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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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Calculated and experimental activation energies to bond rotation (indicated with arrows). C3-C2 bond rotation barriers are decreased when C3 is substituted with carbocation-stabilizing oxygen atoms. Throughout the text gold complexes will be referred to according to the AuXL notation, where L indicates the auxiliary ligand on gold and X indicates the C3 substituents (Me = methyl, O = 1,3-dioxanyl, OMe = methoxy, E = methyl ester).
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Figure 1: Calculated and experimental activation energies to bond rotation (indicated with arrows). C3-C2 bond rotation barriers are decreased when C3 is substituted with carbocation-stabilizing oxygen atoms. Throughout the text gold complexes will be referred to according to the AuXL notation, where L indicates the auxiliary ligand on gold and X indicates the C3 substituents (Me = methyl, O = 1,3-dioxanyl, OMe = methoxy, E = methyl ester).

Mentions: Recently, the M06 flavor of DFT has been shown to accurately describe21,22 transition metal catalyzed organic transformations. To confirm this, we calculated rotational barriers for (Z)-AuOPPh and (Z)-AuOMePMe (Figure 1) and obtained ΔG‡=10.6 kcal mol−1 for (Z)-AuOPPh, in excellent agreement with experiment (ΔG‡=11.0) and ΔG‡=5.8 for (Z)-AuOMePMe, also consistent with experiment (<7.2 kcal mol−1). Previous DFT studies8 used either B3LYP or BP86 functionals. Although these methods have proven valuable for many organometallic studies, we find that they are insufficient to resolve the issues of interest here. For example, B3LYP predicts rotational barriers of ΔG‡=9.1 and 7.9 kcal mol−1, while BP86 predicts ΔG‡=8.9 and 7.4 kcal mol−1 for (Z)-AuOPPh and (Z)-AuOMePMe, respectively. This suggests that both B3LYP and BP86 cannot resolve the effects of the more electron donating PMe3 from the less electron donating PPh3.


A bonding model for gold(I) carbene complexes.

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

Calculated and experimental activation energies to bond rotation (indicated with arrows). C3-C2 bond rotation barriers are decreased when C3 is substituted with carbocation-stabilizing oxygen atoms. Throughout the text gold complexes will be referred to according to the AuXL notation, where L indicates the auxiliary ligand on gold and X indicates the C3 substituents (Me = methyl, O = 1,3-dioxanyl, OMe = methoxy, E = methyl ester).
© Copyright Policy
Related In: Results  -  Collection

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

Figure 1: Calculated and experimental activation energies to bond rotation (indicated with arrows). C3-C2 bond rotation barriers are decreased when C3 is substituted with carbocation-stabilizing oxygen atoms. Throughout the text gold complexes will be referred to according to the AuXL notation, where L indicates the auxiliary ligand on gold and X indicates the C3 substituents (Me = methyl, O = 1,3-dioxanyl, OMe = methoxy, E = methyl ester).
Mentions: Recently, the M06 flavor of DFT has been shown to accurately describe21,22 transition metal catalyzed organic transformations. To confirm this, we calculated rotational barriers for (Z)-AuOPPh and (Z)-AuOMePMe (Figure 1) and obtained ΔG‡=10.6 kcal mol−1 for (Z)-AuOPPh, in excellent agreement with experiment (ΔG‡=11.0) and ΔG‡=5.8 for (Z)-AuOMePMe, also consistent with experiment (<7.2 kcal mol−1). Previous DFT studies8 used either B3LYP or BP86 functionals. Although these methods have proven valuable for many organometallic studies, we find that they are insufficient to resolve the issues of interest here. For example, B3LYP predicts rotational barriers of ΔG‡=9.1 and 7.9 kcal mol−1, while BP86 predicts ΔG‡=8.9 and 7.4 kcal mol−1 for (Z)-AuOPPh and (Z)-AuOMePMe, respectively. This suggests that both B3LYP and BP86 cannot resolve the effects of the more electron donating PMe3 from the less electron donating PPh3.

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.

Show MeSH