Universität zu Köln

Sheehy, Kevin J., Bateman, Lorraine M., Flosbach, Niko T., Breugst, Martin ORCID: 0000-0003-0950-8858 and Byrne, Peter A. (2020). Competition between N and O: use of diazineN-oxides as a test case for the Marcus theory rationale for ambident reactivity. Chem. Sci., 11 (35). S. 9630 - 9648. CAMBRIDGE: ROYAL SOC CHEMISTRY. ISSN 2041-6539

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Abstract

The preferred site of alkylation of diazineN-oxides by representative hard and soft alkylating agents was established conclusively using the(1)H-N-15 HMBC NMR technique in combination with other NMR spectroscopic methods. Alkylation of pyrazineN-oxides (1and2) occurs preferentially on nitrogen regardless of the alkylating agent employed, whileO-methylation of pyrimidineN-oxide (3) is favoured in its reaction with MeOTf. As these outcomes cannot be explained in the context of the hard/soft acid/base (HSAB) principle, we have instead turned to Marcus theory to rationalise these results. Marcus intrinsic barriers (Delta G double dagger 0) and Delta(r)G degrees values were calculated at the DLPNO-CCSD(T)/def2-TZVPPD/SMD//M06-2X-D3/6-311+G(d,p)/SMD level of theory for methylation reactions of1and3by MeI and MeOTf, and used to derive Gibbs energies of activation (Delta G(double dagger)) for the processes ofN- andO-methylation, respectively. These values, as well as those derived directly from the DFT calculations, closely reproduce the observed experimentalN- vs.O-alkylation selectivities for methylation reactions of1and3, indicating that Marcus theory can be used in a semi-quantitative manner to understand how the activation barriers for these reactions are constructed. It was found thatN-alkylation of1is favoured due to the dominant contribution of Delta(r)G degrees to the activation barrier in this case, whileO-alkylation of3is favoured due to the dominant contribution of the intrinsic barrier (Delta G double dagger 0) for this process. These results are of profound significance in understanding the outcomes of reactions of ambident reactants in general.

Item Type:	Journal Article
Creators:	Creators Email ORCID ORCID Put Code Sheehy, Kevin J. UNSPECIFIED UNSPECIFIED UNSPECIFIED Bateman, Lorraine M. UNSPECIFIED UNSPECIFIED UNSPECIFIED Flosbach, Niko T. UNSPECIFIED UNSPECIFIED UNSPECIFIED Breugst, Martin UNSPECIFIED orcid.org/0000-0003-0950-8858 UNSPECIFIED Byrne, Peter A. UNSPECIFIED UNSPECIFIED UNSPECIFIED
URN:	urn:nbn:de:hbz:38-318764
DOI:	10.1039/d0sc02834g
Journal or Publication Title:	Chem. Sci.
Volume:	11
Number:	35
Page Range:	S. 9630 - 9648
Date:	2020
Publisher:	ROYAL SOC CHEMISTRY
Place of Publication:	CAMBRIDGE
ISSN:	2041-6539
Language:	English
Faculty:	Faculty of Mathematics and Natural Sciences
Divisions:	Faculty of Mathematics and Natural Sciences > Department of Chemistry > Institute of Organic Chemistry
Subjects:	no entry
Uncontrolled Keywords:	Keywords Language HARD/SOFT-ACID/BASE PRINCIPLE; HETERONUCLEAR SHIFT CORRELATION; ELECTRON-TRANSFER REACTIONS; NMR COORDINATION SHIFTS; N-15 NMR; SOFT ACIDS; BASES HSAB; REGIOSELECTIVE SYNTHESIS; STRUCTURE ELUCIDATION; FREE-ENERGY Multiple languages Chemistry, Multidisciplinary Multiple languages
Refereed:	Yes
URI:	http://kups.ub.uni-koeln.de/id/eprint/31876