Heteronuclear Overhauser

For compounds that contain a limited number of fluorine atoms, heteronuclear correlation spectroscopy experiments such as F H HETCOR and 2H-19F heteronuclear Overhauser enhancement spectroscopy (HOESY) can provide considerable assistance distinguishing structural isomers and diastereomers as well as for conformational analysis. HOESY experiments have been frequently used for conformational analysis of biomolecules containing fluorine labels.18... 

The HOESY (Heteronuclear Overhauser Effect Spectroscopy) experiment... 

Just as in the COSY type of experiments this cross-relaxation effect is not restricted to protons, but can also involve heteronuclei the acronym HOESY (heteronuclear Overhauser effect) is used in these cases. This can be used, for example, to show that an anion such as IT1., is in close proximity to the ligands of the organometallic compound, as was carried out by Macchioni et al. with a 19F-xH HOESY experiment [24]. 

With the aid of 13C NMR, 6Li NMR and XH HOESY (heteronuclear Overhauser effect spectroscopy) NMR of a-lithiomethoxyallene (106) and l-lithio-l-ethoxy-3-J-butylallene (107) as well as by ab initio model calculations on monomeric and dimeric a-lithiohy-droxyallene, Schleyer and coworkers64 proved that 106 and 107 are dimeric in THF (106 forms a tetramer in diethyl ether) with a nonclassical 1,3-bridged structure. The 13C NMR spectrum of allenyllithium in THF is also in agreement with the allenic-type structure the chemical shift of C2 (196.4 ppm) resembles that of neutral allene (212.6 ppm), rather than C2 of propyne (82.4 ppm). 

An aspect of general interest in organometallic chemistry is the equilibrium between contact and solvent-separated ion pairs, because metal cations which are sun ounded by an individual solvent cage are expected to show different reactivity towards basic centres than those closely attached to carbanions or amines. At the same time, the anionic centre is less shielded in an SSIP than in a CIP and thus expected to be more reactive. In solution, the differentiation by NMR methods between both structural motifs relies in most cases on chemical shift interpretations and, if possible, on heteronuclear Overhauser (NOE) measurements. The latter method is especially powerful in the case of lithium organic compounds, where H, Li or even H, Li NOE can be detected by one- and two-dimensional experiments. ... 

Coordination of [6Li]-a-(phenylthio)benzyllithium with 9 was studied by H Li-HOESY NMR technique (HOESY = heteronuclear Overhauser effect spectroscopy) <1998JOM(550)359>. This interaction results in the formation of contact ion pair and ligand and tetrahydrofuran (THF) solvent molecules compete for three coordination sites. The fourth site is occupied by the anionic benzylic carbon atom in an qMike manner. 

A(-Phenylpyrrole (194) is monolithiated at the 2-position of the heterocyclic ring. This monolithium compound crystallizes as the TMEDA-solvated dimer (195). This structure agrees well with the Li- H 2D heteronuclear Overhauser NMR spectroscopy (2D-HOESY). The structure serves to predict correctly that the second lithiation to a dianion occurs at the ortho position of the phenyl ring loca closest to the lithium in the monoanion. 

The tlirough-space nuclear overhauser effect (NOE) can provide information on sites where metals interact even if the metals do not form stable bonds through which spin coupling can be transferred. For instance, metal-HS interactions can be studied by NOE spectroscopy (NOESY) by measuring interactions between the protons within the HS molecules before and after the addition of metals to understand the conformational changes that occur within the molecules (Kingery et al., 2001). An alternative approach is to measure the heteronuclear overhauser effect (HOE) directly between the metal ion and the HS proton in close proximity by HOE spectroscopy (HOES), as has been demonstrated for organo-Li complexes (Bauer, 1995). 

Bauer, W. (1995). NMR of organolithium compounds general aspects and application of two-dimensional heteronuclear Overhauser effect spectroscopy (HOESY). In Lithium Chemistry, ed. Sapse. A.-M., and Schleyer, P. V. R., Wiley-lnterscience, New York, 125-172. 

A different approach to locate the metal in a lithium organyl, which exploits dipolar rather than scalar interactions, was presented by the group of Berger who appUed both 2D- C, Li HOESY and lD- C Li difference spectroscopy to measure C, Li heteronuclear Overhauser effects and demonstrated that the resulting data can be used for C-Li distance calculation. Ihe major drawback of this technique is that the use of doubly [ Li, C]-labelled samples is mandatory to ensure reliable measurement of very small NOE effects of some 1%. 

The 2D sequence [80,81], referred to as HOESY (heteronuclear Overhauser spectroscopy. Fig. 8.52b), avoids the need for selective proton presaturation but naturally suffers from low sensitivity. The sequence parallels that of NOESY,... 

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