Quantifying Through‐Space Substituent Effects

Rebecca J Burns,Catherine Adam,Ioulia K Mati,Kamila B Muchowska,Scott L Cockroft

doi:10.1002/ange.202006943

Rebecca J Burns, Catherine Adam + Show 3 more

Open Access

https://doi.org/10.1002/ange.202006943

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Journal: Angewandte Chemie	Publication Date: Jul 17, 2020
Citations: 8	License type: CC BY 4.0

Affiliation: University of Edinburgh

Abstract

AbstractThe description of substituents as electron donating or withdrawing leads to a perceived dominance of through‐bond influences. The situation is compounded by the challenge of separating through‐bond and through‐space contributions. Here, we probe the experimental significance of through‐space substituent effects in molecular interactions and reaction kinetics. Conformational equilibrium constants were transposed onto the Hammett substituent constant scale revealing dominant through‐space substituent effects that cannot be described in classic terms. For example, NO2 groups positioned over a biaryl bond exhibited similar influences as resonant electron donors. Meanwhile, the electro‐enhancing influence of OMe/OH groups could be switched off or inverted by conformational twisting. 267 conformational equilibrium constants measured across eleven solvents were found to be better predictors of reaction kinetics than calculated electrostatic potentials, suggesting utility in other contexts and for benchmarking theoretical solvation models.

Highlights

Systematic variation of substituents is often exploited to tune or rationalize chemical behavior and reaction mechanisms.[1]
The transferability of Hammett substituent constants derived from conformational equilibrium constants was
We reasoned that through-space effects would be most likely to dominate over through-bond influences when polar substituents were positioned ortho to the biaryl bond

Summary

Introduction

Supporting information and the ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10. 1002/anie.202006943. 2020 The Authors. Examined across eleven different solvents (Table 1, Figure 4 and Figure 5), and their ability to account for through-space influences on the kinetics of a simple model reaction was assessed (Figure 6)

Results and Discussion

Conclusion

Conflict of interest