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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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Structural and electronic comparison of cationic metal-free and [AuPMe3]+ substituted substrates. (a) On the left, calculated bond distances (Å), and on the right, natural charges for C1, C2, and C3. Parameter A is defined as the ratio of bond distances (C1–C2)/(C2–C3) and correlates to the polarization of the π-electrons along the delocalized C1–C2–C3 system. (b) A comparison of with Au-alkylidene 7 showing the relative natural orbital populations of σ and π contributions to the Au-C bond. On the right calculated bond distances (Å) for 7.
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Figure 2: Structural and electronic comparison of cationic metal-free and [AuPMe3]+ substituted substrates. (a) On the left, calculated bond distances (Å), and on the right, natural charges for C1, C2, and C3. Parameter A is defined as the ratio of bond distances (C1–C2)/(C2–C3) and correlates to the polarization of the π-electrons along the delocalized C1–C2–C3 system. (b) A comparison of with Au-alkylidene 7 showing the relative natural orbital populations of σ and π contributions to the Au-C bond. On the right calculated bond distances (Å) for 7.

Mentions: As a basis for comparison, we began by calculating the structures and natural atomic charges23 of metal-free allyl cations 4, 5, and 6 (Figure 2a). We also defined parameter A as the ratio of the bond lengths: A ≡ (C1-C2)/(C2-C3), so that A indicates roughly whether the partial positive charge on the substrate is more stabilized by its C1 or C3 substituents. The low value of A in 4 (0.926) is indicative of the stabilizing nature of the oxygen lone pairs. The magnitude of A increases with less-donating C3-methyl substituents (A = 0.955) and even further for ester-substituted substrate 6 (A = 0.983). The corresponding gold-coordinated structures AuOPMe, AuMePMe and AuEPMe, all show increased A values as a result of the ability of the gold moiety to stabilize positive charge at C1. In AuMePMe, A is close to 1 (0.993), suggesting that a secondary gold-stabilized carbocation is as stabilized as a tertiary carbocation. For the diester-substituted allyl carbene AuEPMe, the π system is now polarized towards the electron deficient C3 leading to A=1.029. Importantly, the magnitude of stabilization from the gold-moiety increases with increasing electrophilicity of the allyl-cation. This conclusion can also be reached by considering the natural atomic charge on C3. This charge is essentially unaffected in 4 (0.86) versus AuOPMe (0.83), while it is significantly reduced in 5 (0.43) versus AuMePMe (0.34) and 6 (0.21) versus AuEPMe (0.10).


A bonding model for gold(I) carbene complexes.

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

Structural and electronic comparison of cationic metal-free and [AuPMe3]+ substituted substrates. (a) On the left, calculated bond distances (Å), and on the right, natural charges for C1, C2, and C3. Parameter A is defined as the ratio of bond distances (C1–C2)/(C2–C3) and correlates to the polarization of the π-electrons along the delocalized C1–C2–C3 system. (b) A comparison of with Au-alkylidene 7 showing the relative natural orbital populations of σ and π contributions to the Au-C bond. On the right calculated bond distances (Å) for 7.
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Show All Figures
getmorefigures.php?uid=PMC2748951&req=5

Figure 2: Structural and electronic comparison of cationic metal-free and [AuPMe3]+ substituted substrates. (a) On the left, calculated bond distances (Å), and on the right, natural charges for C1, C2, and C3. Parameter A is defined as the ratio of bond distances (C1–C2)/(C2–C3) and correlates to the polarization of the π-electrons along the delocalized C1–C2–C3 system. (b) A comparison of with Au-alkylidene 7 showing the relative natural orbital populations of σ and π contributions to the Au-C bond. On the right calculated bond distances (Å) for 7.
Mentions: As a basis for comparison, we began by calculating the structures and natural atomic charges23 of metal-free allyl cations 4, 5, and 6 (Figure 2a). We also defined parameter A as the ratio of the bond lengths: A ≡ (C1-C2)/(C2-C3), so that A indicates roughly whether the partial positive charge on the substrate is more stabilized by its C1 or C3 substituents. The low value of A in 4 (0.926) is indicative of the stabilizing nature of the oxygen lone pairs. The magnitude of A increases with less-donating C3-methyl substituents (A = 0.955) and even further for ester-substituted substrate 6 (A = 0.983). The corresponding gold-coordinated structures AuOPMe, AuMePMe and AuEPMe, all show increased A values as a result of the ability of the gold moiety to stabilize positive charge at C1. In AuMePMe, A is close to 1 (0.993), suggesting that a secondary gold-stabilized carbocation is as stabilized as a tertiary carbocation. For the diester-substituted allyl carbene AuEPMe, the π system is now polarized towards the electron deficient C3 leading to A=1.029. Importantly, the magnitude of stabilization from the gold-moiety increases with increasing electrophilicity of the allyl-cation. This conclusion can also be reached by considering the natural atomic charge on C3. This charge is essentially unaffected in 4 (0.86) versus AuOPMe (0.83), while it is significantly reduced in 5 (0.43) versus AuMePMe (0.34) and 6 (0.21) versus AuEPMe (0.10).

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