Electronic Structure and Reactivity of Metal Carbenes

Straßner, Thomas

doi:10.1007/b98761

Cited by 44 publications

(9 citation statements)

References 165 publications

(209 reference statements)

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“…Therefore, Au→C π* back-bonding in the transition state for allene stereomutation will be diminished relative to the ground state with a concomitant increase in C→Au σ donation. Because the σ-bonding framework of two-coordinate gold complexes involves a three-center, four-electron L–Au–C σ hyperbond through which both ligands compete for σ donation into a partially occupied sd(σ) gold(I) orbital, greater L→Au σ donation from P relative to IPr would therefore destabilize the transition state for allene stereomutation ( TS1 ) for ( P )Au(π-allene) + relative to (IPr)Au(π-allene) + .…”

Section: Discussioncontrasting

confidence: 65%

“…Our kinetic data for the racemization of 1,3-disubstituted allenes catalyzed by gold phosphine and NHC complexes highlight some notable differences regarding the behavior of (IPr)Au(π-allene) + and ( P )Au(π-allene) + complexes. For example, the energy barriers for associative allene exchange for (IPr)Au(π-allene) + complexes must be significantly larger than for ( P )Au(π-allene) + complexes, which presumably reflects the greater steric shielding of the gold center by the aryl isopropyl groups of the IPr ligand relative to the P ligand . We have previously documented the strong dependence of the rate of intermolecular π-ligand exchange of two-coordinate gold π complexes on the steric bulk of the supporting ligand, in particular the low energy barriers for π-ligand exchange observed for sterically unencumbered Ph 3 PAu π-complexes (Δ G ⧧ ≤ 10 kcal mol –1 ) …”

Section: Discussionmentioning

confidence: 63%

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Kinetics and Mechanism of Allene Racemization Catalyzed by a Gold N-Heterocyclic Carbene Complex

Harris

Nakafuku

et al. 2016

Organometallics

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The kinetics of the racemization of 1,3-disubstituted allenes catalyzed by (IPr)AuOTf (IPr = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidine) has been investigated. The rate of gold-catalyzed allene racemization obeyed the following second-order rate law: rate = k rac [allene] [Au]. An analysis of the rate of the gold-catalyzed racemization of 1-aryl-1,2-butadienes as a function of allene electron donor ability established moderate depletion of electron density on the C1 allenyl carbon atom in the rate-limiting transition state for racemization. Analysis of the temperature dependence of the rate of racemization of 1-(p-tolyl)-1,2-butadiene established the activation parameters ΔH ⧧ = 8.3 ± 1 kcal mol −1 and ΔS ⧧ = −28 ± 4 eu. These observations were in accord with a mechanism for allene racemization involving turnover-limiting, intermolecular allene exchange followed by rapid allene stereomutation. ■ INTRODUCTIONThe proliferation of gold(I)-catalyzed methods for the functionalization of C−C multiple bonds represents one of the most significant developments in homogeneous catalysis over the past decade. 1 In addition to alkynes, alkenes, and allylic alcohols, allenes have proven to be particularly active and versatile substrates for gold-catalyzed functionalization. 2 Goldcatalyzed transformations of allenes include cycloaddition with alkenes and dienes 3 and hydrofunctionalization with carbon and heteroatom nucleophiles. 4 A distinguishing feature of allenes is the potential for axial chirality, and as such, there has been considerable interest in the gold-catalyzed stereospecific functionalization of enantiomerically enriched chiral allenes. 5 Although high levels of chirality transfer have been realized, efficient chirality transfer is often compromised by concomitant gold-catalyzed allene racemization. 6 Alternatively, we 7 and others 8 have exploited gold(I)-catalyzed allene racemization to realize the stereoconvergent enantioselective hydrofunctionalization of chiral racemic allenes and propargylic acetates.Despite the relevance of allene racemization to the gold(I)-catalyzed functionalization of allenes, our understanding of gold(I)-catalyzed allene racemization has been largely restricted to the information gleaned from computational analyses. 9 Toward an experimentally grounded understanding of the coordination chemistry of gold allene complexes, we have investigated the dynamic behavior of well-defined cationic gold π-allene complexes. 10 In particular, variable-temperature NMR analysis of the gold(I) π-4,5-nonadiene complex {(P)Au[η 2 -nPr(H)CCC(H)n-Pr]} + SbF 6 − (P = P(t-Bu) 2 o-biphenyl) and related compounds established facile π-face exchange (ΔG ⧧ 298 < 10 kcal mol −1 ) through chiral η 1 -allene intermediates (Scheme 1). In comparison, allene stereomutation was not detected prior to the onset of intermolecular allene exchange with an energy barrier of ΔG ⧧ 298 ≥ 17.4 kcal mol −1 .We subsequently investigated the kinetics of the racemization of axially chiral 1-aryl-1,2-butadienes catalyzed by c...

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Section: Discussioncontrasting

confidence: 65%

Section: Discussionmentioning

confidence: 63%

Kinetics and Mechanism of Allene Racemization Catalyzed by a Gold N-Heterocyclic Carbene Complex

Harris

Nakafuku

et al. 2016

Organometallics

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“…The carbene-philicity scale ( m CXY ) proposed by Moss − on the basis of kinetic data has been shown to be useful in categorizing carbenes − into electrophilic, ambiphilic, and nucleophilic varieties. There exists a linear free energy relationship between the carbene-philicity and the related Taft substituent parameters.…”

Section: Electrophilicity Scalesmentioning

confidence: 99%

Electrophilicity Index

2006

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“…The excellent σ-donating ability of NHCs compared to phosphine ligands supports stronger metal-ligand bonding, which leads to electron-rich metal centres and confers high stability and catalytic activity to the transition metal complexes [7,21]. In addition, the stereoelectronic properties of NHCs are easily tuneable by modifying the NHC backbone and the exocyclic nitrogen substituents [7,[21][22][23][24][25][26]. All these beneficial features have popularised the use of NHC ligands in transition metal chemistry [13,14,[27][28][29][30][31][32][33][34][35][36][37][38][39][40][41][42][43][44][45][46].…”

Section: Introductionmentioning

confidence: 99%