Computational Studies of Pericyclic Reactions between Aminoalanes

Jan 9, 2009 - Several points are evident from the first five entries. ..... + 2] cycloadditions, violating the precepts of the Woodward−Hoffmann sym...
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Organometallics 2009, 28, 787–794

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Computational Studies of Pericyclic Reactions between Aminoalanes and Ethyne, Ethene, and Dienes. A Reactive Aminoalane That Should Prefer [2 + 2] and [4 + 2] Cyclizations to Dimerization John M. Bailey, Catherine E. Check, and Thomas M. Gilbert* Department of Chemistry and Biochemistry, Northern Illinois UniVersity, DeKalb, Illinois 60115 ReceiVed June 4, 2008

The cyclic aminoalane 1 combines a nearly coplanar core with an endothermic dimerization energy, making it a candidate for computational studies of the chemical importance of the AlN π interaction in pericyclic reactions between aminoalanes and alkenes/dienes. Reaction energy data indicate that 1 should undergo [2 + 2] cycloaddition as readily as or more readily than [4 + 2] cycloaddition, such that treatment of 1 with butadiene might provide multiple bicyclic isomers. However, cyclization between 1 and 1,2dimethylenecyclopentane should form only distal and proximal isomers. The overall reactivity of 1 indicates that the AlN π interaction plays essentially no role in the chemical behavior of aminoalanes. Introduction While the Al-N π bond energy in aminoalanes R2AlNR′2 has been computationally quantified at ca. 40 kJ mol-1,1,2 the physicochemical importance of this value remains controversial. To a large extent, this reflects different views of how much π-bond energy is necessary to qualify the π bond as chemically important. Power has published convincing data that the short Al-N distances and planar geometries around nitrogen, which would usually be considered evidence for the presence of a relevant Al-N π bond, reflect ionic effects.3 More experimental support for this view comes from the observation that all monomeric aminoalanes so far studied by single-crystal X-ray diffraction show “twisting”, i. e., R-Al-N-R′ torsion angles that differ from 0°, the difference generally increasing with the steric bulk of the substituents.3b The ease of twisting around the Al-N bond points to the minimal effect of π bonding. Indirect experimental support comes from the observation that nearly all aminoalanes so far prepared oligomerize; one might expect that a compound with a moderately strong π bond would resist this kinetically if not thermodynamically. We have argued that an alternative approach for determining the importance of heteronuclear π bonds involves computationally examining their effect on chemical reactivity. In particular, we demonstrated that the high barriers for [2 + 2] cycloadditions of aminoboranes4 to alkenes and the low barriers for their [4 + 2] cycloadditions to dienes supports the importance of π bonding in these systems. Phosphinoboranes exhibit much less chemically important π bonds, as their cycloadditions generally display similar barriers.5 * To whom correspondence should be addressed. E-mail: tgilbert@ niu.edu. (1) McGee, A.; Dale, F. S.; Yoon, S. S.; Hamilton, T. P. In Computational Organometallic Chemistry; Cundari, T. R., Ed.; Marcel Dekker: New York, 2001; pp 381-396. (2) Several groups have examined this value for H2AlNH2 with increasing levels of sophistication. For a recent, very high-level approach, see: Grant, D. J.; Dixon, D. A. J. Phys. Chem. A 2006, 110, 12955–12962. (3) (a) Power, P. P. Chem. ReV. 1999, 99, 3463–3503. (b) Brothers, P. J.; Power, P. P. AdV. Organomet. Chem. 1996, 39, 1–69. (4) (a) Bissett, K. M.; Gilbert, T. M. Organometallics 2004, 23, 850– 854. (b) Gilbert, T. M. Organometallics 1998, 17, 5513–5520. (5) Gilbert, T. M.; Bachrach, S. M. Organometallics 2007, 26, 2672– 2678.

It is thus of interest to examine cyclization reactions of aminoalanes. To our knowledge, no such reactions have been reported. We suspect that this arises because examples such as (TIPP)2AlN(H)(DIPP)6 sacrifice reactivity for monomeric stability; i.e. in denying dimerization of the aminoalane, the exceptionally large substituents around the AlN core block attack by all potential substrates. It is possible that other design criteria are not met as well; the presence of electroneutral or electronwithdrawing substituents on N and weakly electron-withdrawing substituents on Al is not optimal for π bond formation and cyclization reactivity, which require coupling a strongly Lewis acidic Al moiety to a strongly Lewis basic N moiety. We report here computational studies of cyclizations of aminoalanes. One part describes studies of several aminoalanes designed to identify one that would prove reactive toward small organic molecules, containing substituents large enough to inhibit dimerization but small enough to minimize twisting of substituents around the molecular core. Meeting these criteria ultimately required design of cyclic aminoalane 1, which contains electron-acceptor substituents on the aluminum and electron-donor substituents on the nitrogen. The second part focuses on cyclization reactions between 1 and alkenes/dienes, showing that both [2 + 2] and [4 + 2] reactions should occur readily, supporting the view that the π bond in aminoalanes exerts little chemical effect. This makes 1 a potential precursor for a range of heterocycles.

Computational Methods All calculations were performed with the Gaussian 98 (G98) suite of programs.7 To determine their resistance to dimerization, the aminoalanes in Table 1, entries 4-8, and their corresponding dimers were initially optimized without constraints at the HF/3-21G level. As there are few conformational options available to the monomers and dimers, this level was deemed adequate for rapid assessment of stationary point structures and, more importantly, for rapid probing of the natures of these structures by analytical frequency analysis. The structures were then reoptimized at the levels given in the table. Entry 7, (F3C)2AlN(t-Bu)2, was an exception, owing (6) Waggoner, K. M.; Ruhlandt-Senge, K.; Wehmschulte, R. J.; He, X.; Olmstead, M. M.; Power, P. P. Inorg. Chem. 1993, 32, 2557–2561.

10.1021/om800515e CCC: $40.75  2009 American Chemical Society Publication on Web 01/09/2009

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Table 1. Predicted Structure Types, π Bond Energies, and Dimerization Energies (all Energies in kJ mol-1) for Various Aminoalanes aminoalane H2AlNH2 Me2AlNH2 Cl2AlNH2 H2AlNMe2 Me2AlNMe2 (F3C)2AlNMe2 (F3C)2AlN(t-Bu)2 1 a

model/basis set CCSD(T)/CBS CCSD(T)/6-311+G(2df,p) MP2/6-31G(d) B3LYP/LANL2DZ* B3LYP/DZP B3LYP/6-311++G(d,p) MP2/6-311++G(d,p) B3LYP/6-31G(d) MPW1PW91/6-31+G(d,p) MPW1PW91/6-311+G(d) MPW1K/BS15

core structure

π bond energya

dimerization energy

planar

44

planar

41

planar planar planar planar planar twisted slightly twisted

34 51 54 43 45

-217 -127 -173 -229 -165 -277

51

49

-247

ref 2 18 19 20 21 this this this this this this

work work work work work work

Estimated as the energy difference between the planar structure and the transition state where the AlC2 plane was fixed to bisect the NC2 plane.

to its preference for a twisted structure (see Results and Discussion). This molecule was optimized several times from starting structures containing coplanar AlC2 and NC2 planes, using increasing levels of model sophistication up to the level noted in the table. No model provided an optimized structure that retained the coplanar conformation. Dimerization energies in Table 1 were corrected using scaled8 zero-point energies (ZPEs) from the frequency analyses. The cyclic aminoalane 1 was selected as the most suitable for cyclization computations, as it displayed a nearly planar C2AlNC2 core and an endothermic dimerization (see Results and Discussion).9 Initial optimizations of aminoaluminacycles 2-7 derived from 1 employed the HF/3-21G model. As above, these have few conformational options; therefore, this approach was viewed as adequate for structural starting points and ZPEs. In a few cases, we explored the relative stabilities of various ring conformations (chair vs boat vs twist boat), finding either that the different starting conformations optimized to identical ones or that the preferred conformation was so much more stable than others as to represent the most logical choice for further optimizations. Initial searches for corresponding transition states employed the HF/6-31+G(d) model, as prior work showed that the presence of diffuse and polarization functions aids in finding saddle points. Frequency analyses at stationary points at these levels provided were used to determine their nature and to give ZPEs. The stationary point structures were then reoptimized using a three-layer ONIOM approach (model/6-311+G(d):model/ 3-21+G*:model/3-21G; hereafter abbreviated as ONIOM3) employing a variety of DFT models and the MP2 model (see the Supporting Information for energy data). The high layer contained the AlNC4 core, the fluorine atoms, and all atoms on the organic molecule/fragment, the middle layer incorporated all other carbon atoms, and the low layer covered the entire molecule. (7) Frisch, M. J. ; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Zakrzewski, V. G.; Montgomery, J. A., Jr.; Stratmann, R. E.; Burant, J. C.; Dapprich, S.; Millam, J. M.; Daniels, A. D.; Kudin, K. N.; Strain, M. C.; Farkas, O.; Tomasi, J.; Barone, V.; Cossi, M.; Cammi, R.; Mennucci, B.; Pomelli, C.; Adamo, C.; Clifford, S.; Ochterski, J.; Petersson, G. A.; Ayala, P. Y.; Cui, Q.; Morokuma, K.; Malick, A. D.; Rabuck, K. D.; Raghavachari, K.; Foresman, J. B.; Cioslowski, J.; Ortiz, J. V.; Baboul, A. G.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Gomperts, R.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Andres, J. L.; Gonzalez, C.; HeadGordon, M.; Replogle, E. S.; Pople. J. A. Gaussian 98, Revision A.11.4; Gaussian, Inc., Pittsburgh, PA, 1998. (8) Scott, A. P.; Radom, L. J. Phys. Chem. 1996, 100, 16502–16513. (9) As a reviewer noted, the presence of a stable minimum for the dimer structure that is endothermic with respect to two molecules of monomer implies the presence of a barrier, and a transition-state structure. Our interest is in finding a monomeric aminoalane that will react with organic substrates in preference to reacting with itself. Since the implication of endothermic dimerization of 1 is that it will not dimerize easily (in contrast to the other aminoalanes in Table 1), we have not searched diligently for this transition state. Our assumption from the data is that 1, if synthesized, will remain monomeric and will contain a planar C2Al-NC2 core. Similar reasoning applies to formation of 7 prox and 7 dist. If the reactions are endothermic, then regardless of the barrier height, separation of the tricyclic product into 1 and cyclopentadiene will be preferred.

As we saw previously, different models gave detectably different predictions for barriers and reaction energies for the same reaction, with values for DFT models varying by up to 45 kJ mol-1. We suggested that such results be used to provide computational “error bars”;10 this approach has proved unsatisfactory, as the error bars have sometimes proved larger than the associated energies. As a result, we do not trust any DFT models that have not been carefully calibrated11 against experimental energies or high-level perturbation theory models using larger molecules.12 As we were unable to do this for the reaction participants in this work, we sought alternative, more trustworthy approaches applicable to larger molecules. We recently reported that13 Vreven and Morokuma’s ONIOM G2R composite methods14 (hereafter OG2Rx; see below) provided “high model chemistry” results that accurately predicted dissociation energies for a range of group 13/15 element donor-acceptor compounds, including several containing over 25 heavy atoms. We therefore opted to apply it to the reactions here. A brief discussion of the method follows. The OG2Rx approaches involve the usual methodology of a composite method, such as the Gn models:15 optimize to an acceptably accurate structure and then use this for vibrational/ temperature corrections and for single-point energy calculations using perturbation theory models of increasing sophistication and increasingly large basis sets. As we previously showed that the MPW1K DFT model16 gives fine agreement with experimental structures with minimal resource cost,17 we used structures from the MPW1K/6-311+G(d):MPW1K/3-21+G*:MPW1K/3-21G optimizations as bases for the single-point calculations. The MPW1K model carries a further advantage for the reactions here in that it (10) Check, C. E.; Gilbert, T. M. J. Org. Chem. 2005, 70, 9828–9834. (11) The M06 suite of DFT models has been so calibrated for several databases of small molecules, showing considerable promise in its energetic predictions. However, the suite still needs to be tested with molecules of the size in this work. We plan to calibrate the M06 suite against the OG2Rx methods in the future using large molecules. See: Zhao, Y.; Truhlar, D. G. Theor. Chem. Acc. 2008, 120, 215–241. (12) A key problem with the B3LYP model, for example, is that its thermochemistry predictions decrease substantially and systematically in accuracy as the number of carbon atoms increases. See ref 10 and: Curtiss, L. A.; Raghavachari, K.; Redfern, P. C.; Pople, J. A. J. Chem. Phys. 2000, 112, 7374–7383. (13) Gille, A. L.; Gilbert, T. M. J. Chem. Theory Comput. 2008, 4, 1681– 1689. (14) (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. (15) Curtiss, L. A.; Raghavachari, K. Theor. Chem. Acc. 2002, 108, 61– 70. (16) Lynch, B. J.; Truhlar, D. G. J. Phys. Chem. A 2001, 105, 2936– 2941. (17) Gilbert, T. M. J. Phys. Chem. A 2004, 108, 2550–2554. (18) Himmel, H.-J. Eur, J. Inorg. Chem. 2003, 2153–2163. (19) Mu¨ller, J. J. Am. Chem. Soc. 1996, 118, 6370–6376. (20) Nakamura, K.; Makino, O.; Tachibana, A.; Matsumoto, K. J. Organomet. Chem. 2000, 611, 514–524. (21) Timoshkin, A. Y.; Bettinger, H. F.; Schaefer, H. F., III J. Am. Chem. Soc. 1997, 119, 5668–5678.

Computational Studies of Pericyclic Reactions was designed to model transition state energies; therefore, it is plausible that it models weakly bound transition state structures well. For the single-point energy calculations, we tested two different ONIOM approaches, designated OG2R2 and OG2R3. Both approximate CCSD(T)/6-311+G(2df,2p) calculations using separate energy determinations (several of which are duplicates) to determine a final molecular energy; however, all are of sizes compatible with typical computational resources and require only a few hours maximum per calculation. In each case, the overall OG2Rx energy is calculated as ∆EOG2Rx ) ∆ECCSD(T) + ∆EMP2large - ∆EMP2small; the methods differ only in how they calculate these terms. For the OG2R2 composite, ∆ECCSD(T) is the energy from a two-layer ONIOM calculation symbolized as ONIOM(CCSD(T)/6-31G(d): MP2/6-31G(d)), ∆EMP2large is the energy from a two-layer ONIOM calculation symbolized as ONIOM(MP2/6-311+G(2df,2p):MP2/ 6-1G(d)), and ∆EMP2small is the energy from a two-layer ONIOM calculation symbolized as ONIOM(MP2/6-31G(d):MP2/6-31G(d)). In these calculations, the high layer contained the Al and N atoms and all carbon atoms of the organic substrate, while the low layer contained all atoms. For the three-layer OG2R3 composite, B3LYP/ 3-21G is used for the low layer, while the low layer above becomes the medium layer; thus, for example, ∆ECCSD(T) is the energy from a calculation symbolized as ONIOM(CCSD(T)/6-31G(d):MP2/631G(d):B3LYP/3-21G). In these calculations, the high layer remained as above, the medium layer contained the high-layer atoms plus the four other carbons attached to the Al and N atoms, and the low layer contained all atoms. Consequently, OG2R2 calculations are likely to be more accurate, as they involve larger basis sets and fewer assumptions regarding layer interactions, while OG2R3 calculations can be applied to larger molecules, since the component involving all atoms is a resource-minimal B3LYP/321G energy calculation. Relative energies, corrected using scaled ZPEs from frequency analyses,8 appear in Table 4; structures appear in Figures 1 -4. One sees that OG2R2 and OG2R3 values generally agree, with the latter predicting slightly higher barriers and slightly less exothermic reactions than the former. This tendency appears more prevalent for larger molecules, suggesting that it arises from inaccuracies of the B3LYP model as molecular size increases.12 Therefore, discussion in the Energetics section below will involve only OG2R2 energies. Comparing the data from OG2Rx models to those from MPW1K optimizations indicates that the DFT model predicts barriers well (as it was designed to do)16 but overestimates reaction exothermicities.

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Figure 1. Optimized structures of 1 (a, b) and its dimer (c) (ONIOM3 MPW1K), showing selected bond distances (Å). Hydrogen atoms and some methyl groups were removed from (b) for clarity.

Results and Discussion Identifying an Aminoalane That Should Resist Dimerization. As implied in the Introduction, basic design criteria for aminoalanes that allow probing of the chemical importance of the Al-N π interaction include the R2AlNR′2 core containing planar geometries around the two core atoms and coplanar R2Al and NR′2 moieties. A further chemical reactivity requirement is that the R and R′ substituents must be large enough to inhibit aminoalane dimerization but small enough that other substrates can access the Al-N bond. The latter provides a significant challenge, as aminoalanes are notorious for their ability to oligomerize. Few monomeric examples have been structurally characterized.3 Data bearing on these issues are collected in Table 1. Several points are evident from the first five entries. One, the π bond energy, estimated as the difference between the energy of the minimum structure and that of the transition state structure where the C2Al plane is fixed to bisect the NC2 plane (Scheme 1), remains fairly constant, even when electron-withdrawing groups such as chloride are placed on aluminum and electron-donating

Figure 2. Optimized (ONIOM3 MPW1K; bond distances in Å) structures of the transition state (left) and product (right) of [2 + 2] cyclization between 1 and (a) ethyne (2ts and 2) and (b) ethene (3ts and 3). Peripheral hydrogen atoms are removed for clarity.

groups such as methyl are placed on nitrogen. Two, no correlation appears between the π bond energy and the dimerization energy. Both observations support the view that the lone pair on the nitrogen atom remains largely localized rather than forming a π bond to the aluminum atom. The nitrogen can act as a Lewis base to an aluminum atom on a different aminoalane nearly as easily as to its bound aluminum atom. Three, the dimerization energy depends significantly on the size of the substituents. The first three entries show a regular decrease in exothermicity as the substituents on the Al atom

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Figure 3. Optimized (ONIOM3 MPW1K) structures of the transition state (left) and product (right) of three of eight possible isomers for [2 + 2] cyclization between 1 and 1,3-butadiene: (a) 4ts and 4 Al prox trans; (b) 4ts and 4 N prox trans; (c) 4ts and 4 Al dist cis. Peripheral hydrogen atoms are removed for clarity. See the text for isomer naming conventions.

increase in size, despite the presence of electron-withdrawing substituents in Cl2AlNH2. However, this effect appears less important for substituents on nitrogen and evidently can be overcome if the substituents on aluminum are exceptionally electron withdrawing, so that the aluminum is exceptionally Lewis acidic, as in (F3C)2AlNMe2. It should be noted that barriers to dimerization are rarely examined, presumably because the processes are barrierless or nearly so.22 Combining this background with that from our study of phosphinoboranes,5 we examined aminoalanes containing electronwithdrawing trifluoromethyl substituents on aluminum and electron donors on nitrogen. (F3C)2AlNMe2 is predicted to adopt a planar, nearly C2V conformation. The estimated AlN π bond energy differs little from that of other aminoalanes, despite the “tuning” of the donor/acceptor properties of the substituents. While use of electron localization models requires caution, we note that the natural bond order (NBO) model23 suggests that 91.0% of the electron density of the AlN π bond “found” by the model lies on the nitrogen atom. In effect, this π bond is largely a nitrogen lone pair. The NBO approach supports Power’s view3 that the short Al-N bonds and planar N atoms (22) Hamilton, T. P.; Shaikh, A. W. Inorg. Chem. 1997, 36, 754–755. (23) Glendening, E. D.; Badenhoop, J. K.; Reed, A. E.; Carpenter, J. E.; Bohmann, J. A.; Morales, C. M.; Weinhold, F. NBO 5.0; Theoretical Chemistry Institute, University of Wisconsin, Madison, WI, 2001.

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Figure 4. Optimized (ONIOM3 MPW1K) structures of the transition state (left) and product (right) of [4 + 2] cyclizations between 1 and (a) 1,3-butadiene (5ts and 5, proximal isomer), (b) 1,2dimethylenecyclopentane (6ts and 6, distal isomer), and (c) 1,3cyclopentadiene (7, distal isomer). Peripheral hydrogen atoms are removed for clarity. Scheme 1. Structures Used To Determine π Bond Energy

in aminoalanes reflects the sizable polarity of the Al-N interaction, in that 88.5% of the σ bond density lies on the nitrogen. Regardless of the model used, when we optimized various starting structures of the more congested (F3C)2AlN(t-Bu)2, the optimization procedure consistently twisted the C2Al and NC2 planes with respect to each other, culminating in structures where the two were nearly perpendicular. As above, this supports the weakness and chemical irrelevance of the AlN π bond in aminoalanes. The result indicated that fixing the planes in place was required, which in turn dictated the presence of a link chain between the moieties. Several cyclic aminoalanes were optimized using various DFT models and modest basis sets. Ultimately we discovered that the six-membered-ring species 1 (Figure 1a), containing trifluoromethyl and pseudo-tert-butyl substituents on aluminum and tert-butyl and pseudo-tert-butyl substituents on nitrogen, combined conformational stability with an endothermic dimerization energy.9 The latter clearly stems from the presence of the sizable

Computational Studies of Pericyclic Reactions

peripheral substituents surrounding the Al-N core; examination of the structure of the dimer shows distortions that probably arise from repulsive interactions (Figure 1). For example, the Al2N2 core is asymmetric, with the Al-N distance within a monomer moiety being 2.041 Å and that between moieties being 2.129 Å. Moreover, the Al-CF3 distances are 2.108 Å, on the long side for Al-C distances. For comparison, the Al-C(ring) distances are 1.997 Å. Both are longer than their counterparts in 1 (2.037 and 1.945 Å, respectively). Given all this, we expect that 1 would remain monomeric in a reaction environment. The Al-N bond distance in 1 lies on the short side of the range of those seen experimentally for monomeric aminoalanes, in line with the value for (TIPP)2AlN(H)(DIPP),6 which like 1 has slightly electron withdrawing substituents on aluminum. Owing to its asymmetry, 1 has two independent torsion angles involving the AlN core. That within the ring has a value of 36.9°, and that external to the ring has a value of 37.6°. The C2Al and NC2 planes are thus significantly skewed (Figure 1b). Estimating the π bond energy in 1 in the usual way (Scheme 1) proved challenging, as the “planes perpendicular” transition state structure exhibited significant distortions within the ring. We found it to be 51 kJ mol-1, in keeping with values for other aminoalanes. The NBO model finds an Al-N π bond in 1, characterized as composed of 4.7% aluminum natural atomic orbitals and 95.3% nitrogen natural atomic orbitals. For comparison, the model characterizes the Al-N σ bond as composed of 8.8% aluminum natural orbitals and 91.2% nitrogen natural orbitals; as for (F3C)2AlNMe2, both interactions are strongly polarized toward the nitrogen atom and the π bond differs only slightly from a pure N lone pair. The NRT (natural resonance theory)24 subroutine within the NBO framework supports the presence of an Al-N π bond in 1, indicating an AlN bond order of 1.86, of which 0.25 (13%) is the covalent contribution and 1.61 (87%) is the ionic contribution. As always, one should view the results of these electronic localization methods skeptically; the related Wiberg method25 predicts an AlN bond order of 0.41. This is obviously flawed, probably reflecting errors arising from the sizable ionic character of the Al-N bond. Optimized Structures of Cyclization Components. Since one goal of this work was to compare [2 + 2] and [4 + 2] cycloadditions, it was necessary to examine baseline cases as well as systems containing multiple cyclization options. Optimized structures of the reaction components for two baseline [2 + 2] cyclizations involving 1 and ethyne (2ts and 2) and ethene (3ts and 3) appear in Figure 2. One sees that the transition states are quite asymmetric, with the C-C π bonds essentially being coordinated to the aluminum in the manner of η2 complexes in transition-metal chemistry. The substrate C-C and reactant Al-N bond distances indicate that the transition states are “early”; therefore, presumably most of the barrier energy is devoted to coordinating the π bond to the Al. The product molecules display generally typical distances, save that the Al-N distance is somewhat long for a dative N-Al interaction. This probably relieves some strain associated with formation of the four-membered heterocyclic ring. DFT-based scans of the potential surface between reactants and product suggest that the cyclizations are concerted; however, this may be artifactual, as DFT models tend to favor concerted pro(24) (a) Glendening, E. D.; Weinhold, F. J. Comput. Chem. 1998, 19, 593–609. (b) Glendening, E. D.; Weinhold, F. J. Comput. Chem. 1998, 19, 610–627. (c) Glendening, E. D.; Weinhold, F. J. Comput. Chem. 1998, 19, 628–646. (25) Wiberg, K. B. Tetrahedron 1968, 24, 1083–1096.

Organometallics, Vol. 28, No. 3, 2009 791 Table 2. Selected Distances (ONIOM3 MPW1K; Å) for the Eight Possible Isomers of [2 + 2] Cyclization between 1 and 1,3-Butadienea Al-N 3ts 4ts 4ts 4ts 4ts 4ts 4ts 4ts 4ts 3 4 4 4 4 4 4 4 4

Al prox cis Al prox trans Al dist cis Al dist trans N prox cis N prox trans N dist cis N dist trans

Al prox cis Al prox trans Al dist cis Al dist trans N prox cis N prox trans N dist cis N dist trans a

Al-C′

N-C

C-C

Transition States 1.913 2.126 2.439 1.921 2.180 2.469 1.921 2.175 2.450 1.915 2.140 2.435 1.923 2.145 2.431 1.945 2.059 2.516 1.952 2.049 2.553 1.935 2.083 2.488 1.942 2.059 2.562

2.186 2.174 2.138 2.144 2.120 2.145 2.156 2.210 2.164

1.388 1.406 1.395 1.399 1.400 1.420 1.430 1.407 1.420

Products 1.974 2.000 1.997 1.993 1.993 1.962 1.970 1.967 1.970

1.513 1.509 1.508 1.519 1.515 1.543 1.557 1.534 1.534

1.527 1.519 1.526 1.526 1.534 1.527 1.543 1.529 1.539

2.117 2.098 2.117 2.090 2.108 2.103 2.097 2.128 2.125

Al-C

See the text for isomer naming conventions.

cesses.26 We have not examined the cyclizations carefully enough using more appropriate models to rule out stepwise mechanisms. Open-chain dienes can react with 1 either using one double bond in [2 + 2] cyclizations or using both in [4 + 2] cyclizations. If they react in the former manner, eight isomers are possible, depending upon whether the unbound vinyl substituent lies near the Al or N, whether the vinyl group lies above the six-membered ring or opposite it (termed proximal and distal), and whether the diene adopts the cis or trans geometry. We optimized the structures of all eight options for the reactions between 1 and 1,3-butadiene (4ts and 4 with additional designators). Rather than show all eight, Figure 3 contains a selection of these.27 For ease of comparison, selected structural data for all eight isomers and for the ethene parent appear in Table 2. The data indicate that the core distances of the transitionstate structures 4ts apparently depend entirely on the position of the vinyl substituent with respect to the aluminum and nitrogen atoms. For example, the Al-C distances for the Altype transition states (average 2.16(2) Å) are generally longer than those for the N-type (2.06(1) Å); this reflects the steric bulk and the electron-withdrawing ability of the vinyl group. The Al-C′ distances behave similarly (2.45(2) vs 2.53(3) Å). The N-C distances may also reflect this, but the differences are not statistically significant: 2.17(3) Å for the N-type transition states and 2.14(2) Å for the Al-type. This decrease in significance probably stems from the fact that the nitrogen and carbon interact weakly at best in the transition state. No statistically significant differences could be found in the bond distances when comparing proximal and distal or cis and trans isomers. Interestingly, the transition states of N-type systems are later than those of the Al-type, as assessed by the fact that (26) For two examples among many, see: (a) Bachrach, S. M.; Gilbert, J. C. J. Org. Chem. 2004, 69, 6357–6364. (b) Leach, A. G.; Goldstein, E.; Houk, K. N. J. Am. Chem. Soc. 2003, 125, 8330–8339. (27) Isomers are designated on the basis of three options: Al means the vinyl substituent lies on the aluminum side of the Al-N bond, and N means it lies on the nitrogen side; prox means the vinyl substituent lies on the six-membered-ring side of the Al-N bond, and dist means it lies on the opposite side; cis means the butadiene adopts a cis geometry, and trans means it adopts a trans geometry.

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Table 3. Selected Distances (ONIOM3 MPW1K; Å) for the Isomers of [4 + 2] Cyclization between 1 and 1,3-Butadiene (Compounds 5ts and 5) and 1,2-Dimethylenecyclopentane (Compounds 6ts and 6)a Al-N 5ts 5ts 6ts 6ts 5 5 6 6

prox dist prox dist

prox dist prox dist

Al-C1

N-C4

C1-C2

C2-C3

MPW1K

C3-C4

1.927 1.929 1.939 1.944

Transition States 2.153 2.274 1.392 2.142 2.265 1.395 2.115 2.267 1.404 2.098 2.250 1.408

1.393 1.391 1.396 1.393

1.373 1.374 1.376 1.380

2.144 2.117 2.155 2.122

1.994 1.986 1.991 1.978

Products 1.508 1.510 1.507 1.512

1.332 1.329 1.335 1.333

1.495 1.499 1.495 1.498

1.482 1.480 1.481 1.480

Table 4. Energetics (kJ mol-1) using Various Models for [2 + 2] and [4 + 2] Cyclizations between 1 and Ethyne, Alkenes, and Dienes

a C1 is the carbon of the diene nearest the aluminum, C4 is the carbon of the diene nearest the nitrogen, and C2 and C3 are the backbone carbons.

the Al-N (1.944(7) vs 1.920(4) Å) and C-C distances (1.419(9) vs 1.400(5) Å) are longer in the former, indicating more extensive bond breaking. This may correlate with the observation that the barriers for N-type cyclizations are larger than those for Al-type (see Energetics section below). The core distances in the product structures 4 exhibit a similar exclusive dependence on the position of the vinyl group with respect to the core atoms. In each case the distance associated with the vinyl-substituted carbon is ca. 0.03 Å longer than its parent counterpart. Whether the butadiene is cis or trans, or proximal or distal, has no detectable impact on the relevant bond distances. [4 + 2] cyclization between 1 and 1,3-dienes provides isomeric products as well, as the formed six-membered ring can be distal or proximal to the original one. We examined two examples, one using butadiene (5ts and 5 with designators) and one using the bulkier 1,2-dimethylenecyclopentane (6ts and 6 with designators). As examples, the proximal isomer reaction components of the former and the distal isomer components of the latter appear in Figure 4. Relevant structural data appear in Table 3. The data show that both the transition states and the proximal and distal product isomers differ little and, indeed, that the bicyclic and tricyclic systems are structurally similar. One clear distinction is that the Al-N distances are longer for the proximal product isomers than for the distal isomers; this probably relieves steric congestion associated with having the six-membered rings atop one another in the former. Less clearly, it appears that the Al-N distances are shorter and the Al-C1 and N-C4 distances longer for the 5ts pair vs the 6ts pair. If true, it is not evident how one rationalizes this, since on steric grounds one predicts that the bulkier dimethylenecyclopentane prefers a transition state with longer Al-C1 and N-C4 bonds. A plausible hypothesis is that the methylene carbon is more electron rich in dimethylenecyclopentane than in butadiene; this makes the carbon more attractive to the Lewis acidic aluminum and, therefore, the Al-C1 bond is shorter. This combines with the stiff backbone in dimethylenecyclopentane to force the N-C4 distance to shrink as well. Comparison of the 5ts C-C distances with those for the free dienes (ca. 1.33 and 1.42 Å) and with the values in the products suggests that the transition states for both reactions are fairly early, which seems consistent with this idea. We also optimized the proximal and distal isomers of the [4 + 2] cyclization between 1 and 1,3-cyclopentadiene (7), as the fixed cis diene is often used as a test molecule for cyclizations. The distal components appear in Figure 4c. As formation of these isomers appears endothermic (see the Energetics section),

OG2R2

OG2R3

[2 + 2] Cyclizations 2ts 11 2 -148 3ts 35 3 -54 4ts Al prox cis 54 4 Al prox cis -47 4ts Al prox trans 51 4 Al prox trans -46 4ts Al dist cis 51 4 Al dist cis -32 4ts Al dist trans 47 4 Al dist trans -35 4ts N prox cis 63 4 N prox cis 11 4ts N prox trans 60 4 N prox trans 14 4ts N dist cis 51 4 N dist cis -23 4ts N dist trans 54 4 N dist trans -14

15 -110 39 -19 48 -20 46 -18 45 -6 42 -8 65 34 56 32 49 2 55 9

17 -105 46 -12 45 -13 52 -11 56 6 49 -2 76 44 68 43 59 16 65 22

[4 + 2] Cyclizations 5ts prox 45 5 prox -43 5ts dist 41 5 dist -45

44 -13 41 -13

54 -2 49 -5

30 -52 35 -47

29 -21 28 -18

46 -6 40 -7

47 67

54 77

72 85

1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1

+ + + + + + + + + + + + + + + + + + + +

C2H2 C2H2 C2H4 C2H4 C4H6 C4H6 C4H6 C4H6 C4H6 C4H6 C4H6 C4H6 C4H6 C4H6 C4H6 C4H6 C4H6 C4H6 C4H6 C4H6

1 1 1 1

+ + + +

cis-C4H6 cis-C4H6 cis-C4H6 cis-C4H6

1 1 1 1

+ + + +

1,2-(H2C)-c-C5H6 1,2-(H2C)-c-C5H6 1,2-(H2C)-c-C5H6 1,2-(H2C)-c-C5H6

f f f f f f f f f f f f f f f f f f f f

f f f f

1 + c-C5H6 f 7 prox 1 + c-C5H6 f 7 dist

f f f f

6ts prox 6 prox 6ts dist 6 dist

they are unlikely to form readily and thus were not further pursued. We were unable to locate stable transition states 7ts for either the distal or proximal isomers. Energetics. Barriers and reaction energies for reactions examined in this work appear in Table 4. As stated in the Computational Methods section, the discussion will involve only the OG2R2 data. [2 + 2] cyclization between 1 and ethyne to form 2 is by far the most efficacious reaction examined, showing a very small barrier and being highly exothermic. Presumably the low barrier reflects the small size and considerable Lewis basicity of the cylinder of π electron density of ethyne; the exothermicity arises from the formation of a strong N-C (sp2) bond at the expense of converting a C-C triple bond to a C-C double bond and breaking a very weak Al-N π interaction. A rough calculation using values of 390, 280,28 and 50 kJ mol-1 for the three interactions and a reaction exothermicity of 110 kJ mol-1 suggests a bond energy of only 50 kJ mol-1 for the Al-C(sp2) bond formed by cyclization. Comparing the energetics of formation of 3 from 1 and ethene with those of formation of vinyl-substituted analogues 4 shows that in general using the larger organic substrate leads to higher barriers and lower exothermicities. This is in keeping with the structural observations above. The 4Al isomers exhibit distinctly lower barriers than their 4N counterparts, as well as larger exothermicities. Again, this agrees with the structural results. Within the four 4Al isomers, no clear barrier differences exist, although formation of 4ts Al distal isomers may be slightly easier than formation of proximal isomers. However, the barriers (28) Lowry, T. H.; Richardson, K. S. Mechanism and Theory in Organic Chemistry, 3rd ed.; Harper & Row: New York, NY, 1987; Chapter 2.3.

Computational Studies of Pericyclic Reactions

Organometallics, Vol. 28, No. 3, 2009 793 Scheme 2

Scheme 3

are small enough that the reactions might be reversible at elevated temperatures. If so, the 4Al proximal isomers are significantly preferred over the distal isomers. Unfortunately, no system shows a preference for cis or trans isomers; therefore, both are likely to form. However, it is possible that rotation of the vinyl group around the C-C single bond will be of low energy; therefore, the two might interconvert. It is thus possible that [2 + 2] cycloaddition of butadiene to 1 will form a single product, despite the isomeric options. In our previous studies of cyclizations of aminoboranes and phosphinoboranes, the above discussion was moot, because the systems always preferred [4 + 2] to [2 + 2] cyclizations with butadiene. Table 4 shows that this does not hold for aminoalane 1. The barriers to [4 + 2] cyclization (5ts prox and 5ts dist) differ little from those for [2 + 2] cyclization to form 4ts Al isomers, and the reaction exothermicities are smaller than those for the proximal isomers. Moreover, no preference exists for formation of 5 prox vs 5 dist (these could interconvert rapidly at room temperature and so could appear as one product). Thus, treatment of 1 with butadiene may provide several products from [2 + 2] and [4 + 2] cyclizations. This unhappy result forced us to examine alternative dienes where the cis conformation was enforced and where steric effects might dictate adoption of the less crowded distal geometry. Test calculations indicated that 1,2-dimethylenecyclopentane bound only weakly in a [2 + 2] cyclization with 1. The results of [4 + 2] cyclization calculations involving these reactants to give 6ts and 6 are given in Table 4. One sees that the barriers are smaller than those for 5 and that the reactions are more exothermic. There may exist a preference for the proximal isomer over the distal isomer as well, although this is unclear. Nonetheless, the data suggest that use of exocyclic dienes in cyclization reactions with 1 will give solely the [4 + 2] products and will do so at relatively low temperatures. This positive result provided motivation to examine the reaction between 1 and 1,3-cyclopentadiene. Surprisingly, this

proved to be sizably endothermic for both the proximal and distal isomers; moreover, we were unable to locate transitionstate structures for either pathway.9 It seemed likely that the reversal of reaction energy stemmed from sizable ring strain energy (RSE) in the tricyclic isomers 7. To test this, we employed the reactions in Scheme 2. Reaction 1 probes the ring strain energy inherent in 1; the calculations (MPW1K/BS1)5 gave RSE < 1 kJ mol-1, typical for most aliphatic six-membered rings. Reaction 2 was then used to estimate the RSE of tricyclic 7 prox. While some components of this reaction were contracted from their ideals (nitrogen atoms should have tert-butyl substituents rather than methyl substituents, for example), the predicted RSE ) 140 kJ mol-1 is consistent with the difference in exothermicities between the reactions forming 6 and 7 (75-95 kJ mol-1), if larger. It thus appears that dienes that will react with 1 in a [4 + 2] cyclization will prove limited to exocyclic systems, although possibly openchain dienes that adopt particular conformations owing to steric or electronic constraints will work also. We plan to test the latter idea in future work.29

Conclusion The computational data indicate that the effect of the AlN π interaction on the chemistry of aminoalanes is essentially nil. In contrast to the case for aminoboranes and phosphinoboranes, [2 + 2] cycloadditions should occur as or more easily as [4 + 2] cycloadditions, violating the precepts of the WoodwardHoffmann symmetry rules. The cyclic aminoalane 1 apparently can be forced to undergo [4 + 2] cycloadditions through use of exocyclic dienes, but the preference is not inherent in its electronic makeup. This suggests that no weight should be given (29) (a) Gilbert, T. M. Organometallics 2005, 24, 6445–6449. (b) Gilbert, T. M. Organometallics 2003, 22, 3748–3752.

794 Organometallics, Vol. 28, No. 3, 2009

to any π interaction of