NMR and Stereochemistry Chem 4010/5326: Organic Spectroscopic Analysis © 2015 Andrew Harned General flow for solving structures C10H20O Molecular weight/formula (MS) Exact Mass: 156.1514 Molecular Weight: 156.2652 Functional groups (IR, NMR) Carbon connectivities (substructures) (NMR) Positions of functional groups within framework (gross structure) (2D NMR, coupling constants) How can this Stereochemical issues be solved??? Relative Stereochemistry (Diastereomers) Can be determined with many of the tools we have already discussed, along with some new ones Bifulco, G.; Dambruoso, P.; Gomex-Paloma, L.; Riccio, R. Chem. Rev. 2007, 107, 3744. NMR Spectroscopy Strategies for determining relative stereochemistry Chemical Shifts – Diastereotopic protons will have different chemical shifts, this will only tell you that diastereomers are present, cannot necessarily tell which is which by inspection only by comparison to known structures – Spatial orientation may place certain protons in shielding/deshielding portions of functional groups Coupling Constants – In acyclic systems, usually cannot tell which is which by inspection – Often must convert to rigid/cyclic structure Both require some knowledge of 3D structure –> Make model(s) NMR Spectroscopy Proximity of Protons Nuclear Overhauser Effect (nOe) – Through space interactions between nuclei, whether or not they are directly coupled – Magnitude decreases as inverse of sixth power of distance – Strongly irradiate one, get larger # in excited state – The others then shift to lower state to compensate and peak increases in intensity – Subtracting the normal spectrum from the NOE spectrum helps with interpretation – Useful for determining stereochemistry – Need rigid system NMR Spectroscopy Proximity of Protons Nuclear Overhauser Effect (nOe) • nOe Difference: Subtract original spectrum from the irradiated spectrum – This leaves only the enhanced protons NMR Spectroscopy Proximity of Protons Nuclear Overhauser Effect (nOe) • nOe Difference: Subtract original spectrum from the irradiated spectrum NMR Spectroscopy Proximity of Protons Nuclear Overhauser Effect (nOe) • nOe Difference: Subtract original spectrum from the irradiated spectrum NMR Spectroscopy Proximity of Protons Nuclear Overhauser Effect (nOe) – 2D experiments NOESY: Nuclear Overhauser Effect Spectroscopy ROESY: Rotating-frame Overhauser Effect Spectroscopy – Look like COSY, but cross-peaks are for through space interactions • cross peaks not observed past ~5 Å NOESY vs. ROESY – For MW <~600 NOE is always positive – For MW 700–1200 NOE goes through zero – For MW >1200 NOE is negative – ROE is always positive, but works best for MW 700–1200 – If given choice for small molecules, run NOESY Theoreticalmaximum NOE Figure from: http://www.columbia.edu/cu/chemistry/groups/nmr/NOE.htm NMR Spectroscopy Proximity of Protons Nuclear Overhauser Effect (nOe) J. Org. Chem. 2008, 73, 2898 1,3-Diol Stereochemistry Derivatization as Acetonide 13C NMR Analysis of acetonide carbons – 1,3-Diols are very common motifs in natural products – Determining the relative stereochemistry can be difficult because many are on acyclic or macrocyclic carbon chains with unknown conformations – Rychnovsky reasoned that converting the 1,3-diols to an acetonide would make the system rigid – Furthermore it was expected that syn-diols would prefer a chair conformation, while anti-diols would prefer a twist-boat conformation – These two would then lead to differences in the 13C NMR spectrum Rychnovsky, S. D.; Rogers, B. N.; Richardson, T. I. Acc. Chem. Res. 1998, 31, 9–17 1,3-Diol Stereochemistry Derivatization as Acetonide 13C NMR Analysis of acetonide carbons equatorial Me axial Me Chart adapted from: Rychnovsky, S. D.; Skalitzky, D. J. Tetrahedron Lett. 1990, 31, 945–948 1,3-Diol Stereochemistry Derivatization as Acetonide 13C NMR Analysis of acetonide carbons – Acetonides of polyproprionate polyols display similar chemical shift patterns Evans, D. A.; Rieger, D. L.; Gage, J. R.Tetrahedron Lett. 1990, 31, 7099–7100. Case Study Macrolactins, Part 1 – The macrolactins were isolated from a deep sea bacterium and displayed interesting biological activity; gross structure determined, but stereochemistry unknown Rychnovsky, S. D.; Skalitzky, D. J.; Pathirana, C.; Jensen, P. R.; Fenical, W. J. Am. Chem. Soc. 1992, 114, 671–677 Absolute Stereochemistry (Enantiomers) Seco, J. M.; Quinoa, E.; Riguera, R. Chem. Rev. 2004, 104, 17. NMR Spectroscopy Enantiomer Determination Determination of Absolute Configuration – Several different methods available – Two main strategies: 1) chiral solvating agent – chiral solvent or additive (e.g. shift reagent) – no covalent linkage – very small differences in δ between the two enantiomers – many times requires both enantiomers of substrate; not always available 2) chiral derivatizing agent – chiral auxiliary – covalent linkage – diastereomeric derivatives made using two enantiomers of auxiliary – does not require both enantiomers of substrate NMR Spectroscopy Enantiomer Determination Chiral Derivatizing Agents The sign (+ or –) of ΔδL1 and ΔδL2 allows for determination of configuration of A – Two main derivitizing agents (both enantiomers needed) – These are the most common, others available but will not discuss, see review (same principles) NMR Spectroscopy Enantiomer Determination Chiral Derivatizing Agents 1) Polar or bulky group to fix a particular conformation 2) A functional group to allow for attachment of substrate 3) A group able to produce an efficient and space-oriented anisotropic effect – Shields/deshields L1 and L2 in each diastereomer NMR Spectroscopy Enantiomer Determination Mosher Analysis with MTPA – Working conformational model, actual conformation may vary – Ph of (R)–MTPA shields L2 – Ph of (S)–MTPA shields L1 NMR Spectroscopy Enantiomer Determination Modified Mosher Analysis – Original method used 19F due to limitations in instruments – Modified method uses 1H or 13C Majority of examples with alcohols, but has been used with other groups (see review) NMR Spectroscopy Enantiomer Determination Modified Mosher Analysis – Example NMR Spectroscopy Enantiomer Determination Modified Mosher Analysis – Example NMR Spectroscopy Enantiomer Determination Modified Mosher Analysis – Example NMR Spectroscopy Enantiomer Determination Modified Mosher Analysis – Conformational model will break down on occasion NMR Spectroscopy Enantiomer Determination Modified Mosher Analysis – Sometimes need to make derivative NMR Spectroscopy Enantiomer Determination Modified Mosher Analysis – Diols possible as well Case Study Macrolactins, Part 2 Authentic samples of each fragment were made and subjected to full Mosher analysis and then compared to degraded material Rychnovsky, S. D.; Skalitzky, D. J.; Pathirana, C.; Jensen, P. R.; Fenical, W. J. Am. Chem. Soc. 1992, 114, 671–677 Absolute Stereochemistry (a bit of UV) Crews, P.; Rodríguez, J.; Jaspars, M. Organic Structure Analysis; Oxford University Press: New York, 1998; pp 349–371. Electromagnetic spectrum Increasing Energy & Frequency & Energy Increasing Increasing Wavelength Increasing Different effects observed in different areas • UV – electronic transitions • IR – bond vibrations • Microwaves – rotational motion • Radiowaves – nuclear spin transitions Taken from: http://www4.nau.edu/microanalysis/ Microprobe/Xray-Spectrum.html Overview of methods Taken from: Crews, P.; Rodriguez, J.; Jaspars, M. Organic Structure Analysis; Oxford University Press: New York, 1998, p 5. Intro to UV-Vis • UV range: 200–400 nm • Visible range: 400–800 nm • Below 200 nm strongly absorbed by air (O2 & CO2) or solvents; must use vacuum techniques to determine (commercial instruments available) • Observe electronic transitions: excitation of an electron from bonding or nonbonding orbital to antibonding orbital • Four types of transitions: Intro to UV-Vis Useful Terminology: • λmax – wavelength where maximum absorbance is observed • Bathochromic (Red) shift – increasing λmax • Hypsochromic (Blue) shift – decreasing λmax • Molar extinction coefficient (ε) – gives an indication of the peak intensity at λmax (how strongly it absorbs the light) Beer-Lambert Law: See Pretsch and Lambert for tables Chiral Chromophores By using plane polarized light with UV wavelengths we can obtain information about the stereochemistry of chiral molecules. • Recall that chiral, molecules will rotate plane polarized light Measuring [α] or [Φ] over a range of wavelengths results in a optical rotatory dispersion (ORD) plot – S-shaped curve Plain curve – chiral compound with no chromophore Cotton effect (CE) occurs with compounds containing a chromophore + CE: peak is at higher λ than trough – CE: peak is at lower λ than trough Zero crossover occurs at λmax Opposite enantiomers display opposite ORD curves of identical magnitude Chiral Chromophores If circularly polarized light is used instead, a circular dichroism (CD) plot is obtained instead Left- and right-handed circularly polarized light is differentially absorbed by chiral molecules and yields elliptically polarized light Plotting [θ] or Δε vs. wavelength gives CD plot – Gaussian curve • can be positive or negative • can be easier to interpret when more than one chromophore is present Sounds like an easy way to determine stereochemistry. But... The Catch How do you interpret the data!!! Rules have been established to interpret the signs of the ORD and CD spectra for carbonyl-containing molecules. Allow for determining constitution, conformation, or configuration... But you need to know two