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A bonding model for gold(I) carbene complexes.

Benitez D, Shapiro ND, Tkatchouk E, Wang Y, Goddard WA, Toste FD - Nat Chem (2009)

Bottom Line: Herein, we propose that the carbon-gold bond in these intermediates is comprised of varying degrees of both sigma and pi-bonding; however, the overall bond order is generally less than or equal to unity.The bonding in a given gold-stabilized intermediate, and the position of this intermediate on a continuum ranging from gold-stabilized singlet carbene to gold-coordinated carbocation, is dictated by the carbene substituents and the ancillary ligand.Experiments show that the correlation between bonding and reactivity is reflected in the yield of gold-catalyzed cyclopropanation reactions.

View Article: PubMed Central - PubMed

Affiliation: Materials and Process Simulation Center, California Institute of Technology, Pasadena, California 91125 USA.

ABSTRACT
The last decade has witnessed dramatic growth in the number of reactions catalyzed by electrophilic gold complexes. While proposed mechanisms often invoke the intermediacy of gold-stabilized cationic species, the nature of bonding in these intermediates remains unclear. Herein, we propose that the carbon-gold bond in these intermediates is comprised of varying degrees of both sigma and pi-bonding; however, the overall bond order is generally less than or equal to unity. The bonding in a given gold-stabilized intermediate, and the position of this intermediate on a continuum ranging from gold-stabilized singlet carbene to gold-coordinated carbocation, is dictated by the carbene substituents and the ancillary ligand. Experiments show that the correlation between bonding and reactivity is reflected in the yield of gold-catalyzed cyclopropanation reactions.

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Arrow pushing in the formation of gold-stabilized carbenes is a useful mnemonic for keeping track of electrons, but it can lead to misconceptions about bonding. While the Au-C bonsd in the intermediate carbene has both σ and π components, the overall bond order is generally less than or equal to unity.
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Figure 4: Arrow pushing in the formation of gold-stabilized carbenes is a useful mnemonic for keeping track of electrons, but it can lead to misconceptions about bonding. While the Au-C bonsd in the intermediate carbene has both σ and π components, the overall bond order is generally less than or equal to unity.

Mentions: The model in Figure 3d for bonding in gold-stabilized carbenes proposes that these intermediates possess highly electron-deficient α-carbons that are stabilized, to varying degrees, by back-donation from the metal to the vacant pπ-orbital of the singlet carbene. This electronic deficiency reduces donation from the filled sp2 σ-orbital of the carbene to the metal, therefore minimizing gold–carbon σ-bonding. Thus, our model suggests that the conversion of a vinylgold intermediate into a gold-stabilized carbene, which is commonly proposed in gold-catalyzed reactions29–34, occurs with an increase in gold–carbon π-bonding and a decrease in the σ-bonding (Figure 4). The bonding situation in these carbene intermediates has often been depicted by two extreme resonance structures: a carbocation with a gold-carbon single bond or a carbene with a gold-carbon double bond. Much like the double “half-bond” model proposed for rhodium-carbenoid intermediates35,36, the depiction of a gold-stabilized carbene with a gold-carbon double bond should not be taken as an indication of a bond order of 2; but rather a means to convey that both σ and π components to the bond are present. To illustrate this, we calculated gold-carbon natural bond orders for AuOPMe (0.53), AuMePMe (0.91), and 7 (1.14). Nucleophilic attack on the now highly electrophilic pπ-orbital of the carbon adjacent to gold restores the gold-carbon sigma bond37–39. In this scenario, divergence towards carbocation-like or carbene-like reactivity may also be influenced by the potential of the nucleophile to intercept the developing positive charge. Alternatively, gold-stabilized carbene intermediates may react with concerted carbene-like reactivity (i.e. cyclopropanation), especially when the gold is coordinated to electron donating ligands.


A bonding model for gold(I) carbene complexes.

Benitez D, Shapiro ND, Tkatchouk E, Wang Y, Goddard WA, Toste FD - Nat Chem (2009)

Arrow pushing in the formation of gold-stabilized carbenes is a useful mnemonic for keeping track of electrons, but it can lead to misconceptions about bonding. While the Au-C bonsd in the intermediate carbene has both σ and π components, the overall bond order is generally less than or equal to unity.
© Copyright Policy
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC2748951&req=5

Figure 4: Arrow pushing in the formation of gold-stabilized carbenes is a useful mnemonic for keeping track of electrons, but it can lead to misconceptions about bonding. While the Au-C bonsd in the intermediate carbene has both σ and π components, the overall bond order is generally less than or equal to unity.
Mentions: The model in Figure 3d for bonding in gold-stabilized carbenes proposes that these intermediates possess highly electron-deficient α-carbons that are stabilized, to varying degrees, by back-donation from the metal to the vacant pπ-orbital of the singlet carbene. This electronic deficiency reduces donation from the filled sp2 σ-orbital of the carbene to the metal, therefore minimizing gold–carbon σ-bonding. Thus, our model suggests that the conversion of a vinylgold intermediate into a gold-stabilized carbene, which is commonly proposed in gold-catalyzed reactions29–34, occurs with an increase in gold–carbon π-bonding and a decrease in the σ-bonding (Figure 4). The bonding situation in these carbene intermediates has often been depicted by two extreme resonance structures: a carbocation with a gold-carbon single bond or a carbene with a gold-carbon double bond. Much like the double “half-bond” model proposed for rhodium-carbenoid intermediates35,36, the depiction of a gold-stabilized carbene with a gold-carbon double bond should not be taken as an indication of a bond order of 2; but rather a means to convey that both σ and π components to the bond are present. To illustrate this, we calculated gold-carbon natural bond orders for AuOPMe (0.53), AuMePMe (0.91), and 7 (1.14). Nucleophilic attack on the now highly electrophilic pπ-orbital of the carbon adjacent to gold restores the gold-carbon sigma bond37–39. In this scenario, divergence towards carbocation-like or carbene-like reactivity may also be influenced by the potential of the nucleophile to intercept the developing positive charge. Alternatively, gold-stabilized carbene intermediates may react with concerted carbene-like reactivity (i.e. cyclopropanation), especially when the gold is coordinated to electron donating ligands.

Bottom Line: Herein, we propose that the carbon-gold bond in these intermediates is comprised of varying degrees of both sigma and pi-bonding; however, the overall bond order is generally less than or equal to unity.The bonding in a given gold-stabilized intermediate, and the position of this intermediate on a continuum ranging from gold-stabilized singlet carbene to gold-coordinated carbocation, is dictated by the carbene substituents and the ancillary ligand.Experiments show that the correlation between bonding and reactivity is reflected in the yield of gold-catalyzed cyclopropanation reactions.

View Article: PubMed Central - PubMed

Affiliation: Materials and Process Simulation Center, California Institute of Technology, Pasadena, California 91125 USA.

ABSTRACT
The last decade has witnessed dramatic growth in the number of reactions catalyzed by electrophilic gold complexes. While proposed mechanisms often invoke the intermediacy of gold-stabilized cationic species, the nature of bonding in these intermediates remains unclear. Herein, we propose that the carbon-gold bond in these intermediates is comprised of varying degrees of both sigma and pi-bonding; however, the overall bond order is generally less than or equal to unity. The bonding in a given gold-stabilized intermediate, and the position of this intermediate on a continuum ranging from gold-stabilized singlet carbene to gold-coordinated carbocation, is dictated by the carbene substituents and the ancillary ligand. Experiments show that the correlation between bonding and reactivity is reflected in the yield of gold-catalyzed cyclopropanation reactions.

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