Here is a story that must drive chemical database quality control personnel nuts. Song, et al. noticed that the reported ¹³C NMR of the natural products cephalosporolide C 1, cephalosporolide J 2 and bassianolone 3 are identical.¹ Given that it is highly unlikely that two diastereomers would have identical NMR spectra, the likelihood that these three have identical spectra seemed remote at best.

Compounds 1 and 2 were synthesized and their structures confirmed by x-ray crystallography. Their ¹³C NMR spectra show clear distinctions, indicating that the isolated “2” is actually 1. Experimental support for the notion that 1 and 3 are actually the same was provided by preparing the diacetylide of 1 and comparing its NMR spectra to that of natural “3”.

Quantum computations confirmed that in fact the natural product thought to be 3 is actually 1. The structures of 1 and 3 were optimized at B3LYP/6-311+G(2d,p) and ¹³C chemical shifts were computed with these geometries at mPW1PW91/6-311+G(2d,p)/CPCM(chloroform). (The computed structures are shown in Figure 1.) The mean absolute deviation (MAD) between the computed and experimental spectra for 1 is 0.97 ppm, while the MAD for the computed spectrum of 3 compared with the experimental values is 2.44 ppm, with a maximum error of 5.13ppm, more than twice the maximum error with structure 1. The authors attribute the misassignments to a faulty initial spectra of authentic cephalosporolide C 1.

1

3

Figure 1. B3LYP/6-311+G(2d,p) optimized geometries of 1 and 3.

References

(1) Song, L.; Lee, K.-H.; Lin, Z.; Tong, R. "Structural Revision of Cephalosporolide J and Bassianolone," J. Org. Chem. 2014, 79, 1493-1497, DOI: 10.1021/jo402602h.

InChIs

1: InChI=1S/C10H16O5/c1-6-2-3-7(11)4-8(12)9(13)5-10(14)15-6/h6,8-9,12-13H,2-5H2,1H3/t6-,8+,9+/m1/s1
InChIKey=JTOYXZZLKBAIEJ-YEPSODPASA-N

2: InChI=1S/C10H16O5/c1-6-2-3-7(11)4-8(12)9(13)5-10(14)15-6/h6,8-9,12-13H,2-5H2,1H3/t6-,8+,9-/m1/s1
InChIKey=JTOYXZZLKBAIEJ-BWVDBABLSA-N

3: InChI=1S/C10H16O5/c1-6(11)2-3-7(12)4-9-8(13)5-10(14)15-9/h6,8-9,11,13H,2-5H2,1H3/t6-,8+,9+/m1/s1
InChIKey=ZTRQRDNTNXPRFW-YEPSODPASA-N

I have posted on the use of computed NMR chemical shifts and coupling constants to help aid in structure identification. The second edition of my book Computational Organic Chemistry has a largely all-new chapter on structure identification aided by computed spectra, especially NMR spectra. In my recent opinion piece speculating on challenges in computational organic chemistry,¹ the first area I highlight is encouraging the larger use of computed spectra as an essential component of structure determination.

While more and more non-traditional computational users are employing quantum computations towards these problems, I suspect that many non-users are a bit wary about stepping into an arena they are not expert in, an arena chock-filled with acronyms and methods and potentially little guidance. While some very nice papers^2-6 and web sites (Chemical Shift Repository (Cheshire) and DP4) do outline procedures for using computations in this fashion, they are not truly designed for the non-specialist.

Well, fear not any longer. Hoye and coworkers, synthetic chemists who have utilized computational approaches in structure determinations for a number of years, have written a detailed step-by-step protocol for using a standard computational approach towards structure determination.⁷ The article is written with the synthetic chemist in mind, and includes a number of scripts to automate many of the steps.

For the specialist, the overall outline of the protocol is fairly routine:

1. Utilize MacroModel to perform a conformational search for each proposed structure, retaining the geometries within 5 kcal mol^-1 of the global minimum.
2. Optimize these conformations for each structure at M06-2x/6-31+G(d).
3. For each conformation of each structure, compute the ¹H and ¹³C chemical shifts, scale them, and determine the Boltzmann weighted chemical shifts
4. Compare these chemical shifts with the experimental values using Mean Absolute Error

The article is straightforward and easily guides the novice user through these steps. Anyone unsure of how to utilize quantum chemical computations in structure determination is well advised to start with this article.

References

(1) Bachrach, S. M. "Challenges in computational organic chemistry," WIRES: Comput. Mol. Sci. 2014, 4, 482-487, DOI: 10.1002/wcms.1185.

(2) Lodewyk, M. W.; Siebert, M. R.; Tantillo, D. J. "Computational Prediction of ¹H and ¹³C Chemical Shifts: A Useful Tool for Natural Product, Mechanistic, and Synthetic Organic Chemistry," Chem. Rev. 2012, 112, 1839–1862, DOI: 10.1021/cr200106v.

(3) Bally, T.; Rablen, P. R. "Quantum-Chemical Simulation of ¹H NMR Spectra. 2. Comparison of DFT-Based Procedures for Computing Proton-Proton Coupling Constants in Organic Molecules," J. Org. Chem. 2011, 76, 4818-4830, DOI: 10.1021/jo200513q.>

(4) Jain, R.; Bally, T.; Rablen, P. R. "Calculating Accurate Proton Chemical Shifts of Organic Molecules with Density Functional Methods and Modest Basis Sets," J. Org. Chem. 2009, 74, 4017-4023, DOI: 10.1021/jo900482q.

(5) Smith, S. G.; Goodman, J. M. "Assigning the Stereochemistry of Pairs of Diastereoisomers Using GIAO NMR Shift Calculation," J. Org. Chem. 2009, 74, 4597-4607, DOI: 10.1021/jo900408d.

(6) Smith, S. G.; Goodman, J. M. "Assigning Stereochemistry to Single Diastereoisomers by GIAO NMR Calculation: The DP4 Probability," J. Am. Chem. Soc. 2010, 132, 12946-12959, DOI: 10.1021/ja105035r.

(7) Willoughby, P. H.; Jansma, M. J.; Hoye, T. R. "A guide to small-molecule structure assignment through computation of (1H and 13C) NMR chemical shifts," Nat. Protocols 2014, 9, 643-660, DOI: 10.1038/nprot.2014.042.

Here is another nice example of the partnership between experiment and computation in ascertaining molecular structure. The Sarpong, Tantillo, Andersen, Berlinck, and Miller groups collaborated on the synthesis, characterization and biosynthesis of some metabolites from Penniculium strains.¹ I will focus here on just the structural identification component of this paper; the synthesis and the biosynthesis are very interesting too!

Cyclopiamine A 1 and cyclopiamine B 2 interconvert through an intermediate that allows for the epimerization at carbon bearing the nitro group.²

Citrinalin A 3 might also seem to undergo the same type of ring opening-ring closing reaction to produce citrinalin B. However, the original proposed structure³ of citrinalin B 4 implies an epimerization at a different carbon (at the ring fusion to the terminal 5 member ring). These authors suggested that perhaps the proper structure of citrinalin B is 5, which differs from citrinalin A only at the carbon bearing the nitro group, analogous to the relationship between 1 and 2.

The low energy conformations of both 4 and 5 (actually the trifluoroacetic acid salts) were optimized at B3LYP/6-31+G(d,p) and the chemical shifts for both ¹H and ¹³C were computed, Boltzmann-weighted and scaled, and then compared with the NMR spectra of authentic citrinalin B. (The lowest energy conformations of 4 and 5 are shown in Figure 1.) The corrected mean absolute deviations for the ¹H and ¹³C chemical shift for the original structure 4 are 0.45 ppm and 2.0 ppm, respectively (with the largest outliers of 2.3 ppm for H and 9.6 ppm for C). These errors are about twice what is observed in comparing the experimental and computed ¹H and ¹³C chemical shifts of 3. The agreement between the computed and experimental values using 5 are much improved, with mean deviations of 0.12 and 1.6ppm, and largest deviations of 0.38 ppm for ¹H and 4.4 ppm for ¹³C. Use of Goodman’s DP4 method indicates a 100% probability that the structure of citrinalin B is 5. This prediction is confirmed by the x-ray structure.

4

5

Figure 1. B3LYP/6-31+G(d,p) optimized lowest energy conformers of 4 and 5.

References

(1) Mercado-Marin, E. V.; Garcia-Reynaga, P.; Romminger, S.; Pimenta, E. F.; Romney, D. K.; Lodewyk, M. W.; Williams, D. E.; Andersen, R. J.; Miller, S. J.; Tantillo, D. J.; Berlinck, R. G. S.; Sarpong, R. "Total synthesis and isolation of citrinalin and cyclopiamine congeners," Nature 2014, 509, 318-324, DOI: 10.1038/nature13273.

(2) Bond, R. F.; Boeyens, J. C. A.; Holzapfel, C. W.; Steyn, P. S. "Cyclopiamines A and B, novel oxindole metabolites of Penicillium cyclopium westling," J. Chem. Soc., Perkin Trans I 1979, 1751-1761, DOI: 10.1039/P19790001751.

(3) Pimenta, E. F.; Vita-Marques, A. M.; Tininis, A.; Seleghim, M. H. R.; Sette, L. D.; Veloso, K.; Ferreira, A. G.; Williams, D. E.; Patrick, B. O.; Dalisay, D. S.; Andersen, R. J.; Berlinck, R. G. S. "Use of Experimental Design for the Optimization of the Production of New Secondary Metabolites by Two Penicillium Species," J. Nat. Prod. 2010, 73, 1821-1832, DOI: 10.1021/np100470h.

InChIs

1: InChI=1S/C26H33N3O5/c1-23(2)12-17(30)20-18(34-5)9-8-16-21(20)28(23)22(31)26(16)13-25(29(32)33)14-27-10-6-7-15(27)11-19(25)24(26,3)4/h8-9,15,19H,6-7,10-14H2,1-5H3/t15-,19+,25+,26-/m1/s1
InChIKey=LUESCPPTDJZZQN-VBPSCBQFSA-N

2: InChI=1S/C26H33N3O5/c1-23(2)12-17(30)20-18(34-5)9-8-16-21(20)28(23)22(31)26(16)13-25(29(32)33)14-27-10-6-7-15(27)11-19(25)24(26,3)4/h8-9,15,19H,6-7,10-14H2,1-5H3/t15-,19+,25-,26-/m1/s1
InChIKey=LUESCPPTDJZZQN-DCWDXPITSA-N

3: InChI=1S/C25H31N3O5/c1-22(2)11-16(29)19-17(33-22)8-7-15-20(19)26-21(30)25(15)12-24(28(31)32)13-27-9-5-6-14(27)10-18(24)23(25,3)4/h7-8,14,18H,5-6,9-13H2,1-4H3,(H,26,30)/t14-,18+,24+,25-/m0/s1
InChIKey=QVBARITWSQCBBV-DTKOROOESA-N

4: InChI=1S/C25H31N3O5/c1-22(2)11-16(29)19-17(33-22)8-7-15-20(19)26-21(30)25(15)12-24(28(31)32)13-27-9-5-6-14(27)10-18(24)23(25,3)4/h7-8,14,18H,5-6,9-13H2,1-4H3,(H,26,30)/t14-,18-,24-,25+/m1/s1
InChIKey=QVBARITWSQCBBV-KUCRYEJDSA-N

5: InChI=1S/C25H31N3O5/c1-22(2)11-16(29)19-17(33-22)8-7-15-20(19)26-21(30)25(15)12-24(28(31)32)13-27-9-5-6-14(27)10-18(24)23(25,3)4/h7-8,14,18H,5-6,9-13H2,1-4H3,(H,26,30)/t14-,18+,24-,25-/m0/s1
InChIKey=QVBARITWSQCBBV-QBDGMKJCSA-N

Here’s another nice example of the use of computed NMR spectra to aid in structure identification. Quercusnin A was identified in an extract of dried sapwood from the oak tree Quercus crispula. The NMR sectrum along with structural comparison to the previously determined extract vescalagin, led the authors to the structure 1.¹

1

To aid in determining the absolute stereochemistry as centers 1’ and A8, the authors employed a computational approach. Conformers of the four diastereomers (RR, RS, SR, SS) were optimized first with molecular mechanics, then the low energy conformers were reoptimized at AM1, and then finally all of the conformers within 6 kcal mol^-1 of the lowest energy structure were reoptimized at PCM(acetone)/B3LYP/6-31G(d,p). The ¹H and ¹³C NMR chemical shifts for all of the structures that contribute greater than 1% to the Boltzmann population were computed at PCM(acetone)mPW1PW91/6-311+G(2d,p)//B3LYP/6-31G(d,p). The DP4 probability (see this post) identified the (1’S,A8R) isomer with 100% probability for matching up with the experimental NMR spectrum. Additionally, the computed ECD spectrum matches nicely with the experimental spectra for this same stereoisomer. The lowest energy conformer of 1 is shown in Figure 1.

1

Figure 1. PCM(acetone)/B3LYP/6-31G(d,p) structure of the lowest energy conformer of 1.

References

(1) Omar, M.; Matsuo, Y.; Maeda, H.; Saito, Y.; Tanaka, T. "New Metabolites of C-Glycosidic Ellagitannin from Japanese Oak Sapwood," Org. Lett. 2014, 16, 1378–1381, DOI: 10.1021/ol500146a.

InChIs

1: InChI=1S/C36H24O22/c37-11-2-8-15(24(44)20(11)40)16-9(3-12(38)21(41)25(16)45)36(53)56-29-14(5-54-32(8)49)55-33(50)10-4-13(39)22(42)26(46)19(10)30-17-6(34(51)57-30)1-7-18(23(17)43)31(58-35(7)52)28(48)27(29)47/h1-4,14,27-31,37-48H,5H2/t14-,27-,28-,29-,30-,31+/m1/s1
InChIKey=PTEZCLBSJBUWFI-LJBUUXJGSA-N

I have not discussed any papers that utilize computations to confirm chemical structure in a while, so here are two recent examples.

Grabow has utilized MP2 and M06-2x computations to confirm the lowest energy conformation of (-)-lupinine 1.¹ The interesting structural aspect of this compound is the possibility of an intramolecular hydrogen bond linking the hydroxyl group with the amine.

1

Using molecular mechanics, the authors identified 57 structures within 50 kJ mol^-1 of each other. These geometries were reoptimized at MP2/6-311++G(d,p) and M06-2x/6-311++G(d,p).
The lowest energy structures had the expected trans ring fusion, with a trans relationship between the hydrogen on the bridgehead carbon (C9) and the hydroxymethyl group. This corresponds to either the (R,R) or (S,S) isomer. The three lowest energy structures are shown in Figure 1. Unfortunately, the geometry for the lowest energy isomer provided in the Supporting Materials is wrong, and the authors did not supply the geometries of the other isomers. This situation is unacceptable! Reviewers and editors must do a better job in policing the Supporting Materials; there is no excuse for not including all of the optimized structures, and better yet, in a more usable format that what has been done here. I have reoptimized these structures at M06-2x/6-31G(d). The lowest energy conformer 1a does possess the expected internal hydrogen bond.

1a (0.0)
1b (10.4)	12 (11.5)

Figure 1. M06-2x/6-31G(d) optimized structures of the three lowest energy conformers of 1, with relative free energies in kJ mol^-1.

Table 1 provides a comparison of the MP2 computed values of important structural parameters along with the experimental values obtained from a microwave experiment. The agreement with the computed values for 1a provides strong evidence that this is the structure of (-)-lupinine.

Table 1. Comparison of MP2 and experimental structural parameters of 1.^a

^aRotational constants (A, B, C) in MHz, centrifugal distortion constants (Δ_J, Δ_JK, Δ_K) in kHz, and nuclear quadrupole coupling tensor elements (χ_aa, χ_bb, χ_cc) in MHz.

The second study utilizes computed NMR chemical shifts to discriminate potential diastereomeric structures. Laurefurenyne A was first assigned the structure 2 based on 1D and 2D NMR experiments. However, based on potential biochemical analogy to other compounds, Paton and Burton² had doubts about this structure. In addition to synthesizing the natural material, they performed an extensive computational study of the chemical shifts of the diastereomers. For each of the 32 possible diastereomers, they performed a Monte Carlo search of the conformational space using molecular mechanics. The structures of all isomers within 10 kJ mol^-1 of the lowest energy structure were reoptimized at ωB97X-D/6-31G(d) with PCM (CHCl₃) and chemical shifts obtained at mPW1PW91/6-311G(d,p). Final chemical shifts were obtained using a Boltzmann weighting. The computed values for 2 were quite off from the experimental values, with a mean unsigned error of 1.5 ppm. A better assessment was provided with the DP4 method, which indicated that 3 has the highest probability of being the correct structure, a structure consistent with the likely biosynthetic pathway.

References

(1) Jahn, M. K.; Dewald, D.; Vallejo-López, M.; Cocinero, E. J.; Lesarri, A.; Grabow, J.-U. "Rotational Spectra of Bicyclic Decanes: The Trans Conformation of (-)-Lupinine," J. Phys. Chem. A 2013, DOI: 10.1021/jp407671m.

(2) Shepherd, D. J.; Broadwith, P. A.; Dyson, B. S.; Paton, R. S.; Burton, J. W. "Structure Reassignment of Laurefurenynes A and B by Computation and Total Synthesis," Chem. Eur. J. 2013, 19, 12644-12648, DOI: 10.1002/chem.201302349.

InChIs

(-)-Lupinine 1: InChI=1S/C11H21NO/c1-11-6-2-3-7-12(11)8-4-5-10(11)9-13/h10,13H,2-9H2,1H3/t10-,11+/m0/s1
InChIKey=WVDUAOYJLFVEMW-WDEREUQCSA-N

Laurefurenyne A 3: InChI=1S/C14H20O4.C2H6/c1-3-4-5-6-12-11(16)8-14(18-12)13-7-10(15)9(2)17-13;1-2/h1,4-5,9-16H,6-8H2,2H3;1-2H3/b5-4-;/t9-,10-,11-,12+,13-,14+;/m1./s1
InChIKey=ZYESDGGYHKCHMJ-SKFGUVSTSA-N

The natural product aquatolide has the proposed structure 1.¹ Before starting to investigate this rather unusual structure – the 2[ladderane] component is rare and likely to be a synthetic challenge – Shaw and Tantillo opted to reassure themselves that the structure is correct.² They computed the chemical shifts of this structure at mPW1PW91/6-311+G(2d,p)//B3LYP/6-31+G(d,p) including PCM to model chloroform. Surprisingly, the mean absolute deviation of the computed ¹³C NMR shifts of 1 with the experimental values is 7.23 ppm, with the largest deviation of 24.3 ppm. The largest deviation between 1 and the experimental ¹H NMR shifts is 1.31 ppm. These large errors suggested that the structure is wrong. Surveying some 60 different possible alternative structures, largely based on other related compounds found in the same plant, they landed on 2. Here the mean absolute deviation of the computed ¹³C chemical shifts is only 1.37 ppm, with a maximum deviation of only 4.3 ppm. Similar dramatic improvement is also seen with the proton chemical shifts. Excellent agreement is also seen in the computed ¹H-¹H coupling constants between those computed for 2 and the experimental spectrum. Crystallization of aquatolide and subsequent determination of the structure using x-ray diffraction confirms that the actual structure of aquatolide is 2.

References

(1) San Feliciano, A.; Medarde, M.; Miguel del Corral, J. M.; Aramburu, A.; Gordaliza, M.; Barrero, A. F. "Aquatolide. A new type of humulane-related sesquiterpene lactone," Tetrahedron Lett. 1989, 30, 2851-2854, DOI: 10.1016/s0040-4039(00)99142-1

(2) Lodewyk, M. W.; Soldi, C.; Jones, P. B.; Olmstead, M. M.; Rita, J.; Shaw, J. T.; Tantillo, D. J. "The Correct Structure of Aquatolide—Experimental Validation of a Theoretically-Predicted Structural Revision," J. Am. Chem. Soc. 2012, DOI: 10.1021/ja3089394

InChIs

1: InChI=1S/C15H18O3/c1-7-5-4-6-15-9(11(7)16)8-10(15)12(14(8,2)3)18-13(15)17/h5,8-10,12H,4,6H2,1-3H3/t8-,9+,10+,12+,15+/m1/s1
InChIKey=JGSDEQLPLHCECO-NIDGLEHPSA-N

2: InChI=1S/C15H18O3/c1-7-5-4-6-15-9-8(10(7)16)11(15)14(2,3)12(9)18-13(15)17/h5,8-9,11-12H,4,6H2,1-3H3/b7-5-/t8-,9-,11+,12-,15+/m0/s1
InChIKey=OKZHLNWYFSWUMD-AETBLWMGSA-N

The use of computed NMR coupling constants is starting to grow. In a previous post I discussed a general study by Rablen and Bally on methods for computing J_HH coupling constants. Now Williamson reports methods to experimentally obtain ¹ J_CC and ³J_CC coupling constants.¹ These were obtained for strychnine. He then computed the coupling constants in two steps. Using the B3LYP/6-31G(d) optimized geometry, first the Fermi contact contribution was computed at B3LYP/6-31+G(d,p) by uncontracting the basis set and adding an additional tighter set of polarization functions. Second, the remaining terms (spin-dipolar, paramagnetic spin-orbit and diamagnetic spin-orbit coupling) were computed with the 6-31+Gd,p) set without modifications. The two computed terms were added to give the final estimate.

A plot of the experimental vs. the DFT computed ¹ J_CC and ³J_CC coupling constants shows
an excellent linear relation, with correlation coefficient of 0.9986 and a slope of 0.98. The mean absolute deviation for the computed and experimental ¹ J_CC and ³J_CC coupling constants is 1.0
Hz and 0.4 Hz, respectively, both well within the experimental error.

I expect that computed NMR spectra will continue to be a growth area, especially for structural identification.

References

(1) Williamson, R. T.; Buevich, A. V.; Martin, G. E. "Experimental and Theoretical Investigation of ¹J_CC and ⁿJ_CC Coupling Constants in Strychnine," Org. Letters 2012, 14, 5098-5101, DOI: 10.1021/ol302366s

InChIs

strychnine:
InChI=1S/C21H22N2O2/c24-18-10-16-19-13-9-17-21(6-7-22(17)11-12(13)5-8-25-16)14-3-1-2-4-15(14)23(18)20(19)21/h1-5,13,16-17,19-20H,6-11H2/t13-,16-,17-,19-,20-,21+/m0/s1
InChIKey=QMGVPVSNSZLJIA-FVWCLLPLSA-N

Here is an interesting twist on using computations in conjunction with experimental NMR to solve for molecular structure. I have blogged a number of times on comparing computed chemical shifts with experimental values to identify structure, and also on using the comparison of computed and experimental coupling constants to accomplish this purpose.

Butts and Bifulco were interested in the structure of conicasterol F 1 and opted to make two sets of comparison.¹ The first uses the traditional approach of comparing the computed and experimental ¹³C chemical shifts. The second comparison uses the distances between protons, coming from the optimized structure and the rotating-frame nuclear Overhauser effect (ROE).

Standard analysis of the NMR spectra of 1 allowed for the determination of all of the stereochemistry except for the epoxy ring at C₈ and C₁₄. The possible options are shown as 1a and 1b. The optimized geometries (MPW1PW91/6-31G) of these two diastereomers are shown in Figure 1.

1a	1b
1a	1b

Figure 1. Optimized geometries of 1a and 1b.

Comparison of 15 distances between protons determined by the ROE experiment and by computation led to a mean absolute error of 7.8% for 1a and 3.0% for 1b, suggesting that the latter is the correct structure. Similar comparison was then made between the experimental chemical shifts of 12 of the carbon atoms with the computed values of the two isomers. The mean absolute error in the chemical shifts of 1a is 3.7ppm, but only 0.8 ppm for 1b. Both methods give the same conclusion: conicasterol F has structure 1b.

References

(1) Chini, M. G.; Jones, C. R.; Zampella, A.; D’Auria, M. V.; Renga, B.; Fiorucci, S.; Butts, C. P.; Bifulco, G., "Quantitative NMR-Derived Interproton Distances Combined with Quantum Mechanical Calculations of ¹³C Chemical Shifts in the Stereochemical Determination of Conicasterol F, a Nuclear Receptor Ligand from Theonella swinhoei," J. Org. Chem., 2012, 77, 1489-1496, DOI: 10.1021/jo2023763.

InChIs

1b: InChI=1/C29H46O4/c1-16(2)17(3)8-9-18(4)21-14-23(31)28-26(21,7)15-24-29(32-24)25(6)12-11-22(30)19(5)20(25)10-13-27(28,29)33-28/h16-18,20-24,30-31H,5,8-15H2,1-4,6-7H3/t17-,18-,20+,21-,22+,23+,24-,25+,26-,27+,28+,29+/m1/s1
InChIKey=XTWHHLLDFQALAM-XAIGNWORBC

A quick note here on the use of computed NMR to determine stereochemical structure. The Garg group synthesized two “oxidized welwitindolines”, compounds 1 and 2.¹ The relative stereochemistry at the C3 position (the carbon with the hydroxy group) was unknown.

Low energy gas-phase conformers of both epimers of 1 and 2 were optimized at B3LYP/6-31+G(d,p). (These computations were done by the Tantillo group.) See Figure 1 for the optimized lowest energy conformers. Using these geometries the NMR chemical shifts were computed at mPW1PW91/6-311+G(d,p) with implicit solvent (chloroform). The chemical shifts were Boltzmann-weighted and scaled according to the prescription (see this post) of Jain, Bally and Rablen.² The computed chemical shifts were then compared against the experimental NMR spectra. For both 1 and 2, the ¹³C NMR shifts could not readily distinguish the two epimers. However, the computed ¹H chemical shifts for the S epimer of each compound was significantly in better agreement with the experimental values; the mean average deviation for the S epimer of 2 is 0.08 ppm but 0.36ppm for the R epimer. As a check of these results, DP4 analysis³ (see this post) of 2 indicated a 100% probability for the S epimer using only the proton chemical shifts or with the combination of proton and carbon data.

1

2

Figure 1. B3LYP/6-31+G(d,p) optimized geometries of the
lowest energy conformations of 1 and 2.

References

(1) Quasdorf, K. W.; Huters, A. D.; Lodewyk, M. W.; Tantillo, D. J.; Garg, N. K., "Total Synthesis of Oxidized Welwitindolinones and (-)-N-Methylwelwitindolinone C Isonitrile," J. Am. Chem. Soc. 2011, 134, 1396-1399, DOI: 10.1021/ja210837b

(2) Jain, R.; Bally, T.; Rablen, P. R., "Calculating Accurate Proton Chemical Shifts of Organic Molecules with Density Functional Methods and Modest Basis Sets," J. Org. Chem. 2009, 74, 4017-4023, DOI: 10.1021/jo900482q.

(3) Smith, S. G.; Goodman, J. M., "Assigning Stereochemistry to Single Diastereoisomers by GIAO NMR Calculation: The DP4 Probability," J. Am. Chem. Soc. 2010, 132, 12946-12959, DOI: 10.1021/ja105035r

InChIs

1: InChI=1/C22H21ClN2O3S/c1-6-20(4)15(23)10-13-17(26)21(20,24-11-29)12-8-7-9-14-16(12)22(28,19(13,2)3)18(27)25(14)5/h6-10,13,28H,1H2,2-5H3/t13-,20+,21+,22-/m0/s1
InChIKey=VDNAFECSPBXWIH-WOHBTAIZBQ

2: InChI=1/C22H21ClN2O3/c1-7-20(4)15(23)11-13-17(26)21(20,24-5)12-9-8-10-14-16(12)22(28,19(13,2)3)18(27)25(14)6/h7-11,13,28H,1H2,2-4,6H3/t13-,20+,21+,22-/m0/s1
InChIKey=GFNPBZSGZFQTJA-WOHBTAIZBX

Bergman and Raymond have prepared a Ga₄L₆^12- host that can encapsulate small monocations and neutral species.^1,2 Figure 1 shows the host with an encapsulated tetraethylammonium ion NEt₄⁺. (Note that the hydrogens have been suppressed for easier viewing. And be sure to click on the structure in order to interact with the 3-D model.)

Figure 1. Structure of the Ga₄L₆^12- host encapsulating NEt₄⁺.

Of interest for readers of this blog is that they have now computed the NMR spectra of the encapsulated species.³ The geometry of the host is fixed to that found in the crystal structure where Cp*Co is the guest and the geometry of the guest (NEt₄⁺, PEt₄⁺ and others) is optimized with molecular mechanics. The complex is then computed at B3LYP with the 3-21G basis set for the host and the G-311(g,p) basis set for the guest. The computed ¹H chemical shifts are actually within 0.1 ppm of experiment, and show the swapping of the relative position of the chemical shifts of the methyl vs methylene proton for the two guests.

This demonstrates the computed NMR shifts can be applied to some very large molecules including organometallics.

References

(1) Caulder, D. L.; Powers, R. E.; Parac, T. N.; Raymond, K. N., "The Self-Assembly of a Predesigned Tetrahedral M₄L₆ Supramolecular Cluster," Angew. Chem. Int. Ed. 1998, 37, 1840-1843, DOI: 10.1002/(SICI)1521-3773(19980803)37:13/14<1840::AID-ANIE1840>3.0.CO;2-D

(2) Biros, S. M.; Bergman, R. G.; Raymond, K. N., "The Hydrophobic Effect Drives the Recognition of Hydrocarbons by an Anionic Metal-Ligand Cluster," J. Am. Chem. Soc. 2007, 129, 12094-12095, DOI: 10.1021/ja075236i

(3) Mugridge, J. S.; Bergman, R. G.; Raymond, K. N., "¹H NMR Chemical Shift Calculations as a Probe of Supramolecular Host-Guest Geometry," J. Am. Chem. Soc. 2011, 133, 11205-11212, DOI: http://dx.doi.org/10.1021/ja202254x