Organometallics 2010, 29, 3541–3545 DOI: 10.1021/om100419j
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Transition Metal Intervention for a Classic Reaction: Assessing the Feasibility of Nickel(0)-Promoted [1,3] Sigmatropic Shifts of Bicyclo[3.2.0]hept-2-enes Osvaldo Gutierrez and Dean J. Tantillo* Department of Chemistry, University of California, Davis, One Shields Avenue, Davis, California 95616 Received May 4, 2010
Density functional theory calculations (B3LYP/LANL2DZ) were used to evaluate the feasibility of Ni(0)-N-heterocyclic carbene-promoted [1,3] sigmatropic rearrangements of bicyclo[3.2.0]hept2-enes to bicyclo[2.2.1]hept-2-enes. We predict that transition metal intervention lowers the barrier and changes the stereoselectivity for such reactions. Moreover, the addition of electron-withdrawing groups to the migrating carbon was found to lower the barrier for oxidative addition, while incorporation of sterically hindered groups was found to increase the driving force for the reaction.
Introduction The high-temperature rearrangement of bicyclo[3.2.0]hept2-ene (1) to bicyclo[2.2.1]hept-2-ene (2, norbornene; Scheme 1; atom numbering shown corresponds to that for 1) is a classic reaction in the field of physical organic chemistry. The mechanism of this apparent [1,3] sigmatropic shift has long been discussed and debated, with the viability of concerted and stepwise mechanisms having been assessed through elegant experimental and theoretical studies by many top physical organic chemists.1-3 The current picture of this reaction (and many substituted versions thereof) is that it proceeds via structures with significant diradical character that reside on flat regions of the potential energy surface connecting reactant and product, making the reaction susceptible to significant dynamic effects.2,3 Given this long (and sometimes contentious) history, as well as our recent work on Ni(0)-N-heterocyclic carbene (NHC)4-catalyzed [1,3] sigmatropic shifts of simpler vinylcyclopropanes examined experimentally by Louie and
co-workers,5,6 we set out to examine how Ni(0)-NHC catalysts might perturb the pathway connecting 1 to 2, with an eye toward the rational design of a transition metal-promoted version of the reaction with a lowered barrier and a well-defined stereochemical course. Although much attention has been paid to the potential of transition metals (with associated ligands) to catalyze and control the regio- and sterochemical outcome of the cycloaddition and electrocyclization flavors of pericyclic reactions,7 there are comparatively few examples of transition metal-promoted sigmatropic shifts.5,6,8
Computational Methods All minima and transition-state structures were located using the B3LYP9 hybrid density functional method with the LANL2DZ10
*To whom correspondence should be addressed. E-mail tantillo@ chem.ucdavis.edu. (1) (a) Berson, J. A.; Holder, R. W. J. Am. Chem. Soc. 1967, 89, 5503– 5504. (b) Berson, J. A.; Salem, L. J. Am. Chem. Soc. 1972, 94, 8917–8918. (c) Berson, J. A. Acc. Chem. Res. 1972, 5, 406–414. (d) Berson, J. A.; Nelson, G. L. J. Am. Chem. Soc. 1970, 92, 1096–1097. (e) Bender, J. D.; Leber, P. A.; Lirio, R. R.; Smith, R. S. J. Org. Chem. 2002, 65, 5396–5402. (f) Woodward, R. B.; Hoffmann, R. The Conservation of Orbital Symmetry; Verlag Chemie: Weinbeim, 1970. (g) Fleming, I. Frontier Orbitals and Organic Chemical Reactions; John Wiley & Sons: New York, 1976. (h) Baldwin, J. E.; Belfield, K. D. J. Am. Chem. Soc. 1988, 110, 296–297. (i) Klarner, F.-G.; Drewes, R.; Hasselmann., D. J. Am. Chem. Soc. 1988, 110, 297–298. For a review, see: (j) Baldwin, J. E.; Leber, P. A. Org. Biomol. Chem. 2008, 6, 36–47. (2) (a) Beno, B. R.; Wilsey, S.; Houk, K. N. J. Am. Chem. Soc. 1999, 121, 4816–4826. (b) Wilsey, S. L.; Houk, K. N.; Zewail, A. H. J. Am. Chem. Soc. 1999, 121, 5772–5786. (c) Suhrada, C. P.; Selcuki, C.; Nendel, M.; Cannizzaro, C.; Houk, K. N.; Rissing, P.-J.; Baumann, D.; Hasselmann, D. Angew. Chem., Int. Ed. 2005, 44, 3548–3552. (3) (a) Carpenter, B. K. J. Am. Chem. Soc. 1995, 117, 6336–6364. (b) Carpenter, B. K. J. Am. Chem. Soc. 1996, 118, 10329–10330. (c) Carpenter, B. K. Angew. Chem., Int. Ed. 1998, 37, 3340–3350. (4) N-Heterocyclic Carbenes in Synthesis; Nolan, S. P., Ed.; WileyVCH: Weinheim, 2006.
(5) Wang, S. C.; Troast, D. M.; Conda-Sheridan, M.; Zuo, G.; LaGarde, D.; Louie, J.; Tantillo, D. J. J. Org. Chem. 2009, 74, 7822– 7833. (6) Zou, G.; Louie, J. Angew. Chem., Int. Ed. 2004, 46, 2227–2229. (7) For recent examples and reviews, see: (a) Jiao, L.; Lin, M.; Yu, Z.-X. Chem. Commun. 2010, 46, 1059–1061. (b) Wender, P. A.; Gamber, G. G.; Hubbard, R. D.; Pham, S. M.; Zhang, L. J. Am. Chem. Soc. 2005, 127, 2836–2837. (c) Wender, P. A.; Christy, J. P.; Lesser, A. B.; Gieseler, M. T. Angew. Chem., Int. Ed. 2009, 48, 7687–7690. (d) Furstner, A.; Stimson, C. C. Angew. Chem., Int. Ed. 2007, 46, 8845–8849. (e) Electrocyclic Reactions; Ansari, F. L.; Qureshi, R.; Qureshi, M. L. Wiley-VCH: Weinheim, 1999. (f) Lautens, M.; Klute, W.; Tam, W. Chem. Rev. 1996, 96, 49–92. (g) Rigby, J. H. Acc. Chem. Res. 1993, 26, 579–585. (h) Schore, N. E. Chem. Rev. 1988, 88, 1081–1119. (8) Selected examples with leading references: (a) Siebert, M. R.; Tantillo, D. J. J. Am. Chem. Soc. 2007, 129, 8686–8687. (b) Fanning, K. N.; Jamieson, A. G.; Sutherland, A. Curr. Org. Chem. 2006, 10, 1007– 1020. (c) Overman, L. E. Angew. Chem., Int. Ed. Engl. 1984, 23, 579–586. (d) Renaldo, A. F.; Overman, L. E. J. Am. Chem. Soc. 1990, 112, 3945– 3949. (e) Murakami, M.; Nishida, S. Chem. Lett. 1979, 927-930. (f) Ryu, I.; Ikura, K.; Tamura, Y.; Maenaka, J.; Ogawa, A.; Somoda, N. Synlett 1994, 941–942. (g) For a mini-review on metal-promoted [1,3] shifts of vinylcylopropanes, see: Wang, S. C.; Tantillo, D. J. J. Organomet. Chem. 2006, 691, 4386–4392. (9) (a) Becke, A. D. J. Chem. Phys. 1993, 98, 1372–1377. (b) Becke, A. D. J. Chem. Phys. 1993, 98, 5648–5652. (c) Lee, C.; Yang, W.; Parr, R. G. Phys. Rev. B 1988, 37, 785–789. (d) Stephens, P. J.; Devlin, F. J.; Chabalowski, C. F.; Frisch, M. J. J. Phys. Chem. 1994, 98, 11623–11627. (10) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 270–283.
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Published on Web 07/20/2010
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Scheme 1
basis set implemented in GAUSSIAN03.11 This methodology was used previously to model a variety of transition metalpromoted reactions,12 including Ni(0)-promoted [1,3] sigmatropic shifts.5 Transition-state structures and minima were characterized with frequency calculations, and intrinsic reaction coordinates (IRC)13 were computed to connect the transitionstate structures with associated minima. In some cases, split basis sets (LANL2DZ for Ni, 6-31G(d) for other atoms) were also used; see text for details. Free energies were calculated at 298 K.
Results and Discussion Successful Ni(0)-promoted [1,3] sigmatropic rearrangements of vinylcyclopropanes to cyclopentenes have been reported by several groups.5,6,8 Although the mechanism and facility of these rearrangements appear to be dependent on the nature of the metal-bound ligands and the substrates, we found the rearrangement of bicyclo[3.2.0]hept-2-ene (1) to bicyclo[2.2.1]hept-2-ene (2) in the presence of Ni(0)-NHC (for our modeling, dimethylimidazolylidene was used, as in ref 5) to proceed in a fashion analogous to that of the Ni(0)-NHCpromoted [1,3] sigmatropic shifts of vinylcyclopropanes to cyclopentenes (Figure 1).5 On the basis of previous work,5,6 the catalyst resting state is assumed to be a Ni(0)-NHC(COD) species (Figure 1). Exchange of the COD ligand with alkene 1 would form catalytically competent η2-alkene complex A1.14 In the analogous rearrangement of vinylcyclopropane, oxidative addition (11) Frisch, M. J.; et al. Gaussian 03, revision D.01; Gaussian, Inc.: Pittsburgh, PA, 2003 (full reference in Supporting Information). (12) (a) Tantillo, D. J.; Hoffmann, R. Helv. Chim. Acta 2001, 84, 1396– 1404. (b) Tantillo, D. J.; Hoffmann, R. J. Am. Chem. Soc. 2001, 123, 9855–9859. (c) Tantillo, D. J.; Carpenter, B. K.; Hoffmann, R. Organometallics 2001, 20, 4562–4564. (d) Tantillo, D. J.; Hietbrink, B. N.; Merlic, C. A.; Houk, K. N. J. Am. Chem. Soc. 2000, 122, 7136–7137. (Additional note: J. Am. Chem. Soc. 2001, 123, 5851). (e) Merlic, C. A.; Walsh, J. C.; Tantillo, D. J.; Houk, K. N. J. Am. Chem. Soc. 1999, 121, 3596–3606. (f) Merlic, C. A.; Hietbrink, B. N.; Houk, K. N. J. Org. Chem. 2001, 66, 6738–6744. (g) Merlic, C. A.; Miller, M. M.; Hietbrink, B. N.; Houk, K. N. J. Am. Chem. Soc. 2001, 123, 4904–4918. (h) Hietbrink, B. N. Ph.D. Thesis, University of California, Los Angeles, 2000. (13) (a) Gonzalez, C.; Schlegel, H. B. J. Phys. Chem. 1990, 94, 5523– 5527. (b) Fukui, K. Acc. Chem. Res. 1981, 14, 363–368. (14) This ligand exchange reaction was predicted to be endergonic by 3.4 kcal/mol (Ni(0)-NHC(COD) þ 1 f A1 þ COD). Calculations using the 16-electron Ni(0)(NHC)2-1 complex gave an energy barrier for the initial oxidative addition step that was 11.8 kcal/mol larger than that found when using A1. Also, replacement of methyl groups in NHC with phenyl groups led to similar results. See Supporting Information for additional details.
Figure 1. Proposed catalytic cycle for the Ni(0)-NHC-promoted [1,3] sigmatropic rearrangement of bicyclo[3.2.0]hept-2-ene (1) to bicyclo[2.2.1]hept-2-ene (2).
leads to a metallacyclobutane intermediate that rearranges to an η3-allyl, η1-alkyl intermediate with a very low barrier.5 However, oxidative addition for A1 (via TS-A1) leads directly to the η3-allyl, η1-alkyl intermediate B1 with an energy barrier of approximately 30 kcal/mol (Figure 2). The 16-electron intermediate B1 then rearranges to T-shaped metallacycle C1 via TS-B1. Intermediate C1 then undergoes reductive elimination via TS-C1 to form σ-complex D1, which is 16.5 kcal/mol higher in energy than reactant complex A1. As for the previously described vinylcyclopropane rearrangement, all three transition-state structures have similar energies.5 The conversion of A1 to D1 is endergonic, but this is primarily a result of the fact that A1 is a π-complex while D1 is a σ-complex. Dissociation of the norbornene product and re-formation of A1 (which may or may not be a stepwise process) would start the next catalytic cycle. The rearrangement of 1 to 2 is slightly exergonic (-4.5 kcal/mol), presumably a result of strain release, but this strain release is not as great as in the vinylcyclopropane-to-cyclopentene rearrangement.5 The overall barrier for the Ni-NHC-promoted rearrangement is 29.9 kcal/ mol, which is approximately 15 kcal/mol lower in energy than that for the uncatalyzed rearrangement.2,15 Moreover, the rearrangement leads to complete suprafacial/suprafacial migration, which would be orbital symmetry forbidden1f for a concerted metal-free rearrangement and which is the minor pathway for the dynamically controlled diradicaloid rearrangement that appears to occur under thermal conditions.1-3 Although the magnitude of the barrier-lowering provided by transition metal intervention in this case is impressive, the overall barrier is still rather large, significantly more so than those found for the rearrangements of simple (15) Cocks, A. T.; Frey, H. M. J. Chem. Soc. A 1971, 2564–2566.
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Figure 2. Computed structures (selected distances in A˚) and relative energies (kcal/mol) for species involved in the Ni(0)-NHCcatalyzed [1,3] sigmatropic shift of [3.2.0]hept-2-ene (1) to norbornene (2). Energies in normal text are free energies computed using B3LYP/LANL2DZ, and energies in italics are free energies computed using B3LYP with the LANL2DZ basis set for Ni and the 6-31G(d) basis set for all other atoms. Metal coordination to the convex face of 1 leads to a more stable complex (by 1.0 kcal/mol) that would need to rearrange to A1 in order to undergo the rearrangement (see Supporting Information).
Figure 3. Reductive elimination transition-state structures and lowest energy minima preceding them (whether or not these minima immediately precede the transition-state structures shown) for entries 4 and 7 of Table 1. Selected distances are shown in angstroms.
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Table 1. Computed Energetics (B3LYP/LANL2DZ) for the Ni(0)-NHC-Promoted [1,3] Sigmatropic Rearrangements of Substituted Bicyclo[3.2.0]hept-2-enes to Bicyclo[2.2.1]hept-2-enesa
system
R1
R2
R3
TS-A
TS-B
TS-C
ΔG‡(free energy span)
ΔGreaction
1 2 3 4 5 6 7 8
H H H H H H H CH3
H H CH3 NO2 F OH CN CN
H CH3 H H H H H H
29.9 37.3 31.7 19.6 30.6 30.5 24.8 24.0
24.9 26.0 28.5 0.7 26.1 21.3 22.4 20.1
26.0 30.2 31.2 18.3 30.6 28.2 26.3 22.5
29.9 37.3 31.7 36.4 30.6 30.5 29.6 28.2
-4.5 -3.5 -3.1 -2.9 -3.4 -2.7 -3.3 -6.7
a All reported free energies (kcal/mol) are in the gas phase. Energies for transition-state structures are relative to the energy of A for each case. The free energy span is the difference in energy between the highest energy transition-state structure and the lowest energy minimum preceding it.16 ΔGreaction is the difference in energy between free reactant and free product.
Figure 4. Ni(0)-NHC-catalyzed [1,3] sigmatropic shift of 5-methyl-7-cyanobicyclo[3.2.0]hept-2-ene to 1-methyl-4-cyanonorbornene (entry 8 of Table 1). Gas phase free energies (kcal/mol) were computed at the B3LYP/LANL2DZ level.
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vinylcyclopropanes.5 Consequently, we examined a variety of substituted systems with the hope that the influence of electronic and steric effects might lead to more tractable barriers (Table 1). It has been shown that introducing groups at the endo-position of the migrating carbon (i.e., R3) can lead to the reversal of the stereochemical outcome in the thermal reaction.1a,d,e This reversal has been rationalized in terms of “subjacent” orbital interactions by Berson1c and in terms of dynamic effects by Carpenter.3 We found that for the Ni(0)-promoted case, introducing steric bulk at the migrating carbon by adding a methyl group facing the bound Ni(0)-NHC (i.e., at position R3; system 2) increased the barrier for the oxidative addition step (TS-A) by 7 kcal/mol over that of the parent system (system 1 vs system 2). This increase is likely due to the fact that the methyl group discourages the Ni atom from approaching the migrating carbon; the Ni 3 3 3 C7 distance (see Scheme 1 for atom numbers) is 2.50 A˚ here versus 2.31 A˚ in TS-A1 for the parent system (Figure 2). However, when the methyl group is added further away from the Ni(0)-NHC group (i.e., at position R2; system 3), the oxidative addition barrier is not nearly as high. Electron-withdrawing groups near the migrating carbon might be expected to facilitate the buildup of charge on this carbon as the cyclobutane ring opens and hence aid in lowering the barrier for oxidative addition. Indeed, this was observed for R2 = NO2 and CN (systems 4 and 7), which had barriers for oxidative addition of 19.6 and 24.8 kcal/mol, respectively. However, the introduction of electron-withdrawing groups led to the formation of low-energy intermediates stabilized either by direct Ni-substituent interactions (e.g., C4, Figure 3) or by withdrawal of charge density from the migrating carbon (leading to longer Ni 3 3 3 C7 distances, e.g., B7, Figure 3). Consequently, the free energy span (energy difference between the highest transition-state structure and lowest minimum preceding it in the catalytic cycle)16 is not lowered (Table 1; compare systems 4 and 7 to system 1).17 Interestingly, the addition of the electron-donating hydroxy group (16) (a) Kozuch, S.; Shaik, S. J. Am. Chem. Soc. 2006, 128, 3355– 3365. (b) Amatore, C.; Jutand, A. J. Organomet. Chem. 1999, 576, 254. (c) Kozuch, S.; Lee, S. E.; Shaik, S. Organometallics 2009, 28, 1303–1308. (d) Kozuch, S.; Shaik, S. J. Phys. Chem. A 2008, 112, 6032–6041. (e) Yu, Z.; Cheong, P. H.; Liu, P.; Legault, C. Y.; Wender, P. A.; Houk, K. N. J. Am. Chem. Soc. 2008, 130, 2378–2379. (17) See Supporting Information for full energy surface diagrams.
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(system 6) had very little effect on the energetics of the catalytic cycle.17 The combination of an electron-withdrawing group (R2 = CN) and a methyl group (here, at the R1 position; entry 8) provided the lowest free energy span, 28.2 kcal/ mol, and the greatest overall driving force, -6.7 kcal/mol (Figure 4). Unfortunately, this free energy span is still substantial.
Conclusions Density functional theory calculations were used to explore the feasibility of the as-yet unexplored Ni(0)-N-heterocyclic carbene-promoted [1,3] sigmatropic shifts of bicyclo[3.2.0]hep2-enes. A mechanism similar to that described previously for the Ni(0)-N-heterocyclic carbene-promoted [1,3] sigmatropic shifts of vinylcyclopropanes was found. Although oxidative addition barriers were found to be somewhat sensitive to substituents on the hydrocarbon framework, low overall barriers were not found for any of the systems explored herein. Nonetheless, the effect of transition metal intervention5,8a,g on these reactions is noteworthy; not only does it reduce the barrier for rearrangement by approximately 15 kcal/mol, it also allows for a sense of stereocontrol that would not be allowed for the metal-free orbital symmetry-controlled process and for a degree of stereocontrol that is difficult to achieve for the metal-free dynamically controlled diradical process. We continue to explore the various means by which transition metals may disturb and direct the course of sigmatropic rearrangements.
Acknowledgment. We are grateful to the University of California-Davis, the American Chemical Society’s Petroleum Research Fund, the National Science Foundation’s CAREER and Partnership for the Advanced Computational Infrastructure (PSC) programs, and the office of Sen. Cedillo and Robert M. Husing for support. We thank Jason Harrison and Claire Knudson for helpful suggestions. Supporting Information Available: Energy surfaces for all catalytic systems presented, energies and coordinates for all minima and transition-state structures, IRC plots, and full ref 11. This material is available free of charge via the Internet at http://pubs.acs.org.
