ARTICLE pubs.acs.org/Organometallics
Does Gold as a Substituent Accelerate [3,3] Sigmatropic Shifts? Young J. Hong and Dean J. Tantillo* Department of Chemistry, University of California, Davis, One Shields Avenue, Davis, California 95616, United States
bS Supporting Information ABSTRACT: Quantum chemical calculations (B3LYP/6-31+G(d,p)-LANL2DZ) were used to assess whether AuL groups attached directly to the π-bonds of 1,5-hexadienes and heterosubstituted analogues lead to a rate acceleration for [3,3] sigmatropic shifts. It is predicted that, in general, they do not, although Au-based species are shown to play an important role in producing reactive 1,5-hexadienes (containing heteroatoms) in the complex rearrangements described recently by Istrate and Gagosz.
A
wide variety of substituents have been shown to accelerate concerted [3,3] sigmatropic shifts, and certain substituents have been shown to cause stepwise mechanisms to prevail.1 π-Complexation of transition-state structures for [3,3] sigmatropic shifts by Pd(II)Ln fragments has also been shown to lead to substantial rate acceleration and to modulate the balance between concerted and stepwise rearrangement pathways.2 Herein, we examine, using quantum chemical calculations,3 whether AuL groups attached directly to 1,5-hexadienes and heterosubstituted analogues can also lead to rate acceleration. Such reactions have been described by Istrate and Gagosz as key steps in multistep Au(I)-catalyzed rearrangements that form furans4 and pyrroles5 (Scheme 1). Although the Au(I) catalyst is clearly essential for these transformations, it is not clear whether it affects the barrier for the [3,3] sigmatropic shift step in each mechanism. Houk, Toste, and co-workers have shown that AuPR3 substituents have negligible effects on barriers for concerted ene reactions of alleneynes (Scheme 2),6 whereas several groups have shown that AuPR3 groups can accelerate pentadienyl cation electrocyclizations.7 The degree to which [3,3] sigmatropic shifts are sensitive to the presence of AuL substituents remains an unresolved issue. Model systems based on the experimental systems shown in Scheme 1 that were examined herein are shown in Figure 1. These include unsubstituted 1,5-hexadiene (1a), vinylallylamine (1b), vinylallylether (1d), protonated versions of the latter two (1c and 1e), 1- and 2-AuL-substituted versions of each (2a 2e and 3a 3e), and a variety of systems closer in structure to the experimental systems shown in Scheme 1 (4c 10c and 11e 14e). For the small model systems, PH3 was used as a model phosphine ligand (L),6 but for the larger systems, PPh3 was also examined. Mesyl groups were used in place of tosyl groups throughout. No evidence for stepwise mechanisms for [3,3] rearrangements was found.3 Computed barriers and reaction energies for [3,3] shifts of systems 1 3 are shown in Table 1 (using conformers of reactants and products that are productive for [3,3] rearrangement). For these small systems, only chairlike transition-state structures were examined.8 Barriers for rearrangements of 1a 1e are consistent with those reported previously (based on both experiment and quantum chemical calculations).9 Although N-substitution increases r 2011 American Chemical Society
Scheme 1
the exothermicity and exergonicity of the rearrangement, it has little effect on the barrier (compare 1a and 1b). N-Protonation further increases the exothermicity and exergonicity and also lowers the barrier considerably (compare 1b and 1c). O-Substitution, both Received: July 23, 2011 Published: October 21, 2011 5825
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Organometallics Scheme 2
Figure 1. Model rearrangement reactions.
without and with protonation, has a more dramatic effect than does N-substitution (compare 1d/1e and 1b/1c). Replacement of vinyl hydrogens with AuPH3 groups, at either a terminal or an internal position, had only small effects on the energetics of these reactions (see 2a 2e and 3a 3e); in all but the systems with OH+ groups, changes to rearrangement barriers were less than 2.5 kcal/mol. Exothermicities/exergonicities were affected more by inclusion of AuPH3 groups, especially for NH2+- and OH+-containing systems. In systems 2c and 2e, the iminium and oxonium substructures formed as the rearrangements proceed appear to be stabilized by hyperconjugation with the C Au bond; note changes to the C N and C O distances upon inclusion of Au, as shown in Tables 2 and 3 and the C Au distances and C C Au and C Au P angles shown in Table 3, which lengthen, decrease, and deviate more from 180°, respectively, as the hyperconjugative interactions increase and tend toward bridging.10 In systems 3c and 3e, donation from a Au lone pair appears to have a similar effect; again, C N and C O distances in iminium and oxonium substructures increase upon
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Table 1. Computed Reaction Energies (in kcal/mol; 1a 1e, B3LYP/6-31+G(d,p); 2a 2e and 3a 3e, B3LYP/6-31+G(d,p)LANL2DZ) of Noncatalyzed and AuL-Catalyzed Rearrangement Reactionsa
reaction
X
ΔE‡
1a
CH2
+34.00
0.00
+35.75
0.00
0.00
1b
NH
+34.66
6.97
+35.81
7.34
0.22
1c
NH2+
+13.60
14.63
+14.82
14.67
0.41
1d 1e
O OH+
+26.38 +1.36
17.64 19.55
+28.06 +2.05
17.54 19.00
0.42 1.26
2a
CH2
+35.49
+1.70
+37.62
+2.18
+0.03
2b
NH
+34.21
7.54
+35.71
7.24
0.21
2c
NH2+
+14.80
24.20
+16.38
23.74
0.42
2d
O
+25.51
20.30
+28.88
18.07
0.44
2e
OH+
+4.61
39.05
+5.99
38.65
1.11
3a
CH2
+33.93
0.00
+34.96
0.00
0.00
3b 3c
NH NH2+
+33.01 +15.71
10.30 22.79
+34.49 +17.27
8.89 24.81
0.24 0.44
3d
O
+24.92
22.96
+26.01
23.62
0.45
3e
OH+
+5.48
31.02
+6.79
31.08
1.02
ΔEreaction
ΔG‡
ΔGreaction
ΔR
a ΔE‡ and ΔG‡ are the electronic energy and the free energy of the transition-state structure relative to those of a reactant for each case, respectively. ΔEreaction and ΔGreaction are the difference in the electronic energy and the free energy between reactant and product, respectively. ΔR is the difference between the breaking bond distance (r1) and the forming bond distance (r2) in the transition-state structure (+: r1 > r2; : r1 < r2).
inclusion of Au (see Tables 2 and 4). The donating ability of a AuPR3 group also appears to be at the heart of its ability to accelerate pentadienyl cation electrocyclizations.7 Note also that the synchronicity of the [3,3] rearrangements, as indicated by the difference in the lengths of the breaking and forming σ-bonds (ΔR in Table 1), is not significantly altered upon inclusion of AuPH3 groups.11,12 Inclusion of solvent (by way of continuum calculations; see the Supporting Information for details) led to predictable changes to barriers and exothermicities/exergonicities. The largest effects were predicted for cationic systems, where barriers increased significantly as solvent polarity was increased; as expected, polar solvent appears to interact more strongly with reactants and products than in the transition-state structures for their interconversion, since charge is more localized in the former (see the Supporting Information for data on all systems). Results for our larger model systems, 4c 10c and 11e 14e, are shown in Tables 5 and 6, Tables S9 S12 in the Supporting Information, and Figures 2 and 3. For the smallest of these, 4c 5c 5826
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Organometallics
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Table 2. Computed Structures (Distances in Angstroms, B3LYP/6-31+G(d,p))
and 11e, activation barriers that are very similar (slightly smaller) than those for the simpler systems (see Table 1) are observed and reactions of the larger systems are more exothermic/exergonic, perhaps a consequence of increasing conjugation between the incipient iminium/oxonium group and the endocyclic CdC π-bond (not present in the smaller systems) as the reactions proceed. Including a sulfonate substituent in the ammonium systems (6c 10c) makes the ammonium group more electrondeficient and lowers the activation barriers by several kcal/mol.
Having an alkyl group at position R2/R (7c 10c and 12e 14e) has a negligible effect. Using PPh3 in place of PH3 (8c, 10c, 13e) also has no significant effect (at least on the [3,3] shift portions of the reaction cascades); this group is rather distant from the region where the [3,3] shift occurs (see Tables S10 and S12, Supporting Information). Finally, having a Z rather than E vinylgold species (9c, 10c, and 14e) does not change the [3,3] shift barrier, although both the reactants and the transition-state structures for the Z-configured systems are 5827
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Table 3. Computed Structures (Distances in Angstroms, B3LYP/6-31+G(d,p)-LANL2DZ)
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Table 4. Computed Structures (Distances in Angstroms, B3LYP/6-31+G(d,p)-LANL2DZ)
a
The product (3b) has a small imaginary frequency ( 17.49 cm 1) associated with PH3 group rotation.
considerably higher in energy than those for the corresponding E-configured systems. All of the computed barriers are
consistent with the experimental results shown in Schemes 1 and 2. 5829
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Table 5. Computed Reaction Energies (in kcal/mol, B3LYP/ 6-31 +G(d,p)-LANL2DZ) of AuL-Catalyzed Rearrangement Reactions of 4c 10ca
reaction
R1
R2
4c 5c 6cchair‑TS 6cboat‑TS 7c 8c 9cb 10cc
H Me SO2Me SO2Me SO2Me SO2Me SO2Me SO2Me
H H H H Me Me Me Me
L
ΔE‡
ΔEreaction
PH3 +13.38 PH3 +13.18 PH3 +9.29 PH3 +14.48 PH3 +9.62 PPh3 +9.52 PH3 +9.80 PPh3 +9.71
27.62 30.76 30.52 27.13 28.85 32.60 22.07 32.45
ΔG‡ +15.30 +14.42 +10.14 +15.67 +10.83 +11.23 +10.76 +10.53
ΔGreaction 28.08 31.14 31.04 27.07 28.85 32.21 23.39 33.45
ΔR 0.42 0.40 0.59 0.69 0.57 0.59 0.67 0.68
a ΔE‡ and ΔG‡ are the ZPE-corrected electronic energy and the free energy of the transition-state structure relative to that of the reactant for each case, respectively. ΔEreaction and ΔGreaction are the difference in the ZPE-corrected electronic energy and the free energy between reactant and product. Positive and negative values in ΔEreaction and ΔGreaction indicate an endothermic and an exothermic reaction, respectively. ΔR is the difference between the breaking bond distance (r1) and the forming bond distance (r2) in the transition-state structure (+: r1 > r2; : r1 < r2). b ΔE‡ and ΔG‡ relative to that of the reactant for 7c are 12.97 and 14.90 kcal/mol, respectively. c ΔE‡ and ΔG‡ relative to that of the reactant for 8c are 12.17 and 14.07 kcal/mol, respectively.
Figure 2. AuPH3-catalyzed cyclization and rearrangement reaction for system 7c. Computed structures (distances in Å) and energies (in kcal/ mol, electronic energies in normal text and free energies in brackets) are shown; B3LYP/6-31+G(d,p)-LANL2DZ.
Table 6. Computed Reaction Energies (in kcal/mol, B3LYP/ 6-31+G(d,p)-LANL2DZ) of AuL-Catalyzed Rearrangement Reactions of 11e 14ea
reaction
R
L
ΔE‡
11echair‑TS 11eboat‑TS 12e 13e 14eb
H H Me Me Me
PH3 PH3 PH3 PPh3 PH3
+5.41 +6.37 +5.56 +5.38 +3.88
ΔEreaction 43.56 43.25 42.21 46.52 47.01
ΔG‡ +7.10 +7.26 +7.17 +6.45 +4.67
ΔGreaction 42.97 42.68 41.27 47.67 47.49
ΔR 1.07 1.11 1.04 1.03 1.18
a ΔE‡ and ΔG‡ are the ZPE-corrected electronic energy and the free energy of the transition state structure relative to that of the reactant for each case, respectively. ΔEreaction and ΔGreaction are the difference in the ZPE-corrected electronic energy and the free energy between reactant and product. Positive and negative values in ΔEreaction and ΔGreaction indicate an endothermic and an exothermic reaction, respectively. ΔR is the difference between the breaking bond distance (r1) and the forming bond distance (r2) in the transition-state structure (+: r1 > r2; : r1 < r2). b ΔE‡ and ΔG‡ relative to that of the reactant for 12e are 7.11 and 8.65 kcal/mol, respectively.
Figure 3. AuPH3-catalyzed cyclization and rearrangement reaction for system 9c. Computed structures (distances in Å) and energies (in kcal/ mol, relative to that of A1 (Figure 2), electronic energies in normal text and free energies in brackets) are shown; B3LYP/6-31+G(d,p)LANL2DZ.
Since the computed barriers for [3,3] shifts are consistently low, even without Au substituents, it seems likely that the main 5830
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Organometallics role of Au in promoting the rearrangements shown in Scheme 1 is to facilitate cyclization to form the 1,5-hexadienes that subsequently rearrange rapidly. To confirm this, cyclization barriers for two representative systems, 7c and 9c, were computed. As shown in Figures 2 and 3, cyclization barriers are indeed predicted to be quite low and transition-state structures for the [3,3] shift steps are lower in energy than those for cyclization. Overall, we conclude that gold-phosphine groups as substituents have only small effects on barriers for [3,3] shifts. They do, however, have significant effects on exothermicities/exergonicities when cationic 1,5-hexadienes are involved.
’ ASSOCIATED CONTENT
bS
Supporting Information. Additional details on computations, including energies and coordinates for all minima and transition-state structures, IRC plots, and full Gaussian reference. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ ACKNOWLEDGMENT We gratefully acknowledge the American Chemical Society’s Petroleum Research Fund and the National Science Foundation’s Partnership for Advanced Computational Infrastructure (Pittsburgh Supercomputer Center) for support. ’ REFERENCES
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studies on pentadienyl cation electrocyclizations, see: Davis, R. L.; Tantillo, D. J. Curr. Org. Chem. 2010, 14, 1561–1577. (8) For system 2a, a boatlike transition-state structure was also examined. The reaction via this boatlike transition-state structure has a higher barrier (by 6.55 kcal/mol in electronic energy and 5.88 kcal/mol in free energy; ΔEbarrier = 42.04, ΔErxn = 1.63, ΔGbarrier = 43.51, ΔGrxn = 2.00). See the Supporting Information for geometries. (9) (a) Cope, A. C.; Hardy, E. M. J. Am. Chem. Soc. 1940, 62, 441–444. (b) For a recent review on the Cope rearrangements, see: Graulich, N. Wiley Interdiscip. Rev.: Comput. Mol. Sci. 2011, 1, 172–190. (c) Quantum chemical calculations on 1b and 1d were performed previously (reported barriers for 1b and 1d at the B3LYP/6-31G(d) level: 32.6 and 27.4 kcal/mol, respectively); see: Yamabe, S.; Okumoto, S.; Hayashi, T. J. Org. Chem. 1996, 61, 6218–6226. (d) Kinetic data for 1d were reported (ΔH‡ = 25.40 ( 0.65 kcal/mol and ΔS‡ = 15.9 ( 1.5 kcal/mol for 1d) in: Burrows, C. J.; Carpenter, B. K. J. Am. Chem. Soc. 1981, 103, 6983–6984. (e) Gajewski, J. J.; Jurayj, J.; Kimbrough, D. R.; Gande, M. E.; Ganem, B.; Carpenter, B. K. J. Am. Chem. Soc. 1987, 109, 1170–1186. (f) Vance, R. L.; Rondan, N. G.; Houk, K. N.; Jensen, F.; Borden, W. T.; Komornicki, A.; Wimmer, E. J. Am. Chem. Soc. 1988, 110, 2314–2315. (g) For kinetic isotope effects for rearrangement of 1d, see: Yoo, H. Y.; Houk, K. N. J. Am. Chem. Soc. 1994, 116, 12047–12048. (10) Recent reviews: (a) Alabugin, I. V.; Gilmore, K. M.; Peterson, P. W. Wiley Interdiscip. Rev.: Comput. Mol. Sci. 2011, 1, 109–141. (b) Tantillo, D. J. Chem. Soc. Rev. 2010, 39, 2847–2854. (11) Data on systems with AuL substituents attached to the other πbond of the 1,5-hexadiene unit (not yet explored experimentally, to our knowledge) can be found in the Supporting Information. For cationic systems with AuL substituents in these positions, somewhat lower barriers for [3,3] sigmatropic shifts were predicted; the corresponding reactions were also predicted to be less exothermic/exergonic. (12) Data on systems with N-heterocyclic carbene, rather than phosphine, ligands can be found in the Supporting Information. Only relatively small changes to predicted barriers and exothermicities for [3,3] sigmatropic shifts were observed upon changing the ligand on Au.
(1) For leading references, see: (a) Dewar, M. J. S.; Jie, C. Acc. Chem. Res. 1992, 25, 537–543. (b) Houk, K. N.; Gonzalez, J.; Li, Y. Acc. Chem. Res. 1995, 28, 81–90. (c) Doering, W. v. E.; Wang, Y. J. Am. Chem. Soc. 1999, 121, 10112–10118. (2) (a) Overman, L. E.; Renaldo, A. E. J. Am. Chem. Soc. 1990, 112, 3945–3949. (b) Siebert, M. R.; Tantillo, D. J. J. Am. Chem. Soc. 2007, 129, 8686–8687. (3) All calculations were performed with Gaussian 03. All geometries and energies reported herein are from the B3LYP method with a split basis set (LANL2DZ for gold and 6-31+G(d,p) for other atoms). B3LYP has been used to study a variety of [3,3] sigmatropic shifts; for leading references, see:1 (a) Wiest, O.; Montiel, D. C.; Houk, K. N. J. Phys. Chem. A 1997, 1101, 8378–8388. (b) Guner, V. A.; Khuong, K. S.; Houk, K. N.; Chuma, A.; Pulay, P. J. Phys. Chem. A 2004, 108, 2959–2965. The B3LYP method was also used previously to study a variety of transitionmetal-promoted reactions; for leading references, see:2b (c) Tantillo, D. J.; Hoffmann, R. J. Am. Chem. Soc. 2001, 123, 9855–9859. (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) Reported energies (in kcal/mol) are electronic energies plus unscaled zero-point energy corrections from frequency calculations at the level used for geometry optimization and free energies at 25 °C. See the Supporting Information for additional computational details and references, including the full Gaussian 03 reference. (4) (a) Istrate, F. M.; Gagosz, F. J. Org. Chem. 2008, 73, 1180–1180. (b) Istrate, F. M.; Gagosz, F. Beilstein J. Org. Chem. 2011, 7, 878–885. (5) Istrate, F. M.; Gagosz, F. Org. Lett. 2007, 9, 3181–3184. (6) Cheong, P. H.-Y.; Morganelli, P.; Luzung, M. R.; Houk, K. N.; Toste, F. D. J. Am. Chem. Soc. 2008, 130, 4517–4526. (7) (a) Shi, F.-Q.; Li, X.; Xia, Y.; Zhang, L.; Yu, Z.-X. J. Am. Chem. Soc. 2007, 129, 15503–15512. (b) Shi, X.; Gorin, D. J.; Toste, F. D. J. Am. Chem. Soc. 2005, 127, 5802–5803. (c) For a recent review on theoretical 5831
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