; C-C-H, 104.4; C-H-C, 139.2.

Scheme 4

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787

Table 4. Energetics (kJ/mol) for the Hydrogen Transfer Reactions between (F3C)2BdN(CH3)2 (1) and H2CdC(H)(Si(CH3)3)a reacn H transfer to CHSi forming 7a

H transfer to CH2 forming 7b

ΔEts(ax)

ΔEts(eq)

ΔE

MPWPW91

54

53

-32

MPW1K MP2//MPW1K OG2R3//MPW1K MPWPW91

84 37 48 86

81 37 50 78

-49 -85 -76 -3

MPW1K MP2//MPW1K OG2R3//MPW1K

115 61 69

105 49 52

-18 -74 -64

model

a ΔEts, ΔEts(ax), and ΔEts(eq) denote transition state barriers, with ax and eq denoting cases where the trimethylsilyl substituent occupies axial and equatorial positions, respectively, in the six-membered-ring transition states. ΔE denotes the overall reaction energy.

Figure 12. Optimized (MPW1K/6-311þþG(2d,2p)) structures (distances in pm) of 9ats ax, 9ats eq, and 9a from the (H f CHR) hydrogen transfer reaction between 1 and 3-chloro-1-butene. Some hydrogen and fluorine atoms were removed from 9ats ax and 9ats eq for clarity. Ring bond angles in 9ats ax (deg): N-B-C, 105.0; B-N-C, 116.4; B-C-C, 115.5; N-C-H, 106.1; C-C-H, 103.6; C-H-C, 145.9. Ring bond angles in 9ats eq (deg): N-B-C, 105.1; B-N-C, 115.0; B-C-C, 115.0; N-C-H, 106.0; C-C-H, 102.3; C-H-C, 146.2.

Figure 11. Optimized (MPW1K/6-311þþG(2d,2p)) structures (distances in pm) of 8ts and 8 from the ene reaction between 1 and 3-chloro-1-butene. Some hydrogen and fluorine atoms were removed from 8ts for clarity. Ring bond angles in 8ts (deg): N-B-C, 109.8; B-N-H, 96.1; B-C-C, 98.5; N-H-C, 156.6; C-C-C, 123.4; C-C-H, 90.4.

is slightly lower than that to 7ats eq; one expects six-membered cycles to prefer having large substituents in equatorial positions. This is apparently irrelevant for 7ats ax and 7ats eq; however, it accounts for the 17 kJ mol-1 difference in barriers between 7bts ax and 7bts eq. As mentioned, the structures of the transition states for these hydrogen transfers are similar to those involving propene. Comparing their energies, one sees that the trimethylsilyl-substituted transition states have barriers 11-12 kJ mol-1 lower than those for the methyl-substituted analogues. This indicates that strong electron donor substituents

assist hydrogen transfer reactions just as they do ene-type reactions. Ene-Type and Hydrogen Transfer Reactions between urger, Hagen, (F3C)2BdN(CH3)2 (1) and 3-Chloro-1-butene. B€ and Pawelke reported6 that H2CdCH(CHMeCl) did not react with 1, attributing this to the electron-withdrawing nature of chloride. As stated above, electron-donating substituents lower barriers; therefore, their reasoning that electron-withdrawing substituents should raise barriers is sound. We were intrigued by this result and hoped that determining the barriers for this nonreaction would provide lower bounds for reactions that could experimentally occur. It was also of interest to see whether the chloride complexed to the boron, thereby cutting off the pericyclic reaction pathways. A variety of optimizations, using MP2 and multiple DFT models, several large basis sets, and polarized continuum solvent models, all failed to provide a stationary point where the boron and chlorine remained bound. If a stationary point was reached, the two were at best in van der Waals contact (distances g400 pm). It appears that the energy of a dative B-Cl interaction is too small to compensate for the loss of the BdN π bond.

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Dutmer and Gilbert

Structures of the reaction components appear in Figures 11 (ene-type), 12, and 13 (hydrogen transfers). Comparing Figures 1 with 11, 2 with 12, and 3 with 13 demonstrates that the presence of the chloride substituent has little impact on the structures of the transition states or products. In each case, the forming B-C distances are slightly longer for

the chloro-substituted systems; therefore, one could argue for slightly earlier transition states resulting from either the electron-withdrawing nature of chloride or its added steric bulk. However, the similarities between other structural parameters make this a tenuous suggestion. The energetic data (Table 5) indicate that pathways forming 8 and 9a exhibit similar barriers and similarly sizable exothermicities. The barrier for the ene-type reaction is only 7 kJ mol-1 greater than that for the reaction between 1 and propene and 23 kJ mol-1 greater than that for the experimentally observed reaction between 1 and 2-methylpropene. Similarly, the barrier to 9ats ax (which, surprisingly, is lower than the barrier to 9ats eq), is 4 kJ mol-1 smaller than that to form 3ats ax, and only 9 kJ mol-1 greater than that predicted for the experimentally observed hydrogen transfer reaction between 1 and trimethylsilylethene. It thus appears that one or both pericyclic reactions between 1 and 3-chloro-1-butene should occur. The description of how this reaction was performed is very brief; therefore, we cannot determine why it was not viable. It is possible that the experimenters used their typical reaction temperature of 4 °C and never warmed the sample much above this. If so, this temperature and the calculated barrier of ca. 56 kJ mol-1 represent lower bounds for experimental reaction viability. Table 5. Energetics (kJ/mol) for Ene-Type and Hydrogen Transfer Reactions between (F3C)2BdN(CH3)2 (1) and 3-Chloro1-butenea reacn

model

ΔEts

ΔE

ene-type forming 8

MPWPW91 MPW1K MP2//MPW1K OG2R3//MPW1K

72 90 41 57

-43 -65 -106 -92

reacn H transfer to CHC forming 9a

Figure 13. Optimized (MPW1K/6-311þþG(2d,2p)) structures (distances in pm) of 9bts ax, 9bts eq, and 9b from the (H f CH2) hydrogen transfer reaction between 1 and 3-chloro-1-butene. Some hydrogen and fluorine atoms were removed from 9bts ax and 9bts eq for clarity. Ring bond angles in 9bts ax (deg): N-B-C, 106.6; B-N-C, 116.1; B-C-C, 109.1; N-C-H, 105.4; C-C-H, 106.7; C-H-C, 144.9. Ring bond angles in 9bts eq (deg): N-B-C, 103.7; B-N-C, 115.0; B-C-C, 110.9; N-C-H, 105.3; C-C-H, 107.8; C-H-C, 146.0.

H transfer to CH2 forming 9b

ΔEts(ax)

ΔEts(eq)

ΔE

MPWPW91

60

63

-32

MPW1K MP2//MPW1K OG2R3//MPW1K MPWPW91

88 44 56 91

93 49 63 100

-51 -93 -83 2

model

MPW1K 121 131 -14 MP2//MPW1K 69 76 -67 OG2R3//MPW1K 81 89 -57 a ΔEts, ΔEts(ax), and ΔEts(eq) denote transition state barriers, with ax and eq denoting cases where the methyl substituent occupies axial and equatorial positions, respectively, in the six-membered-ring transition states. ΔE denotes the overall reaction energy.

Scheme 5

Article

Figure 14. Optimized (MPW1K/6-311þþG(2d,2p)) structures (distances in pm) of 10ts and 10 from the ene reaction between 1 and 2,3,3-trimethyl-1-butene. Some hydrogen and fluorine atoms were removed from 10ts for clarity. Ring bond angles in 10ts (deg): N-B-C, 109.4; B-N-H, 97.7; B-C-C, 109.9; N-H-C, 152.2; C-C-C, 115.6; C-C-H, 95.0.

We hope that this work will drive reinvestigation of this result, involving examination of the reaction at higher temperatures. Ene-Type vs Hydrogen Transfer Reaction Competition in the Reaction between (F3C)2BdN(CH3)2 (1) and 2,3,3-Trimethyl-1-butene. Previous experimental reports have noted special cases where the products of both hydrogen transfer and ene-type reactions form in the same reaction, suggesting energetic competition. We studied the example reaction between aminoborane 1 and H2CdC(CH3)(t-Bu), which experimentally provides a mixture containing 20% ene-type product 10 and 80% hydrogen transfer product 11a, in 75% yield (Scheme 5). For the ene-type reaction, the resultant transition state structure (Figure 14) is very similar to that of the ene-type reaction between 2-methylpropene and 1 (Figure 4), showing similar shortening of the B-C and N-H bonds and lengthening of the B-N distance as compared to the reaction with propene (Figure 1). This reinforces the theory that addition of an electron-donating substituent to the alkene makes the transition state later along the path. The steric bulk of the tert-butyl group in 10ts has little impact on the transition state, but its donor capacity does affect it. Minor steric effects do appear when comparing products; the B-C and adjacent C-C bonds are 1-2 pm longer in 10 than those in 2 and 4, presumably due to the adjacent tert-butyl substituent. We were unable to locate stable six-membered-ring transition states with the tert-butyl substituent axial in the hydrogen transfer reactions. During some optimizations, the ring


789

Figure 15. Optimized (MPW1K/6-311þþG(2d,2p)) structures (distances in pm) of 11ats eq and 11a from the (H f CC2) hydrogen transfer reaction between 1 and 2,3,3-trimethyl-1butene. Some hydrogen and fluorine atoms were removed from 11ats eq for clarity. Ring bond angles in 11ats eq (deg): N-B-C, 107.7; B-N-C, 115.1; B-C-C, 117.9; N-C-H, 107.2; C-C-H, 100.1; C-H-C, 144.5.

inverted, and others located stationary points with no imaginary frequencies. As we had no difficulty finding transition states 5ats ax and 5bts ax, it appears that the stability “tipping point” for axial conformers here lies between methyland tert-butyl-substituted systems. Interestingly, the transition state 11ats eq (Figure 15) exhibits distances quite similar to those of the less hindered 6ats (Figure 7). In particular, the B-C forming distances are essentially identical, while the C-H forming distance in only 3 pm longer in 11ats eq. This indicates that placing the tertbutyl substituent in the equatorial position minimizes its steric interaction with the ring atom, with the only large impact being on the nearby transferring hydrogen atom. A different comparison involves 11ats eq and trimethylsilylsubstituted 7ats eq (Figure 9). The former is more compact than the latter, with the B-C forming distance shorter by around 7.5 pm and the C-H forming distance shorter by ca. 2 pm. This reflects the electron-donating capacity of the trimethylsilyl group, allowing the six-membered transition state ring to relax and lengthen bond distances. The extreme steric repulsions present in 11bts eq are reflected in its distances (Figure 16). In particular, the B-C forming distance of 212.6 pm is by far the longest in this work. Interestingly, product 11b exhibits a B-C bond length of 167.5 pm: long, but only slightly longer than that in 11a, which forms experimentally. It is plausible that 11b could be stable, although it is unlikely to be prepared via a hydrogen

790


Dutmer and Gilbert Table 6. Energetics (kJ/mol) for Ene-Type and Hydrogen Transfer Reactions between (F3C)2BdN(CH3)2 (1) and 2,3,3Trimethyl-1-butene. a reacn

model

ΔEts

ΔE

ene-type forming 10

G3(MP2)mod//MPW1K MPWPW91 MPW1K MP2//MPW1K OG2R3//MPW1K

42 55 70 28 38

-53 4 -14 -65 -58

reacn

model

ΔEts(eq) ΔE

H transfer to CC2 forming 11a G3(MP2)mod//MPW1K 48 -60 MPWPW91 60 -4 MPW1K 84 -22 MP2//MPW1K 39 -70 OG2R3//MPW1K 46 -59 167 81 H transfer to CH2 forming 11b MPWPW91 MPW1K 209 69 MP2//MPW1K 125 -8 OG2R3//MPW1K 147 6 a ΔEts and ΔEts(eq) denote transition state barriers, with eq denoting that the tert-butyl substituent occupies the equatorial position in the sixmembered-ring transition state. ΔE denotes the overall reaction energy.

Figure 16. Optimized (MPW1K/6-311þþG(2d,2p)) structures (distances in pm) of 11bts eq and 11b from the (H f CH2) hydrogen transfer reaction between 1 and 2,3,3-trimethyl-1butene. Some hydrogen and fluorine atoms were removed from 11bts eq for clarity. Ring bond angles in 11bts eq (deg): N-B-C, 99.0; B-N-C, 117.4; B-C-C, 103.3; N-C-H, 103.5; C-CH, 109.2; C-H-C, 152.6.

transfer route. This holds because all models predict that the barrier to formation of 11b by a hydrogen transfer reaction is prohibitive (ca. 150 kJ mol-1), and the process is probably slightly endothermic (Table 6). Thus we will not discuss it further. The OG2R3 model predicts barriers to 10ts and 11ats eq of 38 and 46 kJ mol-1, respectively (Table 6). This difference suggests a preference for formation of 10 over 11a, at odds with experiment. However, the difference is small and could be due to errors in the model. We tested this by calculating the barriers using the (presumably) more accurate G3(MP2)mod//MPW1K approach; this gave values of 38 and 46 kJ mol-1: closer, but still disagreeing with experiment. We used frequency data to determine the free energy barriers at 273 K (the experiments were performed at 277 K), to no avail; the 6 kJ mol-1 preference for the ene-type reaction remained. Optimizations and single point energies using a variety of models and basis sets, and optimization in polarizable continuum solvent (experiment and calculations used CH2Cl2) also maintained this preference. As a result, we hypothesize that the reaction must be reversible and that the product quantities isolated experimentally reflect an equilibrium mixture. The computational data indicate that 11a is slightly more stable than 10; therefore, an equilibrium mixture should contain more of it. The difficulty involves the fact that reversing the ene-type reaction

and crossing the hydrogen transfer barrier involves traversing a barrier of 58 þ 46 = 104 kJ mol-1. This is considerably greater than the initial barriers. However, support for this theory comes from B€ urger et al., who reported32 that 1 reacts with the similarly substituted thiocarbonyl SdC(CH3)(t-Bu) to form the ene-type product, which can be isolated at -40 °C. Upon warming to room temperature, this converts to the hydrogen transfer product, with an activation energy of 112 ( 19 kJ mol-1. This is larger than the 104 kJ mol-1 needed for reversibility of the alkene reaction, so given that the thiocarbonyl and alkene are similar in structure and substitution, it seems plausible that 10 could form first in this process and then rearrange to form 11a. Since the stabilities of the two differ by only 1 kJ mol-1, they could form an equilibrium that would slightly favor 11a, as seen experimentally. Of the reactions between 1 and alkenes described so far in this work, only this one allows a detectable equilibrium; all other systems have matched barriers and exothermicities and thus form only one product.

Conclusions Prior work suggested that aminoborane (F3C)2BN(CH3)2 (1) undergoes Diels-Alder-like [4 þ 2] pericyclic reactions in a concerted fashion, through six-membered-ring transition states akin to those suggested for purely organic cyclizations. This supports characterization of 1 as an inorganic alkene, with a polarized double bond. The work here extends this view, noting the range of ene-type reactions 1 can undergo, employing a six-membered-ring transition state and a concerted mechanism. The computational data show that hydrogen transfer, a reaction unseen in purely organic systems, is competitive with ene-type reactions for reactions where alkene methyls are either improperly located (H2CdCHSi(CH3)3) or are less accessible because of steric bulk (H2CdC(CH3)(t-Bu)). Transition states appear to have their barrier energies dictated largely by steric constraints rather than electronic ones; (32) B€ urger, H.; Pawelke, G.; Rothe, J. J. Organomet. Chem. 1994, 474, 43–48.

Article

the data suggest that hydrogen transfer products of type b will never form solely because the transition state is too hindered. Calculated barriers indicate that, under the typical experimental temperatures near 0 °C, viable reactions exhibit an upper bound barrier of ca. 50 kJ mol-1. The nonreaction between 1 and 3-chloro-1-propene is key to this assignment, as barriers here are predicted to be ca. 57 kJ mol-1. It seems likely that slight warming of this reaction will allow product formation. All of the pericyclic reactions examined are reasonably exothermic; thus, it is unsurprising that products can be isolated. It is interesting that the steric bulk associated with the alkene appears to have little effect on the product structure and reaction exothermicity; electronic issues exert greater control here. That reaction between 1 and H2CdC(CH3)(t-Bu) yields appreciable amounts of both ene-type and hydrogen transfer products and favors the latter is notable, in that calculated barriers predict the opposite. Assuming the calculations model the system accurately, one concludes that the product distribution reflects equilibrium between the processes: the first time such a suggestion has been made. Future work will


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address this point by examining how barriers and reaction energies change as the size of the other (non-methyl) substituent is systematically increased. It will prove interesting to determine how large this substituent must be to make hydrogen transfer the exclusive pathway. Similarly, can this substituent be made sufficiently large to shut down both reactions? Related future work will involve examining whether alkenes other than H2CdC(CH3)2 might form diborylated products and what effect electron-withdrawing substituents in positions other than geminal to the transferring hydrogen might have. One envisions, for example, that H2CdC(CF3)(CH3) might prefer undergoing hydrogen transfer reactions, despite its structural similarity to H2CdC(CH3)2, which exclusively undergoes ene-type reactions. Supporting Information Available: Tables giving optimized (MPW1K/6-311G(2d,2p)) Cartesian coordinates and absolute energies for all compounds described and a figure showing the ONIOM layering selected for OG2R3 calculations. This material is available free of charge via the Internet at http://pubs. acs.org.

Computational Studies of Pericyclic Reactions of Aminoborane (F3C

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