NMR/reactivity correlations

Modem methods based on density-functional theory (DFT) can describe relative activation barriers of organometallic reactions, i.e. relative reactivities, as well as the transition-metal NMR chemical shifts of the reactant complexes involved. It is thus possible to reproduce or rationalize observed correlations between these properties or to predict new ones. NMR/reactivity correlations that could be reproduced theoretically ("intrinsic correlations") are summarized. Newly predicted NMR/ reactivity correlations are discussed for the ethylene polymerization with V(=0-X)R3 or V(=Y)R3 catalysts. When X or Y are varied (X = A1H3, Li+, SbF5, H+ Y = NH, O, S, Se), both... 

With the advent of appropriate DFT-based methods, NMR properties of transition-metal compounds have now become amenable to theoretical computations (8). Suitable density functionals have been identified (9) which permit calculations of transition-metal chemical shifts with reasonable accuracy, typically within a few percent of the respective shift ranges. Thus, it is now possible to investigate possible NMR/reactivity correlations for transition-metal complexes from first principles several such studies have already been undertaken (10,11,12). 

The first part of the present paper summarizes attempts to reproduce or rationalize NMR/reactivity correlations known empirically. The second part is devoted to the search for prospective candidates for new such correlations, with the emphasis on catalytic reactions. 

The first successful reproduction of an empirical NMR/reactivity correlation was achieved for the substitution reaction at a rhodium center, equation 1. For several substituents X at the cyclopentadienyl ring, the logarithm of the observed rate constant... 

The first theoretical rationalization of an NMR/reactivity correlation was offered for an insertion reaction involving iron complexes 2, equation 2. With increasing bulkiness of the alkyl ligands at iron, PPh3-induced insertion of CO into the Fe-... 

It is probably safe to assume that the insertion barriers into the Fe-C(alkyl) bonds will decrease with the BDEs (even though the range of k0 s values, which cover two orders of magnitude, suggest a narrower span of actual insertion barriers than obtained for the BDEs, which vary by 11 kcal/mol). Nevertheless, the data in Table I constitute a plausible rationalization of the observed NMR/reactivity correlation. 

In the preceding chapter it has been shown that the DFT methods currently available can be used to reproduce relative trends in both reactivities and transition-metal NMR chemical shifts. Thus, NMR/reactivity correlations can be modeled theoretically, at least when relative reactivities are reflected in relative energies on the potential energy surfaces (activation barriers, BDEs). It should in principle also be possible to predict new such correlations. This is done in the following, with the emphasis on olefin polymerization with vanadium-based catalysts. 

The results summarized in Figure 3 a were the first theoretical prediction of an NMR/reactivity correlation. Transferred to the experimental system 6, these data... 

However, the two NMR/reactivity correlations depicted in Figure 3 are quantitatively different the slopes of the two linear regression lines differ by one order of magnitude (ca. 1 and 11 cal ppm 1 in Figure 3a and 3b, respectively). Thus, compounds of type 9 or 10 (with alkyl instead of amino rests) would probably hold little promise as active catalysts, despite the expected huge deshielding of the metal center. 

Arylimido derivatives, on the other hand, are deshielded with respect to alkyl imido complexes, cf. V(NTol)(CH2SiMe3)3 (Tol = tolyl), 8 = 1048 (21). Provided the NMR/reactivity correlations hold also when the substituents at the imido nitrogen are varied, one might speculate that suitably derivatized aryl rests (for instance, by introducing electron-withdrawing groups) could produce more deshielded 51V resonances and, at the same time, more active catalysts for ethylene polymerization. Further experimental and theoretical studies in that direction could be rewarding. 

When an elementary reaction that shows such an NMR/reactivity correlation is the rate-determining step of a catalytic cycle, the overall activities of the corresponding catalysts can be related to their chemical shifts. Despite the great potential use, only few such examples are yet known empirically. With the approach detailed in the present paper, theoretical searches for new such correlations are now possible from first principles. Even though NMR/reactivity relations have been predicted only for model compounds, as with the alkylvanadium(V) species discussed, there is a good chance that similar relationships would be observable for the corresponding real systems. Once such a correlation is established, potential catalysts could be readily screened via NMR spectroscopy. It is well possible that in this way active catalysts may be identified which were not or could not be considered in the theoretical computations. 

Garbon-13 NMR shifts correlate with reactivities . Transition states may vary with the nucleophile . Steric effects of substituents are predictable reaction at an occupied position is slow , and the seven-membered ring series is slower than the six. 

There have, however, been attempts to correlate Q-e values and hence reactivity ratios to, for example, c NMR chemical shifts 50 or the results of MO calculations 51153 and to provide a better theoretical basis for the parameters. Most recently, Zhan and Dixon153 applied density functional theory to demonstrate that Q values could be correlated to calculated values of the relative free energy for the radical monomer reaction (PA + Mn — PA ). The e values were correlated to values of the electronegativities of monomer and radical. 

DSP treatments allow one to separate the field and mesomeric effects of substituents on chemical reactivities and physical properties (electronic and NMR spectra, etc.) of organic compounds. In Section 8.3 we will discuss heterolytic dediazoniation of substituted benzenediazonium ions. For this series of reactions the classical Hammett equation completely fails to give useful results (see Fig. 8-1), but the DSP treatment yields a good and mechanistically very meaningful correlation. 

NMR chemical shift data from die protons ortho or para to the electron-withdrawing group can be used to determine the reactivity of the monomer indirecdy.58 Carbon-13 and 19F NMR can be used to probe the chemical shift at the actual site of nucleophilic reaction. In general, lower chemical shifts correlate widi lower monomer reactivity. Carter reported that a compound might be appropriate for nucleophilic displacement if the 13 C chemical shift of an activated Buoride ranges from 164.5 to 166.2 ppm in CDC1359. 

These correlations are of generally poorer precision than those for reactivity and F-nmr shift data, and in the case of the ortho data set they required rejection of data for the substituents CHjCO, NO2, and CO2R (which give large deviations if included). Ihe latter deviations may be associated with ortho chelation effects. In any case, the Kj and X patterns, as noted above, appear essentially as expected. 

There are several systematic nuclear magnetic resonance studies of the interaction between the substituents and the protons and ring atoms of five-membered heterocycles. In some 2-substituted furans, thiophenes, selenophenes, and tellurophenes there is a linear correlation between the electronegativity of the chalcogen and several of the NMR parameters.28 As there also is a good correlation between the shifts of the corresponding protons and carbons in the four heterocycles, the shifts of unknown selenophene and tellurophene derivatives can be predicted when those of thiophene are known. This is of special interest for the tellurophene derivatives, since they are difficult to synthesize. In the selenophene series, where a representative set of substituents can be introduced in the 2- as well as in the 3-position, the correlation between the H and 13C shifts and the reactivity parameters according to Swain and Lupton s two-parameter equation... 

It has been expected that reactivity of metalated pnictides is strongly dependent on the degree of association. One consequence is that the basicity of amides has been correlated with the grade of association by means of extensive 7Li and 16N NMR investigations (16, 17). In contrast, knowledge of structure-reactivity relationships for... 

The major effect of new advanced techniques on catalyst structure is found in zeolite catalysis. NMR techniques, especially MASNMR, have helped to explain aluminum distribution in zeolites and to increase our understanding of critical parameters in zeolite synthesis and crystallization. MASNMR, combined with TEM, STEM, XPS, and diagnostic catalytic reaction probes, has advanced our knowledge of the critical relationship between the structure and reactivity patterns of zeolites in the chemical fuels industry. Throughout the symposium upon which this book is based, many correlations were evident between theoretical quantum mechanical calculations and the structures elucidated by these techniques. 

NMR is predictive of reactivity within a series Within a set of common donors (i.e., galactose) in which the C2 position is constant, the H chemical shift of the anomeric carbon appears to be a good predictor of relative reactivity. Little correlation is found when the C2 position is varied, or between different donor pyranoses. The extent to which this correlation will prove useful remains in question. We envision that its primary applicability may be in trouble shooting a failed synthesis when a complex donor does not behave as predicted. Checking the H NMR of the advanced intermediate may quickly reveal that the RRV value for the pyranose of interest is not as expected, and hence the reactivity is not as expected. 

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