Top Curr Chem (2014) DOI: 10.1007/128_2014_585 # Springer International Publishing Switzerland 2014
Aurophilicity in Gold(I) Catalysis: For Better or Worse? Dieter Weber and Michel R. Gagne´
Abstract This book chapter discusses the effects of aurophilicity on gold catalysis. First, a brief historic account of aurophilicity in organogold chemistry is given, focusing on the pioneering results which set the stage for its association with catalytic intermediates (gold vinyl and gold aryl complexes); this is followed by an introduction to cationic gold(I) as an electrophilic catalyst, and the first isolation of organogold intermediates from catalysis. In the main section, the growing number of reports observing aurophilic interactions in catalysis or illustrative model systems is reviewed in a non-comprehensive tutorial way. The effects of aurophilicity are discussed in the following structures: (1) the geminal diauration of C(sp2)-atoms; (2) geminal diauration of other atoms; (3) σ-π-diauration of terminal alkynes. It is apparent that in most cases efficient catalysis is hindered by aurophilic effects as less active aggregates tend to be formed from more active species [LAu]+, but there are a growing number of reports using aurophilicity as a driving force to access new reactivity and selectivity. Keywords Aurophilicity Geminal diauration Gold acetylide complexes Gold aryl complexes Gold catalysis Gold vinyl complexes Mechanistic proposals Organogold chemistry σ-π-Diauration
D. Weber (*) Max-Planck-Institut fu¨r Kohlenforschung, Kaiser-Wilhelm-Platz 1, 45470 Mu¨lheim an der Ruhr, Germany e-mail: [email protected] M.R. Gagne´ Department of Chemistry, Caudill and Kenan Laboratories, The University of North Carolina at Chapel Hill, Chapel Hill, NC 27599-3290, USA
D. Weber and M.R. Gagne´
Contents 1 Introduction 1.1 A Brief Historic Account of Aurophilicity in Organogold Chemistry 1.2 Gold Catalysis is Electrophilic Catalysis 1.3 Limitations/Challenges of Gold Catalysis 2 First Detailed Mechanistic Studies of Gold-Catalyzed Reactions Including the Isolation of Organogold Intermediates 2.1 Motivation for Detailed Mechanistic Studies in Gold Catalysis 2.2 Triad of Studies Supporting the Viability of Gold Vinyl Intermediates in Gold Catalysis 3 Recent Findings of Aurophilicity in Gold Catalysis and Relevant Model Complexes 3.1 Geminal Diauration of C(sp2)-Atoms 3.2 Geminal Diauration of Other Atoms 3.3 σ-π-Diauration of Acetylenes 4 Conclusion References
1 Introduction 1.1
A Brief Historic Account of Aurophilicity in Organogold Chemistry
Long before gold complexes were identified as promising homogenous catalysts (early 1990s), a variety of organogold(I) compounds were synthesized and studied (for representative reviews, see [1, 2]). For example, the Nesmeyanov Institute of Organoelement Compounds of Russian Academy of Sciences began its studies on “univalent” organogold(I) complexes in 1970 and published more than 100 articles on this topic in Russian and other chemistry journals. The synthesis of alkyl-, alkenyl-, alkynyl-, cyclopropyl-, benzyl-, allyl-, aryl-, cyclopentadienyl-, cymantrenyl-, and ferrocenylgold(I) complexes was described and their reactivity studied. These seminal studies were carried out by Nesmeyanov, Grandberg, and Baukova [3–6]. Other main contributors to this research field were the research groups of contemporaries such as Puddephatt ([7–9] and references therein), Schmidbaur ([10, 11] and references therein), and Kochi [12–17], and shortly thereafter by Gimeno [18], Laguna ([19] and references therein), and many others ([20, 21] and references therein, [22, 23]). Schmidbaur in particular highlighted organogold(I) complexes as tending to form multinuclear aggregates in both their solid state and in solution, and coined the terms aurophilic bonding or aurophilicity, which describe weak attractive forces between two gold centers comparable in strength to hydrogen bonding (5–10 kcal mol–1) [24]. Aurophilic bonding is characterized by an Au–Au distance of 2.8–3.5 Å in X-ray crystal structures. This wide range is a consequence of the mutual approach of two gold centers being characterized by a relatively flat energy profile [11, 25]. Because other closed shell metals have revealed similar behavior, the general terms metallophilicity and metallophilic bonding were also introduced [26].
Aurophilicity in Gold(I) Catalysis: For Better or Worse?
2
AuPPh3 Fe
AuPPh3
1 eq HBF4 • Et2O Et2O, –78°C or rt
1
Fe
– AuPPh3 [BF4] +
Fe
3
2 95% yield
Scheme 1 Synthesis of geminally diaurated gold complex [Fc(AuPPh3)2]+[BF4]– 1 eq HBF4 • Et2O [O(AuPPh3)3]+[BF4] – Fe CHCl3 3 AuPPh3
AuPPh3
1 eq Ph3PAuCl/AgBF4
Fe
Fe
– AuPPh3 [BF4]
1 2
AuPPh3 [BF4]–
1 eq AuPPh3 Fe
AuPPh3 4
1
Scheme 2 Synthetic pathways for [Fc(AuPPh3)2]+[BF4]– via alternative generations of [Ph3PAu]+[BF4]–
Nesmeyanov and Grandberg were among the first to identify this characteristic effect of gold while studying the reactivity of the ferrocenylgold(I)-complex FcAuPPh3 (1) with the Brønsted acid HBF4 (Scheme 1) ([27] and references therein). Treating this species with HBF4 led to the formation of the geminally diaurated complex [Fc(AuPPh3)2]+[BF4]– (2), which could be isolated as a white precipitate in 95% yield, and characterized by X-ray diffraction analysis and NMR. The structure of [Fc(AuPPh3)2]+[BF4]– (2) revealed a hyperconjugated C(sp2)atom with an Au2C-three-center-two-electron (3c-2e) bond [3–6]. The connectivity was stabilized by an aurophilic interaction between the two gold(I) atoms and a second metallophilic bond between one gold(I) atom and the low spin iron(II) center of ferrocene. 1H NMR signals of the diaurated Cp-ring exhibited large downfield shifts relative to the monogold complex (shifting from δ ¼ 3.90, 4.16 ppm to δ ¼ 4.17, 5.43 ppm upon diauration) and the diastereotopic phosphine ligands revealed two resonances at δ ¼ 36.4 and δ ¼ 38.2 ppm (31P NMR). 13C NMR data were not reported. Later studies by the same group showed that [Ph3PAu]+ accompanied by the low coordinating [BF4]– anion was the reactive gold species which intercepts the neutral monogold species to form dinuclear gold complexes. In this way, digold [Fc(AuPPh3)2]+[BF4]– (2) was accessible in several related ways (Scheme 2).
D. Weber and M.R. Gagne´
AuPPh3 [BF4]
+ L or X–
–
AuPPh3 +
[Ph3PAuL] +[BF4 ]– or
AuPPh3
Ph3PAuX
4
5 L = RAuPPh3, PPh3, morpholine X– = CN–, Cl–, I–
Scheme 3 Characteristic reactivity of digold complexes with abstracting ligands L or anions X– [L AuPPh3]+
+
L'
AuPPh3
Aurophilicity in Gold(I) Catalysis: For Better or Worse? Dieter Weber and Michel R. Gagne´
Abstract This book chapter discusses the effects of aurophilicity on gold catalysis. First, a brief historic account of aurophilicity in organogold chemistry is given, focusing on the pioneering results which set the stage for its association with catalytic intermediates (gold vinyl and gold aryl complexes); this is followed by an introduction to cationic gold(I) as an electrophilic catalyst, and the first isolation of organogold intermediates from catalysis. In the main section, the growing number of reports observing aurophilic interactions in catalysis or illustrative model systems is reviewed in a non-comprehensive tutorial way. The effects of aurophilicity are discussed in the following structures: (1) the geminal diauration of C(sp2)-atoms; (2) geminal diauration of other atoms; (3) σ-π-diauration of terminal alkynes. It is apparent that in most cases efficient catalysis is hindered by aurophilic effects as less active aggregates tend to be formed from more active species [LAu]+, but there are a growing number of reports using aurophilicity as a driving force to access new reactivity and selectivity. Keywords Aurophilicity Geminal diauration Gold acetylide complexes Gold aryl complexes Gold catalysis Gold vinyl complexes Mechanistic proposals Organogold chemistry σ-π-Diauration
D. Weber (*) Max-Planck-Institut fu¨r Kohlenforschung, Kaiser-Wilhelm-Platz 1, 45470 Mu¨lheim an der Ruhr, Germany e-mail: [email protected] M.R. Gagne´ Department of Chemistry, Caudill and Kenan Laboratories, The University of North Carolina at Chapel Hill, Chapel Hill, NC 27599-3290, USA
D. Weber and M.R. Gagne´
Contents 1 Introduction 1.1 A Brief Historic Account of Aurophilicity in Organogold Chemistry 1.2 Gold Catalysis is Electrophilic Catalysis 1.3 Limitations/Challenges of Gold Catalysis 2 First Detailed Mechanistic Studies of Gold-Catalyzed Reactions Including the Isolation of Organogold Intermediates 2.1 Motivation for Detailed Mechanistic Studies in Gold Catalysis 2.2 Triad of Studies Supporting the Viability of Gold Vinyl Intermediates in Gold Catalysis 3 Recent Findings of Aurophilicity in Gold Catalysis and Relevant Model Complexes 3.1 Geminal Diauration of C(sp2)-Atoms 3.2 Geminal Diauration of Other Atoms 3.3 σ-π-Diauration of Acetylenes 4 Conclusion References
1 Introduction 1.1
A Brief Historic Account of Aurophilicity in Organogold Chemistry
Long before gold complexes were identified as promising homogenous catalysts (early 1990s), a variety of organogold(I) compounds were synthesized and studied (for representative reviews, see [1, 2]). For example, the Nesmeyanov Institute of Organoelement Compounds of Russian Academy of Sciences began its studies on “univalent” organogold(I) complexes in 1970 and published more than 100 articles on this topic in Russian and other chemistry journals. The synthesis of alkyl-, alkenyl-, alkynyl-, cyclopropyl-, benzyl-, allyl-, aryl-, cyclopentadienyl-, cymantrenyl-, and ferrocenylgold(I) complexes was described and their reactivity studied. These seminal studies were carried out by Nesmeyanov, Grandberg, and Baukova [3–6]. Other main contributors to this research field were the research groups of contemporaries such as Puddephatt ([7–9] and references therein), Schmidbaur ([10, 11] and references therein), and Kochi [12–17], and shortly thereafter by Gimeno [18], Laguna ([19] and references therein), and many others ([20, 21] and references therein, [22, 23]). Schmidbaur in particular highlighted organogold(I) complexes as tending to form multinuclear aggregates in both their solid state and in solution, and coined the terms aurophilic bonding or aurophilicity, which describe weak attractive forces between two gold centers comparable in strength to hydrogen bonding (5–10 kcal mol–1) [24]. Aurophilic bonding is characterized by an Au–Au distance of 2.8–3.5 Å in X-ray crystal structures. This wide range is a consequence of the mutual approach of two gold centers being characterized by a relatively flat energy profile [11, 25]. Because other closed shell metals have revealed similar behavior, the general terms metallophilicity and metallophilic bonding were also introduced [26].
Aurophilicity in Gold(I) Catalysis: For Better or Worse?
2
AuPPh3 Fe
AuPPh3
1 eq HBF4 • Et2O Et2O, –78°C or rt
1
Fe
– AuPPh3 [BF4] +
Fe
3
2 95% yield
Scheme 1 Synthesis of geminally diaurated gold complex [Fc(AuPPh3)2]+[BF4]– 1 eq HBF4 • Et2O [O(AuPPh3)3]+[BF4] – Fe CHCl3 3 AuPPh3
AuPPh3
1 eq Ph3PAuCl/AgBF4
Fe
Fe
– AuPPh3 [BF4]
1 2
AuPPh3 [BF4]–
1 eq AuPPh3 Fe
AuPPh3 4
1
Scheme 2 Synthetic pathways for [Fc(AuPPh3)2]+[BF4]– via alternative generations of [Ph3PAu]+[BF4]–
Nesmeyanov and Grandberg were among the first to identify this characteristic effect of gold while studying the reactivity of the ferrocenylgold(I)-complex FcAuPPh3 (1) with the Brønsted acid HBF4 (Scheme 1) ([27] and references therein). Treating this species with HBF4 led to the formation of the geminally diaurated complex [Fc(AuPPh3)2]+[BF4]– (2), which could be isolated as a white precipitate in 95% yield, and characterized by X-ray diffraction analysis and NMR. The structure of [Fc(AuPPh3)2]+[BF4]– (2) revealed a hyperconjugated C(sp2)atom with an Au2C-three-center-two-electron (3c-2e) bond [3–6]. The connectivity was stabilized by an aurophilic interaction between the two gold(I) atoms and a second metallophilic bond between one gold(I) atom and the low spin iron(II) center of ferrocene. 1H NMR signals of the diaurated Cp-ring exhibited large downfield shifts relative to the monogold complex (shifting from δ ¼ 3.90, 4.16 ppm to δ ¼ 4.17, 5.43 ppm upon diauration) and the diastereotopic phosphine ligands revealed two resonances at δ ¼ 36.4 and δ ¼ 38.2 ppm (31P NMR). 13C NMR data were not reported. Later studies by the same group showed that [Ph3PAu]+ accompanied by the low coordinating [BF4]– anion was the reactive gold species which intercepts the neutral monogold species to form dinuclear gold complexes. In this way, digold [Fc(AuPPh3)2]+[BF4]– (2) was accessible in several related ways (Scheme 2).
D. Weber and M.R. Gagne´
AuPPh3 [BF4]
+ L or X–
–
AuPPh3 +
[Ph3PAuL] +[BF4 ]– or
AuPPh3
Ph3PAuX
4
5 L = RAuPPh3, PPh3, morpholine X– = CN–, Cl–, I–
Scheme 3 Characteristic reactivity of digold complexes with abstracting ligands L or anions X– [L AuPPh3]+
+
L'
AuPPh3
