Determining the π-Acceptor Properties of N-Heterocyclic Carbenes by Measuring the <sup>77</sup>Se NMR Chemical Shifts of Their Selenium Adducts

Liske, Annika; Verlinden, Kathrin; Buhl, Hannes; Schaper, Klaus‐Jürgen; Ganter, C.

doi:10.1021/om400858y

Cited by 333 publications

(311 citation statements)

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“…The latter is dependant on the s-character of the σ-orbital and thus can be obtained directly from the 1 J CH coupling constant of the imidazolium salt [37–39] (which can be regarded as the H + complex of the carbene and therefore π-influences are avoided) or by the N–C–N angle at the carbene [40], which also correlates with the 13 C NMR chemical shift [41]. In 2013, Ganter presented the 77 Se NMR chemical shift of the respective selenoureas as a suitable probe to determine directly the π-influence of the carbene [42–43], as the paramagnetic shift tensor has the largest influence on the 77 Se NMR chemical shift. This method is so far redundant [44] to the method of determining the 31 P NMR chemical shifts of the respective NHC–phosphinidene adducts [45–46].…”

Section: Introductionmentioning

confidence: 99%

Experimental and theoretical investigations on the high-electron donor character of pyrido-annelated N-heterocyclic carbenes

Nonnenmacher¹,

Buck²,

Kunz³

2016

Beilstein J. Org. Chem.

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SummaryRh(CO)2Cl(NHC) complexes of dipyrido-annelated N-heterocyclic carbenes were prepared. From the C–H coupling constant of the respective imidazolium salts and the N–C–N angle of the N-heterocyclic carbene (NHC), a weaker σ-donor character than that of typical unsaturated NHCs is expected. However, the IR stretching frequencies of their Rh(CO)2Cl complexes suggest an electron-donor character even stronger than that of saturated NHCs. We ascribe this to the extremely weak π-acceptor character of the dipyrido-annelated NHCs caused by the conjugated 14 πe− system that thus allows for an enhanced Rh–CO backbonding. This extremely low π-acceptor ability is also corroborated by the 77Se NMR chemical shift of −55.8 ppm for the respective selenourea, the lowest value ever measured for imidazole derived selenoureas. DFT-calculations of the free carbene confirm the low σ-donor character by the fact that the σ-orbital of the carbene is the HOMO−1 that lies 0.58 eV below the HOMO which is located at the π-system. Natural population analysis reveals the lowest occupation of the pπ-orbital for the saturated carbene carbon atom and the highest for the pyrido-annelated carbene. Going from the free carbene to the Rh(CO)2Cl(NHC) complexes, the increase in occupancy of the complete π-system of the carbene ligand upon coordination is lowest for the pyrido-annelated carbene and highest for the saturated carbene.

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Section: Introductionmentioning

confidence: 99%

Experimental and theoretical investigations on the high-electron donor character of pyrido-annelated N-heterocyclic carbenes

Nonnenmacher¹,

Buck²,

Kunz³

2016

Beilstein J. Org. Chem.

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“…[4] Within this group, triazolylidenes embedded in tricyclic architectures exhibit an extraordinary p-acceptor character,q uantitatively assessed by 77 Se NMR spectroscopy [5] in a model ligand L (Figure 1). [4] Within this group, triazolylidenes embedded in tricyclic architectures exhibit an extraordinary p-acceptor character,q uantitatively assessed by 77 Se NMR spectroscopy [5] in a model ligand L (Figure 1).…”

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confidence: 99%

Au^I‐Catalyzed Haloalkynylation of Alkenes

García-Fernández

Llorente

Iglesias‐Sigüenza

et al. 2019

Chemistry A European J

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The formal insertiono fa lkenes into aromatic chloro-a nd bromoalkynes takes place under cationic gold catalysis. This haloalkynylation reactionc an be performed with cyclic, gem-disubstituted and monosubstituted alkenes, using BINAP,t riazolo [4,3-b]isoquinolin-3-ylidenel igands or SPhos, respectively.T he products were isolated in moderatet oe xcellent yieldsa nd with complete diastereo-and regioselectivity;t he halogen atom bondingt he more substituted carbon of the alkene.P reliminary experiments showedt hat the enantioselective haloalkynylation of cyclopentene can be performedw ith (S)-BINAP to afford the insertion productsw ithmoderate to good enantioselectivities.Cationicg old complexes are the most active catalysts for the electrophilic activation of carbon-carbon multiple bonds, being particularlye fficient and selective for alkynes. [1] In 2010, Echavarren and co-workers discoveredt he intermolecular [2+ +2] cycloadditiono fa lkenes and alkynes, one of the most fundamental transformations in this field and av aluablet ool for the synthesiso fc yclobutenes [2] (Scheme 1A). Very recently, the group of Zhang showed that chloroalkynes are also suitable substrates in such [2+ +2] cycloadditions, leading in this case to valuable chlorocyclobutenes [3] (Scheme 1B). The reaction with bromoalkynes,h owever, led to ap oor yield of the cycloadduct in as ingle example.On the other hand, we have been interested in the design of NHCs featuring heterobi(tri)cyclic architectures,w hich offers opportunities for the modulation of their sterica nd electronic properties. [4] Within this group, triazolylidenes embedded in tricyclic architectures exhibit an extraordinary p-acceptor character,q uantitatively assessed by 77 Se NMR spectroscopy [5] in a model ligand L (Figure 1). [6] Combinedw ith ah igh level of steric protection, the electronic properties of this type of NHCs make them particularly attractive for Au I -catalyzed reactions, particularly fort hose in which the activation of the substrate is the rate-limitings tep. [7] Moreover,t he performance of such ligands hasa lready been shown in challenging asymmetric intermolecular cycloaddition reactions of allenamides. [8] On this basis, we decided to explore the behavior of this type of ligand in the Au I -catalyzed cycloaddition of bromoalkynes with alkenes.T he reactiono f( bromoethynyl)benzene 1a with cyclopentene 2 was chosen asaplatform for preliminary studies. The reaction performed with precatalyst A (L·AuCl) under Zhang's conditions {5 mol % B ((IPr·AuCl), IPr = 1,3bis(2,6-diisopropylphenyl)-1,3-dihydro-2H-imidazol-2-ylidene)/ 10 mol %N aBArF (sodium tetrakis[3,5-bis(trifluoromethyl)phenyl]borate) in DCE, [1a] = 0,1 m,4 0 8C} afforded the expected cyclobutene 3,i solated in 46 %y ield and,s urprisingly,t he bromoalkynylation (formal insertion) product 4a was isolated as a minor component in 16 %y ield (Table 1, entry 1). To check whethert his minor product is also formed under Zhang's experimental conditions, the reaction was also performed with IPrAuCl p...

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“…We believe that the study presented here will contribute significantly not only to the intimate comprehension of the intrinsic π bonding characteristics of NHCs, but also to the understanding and perfectioning of the computational tools to study metal–ligand interactions. This could facilitate the analysis of experimental quantification of the NHCs electronic properties, as well as the design of catalytic TM‐NHC complexes with the desirable electronic properties.…”

Section: Discussionmentioning

confidence: 99%

Estimating π binding energy of N‐Heterocyclic carbenes: The role of polarization

Rezabal

Frison

2015

J Comput Chem

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In this work, the tuneability of the π acceptor or donor properties of a set of N-heterocyclic carbenes (NHCs) with a wide spectrum of electronic characteristics is established by means of density functional theory and energy decomposition analysis (EDA) tools. Even though the main orbital interaction contribution to the NHC coordination is the σ donation, a significant contribution of the π interactions to the bond is observed. By means of carefully selected coordination sites, different contributions to the π interactions could be identified and isolated. It includes not only the well known back donation and donation interactions, but also the intrafragment polarization, which has not been considered in previous studies. This can be obtained through the use of the extended transition state method for EDA combined with the natural orbitals for chemical valence and the constrained space orbital variation analysis. The contributions vary with the position of the heteroatoms and the presence of exocyclic substituents; the donation/backdonation π interactions between NHC and the coordination site can range between 2 and 61% of the total π orbital interactions, while the rest is owed to intrafragment polarization. Our results do not only contribute to the understanding of the electronic structure of NHC-based complexes, giving ways to improve their catalytic properties, but also provide comprehension on the modelization methods used to study their donor-acceptor interactions.

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Determining the π-Acceptor Properties of N-Heterocyclic Carbenes by Measuring the ⁷⁷Se NMR Chemical Shifts of Their Selenium Adducts

Cited by 333 publications

References 73 publications

Experimental and theoretical investigations on the high-electron donor character of pyrido-annelated N-heterocyclic carbenes

Experimental and theoretical investigations on the high-electron donor character of pyrido-annelated N-heterocyclic carbenes

Au^I‐Catalyzed Haloalkynylation of Alkenes

Estimating π binding energy of N‐Heterocyclic carbenes: The role of polarization

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Determining the π-Acceptor Properties of N-Heterocyclic Carbenes by Measuring the 77Se NMR Chemical Shifts of Their Selenium Adducts

Cited by 333 publications

References 73 publications

Experimental and theoretical investigations on the high-electron donor character of pyrido-annelated N-heterocyclic carbenes

Experimental and theoretical investigations on the high-electron donor character of pyrido-annelated N-heterocyclic carbenes

AuI‐Catalyzed Haloalkynylation of Alkenes

Estimating π binding energy of N‐Heterocyclic carbenes: The role of polarization

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Determining the π-Acceptor Properties of N-Heterocyclic Carbenes by Measuring the ⁷⁷Se NMR Chemical Shifts of Their Selenium Adducts

Au^I‐Catalyzed Haloalkynylation of Alkenes