Organogold chemistry

Organogold chemistry is the study of compounds containing carbon-gold bonds. They are studied in academic research, but have not received widespread use otherwise. The dominant oxidation states for organogold compounds are I with coordination number 2 and a linear molecular geometry and III with CN = 2 and a planar molecular geometry.[1][2][3] The gold carbide Au2C2 was discovered in 1900.

Gold(I)

Gold(I) complexes are 2-coordinate, linear, diamagnetic, 14 electron species. They exist as adducts LAuR with as ligand L for instance a triphenylphosphine or an isocyanide. The ligand prevents reduction of Au(I) to metallic Au(0) with dimerization of the organic residue. Gold(I) can also exist as the aurate MAuR2 (the ate complex) whereby the cation is usually fitted with a complexing agent to improve stability. The AuR2− anion is also linear just as other M(d10) species such as Hg(Me)2 and Pd(Me)22+. Gold is known to form acetylides (capable of forming polymeric structures), carbenes and carbynes. The classic method for the preparation of LAuR compounds is by reaction of a Grignard reagent with a gold(I) halide. A subsequent reaction with an organolithium R-Li forms the ate complex.

In a special group of compounds, an aryl carbon atom acts as a bridge between two gold atoms. One such compound, (MesAu)5,is formed in a reaction between Au(CO)Cl and the mesityl Grignard. Carbon can be coordinated with gold up to a value to 6. Compounds of the type C(AuL)4 are isolobal with methane and those of type C(AuL)5+ isolobal with the methanium ion.

Gold cyanide compounds (MAu(CN)2) are of some importance to gold cyanidation, a process for the extraction of gold from low-grade ore. The carbon to metal bond in metal cyanides is usually ionic but evidence exists that the C-Au bonding in the gold cyanide ion is covalent.[4]

Gold(III)

Gold(III) complexes are 4 coordinate, square planar, diamagnetic, toxic, 16 electron species. When the formal coordination number is less than 4, ligands such as chlorine can make up for it by forming a bridging ligand. Intramolecular chelation is another strategy. In general gold(III) compounds are toxic and therefore less studied than gold(I). For all practical purposes the chemistry is confined to monoarylgold(III) complexes.

Gold catalysis

Gold-catalyzed reactions fall into two major categories: heterogeneous catalysis including catalysts by gold nanoparticles and thiol-monolayer gold surfaces, and homogeneous catalysis that occurs with gold(I) or gold(III) compounds and is used for organic synthesis.[5][6] Common commercially available gold catalysts are gold(I) chloride, gold(III) chloride, chloroauric acid and a range of gold phosphines such as chloro(triphenylphosphine)gold(I) . To increase the electrophilicity at the gold center, a more dissociated complex can be created through halide abstraction with an additive like silver triflate (AgOTf).

Gold(I) forms pi-complexes with alkene or alkyne bonds, following the Dewar-Chatt-Duncanson model. Gold is certainly not the only metal showing this type of bonding and reactivity, several metal ions isolobal with the simple hydrogen proton (cationic: emptied s-orbital) do as well: mercury(II) and platinum(II). In 2007 Fürstner & Davies [7] proposed the term pi-acid as a cognomen for this type of ion (See also: cation-pi interaction).

Gold(I)-alkene and -alkyne complexes are electrophilic and susceptible toward nucleophilic attack. In oxymercuration the resultant organomercurial species is generated stoichiometrically, and requires an additional step to liberate the product. In the case of gold, protonolysis of the Au-C bond closes the catalytic cycle, allowing the coordination of another substrate. Some advantages of gold(I) catalysis include: air stability (high oxidation potential of Au(I) to Au(III) is a barrier to oxidation), lowered oxophilicity and thus tolerance towards water and alcohols, C-Au bonds that prefer protodeauration over beta-hydride elimination, and relative non-toxicity.[8]

An early example of catalysis by gold was described by Ito in 1986 [9] who reacted benzaldehyde and methyl isocyanoacetate with a chiral ferrocenylphosphine ligand and cationic gold(I) compound (Au(cHexCN)2)BF4 to form a chiral oxazoline. This reaction was a first in more than one way: asymmetric Aldol reactions already existed but this was also the first catalytic one.

In a simple mechanistic picture, gold(I) simultaneously coordinates to two phosphine ligands and the carbon isocyanate group [10] which is then attacked by the carbonyl group. Further studies on the bonding mode of Au(I) indicate that this simple picture may have to be revised.

In 1976, Thomas and coworkers reported conversion of phenylacetylene to acetophenone using tetrachloroauric acid in a 37% yield.[11] In this reaction gold(III) was used as a homogeneous catalyst replacing mercury in oxymercuration. Interestingly, this same study lists a published yield >150%, indicating catalysis that perhaps was not acknowledged by the chemists.

In 1991 Utimoto reacted gold(III)(NaAuCl4) with alkynes and water.[12] Teles identified a major drawback of this method as Au(III) was rapidly reduced to catalytically dead metallic gold and in 1998 returned to the theme of ligand supported Au(I) for the same transformation:[13]

This particular reaction would trigger a lot of academic interest in gold catalysis in the years to come [14] Another example of an Au(III) catalysed reaction is in a forced alkyne / furan Diels-Alder reaction - these do not normally occur - ultimately forming a phenol:[15]

Further mechanistic studies conclude that this is not a concerted transformation, but rather an initial alkyne hydroarylation followed by a series of non-obvious intramolecular rearrangements, concluded with a 6-pi electrocyclization and rearomatization.

Heterogeneous gold catalysis is an older science. Gold is an attractive metal to use because of its stability against oxidation and its variety in morphology for instance gold cluster materials. Gold has been shown to be effective in low-temperature CO oxidation and acetylene hydrochlorination to vinyl chlorides. The exact nature of the catalytic site in this type of process is debated.[16] The notion that gold can catalyse a reaction does not imply it is the only way. However, other metals can do the same job inexpensively, notably in recent years iron (see organoiron chemistry).

Gold catalyzed reactions

Although of no commercial importance, gold catalyzes many organic transformations, usually carbon-carbon bond formation from Au(I), and C-X (X = O, N) bond formation from the Au(III) state, due to that ion's harder Lewis acidity. Hong C. Shen summarized homogeneous reactions forming cyclic compounds into 4 main categories:[17]

The soft gold(I) ion coordinates exclusively to the more pi-basic alkyne, leaving the alkene free to attack as the nucleophile.

Other reactions are the use of gold in C-H bond activation[21] and Itoh Aldol reactions. Gold also catalyses coupling reactions.[22]

References

  1. ↑ Elschenbroich, C. and Salzer, A. (1992) Organometallics : A Concise Introduction. Wiley-VCH: Weinheim. ISBN 3-527-28165-7
  2. ↑ Parish, R. V. (1997). "Organogold chemistry: II reactions". Gold Bulletin 30 (2): 55. doi:10.1007/BF03214757.
  3. ↑ Parish, R. V. (1998). "Organogold chemistry: III applications". Gold Bulletin 31: 14. doi:10.1007/BF03215470.
  4. ↑ Wang, X. B.; Wang, Y. L.; Yang, J.; Xing, X. P.; Li, J.; Wang, L. S. (2009). "Evidence of Significant Covalent Bonding in Au(CN)2−". Journal of the American Chemical Society 131 (45): 16368–70. doi:10.1021/ja908106e. PMID 19860420.
  5. ↑ Gold catalysis for organic synthesis F. Dean Toste (Editor) Thematic Series in the Open Access Beilstein Journal of Organic Chemistry
  6. ↑ Raubenheimer, H. G.; Schmidbaur, H. (2014). "The Late Start and Amazing Upswing in Gold Chemistry". Journal of Chemical Education 91 (12): 2024. doi:10.1021/ed400782p.
  7. ↑ Fürstner, A.; Davies, P. W. (2007). "Catalytic Carbophilic Activation: Catalysis by Platinum and Gold π Acids". Angewandte Chemie International Edition 46 (19): 3410. doi:10.1002/anie.200604335.
  8. ↑ Shen, H. C. (2008). "Recent advances in syntheses of heterocycles and carbocycles via homogeneous gold catalysis. Part 1: Heteroatom addition and hydroarylation reactions of alkynes, allenes, and alkenes". Tetrahedron 64 (18): 3885. doi:10.1016/j.tet.2008.01.081.
  9. ↑ Ito, Y.; Sawamura, M.; Hayashi, T. (1986). "Catalytic asymmetric aldol reaction: Reaction of aldehydes with isocyanoacetate catalyzed by a chiral ferrocenylphosphine-gold(I) complex". Journal of the American Chemical Society 108 (20): 6405. doi:10.1021/ja00280a056.
  10. ↑ Togni, A.; Pastor, S. D. (1990). "Chiral cooperativity: The nature of the diastereoselective and enantioselective step in the gold(I)-catalyzed aldol reaction utilizing chiral ferrocenylamine ligands". The Journal of Organic Chemistry 55 (5): 1649. doi:10.1021/jo00292a046.
  11. ↑ Norman, R. O. C.; Parr, W. J. E.; Thomas, C. B. (1976). "The reactions of alkynes, cyclopropanes, and benzene derivatives with gold(III)". Journal of the Chemical Society, Perkin Transactions 1 (18): 1983. doi:10.1039/P19760001983.
  12. ↑ Fukuda, Y.; Utimoto, K. (1991). "Effective transformation of unactivated alkynes into ketones or acetals with a gold(III) catalyst". The Journal of Organic Chemistry 56 (11): 3729. doi:10.1021/jo00011a058.
  13. ↑ Teles, J. H.; Brode, S.; Chabanas, M. (1998). "Cationic Gold(I) Complexes: Highly Efficient Catalysts for the Addition of Alcohols to Alkynes". Angewandte Chemie International Edition 37 (10): 1415. doi:10.1002/(SICI)1521-3773(19980605)37:10<1415::AID-ANIE1415>3.0.CO;2-N.
  14. ↑ Nugent, W. A. (2012). ""Black Swan Events" in Organic Synthesis". Angewandte Chemie International Edition 51 (36): 8936–49. doi:10.1002/anie.201202348. PMID 22893229.
  15. ↑ Hashmi, A. S. K.; Frost, T. M.; Bats, J. W. (2000). "Highly Selective Gold-Catalyzed Arene Synthesis". Journal of the American Chemical Society 122 (46): 11553. doi:10.1021/ja005570d.
  16. ↑ Hutchings, G. J.; Brust, M.; Schmidbaur, H. (2008). "Gold—an introductory perspective". Chemical Society Reviews 37 (9): 1759. doi:10.1039/b810747p.
  17. ↑ Shen, H. C. (2008). "Recent advances in syntheses of carbocycles and heterocycles via homogeneous gold catalysis. Part 2: Cyclizations and cycloadditions". Tetrahedron 64 (34): 7847. doi:10.1016/j.tet.2008.05.082.
  18. ↑ Reetz, M. T.; Sommer, K. (2003). "Gold-Catalyzed Hydroarylation of Alkynes". European Journal of Organic Chemistry 2003 (18): 3485. doi:10.1002/ejoc.200300260.
  19. ↑ Nieto-Oberhuber, C.; Muñoz, M. P.; Buñuel, E.; Nevado, C.; Cárdenas, D. J.; Echavarren, A. M. (2004). "Cationic Gold(I) Complexes: Highly Alkynophilic Catalysts for theexo- andendo-Cyclization of Enynes". Angewandte Chemie International Edition 43 (18): 2402. doi:10.1002/anie.200353207.
  20. ↑ Gasparrini, F.; Giovannoli, M.; Misiti, D.; Natile, G.; Palmieri, G.; Maresca, L. (1993). "Gold(III)-catalyzed one-pot synthesis of isoxazoles from terminal alkynes and nitric acid". Journal of the American Chemical Society 115 (10): 4401. doi:10.1021/ja00063a084.
  21. ↑ Hoffmann-Röder, A.; Krause, N. (2005). "The golden gate to catalysis". Organic & Biomolecular Chemistry 3 (3): 387–91. doi:10.1039/b416516k. PMID 15678171.
  22. ↑ Wegner, H. A.; Auzias, M. (2011). "Gold for C-C coupling reactions: a Swiss-Army-knife catalyst?". Angewandte Chemie International Edition 50 (36): 8236–47. doi:10.1002/anie.201101603. PMID 21818831.

See also

CH He
CLi CBe CB CC CN CO CF Ne
CNa CMg CAl CSi CP CS CCl CAr
CK CCa CSc CTi CV CCr CMn CFe CCo CNi CCu CZn CGa CGe CAs style="background-color:#98FF98" border-collapse:collapse" | CSe CBr CKr
CRb CSr CY CZr CNb CMo CTc CRu CRh CPd CAg CCd CIn CSn CSb CTe CI CXe
CCs CBa CHf CTa CW CRe COs CIr CPt CAu CHg CTl CPb CBi CPo CAt Rn
Fr CRa Rf Db Sg Bh Hs Mt Ds Rg Cn Uut Fl Uup Lv Uus Uuo
↓
CLa CCe CPr CNd CPm CSm CEu CGd CTb CDy CHo CEr CTm CYb CLu
Ac CTh CPa CU CNp CPu CAm CCm CBk CCf CEs Fm Md No Lr
Chemical bonds to carbon
Core organic chemistry Many uses in chemistry
Academic research, but no widespread use Bond unknown