Changing The Boron Environment – A Powerful Tool To Tune The Reactivity
Shortly after its discovery in 1942, sodium borohydride (NaBH4) has rapidly become recognized as an extremely convenient reductant 1-3. The borohydrides nowadays are considered as an extremely helpful class of substances in modern organic chemistry and material science. Currently, the diverse boron hydrides derivatives are widely used as selective reducing agents in fine organic synthesis 3-5.
Moreover, borohydrides are promising for the hydrogen storage due to their high hydrogen content.6,7 The hydrolysis/alcoholysis processes are certainly significant for the rapid release huge amounts of H2 from borohydrides.
But what factors are exactly influences their reactivity? Researchers from the Laboratory of Metal Hydrides (MH-Lab) have glimpsed in the interplay among boron environment and reactivity of boron hydrides.
Previously it was believed that increased acidity of the proton donor and increased basicity of hydride leads to facilitation of the process of hydrogen release. But, in recent work,11 it was revealed that despite the increase in the rate of the alcoholysis reaction, the substitution of H− by RO− at the boron atom results in a decrease of the basicity of the hydride moiety. In order to explain enhanced reactivity of alkoxyborohydrides, we analyzed the hydride donating ability, equal to free Gibbs energy of the hydride detaching reaction (Figure 1). It was found out that the ability of alkoxyborohydrides to transfer hydride increases upon the consecutive substitution of H− by RO− at the boron atom. This results in a significant facilitation of the cascade BH4– alcoholysis, which energetic profile has a descending sawtooth shape (Figure 2).
Thus, the reactivity of borohydrides is connected with their ability to act as an H– donor that can be increased by B–H bond activation.8 The reactivity of tetra-coordinated borohydrides in reduction processes is substantially determined by the coordination environment of the boron atom. As it has been shown by H. C. Brown 2,3, the modification of parent tetrahydroborate MBH4 provides an effective tool for the fine-tuning its reactivity towards the reactions with hydride transfer as well as the selectivity and stereospecificity of reduction.
As a general rule, the electron withdrawing groups (EWG) increase the electron deficiency (Lewis acidity) of a boron atom, whereas the electron donating groups (EDG) stabilize planar configuration of tricoordinated boranes due to the donation of electron density from the substituents to the empty 2pz orbital of boron atom 9. However, the stability of tetracoordinated boron hydrides [R3BH]− could not be explained solely on the base of electron withdrawing or electron donating ability of the substituent groups 10.
Thermodynamic hydricity, i.e. hydride donating ability (HDA), determined as Gibbs free energy (ΔG°H−) for the reaction in Figure 1 is a very important characteristic of transition metal hydrides that describes their reactivity and is used for the rational design of catalytic reactions 16,17. For that reason, the hydride donating ability (HDA) of borohydrides can also be considered as a measure of their reactivity 18.
However, as it was shown recently by Haiden and Lathem 18, the experimental examination of the hydride donating ability (HDA) of main group hydrides is problematic due to their instability in polar media. Moreover, these compounds have low E–H bond polarizability in comparison with transition metal hydrides and thus the hydride cannot be torn away even in the presence of excess strong Lewis acid. Therefore, the computational methods become extremely helpful in prediction their properties.
Since HDA is a Gibbs free energy (ΔG°H−) of hydride transfer reaction in a chosen media, the lowest HDA value corresponds to the easiest ability of borohydride for hydride transfer to the substrate. Moreover, HDA values are strongly solvent-dependent, in this way switching solvents offers a strategy for changing the Gibbs free energy of hydride transfer (ΔG°H− or HDA) 16. Hence, the evaluation of HDA for various substituted tetracoordinated borohydrides in the media of different polarity in order to pinpoint the ways of B–H bond activation and enhancing the borohydrides reducing power was performed.
However, HDA values for some boranes were previously calculated by Haiden and Lathem 18, but current study was mainly focused on the well-known boron species used for H2 activation or as reducing agents. Some of those compounds are very reactive species, so their reactivity could not be quantified experimentally. Moreover, the analysis of general trends as well as the analysis of substituents impact was not performed.
The hydride donating ability (HDA) values were analyzed for 90 tetracoordinated borohydrides in solvents of different polarity MeCN (ε = 35.7) (HDA varying from 118.2 to 13.4 kcal/mol) and DCM (ε = 8.9) (HDA varying from 124.4 to 18.2 kcal/mol). The analysis of the data obtained shows that the HDA values correlate reasonably well with the Lewis acidity parameters (AN, HA and FA) of parent trigonal boranes (L3B). The substituents at the boron atom provide a useful tool for the increasing a B–H bond reactivity (reduction power) towards the reactions with hydride transfer as well as a fine-tuning of boron hydrides selectivity in the reduction processes.
By varying the number of substituents and their nature, it is possible not only to change the properties of trigonal boranes from highly electrophilic (represented by halogenide- and pseudohalogenide-boranes) to highly nucleophilic (as alkoxy- and amidoboranes), but also to repolarize the boron-bound hydrogen atom and make the proton transfer process more favourable than the hydride transfer. All these findings open new perspectives for further modifications and rational design of new catalytic and reductive systems based on boron hydrides.
These findings are described in the article entitled Hydride donating abilities of the tetracoordinated boron hydrides, recently published in the Journal of Organometallic Chemistry. This work was conducted by Igor E. Golub, Oleg A. Filippov, Natalia V. Belkova, Lina M. Epstein and Elena S. Shubina from the MH-Lab at A.N.Nesmeyanov Institute of Organoelement Compounds of Russian Academy of Sciences (INEOS RAS), Moscow.
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