Strain-promoted cycloaddition to cyclooctyne

cycloadditions &DFT &Diels-Alder Steven Bachrach 19 Feb 2018 No Comments

Click chemistry has been used in a broad range of applications. The use of metal catalysts has limited its application to biological system, but the development of strain-promoted cycloaddition to cyclooctyne has opened up click chemistry to bioorthogonal labeling.

An interesting variation on this is the use of 1,2-benzoquinone 1 and substituted analogues as the Diels-Alder diene component. Escorihuela and co-workers have reported on the use of this diene with a number of cyclooctyne derivatives, measuring kinetics and also using computations to assess the mechanism.1

Their computations focused on two reactions using cyclooctyne 2 and the cyclopropane-fused analogue 3:

Reaction 1

Reaction 2

They examined these reactions with a variety of density functionals along with some post-HF methods. The transition states of the two reactions are shown in Figure 1. A variety of different density functionals and MP2 are consistent in finding synchronous or nearly synchronous transition states.


Rxn1-TS


Rxn2-TS

Figure 1. B97D/6-311+G(d,p) transition states for Reactions 1 and 2.

In terms of activation energies, all of the DFT methods consistently overestimate the barrier by about 5-10 kcal mol-1, with B97D-D3 doing the best. MP2 drastically underestimates the barriers, though the SOS-MP2 or SCS-MP2 improve the estimate. Both CCSD(T) and MR-AQCC provide estimates of about 8.5 kcal mol-1, still 3-4 kcal mol-1 too high. The agreement between CCSD(T), a single reference method, and MR-AQCC, a multireference method, indicate that the transition states have little multireference character. Given the reasonable estimate of the barrier afforded by B97D-D3, and its tremendous performance advantage over SCS-MP2, CCSD(T) and MR-AQCC, this is the preferred method (at least with current technology) for examining Diels-Alder reactions like these, especially with larger molecules.

References

1) Escorihuela, J.; Das, A.; Looijen, W. J. E.; van Delft, F. L.; Aquino, A. J. A.; Lischka, H.; Zuilhof, H., "Kinetics of the Strain-Promoted Oxidation-Controlled Cycloalkyne-1,2-quinone Cycloaddition: Experimental and Theoretical Studies." J. Org. Chem. 2018, 83, 244-252, DOI: 10.1021/acs.joc.7b02614.

InChIs

1: InChI=1S/C6H4O2/c7-5-3-1-2-4-6(5)8/h1-4H
InChIKey=WOAHJDHKFWSLKE-UHFFFAOYSA-N

2: InChI=1S/C8H12/c1-2-4-6-8-7-5-3-1/h1-6H2
InChIKey=ZPWOOKQUDFIEIX-UHFFFAOYSA-N

3: InChI=1S/C9H12/c1-2-4-6-9-7-8(9)5-3-1/h8-9H,3-7H2
InChIKey=rQDNSAFCVPAMWCJ-UHFFFAOYSA-N

4: InChI=1S/C14H16O2/c15-13-11-7-8-12(14(13)16)10-6-4-2-1-3-5-9(10)11/h7-8,11-12H,1-6H2
InChIKey=OQMYZEFKUMPECV-UHFFFAOYSA-N

5: InChI=1S/C15H16O2/c16-14-12-5-6-13(15(14)17)11-4-2-9-7-8(9)1-3-10(11)12/h5-6,8-9,12-13H,1-4,7H2/t8-,9+,12?,13?
InChIKey=NKDGTIVNLDJQKR-RFZWMSCOSA-N

New Procedure for computing NMR spectra with spin-spin coupling

Grimme &NMR Steven Bachrach 05 Feb 2018 No Comments

Computed NMR spectra have become a very useful tool in identifying chemical structures. I have blogged on this multiple times. A recent trend has been the development of computational procedures that lead to computed spectra (again, see that above link). Now, Grimme, Neese and coworkers have offered their approach to computed NMR spectra, including spin-spin splitting.1

Their procedure involves four distinct steps.

  1. Generation of the conformer and rotamer space. This is a critical distinctive element of their method in that they take a number of different tacks for sampling conformational space to insure that they have identified all low-energy structures. This involves a combination of normal mode following, genetic structure crossing (based on genetic algorithms for optimization), and molecular dynamics. Making this all work is their choice of using the computational efficient GFN-xTB2 quantum mechanical method.
  2. The low-energy structures are then subjected to re-optimization at PBEh-3c and then single-point energies obtained at DSD-BLYP-D3/def2-TZVPP including treatment of solvation by COSMO-RS. The low-energy structures that contribute 4% or more of the Boltzmann-weighted population are then carried forward.
  3. Chemical shifts and spin-spin coupling constants are then computed with the PBE0 method and the pcS and pcJ basis sets developed by Jensen for computing NMR shifts.3
  4. Lastly, the chemical shifts and coupling constants are averaged and the spin Hamiltonian is solved.

The paper provides a number of examples of the application of the methodology, all with quite good success. The computer codes to run this method are available for academic use from xtb@thch.uni-bonn.de.

References

1) Grimme, S.; Bannwarth, C.; Dohm, S.; Hansen, A.; Pisarek, J.; Pracht, P.; Seibert, J.; Neese, F., "Fully Automated Quantum-Chemistry-Based Computation of Spin–Spin-Coupled Nuclear Magnetic Resonance Spectra." Angew. Chem. Int. Ed. 2017, 56, 14763-14769, DOI: 10.1002/anie.201708266.

2) Grimme, S.; Bannwarth, C.; Shushkov, P., "A Robust and Accurate Tight-Binding Quantum Chemical Method for Structures, Vibrational Frequencies, and Noncovalent Interactions of Large Molecular Systems Parametrized for All spd-Block Elements (Z = 1–86)." J. Chem. Theory Comput. 2017, 13, 1989-2009, DOI: 10.1021/acs.jctc.7b00118.

3) Jensen, F., "Basis Set Convergence of Nuclear Magnetic Shielding Constants Calculated by Density Functional Methods." J. Chem. Theory Comput. 2008, 4, 719-727, DOI: 10.1021/ct800013z.

Isotope Controlled Selectivity

Borden &Isotope Effects &Schreiner &Tunneling Steven Bachrach 22 Jan 2018 No Comments

I seem to be recently flooded with papers dealing with tunneling in organic systems. Well, here’s one more! Kozuch, Borden, Schreiner and co-workers seek out systems whereby isotopic substitution might lead to reaction selectivity.1 Their base system is cyclopropylmethylcarbene 1, which can undergo three different reactions: (a) the ring can expand to give 1-methylcyclobut-1-ene 2, (b) a hydrogen can shift from the terminal methyl group to give vinylcyclopropane 3, or (c) the methane hydrogen can shift to produce ethylidenecyclopropane 4. This last option can be neglected since its barrier (20.5 kcal mol-1) is so much higher than for the other two, 7.5 kcal mol-1 for the ring expansion and 12.1 kcal mol-1 for the [1,2]H-shift converting 13.

At high temperature, the ring expansion to 2 will dominate, but at low temperature the hydrogen shift to 3 might dominate by tunneling through the barrier due to the low mass and short distances involved. The reaction rates were computed using B3LYP/6-31G(d,p) and small-curvature tunneling. At low temperature, the rate for the hydrogen shift is 10 orders of magnitude faster than the ring expansion. Thinking that deuterium substitution of the terminal methyl group might slow down the rate of the [1,2]-shift, they computed the rates for the reactions of 1-d3, and in fact the rate of this shift does reduce by 104 but it is still much faster than the rate for ring expansion. What is needed is a system where the rate for ring expansion is slower than the rate for hydrogen migration but faster than the rate of deuterium migration.

They examine a number of different substituents that may help to lower the barrier for the ring expansion. The methoxy derivative 5 turns out to suit the bill perfectly. The methoxy group reduces the barrier for ring expansion from 7.5 kcal mol-1 with 1 to 2.5 kcal mol-1 with 5. With hydrogenated 5, the [1,2]H-shift is 103 times faster than ring expansion, but with deuterated 5, ring expansion is twice as fast as the deuterium migration.

The authors call this isotope controlled selectivity (ICS), and this is the first example of this type of control.

References

1. Nandi, A.; Gerbig, D.; Schreiner, P. R.; Borden, W. T.; Kozuch, S., Isotope-Controlled Selectivity by Quantum Tunneling: Hydrogen Migration versus Ring Expansion in Cyclopropylmethylcarbenes. J. Am. Chem. Soc. 2017, 139, 9097-9099, DOI: 10.1021/jacs.7b04593.

InChIs

1: InChI=1S/C5H8/c1-2-5-3-4-5/h5H,3-4H2,1H3
InChIKey=KJIJNBZLGHBOTI-UHFFFAOYSA-N

2: InChI<=1S/C5H8/c1-5-3-2-4-5/h3H,2,4H2,1H3
InChIKey=AVPHQXWAMGTQPF-UHFFFAOYSA-N

3: InChI=1S/C5H8/c1-2-5-3-4-5/h2,5H,1,3-4H2
InChIKey=YIWFBNMYFYINAD-UHFFFAOYSA-N

4: InChI=1S/C5H8/c1-2-5-3-4-5/h2H,3-4H2,1H3
InChIKey=ZIFNDRXSSPCNID-UHFFFAOYSA-N

5: InChI=1S/C6H10O/c1-3-6(7-2)4-5-6/h4-5H2,1-2H3
InChIKey=YMBSTCICUAORNN-UHFFFAOYSA-N

6: InChI<=1S/C6H10O/c1-5-3-4-6(5)7-2/h3-4H2,1-2H3
InChIKey=QBLNAZHAVPMLHB-UHFFFAOYSA-N

7: InChI<=1S/C6H10O/c1-3-6(7-2)4-5-6/h3H,1,4-5H2,2H3
InChIKey=FHYLDABSPVPDTJ-UHFFFAOYSA-N

Antiaromatic compounds stabilized by benzenoid fusion

Aromaticity Steven Bachrach 08 Jan 2018 No Comments

Antiaromatic compounds by definition are unstable and so difficult to prepare. One approach to increase their stability is to fuse aromatic ring(s) onto the antiaromatic system. I discuss in this blog post two different scaffolds where this approach has been successful in preparing molecules that express some degree of antiaromaticity. In addition, I mention a technique to aid in evaluating the aromatic/antiaromatic character.

Pentalene 1 is a formal 8-π electron system and would be antiaromatic. To avoid this antiaromatic character, the double bonds are localized. Fusing benzenoid rings to pentalene to give dibenzo[a,e]penatalene 2 has been done, but the central rings avoid antiaromatic character by expressing the Kekule structure shown below.

Yasuda and coworkers report the preparation of mesityl-substituted dibenzo[a,f]penatalene
3.1 Resonance structures of 3¸ shown below, either have only one aromatic ring, or have two aromatic rings along with a trimethylenemethane (TMM) diradical component. Thus, one might expect 3 to express more antiaromatic character than 2.

NICS(1) values, computed at B3LYP/6-31G**, for 2 are -6.23 for the 6-member ring and +5.87 ppm for the 5-member ring, showing reduced aromaticity of the former ring. In sharp contrast, the NICS(1) values for 3 are +7.48 for the 6-member ring and +25.5 ppm for the 5-member ring, indicating substantial antiaromatic character for both rings. The calculated spin density distribution shows largest unpaired density on the expected carbon atoms based on the resonance structures involving the TMM fragment.

Xia and coworkers have prepared substituted analogues of the three structural isomers whereby three naphthylene units are fused together creating two cyclobutadienoid rings.2 These three frameworks are molecules 4-6. The 4-member rings are formally antiaromatic, tempered by the fused aromatic naphthylene groups. The question is then how does the different attachment geometry manifest in aromatic and/or antiaromatic character?

The computations take advantage of the NICS-XY method – well, a variation of this method. I had meant to write a post about the NICS-XY method when Stanger published it,3 but I just never got around to it. The idea is that NICS is evaluated typically at a single point, and just which point to use has been the subject of some discussion. Instead, Stanger proposes the NICS-XY method as a grid of points perpendicular to the plane of the molecule, typically in the plane bisecting the molecule. Trends in the values as one moves across the ring and perpendicular to the ring could assist in identifying aromatic/antiaromatic behavior.

Xia computed the NICSπZZ along a line in the molecular plane bisecting the rings. This is shown in the figure below, which I have reproduced from the article. For example, for 4, which is compound 1 in the Xia paper and the figure below, the NICS values are taken along the line that horizontally bisects the molecule. In ring A, the values are negative, indicative of an aromatic ring. Across ring B, the values are still negative, but not as negative as for ring A, indicating a diminished aromaticity. In ring C, the values are positive, as one would expect for the antiaromatic cyclobutadienoid ring.


Figure taken from J. Am. Chem. Soc. 2017, 139, 15933-15939.

The authors highlight two trends. First, in the linear fusion (see the inset above), the aromatic ring fused to the cyclobutadienoid ring expresses diminished aromaticity. This can be understood in the following way. In naphthalene, the C2-C3 bond is longer than the C1-C2 bond. When the cyclobutadienoid is fused at the C2-C3 bond, it can lengthen even more to weaken the antiaaromaticity of the 4-member ring, and this consequently reduces the aromaticity of the 6-member ring. Fusion of the cyclobutadienoid ring at C1-C2, the shorter bond, causes a higher degree of antiaromaticity in the 4-member ring. The lengthening of this C1-C2 bond to try to reduce the antiromaticity of the 4-member ring leads to greater bond equalization in the 6-member ring, and its consequently greater aromatic character.

References

1. Konishi, A.; Okada, Y.; Nakano, M.; Sugisaki, K.; Sato, K.; Takui, T.; Yasuda, M., "Synthesis and Characterization of Dibenzo[a,f]pentalene: Harmonization of the Antiaromatic and Singlet Biradical Character." J. Am. Chem. Soc. 2017, 139, 15284-15287, DOI: 10.1021/jacs.7b05709.

2. Jin, Z.; Teo, Y. C.; Teat, S. J.; Xia, Y., "Regioselective Synthesis of [3]Naphthylenes and Tuning of Their Antiaromaticity." J. Am. Chem. Soc. 2017, 139, 15933-15939, DOI: 10.1021/jacs.7b09222.

3. Gershoni-Poranne, R.; Stanger, A., "The NICS-XY-Scan: Identification of Local and Global Ring Currents in Multi-Ring Systems." Chem. Eur. J. 2014, 20, 5673-5688, DOI: 10.1002/chem.201304307.

InChIs

1: InChI=1S/C8H6/c1-3-7-5-2-6-8(7)4-1/h1-6H
InChIKey=GUVXZFRDPCKWEM-UHFFFAOYSA-N

2: InChI=1S/C16H10/c1-3-7-13-11(5-1)9-15-14-8-4-2-6-12(14)10-16(13)15/h1-10H
InChIKey=OZEPXROCWSMGGM-UHFFFAOYSA-N

3: InChI=1S/C16H10/c1-3-7-14-11(5-1)9-13-10-12-6-2-4-8-15(12)16(13)14/h1-10H
InChIKey=XOERMEAUYMRNNZ-UHFFFAOYSA-N

4: InChI=1S/C30H16/c1-2-6-18-10-24-23(9-17(18)5-1)27-13-21-15-29-25-11-19-7-3-4-8-20(19)12-26(25)30(29)16-22(21)14-28(24)27/h1-16H
InChIKey=CHDMCKMZQIHGAH-UHFFFAOYSA-N

5: InChI=1S/C30H16/c1-3-7-19-15-27-25(13-17(19)5-1)23-11-9-22-21(29(23)27)10-12-24-26-14-18-6-2-4-8-20(18)16-28(26)30(22)24/h1-16H
InChIKey=LPXGODOTGXTPRU-UHFFFAOYSA-N

6: InChI=1S/C30H16/c1-2-7-19-13-26-25(12-18(19)6-1)27-15-21-9-10-22-24-11-17-5-3-4-8-20(17)14-29(24)30(22)23(21)16-28(26)27/h1-16H
InChIKey=BKMGPFRQJXDFJQ-UHFFFAOYSA-N

azatriquinacene, a novel aromatic

Aromaticity Steven Bachrach 11 Dec 2017 No Comments

The range of aromatic compounds seems limitless. Mascal and co-workers have prepared the azatriquinacene 1 in a remarkably simple fashion.1 The molecule is a zwitterion, with the carbon atoms forming a 9-center, but 10 π-electron ring, and the quaternary nitrogen sitting above it. The carbon ring satisfies Hückel’s rule (4n+2) and so should be aromatic. The capping nitrogen should help to keep the carbon ring fixed in a shallow bowl.

As expected, the molecule in fact turns out to possess an aromatic 10 π-electron ring. The B3LYP/6-311++G(d,p) geometry is shown in Figure 1. There is little bond alternation among the C-C distances: the mean deviation is only 0.015 Å with the largest difference only 0.024 &Aring. The x-ray crystal structure shows the same trends. The NICS(1) value is -12.31 ppm, larger even than that of benzene (-10.22 ppm).

Figure 1. B3LYP/6-311++G(d,p) geometry of 1.

References

1) Hafezi, N.; Shewa, W. T.; Fettinger, J. C.; Mascal, M., "A Zwitterionic, 10 π Aromatic Hemisphere." Angew. Chem. Int. Ed. 2017, 56, 14141-14144, DOI: 10.1002/anie.201708521.

InChIs

1: InChI=1S/C10H9N/c1-11-8-2-3-9(11)6-7-10(11)5-4-8/h2-7H,1H3
InChIKey=ZXZPLDVSQUVKTH-UHFFFAOYSA-N

Heavy-atom tunneling

Tunneling Steven Bachrach 04 Dec 2017 1 Comment

Though recognized to occur in organic systems, the breadth of involvement of heavy-atom tunneling has not been established. Doubleday, Greer and coworkers have examined 13 simple organic reactions sampling pericyclic reactions, radical rearrangements and SN2 reactions for heavy-atom tunneling.1 A few of these reactions are shown below.

Reaction rates were obtained using the small curvature tunneling approximation (SCT), computed using Gaussrate. Reaction surfaces were computed at B3LYP/6-31G*. The tunneling correction to the rate was also estimated using the model developed by Bell: kBell = (u/2)/sin(u/2) where u = hν/RT and ν is the imaginary frequency associated with the transition state. The temperature was chosen so as to give a common rate constant of 3 x 10-5 s-1. Interestingly, all of the examined reactions exhibited significant tunneling even at temperatures from 270-350 K (See Table 1). The tunneling effect estimated by Bell’s equation is very similar to that of the more computationally demanding SCT computation.

Table 1. Tunneling contribution to the rate constant

Reaction

% tunneling

95

35

17

28

CN + CH3Cl → CH3CN + Cl (aqueous)

45

This study points towards a much broader range of reactions that may be subject to quantum mechanical tunneling than previously considered.

(Note: The original post had swapped some of the values in Table 1. These have now been corrected. My thanks to Drs. Greer and Doubleday for bringing this to my attention.)

References

1. Doubleday, C.; Armas, R.; Walker, D.; Cosgriff, C. V.; Greer, E. M., "Heavy-Atom Tunneling Calculations in Thirteen Organic Reactions: Tunneling Contributions are Substantial, and Bell’s Formula Closely Approximates Multidimensional Tunneling at ≥250 K." Angew. Chem. Int. Ed. 2017, 56, 13099-13102, DOI: 10.1002/anie.201708489.

Perspective on Tunneling Control

Schreiner &Tunneling Steven Bachrach 13 Nov 2017 No Comments

Over the past nine years the Schreiner group, often in collaboration with the Allen group, have produced some remarkable studies demonstrating the role of tunneling control. (I have made quite a number of posts on this topics.) Tunneling control is a third mechanism for dictating product formation, in tandem with kinetic control (the favored product is the one that results from the lowest barrier) and thermodynamic control (the favored product is the one that has the lowest energy). Tunneling control has the favored product resulting from the narrowest mass-considered barrier.

Schreiner has written a very clear perspective on tunneling control. It is framed quite interestingly by some fascinating quotes:

It is probably fair to say that many organic chemists view the concept of tunneling, even of hydrogen atoms, with some skepticism. – Carpenter 19832

Reaction processes have been considered as taking place according to the laws of classical mechanics, quantum mechanical theory being only employed in calculating interatomic forces. – Bell 19333

Schreiner’s article makes it very clear how critical it is to really think about reactions from a truly quantum mechanical perspective. He notes the predominance of potential energy diagrams that focus exclusively on the relative energies and omits any serious consideration of the reaction coordinate metrics, like barrier width. When one also considers the rise in our understanding of the role of reaction dynamics in organic chemistry (see, for example, these many posts), just how long will it take for these critical notions to penetrate into standard organic chemical thinking? As Schreiner puts it:

It should begin by including quantum phenomena in introductory textbooks, where they are, at least in organic chemistry, blatantly absent. To put this oversight in words similar to those used much earlier by Frank Weinhold in a different context: “When will chemistry textbooks begin to serve as aids, rather than barriers, to this enriched quantum-mechanical perspective?”4

References

1) Schreiner, P. R., "Tunneling Control of Chemical Reactions: The Third Reactivity Paradigm." J. Am. Chem. Soc. 2017, 139, 15276-15283, DOI: 10.1021/jacs.7b06035.

2) Carpenter, B. K., "Heavy-atom tunneling as the dominant pathway in a solution-phase reaction? Bond shift in antiaromatic annulenes." J. Am. Chem. Soc. 1983, 105, 1700-1701, DOI: 10.1021/ja00344a073.

3) Bell, R. P., "The Application of Quantum Mechanics to Chemical Kinetics." Proc. R. Soc. London, Ser. A 1933, 139 (838), 466-474, DOI: 10.1098/rspa.1933.0031.

4) Weinhold, F., "Chemistry: A new twist on molecular shape." Nature 2001, 411, 539-541, DOI: 10.1038/35079225.

Heavy atom tunneling in semibullvalene

Borden &Tunneling Steven Bachrach 02 Nov 2017 No Comments

Another prediction made by quantum chemistry has now been confirmed. In 2010, Zhang, Hrovat, and Borden predicted that the degenerate rearrangement of semibullvalene 1 occurs with heavy atom tunneling.1 For example, the computed rate of the rearrangement including tunneling correction is 1.43 x 10-3 s-1 at 40 K, and this rate does not change with decreasing temperature. The predicted half-life of 485 s is 1010 shorter than that predicted by transition state theory.

Now a group led by Sander has examined the rearrangement of deuterated 2.2 The room temperature equilibrium mixture of d42 and d22 was deposited at 3 K. IR observation showed a decrease in signal intensities associated with d42 and concomitant growth of signals associated with d22. The barrier for this interconversion is about 5 kcal mol-1, too large to be crossed at this temperature. Instead, the interconversion is happening by tunneling through the barrier (with a rate about 10-4 s-1), forming the more stable isomer d22 preferentially. This is exactly as predicted by theory!

References

1. Zhang, X.; Hrovat, D. A.; Borden, W. T., "Calculations Predict That Carbon Tunneling Allows the Degenerate Cope Rearrangement of Semibullvalene to Occur Rapidly at Cryogenic Temperatures." Org. Letters 2010, 12, 2798-2801, DOI: 10.1021/ol100879t.

2. Schleif, T.; Mieres-Perez, J.; Henkel, S.; Ertelt, M.; Borden, W. T.; Sander, W., "The Cope Rearrangement of 1,5-Dimethylsemibullvalene-2(4)-d1: Experimental Evidence for Heavy-Atom Tunneling." Angew. Chem. Int. Ed. 2017, 56, 10746-10749, DOI: 10.1002/anie.201704787.

InChIs

1: InChI=1S/C8H8/c1-3-6-7-4-2-5(1)8(6)7/h1-8H
InChIKey=VEAPRCKNPMGWCP-UHFFFAOYSA-N

d42: InChI=1S/C10H12/c1-9-5-3-7-8(4-6-9)10(7,9)2/h3-8H,1-2H3/i5D
InChIKey=WUJOLJNLXLACNA-UICOGKGYSA-N

d22: InChI=1S/C10H12/c1-9-5-3-7-8(4-6-9)10(7,9)2/h3-8H,1-2H3/i7D
InChIKey=WUJOLJNLXLACNA-WHRKIXHSSA-N

Review of the Activation Strain/Distortion-Interaction Model

Houk Steven Bachrach 16 Oct 2017 No Comments

Bickelhaupt and Houk present a nice review of their separately developed, but conceptually identical model for assessing reactivity.1 Houk termed this the “distortion/interaction” model,2 while Bickelhaupt named it “activation strain”.3 The concept is that the activation barrier can be dissected in a distortion or stain energy associated with bringing the reactants into the geometry of the transition state, and the interaction energy is the stabilization energy afforded by the molecular orbital interactions of the reactant components with each other in the transition state.

The review discusses a broad range of applications, including SN2 and E2 reactions, pericyclic reactions (including Diels-Alder reactions of enones and the dehdydro Diels-Alder reaction that I have discussed in this blog), a click reaction, a few examples involving catalysis, and the regioselectivity of indolyne (see this post). They also discuss the role of solvent and the relationship of this model to Marcus Theory.

I also want to mention in passing a somewhat related article by Jorgensen and co-authors published in the same issue of Angewandte Chemie as the above review.4 This article discusses the paucity of 10 electron cycloaddition reactions, especially in comparison to the large number of very important cycloaddition reactions involving 6 electrons, such as the Diels-Alder reaction, the Cope rearrangement, and the Claisen rearrangement. While the article does not focus on computational methods, computations have been widely used to discuss 10-electron cycloadditions. The real tie between this paper and the review discussed above is Ken Houk, whose graduate career started with an attempt to perform a [6+4] cycloaddition, and he has revisited the topic multiple times throughout his career.

References

1. Bickelhaupt, F. M.; Houk, K. N., "Analyzing Reaction Rates with the Distortion/Interaction-Activation Strain Model." Angew. Chem. Int. Ed. 2017, 56, 10070-10086, DOI: 10.1002/anie.201701486.

2. Ess, D. H.; Houk, K. N., "Distortion/Interaction Energy Control of 1,3-Dipolar Cycloaddition Reactivity." J. Am. Chem. Soc. 2007, 129, 10646-10647, DOI: 10.1021/ja0734086

3. Bickelhaupt, F. M., "Understanding reactivity with Kohn-Sham molecular orbital theory: E2-SN2 mechanistic spectrum and other concepts." J. Comput. Chem. 1999, 20, 114-128

4. Palazzo, T. A.; Mose, R.; Jørgensen, K. A., "Cycloaddition Reactions: Why Is It So
Challenging To Move from Six to Ten Electrons?" Angew. Chem. Int. Ed. 2017, 56, 10033-10038, DOI: 10.1002/anie.201701085.

More applications of computed NMR spectra

NMR Steven Bachrach 04 Oct 2017 No Comments

In this post I cover two papers discussing application of computed NMR chemical shifts to structure identification and (yet) another review of computational techniques towards NMR structure prediction.

Grimblat, Kaufman, and Sarotti1 take up the structure of rubriflordilactone B 1, which was isolated from Schisandra rubriflora. The compound was then synthesized and its x-ray structure reported, however its NMR did not match with the natural extract. It was suggested that there were actually two compounds in the extract, the minor one was less soluble and is the crystallized 1, and a second compound responsible for the NMR signal.

The authors looked at all stereoisomers of this molecule keeping the three left-most rings intact. The low energy rotamers of these 32 stereoisomers were then optimized at B3LYP/6-31G* and the chemical shifts computed at PCM(pyridine)/mPW1PW91/6-31+G**. To benchmark the method, DP4+ was used to identify which stereoisomer best matches with the observed NMR of authentic 1; the top fit (92.6% probability) was the correct structure.

The 32 stereoisomers were then tested against the experimental NMR of the natural extract. DP4+ with just the proton shifts suggested structure 2 (99.8% probability); however, the 13C chemical shifts predicted a different structure. Re-examination of the reported chemical shifts identifies some mis-assigned signals, which led to a higher C-DP4+ prediction. When all 128 stereoisomers were tested, structure 2 had the highest DP4+ prediction (99.5%), but the C-DP4+ prediction remained problematic (10.8%). Analyzing the geometries of all reasonable alternative for agreement with the NOESY spectrum confirmed 2. These results underscore the importance of using all data sources.

Reddy and Kutateladze point out the importance of using coupling constants along with chemical shifts in structure identification.2 They examined cordycepol A 3, obtained from Cordyceps ophioglossoides. They noted that the computed chemical shifts and coupling constants of originally proposed structure 3a differed dramatically from the experimental values.

They first proposed that the compound has structure 3b. The computed coupling constants using their relativistic force field.3 The experimental coupling constants for the proton H1 are 13.4 and 7.1 Hz. The computed values for 3a are 8.9 and 1.6 Hz, and this structure is clearly incorrect. The coupling constants are improved with 3b, but the 13C chemical shifts are in poor agreement with experiment. So, they proposed structure 3c, the epimer at both C1 and C11 of the original structure.

They optimized four conformations of 3c at B3LYP/6-31G(d) and obtained Boltzmann-weighted chemical shifts at mPW1PW91/6-311+G(d,p). The RMS deviation of the computed 13C chemical shifts relative to the experiment is only 1.54 ppm, and more importantly, the computed coupling constants of 13.54 and 6.90 Hz are in excellent agreement with the experiment values.

Lastly, Grimblat and Sarotti present a review of a number of methods for using computed NMR chemical shifts towards structure prediction.4 These methods include CP3, DP4, DP4+ (all of which I have posted on in the past) and an artificial neural network approach of their own design. They discuss a number of interesting cases where each of these methods has been crucial in identifying the correct chemical structure.

References

1. Grimblat, N.; Kaufman, T. S.; Sarotti, A. M., "Computational Chemistry Driven Solution to Rubriflordilactone B." Org. Letters 2016, 18, 6420-6423, DOI: 10.1021/acs.orglett.6b03318.

2. Reddy, D. S.; Kutateladze, A. G., "Structure Revision of an Acorane Sesquiterpene Cordycepol A." Org. Letters 2016, 18, 4860-4863, DOI: 10.1021/acs.orglett.6b02341.

3. (a) Kutateladze, A. G.; Mukhina, O. A., "Minimalist Relativistic Force Field: Prediction of Proton–Proton Coupling Constants in 1H NMR Spectra Is Perfected with NBO Hybridization Parameters." J. Org. Chem. 2015, 80, 5218-5225, DOI: 10.1021/acs.joc.5b00619; (b) Kutateladze, A. G.; Mukhina, O. A., "Relativistic Force Field: Parametrization of 13C–1H Nuclear Spin–Spin Coupling Constants." J. Org. Chem. 2015, 80, 10838-10848, DOI: 10.1021/acs.joc.5b02001.

4. Grimblat, N.; Sarotti, A. M., "Computational Chemistry to the Rescue: Modern Toolboxes for the Assignment of Complex Molecules by GIAO NMR Calculations." Chem. Eur. J. 2016, 22, 12246-12261, DOI: h10.1002/chem.201601150.

InChIs

1: InChI=1S/C28H30O6/c1-13-9-20(32-26(13)30)25-14(2)24-17-6-5-15-12-28-21(8-7-16(15)18(17)10-19(24)31-25)27(3,4)33-22(28)11-23(29)34-28/h5-9,14,19-22,24-25H,10-12H2,1-4H3/t14-,19+,20-,21-,22+,24-,25-,28+/m0/s1
InChIKey=JGSLSHOXBXVVTQ-NEUKEVNNSA-N

2: InChI=1S/C28H30O6/c1-13-9-20(32-26(13)30)25-14(2)24-17-6-5-15-12-28-21(8-7-16(15)18(17)10-19(24)31-25)27(3,4)33-22(28)11-23(29)34-28/h5-9,14,19-22,24-25H,10-12H2,1-4H3/t14-,19-,20-,21-,22+,24+,25-,28+/m0/s1
InChIKey=JGSLSHOXBXVVTQ-WQIRXNRDSA-N

3c: InChI=1S/C16H28O2/c1-6-11(2)9-14-16(5)12(3)7-8-13(16)15(4,17)10-18-14/h9,12-14,17H,6-8,10H2,1-5H3/b11-9-/t12-,13-,14-,15-,16+/m0/s1
InChIKey=WPQIVUHVYBQTBG-AWEVENECSA-N

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