Elsevier

Tetrahedron Letters

Volume 60, Issue 3, 17 January 2019, Pages 310-321
Tetrahedron Letters

The DFT study on Rh–C bond dissociation enthalpies of (iminoacyl)rhodium(III)hydride and (iminoacyl)rhodium(III)alkyl

https://doi.org/10.1016/j.tetlet.2018.12.042Get rights and content

Highlights

  • The B97D3 method gave both the lowest RMSE and the highest accuracy for relative and absolute Rh–C BDEs.

  • When R1 is –C6H4R, anti-spin-delocalization effect was found in meta-NO2, para-NO2 and meta-CHO of R.

  • For R1 = cycloalkyl, there is a stronger substituent effect on Rh–C BDEs than R1 = n-alkyl.

  • For R2 substituted by EWGs, EDGs and CEGs, the linear relationship between the Rh–C BDEs with σp+ values was found.

  • When –CH2CH2R2 group is substituted by cycloalkyl, the Rh–C BDEs decrease obviously with the increase of ring size.

Abstract

Rhodium transition-metal-organic cooperative catalysis, which has been intensively studied by many chemists, represents a great success in C–H bond activation because of high efficiencies and selectivities. Typically, in the reaction mechanism of aldehyde and alkene catalyzed by Rh(I) complex and 2-amino-3-picoline, two kinds of metala-cyclic transition-metal complexes of (iminoacyl)rhodium(III)hydride and (iminoacyl)rhodium(III) alkyl are generally formed. The two complexes play an important role in the overall reaction, in which the Rh–C bond formations are involved. So it is meaningful to understand the strength of Rh–C bond, which can be measured by the homolytic bond dissociation enthalpies (BDEs). To this end, we first calculated 16 relative Rh–C BDEs of Tp′Rh(CNneopentyl)RH (Tp′ = hydridotris-(3,5-dimethylpyrazolyl)borate) by 19 density functional theory (DFT) methods. Furthermore, the 5 absolute Rh–C BDEs of Rh transition-metal complexes were also calculated. The results show that the B97D3 is the most accurate method to predict the relative and absolute Rh–C BDEs and the corresponding RMSE values are the smallest of 2.8 and 3.3 kcal/mol respectively. Therefore, the Rh–C BDEs of (iminoacyl)rhodium(III)hydride and (iminoacyl)rhodium(III)alkyl as well as the substituent effects were investigated by using the B97D3 method. The results indicated that the different substituents exhibit different effects on different types of Rh–C BDEs. In addition, the analysis including the natural bond orbital (NBO) as well as the energies of frontier orbitals were performed in order to further understand the essence of the Rh–C BDE change patterns.

Graphical abstract

The Rh–C BDEs of metala-cyclic transition-metal complexes (iminoacyl)rhodium(III)hydride and (iminoacyl)rhodium(III)alkyl were performed by the B97D3 method.

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Introduction

Rhodium transition-metal-organic cooperative catalysis, which has been intensively studied by many chemists, represents a great success in C–H bond activation because of high efficiencies and selectivities [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15]. For example, Jun et al. [14] developed an effective general intermolecular hydroacylation of alkene with aliphatic aldehyde using a catalyst mixture of Rh(I) complex and 2-amino-3-picoline, and the ketones were obtained by C–H bond activation. Willis et al. [15] have developed a new method for intermolecular hydroacylation of commercially available methylsulfanyl-substituted aldehydes and functionalized alkenes based on the proposed formation of chelation-stabilized acyl–rhodium intermediates, and the hydroacylation adducts are in good to excellent yields. Therefore, the mechanism of rhodium transition-metal-organic cooperative catalysis in C–H bond activation has been researched by many chemists [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29], [30], [31], [32], [33], [34]. Typically, the promoted mechanism of aldehyde and alkene catalyzed by Rh(I) complex and 2-amino-3-picoline is shown in Fig. 1 [1]. Three stages are included: the C–H activation to form rhodium hydride (stage I), the alkene insertion into the Rh-H bond to give the Rh-alkyl complex (stage II), and the C–C bond formation (stage III).

It can be seen that the intermediates of two metala-cyclic transition-metal complexes (iminoacyl)rhodium(III)hydride (G) and (iminoacyl)rhodium(III) alkyl (I) play an important role in the overall reaction, in which the Rh–C bond formations are involved. Therefore, the strength of the Rh–C bond which can be measured by the homolytic bond dissociation enthalpy (BDE) is very important. Unfortunately, the experimental determination of the absolute BDE values of the polyatomic transition-metal complexes is exceptionally difficult, which can be achieved only when the demanding criteria are met [35], [36], [37]. For example, Jones et al. [37] obtained the experimental relative (to R = Ph) Rh–C BDEs, not the absolute ones of complexes Tp′Rh(CNneopentyl)RH (R = aryl, alkyl, alkenyl, etc.) by using a combination of kinetic techniques. With the rapid development of density functional theory (DFT) method in quantum chemistry, the high-precision BDE calculations of the transition-metal complexes can be performed [38], [39], [40], [41]. Qi et al. [38] found that the TPSS/LANL2DZ + p method can give a precision of ca. 2.2 kcal/mol for the calculation of the Co-C BDEs of 28 structurally unrelated organocobalts, and the correlation coefficient between the experimental and theoretical Co-C BDEs is 0.9034. Shi et al. [39] found that TPSS method can give a precision of ca. 1.2 kcal/mol for the calculation of the Ir-H BDEs of 17 different complexes, and the correlation coefficient between the experimental and theoretical Ir-H BDEs is 0.9873. As to the Rh–C BDE calculations by DFT method, there are a few reports by theoretical chemists [40], [42], [43]. For instance, Perutz et al. [40] used B3PW91 method to calculate the Rh–C BDE values of Rh(η5-C5H5)(H)(ArF)(PH3) (ArF = C6FnH5−n, n = 0–5), which fits a linear function of the number of fluorine substituents, with different coefficients corresponding to ortho-, meta-, and para-fluorine. Hasanayn et al. [43] used B3LYP method to calculate the Rh–C BDEs of cis- and trans-Rh(PMe3)2(CO)X (X = F, Cl, Br, I, CN) and the results indicated that the largest BDE exhibited when X = F.

In the present study, the Rh–C BDEs as well as the substituent effects of transition-metal complexes G and I in the rhodium transition-metal-organic cooperative catalysis mechanism, which can also be found in C–H activation of alcohols and alkenes, C–C activation of ketones and alkenes etc, were systematically studied by using DFT methods. It is helpful to further understand the Rh–C formations of complexes G and I in rhodium transition-metal-organic cooperative catalysis and can provide more guidance for the experimental researches.

Section snippets

Computational method

The homolytic bond dissociation enthalpy (BDE) of Rh–C bonds of intermediates (complexes G and I) is described as the enthalpy change of the following reaction at 1 atm and 298.15 K [44] in the gas phase:

The enthalpy of each species can be calculated by the following equation:H298K=E+ZPE+Htrans+Hrot+Hvib+RT

In this equation, ZPE represents the zero point energy. The Htrans, Hrot, and Hvib are the standard temperature correction terms calculated with equilibrium statistical mechanics with

The DFT precisions of Rh–C BDEs

At first, the 16 Rh transition-metal complexes Tp′Rh(CNneopentyl)RH (R = alkyl, alkenyl, aryl, cyanoalkyl, etc.) structurally similar to complexes G and I, which only have experimental relative (to R = Ph) Rh–C BDE values [37], [42], [78], were selected as our training set. Then, we selected 19 DFT methods to calculate the 16 Rh–C BDEs and the results are listed in the Supporting Information. Among these DFT methods, the TPSS, BP86, B3PW91 etc. were included which were testified to be suitable

Conclusions

Rhodium transition-metal-organic cooperative catalysis represents a great success in C–H bond activation because of high efficiencies and selectivities, in which the formations of Rh–C bonds are involved in complexes G and I. It is meaningful to understand the strength of the Rh–C bond, which can be measured by the BDE values. In our present study, the 16 relative Rh–C BDEs of transition-metal complexes Tp′Rh(CNneopentyl)RH were selected as the training set and calculated by using 19 DFT

Acknowledgements

This project is sponsored by the Shanghai University of Engineering Science Innovation Funds for Graduate Students (No.17KY0409, No.18KY0412). We also thank the Shanghai Supercomputer Center for the computational resources.

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