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Investigation of transition metal-catalyzed nitrene transfer reactions in water.

ABSTRACT: Transition metal-catalyzed nitrene transfer is a powerful method for incorporating new CN bonds into relatively unfunctionalized scaffolds. In this communication, we report the first examples of site- and chemoselective CH bond amination reactions in aqueous media. The unexpected ability to employ water as the solvent in these reactions is advantageous in that it eliminates toxic solvent use and enables reactions to be run at increased concentrations with lower oxidant loadings. Using water as the reaction medium has potential to expand the scope of nitrene transfer to encompass a variety of biomolecules and highly polar substrates, as well as enable pH control over the site-selectivity of CH bond amination.

SUBMITTER: Alderson JM

PROVIDER: S-EPMC6657797 | biostudies-literature | 2018 Oct

REPOSITORIES: biostudies-literature

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Investigation of transition metal-catalyzed nitrene transfer reactions in water.

Alderson Juliet M JM Corbin Joshua R JR Schomaker Jennifer M JM

Bioorganic & medicinal chemistry 20180411 19

Transition metal-catalyzed nitrene transfer is a powerful method for incorporating new CN bonds into relatively unfunctionalized scaffolds. In this communication, we report the first examples of site- and chemoselective CH bond amination reactions in aqueous media. The unexpected ability to employ water as the solvent in these reactions is advantageous in that it eliminates toxic solvent use and enables reactions to be run at increased concentrations with lower oxidant loadings. Using water as t ...[more]

PMID: 29681485

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Project description:Reported herein are thermochemical studies of hydrogen atom transfer (HAT) reactions involving transition metal H-atom donors M(II)LH and oxyl radicals. [Fe(II)(H(2)bip)(3)](2+), [Fe(II)(H(2)bim)(3)](2+), [Co(II)(H(2)bim)(3)](2+), and Ru(II)(acac)(2)(py-imH) [H(2)bip = 2,2'-bi-1,4,5,6-tetrahydropyrimidine, H(2)bim = 2,2'-bi-imidazoline, acac = 2,4-pentandionato, py-imH = 2-(2'-pyridyl)imidazole)] each react with TEMPO (2,2,6,6-tetramethyl-1-piperidinoxyl) or (t)Bu(3)PhO(*) (2,4,6-tri-tert-butylphenoxyl) to give the deprotonated, oxidized metal complex M(III)L and TEMPOH or (t)Bu(3)PhOH. Solution equilibrium measurements for the reaction of [Co(II)(H(2)bim)(3)](2+) with TEMPO show a large, negative ground-state entropy for hydrogen atom transfer, -41 +/- 2 cal mol(-1) K(-1). This is even more negative than the DeltaS(o)(HAT) = -30 +/- 2 cal mol(-1) K(-1) for the two iron complexes and the DeltaS(o)(HAT) for Ru(II)(acac)(2)(py-imH) + TEMPO, 4.9 +/- 1.1 cal mol(-1) K(-1), as reported earlier. Calorimetric measurements quantitatively confirm the enthalpy of reaction for [Fe(II)(H(2)bip)(3)](2+) + TEMPO, thus also confirming DeltaS(o)(HAT). Calorimetry on TEMPOH + (t)Bu(3)PhO(*) gives DeltaH(o)(HAT) = -11.2 +/- 0.5 kcal mol(-1) which matches the enthalpy predicted from the difference in literature solution BDEs. A brief evaluation of the literature thermochemistry of TEMPOH and (t)Bu(3)PhOH supports the common assumption that DeltaS(o)(HAT) approximately 0 for HAT reactions of organic and small gas-phase molecules. However, this assumption does not hold for transition metal based HAT reactions. The trend in magnitude of |DeltaS(o)(HAT)| for reactions with TEMPO, Ru(II)(acac)(2)(py-imH) << [Fe(II)(H(2)bip)(3)](2+) = [Fe(II)(H(2)bim)(3)](2+) < [Co(II)(H(2)bim)(3)](2+), is surprisingly well predicted by the trends for electron transfer half-reaction entropies, DeltaS(o)(ET), in aprotic solvents. This is because both DeltaS(o)(ET) and DeltaS(o)(HAT) have substantial contributions from vibrational entropy, which varies significantly with the metal center involved. The close connection between DeltaS(o)(HAT) and DeltaS(o)(ET) provides an important link between these two fields and provides a starting point from which to predict which HAT systems will have important ground-state entropy effects.

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Investigation of transition metal-catalyzed nitrene transfer reactions in water.

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Investigation of transition metal-catalyzed nitrene transfer reactions in water.

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